"Science, Faculty of"@en . "Chemistry, Department of"@en . "DSpace"@en . "UBCV"@en . "Zwarich, Ronald James"@en . "2011-06-24T20:41:18Z"@en . "1968"@en . "Doctor of Philosophy - PhD"@en . "University of British Columbia"@en . "The fundamental vibrations of carbazole have been assigned from polarized infrared, Raman and fluorescence spectra. A crude normal coordinate calculation using a force field transferred from phenanthrene for the in-plane problem, and from benzene and anthracene for the out-of-plane problem gave satisfactory agreement with the observed frequencies. From a study of the polarized absorption spectrum of carbazole in a single crystal matrix of fluorene at about 15\u00C2\u00B0K, the lowest-energy transition is assigned \u00C2\u00B9A\u00E2\u0082\u0081\u00E2\u0086\u0090\u00C2\u00B9A\u00E2\u0082\u0081, and the excited state vibrations are analysed. In fluorene, an impurity with a weak absorption at 3200 \u00C3\u0085, which previous workers inadvertently attributed to fluorene itself, was characterized as benz[f] indan. From the polarized absorption and fluorescence spectra, the transition is assigned \u00C2\u00B9A\u00E2\u0082\u0081\u00E2\u0086\u0090\u00C2\u00B9A\u00E2\u0082\u0081 and vibrational analyses of these spectra are given.\r\nFrom a study of the absorption spectrum of a-fluorene crystal (ab face), the solution absorption spectrum is given the following interpretation: the 3000 \u00C3\u0085 system of medium intensity and the stronger 2600 \u00C3\u0085 system are long-axis polarized, and a rather weak band at 2730 \u00C3\u0085 is short-axis polarized. Vibrational analyses of the absorption and fluorescence spectra in a polycrystalline n-heptane matrix at about 15\u00C2\u00B0K are presented. A vibrational analysis of the intense blue phosphorescence induced in fluorene by the deliberate addition of dibenzothiophene at about 6\u00C2\u00B0K shows that the intervals are identical with those in the fluorescence and phosphorescence of fluorene in n-heptane. An assignment of the fundamentals of fluorene from polarized infrared and Raman spectra is reported.\r\nPolarized infrared and Raman spectra of dibenzothiophene-h\u00E2\u0082\u0088 and -d\u00E2\u0082\u0088. are utilized in an assignment of their fundamental vibrations. The in-plane frequencies were calculated using the force field transferred from phenan-threne. The lowest-energy transition of dibenzothiophene in fluorene at about 15\u00C2\u00B0K is assigned \u00C2\u00B9A\u00E2\u0082\u0081\u00E2\u0086\u0090\u00C2\u00B9A\u00E2\u0082\u0081. Vibrational analyses of the absorption spectrum in n-heptane and of the fluorescence and phosphorescence spectra in the crystal, n-heptane, and hexamethylbenzene are presented. An intense blue phosphorescence observed at 6\u00C2\u00B0K in a biphenyl crystal following the deliberate addition of either carbazole or dibenzothiophene was identified as biphenyl phosphorescence originating from energy trapping centers created in the crystal lattice. The triplet band energy of biphenyl estimated from preliminary temperature dependence studies of the phosphorescence and delayed fluorescence agrees satisfactorily with Hirota's independent measurement."@en . "https://circle.library.ubc.ca/rest/handle/2429/35750?expand=metadata"@en . "A STUDY OF THE SPECTRA OF SOME ORGANIC COMPOUNDS BY RONALD JAMES ZWARICH B.Sc. (Hons.) U n i v e r s i t y of B r i t i s h Columbia, 1963 A THESIS SUBMITTED' IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the req u i r e d standard The U n i v e r s i t y of B r i t i s h Columbia December, 1968 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e Head o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h C o l u m b i V a n c o u v e r 8, Canada D e p a r t m e n t A b s t r a c t The fundamental v i b r a t i o n s of carbazole have been assigned from p o l a r i z e d i n f r a r e d , Raman and fl u o r e s c e n c e s p e c t r a . A crude normal coordinate c a l c u l a t i o n using a f o r c e f i e l d t r a n s f e r r e d from phenanthrene f o r the in-plane problem, and from benzene and anthracene f o r the out-of-plane problem gave s a t i s f a c t o r y agreement with the observed fre q u e n c i e s . From a study of the p o l a r i z e d absorption spectrum of carbazole i n a s i n g l e c r y s t a l matrix of f l u o r e n e at about 15\u00C2\u00B0K, the lowest-energy t r a n s i t i o n i s assigned ^ A ^ - \u00C2\u00AB \u00E2\u0080\u0094 a n c * the e x c i t e d s t a t e v i b r a t i o n s are analysed. o In f l u o r e n e , an i m p u r i t y w i t h a weak abso r p t i o n at 3200 A, which previous workers i n a d v e r t e n t l y a t t r i b u t e d to f l u o r e n e i t s e l f , was c h a r a c t e r -i z e d as b e n z [ f ] i n d a n . From the p o l a r i z e d absorption and f l u o r e s c e n c e s p e c t r a , the t r a n s i t i o n i s assigned *Aj-\u00C2\u00AB\u00E2\u0080\u0094^^\> a n c* v i b r a t i o n a l analyses of these s p e c t r a are given. From a study of the absorption spectrum of a-fluorene c r y s t a l (ab f a c e ) , the s o l u t i o n a b s o r p t i o n spectrum i s given the f o l l o w i n g i n t e r p r e t a t i o n : o o the 3000 A system of medium i n t e n s i t y and the stronger 2600 A system are o long-axis p o l a r i z e d , and a r a t h e r weak band at 2730 A i s s h o r t - a x i s p o l a r i z e d . V i b r a t i o n a l analyses of the a b s o r p t i o n and f l u o r e s c e n c e s p e c t r a i n a p o l y c r y s t a l l i n e n-heptane matrix at about 15\u00C2\u00B0K are presented. A v i b r a t i o n a l a n a l y s i s of the intense blue phosphorescence induced i n fluorene by the d e l i b e r a t e a d d i t i o n of dibenzothiophene at about 6\u00C2\u00B0K shows that the i n t e r v a l s are i d e n t i c a l w i t h those i n the fluorescence and phosphorescence of f l u o r e n e i n n-heptane. An assignment of the fundamentals of f l u o r e n e from p o l a r i z e d i n f r a r e d and Raman sp e c t r a i s r e p o r t e d . P o l a r i z e d i n f r a r e d and Raman spec t r a of dibenzothiophene-h and -d. are u t i l i z e d i n an assignment of t h e i r fundamental v i b r a t i o n s . The in-plane frequencies were c a l c u l a t e d using the f o r c e f i e l d t r a n s f e r r e d from phenan-threne. The lowest-energy t r a n s i t i o n of dibenzothiophene i n f l u o r e n e at about 15\u00C2\u00B0K i s assigned * A j - * \u00E2\u0080\u0094 . V i b r a t i o n a l analyses of the absorption spectrum i n n-heptane and of the fluo r e s c e n c e and phosphorescence sp e c t r a i n the c r y s t a l , n-heptane, and hexamethylbenzene are presented. An i n t e n s e blue phosphorescence observed at 6\u00C2\u00B0K i n a biphenyl c r y s t a l f o l l o w i n g the d e l i b e r a t e a d d i t i o n of e i t h e r carbazole or dibenzothiophene was i d e n t i f i e d as biphenyl phosphorescence o r i g i n a t i n g from energy t r a p p i n g centers created i n the c r y s t a l l a t t i c e . The t r i p l e t band energy of biphenyl estimated from p r e l i m i n a r y temperature dependence s t u d i e s of the phosphorescence and delayed f l u o r e s c e n c e agrees s a t i s f a c t o r i l y w i t h H i r o t a ' s independent measurement. - i v -Table of Contents A b s t r a c t 1 1 Table of Contents i v L i s t of Figures l x L i s t of Tables x i v Acknowledgements .... x v m Chapter 1 General I n t r o d u c t i o n \u00E2\u0080\u00A2 1 A. Introductory Remarks 1 B. The Theory of V i b r a t i o n a l - E l e c t r o n i c I n t e r a c t i o n s 3 C. The Franck-Condon P r i n c i p l e 8 D. The Theory of Excitons i n Molecular C r y s t a l s 9 E. Coupling C r i t e r i a and D i s s i p a t i o n of E x c i t a t i o n Energy i n Molecular C r y s t a l s 15 F. Mixed Molecular C r y s t a l Systems 17 G. The Theory of Molecular V i b r a t i o n s 18 H. The I n t e n s i t y of I n f r a r e d Bands 24 I. Determination of Force Constants 25 Chapter 2 M a t e r i a l s and Experimental Methods 28 A. P u r i f i c a t i o n and Synthesis of M a t e r i a l s 28 B. P r e p a r a t i o n of Samples 33 C. Cryostats 3 4 D. C a l i b r a t i o n of Gas Thermometer 35 E. Apparatus .; 3^ F. I n f r a r e d Spectra 3 ^ G. Raman Spectra 3 ^ - v -H. Measurement of Spectra 4 0 I . C r y s t a l Data and O p t i c a l P r o p e r t i e s of Some Organic Compounds \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 4 ^ Chapter 3 A V i b r a t i o n a l Assignment of Carbazole from I n f r a r e d , Raman, and Fluorescence Spectra A. I n t r o d u c t i o n B. S e l e c t i o n Rules Chapter 4 The Absorption Spectrum of Carbazole i n a Fluorene M a t r i x A. I n t r o d u c t i o n C. D i s c u s s i o n . Chapter 5 U l t r a - v i o l e t Spectra of Benz[f]indan Impurity i n a Fluorene C r y s t a l A. I n t r o d u c t i o n B. The Mixed C r y s t a l Systems C. The Absorption Spectrum 48 48 49 C. The I n f r a r e d Spectrum 5 3 58 62 D. The Raman Spectra \u00E2\u0080\u00A2 E. The Fluorescence Spectrum F. Assignment of Fundamentals 69 G. Normal Coordinate C a l c u l a t i o n 72 H. Conclusion ^9 82 82 B. Spectrum ^ 3 88 92 92 93 95 D. The Fluorescence Spectrum 99 - v i -E. D i s c u s s i o n 102 Chapter 6 A Study of Some E x c i t e d S i n g l e t and T r i p l e t E l e c t r o n i c States o f Fluorene \u00E2\u0080\u00A2\u00E2\u0080\u00A2 A. I n t r o d u c t i o n 106 B. The Pure C r y s t a l Spectra 1 0 7 1. S e l e c t i o n Rules 1 0 7 2. C r y s t a l Energies 109 3. C a l c u l a t i o n of the C r y s t a l S p l i t t i n g s ; D i p o le-Dipole Approximation 109 4. Room Temperature Absorption 113 5. Absorption at Low Temperature H 6 6. Fluorescence at Low Temperature H 7 C. Spectra i n a M a t r i x 1. The Absorption Spectrum 120 2. The Fluorescence Spectrum 122 3. The T r i p l e t State I 2 5 Chapter 7 A V i b r a t i o n a l Assignment of Fluorene from the I n f r a r e d and Raman Spectra A. I n t r o d u c t i o n I 3 2 B. S e l e c t i o n Rules i 3 2 C. The I n f r a r e d Spectrum I 3 4 D. Raman Spectra 1^0 / \u00E2\u0080\u00A2 14^ E. Assignment of Fundamentals 1. The Low-Energy Region i 4 5 2. A 1 Symmetry i 4 6 - v i i -3. B 1 Symmetry : .147 4. A 2 Symmetry 148 5. B 2 Symmetry 148 F. D i s c u s s i o n 149 Chapter 8 A V i b r a t i o n a l Assignment of Dibenzothioptiene-h and Dibenzothiophene-d from I n f r a r e d and Raman Spectra... 153 o A. I n t r o d u c t i o n 153 B. S e l e c t i o n Rules - 153 C. The I n f r a r e d Spectrum 156 D. Raman Spectra 163 E. Raman A c t i v e L a t t i c e Modes 168 F. Assignment of Fundamentals . .\u00E2\u0080\u00A2 . .... 170 G. I n f r a r e d and Raman Spectra of Dibenzothiophene-dg 172 \u00E2\u0080\u00A2H. The Assignment of the Fundamentals of Dibenzothiophene-d 0 183 o I. Normal Coordinate C a l c u l a t i o n 186 J . D i s c u s s i o n 187 Chapter 9 A Study of Some E l e c t r o n i c States of Dibenzothiophene. 191 A. I n t r o d u c t i o n 191 B. Absorption Spectrum 191 C. Fluorescence Spectra 194 D. Phosphorescence Spectra 1^8 E. D i s c u s s i o n 203 Chapter 10 A Study of Biphenyl C r y s t a l Phosphorescence Induced by I m p u r i t i e s 206 - V l l l -A. I n t r o d u c t i o n 206 B(. Carbazole i n Biphenyl 207 C. Dibenzothiophene i n Biphenyl 213 Appendix I Absorption and Fluorescence Spectra of Carbazole i n a Biphenyl M a t r i x 219 Appendix I I A Comparison of the Fundamental V i b r a t i o n s of Carba-z o l e , Fluorene and Dibenzothiophene 227 References 230 - i x -17. C a l c u l a t e d normal coordinates f o r two A^ fundamentals of carbazole 18. The p o l a r i z e d absorption spectrum of carbazole i n flu o r e n e at about 15\u00C2\u00B0K \" \u00E2\u0080\u00A2 L i s t of Figures 1. P o s s i b l e s t r u c t u r e s f o r the imp u r i t y g i v i n g the weak absorp-o t i n g at 3200 A. I b e n z [ f ] i n d a n ; I I benz[e]indan; I I I 2,3-dihydrophenalene ' 30 2. NMR spectrum of b e n z [ f ] i n d a n 31 3. Helium gas thermometer 35 4. E x t e r n a l o p t i c a l system used f o r absorption and lumines-cence experiments 3 7 5. Biphenyl u n i t c e l l 4 3 6. Fluorene u n i t c e l l . . 43 7. Bond lengths and bond angles of carbazole 4 5 8. Carbazole u n i t c e l l 4 3 \u00E2\u0080\u00A2 9. Dibenzothiophene u n i t c e l l 46 10. Bond lengths and bond angles of dibenzothiophene 4 7 11. Low frequency i n f r a r e d s p e c t r a of carbazole 51 12. I n f r a r e d s p e c t r a of carbazole 13. Some low frequency Raman i n t e r v a l s of carbazole 14. Raman s p e c t r a of carbazole 15. The fluore s c e n c e spectrum of carbazole i n a s i n g l e c r y s t a l m a t r i x of f l u o r e n e at about 15\u00C2\u00B0K 16. In-plane i n t e r n a l coordinate d e f i n i t i o n s of carbazole 59 61 63 73 81 84 - X -19. A microdensitometer t r a c i n g of the p o l a r i z e d absorption spectrum of benz[ f ] i n d a n i n fluorene at about 25\u00C2\u00B0k 96 20. A microdensitometer t r a c i n g of the p o l a r i z e d fluorescence spectrum of benz[f]indan i n fluorene at about 15\u00C2\u00B0K 102 21. Factor group s p l i t t i n g s a s s o c i a t e d w i t h the long- and s h o r t - a x i s of flu o r e n e c a l c u l a t e d f o r t r a n s i t i o n moments of 0.237 A. H 2 22. The room temperature absorption s p e c t r a of flu o r e n e H 3 23. The absorption s p e c t r a of flu o r e n e at l i q u i d helium temperatures (a) i n a n-heptane matri x and (b) f o r b-p o l a r i z a t i o n i n an ab c r y s t a l s e c t i o n 116 24. The fluorescence s p e c t r a of fluorene at ^ 15\u00C2\u00B0K f o r a be s e c t i o n o f the s i n g l e c r y s t a l and i n a n-heptane matrix ... H 8 25. The t r i p l e t - t r i p l e t absorption spectrum of flu o r e n e i n n-heptane at > 15\u00C2\u00B0K 127 26. The phosphorescence of f l u o r e n e i n the c r y s t a l at \u00C2\u00B0o 6\u00C2\u00B0K when induced by the a d d i t i o n of dibenzothiophene impurity and i n n-heptane at ^ 15\u00C2\u00B0K 128 27. The i n f r a r e d spectrum of f l u o r e n e i n a cyclohexane s o l u t i o n i n the low-frequency r e g i o n 134 28. The p o l a r i z e d i n f r a r e d s p e c t r a o f flu o r e n e s i n g l e c r y s t a l at low frequencies 135 29. I n f r a r e d s p e c t r a of flu o r e n e i n the CH s t r e t c h i n g r e g i o n . . . 1^5 30. The p o l a r i z e d i n f r a r e d s p e c t r a of a f l u o r e n e s i n g l e c r y s t a l 135 31. The Raman s p e c t r a of a f l u o r e n e s i n g l e c r y s t a l 141 - x i -32. The Raman s p e c t r a of a f l u o r e n e s i n g l e c r y s t a l showing A 2 and modes 142 33. The p o l a r i z e d i n f r a r e d spectrum of dibenzothiophene i n the low frequency r e g i o n 157 34. The p o l a r i z e d i n f r a r e d s p e c t r a of a dibenzothiophene s i n g l e c r y s t a l 158 35. The i n f r a r e d spectrum o f dibenzothiophene i n cyclohexane and benzene s o l u t i o n s i n the low frequency r e g i o n 159 36. The i n f r a r e d spectrum of dibenzothiophene i n carbon d i s u l p h i d e , carbon t e t r a c h l o r i d e and t e t r a c h l o r o e t h y l e n e s o l u t i o n s 159 37. The Raman sp e c t r a of a dibenzothiophene s i n g l e c r y s t a l .... 164 38. The Raman s p e c t r a of a dibenzothiophene s i n g l e c r y s t a l 165 39. The i n f r a r e d spectrum of dibenzothiophene-d s i n g l e c r y s t a l o i n the low frequency region 173 40. The i n f r a r e d spectrum of dibenzothiophene s i n g l e c r y s t a l . . . 174 41. I n f r a r e d spectrum of dibenzothiophene-d i n carbon d i s u l p h i d e , carbon t e t r a c h l o r i d e and t e t r a c h l o r o e t h y l e n e s o l u t i o n s 175 42. I n f r a r e d spectrum o f dibenzothiophene-dg i n cyclohexane s o l u t i o n i n the low frequency region : 176 43. Raman s p e c t r a of dibenzothiophene-d 181 44. Raman s p e c t r a o f dibenzothiophene-d 182 o 45. In-plane i n t e r n a l coordinates o f dibenzothiophene 187 - x i i -46. Absorption s p e c t r a of dibenzothiophene i n n-heptane and fluo r e n e at about 15\u00C2\u00B0K 192 47. Dibenzothiophene fluo r e s c e n c e i n (a) a flu o r e n e m a t r i x , (b) the c r y s t a l , and (c) an n-heptane s o l u t i o n at about 15\u00C2\u00B0K.... 195 48. Phosphorescence s p e c t r a of dibenzothiophene i n (a) hexamethyl-benzene, (b) the c r y s t a l , and (c) n-heptane s o l u t i o n at about 15\u00C2\u00B0K 199 49. Phosphorescence s p e c t r a of biphenyl c r y s t a l s doped with carbazole and dibenzothiophene 208 50. Temperature v a r i a t i o n of the i n t e n s i t y of the phosphores-cence and of the delayed fluorescence from a biphenyl c r y s t a l doped with carbazole 211 51. P l o t s of l o g ( i n t e n s i t y r a t i o s ) vs. 1/T f o r the carbazole/ 212 b i p h e n y l system 52. Temperature v a r i a t i o n o f the i n t e n s i t y of the phosphores-cence and of the delayed fluorescence from a biphe n y l c r y s t a l doped with dibenzothiophene-hg 215 53. P l o t s of log ( i n t e n s i t y r a t i o s ) vs. 1/T f o r the dibenzo-thiophene-h /biphenyl system 216 o 54. T r i p l e t - t r i p l e t a bsorption i n the biphenyl c r y s t a l doped w i t h dibenzothiophene-h at about 10\u00C2\u00B0K and the t r i p l e t -o t r i p l e t absorption spectrum of biphenyl i n a r i g i d g l a s s at 77\u00C2\u00B0K 217 55. Temperature v a r i a t i o n of the i n t e n s i t y of the phosphores-cence and of the delayed fluorescence from a biphenyl c r y s t a l doped with dibenzothiophene-d 218 - X l l l -56. The p o l a r i z e d absorption spectrum of carbazole i n biphenyl at about 15\u00C2\u00B0K . 222 57. The fluorescence spectrum of carbazole i n biphenyl at about 15\u00C2\u00B0K 226 - x i v -L i s t of Tables 1. D i r e c t i o n cosines of biphenyl 42 2. D i r e c t i o n cosines of f l u o r e n e .44 3. D i r e c t i o n cosines of carbazole 44 4. D i r e c t i o n cosines of dibenzothiophene 47 5. D i r e c t i o n cosines of dibenzothiophene 47 6. C o r r e l a t i o n t a b l e showing the s e l e c t i o n r u l e s f o r the i s o l a t e d molecule and f o r the c r y s t a l 7. The i n f r a r e d spectrum of carbazole 53 8. R e l a t i v e i n f r a r e d band i n t e n s i t i e s of carbazole along the c r y s t a l axes c a l c u l a t e d f o r the o r i e n t e d gas model ... 56 9. The Raman spectrum of carbazole 59 10. The fluo r e s c e n c e spectrum of carbazole i n f l u o r e n e at about 15\u00C2\u00B0K 64 11. The i n - p l a n e f o r c e constants of carbazole 75 12. Values of the out-of-plane f o r c e constants of benzene .... 77 13. A.comparison of the observed and c a l c u l a t e d fundamentals of carbazole 78 14. The absorption spectrum of carbazole i n f l u o r e n e at about 15\u00C2\u00B0K 85 15. C o r r e l a t i o n between fundamentals of the ground and f i r s t e x c i t e d s t a t e of carbazole 90 16. D i s t r i b u t i o n of the molecular fundamentals of b e n z [ f ] -indan ' 95 17. The p o l a r i z e d absorption spectrum of benz[f]indan i n f l u o r e n e 97 - X V -18. The p o l a r i z e d f l u o r e s c e n c e spectrum of b e n z [ f ] i n d a n i n fl u o r e n e 100 19. C o r r e l a t i o n between fundamentals of the ground s t a t e and f i r s t e x c i t e d e l e c t r o n i c s t a t e s of b e n z [ f ] i n d a n 104 20. The e x c i t e d - s t a t e c r y s t a l wavefunctions and t h e i r i r r e d u c -i b l e r e p r e s e n t a t i o n s i n the space group c o r r e l a t e d w i t h the f r e e molecule t r a n s i t i o n s 108 21. D i p o l e - d i p o l e i n t e r a c t i o n sums f o r c r y s t a l l i n e f l u o r e n e o over a sphere of 40 A rad i u s f o r u n i t t r a n s i t i o n moments.. I l l 22. C a l c u l a t e d c r y s t a l l e v e l s of f l u o r e n e 113 23. The abso r p t i o n spectrum taken at about 15\u00C2\u00B0K of flu o r e n e c r y s t a l (ab face) and of f l u o r e n e i n an :n-heptane m a t r i x . . 120 24. The fl u o r e s c e n c e spectrum of f l u o r e n e i n n-heptane at 15\u00C2\u00B0K 123 25. The t r i p l e t - t r i p l e t a b sorption spectrum of f l u o r e n e i n n-heptane at about 15\u00C2\u00B0K 126 26. The phosphorescence spectrum of f l u o r e n e i n a c r y s t a l .doped w i t h dibenzothiophene. at about 6\u00C2\u00B0K 130 27. C o r r e l a t i o n t a b l e showing s e l e c t i o n r u l e s f o r the i s o l a t e d molecule and f o r the c r y s t a l of f l u o r e n e 133 - x v i -28. The r e l a t i v e band i n t e n s i t i e s of flu o r e n e along the c r y s t a l axes c a l c u l a t e d i n the oriented-gas approximation . . \u00E2\u0080\u0094 ... 137 29. The i n f r a r e d spectrum of f l u o r e n e 137 30. The Raman spectrum of flu o r e n e near the e x c i t i n g l i n e . . . . . 143 31. The Raman spectrum of f l u o r e n e . 144 32. The assignments o f the fundamental v i b r a t i o n s of f l u o r e n e . 149 33. C o r r e l a t i o n t a b l e showing the s e l e c t i o n r u l e s f o r the i s o -l a t e d molecule and the c r y s t a l o f dibenzothiophene ....... 154 34. R e l a t i v e i n f r a r e d band i n t e n s i t i e s along the c r y s t a l axes of dibenzothiophene c a l c u l a t e d f o r theoriented-gas model.. 1^5 35. The i n f r a r e d spectrum of dibenzothiophene i n the c r y s t a l and s o l u t i o n .1^0 36. Raman s p e c t r a of dibenzothiophene i n carbon t e t r a c h l o r i d e s o l u t i o n and a s i n g l e c r y s t a l ;. 37- Raman spectrum of dibenzothiophene near the e x c i t i n g l i g h t 168 38. R e l a t i v e i n t e n s i t i e s of Raman a c t i v e modes due to r o t a t i o n -a l o s c i l l a t i o n s about the x_, y_, z_-molecular axes among the various p o l a r i z e d s p e c t r a 39. The i n f r a r e d spectrum o f dibenzothiophene-dg 177 40. The Raman spectrum of dibenzothiophene-d Q c r y s t a l near the e x c i t i n g l i n e 181 41. The Raman spectrum of dibenzothiophene-dg c r y s t a l ........ 182 42. Moments of i n e r t i a about the molecular axes of dibenzo-thiophene 186 43. In-plane f o r c e constants o f dibenzothiophene , 187 - x v i i -44. A comparison of the observed and c a l c u l a t e d fundamentals f o r dibenzothiophene-h and - d Q 189 o o 45. Absorption s p e c t r a of dibenzothiophene i n hexamethyl-benzene (11MB) and n-heptane at about 15\u00C2\u00B0K 193 46. Dibenzothiophene fluorescence i n a fluorene m a t r i x , the c r y s t a l and n-heptane s o l u t i o n at about 15\u00C2\u00B0K 196 47. Phosphorescence s p e c t r a of dibenzothiophene i n n-heptane s o l u t i o n , hexamethylbenzene (HMB) ma t r i x , and c r y s t a l at about 15\u00C2\u00B0K . s 200 48. The absorption spectrum of carbazole i n biphenyl at about 15\u00C2\u00B0K ... 219 49. The fluorescence spectrum of carbazole i n biphenyl at about 15\u00C2\u00B0K 223 50. A comparison o f the assignments of the fundamentals f o r ca r b a z o l e , f l u o r e n e , dibenzothiophene-h , and d i b e n z o t h i o -8 phene-dg . . . 227 - XV111 -Acknowledgements To Dr. A. Bree, t h e s i s s u p e r v i s o r , to whom I express my a p p r e c i a t i o n f o r h i s guidance and continuous support throughout the d u r a t i o n of t h i s work, to Miss V.V. V i l k o s , f o r v a l u a b l e a s s i s t a n c e w i t h experimental procedures and f o r many h e l p f u l d i s c u s s i o n s , t o Dr. K.B. Harvey, who r e a d i l y made the i n f r a r e d equipment a v a i l a b l e , and to Mr. R. Green, f o r i n s t r u c t i o n i n the use of t h i s equipment, to Dr. R. S c h a f f r i n and Dr. J . T r o t t e r , who generously made the c r y s t a l s t r u c t u r e data of dibenzothiophene a v a i l a b l e , to S. Whitlow, who loc a t e d the c r y s t a l axes i n the melt-grown samples of carbazole and dibenzothiophene, t o Dr. M. Kurahashi, who s u p p l i e d d e t a i l s o f the carbazole c r y s t a l s t r u c t u r e p r i o r to p u b l i c a t i o n , to Dr. S.C. Wait, J r . , f o r p r o v i d i n g d e t a i l s of his.work before i t was p u b l i s h e d , to many other i n d i v i d u a l s , f o r t h e i r a s s i s t a n c e and c o n t r i b u t i o n s i n the p r e p a r a t i o n of t h i s work, thank you. Chapter 1 General I n t r o d u c t i o n A. Introductory Remarks In the past 20 years the spectra of organic molecular c r y s t a l s have been e x t e n s i v e l y s t u d i e d to gain a more complete understanding of i n t e r -molecular as w e l l as i n t r a m o l e c u l a r p r o p e r t i e s . Progress was g r e a t l y f a c i l i t a t e d by the i n t r o d u c t i o n of high speed computers which could be used to t e s t t h e o r i e s i n v o l v i n g complex c a l c u l a t i o n s of molecular o r b i t a l s , of m u l t i p o l e i n t e r a c t i o n s i n c r y s t a l s , a n d of v i b r a t i o n a l f r e q u e n c i e s . The development of intense UV l i g h t sources f o r absorption'and emission s t u d i e s , l a s e r Raman spectrometers, and i n f r a r e d p o l a r i z e r s and spectrometers extending i n t o the f a r i n f r a r e d has made a v a i l a b l e more complete sets of experimental data which i n t u r n has r e s u l t e d i n more accurate i n t e r p r e t a -t i o n s of many phenomena a s s o c i a t e d with organic molecules and c r y s t a l s . The present i n v e s t i g a t i o n i s concerned with the e l e c t r o n i c and v i b r a t i o n -a l p r o p e r t i e s of some t r i c y c l i c aromatic and heteroaromatic compounds. The aim of t h i s research i s to use the already w e l l - e s t a b l i s h e d experimental techniques of i n f r a r e d , Raman and low-temperature UV spectroscopy to gather the data and to i n t e r p r e t the data w i t h i n the framework of the e x i s t i n g t h e o r i e s . Thus t h i s f i r s t chapter o u t l i n e s r e l e v e n t p a r t s of the t h e o r i e s of v i b r a t i o n a l - e l e c t r o n i c i n t e r a c t i o n s , excitons i n molecular c r y s t a l s , and normal coordinate a n a l y s i s . More s p e c i f i c d e t a i l s of the theory are given - 2 -as they are needed and used to e l u c i d a t e some aspect of the phenomenon under i n v e s t i g a t i o n . Chapter 2 describes the general experimental techniques t h a t were used to p u r i f y the m a t e r i a l s , grow and cut the s i n g l e c r y s t a l samples, and o b t a i n the s p e c t r a . The content o f chapters 3-10 i s comprised of the Raman, i n f r a r e d , luminescence and absorption s p e c t r a i n v a r y i n g degrees o f completeness w i t h respect to the number of types of s p e c t r a and the extensiveness of the i n v e s t i g a t i o n of a p a r t i c u l a r type o f spectrum f o r each of the molecules c a r b a z o l e , b e n z [ f ] i n d a n , f l u o r e n e , and dibenzo-thiophene. The a c t u a l d i v i s i o n of experimental r e s u l t s among the chapters and the sequence o f the chapters was decided upon according to the compound being i n v e s t i g a t e d and the r e l a t i v e c e r t a i n t y of i t s v i b r a t i o n a l assignment. Thus chapter 3 contains a f a i r l y complete d i s c u s s i o n o f the v i b r a t i o n s of carbazole from the i n f r a r e d , Raman and fluorescence s p e c t r a as w e l l as the c a l c u l a t e d f r e q u e n c i e s . The v i b r a t i o n s and p o l a r i z a t i o n p r o p e r t i e s of the f i r s t e x c i t e d s i n g l e t s t a t e o f carbazole are d e a l t w i t h i n chapter 4. In chapter 5, the d i s c u s s i o n of the i m p u r i t i e s of f l u o r e n e prepares the way f o r the study o f some e x c i t e d s i n g l e t and t r i p l e t e l e c t r o n i c s t a t e s of fl u o r e n e which f o l l o w s i n chapter 6. The v i b r a t i o n a l assignment of fl u o r e n e in chapter 7 from the i n f r a r e d and Raman s p e c t r a i s c o r r e l a t e d w i t h the assignments f o r carbazo l e . The i n v e s t i g a t i o n of the v i b r a t i o n s of dibenzo-thiophene and dibenzothiophene-dg i s reported f o r the f i r s t time. A study o f some e x c i t e d t r i p l e t and s i n g l e t s t a t e s of dibenzothiophene i s contained in chapter 9. F i n a l l y , chapter 10 deals w i t h some unusual energy-trapping e f f e c t s that arose from a study of the phosphorescence s p e c t r a o f the s i n g l e c r y s t a l systems made up of carbazole and dibenzothiophene d e l i b e r a t e l y added as i m p u r i t i e s to biphenyl or fluorene m a t r i c e s . B. The Theory of V i b r a t i o n a l - E l e c t r o n i c I n t e r a c t i o n s The i n t e n s i t y of an o p t i c a l ( e l e c t r i c d i p o l e ) t r a n s i t i o n i s p r o p o r t i o n a l to. the square of the t r a n s i t i o n moment between the i n i t i a l and f i n a l s t a t e s . The t r a n s i t i o n moment between two s t a t e s i and j i s M\u00C2\u00B1j =y*YiCxiX)M(x,X)\u00C2\u00A5j(x,X)dxdX , . (1.1) where 1^(x,X) and ^ (x,X) are exact s o l u t i o n s o f the Schrodinger equation f o r the complete Hamiltonian. The c o l l e c t i v e coordinate symbols x and X form a complete set of i n t e r n a l coordinates and l o c a t e r e s p e c t i v e l y a l l of the el e c t r o n s and n u c l e i . M(x,X) i s the d i p o l e moment operator M(x,X) = e\u00C2\u00A3r. - eEZ R (1.2) l where r . and R are the p o s i t i o n v e c t o r s of the i t h e l e c t r o n and cth nucleus r e s p e c t i v e l y . In polyatomic systems exact s o l u t i o n s are not known s i n c e the equations of motion of three or more i n t e r a c t i n g p a r t i c l e s cannot be sol v e d . The Born-Oppenheimer approximation\"''is used t o o b t a i n a s i m p l i f i c a t i o n of the mathematical d e s c r i p t i o n of the system. This procedure separates the e l e c t r o n i c and nuclear motions. The a d i a b a t i c wavefunctions are u s u a l l y f u r t h e r s i m p l i f i e d by f i x i n g the nuclear coordinates which appear as para-meters i n the e l e c t r o n i c wavefunctions. Thus the zeroth-order v i b r o n i c wave-f u n c t i o n f o r the pth v i b r a t i o n a l l e v e l of the i t h e l e c t r o n i c s t a t e can be w r i t t e n as the product f u n c t i o n \u00C2\u00A5.\"(x,X) = i>. (x ,X ' )x- (X) (1.3) l y l o l y - 4 -where \u00C2\u00B0), r ( E i \ ) , r(i|j\u00C2\u00B0) must c o n t a i n the t o t a l l y symmetric r e p r e s e n t a t i o n of the p o i n t group o f the molecule. For f i n i t e X^ the d i r e c t products of r ( ^ ) , r(3H/3Q^) o, r(u>\u00C2\u00B0), and r ( x k y ) , r ( Q q ) , T(X_Q ) must simultaneously c o n t a i n the t o t a l l y symmetric representa-t i o n . These requirements imply t h a t r ( ^ ) x r(3H/3Q ) o = r = r(Er.) (1.10) Further, s i n c e the Hamiltonian i s i n v a r i a n t to the operations o f the p o i n t group of the molecule, (3H/9Q^) o has the same symmetry p r o p e r t i e s i n -electron space t h a t has i n nuclear space. In equation (1.10), r C ^ ) ar>d T (IJJ\u00C2\u00B0) are s p e c i f i e d , and T(3H/3Q^) o can be determined. This i s equivalent to s p e c i f y i n g the type of v i b r a t i o n that can mix the e l e c t r o n i c s t a t e w i t h <\u00E2\u0080\u00A2 A q u a n t i t a t i v e treatment of the v i b r a t i o n a l borrowing v i a the 606 and. -1 1 1595 cm e 2g v i b r a t i o n s between the allowed (1800A) e l e c t r o n i c s t a t e 1 \u00C2\u00B0 i and the forbidden (2650A) and (2000A) e l e c t r o n i c s t a t e s of benzene has been made.^ The c a l c u l a t i o n i n v o l v e s an e v a l u a t i o n of A,,. k l The term i n H which depends on both the e l e c t r o n s i and the n u c l e i a i s 1 E Z e 2 i o \u00C2\u00B1c\u00C2\u00A3\u00E2\u0080\u0094 (1.11) - i o and t h e r e f o r e \u00E2\u0080\u0094 - = X {ZZ e 2 ____ . \u00C2\u00A3ia }-=\u00E2\u0080\u00A2?' h. \u00E2\u0080\u009E '(1.12) % i = l cr a 9Q 3 i = 1 1 n q r . - 8 -where r i s the p o s i t i o n v e c t o r of nucleus a. The nuclear displacements i n 8 r o the normal modes are thus represented by the set of d i p o l e s eZ^ -r^r-q i n t e r a c t i n g w i t h the e l e c t r o n s i . Since equation (1.12) i s the sum of one e l e c t r o n operators h^ the c a l c u l a t i o n of the i n t e g r a l i n 1.8 reduces t o the N i n t e g r a t i o n of 1 h. with the t r a n s i t i o n d e n s i t y , p, .. 1=1 P =JJ\u00C2\u00B0 \u00C2\u00A5jdx' k l I k l (1.13) The r e s u l t s of the c a l c u l a t i o n ^ show that the^E, s t a t e i s mixed most l u 1 -1 e f f e c t i v e l y w i t h the s t a t e by the 6 0 6 cm v i b r a t i o n and with the ^B^ s t a t e by the 1595 cm * v i b r a t i o n , l u 1 C. The Franck-Condon P r i n c i p l e The Franck-Condon p r i n c i p l e determines the i n t e n s i t y d i s t r i b u t i o n of an e l e c t r o n i c t r a n s i t i o n among i t s v i b r a t i o n a l members. The p r i n c i p l e s t a t e s that the nuclear p o s i t i o n s and v e l o c i t i e s do not change during an e l e c t r o n i c t r a n s i t i o n . That i s , the s t a r t i n g c o n f i g u r a t i o n i n the new e l e c t r o n i c s t a t e represents a displacement from the new e q u i l i b r i u m nuclear c o n f i g u r a t i o n without change of symmetry. I f the displacement of the e q u i l i b r i u m nuclear c o n f i g u r a t i o n i s zero (A=0) and i f the p o t e n t i a l energy surfaces have the same shape i n both e l e c t r o n i c s t a t e s , then a l l the i n t e n s i t y i s concentrated i n the ( 0 - 0 ) band. I f the displacement i s non-zero (A,^0), t r a n s i t i o n s t o higher v i b r o n i c s t a t e s become more probable. The steepness of the p o t e n t i a l energy curve i n the e x c i t e d s t a t e determines the number of excited' s t a t e v i b r a t i o n a l wavefunctions X ^ C Q J A ) which w i l l have app r e c i a b l e overlap with the ground s t a t e v i b r a t i o n a l wavefunctions XQQ(Q) a n d therefore the length of - 9 -the p r o g r e s s i o n i n an e l e c t r o n i c t r a n s i t i o n . The dominant p r o g r e s s i o n that appears i n the t r a n s i t i o n s o f polyacene aromatic hydrocarbons has a spacing of about 1400 cm For example the spacing i n the naphthalene spectrum i s 1378 cm * and i n anthracene 1401 cm Frequencies of about 1400 cm * are a t t r i b u t e d to v i b r a t i o n s which are e s s e n t i a l l y mixtures of C-C ( s k e l e t a l ) and in-plane C-H bending modes. A comparison of the s p e c t r a of f u l l y deuterated naphthalene and anthracene with the undeuterated species shows that d e u t e r a t i o n has no marked e f f e c t on the s p e c t r a beyond a zero p o i n t energy s h i f t . ^ One c o n c l u s i o n to be drawn from t h i s observation i s that shape changes i n the e x c i t e d s t a t e can be described predominantly i n terms of C-C bond lengths such that no l o s s of covering symmetry occurs. A number of q u a n t i t a t i v e a p p l i c a t i o n s of the Franck-Condon p r i n c i p l e have been made. From a knowledge of the i n t e n s i t y d i s t r i b u t i o n i n the benzene o band system at 2650A and the v i b r a t i o n s a c t i v e i n forming the progression (ground s t a t e , 992 cm e x c i t e d s t a t e , 923 cm ^ ) , the extension of the ' :\" ' \u00C2\u00B0 7 C-C bond length of benzene i n the e x c i t e d s t a t e was c a l c u l a t e d to be 0.036 A. The lowest benzene t r i p l e t s t a t e C-C bond length extension r e l a t i v e to the \u00C2\u00B011 ground s t a t e was c a l c u l a t e d to be 0.036A as w e l l . The i n t e n s i t y d i s t r i -b u t i o n among the v i b r a t i o n a l members of a t r a n s i t i o n has been c a l c u l a t e d from the changes i n geometry, the frequencies and the corresponding normal A c +u ! \ -u \u00E2\u0080\u00A2 +u \u00E2\u0080\u00A2 10,12,13,14 modes of the r e l e v a n t v i b r a t i o n s i n the p r o g r e s s i o n . D. The Theory of Excitons i n Molecular C r y s t a l s E s p e c i a l l y f o r the weaker t r a n s i t i o n s , the a b s o r p t i o n s p e c t r a i n the c r y s t a l resemble the s o l u t i o n s p e c t r a i n band shape and frequency, For example, there i s no d i f f i c u l t y i n c o r r e l a t i n g the p r i n c i p a l f e a t u r e s of - 10 -the c r y s t a l and s o l u t i o n a b s o r p t i o n system of medium i n t e n s i t y i n anthracene.''' However, s i g n i f i c a n t d i f f e r e n c e s between the c r y s t a l and s o l u t i o n spectra do occur. One' notable d i f f e r e n c e i s the frequency s h i f t o f the s p e c t r a p o l a r i z e d along d i f f e r e n t d i r e c t i o n s of c r y s t a l symmetry. That i s , the s o l u t i o n bands are s p l i t i n the c r y s t a l . Small changes i n the i n t e n s i t y p a t t e r n and a general displacement t o the red of the s o l u t i o n band system a l s o a r i s e . The o r g a n i c s o l i d s under i n v e s t i g a t i o n i n t h i s work represent examples of molecular c r y s t a l s . In the zeroth order of approximation molecules do not i n t e r a c t and the o r d e r l y a r r a y of n o n - i n t e r a c t i n g molecules i s r e f e r r e d to as an \" o r i e n t e d gas\". The s i m i l a r i t y between the c r y s t a l and s o l u t i o n s p e c t r a suggests that the p o l a r i z a t i o n , s p l i t t i n g , displacement and i n t e n -s i t y d i s t r i b u t i o n of the bands i n the c r y s t a l can be accounted f o r using p e r t u r b a t i o n methods based on the o r i e n t e d gas model. A d e t a i l e d considera-t i o n o f t h i s theory i s a v a i l a b l e i n a number of books'^ ^ and only a b r i e f o u t l i n e of the e s s e n t i a l ideas i s given below. In the unperturbed problem, the c r y s t a l i s represented by a system of o r i e n t e d but n o n - i n t e r a c t i n g molecules. The energy l e v e l s of the system are the same as those of the f r e e molecule and the i n t e n s i t y of the a b s o r p t i o n of l i g h t along the c r y s t a l l o g r a p h i c axes i s r e l a t e d to the f r e e molecule t r a n s i t i o n moment by the square of the d i r e c t i o n c o s i n e s . In r e a l c r y s t a l s the molecules are weakly i n t e r a c t i n g and t h e r e f o r e the energy l e v e l s c o r r e c t to the f i r s t order of approximation are eigenvalues of the Hamiltonian (1.14) - 11 -f o r a r i g i d l a t t i c e . i s the Hamiltonian f o r the k t h molecule, and i s the p e r t u r b a t i o n operator between molecules k and 1. The unperturbed or ground s t a t e ($-.) of the c r y s t a l i s given as the product f u n c t i o n of the f r e e molecule wavefunctions f o r a c r y s t a l of N molecules. E l e c t r o n exchange i s neglected s i n c e overlap between molecular wavefunctions even of neighbouring molecules i s very s m a l l . Then the energy of the ground s t a t e to a f i r s t order of approxima-t i o n i s E G = S + 5 j / V l V k 5 l d T (1'16^ ' k k l ^ k where w^ i s the molecular ground s t a t e energy. An unperturbed e x c i t e d s t a t e of a c r y s t a l c o n s i s t s of only one molecule i n an e x c i t e d s t a t e , w h i l e a l l the other molecules are i n t h e i r ground s t a t e , so t h a t a l o c a l i z e d e x c i t a t i o n wavefunction i s r | H |<{>r > _ < r | l|<}\u00C2\u00BBr > E | = 0 ( 1 . 1 9 ) -p j q ip j q The problem of f i n d i n g the c o r r e c t l i n e a r combinations may be s i m p l i f i e d c o n s i d e r a b l y by making use of the symmetry of the c r y s t a l and so the sec u l a r equation need not be solved i n t h i s form. 2 1 2 2 Space group theory has been discussed by Winston and Halford,'' Winston, 2 3 and Koster. The r o l e o f the space group, s i t e group, f a c t o r group, t r a n s l a t i o n group and molecular p o i n t group i n the theory of molecular 2 2 c r y s t a l s was set out i n a paper by Winston. The Hamiltonian has the f u l l symmetry o f the c r y s t a l and i s , t h e r e f o r e , i n v a r i a n t under the operations of the space group. Linear combinations of the l o c a l i z e d e x c i t a t i o n wavefunctions can be constructed so that they transform l i k e i r r e d u c i b l e r e p r e s e n t a t i o n s of the space group. The molecules form h t r a n s l a t i o n a l l y equivalent sets i n the c r y s t a l and assum-ing the c r y s t a l i s so large that i t may be regarded as having an i n f i n i t e number of u n i t c e l l s i n a l l d i r e c t i o n s , p e r i o d i c boundary c o n d i t i o n s may be a p p l i e d . Then f u n c t i o n s of the form * ? ( k ) = v/h7N \u00C2\u00A3 e x p ( i k - r . ) 4>T ( 1 . 2 0 ) l V r - - i p i p / p belonging to the kth r e p r e s e n t a t i o n of the t r a n s l a t i o n group are constructed r from the product f u n c t i o n s $^ i n which the molecules i n the i t h t r a n s l a t i o n a l l y - 13 -e q u ivalent set are i n t h e i r r t h e x c i t e d s t a t e . The v e c t o r s r. and k are - i p -t r a n s l a t i o n v e c t o r s and vvavevectors r e s p e c t i v e l y . x Further c l a s s i f i c a t i o n of the *^(k) i s p o s s i b l e only f o r s p e c i a l values of k. In p a r t i c u l a r , the value k = 0 i s important i n o p t i c a l t r a n s i t i o n s . In t h i s case, a l l the u n i t c e l l f u n c t i o n s are i n phase and a t t e n t i o n may be focussed on only one u n i t c e l l . The f u n c t i o n s *?(0) = A / \u00C2\u00A5 E f (1.21) P are c a l l e d one s i t e e xcitons and correspond to the spread of the e x c i t a t i o n throughout the c r y s t a l v i a one t r a n s l a t i o n a l l y equivalent set of molecules, Now the f u n c t i o n s $_(0) can be combined to form eigenfunctions of the c r y s t a l which transform l i k e i r r e d u c i b l e r e p r e s e n t a t i o n s of the f a c t o r group. The number of combinations corresponds to the number of molecules i n the u n i t c e l l . The energy above the ground s t a t e of the ath c r y s t a l s t a t e a r i s i n g from one f r e e molecule s t a t e i s A E a C r ) = Aw T + D r + E ' l r . (1.22) where D1\" = Z''{ - < r c |V I r r >} (1.23) p \u00C2\u00B0L pq p q p q 1 pq 1 p q I T . = (1-24) i p , j q i p j q 1 i p . j q 1 ip j q and the prime on the summation symbol i n d i c a t e s that i p ^ j q . The q u a n t i t y x D i s a coulomb i n t e r a c t i o n term and represents the change i n b i n d i n g energy of a molecule i n a c r y s t a l upon e x c i t a t i o n . The r e s u l t i s a s h i f t i n the bands of the c r y s t a l to longer wavelengths r e l a t i v e t o those;of the f r e e - 14 -i\" molecule. The q u a n t i t y I. . i s a resonance i n t e r a c t i o n i n t e g r a l and i s as s o c i a t e d with the m i g r a t i o n of energy from molecule to molecule i n the c r y s t a l . Many organic c r y s t a l s have two molecules per u n i t c e l l . The energy d i f f e r e n c e between the two c r y s t a l s t a t e s a r i s i n g from the r t h molecular s t a t e i s 2 \u00C2\u00A3 I.r q ip\u00C2\u00BB2q This i s known as the Davydov or f a c t o r group s p l i t t i n g . The s p l i t t i n g i s the r e s u l t of the i n t e r a c t i o n between t r a n s l a t i o n a l l y i n e q u i v a l e n t molecules i n the c r y s t a l . E v a l u a t i o n of the i n t e r a c t i o n i n t e g r a l i s c a r r i e d out by expanding 24 the operator V. as a s e r i e s o f p o i n t m u l t i p o l e - m u l t i p o l e terms. The 1 i p , j q * . leadi n g term i n the expansion i s the d i p o l e - d i p o l e i n t e r a c t i o n which can be c a l c u l a t e d from c r y s t a l l o g r a p h i c data and from in f o r m a t i o n extracted from the s o l u t i o n spectrum of the molecule. The d i p o l e - d i p o l e approximation 25 was s u c c e s s f u l l y a p p l i e d to the anthracene c r y s t a l . The shape dependence of the d i p o l e - d i p o l e sums was st u d i e d and the summation over a sphere was 26 found to give best agreement w i t h experiment. D i f f e r e n t s t a t e s of the molecule o f t e n belong to the same representa-t i o n of the s i t e group and thus give r i s e to s t a t e s w i t h the same f a c t o r group r e p r e s e n t a t i o n . Such s t a t e s are normally separated by a wide energy gap so th a t t h e i r i n t e r a c t i o n may be neglected or t r e a t e d to a second 27 order of approximation by p e r t u r b a t i o n theory. C r a i g and Walmsley have shown that although the e f f e c t on the energy may not be l a r g e , the e f f e c t on the p o l a r i z a t i o n r a t i o i s important. For example, i n anthracene the - 15 -c a l c u l a t e d p o l a r i z a t i o n r a t i o I ^ ; l a to a f i r s t order of approximation i s 7.8:1 and t o a second order of approximation 2.3:1 f o r a short a x i s 25 t r a n s i t i o n . In the preceding d i s c u s s i o n , the Born-Oppenheimer s e p a r a b i l i t y of the e l e c t r o n i c and v i b r a t i o n a l wavefunctions was assumed and only the e l e c t r o n i c x wavefunctions used i n d e f i n i n g the resonance i n t e g r a l I. . . However, i p , j q v i b r a t i o n a l e f f e c t s on the t r a n s f e r of energy i n the c r y s t a l are important. In weakly coupled systems, many v i b r a t i o n s occur before the e x c i t a t i o n moves to another molecule, and so the molecular v i b r a t i o n s i n the c r y s t a l have f a c t o r group s p l i t t i n g i n each v i b r o n i c band depends on i t s i n t e n s i t y i n the spectrum as determined by the Franck-Condon p r i n c i p l e . In the case that e l e c t r o n i c c o upling i s s t r o n g , the e x c i t a t i o n does not remain l o c a l i z e d f o r a v i b r a t i o n a l p e r i o d and the d e t a i l of the v i b r a t i o n a l s t r u c t u r e i s washed out. E. Coupling C r i t e r i a and D i s s i p a t i o n of E x c i t a t i o n Energy i n Molecular C r y s t a l s . Coupling c r i t e r i a were de r i v e d by Simpson and Peterson. 28 In the weak c o u p l i n g l i m i t , the i n t e r a c t i o n energy i s much l e s s than the separa-t i o n o f v i b r a t i o n a l l e v e l s and can be w r i t t e n i n the form 2C/A \u00C2\u00AB 1 (1.25) where 2C i s the e l e c t r o n i c f a c t o r group s p l i t t i n g obtained by co n s i d e r i n g the i n t e n s i t y as concentrated i n one molecular band, and A i s the width of - 16 -the e n t i r e e l e c t r o n i c v i b r a t i o n a l spectrum. The strong coupling i n e q u a l i t y i s 2C/A \u00C2\u00BB 1 (1.26) The d i s s i p a t i o n of r a d i a t i o n absorbed by a c r y s t a l can be conveniently d i v i d e d i n t o two c a t e g o r i e s . F i r s t l y , the e x c i t a t i o n energy of the c r y s t a l may be transformed i n t o heat, and secondly, i t may be r e - r a d i a t e d ( c r y s t a l luminescence). Even f o r the second case when e x c i t e d v i b r o n i c s t a t e s of the c r y s t a l are created, the time s c a l e of events i s favourable f o r the d i s s i p t i o n of the v i b r a t i o n a l energy i n t o l a t t i c e modes so th a t luminescence o r i g i n a t e s -12 from the pure e l e c t r o n i c s t a t e . The phonon p e r i o d i s about 10 sec, which -8 i s very small compared with the normal l i f e t i m e (a. 10 sec) of an e x c i t e d e l e c t r o n i c s t a t e . In a c r y s t a l f r e e from defects and undisturbed by l a t t i c e v i b r a t i o n s , an e x c i t o n can propagate r a p i d l y . The c o n d i t i o n s under which such a \" f r e e \" 29 e x c i t o n w i l l luminesce have been discussed p r e v i o u s l y . G e n e r a l l y , the luminescence of the e x c i t o n depends upon the s t r u c t u r e o f the B r i l l o u i n zone and r a t e s of the decay processes compared with the l i f e t i m e of the e x c i t o n . Another p o s s i b l e decay route f o r the e x c i t o n other than the transforma-t i o n i n t o heat and r a d i a n t energy i n v o l v e s the e x c i t o n trapped or l o c a l i z e d at an im p u r i t y molecule or l a t t i c e defect which has energy l e v e l s lower than the e x c i t o n band. The trapped e x c i t o n w i t h i t s energy l o c a l i z e d on one molecule behaves as a s i n g l e i s o l a t e d molecule and such a molecule may r a d i a t e the energy or undergo other i n t e r n a l processes. - 17 -F. Mixed Molecular C r y s t a l Systems Mixed c r y s t a l systems which c o n s i s t of an impurity (guest) introduced i n small amounts i n t o a host l a t t i c e have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . 30 The c l a s s i c study of naphthalene i n durene i s w e l l known. The r e q u i r e -ments of a mixed c r y s t a l system are that the host be o p t i c a l l y transparent i n the r e g i o n of i n t e r e s t and that the guest molecules have a s i z e , shape,, and other p h y s i c a l p r o p e r t i e s s i m i l a r to the host molecule i n order to s u b s t i t u t e at l a t t i c e s i t e s . The e a r l i e s t mixed c r y s t a l s t u d i e s considered guest-host i n t e r a c t i o n s to be very weak and i n t e r p r e t e d the absorption s p e c t r a on the b a s i s of the o r i e n t e d gas model. The experimental observation i s that the e x c i t e d s t a t e s of pure c r y s t a l s are markedly modified by i n t e r m o l e c u l a r i n t e r a c t i o n s and i t i s expected that the energies of mixed c r y s t a l s t a t e s w i l l a l s o show some dependence on i n t e r m o l e c u l a r i n t e r a c t i o n s . In pure crystal's resonance exchange i n t e r a c t i o n takes p l a c e between i d e n t i c a l molecular l e v e l s and i s t r e a t e d to a f i r s t order of p e r t u r b a t i o n theory. On the other hand, second order e f f e c t s i n pure c r y s t a l s depend on the i n t e r a c t i o n between 27 d i f f e r e n t l e v e l s of the molecule. The mixing of d i f f e r e n t e x c i t e d s t a t e s by c r y s t a l forces causes a s i g n i f i c a n t r e d i s t r i b u t i o n of i n t e n s i t y of a b s o r p t i o n along two symmetry axes of the c r y s t a l . In analogy with the treatment of pure c r y s t a l s p e c t r a , second order e f f e c t s i n mixed c r y s t a l s are expected to occur s i n c e the i n t e r a c t i o n takes place between d i f f e r e n t 31 e x c i t e d s t a t e s of the guest and host. Choudhury and Ganguly examined the p o l a r i z e d a b s o r p t i o n spectrum of tetracene i n anthracene.' The e x p e r i -i mentally determined p o l a r i z a t i o n r a t i o I ^ ; I a of 1.9:1 compared with the o r i e n t e d gas value 7 .8:1 i n d i c a t e s appreciable i n t e r a c t i o n . The theory of e l e c t r o n i c s p e c t r a of mixed c r y s t a l s i n the d i p o l e - d i p o l e approximation has - 18 -32 been a p p l i e d t o a number of systems. G. The Theory of Molecular V i b r a t i o n s 33 34 The c l a s s i c a l treatment of the v i b r a t i o n s of polyatomic molecules ' i n v o l v e s s e t t i n g up expressions f o r the p o t e n t i a l and k i n e t i c energies of the molecules i n terms of a s u i t a b l e coordinate system of the atoms. The expressions f o r the k i n e t i c and p o t e n t i a l energy are used to formulate the wave equation which i s then solved f o r the v i b r a t i o n s , r o t a t i o n s , and t r a n s l a t i o n s . In the Born-Oppenheimer approximation the e l e c t r o n i c and nuclear motions are separated. Further, i t i s assumed t h a t the e l e c t r o n i c energy i s known and only the nuclear motions.are to be determined. The k i n e t i c energy of the molecule i s given by 21 =J\u00C2\u00B1 ma L ( - d T - ) + (~d~ ) + (-^-) ] (1.27) where (x, ,x., , x\u00E2\u0080\u009E ) are the displacements of the ath atom from e q u i l i b r i u m . ^ l a ' \u00E2\u0080\u00A2 2a' 3cr r n ; m i s the mass of the atom and N i s the number of atoms i n the molecule, a In terms of mass weighted c a r t e s i a n coordinates (q^, q 2 ? I j ; \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 - l ^ ) defined by q 1 = Vm^ x n , q 2 = x^, q 3 =\u00E2\u0080\u00A2 vfiu, x ^ , q 4 = vV2 x 1 2 . . . the k i n e t i c energy i s 3N 2T = . I. q (1 .28) 1 = 1 l i In m a t r i x n o t a t i o n 2T - q + E q where q i s a column matri x of the q^ and E i s the i d e n t i t y m a t r i x . For small displacements the p o t e n t i a l energy may be expressed as a - 19 -power s e r i e s expansion of the form 3 N 3 V 3 ? 9 2V 2V = 2V + 2 E (_!\u00E2\u0080\u00A2 ) q. + E (^J! ) q.q. + o . , v\u00C2\u00B0q. o n i . - . 9q. q. ' o n i n j 1=1 n i i>J=l i J (1.29) 3N 3N-= 2V + 2 ^ f.q. + \u00E2\u0080\u00A2 E - f..q.q. + ... (1.30) o . , i n i . . i 1 1 n l n l 1=1 i , J = l J J By choosing the p o t e n t i a l energy of the e q u i l i b r i u m c o n f i g u r a t i o n o f the atoms t o be zero, and noting that at the p o t e n t i a l energy minimum 9V (_L ) = f. = o (1.31) 9q. o l n i then f o r small amplitudes of v i b r a t i o n , the higher terms may be neglected-and the f i n a l expression f o r the p o t e n t i a l energy i s given by the f i r s t non-zero term. 3N _2 3N 2V - E f \ ) q.q. = E f..q.q. (1.32) . . n v9q.9q. o n i n i . . 11 i 1 i , j = l n i H j J \u00E2\u0080\u00A2 i , j \u00E2\u0080\u00A2 . J J The dependence of the p o t e n t i a l energy on only the quadratic terms i m p l i e s that the v i b r a t i o n a l motion i s harmonic. In t h i s approximation the f o r c e constants are defined by ' . 2 f. . = (I \ \u00E2\u0080\u00A2) (1.33) 13 ^ q ^ ^ o v J In m atrix n o t a t i o n the p o t e n t i a l energy i s 2V = q + f q where f i s a symmetric m a t r i x w i t h f j ^ . The for c e constant m a t r i x f,.has -|(N) (N+l) independent terms. - 20 -A system o f p a r t i c l e s i n motion must obey Lagrange's equations. In terms o f mass weighted c o o r d i n a t e s , these are d 3T 3V -TT- \u00E2\u0080\u0094\u00E2\u0080\u0094\u00E2\u0080\u0094 + -r\u00E2\u0080\u0094 = 0 (1-34) dt 3q 3q \u00C2\u00AB\u00E2\u0080\u00A2 J S u b s t i t u t i o n of the expressions f o r T and V y i e l d s 3N equations 3N q. + \u00C2\u00A3 f. . q . = 0 (1.35) J i = l 1 3 1 This set o f 3N simultaneous second-order l i n e a r d i f f e r e n t i a l equations has s o l u t i o n s of the form q i = A . c o s (A \" t + e) (1.36) where A^ i s the amplitude o f the v i b r a t i o n , A a.frequency f a c t o r and e a phase f a c t o r . S u b s t i t u t i o n of t h i s s o l u t i o n i n t o the d i f f e r e n t i a l equations r e s u l t s i n a set of equations i n the unknown amplitudes A^ 3N \u00C2\u00A3 ( f . . - 6.. A.)A. = 0 (1.37) i - 1 i J i J J i or i n matri x n o t a t i o n (f - E A)A = 0 The t r i v i a l s o l u t i o n s f o r which A. = 0 correspond to t r a n s l a t i o n s and r o t a t i o n s of the molecule as a whole. Non-zero s o l u t i o n s occur f o r values of A_. which s a t i s f y the determinantal or sec u l a r equation |f - E A | - 0 (1.38) - 2 1 -Development of the equations i n mass weighted C a r t e s i a n coordinates i s f a i r l y simple, but i t i s u s u a l l y not solved i n t h i s form. Among the 3 N p o s s i b l e values of X., the s i x values corresponding to the t r a n s l a t i o n s and r o t a t i o n s are zero, and more important, the f o r c e constants expressed i n t h i s system l a c k g e n e r a l i t y s i n c e they are a p p l i c a b l e to a s p e c i f i c molecule and are not t r a n s f e r a b l e among s i m i l a r molecules. I t i s t h e r e f o r e d e s i r a b l e to d e f i n e i n t e r n a l coordinates i n a way such that they d e s c r i b e the motions of the atoms, and are independent o f the t r a n s l a t i o n s and r o t a t i o n s of the molecule as a whole. The f o r c e constants should a l s o be t r a n s f e r a b l e among s i m i l a r molecules having s i m i l a r chemical bonds. A set of i n t e r n a l coordinates i n terms of bond s t r e t c h i n g , valence-angle 3 3 bending, out-of^plane bending, and bond t o r s i o n has been d e f i n e d . This set i s u s u a l l y r e f e r r e d to as valence c o o r d i n a t e s . From a diagram of the molecule, the p o t e n t i a l energy can be w r i t t e n down i n terms o f the i n t e r n a l valence coordinates (R^) and the k i n e t i c energy i n terms of C a r t e s i a n coordinates. Using the t r a n s f o r m a t i o n from i n t e r n a l coordinates t o C a r t e s i a n coordinates R = Bx ( 1 . 3 9 ) the k i n e t i c energy can be expressed i n terms of i n t e r n a l c o o r d i n a t e s . The elements of the B-matrix are r e a d i l y obtained by the Wilson s-vector technique. In matrix n o t a t i o n , the k i n e t i c and p o t e n t i a l energies i n i n t e r n a l coordinates are . 1 1 . 2 1 ' = B +5 M 2F,M2 B - 1 R = R ^ G ^ R ( 1 . 4 0 ) - 22 -2V = R +F R (1-41) where M i s a diagonal m a t r i x made up of the masses of the atoms. As be f o r e , a p p l i c a t i o n o f Lagrange's equation and s o l u t i o n s of the form R i = A i cos (At + e) (1.42) r e s u l t s i n the s e c u l a r determinant GF - AEl = 0 (1.43) The 3N - 6 values o f A . y i e l d 3N - 6 i n t e r n a l v i b r a t i o n s of the molecule. J In i n t e r n a l coordinates, the p o t e n t i a l and k i n e t i c energy matrices c o n t a i n cross terms which complicate the v i b r a t i o n a l problem. However, there i s a m a t r i x L which transforms i n t e r n a l coordinates to normal coordinates i = 1,2,3, ..3N-6 i n which a l l the cross terms i n the p o t e n t i a l and k i n e t i c energy expressions are e l i m i n a t e d . In matrix n o t a t i o n , the t r a n s -formation i s defined as R = L Q (1.44) and must s a t i s f y the c o n d i t i o n s 21 = R +G - 1R = Q + L + G _ 1 L Q = Q +E Q (1.45) and 2y = R +F -R = Q +L +F L Q = Q +A Q (1.46) Therefore L + G _ 1 L = E (1-47) and L +F L = A (1 -48) - 23 -where E i s the i d e n t i t y m a t r i x and A a diagonal m a t r i x . The s i g n i f i c a n c e o f each normal coordinate i s that i t describes a e e r t a i n change i n the arrangement of the atoms of a molecule with respect to each other. Further, corresponding to each normal coordinate there i s a s o l u t i o n of the sec u l a r equation such th a t \u00E2\u0080\u00A2 t h e atoms move with the same frequency and phase but d i f f e r i n g amplitudes. That i s , a l l the atoms of a molecule reach t h e i r maximum or minimum displacement at.the same time. These c h a r a c t e r i s t i c s d e f i n e a normal mode of v i b r a t i o n . I t s a s s o c i a t e d frequency i s known as a fundamental frequency of the molecule. A normal mode i s e a s i l y v i s u a l i z e d by expressing the normal coordinates i n terms of C a r t e s i a n displacement coordinates by the tra n s f o r m a t i o n 3 N Q. = I .I a.. x. (1.49) l . . . l i a i a j = l a=l The c o e f f i c i e n t s a., describe the change of the C a r t e s i a n coordinates x. per u n i t change of normal coordinate ?x. a\" 1' = - i a (1.50) 3 Q i An approximate d e s c r i p t i o n of- the normal modes i s d e r i v e d by equating i t with that valence coordinate which makes the l a r g e s t c o n t r i b u t i o n to the p o t e n t i a l energy. In valence coordinates the p o t e n t i a l energy of a normal mode i s ; 3N-6 3N-6 A. = E E F: . , L.. L. , . , (1.51) 24 The meaning of each L.. of the L-matrix i s the change of valence coordinate R per u n i t change of normal coordinate Q 3R. V = 3 Q T C 1 ' 5 2 ) The l a r g e s t terms i n the p o t e n t i a l energy matrix F are the diagonal terms. In t h i s case the l a r g e s t term i n X. i s the r e s u l t of a la r g e f o r c e constant 3^V 2 2 (f \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 - -^K\u00E2\u0080\u0094?) or a large L-matrix element (L.. = (\u00E2\u0080\u00A2--\u00E2\u0080\u0094 ) . The valence v n 9R ^ & ^ n ^9Qi ^ coordina/te R i s chosen as the approximate d e s c r i p t i o n of the mode. The I n t e n s i t y of I n f r a r e d Bands The L-matrix i s u s e f u l i n accounting f o r the i n t e n s i t y of a normal mode i n terms of i n t e r n a l coordinates and thus provides a more e a s i l y v i s u a l i z e d 33 e x p l a n a t i o n f o r the i n t e n s i t y o f a absorption band. The experimentally a c c e s s i b l e i n t e g r a t e d i n t e n s i t y c o e f f i c i e n t i f we de f i n e S to be p r o p o r t i o n a l to the square of the t r a n s i t i o n d i p o l e moment f o r a v i b r a t i o n i s ) P(Q) X j C Q - W j ] 2 (1-53) For small displacements the d i p o l e moment operator i s approximated by the s e r i e s expansion 3N-6 .. 3N-6 ' u(Q) = \u00C2\u00A3 u + E ( \u00C2\u00B1 ) ( L + ... (1.54) 3u I f \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i s assumed to be constant, over the whole amplitude of the v i b r a t i o n 3Q. . . G\, the expression f o r S_. becomes 25 S. -ij^.n. X ^ Q ^ d Q . ] 2 ( | - ) 2 (1.55) This can be evaluated i f the v i b r a t i o n a l wavefunctions x are assumed to be 33 harmonic o s c i l l a t o r wavefunctions. The f i n a l r e s u l t i s i 3c 90. o where c i s the v e l o c i t y of l i g h t . Now the o r i g i n of the i n t e n s i t y can be understood by developing 4^ \u00E2\u0080\u0094 i n terms of i n t e r n a l coordinates j ai,' 3 N ~ 6 a,, 9 R- 3N-6 \u00C2\u00AB 3Q. \" . . 3R. 3Q- \" . . 3R. L i j U - 5 / J x j 1=1 l x j 1=1 j J 9y A l l 3N-6 of the q u a n t i t i e s Q^\u00E2\u0080\u0094 y i e l d a set of l i n e a r homogeneous equations 9y J i n the unknowns . The s o l u t i o n of the equations gives the l a r g e s t 1 3y term c o n t r i b u t i n g to - r r r \u00E2\u0080\u0094 and thus the source of the i n t e n s i t y of the 3Q. i n f r a r e d a b s o r p t i o n band. I. Determination of Force Constants In g e n e r a l , i t i s not p o s s i b l e to determine the f o r c e constants (F-matrix) from e x p e r i m e n t a l l y observed fundamental frequencies s i n c e the 3N-6 equations which form the s e c u l a r equation have y(3N-6) (3N-6+1) unknowns. Three methods of at t a c k commonly used to o b t a i n f o r c e constants 33 are; ' (a) the use of frequencies from i s o t o p i c a l l y s u b s t i t u t e d molecules, (b) the use of symmetry to f a c t o r the s e c u l a r equation, and (c) the use of s i m p l i f i e d f o r c e f i e l d s . The f o r c e constants depend on the e l e c t r o n i c s t r u c t u r e o f a molecule which i n t u r n i n v o l v e s the charges and c o n f i g u r a t i o n of the n u c l e i but not - 26 -the masses. Presumably, the i s o t o p i c s u b s t i t u t i o n of one or more atoms does not a l t e r the e l e c t r o n i c s t r u c t u r e and t h e r e f o r e the change i n f o r c e constants w i l l be n e g l i g i b l e . The k i n e t i c energy does depend on the masses so that i s o t o p i c s u b s t i t u t i o n a l t e r s the G-matrix and consequently the f r e q u e n c i e s . The most favourable mass change i s achieved by r e p l a c i n g hydrogen by deuterium. The a l t e r e d frequencies and G-matrix elements provide a d d i t i o n a l equations i n the \u00E2\u0080\u0094 (3N-6)(3N-6+1) f o r c e constants. The symmetry of a molecule o f t e n makes i t p o s s i b l e to u t i l i z e group t h e o r e t i c a l methods t o f a c t o r i z e both the G and F matrices and hence the 33 s e c u l a r equation i n t o b lock form. Each block belongs to a d i f f e r e n t i r r e d u c i b l e r e p r e s e n t a t i o n of the point group of the molecule so that there are no o f f diagonal terms connecting d i f f e r e n t symmetry b l o c k s . In the symmetrized s e c u l a r equation each block may be t r e a t e d as a separate problem. A number of approximations are used to d e s c r i b e the general q u a d r a t i c f o r c e f i e l d . The c e n t r a l f o r c e f i e l d i s one o f the simplest approximations . used to reduce the number of independent f o r c e constants. A l l the i n t e r n a l coordinates are d e f i n e d as d i s t a n c e s between p a i r s of atoms both bonded and non-bonded, and each p a i r of atoms i s connected by a f o r c e . The c e n t r a l f o r c e f i e l d y i e l d s a diagonal p o t e n t i a l energy m a t r i x , but has been u n s a t i s f a c t o r y i n a p p l i c a t i o n to a l l but the simplest molecules. The valence f o r c e f i e l d i s an approximation most compatible with the chemical p o i n t of view. I t assumes that the p o t e n t i a l energy of a molecule can be s a t i s f a c t o r i l y described i n terms of changes i n length of chemical bonds and changes.in,angles between chemical bonds. The number of..force constants i s u s u a l l y l e s s than the number of frequencies and they can always be determined from the s e c u l a r equation. However, the neglect of terms o f f - 27 -the diagonal i n the F-matrix renders the approximation too i n a c c u r a t e to be u s e f u l i n most cases. An approximation which i s simpler than the general f o r c e f i e l d and combines the f e a t u r e s of both the c e n t r a l and valence f o r c e f i e l d i s known 35 as the Urey-Bradley f o r c e f i e l d , I t has terms i n v o l v i n g bond length changes and angle bends, and i n c l u d e s i n t e r a c t i o n terms between non-bonded atoms as diagonal e n t r i e s i n the F m a t r i x . The number of f o r c e constants i n a Urey-Bradley f o r c e f i e l d i s l e s s than ^(3N-6)(3N-6+1) thereby s i m p l i f y i n g the problem c o n s i d e r a b l y . In a normal coordinate a n a l y s i s the Urey-Bradley f o r c e f i e l d i s incorporated i n t o the s e c u l a r equation by the t r a n s f o r m a t i o n 1 F... = z 1... *. (1.58) j k 1 j k l 1 v 1 or i n m a t r i x n o t a t i o n Z$ (1-59) where Z i s a c o n s t r a i n t matrix and |_ the Urey-Bradley f o r c e constants. Chapter 2 M a t e r i a l s and Experimental Methods A. P u r i f i c a t i o n and Syntheses of M a t e r i a l s P r e l i m i n a r y experiments c a r r i e d out on Eastman Kodak White Label f l u o r e n e i n d i c a t e d that s u b l i m a t i o n reduced but d i d not e l i m i n a t e peaks i n o o the s o l u t i o n a b s o r p t i o n spectrum at 3470 A and 3380 A ( p o s s i b l y due to phenanthrene and carbazole, r e s p e c t i v e l y ) and that zone r e f i n i n g reduced o but d i d not e l i m i n a t e the peak at 3200.A. Therefore, commercial fluorene was p u r i f i e d i n the f o l l o w i n g way. Fluorene was t r e a t e d w i t h maleic anhydride to remove the anthracene according to a modified procedure p r e s c r i b e d f o r removing anthracene from phenanthrene. 0^ A second i m p u r i t y , p o s s i b l y carbazole, was removed by d i s s o l v i n g the r e s u l t i n g m a t e r i a l i n petroleum ether, and shaking the s o l u t i o n s e v e r a l times with small q u a n t i t i e s of concentrated s u l p h u r i c a c i d u n t i l the a c i d l a y e r no longer d i s c o l o u r e d . The mixture was washed with d i s t i l l e d water and the f l u o r e n e recovered by evaporating the s o l v e n t . This was f o l l o w e d by slow s u b l i m a t i o n to separate the f l u o r e n e from any i n v o l a t i l e i m p u r i t i e s that may have been added during the maleic anhydride treatment. A 2 gm sample was next chromatographed on a 200 gm s i l i c a g e l column (25\" x 1 1/4\" dia.) with petroleum ether as eluent, successive 1.00 ml f r a c t i o n s being checked f o r an impurity a b s o r p t i o n o peak at 3200 A i n a 5 cm c e l l . S u f f i c i e n t i m p u r i t y was c o l l e c t e d before the - 29 -37 f l u o r e n e was e l u t e d to confirm i t s i d e n t i t y as b e n z [ f ] i n d a n . The l a s t stage of p u r i f i c a t i o n was zone r e f i n i n g , the f l u o r e n e undergoing 86 passes. o The f i n a l sample was o p t i c a l l y c l e a r to n e a r l y 3000 A; there were no o a b s o r p t i o n l i n e s above 3052 A i n a 1 mm t h i c k sample he l d at about 10\u00C2\u00B0K although a weak fluorescence w i t h a doubled o r i g i n at 29,519 and 29,505 cm * p e r s i s t e d . The i d e n t i t y of t h i s r e s i d u a l i m p u r i t y was not e s t a b l i s h e d . The i m p u r i t y separated from f l u o r e n e by chromatography was shown to be b e n z [ f ] i n d a n from the f o l l o w i n g evidence: ( i ) The i m p u r i t y was e l u t e d from 37 the chromatography column before f l u o r e n e . ( i i ) Following two r e c r y s t a l -l i z a t i o n s from n-hexane, the s o l u t i o n absorption spectrum agreed with that 37 of b e n z [ f ] i n d a n , ( i i i ) The mass spectrum showed the parent mass peak at 168. ( i v ) A n a l y s i s found C, 92.02%; H, 7.79% while benz[f]indan r e q u i r e s C, 92.80%; H, 7.20%. Thus the i m p u r i t y was a hydrocarbon of molecular weight 168 with the u l t r a - v i o l e t a b s o r p t i o n spectrum i n d i c a t i n g the presence of a naphthalene r i n g . The three s t r u c t u r e s of F i g . 1 are c o n s i s t e n t w i t h t h i s data. Again 38 i t i s known that I and I I I are c o l o u r l e s s s o l i d s m e l t i n g at 94\u00C2\u00B0C and 64\u00C2\u00B0C r e s p e c t i v e l y , w h ile I I i s a l i q u i d at room temperature. A l l the above t e s t s were c a r r i e d out on the small amount of i m p u r i t y ( i n i t i a l l y about 20 mg) separated by chromatography. A f a i n t y ellow colour that had slowly b u i l t up i n the sample (probably by a e r i a l o x i d a t i o n ) was removed by sublimation i n a stream of n i t r o g e n e n t r a i n e r gas when the m a t e r i a l melted over the range 86-91\u00C2\u00B0C; there was not enough m a t e r i a l f o r f u r t h e r p u r i f i c a t i o n steps. A high r e s o l u t i o n n.m.r. spectrum ( F i g . 2) was recorded on the remaining (impure) m a t e r i a l where the presence of groups of two and four equivalent protons on the aromatic r i n g , and two and four equivalent protons on the a l k y l s i d e chain were revea l e d . These fea t u r e s were c o n s i s t e n t with e i t h e r - 30 -s t r u c t u r e I or I I I , but the melt i n g p o i n t i n d i c a t e d that s t r u c t u r e I was c o r r e c t . Thus we have shown th a t b e n z [ f ] i n d a n i s a common imp u r i t y i n f l u o r e n e , present i n our samples (from Eastman Kodak and Matheson, Coleman 37 and B e l l ) as w e l l as i n Johnson's. The commercial m a t e r i a l showed a sharp, b - p o l a r i z e d a b s o r p t i o n at 32,789 cm * i n an ab face which was a l t o g e t h e r absent i n the s y n t h e t i c sample; t h i s marks the presence o f yet another unknown im p u r i t y i n commercial f l u o r e n e . y I II III F i g . 1 P o s s i b l e s t r u c t u r e s f o r the im p u r i t y g i v i n g the weak abso r p t i o n at 3200 A. I b e n z [ f ] i n d a n ; I I benz[e]indan; I I I 2,3-dihydrophenalene. Fluorene was a l s o prepared from b e n z i l i c a c i d . The synthesis was ' 39 e s s e n t i a l l y the second method fo l l o w e d by Kanda e_t al_. except t h a t , i n the l a s t step, fluorenbne was reduced to fl u o r e n e by a m o d i f i c a t i o n of the 40 0 Wolff-Kishner r e a c t i o n . Although there were no impurity peaks at 3200 A, o o 3380 A or 3470 A, t h i s s y n t h e t i c m a t e r i a l was contaminated with anthracene which was removed by treatment with maleic anhydride. Following chromato-graphy and zone r e f i n i n g , the f i n a l s y n t h e t i c m a t e r i a l showed no ab s o r p t i o n H(l,3) H (5,6,7,8.) 8 9 I F i g . 2 NMR spectrum of b e n z [ f ] i n d a n . - 32 -o or fluorescence o r i g i n s above 3052 A i n a 1.5 mm t h i c k n e s s (be face) at 10\u00C2\u00B0K. However, even t h i s sample was not completely pure s p e c t r o s c o p i c a l l y s i n c e at low temperature there was a very f a i n t green phosphorescence and s o l i d f l u o r e n e , i f i t phosphoresces at a l l , i s expected to phosphoresce b l u e . Eastman Kodak white l a b e l b i p h e n y l was subjected to e s s e n t i a l l y the same p u r i f i c a t i o n procedure as f l u o r e n e . The chromatography was e l i m i n a t e d and the sample passed 46 times through a zone r e f i n e r . No i m p u r i t y absorp-t i o n l i n e s appeared i n a 2 mm t h i c k sample at about 10\u00C2\u00B0K; however, phenanth-41 . rene f l u o r e s c e n c e , - although very weak, was detected on a photographic p l a t e exposed f o r 2 hr. White l a b e l carbazole s u p p l i e d by Eastman Kodak was freed from anthra-cene by treatment w i t h maleic anhydride. The sample was then chromatographed 42 f o l l o w i n g a technique developed by Sangster on s i l i c a g e l using petroleum ether as an eluent. The b r i g h t blue fluorescence of the s t a r t i n g m a t e r i a l was replaced by a deep v i o l e t fluorescence i n the p u r i f i e d sample. Eastman Kodak white l a b e l dibenzothiophene was used a f t e r undergoing 204 passes i n a zone r e f i n e r . 43 Perdeuterated dibenzothiophene was synthesized from biphenyl-d-^Q s u p p l i e d by Merck, Sharp and Dohm. F i n a l p u r i f i c a t i o n of the product was achieved by s u b l i m a t i o n i n an apparatus which c o n s i s t e d of a pyrex tube connected to a n i t r o g e n c y l i n d e r and i n s e r t e d i n t o a furnace constructed so that a temperature gradient was set up along the h o r i z o n t a l pyrex tube. The sample was placed i n the h o t t e s t part of the tube, and a gentle flow of n i t r o g e n deposited the m a t e r i a l s at v a r y i n g distances along the tube accord-ing t o t h e i r r a t e and temperature of s u b l i m a t i o n . A l i g h t blue f l u o r e s c i n g i m p u r i t y was deposited i n the c o o l e s t part of the tube. Mass s p e c t r a l a n a l y s i s showed that the f i n a l product contained about 1 7 % dibenzothiophene-\" 1^ >d7<> - 33 -S p e c t r o q u a l i t y cyclohexane ( B r i t i s h Drug Houses), benzene (Fischer) and n-heptane (Matheson, Coleman and B e l l ) were used w i t h o u t f u r t h e r p u r i f i c a t i o n . B. P r e p a r a t i o n of Samples A l l c r y s t a l s were grown i n evacuated pyrex tubes from the melt by 44 s l o w l y lowering the sample through a Bridgman furnace. Pure c r y s t a l s of c a r b a z o l e , f l u o r e n e , dibenzothiophene-hg, and dibenzothiophene-dg were used f o r i n f r a r e d and Raman s t u d i e s . Mixed c r y s t a l s systems i n which the -2 -3 guest/host concentrations v a r i e d from about 10 M/M to 10 : M/M were used f o r a b s o r p t i o n and luminescence experiments.. The guest m a t e r i a l s were carbazole, dibenzothiophene-h D, dibenzothiophene-d Q, and b e n z [ f ] i n d a n ; the o o host m a t e r i a l s were f l u o r e n e , b i p h e n y l , and hexamethylbenzene. S t r a i n - f r e e , s i n g l e c r y s t a l p o r t i o n s of an ingot were s e l e c t e d by examining the sample f o r complete, uniform e x t i n c t i o n under a p o l a r i z i n g microscope. The appropriate c r y s t a l face was i d e n t i f i e d by conoscopic examination using o p t i c a l data f o r f l u o r e n e and biphenyl summarized by 45 W i n c h e l l . The conoscopic f i g u r e s shown by carbazole are the same as those of f l u o r e n e although the changed naming of the c r y s t a l axes as i l l u s t r a t e d i n F i g . 6 and F i g . 8 should be noted. The o p t i c a l p r o p e r t i e s of dibenzothiophene were e s t a b l i s h e d on a sample f o r which the c r y s t a l axes (a_,b_,c) were known from X-ray a n a l y s i s . The r e s u l t s of t h i s examin-a t i o n are i n c l u d e d i n the s e c t i o n on c r y s t a l s t r u c t u r e s and o p t i c a l p r o p e r t i e s . A f t e r the d e s i r e d c r y s t a l face was i d e n t i f i e d , i t was c a r e f u l l y prepared by p l a n i n g w i t h a razor blade. Next, the sample was placed on - 34 -\u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 i the prepared face i n a brass r i n g with the d e s i r e d c r y s t a l t h i c k n e s s and packed with P l a s t e r o f P a r i s . When the p l a s t e r had s e t , the c r y s t a l was ground on f i n e emery, paper to the thickness of the brass r i n g . With care, samples 0.15 mm t h i c k were prepared but r e q u i r e d f u r t h e r p o l i s h i n g i n order t o be s u i t a b l e f o r i n f r a r e d experiments. F i n a l l y , to prevent l i g h t losses from s c a t t e r i n g and to achieve very t h i n samples ( < 0.1 mm), the surfaces were p o l i s h e d w i t h a s u i t a b l e solvent such as acetone, ethanol, e t c . on a brass block wrapped with s o f t t i s s u e paper. C. - Cryostats The bulk of the absorption and luminescence experiments were c a r r i e d out 46 using a l i q u i d helium c r y o s t a t of the standard Duerig and Mador- design mainly because i t s d i s m a n t l i n g and assembly were accomplished w i t h ease. The sample was cooled by conducting away i t s thermal energy to l i q u i d helium through GE 7031 cement and a short length of copper. The temperature of the c r y s t a l d i d depend on the i n c i d e n t l i g h t i n t e n s i t y and was estimated to be about 15\u00C2\u00B0K. The c a p a c i t y of the helium r e s e r v o i r was s u f f i c i e n t to h o l d enough l i q u i d helium to l a s t 6-8 hours. A l i q u i d helium c r y o s t a t somewhat modified f o l l o w i n g the suggestion of 47 Roberts i n which the sample was cooled by helium heat exchange gas i n contact w i t h the l i q u i d helium c o n t a i n e r had two main advantages. An experiment c o u l d be conducted over a range of temperatures, and samples could be supported l o o s e l y i n such a way that a minimum of s t r a i n was i n v o l v e d i n the c o o l i n g and warming process. Considerable d i f f i c u l t y was experienced i n a c h i e v i n g vacuum t i g h t s e a l s using indium 0-rings between the s i l i c a windows and the metal frame. The most s u c c e s s f u l procedure r e q u i r e d c l e a n i n g - 35 -the metal surfaces w i t h s t e e l wool and the indium O-rings with s i l v e r s o l d e r f l u x (Stay B r i t e ) . Before each experiment, a l l vacuum t i g h t s e a l s were checked with a B a l z e r s leak d e t e c t o r using Freon 12 as a t e s t gas. D. C a l i b r a t i o n o f Gas Thermometer The temperature of the samples was estimated to w i t h i n about a degree during the c o o l i n g and warming up process by monitoring the pressure of 48 the helium heat exchange gas i n the sample chamber. A schematic drawing of the gas thermometer i s d i s p l a y e d i n F i g . 3. connect ing t u b e pressure gauge V sample c h a m b e r F i g , 3 Helium gas thermometer This d i s c u s s i o n assumes that helium behaves i d e a l l y and that the volume of the connecting l i n e s which s u f f e r a temperature gradient i s small enough to be ignored. At f i l l i n g , and V\"2 are at and T^; i n use, i s at and T^ wh i l e V\"2 i s at P 2 and T 2 > Then at T , P1(V1 + v2) RT, = n (2.1) - 36 -and at T 2, P 2 V 2 x> \u00E2\u0080\u0094 = n _ x (2-2) RTj ' RT 2 whsre n i s the t o t a l number of moles of helium i n and V 2 and (n-x) i s the number of moles of helium i n V\"2 at T^. Thus p fV + V ) P V P V l l 1 2' 2 1 2 2 - - \u00E2\u0080\u0094 \u00E2\u0080\u0094 = - + f2 31 RT^ RT RT 2 1 J P V T 2 2 1 o r T 2 P~V + p v - P V C2-4) 1 1 1 2 \u00E2\u0080\u00A2 2 1 P T V 2 1 1 hence T, = \u00E2\u0080\u00A2=\u00E2\u0080\u0094: = =;\u00E2\u0080\u0094 where v = rr~ (2.5) l v + 1 ~ 2 V 2 The c o n d i t i o n s p r e v a i l i n g i n the c r y o s t a t when i n use are such that v < 1 and P 2 << P so that T 2 = 4^T) ] p 2 ( 2 ' 6 ) T l or T 2 = k P 2 where k = -p ( v + 1 ) C2-7) The sample chamber was f i l l e d w i t h helium to about one atmosphere at room temperature and the tap c l o s e d . On c o o l i n g , two c a l i b r a t i o n p o i n t s were obtained: one at l i q u i d n i t r o g e n temperature (77\u00C2\u00B0K) and the other at l i q u i d helium temperature (4.2\u00C2\u00B0K). Other temperatures i n the'range 4.2\u00C2\u00B0K to 77\u00C2\u00B0K were then found approximately from the measured(P ) by l i n e a r i n t e r p o l a t i o n . _ 37 -E. Apparatus Absorption and Prompt Luminescence Studies A l l low-temperature p o l a r i z e d absorption and emission s p e c t r a were photo-, graphed with a H i l g e r and Watts E201 large L i t t r o w Spectrograph with quartz o p t i c s on Kodak 103 a-0 and 103 a-F p l a t e s . The e x t e r n a l o p t i c a l system d i s p l a y e d i n F i g . 4 was constructed w i t h fused s i l i c a lenses. A lens focused the l i g h t source on to the sample; then the tr a n s m i t t e d or emitted l i g h t was p o l a r i z e d and focused on the s l i t by a Wollaston u n i t . The Wollaston u n i t c o n s i s t e d of a prism with two ad j u s t a b l e planoconvex lenses on e i t h e r s i d e to ensure passage of p a r a l l e l l i g h t through the prism and proper focus-i n g at the s l i t . The two beams were separated by 4 mm p r o v i d i n g ample space f o r an i r o n arc c a l i b r a t i o n between them. LIGHT SOURCE LENS SAMPLE HOLDER WOLLASTON UNIT SLIT 18 cm- 18cm 32cm- \u00E2\u0080\u00A232cm F i g . 4 E x t e r n a l o p t i c a l system used f o r abso r p t i o n and luminescence experiments. The c r y s t a l s were a l i g n e d i n the o p t i c a l t r a i n using the p r i n c i p l e of 49 the p o l a r i z i n g microscope. With a v i s i b l e - l i g h t source and no sample i n pl a c e , a piece of p o l a r o i d was placed before the sample h o l d e r , and r o t a t e d - 38 -to e x t i n g u i s h one beam from the Wollaston prism as i n d i c a t e d by a minimum current from a p h o t o m u l t i p l i e r mounted on a Beckman DU monochromator.. Then the s i n g l e c r y s t a l sample was i n s e r t e d i n t o the o p t i c a l t r a i n and the complete holder r o t a t e d u n t i l a new minimum i n the photocurrent was reached. In t h i s way the sample and Wollaston e x t i n c t i o n d i r e c t i o n s were a l i g n e d to w i t h i n one degree. The room-temperature c r y s t a l s p e c t r a were measured on the modified Beckman DU spectrometer.\"^ A PEK 75 W Xenon lamp was used as continuous l i g h t source f o r absorp-o t i o n experiments. Fluorescence e x c i t a t i o n was accomplished w i t h the 3131 A mercury l i n e i s o l a t e d by 5 cm of potassium chromate s o l u t i o n (0.200 gm/1) and 3 mm Corning f i l t e r CS 7-54 from a PEK 100( W mercury lamp. Some phosphorescence spectra were photographed w i t h a J a r r e l l - A s h 3/4 o meter, f/6.3 g r a t i n g spectrograph with a d i s p e r s i o n of 22 A/mm i n 1st order. Delayed Luminescence Studies Delayed luminescence e x c i t a t i o n was provided by the l i g h t from a PEK model 401 mercury lamp operated at an output of 100W focused on to a sample plac e d i n the heat exchange chamber of the modified l i q u i d helium c r y o s t a t . The prompt fluorescence was separated from the delayed emission by means of a s l o t t e d r o t a t i n g can phosphoroscope mounted around the outside of the lower p o r t i o n of the helium c r y o s t a t . The emission was passed through a Spex model 1700-11 (Czerny-Turner, 3/4 meter, f/6 spectrometer/spectrograph) w i t h o o a g r a t i n g blazed at 7500 A and a d i s p e r s i o n of 10 A/mm. The s l i t width was 250 microns. The emission was detected by an RCA 1P28 p h o t o m u l t i p l i e r mounted at the e x i t s l i t . The photocurrent was a m p l i f i e d by a K e i t h l y 414 micro-microammeter, and d i s p l a y e d on a B r i s t o l chart r e c o r d e r . The i n t e n s i t y of the delayed f l u o r e s c e n c e and'phosphorescence was monitored over a wide - 39 - : range of temperatures. The l i f e t i m e s were measured from the decay of the p h o t o m u l t i p l i e r s i g n a l from an intense emission peak; a f t e r c u t t i n g o f f the l i g h t source, the decay was followed on a recorder operating at a speed of 40 in./min. F. I n f r a r e d Spectra The i n f r a r e d s p e c t r a were recorded on Perkin-Elmer spectrometers (models 301 and 421). P o l a r i z e d spectra were obtained using Perkin-Elmer gold wire g r i d p o l a r i z e r s on s i l v e r bromide or polyethylene s u b s t r a t e s . A number of s u i t a b l e s o l v e n t s were a v a i l a b l e f o r the f a r i n f r a r e d r e g i o n , ^ but c y c l o -hexane proved to be most s u i t a b l e f o r organic molecules. A 1 cm c e l l equipped w i t h p o l y e t h y l e n e windows and f i l l e d w i t h a saturated s o l u t i o n of the organic compound i n cyclohexane was placed i n the sample beam. A matching c e l l f i l l e d w i t h cyclohexane was u t i l i z e d to attenuate the reference beam. G. Raman Spectra o Raman spec t r a of carbazole were e x c i t e d by the 4880 A l i n e from an argon i o n gas l a s e r operated at a nominal output power of 50 mW. The o 4880 A l i n e was i s o l a t e d from other l i n e s i n the discharge using an i n t e r -o ference f i l t e r with a 10 A band width. The spectra were detected by an RCA 1P28 p h o t o m u l t i p l i e r mounted at the e x i t s l i t o f the Spex model 1700-11 spectrometer. The p h o t o m u l t i p l i e r output was a m p l i f i e d by a K e i t h l y 414 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 i micro-microammeter and recorded by a B r i s t o l r e c o r d e r . A1 second method, s l i g h t l y more s e n s i t i v e but l e s s s a t i s f a c t o r y from the p o i n t of view of operating c o n d i t i o n s was a l s o used t o amplify the signal.: The l a s e r l i g h t was chopped at about 1 kc and the p h o t o m u l t i p l i e r output was sent to a recorder through a narrow band a m p l i f i e r and phase s e n s i t i v e d e t e c t o r . The f i \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 I - 40 -l i m i t of the d e t e c t i o n was set by the o p t i c a l noise a r i s i n g from ghosts and s a t e l l i t e s at the l a s e r frequency s c a t t e r e d from the g r a t i n g . The s p e c t r a l band pass was set at about 5 cm ^ and the r e p r o d u c i b i l i t y i n l i n e measurement was u s u a l l y l e s s than t h i s . The c r y s t a l and s o l u t i o n Raman spectra o f f l u o r e n e and dibenzothiophene were e x c i t e d by a helium-neon gas l a s e r operating at 90 mW, and were recorded using a Cary 81 spectrometer. The s l i t width corresponded to a s p e c t r a l band pass of 5 cm ^. H. Measurement of Spectra S p e c t r a l band p o s i t i o n s were measured from photographic enlargements (Kodabromide 5A paper) of the s p e c t r o s c o p i c p l a t e s with the a i d of a measuring box. The measuring box c o n s i s t e d of a h a i r l i n e mounted i n p l e x i -g l a s s and attached to an i l l u m i n a t e d box so t h a t i t t r a v e l l e d at r i g h t angles to a p r e c i s e l y engraved r u l e r . The accuracy i n measurement between l i n e s was estimated to be 0.1 mm. At f i r s t , the method used to convert l i n e p o s i t i o n s on a p r i n t to vacuum wavenumbers assumed that the d i s p e r s i o n of the prism spectrograph was l i n e a r i n wavenumber over small d i s t a n c e s . Two i r o n arc l i n e s were chosen near to an unknown l i n e , i d e n t i f i e d i n Angstroms 52 using i r o n arc charts i n Brode, converted to vacuum wavenumbers by con-53 s u i t i n g Kayser's Tables, and f i n a l l y the p o s i t i o n of the unknown l i n e determined by i n t e r p o l a t i o n between the two standard l i n e s . The accuracy of the method was estimated to be about 1 cm *. . During the l a t t e r h a l f o f the work done f o r t h i s t h e s i s , a computer programme was a v a i l a b l e to evaluate the l i n e p o s i t i o n s . A c a l i b r a t i o n curve f o r each s t r i p was obtained by a l e a s t squares f i t to a t h i r d degree polynomi - 41 -X = A +.Bx + Cx 2 + Dx 3 from a number of i r o n arc l i n e s . The a c t u a l degree of the equation depends on the number of i r o n l i n e s . Any misassigned i r o n l i n e s were detected by the poor f i t to the d i s p e r s i o n equation and r e j e c t e d by the programme. The p o s i t i o n s of the s p e c t r a l l i n e s were computed from- the c a l i b r a t i o n curve, c o r r e c t e d t o vacuum wavelength using E d l i n ' s formula, and then converted to wavenumbers. The accuracy of the two methods was comparable; the l a t t e r method however expedited the tedious procedure of measuring l i n e s . Line i n t e n s i t i e s r e q u i r e d f o r the v i b r a t i o n a l a n a l y s i s of absorption and fluorescence s p e c t r a were estimated from t r a c i n g s of p l a t e s made with a Joyce-Loebl model MK I I I C double beam recording microdensitometer. I , C r y s t a l data and o p t i c a l p r o p e r t i e s of some organic compounds. Biphenyl ( C 1 2 H 1 0 ) C r y s t a l d a t a ^ 4 Molecular weight = 154.2; m e l t i n g p o i n t = 71\u00C2\u00B0C. M o n o c l i n i c , a. = 8.12, _b = o \u00E2\u0080\u00A2 5.64, c_ = 9.47 A, 3 = 95.4\u00C2\u00B0. Space group p 2 1 / a ' T w o molecule per u n i t c e l l . P e r f e c t cleavage ab_ plane; secondary cleavage (201) very n e a r l y c o i n c i d e n t with the molecular plane. 45 O p t i c a l p r o p e r t i e s Optic a x i a l plane a_c; acute b i s e c t r i x n e a r l y p a r a l l e l t o the long molecular a x i s , Z A C = -20.5\u00C2\u00B0. The p l a n a r i t y of the biphenylmolecule has been a subject of d i s c u s s i o n f o r some time. E l e c t r o n d i f f r a c t i o n s t u d i e s show c o n c l u s i v e l y that the f r e e molecule i s non-planar t o the extent that the d i h e d r a l angle i s 4 2 \u00C2\u00B0 ^ , and a.recent c a r e f u l study of the c r y s t a l revealed - s l i g h t deviations-from - 42 -molecular p o s i t i o n s are i l l u s t r a t e d i n F i g . 5 and the r e l a t i n g the molecular and c r y s t a l axes are l i s t e d i n Table 1 D i r e c t i o n cosines of biphenyl X Y. z * a 0.8289 0.5563 -0.0163 a \u00E2\u0080\u00A2 0.7986 0.5177 0.3031 b 0.5465 -0.8376 -0.0066 -0.2522 -0.1744 0.9529 * c_ 0.0125 -0.0004 0.9996 Note: The c a x i s i s the [102] d i r e c t i o n and a i s perp e n d i c u l a r to b and c_ . Fluorene ( C ^ H ^ ) 57 C r y s t a l data Molecular weight = 166.2; m e l t i n g p o i n t = 116-7\u00C2\u00B0C. Orthorhombic, a_ = 8,49, b_ = 5,721, \u00C2\u00A3 = 1 8 . 9 7 A. Space group Pnam (\u00C2\u00A3>2hJ ' Four molecules P e r u n i t c e l l . P e r f e c t cleavage ab plane. 45 O p t i c a l p r o p e r t i e s Optic a x i a l plane ab; acute b i s e c t r i x / / c . The molecule possesses a plane o f symmetry which i s p a r a l l e l to the ab c r y s t o l l o g r a p h i c plane and places the long a x i s of the molecule p a r a l l e l to the c_ a x i s of the c r y s t a l . The d i s p o s i t i o n of the molecules i n the u n i t c e l l i s shown i n F i g , 6 and the d i r e c t i o n cosines are t a b u l a t e d i n Table 2. p l a n a r i t y . The d i r e c t i o n cosines Table 1. -\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2-45 ~ F i g . 5 Biphenyl u n i t c e l l y 1 2 i C / / \" I D V a Fig.' 6. Fluorene u n i t c e l l _ 44 -Table 2 D i r e c t i o n cosines of f l u o r e n e X Y. z a_ 0.821 0 0.571 b 0.571 0 -0.821 c_ 0 1 0 Carbazole (C,\u00E2\u0080\u009EH N) C r y s t a l data Molecular weight = 167.2; m e l t i n g p o i n t = 247\u00C2\u00B0C. Orthorhombic, a =7.76,' \u00C2\u00B0 16 b_ = 19.15, c_ = 5.74 A. Space group Pnam \u00E2\u0080\u00A2 F \u00C2\u00B0 u r molecules per u n i t c e l l . P e r f e c t cleavage plane a c , ^ p l a t e - l i k e c r y s t a l s from ethanol w i t h 59 w e l l developed ax f a c e . O p t i c a l p r o p e r t i e s Optic a x i a l plane ab_; acute b i s e c t r i x //b_. The molecule i s s l i g h t l y non-planar i n the c r y s t a l being bent about the 59 z_ a x i s to the extent that the angle between the two benzene r i n g s i s 178.4. Since the d i s t o r t i o n from p l a n a r i t y i s small i t w i l l be a s u f f i c i e n t l y c l o s e approximation to t r e a t the molecule as though i t r e t a i n s f u l l covering symmetry. The o r i e n t a t i o n o f the molecular axies w i t h respect t o the c r y s t a l axes i s i l l u s t r a t e d i n F i g . 8 and s p e c i f i e d by the d i r e c t i o n cosines i n 59 Table 3. The molecular dimensions are shown i n F i g . 7. Table 3 D i r e c t i o n cosines of carbazole a 0.8771 0 0,4803 b 0 1 0 c 0.4803 0 0.8771 - .45 ^ F i g . 8 Carbazole u n i t c e l l F i g . 9 Dibenzothiophene u n i t c e l l Dibenzothiophene ( C . i S) -\u00E2\u0080\u00A2 1/ o C r y s t a l d a t a ^ Molecular weight = 183.4; melt i n g p o i n t = 99\u00C2\u00B0C. M o n o c l i n i c , \u00C2\u00A7_ = 8.667, O , r b = 5.998, c = 18.705 A, 3 = 113.91\u00C2\u00B0. Space group ? 2 ^ ( C ^ ) . Four molecules per u n i t c e l l . O p t i c a l p r o p e r t i e s Optic a x i a l plane ac_; e i t h e r the acute or obtuse b i s e c t r i x (the exact choice was not determined) c o i n c i d e s w i t h the b i s e c t o r of the angle 3 . F i g . 9 d i s p l a y s the p o s i t i o n of the molecules i n the u n i t c e l l , and the r e l a t i o n s h i p of the molecular axes to the c r y s t a l axes. The c e l l d e fined by the orthogonal edges r_, b, and s_, i s o u t l i n e d with a dotted l i n e i n F i g . 9; - 47 -r b i s e c t s 3 and rb l o c a t e s the p r i n c i p l e cleavage plane of the melt-grown s a m p l e s . ^ The o r i e n t a t i o n of the molecular axes w i t h respect to the a, 1) and r , b, s axes are given by the d i r e c t i o n cosines i n Tables 4 and 5. Th mean molecular parameters are shown i n F i g . 10 f o r a planar molecule although the molecule i s s l i g h t l y bent about the z a x i s so that the angle between the benzene r i n g s i s 178.7\u00C2\u00B0. Table 4 D i r e c t i o n cosines of dibenzothiophene X z a_ 0.7534 0.5133 -0.4085 b 0.4712 0.0092 0.8820 0.4587 0.8581 0.2350 Table 5 D i r e c t i o n cosines of dibenzothiophene X X z. r. -0.1235 0.9912 0.0704 b_ 0.4715 0.0092 -0.8819 J2. 0.8732 0.1324 0.4685 F i g . 10 Bond lengths and bond angles of dibenzothiophene Chapter 3 A V i b r a t i o n a l Assignment of Carbazole from I n f r a r e d , Raman, and Fluorescence Spectra A. I n t r o d u c t i o n A number of f a i r l y complete v i b r a t i o n a l assignments have been made 62 71 f o r aromatic hydrocarbons w i t h at l e a s t three r i n g s such as anthracene , 41,72-76 , . 77 78 79 phenanthrene , phenazme , pyrene , and perylene . These assignments have been based l a r g e l y on the r a t h e r complex i n f r a r e d spectra of s i n g l e c r y s t a l samples. In most cases, the a n a l y s i s of these sp e c t r a r e l i e s on the assumptions that the stronger l i n e s mark the presence of fundamentals and that departures from the oriented-gas p r e d i c t i o n s are not severe as to reverse p o l a r i z a t i o n r a t i o s . The usual outcome i s that the s e l e c t i o n of the l a s t few fundamentals i n each symmetry block n e c e s s a r i l y i n v o l v e s a r a t h e r a r b i t r a r y choice between many l i n e s of moderate strength that i n d i c a t e the presence of e i t h e r weak fundamentals or combinations. 73 74 One method that has been used ' to r e s o l v e t h i s dilemma i s to c a l -c u l a t e approximate values f o r the fundamentals using s i m p l i f i e d f o r c e f i e l d s which have been e x t r a p o l a t e d from those of smaller molecules. The few fundamentals missing from the experimental assignment are located from these roughly c a l c u l a t e d f r e q u e n c i e s . There i s a r e a l danger here that the f i n a l c a l c u l a t i o n s may c a r r y the r e s u l t of an i n i t i a l misassignment. A recent c a l c u l a t i o n * ^ has attempted to apply a u n i f i e d valence f o r c e f i e l d s imultaneously to benzene, naphthalene, anthracene and some deuterated - 49 -d e r i v a t i v e s ; i n t h i s case the e f f e c t s of any inadvertent misassignments are probably minimal, An a l t e r n a t i v e procedure was a p p l i e d to carbazole. For molecules l a c k i n g an i n v e r s i o n center, the data d e r i v e d from the i n f r a r e d s p e c t r a may be u s e f u l l y supplemented from f l u o r e s c e n c e and Raman s t u d i e s . Because of the d i f f e r e n t forms that the t r a n s i t i o n moment i n t e g r a l s take there i s a chance t h a t fundamentals that are weak i n an i n f r a r e d spectrum may be r e l a t i v e l y strong i n fluorescence or Raman s p e c t r a . In t h i s chapter, as complete an assignment of the molecular fundamentals of carbazole as p o s s i b l e was made from experimental r e s u l t s , and the data was used to t e s t the f o r c e f i e l d s that have been employed s u c c e s s f u l l y on other molecules by r 64,65,73 C a l i f a n o B. S e l e c t i o n Rules 8 0 The carbazole molecular axes x, v_, and z_ ( F i g . 8) have been chosen such that z_ i s the in- p l a n e a x i s passing through the n i t r o g e n atom and transforms l i k e A^ wh i l e x i s the molecular normal spanning the B^ i r r e d u c -i b l e r e p r e s e n t a t i o n of the molecular p o i n t group. As p r e v i o u s l y pointed 59 out, although the carbazole molecule i s s l i g h t l y non-planar i n the c r y s t a l , i t - w i l l be a s u f f i c i e n t l y c l o s e approximation t o t r e a t the molecule as r e t a i n i n g f u l l covering symmetry. However, t h i s molecular d i s t o r t i o n may induce the i n f r a r e d forbidden bands ( i n C^) to appear weakly along the b c r y s t a l axes. The s e l e c t i o n r u l e s f o r the f r e e molecule and c r y s t a l may be deduced from Table 6. Of the twenty one A^ fundamentals, there are four C-H and one N-H s t r e t c h i n g modes so th a t s i x t e e n are expected below 2000 cm *. There are a l s o four C-H s t r e t c h e s of B symmetry. Each f r e e molecule s t a t e - 50 -gives r i s e t o four c r y s t a l s t a t e s , each with the value f o r the k_ v e c t o r equal to zero. Two of these c r y s t a l s t a t e s are Raman a c t i v e . The remaining c r y s t a l s t a t e s are i n f r a r e d a c t i v e i n the case o f A^ or f r e e molecule v i b r a t i o n s , although only one may appear i n the i n f r a r e d spectrum ( p a r a l l e l to the b_ c r y s t a l a x i s ) f o r B^ v i b r a t i o n s . A^ f r e e molecule modes may give r i s e to very weak b p o l a r i z e d bands i n the i n f r a r e d spectrum si n c e i n t e r -molecular i n t e r a c t i o n s i n the c r y s t a l mix A^ and s t a t e s of the i s o l a t e d molecule. The numbers of l a t t i c e frequencies and t h e i r d i s p o s i t i o n amongst the i r r e d u c i b l e r e p r e s e n t a t i o n s of the f a c t o r group are a l s o shown i n Table 6. Table 6. C o r r e l a t i o n t a b l e showing the s e l e c t i o n r u l e s f o r the i s o l a t e d 3. molecule and f o r the c r y s t a l . Molecular group S i t e group Factor group C 2 v C s D 2 h N Bases Bases n A g 2g aa,bb,cc 3 21 xx,yy,zz;z A i ac 3 10 xz;x B 1 l u c 2 B ? 3u a 2 ab 3 9 xy_ 20 yz;y ^ A\" B2 B3g A u .be 3 3 2u b 2 N i s the number of fundamentals i n the f r e e molecule and n i s the number of l a t t i c e frequencies with k = 0. i r 100 200 300 400 500 \u00E2\u0080\u00A2 600 cm\" F i g . 11 Low frequency i n f r a r e d s p e c t r a of c a r b a z o l e . (a) ac_ s e c t i o n ; c r y s t a l t h i c k n e s s .19 mm below 160 cm - 1 and .053 mm above 160 cm - 1 ( f o r the r e g i o n 160-250 cm - 1 low t r a n s m i s s i o n i s from a sample 1 mm t h i c k ) ; f u l l l i n e / / a , broken l i n e / / c . (b) be s e c t i o n ; c r y s t a l t h i c k n e s s 1 mm below 160 cm - 1 and .15 mm above 160 cm~l; f u l l l i n e //b_, broken l i n e / / c . (b) 70 p t h i c k b_c s e c t i o n ; f u l l l i n e //b_, broken l i n e //c_. - 53 -C. The I n f r a r e d Spectrum The p o l a r i z e d i n f r a r e d spectra were recorded w i t h l i g h t i n c i d e n t on ac_ and bc_ faces and are shown as F i g s . 11 and 12. Table 7 l i s t s the observed frequencies of the bands and t h e i r assignments made on the assumption that d e v i a t i o n s from an o r i e n t e d gas model are s m a l l . In t h i s approximation the r e l a t i v e i n f r a r e d band i n t e n s i t i e s along the c r y s t a l axes (see Table 8) are given as the square of the d i r e c t i o n cosines of the molecular axes on the * i 59 c r y s t a l axes. Table 7 The i n f r a r e d spectrum of carbazole //a_- //b_ //c Symmetry 69 m 127 s 218 ms 310 w 438 vs 570 ms 658 m 103 vw 129 vw 49 vw 104 mw 148 m 221 s lu 3u 2u l u bgu+comp. B^ b o 2u A, 505 m 548 w 576 vw 616 mw 379 sh 418 mw 451 s 562 m 603 sh 658 m A f - 54 -//a //b //\u00C2\u00A3 Symmetry 720 vs 723 s B l 733 sh B2 737 ms B2 742 vs 740 m B l 750 sh 751 vs A l 756 m B2 772 m B2 835 s B2 851 ms 852 s A l 859 w B2 873 sh 873 sh B l . 878 m B2 909 vw B2 910 s 910 s A l 924 vs 928 s B l 966 mw A l 979 w B2 995 s B2 1010 m 1006 ms A l 1022 ms B2 1062 w B2 1070 w A l 1102 vw 1109 ms A l 1123 sh B2 1136 s A l 1146 sh B2 1152 w B l 1158 ms B2 1173 ms B2 1204 ms B2 1203 ms /' . 1200 s A l 1223 m 1223 m A 1 ? - 55 -/ / a //b / / c symmetry 1286 vw 1305 mw 1332 ms 1349 sh 1418 sh 1444 ms 1478 vw 1577 sh 1609 sh 1620 ms 1788 m 1899 m 1233 s 1270 m 1320 s 1357 mw 1380 vs 1422 ms 1452 vs 1490 s 1540 mw 1560 mw 1575 w 1594 vw 1643 sh 1667 mw 1694 s 1739 m 1749 m 1819 m 1868 m 1284 s 1307 m 1327 s 1346? 1418 sh 1442 s 1480 m 1509 vw 1537 vw 1577 w 1610 sh 1620 s 1788 m 1899 m V B ? 1 A, A, A, B, - 56 -//a //b / / \u00C2\u00A3 symmetry 1912 mw B2 1916 w 1916 mw A l 1940 mw B2 2467 mw B2 2597 w 2597 mw A l 2663 w B2 2728 mw B2 2774 w B2 2825 mw B2 2940 m B2 2973 mw 3039 ms A l 3017 sh 3017 sh B l 3030 m B2 3050 vs B2 3027 m 3055 s A l 3075 mw 3077 ms A l 3082 sh 3084 m A l 3094 m B2 3150 mw B2 3410 vs 3421 vvs A l Table 8, R e l a t i v e i n f r a r e d band i n t e n s i t i e s of carbazole along the c r y s t a l axes c a l c u l a t e d f o r the o r i e n t e d gas model. a 0.231 b 0.000 c 0.769 0.769 0.000 0.231 0.000 1.000 0.000 - 57 L a t t i c e modes, r e f e r r e d to here and i n Tables 7 and 9 using lower case symbols, are expected at low frequencies as l i n e s p o l a r i z e d along only one c r y s t a l a x i s . Thus the two \u00C2\u00A3 p o l a r i z e d l i n e s at 49 and 104 cm * are assigned as b^ u, the two b_ p o l a r i z e d l i n e s at 103 and 129 cm * as b^u and the two \u00C2\u00A3 p o l a r i z e d l i n e s at 69 and 127 cm''1 as b ^ l a t t i c e modes. The l i n e at 127 cm * was strong and q u i t e broad and was thought to be made up of two c o n t r i b u t i o n s : one derived from the b_ l a t t i c e mode mentioned above 3u and another (weaker) c o n t r i b u t i o n marking the presence of the B^ u c r y s t a l component of a f r e e molecule fundamental. The spectrum of a s o l u t i o n of carbazole i n p y r i d i n e was measured i n the low energy r e g i o n and the lowest frequency l i n e was observed weakly at 140 cm * i n a c e l l of length 1 mm. The weakness of the spectrum was due to the low con c e n t r a t i o n a v a i l a b l e and the c e l l length was l i m i t e d by the small amount of l i g h t t r a n s m i t t e d b y the s o l v e n t . I f the solvent s h i f t i s about the same i n s o l u t i o n as i n the c r y s t a l then the s o l u t i o n l i n e at 140 cm * should be the mean of the frequencies of the four c r y s t a l components, only two of which are observed -1 -1 at 148 cm and at about 127 cm The assignments of the l i n e s at 658, 1223 and 1418 cm * are u n c e r t a i n s i n c e the components i n a_ and _c p o l a r i z a t i o n are of very n e a r l y equal i n t e n s i t y . I t may be that these l i n e s are complex i n that they contain combinations o f f r e e molecule symmetry opposite to t h a t of the main c o n t r i b u t i o n or that c r y s t a l induced mixing with the strong l i n e s at 723, 1200, and 1442 cm ^ r e s p e c t i v e l y d i s t u r b s the p o l a r i z a t i o n r a t i o . I f the l a t t e r i n t e r p r e t a t i o n i s c o r r e c t the l i n e at 1223 cm * should be reassigned as wi t h s u f f i c i e n t i n t e n s i t y mixed-in to reverse the p o l a r i z a t i o n r a t i o . Only the more prominent l i n e s above 2000 cm ^ are included in' Table 7. - 58 -The i n t e r p r e t a t i o n suggested i n the Table, that the strong C-H s t r e t c h i n g fundamentals of A^ symmetry have s u f f e r e d a l a r g e f a c t o r group s p l i t t i n g need not be c o r r e c t . The a l t e r n a t i v e i s to suppose that combinations of moderate i n t e n s i t y e x i s t at these energies; c e r t a i n l y the even stronger N-H s t r e t c h at 3420 cm ^ does not show so l a r g e a s p l i t t i n g . D. The Raman Spectra I f g and a' are p o l a r i z a b i l i t y tensors defined with respect to the p r i n c i p a l axes x,%_,z_ of the molecule and a_,b_,c^ of the c r y s t a l r e s p e c t i v e l y , then i n the o r i e n t e d gas model ct' = U a U (3.1) where U defin e s the tr a n s f o r m a t i o n from the molecular to the c r y s t a l frames. In t h i s approximation the i n t e n s i t i e s (I) of the Raman s c a t t e r i n g of the c r y s t a l are r e l a t e d to those of the i s o l a t e d molecule by the equations aa bb cc _ ac_ 592 .053 .710 \" i X X 1.000 I yy 053 .592 .710 i zz 177 .177 .290 I L x z J and ab_ f b c . \".769 .23l\" I xy. .231 .769 I - J Ly_zj (3.2) The Raman l i n e s observed i n a l l p o s s i b l e c r y s t a l o r i e n t a t i o n s are c o l l e c t e d i n Table 9 and the spectra are shown i n F i g s . 13 and 14. The sp e c t r a i n F i g . 14 have been adjusted to make the most prominent l i n e i n .- 59 -each of the sp e c t r a of equal strength and so the r e l a t i v e strengths of l i n e s f o r d i f f e r e n t o r i e n t a t i o n s of the sample should not be c l o s e l y compared. However i n F i g . 13 -the three d i f f e r e n t spectra have not been adjusted and were recorded under as n e a r l y the same c o n d i t i o n s as p o s s i b l e . (ac) n j _ - 2 0 0 O (ab) (be) 2 0 0 O 2 0 0 1 o 2 0 0 F i g . 13 Some low frequency Raman i n t e r v a l s of carbazole. The nomenclature i s d e fined i n Table 9. Table 9. The Raman spectrum of carbazole, b(aa)c a(bb)c a(cc)b a(ca)b b(ab)c b(cb)a symmetry 52? 78 ? 52 78 104 28 34 103 3g 3g l g a ? l g a ? g - 60 -b(aa)c a(bb)c a(cc)b a(ca)b b(ab)c b(cb)a symmetry 107 108 108 a g 220 220 221 305 299 A l A 2 V \u00E2\u0080\u00A2 431 430 432 A l 569 658 569 658 569 V A l 743 742 743 A l 860 860 V 889 889 1015 1017 1014 A l 1107 1105 1208 1226 1108 1212 A l A l A l 1291 1291 1292 A l 1314 1314 1314 A l 1339 1338 1340 A l 1455 1455 1483 1455 A l A l 1576 1575 1629 . 1577 A l A l 3061 3060 3062 A l 3421 3420 3422 A l The column headings i n d i c a t e from l e f t to r i g h t i n s i d e the parenthesis the p o l a r i z a t i o n of the i n c i d e n t and s c a t t e r e d l i g h t and from l e f t to r i g h t outside the parenthesis the propagation d i r e c t i o n s of the i n c i d e n t and s c a t t e r e d l i g h t . The l i n e s r e p r e s e n t i n g l a t t i c e modes are e s p e c i a l l y strong and were recorded at reduced s e n s i t i v i t y . The very weak l i n e s 52 and 78 cm ^ from the e x c i t i n g l i n e i n the (aa) spectrum are probably components of the much I \u00E2\u0080\u0094 r -200 JL UA 1 .1 1 \u00E2\u0080\u0094 1 ' i ' r A. ft / A , A, /Z. (aa) - i r-(bb) (cc) 1 O 2 0 0 4 0 0 6 0 0 8 0 0 IOOO ' 1200 ' I4CO \" 1600 7 ^OOO 32CO ' 3 4 0 0 F i g . 14 Raman sp e c t r a of carbazol e . The nomenclature i s defined i n Table 9. i - 62 -stronger l i n e s i n the (ab) spectrum a r i s i n g from imperfect c r y s t a l alignment. However the 107 cm 1 i n t e r v a l i n the. (ac) spectrum i s of a medium strength which could not be f u r t h e r reduced by small angular adjustments of the c r y s t a l or the a n a l y s i n g p o l a r o i d and so i s probably not a component of the very strong l i n e s i n the (aa) or (cc) s p e c t r a . P o s s i b l y , the l i n e s at 104 cm (ab) and 103 cm 1 (be) are two f a c t o r group components of a molecular funda-mental (taken to be from energy c o n s i d e r a t i o n s ) . Because of low v o l a t i l i t y (the m e l t i n g p o i n t of carbazole i s 247\u00C2\u00B0C) the sample, always f r e s h l y p o l i s h e d w i t h acetone before each s e r i e s of spectra were measured, h e l d a f i n e surface f i n i s h f o r s e v e r a l days. Even so, the s c a t t e r i n g of the e x c i t i n g l i g h t w i t h i n the s i n g l e monochromator was so severe that only two q u i t e weak n o n - t o t a l l y symmetric fundamentals were recorded. I t i s p o s s i b l e that the 305 cm 1 i n t e r v a l i n the (cc) spectrum i s the Ag f a c t o r group component of a B^ molecular v i b r a t i o n although the B_ f a c t o r group component was not r e p r o d u c i b l y measured i n the (cc) spectrum and consequently was not i n c l u d e d i n Table 9. I n t e r v a l s of 744, 1014, 1216, 1286, 1313, 1356, 1487, 1572 and 1630 cm\"1 81 have been reported i n the Raman spectrum of a s o l u t i o n of carbazole i n acetone. The present study has not only shown that a l l these i n t e r v a l s represent t o t a l l y symmetric v i b r a t i o n s and found more l i n e s but a l s o allows an estimate to be made of the r e l a t i v e e f f e c t i v e n e s s of each of the molecular t r a n s i t i o n moment operators. For example, v i b r a t i o n s that appear e x c l u s i v e l y through the operator a are seen at 220, 1226, 1483 and 1629 cm E, The Fluorescence Spectrum The fluorescence spectrum of carbazole i n f l u o r e n e at a temperature estimated to be about 15\u00C2\u00B0K i s reproduced i n F i g . 15 and an a n a l y s i s of the 27,000 27,500 28,000 28,500 29,000 29,500 cm\" F i g . 15 The fluorescence spectrum of carbazole i n a s i n g l e c r y s t a l m a t r i x of f l u o r e n e at about 15 .\. - 64 -spectrum i s shown i n Table 10. In F i g . 15 the e l e c t r o n i c o r i g i n i s almost completely reabsorbed. From the absorption spectrum (see next chapter) i t i s known that the o r i g i n i s s t r o n g l y b p o l a r i z e d so that the e l e c t r o n i c t r a n s i t i o n under i n v e s t i g a t i o n may be assigned \u00E2\u0080\u0094>\u00E2\u0080\u00A2 '''A^ . A study of the fluore s c e n c e spectrum of carbazole i n biphenyl (Appendix I) confirmed the assignment of the e l e c t r o n i c o r i g i n , but d i d not y i e l d any a d d i t i o n a l i n f o r -mation about the v i b r a t i o n a l a n a l y s i s . However each v i b r o n i c band i n the biphenyl m a t r i x e x h i b i t e d m u l t i p l e t s t r u c t u r e c o n s i s t i n g o f two l i n e s separated by 22 cm 1 on the average. The fluorescence spectrum of phenanthrene i n biphenyl a l s o shows a doublet s p l i t t i n g which was a t t r i b u t e d to e n e r g e t i -c a l l y i n e q u i v a l e n t t r a p p i n g s i t e s a r i s i n g from guest-host i n t e r a c t i o n s mainly 8 2 of a r e p u l s i v e nature. Table 10. The fluorescence spectrum o f carbazole i n f l u o r e n e at about 15\u00C2\u00B0K. //b //c_ Int Remarks 691 o r i g i n , reabsorbed 30 m 30, l a t t i c e 48 m 48, l a t t i c e 81 m 81, l a t t i c e 108 mw 108, l a t t i c e 138 w 30 + 108? 219 vs 219, Ax 250 m 219 + 3 0 + 1 427 m 427, A x 435 ms 2 x 219 - 3 475 w 2 x 219 + 30 + 7 549 ms 549, B 2 616 m 616, B 2 654 vs 654, k - 65 -//b //c_ Int Remarks 687 m 654 + 30 + 3 743 mw 743, A 771 w 219 + 549 + 3 830 w 219 + 616 - 5 874 s 219 + 654 + 1 888 ms 888, A 963 vw 219 + 743 + 1 996 w 996, I 52 1000 s 1009, A l 1040 w 1009 + 3 0 - 1 1098 w 2 x 219 + 654 + 6 1118 m 1118, R2 1151 mw 1151, A l 1204 mw 1204, A l 1210 s 1210, B2 1231 mw 219 + 1009 + 3 1237 m 1237, B2 1287 vs 1287, A l 1310 vs 2 x 654 + 2 1336 vs 1336, A l 1391 s 1391, B2 1427 w 219 + 1210 - 2 1453 mw 1453, A j ? ; 2 x 219 1461 m 1461, B2 1482 mw 1482, A l 1487 m 1487, B2 1505 m 219 + 1287 - 1 1528 m 219 + 2 x 654 + 1 1555 w 219 + 1336 1577 vw 1577, A l 1607 s 1607, B2 1625 s 1625, A l 66-//b //\u00C2\u00A3 Int Remarks 1665 ms 654 + 1009 + 2 1680 w 219 + 1461 1709 w 219 + 1487 + 3 1751 vw 743 + 1009 - 1 1802 vw 654 + 1151 - 3 1836 \u00E2\u0080\u00A2 w 549 + 1287 1841 w 219 + 1625 - 3 1886 w 219 + 654 + 1009 + 4 1900 vw 616 + 1287 - 3 1941 m 654 + 1287 1965 2 Symmetry There i s l e s s i n f o r m a t i o n a v a i l a b l e to determine the fundamentals and the assignment i s correspondingly l e s s c e r t a i n . However, from the f l u o r -escence spectrum fundamentals were d e f i n i t e l y l o c a t e d at 549, 616, 996, 1118, 1210, 1391, 1461, 1487, and 1607 cm\"1. The assignment of a B 2 fundamental at 1237 cm 1 from fluorescence r e s t s on the b a r e l y s i g n i f i c a n t energy d i f f e r e n c e between the b_ p o l a r i z e d l i n e at 1231 cm 1 and th a t i n c_ p o l a r i z a t i o n at 1237 cm 1. R e l i a b l e B 2 fundamentals were observed as strong l i n e s i n the i n f r a r e d spectrum at 505, 616, 835, 995, 1158, 1233, 1320, 1380, 1452 and 1594 cm\"1. Frequencies common to both sets number fourteen so that two more fundamentals must be chosen from the somewhat l e s s intense b p o l a r i z e d i n f r a r e d l i n e s at 737, 1022, 1270, and 1694 cm\"1. The l i n e at 737 cm 1 i s not p a r t i c u l a r l y strong and f a l l s i n a re g i o n of i n t e n s e absorption p o l a r i z e d i n the ac_ plane; any c r y s t a l i m p e r f e c t i o n or s t r a i n need only be s l i g h t to induce components of the intense l i n e s i n t o the b_ absorption spectrum. However, t h i s does not seem to have occurred s i n c e the b_ p o l a r i z e d l i n e s do not co i n c i d e i n energy with those of a or \u00C2\u00A3 p o l a r i z a t i o n . Because the l i n e at 737 cm 1 i s the strongest of that group i t i s thus taken as a fundamental, the l i n e s at 756, and 772 cm\"1 presumably marking the presence of combinations. Although the r a t h e r broad l i n e at 1694 cm 1 must co n t a i n considerable combination a b s o r p t i o n , the problem i s to decide whether or not the r a t h e r - 72 -co n s i d e r a b l e l i n e s trength i s d e r i v e d from a f o r t u i t o u s coincidence of many combinations at that energy. The i n f r a r e d spectrum of f l u o r e n e and of dibenzothiophene (vide i n f r a ) show only a number of f a i r l y weak l i n e s i n t h i s r e gion and so the t e n t a t i v e c o n c l u s i o n i s reached t h a t there i s no strong fundamental at 1694 cm 1. A frequency as high as 1694 cm 1 should not be l i g h t l y discarded f o r i t s i n c l u s i o n i n a l i s t of fundamentals would have a profound e f f e c t on any c a l c u l a t e d f o r c e f i e l d . The choice of 1022 cm 1 as the l a s t fundamental was q u i t e a r b i t r a r y and was based on the b e l i e f that the l i n e at 1270 cm 1 as a combination had the g r e a t e r opportunity t o . a c q u i r e i n t e n s i t y from the more numerous fundamentals nearby. Using l i n e s t r e n g t h as the only c r i t e r i o n , the f o u r C-H s t r e t c h i n g fundamentals were assigned as 3050, 3030, 2940 and 3094 cm i n decreasing order of p r o b a b i l i t y . The very strong l i n e at 3050 cm - 1 dominates t h i s r e g i o n of the spectrum and may represent a case of near degeneracy of more than one fundamental. B^ Symmetry The presence of B^ fundamentals were observed i n the a_ p o l a r i z e d i n f r a r e d spectrum at 148, 310, 438, 570, 720, 742, 924, and 1152 cm\"1. The p o s i t i o n s of the l a s t two B^ fundamentals could not be e s t a b l i s h e d from the i n f r a r e d spectrum although i t i s p o s s i b l e that one of the two could be near 880 cm\"1 where shoulders appear i n the i n f r a r e d and Raman s p e c t r a . A^ Symmetry Only one A\u00E2\u0080\u009E fundamental was found at 299 cm 1 from the Raman spectrum. G. Normal Coordinate C a l c u l a t i o n Wilson's GF matrix method was employed to c a l c u l a t e the carbazole - 73 fundamental fre q u e n c i e s . The c a l c u l a t i o n s were performed on an IBM model 7044 computer using a somewhat modified v e r s i o n of a normal-coordinate program 88 w r i t t e n by Schachtschneider. The G-matrix was constructed from the bond lengths and bond angles shown i n F i g . 7,. and the molecular, symmetry was used to advantage i n d e r i v -i n g the symmetry coordinates r e q u i r e d to f a c t o r the s e c u l a r equation. The redundant coordinates were handled d i r e c t l y by the program. In-plane c a l c u l a t i o n F i g . 16 In-plane i n t e r n a l coordinate d e f i n i t i o n s of carbazole The f o l l o w i n g valence-type p o t e n t i a l was used: 2V = z [ K D R 2 + 2F!J R.R + 2F* R.R. _ + 2F!J R.R. ,] j R J Ro- j j+1 Rm j j + 2 Rp j j+3J + f/i * 2 F r W l ] + f V'V' * 2 F * V j . l l \u00C2\u00A33.3) ~ 74 The f o r c e f i e l d was t r a n s f e r r e d from phenanthrene and t h e r e f o r e the symbolism used by C a l i f a n o was r e t a i n e d i n l a b e l l i n g the in-plane i n t e r n a l coordinates shown i n F i g . 16. I t should be noted t h a t , w i t h the C-C bond l a b e l l e d z by C a l i f a n o of the phenanthrene molecule removed, the s t r u c t u r e c o l l a p s e s i n t o t h a t of carbazole. The correspondence between the i n t e r n a l coordinate symbols i n the p o t e n t i a l energy expression and those i n F i g . 16 i s as f o l l o w s : Rj = th eCC or CN bond s t r e t c h i n g coordinates q,riS,1,t,u,v,w r.. = the CH and NH bond s t r e t c h i n g coordinates = the angle bending coordinates a, B,Y, <$, e , C, 6,K ,T ,X,tn ^ = the CCH bending coordinates T,V,cr,* o,m,p = ortho, meta, para, r e s p e c t i v e l y The l e t t e r s K,H, and F r e f e r r e s p e c t i v e l y to the s t r e t c h i n g , bending, and i n t e r a c t i o n f o r c e constants. Except f o r minor v a r i a t i o n s , the f o r c e constants l i s t e d i n Table 11 were t r a n s f e r r e d d i r e c t l y from phenanthrene. The CC s t r e t c h i n g f o r c e constants of phenanthrene were p l o t t e d against bond length and from t h i s graph values of were found from the known bond lengths of carbazole. The angle bending f o r c e constants were a l t e r e d r a t h e r a r b i t r a r i l y i n an e f f o r t to conform with the small v a r i a t i o n of f o r c e constant w i t h valence angle given i n reference 73. A l l other f o r c e constants were as defined p r e v i o u s l y . The c a l c u l a t e d frequencies are compared with the observed values i n Table 13. - 75 -Table 11 The in-plane f o r c e constants of carbazol e . Type Force Constant Value o K\u00E2\u0080\u009E u 5.07 mdyne/A K N H 6.50 K c c K r 7.00 K = K = K = K = K = K 6.3 1 q s t u w K 5.10 v o H\u00E2\u0080\u009E\u00E2\u0080\u009E\u00E2\u0080\u009E H = H = H = H. 0.94 mdyne A CCC a. & y 6 H = H 0.91 T K H = H.. 1.00 6 to H = H 0.70 UR..R H 1.03 CNC e H \u00E2\u0080\u009E r r H = H, \u00E2\u0080\u00A2 0.50 HCC a X H = H 0.55 U TT H , = H , 0.45 HHNC H X 1 1 \u00C2\u00B0 - 5 \u00C2\u00B0 CC r s 1 t \u00C2\u00B0 F ^ ortho F = F = F = F 0.80 mdyne/A CC q r s q 1 F W = F W 0.07 t q w t F = F 0.61 v v F J \u00C2\u00A3 ortho F U = F U 0.61 CN q w F ^ meta F q = F t = F 1 = F + q -0.32 CC s s r t F r = F 1 = F V -0.06 w w 1 w F w -0.12 w F v = F * = F q 0.15 q t w F?l meta F V = F r -0.12 CC u u F t 0.15 u CC 1 t V \u00C2\u00B1 para F = F , 0.30 X r q r ' S = F w u F S = F t -0.07 - 76 -Type Force Constant Value F 5 V 0, .03 _CN Fcc p a r a F S u F t u = F 1 u 0, -0, .03 .07 ccc CC F \" s = F Y s = F l - V - F Y = r F 6 = r F t = Ft 0. . 23 mdyne 't = F K q = F 8 q = F n q 0, .23 F T w = F K w = F 9 w = F W w p w V = F 5 V 0, .41 ccc CN F n u F \u00C2\u00A3 u = F 8 u 0, 0, .23 .41 \u00E2\u0080\u009EHCC CC F \u00C2\u00B0 q = F \u00C2\u00B0 r = F y r = F y s = F 11 s - V = F / = F t X 0 .18 \u00E2\u0080\u009EHNC CN F x u 0. .18 CCC CCC F a Y = F a Y = F B a = F 6 K 0 -0.049 mdyne/A F T K = F 9 = F \" = F ? =\u00E2\u0080\u00A2 F 5 10 = F W K 0 .057 CCC CCN -0 .49 DCH CH 0.068 mdyne/A Out-of-plane c a l c u l a t i o n The out-of-plane motions are described by Y (CH wagging) and

2 + 2q R 2E 0.012 0.011 q -0.0100 -0.005 0 o 1 -0.022 -0.020 t 0.014 0.010 m o > -0.017 -0.015 P The d i f f i c u l t i e s encountered i n the t r a n s f e r of the benzene f o r c e f i e l d 87 to anthracene by Evans and S c u l l y were a l s o noted and are recounted b r i e f l y here. S l i g h t changes i n the f o r c e constants a s s o c i a t e d w i t h y a r e expected s i n c e the hydrogens i n the more complex molecules have d i f f e r e n t environments. F u r t h e r , the f o r c e constants a s s o c i a t e d w i t h the independent displacements of the carbon atoms at t r i g o n a l carbons where an i n t e r n a l c oordinate d e f i n i t i o n s i m i l a r to y i s a p p l i c a b l e are unknown and are assumed to have the same value as y I n the benzene case, the i n t e r n a l coordinates 1 s depending on the combination of carbon and hydrogen atoms attached t o the C-C bond about which the t o r s i o n i s t a k i n g p l a c e . I t was assumed t h a t a l l the t o r s i o n s were equal, and were a c c o r d i n g l y assigned the same value of the f o r c e constant. F i n a l l y , w i t h respect to the i n t e r a c t i o n constants, the convention a p p l i e d to benzene was adhered t o , that i s , only i n t r a - r i n g terms were taken i n t o account. I t was recognized that two- r i n g s may have an i n t e r n a l coordinate i n common which w i l l r e s u l t i n an i n t e r - r i n g i n t e r a c t i o n . The c a l c u l a t e d out-of-plane frequencies are compared with the observed values i n Table 13. Table 13. A comparison of the observed and c a l c u l a t e d fundamentals of carbazole. Species Observed C a l c u l a t e d Species Observed C a l c u l a t e d 3421 3436 B2 3084 3091 3094? 3091 3077 3068 3050 3068 3055 3046 3030 3046 3039 3024 2940 3024 1625 1648 1594 1635 1576 1612 1490 1623 1481 1535 1452 1539 1449 1491 1380 1511 1334 1419 1320 1431 1288 1388 1233 1424 1205 1302 1204 1296 1136 1240 1158 1252 1107 1213 1118 1226 1012 1133 1022? b 1152 910 1041 995 1051 856 857 835 1001 747 759 737 ? b 847 658 629 616 620 425 409 548 559 220 233 505 472 - 79 -Species Observed C a l c u l a t e d Species Observed C a l c u l a t e d Calc. 1\u00C2\u00B0 C a l c . 2 d C a l c . l 0 C a l c . 2 d A 2 989 1088 B 1089 1237 939 1049 926 935 1046 893 973 880? 895 982 771 825 741 831 903 691 754 722 737 782 467 588 566 567 676 410 478 445 433 519 299 231 303 310 342 414 104? 106 147 222 290 139 108 134 The observed q u a n t i t i e s are means of the f a c t o r group components where these are r e l i a b l y known. See t e x t f o r a more complete d i s c u s s i o n . C a l c u l a t i o n 1 frequencies based on modified f o r c e constants. 86 C a l c u l a t i o n 2 frequencies based on Whiffen's f o r c e constants. H. Conclusion As expected, i t i s only i n the A^ symmetry b l o c k , where f a i r l y complete i n f o r m a t i o n was a v a i l a b l e from i n f r a r e d , Raman and fluorescence s p e c t r a , that a f i r m assignment of fundamentals was p o s s i b l e . A comparison of the observed and c a l c u l a t e d frequencies i s shown i n Table 13. For those symmetry bloc k s where even only one experimental assignment i s u n c e r t a i n or missing the comparison provides no t e s t of the v a l i d i t y of the f o r c e f i e l d s i n c e the ordering of the observed fundamentals i s important: f o r example, i f 1694 cm 1 replaces 737 cm 1 as a B fundamental then a l l the frequencies g r e a t e r than 737 cm 1 occupy a d i f f e r e n t p l a c e i n Table 13 and - 80 \" the h o r i z o n t a l comparison becomes q u i t e d i f f e r e n t . The agreement between observed and c a l c u l a t e d frequencies f o r the species i s s a t i s f a c t o r y . No attempt was made to modify the f o r c e f i e l d to achieve a b e t t e r f i t . I t i s obvious that a f i t over twenty one v i b r a t i o n s provides no t e s t f o r a f i e l d w i t h a g r e a t e r number of a d j u s t a b l e parameters. However, i t i s tempting to assume that the f o r c e f i e l d i s e s s e n t i a l l y c o r r e c t and to use the c a l c u l a t e d frequencies as a guide to complete the assignment, when the 737 cm 1 fundamental would be rep l a c e d by one at 1270 cm 1 (see e a r l i e r d i s c u s s i o n on t h i s p o i n t ) . A mere d e t a i l e d Raman study i s c l e a r l y r e q u i r e d . The c a l c u l a t i o n f u r t h e r p r e d i c t s the appearance of the tenth fundamental near 220 cm 1 where i t could very e a s i l y be obscured i n the i n f r a r e d and Raman by the strong A^ fundamental already present and makes more c r e d i b l e the suggestion that the (ab) and (be) Raman l i n e s at 104 cm 1 represent an A^ fundamental. I t i s r a t h e r unreasonable t o expect the approximate c a l c u l a t i o n s c a r r i e d out i n t h i s work t o give an accurate d e s c r i p t i o n of the normal modes of motion and t h i s i s e s p e c i a l l y t r u e i n the crowded energy r e g i o n above 1000 cm f o r the A^ s p e c i e s . The two strongest l i n e s i n the f l u o r e s c e n c e spectrum i n v o l v e the A^ fundamentals at 220 and 658 cm 1. Since these modes occupy a r e g i o n where the d e n s i t y of v i b r a t i o n a l s t a t e s i s low the c a l c u l a t e d normal modes (shown i n F i g . 17) should g i v e at l e a s t a q u a l i t a t i v e idea of the chang i n the geometry i n going to the e x c i t e d e l e c t r o n i c s t a t e . In the lowest energy e l e c t r o n i c absorption system of phenanthrene^\"'''^ the t o t a l l y symmetric fundamental at 674 cm 1 i n the upper e l e c t r o n i c s t a t e acts as a strong f a l s e o r i g i n . There i s no evidence that the 658 cm 1 fundamental of 27 carbazole forms a f a l s e o r i g i n e i t h e r i n fluorescence or absorption. - 81 -V \u00E2\u0080\u00A2= 233 cm.\" -I F i g . 17 C a l c u l a t e d normal coordinates f o r two A, fundamentals of carbazole Z/= 6 2 9 cm.\" -I F i n a l l y , i t may be of i n t e r e s t to note that s e v e r a l of the v i b r a t i o n a l l e v e l s of carbazole show unusually large f a c t o r group s p l i t t i n g s . Those w i t h a s p l i t t i n g g r e a t e r than 10 cm 1 are ( f a c t o r group symmetry symbols i n parentheses): v i b r a t i o n s 431 ( A ) , 418 ( B ^ ) ; 1455 ( A ) , 1442 ( B l u ) , 1444 (B\u00E2\u0080\u009E ); and B 1 v i b r a t i o n s 148 (B, ) , 127 (B\u00E2\u0080\u009E ); 451 (B, ), 438 (B, ). v iu ' 1 v luJ ' m v l u v 3u Thus any given v i b r a t i o n need not. appear at the same frequency i n the Raman as i n the i n f r a r e d spectrum. F u r t h e r , t h i s a l s o provides a u s e f u l c r i t e r i o n 27 f o r the assignment of fundamentals s i n c e from Craig's e a r l i e r arguments t r a n s i t i o n s that are normally forbidden but appear by i n t e n s i t y s t e a l i n g have a s p l i t t i n g determined by the c o e f f i c i e n t of mixing. Chapter 4 The Absorption Spectrum of Carbazole i n a Fluorene M a t r i x A. I n t r o d u c t i o n The spectrum of carbazole i n s o l u t i o n shows three general regions o f o a b s o r p t i o n i n the quartz u l t r a v i o l e t : a moderately weak system near 3300 A o (f = 0.042) running i n t o a system of medium strength near 2900 A (f = 0.15) and followed by a stronger and more complex system which probably contains two o o separate e l e c t r o n i c t r a n s i t i o n s at 2550 A and 2300 A. A p h o t o s e l e c t i o n study of the p o l a r i z a t i o n of the absorption bands of carbazole i n ethanol (93\u00C2\u00B0K) and cyclohexane (293\u00C2\u00B0K) s o l u t i o n s assigned these systems as s h o r t , long, long, and short a x i s t r a n s i t i o n s r e s p e c t i v e l y . A l a t e r magnetophoto-s e l e c t i o n study o f carbazole i n a d i l u t e s o l u t i o n of d i e t h y l ether at 77\u00C2\u00B0K o o supported the assignment of the 3000 A and 2900 A as being predominantly 85 91 92 short and long a x i s t r a n s i t i o n s r e s p e c t i v e l y . T h e o r e t i c a l s t u d i e s '\" have r e c e n t l y been c a r r i e d out and a l l three p r e d i c t that the lowest-energy t r a n s i t i o n would be weak and p o l a r i z e d along*'-.the short-molecular a x i s (A^-*\u00E2\u0080\u0094 A^) while the second should be stronger and long-axis p o l a r i z e d ( E ^ \" * ' \u00E2\u0080\u0094 ^ ) ; as i s u s u a l , the c a l c u l a t e d i n t e n s i t i e s were somewhat over-estimated. o The present work i s concerned with the weak 3300 A absorption system of carbazole i n a fl u o r e n e matrix. S i n g l e c r y s t a l s of fl u o r e n e provide a very u s e f u l m a t r i x f o r studying the p o l a r i z e d fluorescence and absorption - 83 _ s p e c t r a of molecules which have absorption systems at wavelengths greater o than about 3000 A and which f u l f i l the c o n d i t i o n s f o r the formation of a 30 s u b s t i t u t i o n a l s o l i d s o l u t i o n w i t h f l u o r e n e . Molecules that have been 68 83 st u d i e d i n such a mixed c r y s t a l system with f l u o r e n e i n c l u d e anthracene ' 41 76 93 phenanthrene ' , and pyrene ' . The e.s.r. spectrum of the lowest t r i p l e t 94 95 s t a t e of pyrene ' added as an impurity to f l u o r e n e has a l s o been re p o r t e d . The value of f l u o r e n e as a matrix r e s t s on the f a c t that the c r y s t a l has high symmetry. Each molecule occupies a s p e c i a l s i t e (C^,) i n 57 an orthorhombic u n i t c e l l such that the long a x i s of the molecule c o i n -cides w i t h the \u00C2\u00A3 c r y s t a l a x i s . Thus the t r a n s i t i o n s a c t i v e along the long and short molecular axes are completely r e s o l v e d by the examination of any c r y s t a l s e c t i o n c o n t a i n i n g the \u00C2\u00A3 a x i s (see F i g . 6) i n p o l a r i z e d l i g h t . The a b s o r p t i o n spectrum of carbazole i n biphenyl (see appendix I) was a l s o s t u d i e d , but i s not discussed here since i t d i d not y i e l d any a d d i t i o n a i n f o r m a t i o n not already a v a i l a b l e i n the study of the absorption spectrum of carbazole i n f l u o r e n e . B. The Spectrum The p o l a r i z e d a b s o r p t i o n spectrum of carbazole i n a f l u o r e n e m a t r i x i s shown i n F i g . 18 and the a n a l y s i s of the spectrum i s set out i n Table 14. The e l e c t r o n i c o r i g i n was l o c a t e d as the only resonance l i n e i n the spectrum The strength of the o r i g i n as shown i n F i g . 18 i s reduced c o n s i d e r a b l y by the occurrence of simultaneous emission; indeed, by e x t r a p o l a t i n g back the short Franck-Condon progressions i n the t o t a l l y symmetric frequencies 218, _1 1236 and perhaps 648 cm i t i s obvious that the o r i g i n represents by f a r the most probable t r a n s i t i o n at these energies. - 85 Table 14 The absorption spectrum of carbazole i n f l u o r e n e at about. 15\u00C2\u00B0K. //b //c Int Remarks 29 691 s o r i g i n 32 s 32', l a t t i c e 218 s 218, A l 418 m 418, A l 433 m 2 x 218 - 3 497 sh see t e x t 512 ms 512, B2 574 m 574, B2 648 vs 648, A l 680 w 648 + 32 717 ms 717, A l 732 w 218 + 512 + 2 789 w 218 + 574 - 3 824 w 824, B 2 * 864 ms 218 + 648 - 2 921 s 921, B2 932 mw 218 + 717 - 3 983 s 983, A l 989 ms 989, B 2; 418 + 574 - 3? 1014 ms 983 + 3 2 - 1 1033 ms 1033, B2 1118 s 1118, B2 1136 vw 418 + 717 + 1 1159 m 512 + 648 - 1 1201 ms 218 + 983 1204 vw 218 + 989 - 3 1236 vs 1236, A l 1296 i s 1296, A j ? ; 2 x 648? '1318 s 1318, B2 1336 m 218 + 1118 1451 m 218 + 1236 - 3 1469 m 1469, B2 - 86 //b_ //c_ Int Remarks 1484 m 1484, B 2 1511 m 218 + 2 x 648 - 3 1548 s 1548, B 2 1551 m 1551, 1566 ms 648 + 9 2 1 - 3 1591 ms 1591, k 1629 m 648 + 9 8 3 - 2 1636 m 648 + 9 8 9 - 1 1653 mw 418 + 1236 - 1 1682 w 648 + 1033 + 1 1699 w 717 + 9 8 3 - 1 1767 ms 648 + 1118 + 1 1807 w 218 + 1591 - 2 1808 w 574 + 1236 - 2 1848 mw 218 + 648 + 9 8 3 - 1 1882 m 648 + 1236 - 2 1915 w 218 + 717 + 983 - 3 1948 mw 717 + 1236 - 5; 3 x 648 + 4 1981 w 2 x 218 1551 - 6 1984 ms 218 + 648 + 1118 2012 w 2 x 648 + 717 - 1 2093 w 218 + 648 + 1236 - 9 2152 w 921 + 1236 - 5 2159 vw 218 + 3 x 648 - 3 2194 w 648 + 1548 - 2 2217 mw 983 + 1236 - 2 2220 mw 989 + 1236 - 5 2239 w 648 + 1591 - 3 2276 w 2 x 648 + 983 - 3 2307 ' w 418 + 648 + 1 2 3 6 + 5 2318 w 2 x 648 + 1033 - 9 2347 w 1118 + 1236 - 7 2413 ' w 218 + 648 + 1548 - 1 - 87 -//b //c Int ' Remarks 2434 w 218 + 983 + 1236 - 3 2465 w 2 x 1236 - 7 2495 vw 218 + 2 x 648 + 983 2530 mw 2 x 648 + 1236 - 2 2785 vw 1236 + 1551 - 3 2823 vw 1236 + 1591 - 4 2863 vw 648 + 983 + 1236 - 4 2883 vw 2 x 648 + 1591 - 4 \u00E2\u0080\u00A2The p o s i t i o n of the o r i g i n i s given i n cm and a l l other e n t r i e s show d i f f e r e n c e s from the o r i g i n . - 88 -That there i s a p r o g r e s s i o n up to the second overtone i n the 648 cm 1 fundamental i s u n c e r t a i n . The i n t e n s i t y of the 1296 cm i n t e r v a l seems too high to be the second member i n a progression e s p e c i a l l y when the t h i r d member i s f a i r l y weak. Both assignments of the 1296 cm 1 i n t e r v a l - as a fundamental and as the overtone of the 648 cm 1 fundamental - are included i n Table 14; i n the absence of f u r t h e r experimental evidence the f i n a l choice i s l e f t open. The l i n e 1014 cm 1 to the blue of the o r i g i n was unusually broad. The impression gained from a c l o s e study of enlargements taken from the o r i g i n a l p l a t e s was t h a t the s i n g l e broad l i n e had n e a r l y s p l i t i n t o two and would have done so i f the sample co u l d have been f u r t h e r cooled. Since there i s no reasonable assignment as a combination f o r a second l i n e , t h i s i n d i c a t e s the p o s s i b l e presence of an fundamental at 1014 cm 1 i n the e x c i t e d e l e c t r o n i c s t a t e . The 1469 cm 1 i n t e r v a l was not assigned as the combination 2 x 218 + 1003 s i n c e 217 + 1003 d i d not appear at a l l . The assignment as a combination i s o n l y reasonable i f some s o r t of i n t e r a c t i o n w i t h the 1484 cm 1 funda-mental i s assumed. C. D i s c u s s i o n The r e g i o n to the immediate blue of the o r i g i n was q u i t e d i f f u s e and only the 32 cm 1 i n t e r v a l entered i n Table 14 could be p i c k e d out f o r measure-ment. The greater p o l a r i t y of carbazole leads to a l a r g e r b i n d i n g energy as i m p l i e d by the very d i f f e r e n t melting p o i n t s of the two s o l i d s , v i z . the melt i n g p o i n t of carbazole i s 247\u00C2\u00B0C and of fluorene i s 116\u00C2\u00B0C. Thus there i s probably an a p p r e c i a b l e c o n t r a c t i o n i n the u n i t c e l l dimensions around - 89 -the s i t e occupied by the carbazole molecule and the t r a n s l a t i o n a l symmetry i n the f l u o r e n e c r y s t a l i s l o s t at the impurity so that k_ i s no longer a good quantum number. The d i f f u s e n e s s then i s created by the appearance of a large number of weak l o c a l i z e d l a t t i c e modes that r e t a i n c l a s s i f i c a t i o n only i n the s i t e group and t h a t become a c t i v e because of a small s h i f t i n the e q u i l i b r i u m o r i e n t a t i o n i n the l a t t i c e on e x c i t a t i o n . The very high p o l a r i z a t i o n r a t i o l i m i t e d only by experimental misalignment of c r y s t a l and Wollaston axes i n d i c a t e s that the s i t e symmetry i s preserved. The assumption i s made i n the i n t e r p r e t a t i o n of the spectrum that i t i s the long a x i s of the carbazole molecule that i s d i r e c t e d along c_ and not the short or normal a x i s . The pure e l e c t r o n i c o r i g i n was p o l a r i z e d along the b_ a x i s of the m a t r i x so-that the t r a n s i t i o n may be assigned * A ^ o \u00E2\u0080\u0094 i n agreement with the 85 91 92 c a l c u l a t i o n s . ' ' The second e x c i t e d s t a t e was not observed. A c o r r e l a t i o n between fundamentals of the ground and f i r s t e x c i t e d e l e c t r o n i c s t a t e s i s given i n Table 15. The 497 cm 1 i n t e r v a l occurred i n c_ p o l a r i z a t i o n as a shoulder (see Table 18). I t may mark the presence of a combination of low frequency B^ and A^ fundamentals but t h i s p o s s i b i l i t y i s considered u n l i k e l y s i n c e combinations of the corresponding ground s t a t e fundamentals f a l l near 400 and 600 cm 1 ; admittedly knowledge of the A^ fundamentals i s very incomplete. The a l t e r n a t i v e i s to a s s i g n a B^ fundamental at, 497 cm 1 n e a r l y degenerate with the one at 512 cm 1 and the corresponding entry i s made i n Table 15. There i s no long progession i n any fundamental i n the absorption spectrum so that any change of shape i n going between the two e l e c t r o n i c s t a t e s must be s m a l l . However, i f the normal co-ordinates are the same or n e a r l y the - 90 -Table 15 C o r r e l a t i o n between fundamentals-of the ground and f i r s t e x c i t e d c l s t a t e of carbazole. Species E x c i t e d State Ground State Species E x c i t e d State Ground State A l 218, s 219, vs B2 418, m 427, m 497?, sh 505 b 648, vs 654, vs 512, ms 549, ms 717, ms 743, mw 574, m 616, m 856 b 824, w 835 b 888, ms 921, s 996, w 983, s 1009, s 989?, ms 1022? b 1107 b 1033, ms 1118, m 1014?, ms 1151, mw 1158 b 1024, mw 1118, s 1210, s 1236, vs 1287, vs 1237, m 1296, s 1336, vs 1320 b 1453, mw 1318, s 1391, s 1482, mw 1469, m 1461, m 1551, m 1577, vw 1484, m 1487, m 1591, ms 1625, s 1548, s 1607, s When a v a i l a b l e , ground s t a t e data were taken from the fluorescence spectrum of carbazole i n a f l u o r e n e m a t r i x . T h i s . l i n e d i d not appear i n the f l u o r e s c e n c e spectrum and the frequency entered was measured from the i n f r a - r e d and Raman sp e c t r a . - 91 -same i n the two e l e c t r o n i c s t a t e s , then the c o r r e l a t e d frequencies should p l a y s i m i l a r r o l e s and have r e l a t i v e l y the same i n t e n s i t i e s i n the absorp-t i o n as i n the fluorescence spectrum. Table 15 shows that t h i s i s not the case. Some fundamentals appearing with appreciable i n t e n s i t y i n fluorescence (e.g. A fundaments at 888, 1204, 1453 and 1482 cm - 1 and the B 0 fundamental at 1237 cm *) have no counterpart i n the absorption spectrum while the fundamentals at 921, 824 and p o s s i b l y 989 and 497 cm 1 are much more prominent i n a b s o r p t i o n than the a s s o c i a t e d ground s t a t e fundamentals i n fluor e s c e n c e . Thus i t may be concluded t h a t w h i l e the molecular geometry has undergone l i t t l e change f o l l o w i n g the e l e c t r o n i c t r a n s i t i o n , the f o r c e - f i e l d has. The nature of the change (and the corresponding change i n bond order) could perhaps be determined i f s u f f i c i e n t fundamentals could be found to al l o w a normal co-ordinate a n a l y s i s to be c a r r i e d out f o r the e x c i t e d e l e c t r o n i c s t a t e . Chapter 5 U l t r a - v i o l e t Spectra of Benz[f]indan Impurity i n a Fluorene C r y s t a l A. I n t r o d u c t i o n U n f o r t u n a t e l y , commercially a v a i l a b l e f l u o r e n e contains a large number of i m p u r i t i e s . For i n s t a n c e , the intense blue fluorescence i s caused by 96 97 anthracene present as an i m p u r i t y ' and t r a c e amounts of dibenzfuran have 37 97 been detected. '' There i s some doubt whether carbazole i s a l s o present or not (see, f o r example references 96>97) although t h i s may vary according t o the source Of supply. The f l u o r e s c e n c e and a b s o r p t i o n s p e c t r a of f l u o r e n e i n very d i l u t e 96 97 96 s o l u t i o n s are now r e l i a b l y known. ' Nurmukhametov and Gobov a l s o reported the corresponding s p e c t r a of the p u r i f i e d s o l i d f i n d i n g them o o c o n s i d e r a b l y r e d - s h i f t e d from 3015 A i n an n-heptane s o l u t i o n to 3209 A i n the s o l i d . Since the f l u o r e s c e n c e l i n e s c o i n c i d e d w i t h those recorded 98 99 by Barbaron and P e s t e i l , ' the Russian workers concluded that the spectrum was indeed that of f l u o r e n e even though there was an appreciable change i n the v i b r a t i o n a l i n t e r v a l s and i n t e n s i t y p a t t e r n . In a l a t e r study of the f l u o r e s c e n c e of f l u o r e n e c r y s t a l s at 4\u00C2\u00B0K, B e n a r r o c h e 1 ^ found the fluorescence emission to be comprised of two d i s t i n c t p a r t s ; a weak spectrum I with i t s o o o r i g i n near 3051 A and a strong spectrum I I having i t s o r i g i n at 3209 A. Spectrum I I was again i n the one measured by Barbaron and P e s t e i l , and Benarroche a t t r i b u t e d both fluo r e s c e n c e s p e c t r a I and I I to f l u o r e n e . - 93 -Parker, Hatchard and J o y c e 1 ^ 1 have reported that the delayed f l u o r e s -cence i s q u i t e d i f f e r e n t from the prompt fluorescence of a s o l u t i o n of c a r e f u l l y p u r i f i e d f l u o r e n e i n ethanol. Although t h e i r spectrum was measured at low r e s o l u t i o n , i t i s probable that the delayed fluorescence comes from benz [ f ] i n d a n i m p u r i t y . The purpose of t h i s chapter i s to show that the fluorescence spectrum I I i n f a c t a r i s e s from b e n z [ f ] i n d a n which i s known to be another p e r s i s t e n t impurity i n commercial f l u o r e n e (see reference 37 and chapter 2) and to prepare the way f o r the f o l l o w i n g chapter i n which some e x c i t e d e l e c t r o n i c s t a t e s of fl u o r e n e are disc u s s e d . Removal of the be n z [ f ] i n d a n from commercial f l u o r e n e by c a r e f u l chromatography provides a c r y s t a l having o no d e t e c t a b l e absorption above 3052 A even when the sample i s cooled t o about 10\u00C2\u00B0K, but i t does show a sharp b_-polarized absorption l i n e at 32 789 cm i n an ab face which i s not observed i n a s y n t h e t i c a l l y prepared sample. This i n d i c a t e s the presence of yet another unsuspected and unknown impurity i n commercial f l u o r e n e which probably gives r i s e to the weak fluorescence . , . 100 spectrum I reported by Benarroche. B. The Mixed C r y s t a l Systems As mentioned e a r l i e r , the fo u r f l u o r e n e molecules occupy s p e c i a l s i t e s i n an orthorhombic u n i t c e l l having space group P (D\u00E2\u0080\u009E 1 ( S . The molecular \u00E2\u0080\u00A2 a r r nam 2h J plane of symmetry c o n t a i n i n g the short in-plane a x i s and molecular normal c o i n c i d e s w i t h the ab c r y s t a l m i r r o r plane so t h a t the long molecular a x i s i s e x a c t l y p a r a l l e l to _c. The s t r u c t u r e of the benz[ f ] i n d a n molecule i s shown i n F i g . 1. I t has the e l e c t r o n i c p r o p e r t i e s of a s u b s t i t u t e d naphthalene although looked at - 94 -i n t h i s way i t i s named r a t h e r awkwardly 2,3~dihydro-lH-cyclopenta[b]naphthalene. The molecule i s assumed to be p l a n a r w i t h symmetry, when the 69 normal v i b r a t i o n s may be c l a s s i f i e d as i n d i c a t e d i n Table 16. In an a l y z i n g the s p e c t r a we make the f u r t h e r assumption that the guest molecule s u b s t i t u t e s f o r a host molecule i n each o f the two matrices used. I t i s to be expected that b e n z [ f ] i n d a n would occupy a s u b s t i t u t i o n a l s i t e i n flu o r e n e s i n c e i t i s a p e r s i s t e n t i m p u r i t y that i s q u i t e d i f f i c u l t to remove having a shape and other p h y s i c a l p r o p e r t i e s very s i m i l a r t o those of f l u o r e n e and of anthra-cene another common i m p u r i t y . I t i s c l e a r that one of the molecular axes of b e n z [ f ] i n d a n i s a l i g n e d w i t h the c_ a x i s of the f l u o r e n e c r y s t a l s i n c e the l i n e s of the abso r p t i o n spectrum are completely p o l a r i z e d ; then the above assumption i s equivalent to the s u p p o s i t i o n that i t i s the long a x i s of the guest t h a t i s d i r e c t e d along c_ and not the short or normal a x i s . However, the spectrum of ben z [ f ] i n d a n i n a mixed c r y s t a l w i t h b i p h e n y l was a l s o measured as a check. The b i p h e n y l c r y s t a l 5 4 ' ^ belongs to the monoclinic space group p 2 ^ / a ( C 2 n ^ ) with the two molecules i n the u n i t c e l l occupying s i t e s of symmetry . The oriented-gas r a t i o of a lon g - a x i s p o l a r i z e d t r a n s i t i o n i n the be' face of bi p h e n y l i s 1^:1 \u00C2\u00BB= 1:2x10^ and of a s h o r t - a x i s p o l a r i z e d t r a n s i t i o n i s I K : I \u00E2\u0080\u00A2 i= 23:1. The a x i s _c' i s the perpendicular to the ab plane. - 95 -Table 16. D i s t r i b u t i o n of the fundamental v i b r a t i o n s of molecular b e n z [ f ] i n d a n . 2v E C 2 ( z ) \u00C2\u00B0v(yz) N n (!) A i 1 1 l 1 23 18 A 2 1 1 - l -1 12 11 ( R y . *) B l 1 -1 - l 1 13 11 > y) B 2 1 -1 l -1 21 17 N i s the number of fundamentals i n each symmetry species w h i l e n i s the number w i t h energy l e s s than 2000 cm~l C. The Absorption Spectrum 37 The ab s o r p t i o n spectrum of be n z [ f ] i n d a n i n s o l u t i o n shows three o general regions of abso r p t i o n above 2000 A . At low energy there i s a weak o system (f=0.0108) beginning at 3200 A which runs i n t o another system of o medium s t r e n g t h (f=0.115) at about 2800 A. A t h i r d intense system (f=1.13) o occurs at 2300 A. These general f e a t u r e s are q u i t e analogous to those of the spectrum of naphthalene. This chapter i s concerned with the weak o system at 3200 A, the only one of the three to be seen before c u t - o f f by the matrices occurs. The p o l a r i z e d absorption spectrum of benz[ f ] i n d a n i n a f l u o r e n e matrix i s shown i n F i g . 19 and the a n a l y s i s of the spectrum i s set out i n Table 17. Table 17. The p o l a r i z e d absorption spectrum of be n z [ f ] i n d a n i n f l u o r e n e . //b //c I n t . Remarks -79 w l a t t i c e -48 mw l a t t i c e 31 158 vs o r i g i n 134 mw l a t t i c e ; : s e e t e x t 190 m 190, A :; -see t e x t 250 m 250, B 2 363 ms 363, B 2 402 s 402, A x 510 ms 510, B 2 536 m 536, B 2 590 w 590, A x?; 190 + 4 0 2 - 2 704 vs 704, A 1 725 w see t e x t 759 vw 363+ 402 - 6 770 w 770, A x 804 m 2 x 402 838 m 838, A 848 m 848, B 2; : 250 + 590 + 8? 874 ms 874, B 2 905 ' vw 402 + 510 - 7 950 mw 250 + 704 - 4 973 vs 973, Aj 989 mw 989, A 2 1048 m ^1048, A 2 1060 m 363 + 704 - 7 1087 w 363 + 725 - 1 1093 w 250 + 838 + 5 1101 ms 402 + 704 - 5 1114 mw 1114, B 2 1161 mw 2 x 402 + 363 - 6 1165 vw 190 + 973 + 2 - 98 _ //b //\u00C2\u00A3 I n t . Remarks 1207 m 510 + 704 - 7 1223 w 250 + 973 1234 mw 536 +.704 - 6 1259 m 1259, B 2 1294 mw 1294, B 2 1323 m 2 x 402 + 510 + 9 1334 mw 363 + 973 - 2 1381 vs 1381, A ; 402 + 9 7 3 + 6 1408 s 2 x 704 1449 s 1449, A 1481 m 510 + 973 - 2 1509 w 536 + 973 1523 mw 1523, B 2 1545 mw 704 + 848 - 7 1572 m 704 + 874 - 6 a The energy of the e l e c t r o n i c o r i g i n i s i n cm 1 and a l l other l i n e p o s i t i i are given as d i f f e r e n c e s from the o r i g i n . A bsorption around the e l e c t r o n i c or i g i n , although complex, was c l e a r l y p o l a r i zed along the \u00C2\u00A3 a x i s of the matrix The d i f f u s e n e s s : i s probably caused by the l a r g e number of l o c a l i z e d l a t t i c e modes th a t may appear due to the l o s s of t r a n s l a t i o n a l symmetry i n the f l u o r e n e c r y s t a l at the s i t e occupied by the b e n z [ f ] i n d a n molecule. Apart from the o r i g : i n i t s e l f , l i n e s stand out from t h i s r e g i o n of d i f f u s e n e s s at 48 and 79 cm\"1 to the red and 134 cm 1 to the blue of the o r i g i n ; these are assumed to represent l a t t i c e modes having an e s p e c i a l l y high Franck-Condon allowedness. Although the temperature of t h i s p a r t i c u l a r sample was estimated to be about 25\u00C2\u00B0X, the l i n e s remained sharp having an average width of about 5 cm 1 . The l i n e 190 cm \" to the blue of the o r i g i n appeared only i n the - 99 -fluorene matrix (and not i n biphenyl or n_-heptane matrices) . This i s taken to mark the presence of an molecular fundamental that i s induced to appear through i n t e r a c t i o n s w i t h phonons of the matrix. The 134 cm 1 i n t e r v a l could be given a s i m i l a r i n t e r p r e t a t i o n although, i n t h i s case, a sharp d i s t i n c t i o n between i n t r a - and i n t e r m o l e c u l a r modes i s probably not s i g n i -f i c a n t . The assignment of the 590 cm 1 i n t e r v a l as an A^ fundamental was made from i t s appearance i n the spectrum i n a biphe n y l m a t r i x . In t h i s case, the assignment as a combination was not tenable s i n c e the 190 cm 1 i n t e r v a l was not observed. Apart from t h i s f e a t u r e and from the f a c t that b e n z [ f ] i n d a n may apparently occupy more than one s i t e i n biphenyl and n-heptane m a t r i c e s , the s p e c t r a are very s i m i l a r . For t h i s reason the s p e c t r a (absorption and fluorescence) i n f l u o r e n e only are given i n t h i s t h e s i s . The 725 cm 1 i n t e r v a l i s thought to represent an example of Fermi resonance. The evidence f a v o u r i n g t h i s p o i n t of view i s that ( i ) i t i s a weak l i n e adjacent to the p a r t i c u l a r l y strong one at 704 cm 1 and ( i i ) no corresponding l i n e was observed i n the fluore s c e n c e spectrum. D. The Fluorescence Spectrum The p o l a r i z e d fluorescence spectrum of b e n z [ f ] i n d a n i n fluorene i s shown i n F i g . 20 with the a n a l y s i s i n Table 18. I t i s seen t h a t the fluorescence spectrum, l i k e the absorption spectrum, i s very n e a r l y completely p o l a r i z e d ; although expected, t h i s behaviour i s not o f t e n observed f o r aromatic molecules i n s o l i d s o l u t i o n s . The one exception i s the l i n e 1280 cm 1 to the red of the o r i g i n and, i n Table 18, t h i s has been i n t e r p r e t e d as a r i s i n g from an a c c i d e n t a l degeneracy. - 100 -Table 18. The p o l a r i z e d fluorescence spectrum of be n z [ f ] i n d a n i n f l u o r e n e . //b //\u00C2\u00A3 I n t . Remarks 31 158 vs o r i g i n 200 mw 200, A^; see t e x t 25S w 258, B 2 399 m 399, B 2 409 mw 409, A : 560 m 560, B 2 750 s 750, A 1 803 ms 803, A 1 897 mw 897, B 2 948 vw 200 + 750 - 2 990 vw 990, B 2 1018 mw 1018, A 1062 vw 1062, B 2 1122 vw 1122, B 2 1146 - mw 399 + 7 5 0 - 3 1158 mw 1158, A 2; 409 + 750 - 1? 1195 w 399 + 8 0 3 - 7 1280 w 1280, B 2; 258 + 1018 + 4? 1280 vw 1280, A x 1305 mw 560 + 7 5 0 - 5 1358 w 560 + 8 0 3 - 5 1373 m 1373, A^; see t e x t 1398 vs 1398, A1 1454 m 1454, A : 1497 mw 2 x 750 - 3 1544 \u00E2\u0080\u00A2 mw 750 + 8 0 3 - 9 1575 . w 200 + 1373 + 2 1598 mw 200 + 1398; 2 x 803 - 8 1765 w 750 + 1018 - 3 1774 vw 399 + 1373 + 2 1795 w 399 + 1398 - 2 - 101 -//b //c I n t . Remarks 1853 2296 1815 vw 803 + 1018 - 6 w 399 + 1454 1905 w 750 + 1158 - 3 1956 vw 803 + 1158 - 5 2143 m 750 + 1398 - 5 2198 m 803 + 1398 - 3; 2252 w 3 x 750 + 2; 803 w 897 + 1398 + 1 2413 vw 1018 + 1398 - 3 2554 vw 1158 + 1398 - 2 2792 mw 2 x 1398 - 4 2851 w 1398 + 1454 - 1 a -1 The energy o f the e l e c t r o n i c o r i g i n i s i n cm and a l l other l i n e p o s i -t i o n s are given cm~l as d i f f e r e n c e s from the o r i g i n . The l i n e 200 cm 1 to the red of the o r i g i n appeared only i n f l u o r e n e and i s assigned as an fundamental induced by the matrix i n an analogous manner to the 190 cm 1 l i n e i n the abso r p t i o n spectrum. From an i n s p e c t i o n of the fluorescence spectrum alone, the 1373 cm 1 i n t e r v a l may have been assigned as an A^ combination of (unknown) non-t o t a l l y symmetric fundamentals a c q u i r i n g i n t e n s i t y through a Fermi r e s -onance with the Franck-Condon allowed fundamental at 1398 cm 1 . I f t h i s were so, then the same i n t e n s i t y r a t i o f o r these two l i n e s should be 102 observed i n the Raman spectrum. Both l i n e s have been recorded i n the Raman spectrum (the data have been entered i n Table 19 where i t i s found th a t the l i n e at 1375 cm 1 i s stronger than the one at 1394 cm \ the reverse of the s i t u a t i o n i n f l u o r e s c e n c e . Therefore i t may be concluded that both l i n e s mark the presence of A^ fundamentals. 102 -, , ! ! ! 2 9 0 0 0 2 9 5 0 0 3 0 0 0 0 3 0 5 0 0 3 I O O O C m \" 1 F i g . 20 A microdensitometer t r a c i n g of the p o l a r i z e d fluorescence spectrum of be n z [ f ] i n d a n i n fl u o r e n e at about 15\u00C2\u00B0K. The l i n e s marked I designate fluorescence from the im p u r i t y with a doubled o r i g i n at 29 519 cm~l and 29 505 cm-1 (see t e x t ) . The o r i g i n was traced from a d i f f e r e n t exposure on the p l a t e . E. D i s c u s s i o n Since the absorption and 'fluorescence s p e c t r a were completely p o l a r i z e d , i t may be concluded that an a x i s of the benz[f]indan molecule l i e s along the c_ a x i s of the fl u o r e n e c r y s t a l . I f the assumption that b e n z [ f ] i n d a n occupies a s u b s t i t u t i o n a l s i t e i n the l a t t i c e i s c o r r e c t then the pure 1 1 e l e c t r o n i c t r a n s i t i o n i s long-axis p o l a r i z e d A ~ i \u00E2\u0080\u0094 A^. This i s a l s o the expected assignment by analogy with naphthalene where the weak, low-energy _ 103 _ t r a n s i t i o n i s k n o w n ^ ' t o be \u00E2\u0080\u00A2<\u00E2\u0080\u0094''\"A . However, the spectrum of the 2u g \u00E2\u0080\u00A2 1 be face i n a biphenyl matrix was measured and the e l e c t r o n i c o r i g i n was almost completely c_ p o l a r i z e d again c o n s i s t e n t w i t h the long-axis assignment. The f i r s t a b s o r p t i o n system was measured only about 1600 cm 1 beyond the o r i g i n when complete abs o r p t i o n by the matrix occurred. The second e l e c t r o n i c t r a n s i t i o n was not observed. A c o r r e l a t i o n between fundamentals of the ground and f i r s t e x c i t e d s t a t e i s presented i n Table 19. Data taken from the sp e c t r a i n biphenyl and n-heptane matrices are a l s o i n c l u d e d f o r comparison although i t should be noted that the n-heptane sample was p o l y c r y s t a l l i n e so that i n f o r m a t i o n concerning l i n e p o l a r i z a t i o n was not a v a i l a b l e . The c o r r e l a t i o n between the fundamentals of the ground and e x c i t e d e l e c t r o n i c s t a t e s was based on the r e l a t i v e strengths of l i n e s i n the fluore s c e n c e and absorption s p e c t r a , i t being assumed that the f o r c e f i e l d s of the molecule i n the two e l e c t r o n i c s t a t e s are very s i m i l a r ; the absence of progressions i n the spec t r a show' tha t there can be only very small changes i n bond lengths (and strengths) i n the t r a n s i t i o n . V i b r a t i o n a l modes were g e n e r a l l y reduced i n energy f o l l o w i n g the e l e c t r o n i c e x c i t a t i o n presumably because the e l e c t r o n was t r a n s f e r r e d from a bonding to an anti-bonding o r b i t a l which l e d , i n t u r n , to a small decrease i n the average f o r c e constant. 102 Raman, data have a l s o been included i n Table 19. D e p o l a r i z a t i o n r a t i o s were not a v a i l a b l e and the assignments were made by f i n d i n g the c l o s e s t energy f i t to the flu o r e s c e n c e data. However, those Raman-active modes assigned A^.symmetry by comparison with the fluorescence a n a l y s i s were, i n g e n e r a l , the s t r o n g e s t . When a fundamental was assigned from the absorption and not the fluore s c e n c e s p e c t r a , a Raman-active v i b r a t i o n w i t h a - 104 -somewhat gre a t e r energy than that i n the e x c i t e d e l e c t r o n i c s t a t e was i n s e r t e d i n Table 19; the few sp e c u l a t i o n s of t h i s s o r t attempted gain some support from the f a c t that those fundamentals assumed to be were 102 strong i n the Raman spectrum while those taken to be were weaker. The Raman i n t e r v a l s 1577 and 1433 cm 1 may als o mark the presence of A^ funda-mentals because of t h e i r r e l a t i v e l y great strength of 6 and 5 a r b i t a r y . , . . . , 102 m c e n s i t y u n i t s , r e s p e c t i v e l y . Table 19. C o r r e l a t i o n between fundamentals of the ground and f i r s t e x c i t e d e l e c t r o n i c s t a t e s of benz[ f ] i n d a n Symmetry A^ Ex c i t e d State Ground State Absorption i n Fluorescence i n . : \u00E2\u0080\u0094 Raman Fluorene Biphenyl n-heptane Fluorene Biphenyl n-heptane 190 m 200 mw 194 (3) 402 s 397 vs 396 vs 409 mw 405 mw 407 (3) 590? w 586 w 647 (2) 704 vs 702 vs 707 vs 750 s 748 s 747 s 738 (4) 770 w 765 w 803 ms 799 ms 797 s 813 (2) 838 in 843 (3) 973 vs 971 vs 974 vs 1018 mw 1021 mw 1016 mw 1014 (3) 1158 mw 1163 mw 1154 m 1171 (2) 1280 vw 1280 w 1272 m 1296 (3) 1373 m 1379 m 1370 s 1.373 (8) 1381 vs 1381 vs 1391 s 1398 vs 1396 vs 1393 vs 1394 (6) 1449 s 1444 vs 1445 s 1454 m 1456 m 1450 m 1457 (5) - 105 -Symmetry B E x c i t e d State Ground State Absorption i n Fluorescence i n \u00E2\u0080\u0094 : Raman Fluorene Biphenyl n-heptane Fluorene Biphenyl n-heptane 250 m 247 m 251 vw 258 w 258 w 252 (2) 363 ms 358 s 359 s 399 m 401 m 393 w 389 (1) 510 ms 509 s 509 m 560 m 562 m 555 mw 550 (4) 536 m 532 m 581 (1) 848 m 853 m 853 w 872 (1) 874 ms 873 ms 877 w 897 mw 903 mw- 906 vw 989 mw 996 mw 990 vw 992 (2) 1048 m 1046 ms 1046 vw 1062 vw 1067 mw 1063 w 1114 mw 1109 m 1122 vw 1259 m 1254 m 1261 vw 1280 w 1294 mw 1523 mw Chapter 6 A Study of Some Ex c i t e d S i n g l e t and T r i p l e t E l e c t r o n i c States of Fluorene A. I n t r o d u c t i o n 89 98 99 100 91 104 Although s e v e r a l experimental ' ' ' and t h e o r e t i c a l ' analyses of the spectrum of fluorene have been r e p o r t e d , no f i r m assignments f o r the \u00E2\u0080\u00A2 * i -, \u00E2\u0080\u00A2 * * * 4-u i i. u A x. * A- \u00E2\u0080\u00A2 104,105,106 e x c i t e d e l e c t r o n i c s t a t e s of the molecule have been made. Most d i s c u s s i o n has centered on the s o l u t i o n absorption spectrum (see F i g . 22) which shows three d i s t i n c t groups of bands i n the quartz u l t r a v i o l e t . The moderately o intense band near 3000 A (f=0.053) i s f o l l o w e d by a stronger more complex o \u00E2\u0080\u00A2 r e g i o n of absorption at about 2600 A (f=0.361) and an even stronger system \u00C2\u00B0 106 beginning near 2300 A. P i a t t has suggested that the lowest-energy system of f l u o r e n e should be c o r r e l a t e d w i t h the corresponding low-energy system of carbazole or of phenanthrene although the band i n t e n s i t i e s are somewhat d i f f e r e n t . The lowest-energy t r a n s i t i o n i n carbazole (Chapter 4) and p h e n a n t h r e n e , ^ ' ' ' i s now known to be d i r e c t e d along the short in-plane mole-c u l a r a x i s i n agreement w i t h P i a t t ' s contention and t h i s lends support to o the idea that the 3000 A band of f l u o r e n e should a l s o be s h o r t - a x i s p o l a r i z e d (^A^-t\u00E2\u0080\u0094*Aj1 . Other t h e o r e t i c a l s t u d i e s ^ 1 have p r e d i c t e d two weak t r a n s i t i o n s (one long and one s h o r t - a x i s p o l a r i z e d ) followed by a strong l o n g - a x i s t r a n s i t i o n (^ B ^A^). Since the c a l c u l a t e d t r a n s i t i o n s were c l o s e l y spaced i n energy, the ordering of the weak t r a n s i t i o n s depended on the approximations used. o Two attempts to measure the p o l a r i z a t i o n of the 3000A band have been - 107 -r e p o r t e d . U n f o r t u n a t e l y , the c r y s t a l samples used i n the f i r s t e x p e r i m e n t s ^ were contaminated with an impurity (see Chapter 5) shown to have been b e n z [ f ] i n d a n . The i m p u r i t y a b s o r p t i o n was about 2000 cm 1 to the red of the f i r s t f l u o r e n e band and i t was l a r g e l y t h i s b e n z [ f ] i n d a n 98-100 system that was measured and i n a d v e r t e n t l y a t t r i b u t e d to f l u o r e n e . 89 The second determination i n v o l v e d a magnetophotoselection study of f l u o r e n e at 77\u00C2\u00B0K i n a d i l u t e d i e t h y l ether s o l u t i o n , where t r a c e i m p u r i t i e s i n f l u o r e n e should be unimportant. This method u t i l i z e d a broad-band e x c i t a -t i o n technique that d i d not permit an independent measurement on each o v i b r o n i c band. The 3000 A system was found to be predominantly long-axis p o l a r i z e d , yet t h i s r e s u l t was i n t e r p r e t e d as a r i s i n g from an e l e c t r o n i c s h o r t - a x i s t r a n s i t i o n having strong v i b r o n i c coupling w i t h a long-axis t r a n s i -t i o n at higher energy i n order to remain i n agreement with P i a t t ' s e a r l i e r c o r r e l a t i o n . The aim of t h i s work was to d e r i v e as much inf o r m a t i o n as p o s s i b l e about the a b s o r p t i o n and f l u o r e s c e n c e s p e c t r a , paying p a r t i c u l a r a t t e n t i o n to any observations which might a i d the assignment of the low-energy e x c i t e d s i n g l e t s t a t e s . I t w i l l be seen t h a t such an assignment may be made from an examin-a t i o n of the c r y s t a l s p e c t r a . B. The Pure C r y s t a l Spectra 1. S e l e c t i o n Rules 16 Fluorene forms orthorhombic c r y s t a l s of space group P (D\u00E2\u0080\u009E, ) having J r to i n a m v 2\\J a four molecules i n a u n i t c e l l . ^ The molecules are numbered so that the 3. b c screw r o t a t i o n s C ~ , C~, and C~ performed on molecule 1 generate molecules 2, 3, and 4 r e s p e c t i v e l y . Each f r e e molecule s t a t e gives r i s e to f o u r zero wave v e c t o r c r y s t a l - 108 -fu n c t i o n s which form a b a s i s of the i r r e d u c i b l e r e p r e s e n t a t i o n s of the D 2h f a c t o r group. The combinations of the l o c a l i z e d one s i t e e x c i t o n f u n c t i o n s having the c o r r e c t t r a n s f o r m a t i o n p r o p e r t i e s are: a Y = 1 / 2 C * ! + * 2 + \u00E2\u0080\u00A2 3 3 Y = l/2(cb 1 - * 2 -+ V (6.1) 6 Y = 1/2(4,1 \" *2 + *3 e Y = 1 / 2 C * ! + * 2 \" *3 - V The c r y s t a l s t a t e s can be f u r t h e r c l a s s i f i e d according t o whether the f r e e molecule s t a t e from which they are de r i v e d i s symmetric or antisymmetric to the s i t e group operation a . The c o r r e l a t i o n of the f r e e molecule s t a t e s w i t h the c r y s t a l s t a t e s were found from symmetry c o n s i d e r a t i o n s and the r e s u l t s are c o l l e c t e d i n Table 20. Table 20. The e x c i t e d - s t a t e c r y s t a l wavefunctions and t h e i r i r r e d u c i b l e r e p r e s e n t a t i o n s i n the space group c o r r e l a t e d w i t h the f r e e molecule t r a n s i t i o n s . Free molecule Molecular C r y s t a l wavefunctions D2h f a c t o r group C r y s t a l \u00E2\u0080\u00A2 s e l e c t i o n r u l e s t r a n s i t i o n s f o r wavevector (k=Q) r e p r e s e n t a t i o n s e l e c t i o n r u l e s z_ x A + A 1 1 B l - A l Y i Y l g 2u 3u forbidden forbidden allowed //b. allowed //a forbidden y 1 A 1 A A \u00C2\u00AB\u00E2\u0080\u00A2 A 1 1 B 2,- A : Y I Y A t B B l u 2g 3g forbidden allowed //c_ forbidden forbidden - 109 -Thus, a long-axis t r a n s i t i o n i n the i s o l a t e d molecule gives r i s e to no_ absorption i n an ab cleavage plane and a s h o r t - a x i s t r a n s i t i o n gives a b s o r p t i o n only i n the ab plane w i t h an oriented-gas r a t i o ( f ^ / f ^ ) of 2:1. 2. C r y s t a l Energies Using the already enumerated c r y s t a l wavefunctions as the ba s i s s e t , then to a f i r s t order of approximation, we can express the energies of the r r st a t e s with respect to [w + D ] (that i s , the sum of the f r e e molecule t r a n s i t i o n energy and the change i n bi n d i n g energy o f a molecule i n a c r y s t a l upon e x c i t a t i o n ) as f o l l o w s : A* CD = z'iTn \u00E2\u0080\u00A2 n T u + z i j 3 + zi^ 4 AE*(r) = Z'lTn - U\2 - n\3 + i f ^ (6.2) A E 6 ( r ) = z ' l ^ - Z i J 2 + Z l J 3 - Z i J 4 AE^(r) = l < ? n + H [ 2 - E I - 3 + Z i J 4 The i n t e r m o l e c u l a r resonance energy i s summed over the sets of molecules i n d i c a t e d by the second s u b s c r i p t s and r = y_ or z depending on whether the f r e e molecule t r a n s i t i o n i s long or s h o r t - a x i s p o l a r i z e d . 3. C a l c u l a t i o n o f the C r y s t a l S p l i t t i n g s ; D i p o l e - D i p o l e Approximation. C a l c u l a t i o n of the c r y s t a l s t a t e energies r e q u i r e s the e v a l u a t i o n of the i n t e r m o l e c u l a r coupling energy between i d e n t i c a l molecules k and 1 with - 0 0 X I * ground s t a t e s ^ and e x c i t e d s t a t e s t,^, t ^ . The matrix elements have the.form r r o r i , , i r o . ' ,, _.. J k i = ^hWiKh\" (6-3) - no -The operator i s the i n t e r m o l e c u l a r p o t e n t i a l energy V, , = - 2 Z^e 2 - '2 1 e2 + 2 e 2 + 2 V z e 2 (6.4) k l r \u00E2\u0080\u00A2 f \u00E2\u0080\u00A2 g r f g f . l g . i \u00E2\u0080\u0094 i , J f 3 g , J r,.. \u00E2\u0080\u00A2 ' r . > J r . . \u00E2\u0080\u00A2 r,. f j g i i j tg where f,g are the n u c l e i , Z^ and Z the nuclear charges, and i and j the el e c t r o n s of the kth and 1th 'molecules r e s p e c t i v e l y . One method used to evaluate the i n t e g r a l 1 ^ i s t o expand the operator i n a s e r i e s of p o i n t m u l t i p o l e s . This method has the advantage that the molecular wavefunctions need not be s p e c i f i e d . The f i r s t non-vanishing term i n the expansion gives the p o t e n t i a l energy of i n t e r a c t i o n i n the form V k l =2)4 - V ( \" k ^ C V r k l ) ] (6.5) E k l r k l where M, and M, are the f r e e molecule t r a n s i t i o n moments. The f i n a l form -k \u00E2\u0080\u00941 of the expression used i n the program f o r computing the d i p o l e i n t e r a c t i o n sum i s S l k l = 4^- - V ( Q k - I k l ) ( Q t . r k l ) ] (6.6) * k l r k l where M = eQ ( e l e c t r o n i c charge times t r a n s i t i o n length i n centimeters) has been s u b s t i t u t e d i n t o equationft-S^The value of Q i s r e l a t e d t o the o s c i l l a t o r s t rength f at the energy v ( i n cm 1) and can be obtained from s o l u t i o n i n t e n s i t y measurements f = 1.085 x 1 0 n Q 2 v (6.7) - I l l -Equation (6.6) was used to compute the d i p o l e - d i p o l e i n t e r a c t i o n sums f o r fluo r e n e from c r y s t a l s t r u c t u r e data f o r a u n i t t r a n s i t i o n d i p o l e o length (Q = 1) over a sphere of 40 A r a d i u s . The r e s u l t s are l i s t e d i n Table 21. Table 21. D i p o l e - d i p o l e i n t e r a c t i o n sums f o r c r y s t a l l i n e f luorene over a \u00C2\u00B0 a b sphere of 40 A r a d i u s f o r u n i t t r a n s i t i o n moments. ' k>* Z I k l E I k l E I k l 1,1 -703 2305 -1602 1,2 -2405 -4310 -907 1,3 -196 1077 178 1,4 -545 -1095 -550 x,y,z are the molecular axes shown i n F i g . 6 and l a b e l the d i r e c t i o n of the t r a n s i t i o n moment connecting the ground s t a t e with the e x c i t e d s t a t e under c o n s i d e r a t i o n . b C r y s t a l d a t a , D.M. Burns and J . I b a l l , Proc. Roy. Soc. (London) A227 200 (1955). An estimate of the c r y s t a l t r a n s i t i o n energies a s s o c i a t e d w i t h the long-and s h o r t - a x i s p o l a r i z e d t r a n s i t i o n s of the f r e e molecule w i l l a s s i s t i n the i n t e r p r e t a t i o n of the experimental r e s u l t s d i s c u s s e d i n the next s e c t i o n . o The o s c i l l a t o r s t r e n g t h of the 0-0 band o f the 3000 A absorption system i n fl u o r e n e s o l u t i o n was estimated to be 0.02 g i v i n g a t r a n s i t i o n d i p o l e length o of 0.237 A. The c a l c u l a t e d p o s i t i o n s of the f l u o r e n e c r y s t a l l e v e l s f o r the o t r a n s i t i o n d i p o l e lengths 1.0 and 0.237 A are given i n Table 22. The r e s u l t s of the c a l c u l a t i o n are d i s p l a y e d g r a p h i c a l l y i n F i g . 21. 112 5 0 0 B 2 g 4 0 0 300 H 200 H O o 100 H -200 B | u ( / / \u00C2\u00A3 ) B _ / 3 < 3 - B o ( / / b ) B i g B 3 u ( / / g ) A g F i g . 21. Factor group s p l i t t i n g s a s s o ciated w i t h the long- and s h o r t - a x i s o of f l u o r e n e molecule c a l c u l a t e d f o r t r a n s i t i o n moments of 0.237 A. - 113 -Table 22. C a l c u l a t e d c r y s t a l l e v e l s of f l u o r e n e C r y s t a l S h o r t - a x i s assignment Long-axis assignment l e v e l Q = 1.0 Q = 0.237 Q = 1.0 Q = 0.237 A E a ( r ) -2881 -161 '-2023 -103 A E P ( r ) -1423 -80 4443 249 A E 6 ( r ) 33 2 8787 492 A E \u00C2\u00A3 ( r ) -2137 -120 -1987 -101 4. Room Temperature Absorption F i g , WAVELENGTH ( A ) 24CO 2200 4 0 4 2 W A V E N U M B E R ( k K ) 4 4 \u00E2\u0080\u0094 i \u00E2\u0080\u0094 4 6 The room-temperature absorption s p e c t r a of f l u o r e n e : f u l l l i n e , ab c r y s t a l s e c t i o n //b_; dotted l i n e , ab c r y s t a l s e c t i o n //a_; broken l i n e s o l u t i o n spectrum i n iso-octane (taken from \"Catalogue of U l t r a v i o l e t Spectrograms\" American Petroleum I n s t i t u t e ) . - 114 -The a b s o r p t i o n spectrum at 295\u00C2\u00B0K i s shown i n F i g . 22. The t h i n , s i n g l e c r y s t a l s needed could only be grown by s u b l i m a t i o n i n a flow of n i t r o g e n e n t r a i n e r gas, when f l a k e s - w e r e produced i n which the prominent ab cleavage plane i s the well-developed face. C r y s t a l thicknesses were measured w i t h a Berek compensator once the r e f r a c t i v e i n d i c e s had been 107 determined by i n t e r f e r o m e t r y ; the b i r e f r i n g e n c e was found to be 0.097 o at 4500 A. The curves shown i n F i g . 22 represent the mean values of data obtained from f o u r c r y s t a l s whose thicknesses ranged from 0.5 to 2.5 u. The e r r o r spanning these separate measurements i s about 20%, probably a r i s i n g from r a p i d s u b l i m a t i o n of the t h i n c r y s t a l s and from s t r a y l i g h t w i t h i n the monochromator f o r the t h i c k e r samples. Molar e x t i n c t i o n co-e f f i c i e n t s fr , e, and e j i n the c r y s t a l are d e f i n e d so that 3e , . . = v a b c 3 s o l u t i o n C\u00C2\u00A3a + e b + Ec>\" I t should be noted that the oriented-gas estimate f o r the i n t e n s i t y of the c - p o l a r i z e d o r i g i n i s about 27,000 1 mole *cm thus the lack o f absorp-t i o n i n the s u b l i m a t i o n f l a k e may be given the f o l l o w i n g i n t e r p r e t a t i o n . The moderately intense and strong t r a n s i t i o n s observed i n the s o l u t i o n o o spectrum at about 3000 A and 2600 A must both be p o l a r i z e d along the long a x i s of the molecule and may be assigned ^T^ <- ^A^. The very weak t r a n s i -t i o n observed at about 35 520 cm 1 i n the s u b l i m a t i o n f l a k e and p o s s i b l y at about 36 700 cm 1 i n the s o l u t i o n spectrum, i s t h e r e f o r e s h o r t - a x i s p o l a r i z e d and may be assigned *A^ <- ^A^. The few weak bands to the red of 35 520 cm 1 i n the c r y s t a l spectrum may mark the appearance of n o n - t o t a l l y symmetric v i b r a t i o n s b u i l t on the lowest-energy e l e c t r o n i c t r a n s i t i o n at about 33 050 cm 1 i n S i t e Group (C ) Factor Group Bases Species Species Species Bases xx,yy,zz; z A l A' A . g *g aa,bb,cc ab xz ; x B l 2u b B, a 3u \u00E2\u0080\u0094 A u - 134 -C. I n f r a r e d Spectra P o l a r i z e d s p e c t r a were recorded w i t h l i g h t i n c i d e n t on the ab_, bc_ and ac faces of a s i n g l e c r y s t a l of f l u o r e n e and a set of independent s p e c t r a are shown i n F i g s . 28-30. To the extent t h a t our s i n g l e - c r y s t a l s e c t i o n was somewhat t h i c k e r than Witt's o r i e n t e d , m u l t i r- c r y s t a l l i n e sample, our ab s p e c t r a f o r the higher-energy r e g i o n (shown i n Fig.30) confirm h i s 117 r e s u l t s and supplement them s i n c e the weaker l i n e s have become more prominent. There was one d i f f e r e n c e i n the low-energy s p e c t r a (see F i g . 28) where the s p l i t l i n e centered at 260 cm 1 i s s l i g h t l y more intense i n 117 a_ than b_ p o l a r i z a t i o n . This i s a r e v e r s a l of Witt's observation and may be a t t r i b u t e d to the f a c t that the l i n e f e l l at the l i m i t of h i s spectrometer. There was some d i f f i c u l t y i n d i s t i n g u i s h i n g l a t t i c e modes from low-frequency molecular fundamentals, so the spectrum of f l u o r e n e i n a cyclohexane s o l u t i o n , measured from 50 to 600 cm \ i s shown i n F i g . 27 the s o l u t i o n spectrum at higher energies has been reported by W i t t . 100 200 300 400 500 600 cm\"1 F i g . 27 The i n f r a r e d spectrum of f l u o r e n e i n a cyclohexane s o l u t i o n i n the low-frequency r e g i o n ; the broken curve represents the spectrum i n a d i l u t e s o l u t i o n . 600 cm-' F i g . 28 The p o l a r i z e d i n f r a r e d s p e c t r a of flu o r e n e s i n g l e c r y s t a l at low _ l frequencies. The ab s e c t i o n was 1 mm t h i c k f o r the range 50-160 cm and 0.1 mm t h i c k f o r 160-650 cm\"1, and the ac s e c t i o n was. 0.79 mm thick, f o r tlie range 5Q-16Q cm-1 and Q.15 mm f o r 160-65Q cm~l. F i g 3 2 0 0 Wavenumber (cm - 1) 29' 'The' i n f r a r e d s p e c t r a - i n the CH s t r e t c h i n g r e g i o n : (a) an ab c r y s t a l s e c t i o n , (b) a be c r y s t a l s e c t i o n , and (c) i n a carbon t e t r a c h l o r i d e s e c t i o n . - 137 -Table 29 l i s t s the observed frequencies of the bands and t h e i r a s s i g n -ments made on the assumption that d e v i a t i o n s from an oriented-gas model are s m a l l . In t h i s approximation, the r e l a t i v e l i n e strengths along the c r y s t a l axes are given as the squares of the d i r e c t i o n cosines of molecular axes on the c r y s t a l axes (see Table 28) when, q u a l i t a t i v e l y , modes are expected to be most intense along b, B along a, and B along c_. - 1 \u00E2\u0080\u00A2 ^ Table 28. The r e l a t i v e band i n t e n s i t i e s o f f l u o r e n e along the c r y s t a l axes c a l c u l a t e d i n the oriented-gas approximation. A l B l B2 a 0, .326 0. .674 0.000 b 0. .674 0. .326 0.000 c 0, .000 0, .000 1.000 Table 29. The i n f r a r e d spectrum o f fluorene S o l u t i o n //a_ //b_ //c Symmetry5 46 vw b_ 3u 70 ms b\u00E2\u0080\u009E 3u 80 m b n 2u 100 w b\u00E2\u0080\u009E 2u 101 vw b, l u 120 m 111 s 126 ms 130 vw b. ; l u 150 vvw see t e x t 215 mw 216 vw 218 mw A 247 vs 255 vs 265 vs Bx 410 s 410 ms 408 m see t e x t - 138 -S o l u t i o n //a //b //\u00C2\u00A3 Symmetry1 467 m 476 w 471 m see t e x t 487 mw 487 m B2 542 m B2 618 ms B2 621 m 627 w 629 m A l 693 mw 694 s 692 m B l 722 w B2 737 vs 735 vs B l 738 vs A l \u00E2\u0080\u00A2 773 w B2 841 w B l 851 w 854 ms 859 ms A l 865 vw B2 873 w 873 vw B l 904 mw B2 910 mw 910 w B l 951 s 949 s 952 s B 1 975 vw A l 990 vw A l 1001 m 994 vs B2 1019 vw 1016 ms 1016 ms A l 1026 w 1023 vs _ B2 1056 vw A l 1089 w 1089 ms 1089 s A l 1103 s B2 1124 w 1120 w B2 1137 vw B2 1143 vw A l 1147 w 1152 s 1154 vw B l 1185 m 1188 s 1184 s A l 1194 sh 1188 vs B2 1213 mw 1215 m 2 _ 139 -S o l u t i o n //a //b //c Symmetry3-1230 w 1295 ra 1311 m 1331 w 1377 vw 1399 s 1410 w 1446 s 1450 s 1477 s 1575 w 1231 m '2902 m 2927 m 3023 m 3045 m 1319 w 1340 w 1380 w 1397 vs 1406 m 1426 w 1440 vs 1486 vw 1570 w 1592 mw 1643 w 2907 mw 2920 w 3016 mw 3048 m 1231 m 1291 w 1319 w 1340 vw 1380 w 1392 vs 1426 w 1440 vs 1486 vw 1570 m 1592 mw 1643 w 2921 ms 3020 s 3028 sh 3048 ms 1294 s 1303 vs 1336 s 1382 w 1417 w 1440 s 1471 s 1521 m 1582 vw 1602 vw 1635 w 3006 m 3040 s 3062 s B2 see t e x t A l B2 see t e x t B l B 2 \u00E2\u0080\u00A2 2 FL - 1 4 0 -S o l u t i o n //a //b /'/\u00C2\u00A3_ Symmetry3-3 0 7 0 s 3 0 6 3 m 3 0 6 4 s A l 3 0 7 2 sh 3 0 7 2 sh V 3 0 8 4 sh B 2 3 0 9 6 vw 3 0 9 4 ms A l 3 1 5 3 m B 2 a L a t t i c e modes are d e s i j jnated using lower-case symbols to d i s t i n g u i s h them from molecular v i b r a t i o n s D. Raman Spectra The Raman s p e c t r a of f l u o r e n e as a s o l u t i o n , , a m e l t 1 ^ 2 and a 1 1 9 1 2 0 powder ' have been used to make a t e n t a t i v e i d e n t i f i c a t i o n of a few fundamentals; however, no p o l a r i z a t i o n data have been a v a i l a b l e . P o l a r i z e d s p e c t r a o f a s i n g l e c r y s t a l and d e p o l a r i z a t i o n r a t i o s of the l i n e s measured f o r a carbon t e t r a c h l o r i d e s o l u t i o n (see Tables 3 0 and 3 1 , and F i g s . 3 1 and 3 2 ) a r e reported here f o r the f i r s t time. The assignment of the t o r s i o n a l l a t t i c e modes given i n Table 3 0 was made using the s e l e c t i o n r u l e s l i s t e d i n Table 2 7 . The i n t e n s i t i e s of the l i n e s corresponding to i n t r a m o l e c u l a r v i b r a t i o n s may be estimated f o r the orieiited-gas model by transforming the components of the p o l a r i z a b i l i t y tensor from the molecular frame Q c , y_, z) to the c r y s t a l frame (a, b_, c j . In t h i s approximation, the i n t e n s i t y of a lone i n the c r y s t a l s p e c t r a i s r e l a t e d to i t s free-molecule i n t e n s i t y by the equations: aa bb cc ab. . 4 5 4 . 1 0 6 . 8 8 0 . 1 0 6 . 4 5 4 . 8 8 0 1 . 0 0 0 . 2 2 0 . 2 2 0 . 1 2 1 xx zz L?\"xz. and I ac L : b c . 6 7 4 . 3 2 6 . 3 2 6 . 6 7 4 I xy I L y z J -.141 wavenumber (cm-1) F i g . 31 The Raman s p e c t r a of a flu o r e n e s i n g l e c r y s t a l . The nomenclature i s defined i n Table 32. A l l s p e c t r a were recorded under i d e n t i c a l c o n d i t i o n s except that the i n t e n s i t y of the (cc) spectrum has been reduced here by a f a c t o r of ten. The l i n e s at 221, 1480 and 1612 cm 1 e v i d e n t l y appear only through the e f f e c t i v e n e s s o f the operator a yy The s p e c t r a i n F i g . 31 were measured under c o n d i t i o n s that were as n e a r l y the same as p o s s i b l e . However, i t would appear t h a t , i f the assumption of n e g l i g i b l e i n t e r m o l e c u l a r i n t e r a c t i o n i s t o be r e l i e d upon, the i n t e n s i t y of the (ab) spectrum should be appr e c i a b l y increased before any comparison between the sp e c t r a i s made s i n c e the 743 and 1022 cm 1 i n t e r v a l s may be '. .7142 ' (ac) i 1 1 * 1 1 1 1 1 1 1 1 1 1 1 1 i 1 i - 2 G O O 2 C O 4 0 0 G O O 8 C O I O O O I 2 C O I 4 C O 1 6 0 0 wavenumber (cm\"') F i g . 32 The Raman s p e c t r a o f a flu o r e n e s i n g l e c r y s t a l showing the A^ and modes. The nomenclature i s defined i n Table 32. taken to represent modes from t h e i r low d e p o l a r i z a t i o n r a t i o s i n s o l u t i o n . The l i n e s at S46 and 854 cm 1 mark the presence of two molecular fundamentals s i n c e they both have A f a c t o r group components i n the (bb) spectrum. The reg i o n 1400 cm 1 to the red of the e x c i t i n g l i n e was als o complicated and was i n t e r p r e t e d as f o l l o w s : The 1408 cm 1 i n t e r v a l i n the (aa) and (bb) sp e c t r a represented the A f a c t o r group component and the 1400 cm 1 i n t e r v a l B component of a s i n g l e A 1 molecular v i b r a t i o n . 143 -Table 30. The Raman spectrum of fluorene near the e x c i t i n g l i n e . b(aa)c a(bb)c a(ba)c a(ca)c b(cb)c Symmetry 24 3g 31 big 38 44 '\ 66 \"lg 77 b 3 g 88 r \ 88 89 a g The column headings i n d i c a t e , from l e f t to r i g h t i n s i d e the parentheses, the p o l a r i z a t i o n of the i n c i d e n t and s c a t t e r e d l i g h t and, from l e f t t o r i g h t o u t s ide the parentheses, the p r o p a g a t i o n ' d i r e c t i o n s of the i n c i d e n t and s c a t t e r e d l i g h t , r e s p e c t i v e l y . No (cc) Raman s c a t t e r i n g was detected. The l i n e s at 846 and 854 cm 1 mark the presence of two molecular fundamentals s i n c e they both have A f a c t o r group components i n the (bb) spectrum. The region 1400 cm 1 to the red of the e x c i t i n g l i n e was a l s o complicated and was i n t e r p r e t e d as f o l l o w s : The 1408 cm 1 i n t e r v a l i n the (aa) and (bb) s p e c t r a represented the A f a c t o r group component and the 1400 cm i n t e r v a l the component of the s i n g l e A^ molecular v i b r a t i o n . The entry i n Table 31 i s the mean of these v a l u e s . i / - 144 -Table 31. The Raman spectrum of flu o r e n e CC1. jya C r y s t a l Symmetry 221 A l 258 V 280 mw /v>. 8 285 A 2 416 mw .14 421 A l 546 w u^, the b - p o l a r i z e d l i n e s at 80 and 100 cm 1 as ^2U' a n c* t^ i e w e a ^ c - p o l a r i z e d l i n e s at 101 and 130 cm 1 as b ^ u modes. The a - p o l a r i z e d l i n e at 111 cm 1 and the b - p o l a r i z e d l i n e at 126 cm 1 were taken as f a c t o r group components of a molecular fundamental which appeared i n the s o l u t i o n spectrum at 120 cm 1. The very weak 150 cm 1 l i n e i n a - p o l a r i z a t i o n i s probably a b ^ u combination of the l a t t i c e modes at 70 (b_ ) and 88 cm 1 (a ), although a b e t t e r energy f i t may be found.for ou g - 146 -t h i s combination when the missing g modes are i d e n t i f i e d . I t i s i n t e r e s t i n g to -compare the l a t t i c e modes observed i n f l u o r e n e 58 w i t h those f o r car b a z o l e , s i n c e the carbazole c r y s t a l belongs to the same space group as f l u o r e n e and has i t s four molecules disposed i n the u n i t c e l l i n almost i d e n t i c a l f a s h i o n . However, i t should be noted that the axes b_ and c of the carbazole c r y s t a l should be interchanged before the comparison i s made. In ge n e r a l , there i s an increase i n lattice-mode frequency between flu o r e n e and car b a z o l e , and , i n p a r t i c u l a r , the f a i r l y prominent Raman l i n e s at 66, 77, 88 and 89 cm 1 i n flu o r e n e have counterparts i n the carbazole spectrum that are somewhat grea t e r than 100 cm 1 . This i m p l i e s that the r e l e v a n t f o r c e constants must be greater i n car b a z o l e , a r e s u l t which i s i n harmony w i t h ( i ) the very much higher m e l t i n g - p o i n t of car b a z o l e , \u00C2\u00B03 and ( i i ) the observation that carbazole has the smaller u n i t c e l l (853 A i n \u00C2\u00B03 carbazole and 921 A i n f l u o r e n e ) . 2. A^ Symmetry (22 fundamentals) Prominent b - p o l a r i z e d l i n e s were observed i n the i n f r a r e d spectrum at 217, 628, 857, 1016, 1089, 1186, 1231, 1291, 1319, 1440, 1570 and about 1592 cm 1 and were assigned as A^ fundamentals. The f a i r l y intense i n t e r v a l s 221, 421, 743, 846, 1022, 1238, 1296, 1329, 1349, 1480, 1578 and 1612 cm\"1 i n the Raman spectrum, which e i t h e r had a low d e p o l a r i z a t i o n r a t i o or appeared i n the (cc) spectrum, were a l s o taken to represent A^ fundamentals. There are 16 d i f f e r e n t e n t r i e s i n these combined l i s t s , many of them common w i t h i n the l i m i t s of experimental accuracy (about 5 cm 1 f o r each entry) and of a probable f a c t o r group s p l i t t i n g . The missing fundamental i n the range below 2000 cm 1 probably l i e s at 1397 cm 1 , which i s c e r t a i n l y a re g i o n of strong i n f r a r e d absorption. W i t t 1 1 ^ has shown that the band contour i n - 147 -the vapour spectrum i s c o n s i s t e n t with an assignment, and the Raman d e p o l a r i z a t i o n r a t i o , i n a c a r e f u l remeasurement, was 0.13. However, i n the i n f r a r e d spectrum of the c r y s t a l , i t s i n t e n s i t y i s gr e a t e r i n a_ r a t h e r 117 than b p o l a r i z a t i o n , a r e s u l t found both by Witt and i n the present work; the same s o r t of disturbance seems to have a f f e c t e d the 1340 cm 1 i n f r a r e d l i n e . This seems to be one c l e a r example where i n t e r m o l e c u l a r i n t e r a c t i o n s are s u f f i c i e n t l y severe to reverse an oriented-gas p r e d i c t i o n . Another example occurs near 400 cm 1 . Both the Raman and fluorescence spectrum show that there i s an A^ fundamental at 421 cm while the i n f r a r e d s p e c t r a i n d i c a t e s that a B^ fundamental l i e s at 409 cm 1 with the A^ fundamental -1 117 at 474 cm . Indeed, the i n f r a r e d assignment i s the one presented by Witt I t i s not necessary t o suppose that the i n t e r a c t i o n s i n the c r y s t a l are so extreme as to cause a broadening of the A^ molecular s t a t e to cover the range 408-476 cm 1 ; r a t h e r i t i s s u f f i c i e n t to suppose that the c r y s t a l bands a r i s i n g from the 421 (A^) and 474 (B^) molecular s t a t e s remain narrow (probably not more than 10 cm 1 wide) yet mix s t r o n g l y under the a c t i o n of the c r y s t a l environment. The 5 CH s t r e t c h i n g fundamentals of A^ symmetry were observed i n the i n f r a r e d spectrum at 2920, 3018, 3064, and 3094 cm 1 . The assignments i n the higher-freo^ency r e g i o n c o n t a i n a gr e a t e r u n c e r t a i n t y , s i n c e the weaker l i n e s may represent combinations and the strong regions of absorption an 117 approach to a c c i d e n t a l degeneracy. Witt has assigned the symmetry s t r e t c h i n g v i b r a t i o n of the CH^ group at 2905 cm 1 with the asymmetric (B^) s t r e t c h at 2920 cm 1 on the b a s i s of the band contours of the vapour spectrum. 3. B^ Symmetry (11 fundamentals) Eight B^ fundamentals are i d e n t i f i e d i n the p o l a r i z e d i n f r a r e d spectrum - 148 -at 119, 260, 693, 735, 910, 951, 1153 and 2905 cm\" 1, although, as noted 117 above, the r e s u l t from the vapour spectrum may cause the l a s t assignment to be m o d i f i e d . Two of the remaining three fundamentals may be seen as weak l i n e s at 841 and 873 cm 1 and the l a s t i s at 474 cm 1 as i n d i c a t e d i n the previous s e c t i o n . Since some of these out-of-plane modes show an a p p r e c i a b l e s p l i t t i n g , only the mean values are given above. The modes are not prominent i n the Raman spectrum so t h a t l i t t l e c o r r o b o r a t i o n of these assignments i s a v a i l a b l e from t h i s source. 4. A 2 Symmetry (10 fundamentals) Only two fundamentals were lo c a t e d from the Raman s p e c t r a at 285 and 787 cm 1 ; no other r e l e v a n t i n f o r m a t i o n i s a v a i l a b l e . 5. &2 Symmetry (20 fundamentals) Of the 16 B 2 fundamentals expected below 2000 cm 1 , 13 of the strongest l i n e s i n the \u00C2\u00A3-polarized spectrum at 487, 542, 618, 994, 1023, 1103, 1188, 1215, 1303, 1336, 1440, 1471 and 1521 cm\"1 are assigned to t h i s species. No combination could be found f o r the 487 cm 1 l i n e although, to be c e r t a i n t h a t there i s none, more complete knowledge of the low-frequency A^ fundamentals i s r e q u i r e d . The assignment of the other low-frequency l i n e of only moderate stre n g t h as a fundamental i s confirmed -by the Raman data. The Raman s p e c t r a a l s o suggest the presence of B ? fundamentals at 1146 and 1193 cm 1 which may c o r r e l a t e w i t h the i n f r a r e d bands at 1137 and 1188 cm 1. The weak l i n e s at 722 and 904 cm 1 are f a i r l y w e l l i s o l a t e d from other s t r o n l i n e s and are taken to mark the presence of the l a s t two fundamentals. The l i n e at 1120 cm 1 was not s e l e c t e d as a fundamental s i n c e i t was not present i n the s o l u t i o n spectrum and was thought to represent a combination band d e r i v i n g i t s i n t e n s i t y i n the c r y s t a l from the strong l i n e at 1103 cm 1 - 149 -The fo u r CH s t r e t c h i n g fundamentals were chosen on the b a s i s of l i n e s t r e n g t h at 3040 and 3062 cm\" 1, and p o s s i b l y 3006 and 3084 cm\"1. F. D i s c u s s i o n The assignments made i n the previous s e c t i o n are l i s t e d i n Table 32 where 117 a comparison w i t h Witt's e a r l i e r assignment i s given. Table 32. The assignments of the fundamental v i b r a t i o n s of fl u o r e n e Species Present Work Ref. 1 Species Present Work Ref. 1 217 42 1 a 628 743 a 857 1016 1089 1186 1231 1291 1319? 1349 1397 1440 1480 1570 1612 a 2920? 3018 3048 262 472 633 1 738?* 860 1022 \u00E2\u0080\u00A2 1092 1 1190 1234 1324 1343 1400 1448 1576 1610? J 2905 3019? 3027? 487 542 618 722? 904? 994 1023 1103 1146 a 1188 1215 1303 1336 1440 1471 1521 3006? 3040 3062 414? 490 624 ? ? 1 000 1025 1109 1156 1160 1198? 1300 1311 1451 1478 3003? 3041 3062 - 150 -Species Present Work Ref. 1 Species Present Work Ref. 1 3064 3047? 3084? 3093? 3094? 3071? A 0 B, 119 ? 2 1 287 260 212? a 473 4 1 l a 693 545? 735 698 788 841 740 873 842? 910 1 951 914 1153 954 2905? 2920 Data taken from the Raman spectrum; a l l other e n t r i e s are from the i n f r a r e d spectrum. Apart from d i f f e r e n c e s that have already been mentioned, a comparison w i t h Witt's assignment y i e l d s the f o l l o w i n g observations. ( i ) I f , i n the A^ symmetry b l o c k , Witt's two queries are l i f t e d out and the prominent Raman l i n e s at 1296 and 1480 cm 1 are i n s e r t e d i n t h e i r p l a c e , good agree-ment i s reached. ( i i ) W i tt's assignment of the fundamentals i s remarkably accurate c o n s i d e r i n g that he d i d not have a v a i l a b l e the l e s s -crowded c_-polarized spectrum where only modes appear. Minor discrepancies occur i n the complex regions around 1150 and 1300 cm 1. More important d i f f e r e n c e s occur at low-frequencies: we f i n d no evidence f o r a B^ funda-mental at 414 cm 1 , yet have such evidence (both i n the i n f r a r e d and Raman spectrum) f o r one at 542 cm 1. ( i i ) The B symmetry block comes i n t o good - 15.1 -agreement once i t i s recognized that the Raman l i n e at 546 cm 1 has, i n f a c t , symmetry and th a t the ord e r i n g i n energy may be rearranged. A comparison with the carbazole assignment i s a l s o rewarding (see Appendix I I ) . In A^ symmetry two p o i n t s a r i s e . In carbazole the f a i r l y prominent 1327 cm 1 l i n e was taken to represent an A^ combination with a more prominent l i n e at 910 cm 1 being accepted as the fundamental; i n f l u o r e n e , the l i n e corresponding to 910 cm 1 i s missing and the assignment has been reversed. I t i s not p o s s i b l e to make a s a t i s f a c t o r y choice between these a l t e r n a t i v e types of assignment on the b a s i s of the data p r e s e n t l y a v a i l a b l e . The second p o i n t a r i s e s from the absence of an A^ fundamental at 1397 cm 1 i n the carbazole spectrum; thus the normal coordinate may be p r i m a r i l y represented by the CH^ s c i s s o r i n g motion. This argument i s 121 r e i n f o r c e d by d e u t e r a t i o n e f f e c t s . Using s i m i l a r arguments, i t may be deduced t h a t the normal coordinate f o r the 693 cm 1 B^ fundamental i s made up l a r g e l y of the out-of-plane r o c k i n g motion of the CH^ group. In view of the assignment o f the A^ and B^ fundamentals from i n f r a r e d and Raman data, a somewhat novel i n t e r p r e t a t i o n ' o f the medium stre n g t h 227 cm 1 i n t e r v a l which appears i n both the fluorescence and phosphorescence spectrum i s p o s s i b l e . The a n a l y s i s of the fluorescence and phosphorescence shows that the prominent v i b r a t i o n s are e i t h e r of A^ or B^ symmetry with the gr e a t e s t i n t e n s i t y r e s i d i n g i n the A^ modes. The 227 cm 1 does not represent a fundamental of e i t h e r the A. or B\u00E2\u0080\u009E s p e c i e s , and t h e r e f o r e must be the 1 z overtone of the B^ fundamental at 119 cm 1 i n the fluorene c r y s t a l . The overtone i s symmetry allowed s i n c e i t belongs to the t o t a l l y symmetric represe a t i o n o f the C,, molecular po i n t group; thus the problem remaining i s to account f o r i t s i n t e n s i t y . In the harmonic approximation, the r a t i o of the i n t e n s i t y of the f i r s t overtone to that of the o r i g i n band i s given by the - 152 -122 square of the r a t i o s of the overlap of two harmonic o s c i l l a t o r s ^,0 _ rR(2,0) ,2 = 1 rv' - v\"-,2 n n LRfn.0 ,l J 2 Lv' + v\" J 1 J I 0 j 0 LR(0, ) J 2 Lv' + v where v' and v\" are r e s p e c t i v e l y the ground and e x c i t e d s t a t e frequencies of a p a r t i c u l a r normal mode. Equation (7.1) i s v a l i d i f the o r i g i n of the ground and.excited e l e c t r o n i c s t a t e are not d i s p l a c e d , a reasonable assumption i n t h i s case s i n c e no p r o g r e s s i o n i s observed.. A long 20-cm 1 91 sequence i s observed i n the vapour spectrum. I f t h i s sequence i n v o l v e s the 119-cm 1 fundamental, the i n t e n s i t y of the overtone i n the e l e c t r o n i c spectrum according to equation (7.1) i s about 0.5% of the o r i g i n band. Apparently, the presence of other i n t e r a c t i o n s i s r e q u i r e d to account f o r the f u l l i n t e n s i t y of t h i s band. Chapter 8 A V i b r a t i o n a l Assignment of Dibenzothiophene-h c and o Dibenzothjophene-d from I n f r a r e d and Raman Spectra o A. I n t r o d u c t i o n An assignment of the fundamental v i b r a t i o n s of dibenzothiophene based on i n f r a r e d and Raman st u d i e s has not been p r e v i o u s l y attempted. However, the c r y s t a l s t r u c t u r e of dibenzothiophene has been determined^ 1 and large c r y s t a l s are e a s i l y grown from the melt so that i t i s p o s s i b l e to obt a i n p o l a r i z e d i n f r a r e d and Raman data. I t i s a l s o f o r t u n a t e that dibenzothiophene-d i s r e a d i l y synthesized from b i p h e n y l - d ^ so that a more complete study of the v i b r a t i o n s of dibenzothiophene i s p o s s i b l e . The assignment of the fundamental v i b r a t i o n s from t h i s data provides not only a u s e f u l comparison w i t h the assignments of carbazole and f l u o r e n e , but a l s o a f u r t h e r check 73 on the t r a n s f e r a b i l i t y of the phenanthrene f o r c e f i e l d which proved s a t i s f a c t o r y i n the carbazole normal coordinate c a l c u l a t i o n . B. S e l e c t i o n Rules Dibenzothiophene c r y s t a l l i z e s i n the monoclinic system w i t h space group ^2h ^ 2 j / c ) T h e p o s i t i o n s of the fou r molecules i n the u n i t c e l l , and the r e l a t i v e d i s p o s i t i o n s of the molecular axes (x_>y,z) , the e x t i n c t i o n d i r e c t i o n s (j_,b_,s_) i n the c r y s t a l , and the c r y s t a l l o g r a p h i c axes ( 6 0 0 TOO \u00E2\u0080\u00A2> \u00E2\u0080\u00A2 wavenumber (cm-') F i g . 33 The p o l a r i z e d i n f r a r e d spectrum of dibenzothiophene i n the low frequency r e g i o n . Dotted l i n e / / r ; s o l i d l i n e //b_. 400 6QO 800 IOOO 1200 1400 1600 1800 Cm\"1 F i g . 34 The i n f r a r e d s p e c t r a of dibenzothiophene s i n g l e c r y s t a l , (a) Dotted l i n e //s_; s o l i d l i n e / / r _ . (b) Dotted l i n e //b; s o l i d l i n e / / r . - 159 -100 200 300 400 500 600 cm\"1 F i g . 35 The i n f r a r e d spectrum of dibenzothiophene i n cyclohexane ( s o l i d l i n e ) and benzene (broken l i n e ) s o l u t i o n s i n the low frequency r e g i o n . II \u00E2\u0080\u0094I , , , : , , , , yf. ! :\u00E2\u0080\u0094 6 0 0 8 0 0 IOOO 1200 W O O 1600 1800 2 C O O 3 C O O 3 2 C O . wavenumber (cm-1) F i g . 36 The i n f r a r e d spectrum of dibenzothiophene i n carbon d i s u l p h i d e , carbon t e t r a c h l o r i d e and t e t r a c h l o r o e t h y l e n e s o l u t i o n s . _ 160 Table 35. The i n f r a r e d spectrum of dibenzothiophene i n the c r y s t a l and s o l u t i o n . S o l u t i o n / / r \u00E2\u0080\u00A2//b //s Symmetry 53 w 53 w b u 72 vw? a u 89 m a u 104 w 101 w b u 109 vw a u 121 s 138 vw 137 ms 138 s B 210 m 213 s 218 mw k 224 s 229 w 224 ms 228 ms B 279 w B 2 comb? 282 m A 1 comb? 405 m 403 s Aj 416 s 425 w 417 m 424 s Bj 433 vw 433 w B 1 comb 456 vw A^ comb 462 vw B 2 495 s 494 m A 1 520 ms 499 m 496 m B 2 508 sh 559 m 561 mw 560 vw B 2 608 m 612 m B 2 701 m 704 s 705 w B 2 706 mw 712 sh 711 sh A 726 vw 724 vw B, 739 vs 735 vs 744 vs Bj 753 in ? 768 s 770 s 773 vw , 772 s A. 794 vw B1 850 w . 854 vw _ Aj - 161 -S o l u t i o n / / r //b_ //s Symmetry 859 mw B l 865 m 872 mw 866 m B2 891 w 900 w 890 w B l 930 mw 934 s 934 s 934 vw A l 940 s B l 966 s B2 970 s 973 m V 992 w 988 m 992 m A i 1000 sh 1001 m 1000 vw B2 1024 s 1028 s 1026 s 1026 mw A l 1054 w 1053 vw A l 1063 vw B2 1066 m 1068 s A l 1072 s 1074 s 1072 m B2 1105 vw A l 1121 vw 1120 sh B2 1131 m 1134 s 1132 s 1132 vw A l 1145 sh B2 1157 m 1155 mw 1156 ms 1155 m A r B 1 ? 1166 w 1163 vw V 1179 vw B2 1196 w 1201 m 1203 ms 1203 m A l 1226 s 1232 m 1233 vs 1235 s A. i 1262 w 1268 ms 1265 mw 1270 w . B2 1282 vw A l 1305 sh 1314 s 1313 ms B2 1309 s 1317 s 1320 m A l 1334 vw 1335 m 1533 vw A l 1350 vw 1353 m B2 1393 vw B2 1412 w 1415 w 1424 s 1417 vs 1425 s A l 1442 s 1442 s B2 - 162 -S o l u t i o n //b / / s Symmetry 1454 m 1466 w 1510 w 1555 w 1594 s 1620 w 1676 vw 1703 vw 1780 w 1815 vw 1864 vw 1899 w 1935 w 3010 vw 3025 w 1462 s 1497 sh 1514 m 1564 vw 1589 s 1600 w 1623 mw 1645 vw 1662 mw 1681 m 1706 m 1739 w 1780 w 1792 w 1816 m 1826 w 1871 m 1896 sh 1907 m 1937 ms 2880 w 3020 s 1475 mw 1493 mw 1557 m 1577 vw 1594 s 1627 mw 1638 sh 1677 mw 1705 mw 1734 vw 1781 ms 1793 sh 1817 w 1826 w 1897 sh 1907 ms 1934 mw 1973 w 2893 w 3000 sh 3025 sh 1476 vw 1592 mw 1629 vw 1708 vw 1782 m 1793 sh 1829 vw 1899 sh 1909- mw 1937 w 2995 vw 3025 vw A, A, 3055 sh - 163 -S o l u t i o n / / r //b //s Symmetry 3063 vs 3055 vs 3055 vs 3055 ms 3121 vw 3117 mw D. Raman Spectra Raman sp e c t r a of dibenzothiophene c r y s t a l recorded f o r a l l p o s s i b l e p o l a r i z a t i o n arrangements o f the i n c i d e n t and s c a t t e r e d l i g h t are reproduced i n F i g . 37 and 38. The d e p o l a r i z a t i o n r a t i o s of the Raman l i n e s which were measured i n a carbon t e t r a c h l o r i d e s o l u t i o n are shown i n Table 37 together with the c r y s t a l data and the symmetry assignments of the bands. _ 164 _ (rr) | r ~ 1 1 \u00E2\u0080\u00A2 1 1 1 1 1 1 1 1 1 1 1 1 1\u00E2\u0080\u00947/ ' | ' 1 O 2 0 0 4 0 0 6 0 0 8 0 0 IOOO 1200 1400 1600 3 0 0 0 32CO wavenumber ( cm - 1) F i g . 37 Raman sp e c t r a of dibenzothiophene (sb) \u00E2\u0080\u0094 i \u00E2\u0080\u0094 \u00E2\u0080\u00941\u00E2\u0080\u0094'\u00E2\u0080\u00941\u00E2\u0080\u0094\u00E2\u0080\u00A2\u00E2\u0080\u00941\u00E2\u0080\u0094 eoo 000 120c w a v e n u m b e r ( c m - ' ) \u00E2\u0080\u00941\u00E2\u0080\u0094\u00E2\u0080\u00941\u00E2\u0080\u00941\u00E2\u0080\u00947,\u00E2\u0080\u00941\u00E2\u0080\u0094'\u00E2\u0080\u00941 w o o e o o 3000 3200 400 600 F i g . 38 Raman s p e c t r a of dibenzothiophene s i n g l e c r y s t a l . Table 36. Raman spectrum of dibenzothiophene i n carbon t e t r a c h l o r i d e s o l u t i o n and a s i n g l e c r y s t a l . CC1, P 277 w '406 ms ~ 0.75 0.17 C r y s t a l 138 171 218 235 286 '308 410 Symmetry A l 1 A l A, - 166 -CC1, P 704 s 1025 vs 1070 w 1134 ms 1160 vw 1230 ms 1318 vs 1480 m 1560 w 1605 s 3066 mw 0.14 0.09 0.53 0.24 ^ 0.75 0.21 0.21 0.31 0.54 0.44 0.0 C r y s t a l 421 438 498 505 704 730 747 771 939 1002 1027 1072 1122 1137 1172 1204 1236 1294 1315 1321 1337 1422 1433 1480 1558 1566 1580 1590 1601 1618 3060 Symmetry A l V A l A, A, - 167 -According to the oriented-gas p r e d i c t i o n s i n equation (8.1), the t o t a l l y symmetric v i b r a t i o n s are expected to dominate the ( r r ) spectrum, and the A^ as w e l l as the modes should appear w i t h about equal r e l a t i v e strengths i n both the (bb) and (ss) s p e c t r a . I t should be noted, however, that the t o t a l l y symmetric v i b r a t i o n s have the gr e a t e s t i n t r i n s i c s t r e n g t h (see the s o l u t i o n data i n Table 3~6) . With the a i d of the d e p o l a r i z a t i o n r a t i o s , the A^ l i n e s i n Table 36 were f i r m l y assigned. The B^ bands were assigned wi t h p a r t i c u l a r reference to the (rb) spectrum i n which they are p r e d i c t e d to have most of t h e i r r e l a t i v e i n t e n s i t y , a f t e r s u b t r a c t i n g out the c o n t r i b u t i o n s from the t o t a l l y symmetric l i n e s . On the b a s i s of the Raman s p e c t r a i n F i g . 37 and 38, the 235 cm 1 band cannot be assigned. However, i t was tempting to assign the l i n e as a B^ species s i n c e a 226 cm 1 band appeared i n the i n f r a r e d spectrum. I f t h i s were the case, however, the 235 cm 1 l i n e should appear i n the (bb) spectrum and not i n the (rs) spectrum. I t i s u n l i k e l y that misalignment of the c r y s t a l can account f o r the observed r e s u l t s so that appreciable c r y s t a l mixing of the A-^ and B^ fundamentals has probably taken p l a c e . The appearance of a l i n e at 287 cm 1 i n the (rb) and (rs) s p e c t r a favours a B^ assignment, but t h i s i s brought i n t o question by i t s appearance i n the (ss) and (sb) s p e c t r a as w e l l . The s i t u a t i o n i s f u r t h e r complicated by a comparison w i t h the 299 cm 1 carbazole band and a 288 cm 1 f l u o r e n e band both of which have A^ symmetry. The weak s a t e l l i t e bands at 1122, 1294, 1337, and 1618 cm f o r example, are probably combination bands d e r i v i n g t h e i r i n t e n s i t y from the strong bands nearby. _ 168 _ E. Raman A c t i v e L a t t i c e Modes The l a t t i c e v i b r a t i o n s shown i n F i g . 37 and 38 are q u i t e s t r o n g , and were recorded under c o n d i t i o n s which reduced the i n t e n s i t y o f the bands by a f a c t o r of about three compared with the r e s t of the spectrum. The l a t t i c e frequencies and t h e i r assignments are t a b u l a t e d i n Table 37. Table 37. Raman spectrum of dibenzothioph ene near the e x c i t i n g l i n e . (bb) ( r r ) (ss) (IS) (bl) (bs) Symmetry 34 33 32 a ,b g ;.e 49 a . g 64 64 b . g 78 79 78 79 81 80 a ,b g'/g 104 104 104 104 126 104 a ,b g g b ? . g In a recent paper, the Raman a c t i v e l a t t i c e v i b r a t i o n s of anthracene and naphthalene which are a l l due to l i b r a t i o n a l modes (hindered r o t a t i o n s ) were adequately accounted f o r by co n s i d e r i n g the r o t a t i o n a l o s c i l l a t i o n s about the three p r i n c i p a l axes of the molecule. I t i s i n t e r e s t i n g to use t h i s method i n i n t e r p r e t i n g the low frequency Raman a c t i v e modes. In monoclinic c r y s t a l s w i t h four molecules i n the u n i t c e l l , s i x Raman a c t i v e l i b r a t i o n a l modes appear i n three p a i r s ; each p a i r c o n s i s t s of an a and b mode (see Table 33) . The r o t a t i o n a l o s c i l l a t i o n s about the x_, y_, z_ axes of the molecule transform l i k e the B^, B^, and i r r e d u c i b l e r e p r e s e n t a t i o n s of the C, molecular po i n t group. Since a r o t a t i o n a l o s c i l l a t i o n about a - 169 -p r i n c i p a l molecular a x i s and an i n t r a m o l e c u l a r v i b r a t i o n belonging to the same symmetry species are expected to behave i n the same manner, the r e l a t i v e i n t e n s i t y d i s t r i b u t i o n of a l i b r a t i o n a l mode among the various p o l a r i z e d s p e c t r a can be deduced from-equation (8.1). For c l a r i t y the appropriate e n t r i e s are tabu l a t e d i n Table 38. Table 38. R e l a t i v e i n t e n s i t i e s of Raman a c t i v e modes due o s c i l l a t i o n s about the x_, z -molecular axes various p o l a r i z e d s p e c t r a . w w w ( r r ) 0. .0195 0. . 0005 0. ,0599 (bb) 0, .0003 0. .6916 0. ,0001 (ss) 0. .0154 0. .6691 0. .0534 (rb) 0. .7562 0. .0202 0. .2174 (rs) 0. .2244 0. .0000 0. ,7210 (bs) 0. .0127 0. .3016 0, .0050 to r o t a t i o n a l among the Tr e a t i n g the l i b r a t i n g molecule as a harmonic o s c i l l a t o r , the frequency of o s c i l l a t i o n (v) i s 1 k 1 / 2 v = 2 7 ( T ) (8 . 2 ) where k i s the force constant and I the moment of i n e r t i a . Since the moments of i n e r t i a of the dibenzothiophene molecule about the x, z_, y_ axes decrease i n that order, equation (8.2) p r e d i c t s that the l i b r a t i o n a l - 170 -frequencies a l s o increase i n the same order. I f only the three strongest l a t t i c e modes are considered, t h i s simple treatment of the o r i g i n s of the l i b r a t i o n a l modes i s i n agreement with the f o l l o w i n g observations: ( i ) The lowest frequency l i n e at 33 cm 1 was c o r r e c t l y p r e d i c t e d to appear w i t h most of i t s i n t e n s i t y i n the (rb) spectrum and some of i t s i n t e n s i t y i n the (rs) spectrum; ( i i ) the intermediate frequency at 79 cm 1 w i t h most o f i t s i n t e n s i t y i n the (rs) spectrum and w i t h some i n t e n s i t y i n the (rb) spectrum; and ( i i i ) the highest frequency at 104 cm 1 appeared with about equal i n t e n s i t y i n the (bb) and (ss) s p e c t r a , and w i t h reduced i n t e n s i t y i n the (sb_) spectrum. F. Assignment of Fundamentals The c r i t e r i a f o r s e l e c t i n g a fundamental were p r e v i o u s l y discussed i n connection w i t h the assignment of the carbazole and flu o r e n e fundamental v i b r a t i o n s . A^ Symmetry There are 20 fundamental v i b r a t i o n s belonging to the A^ s p e c i e s ; 16 of these modes are expected to have frequencies l e s s than 2000 cm 1. In the i n f r a r e d spectrum, the strong l i n e s at 215, 403, 494, 712, 934, 1026, 1068, 1133, 1203, 1233, 1318, 1421, 1557 and 1593 cm\"1 were assigned as A 1 funda-mentals. These assignments were supported by the Raman bands of A^ symmetry observed at 218, 409, 498, 704, 1027, 1071, 1136, 1238, 1321, 1479, 1559, and 1601 cm 1. The frequency d i f f e r e n c e s between the bands common to both the i n f r a r e d and Raman spectrum are w i t h i n the allowable l i m i t s o f experimental accuracy and f a c t o r group s p l i t t i n g . The two sets of v i b r a t i o n s account f o r 15 of the 16 fundamentals. The l a s t A^ , fundamental was located as the strongest, l i n e i n the phosphorescence spectrum at 849 cm 1. The - 171 -counterpart of the 849 cm 1 i n t e r v a l i n the i n f r a r e d spectrum i s the weak band at 854 cm Although a f a i r l y strong i n f r a r e d l i n e at 1156 cm 1 appears to have A^ symmetry, i t was r e j e c t e d as a fundamental f o r the f o l l o w i n g reasons: ( i ) The d e p o l a r i z a t i o n r a t i o of the Raman l i n e at 1160 cm 1 i n d i c a t e d that i t was n o n - t o t a l l y symmetric, ( i i ) the band d i d not have any a c t i v i t y i n e i t h e r the fl u o r e s c e n c e or phosphorescence spectrum. Three of the four CH s t r e t c h i n g frequencies were t e n t a t i v e l y placed at 3000, 3025, and 3055 cm 1 . B e t t e r resolved i n f r a r e d s p e c t r a of t h i n n e r c r y s t a l s are needed to improve t h i s assignment. B 2 Symmetry The 15 B 2 fundamentals with frequencies l e s s than 2000 cm 1 were assigned from the r - p o l a r i z e d i n f r a r e d spectrum. Bands which were e i t h e r f a i r l y i n t e n s e or w e l l separated from other stronger bands were observed at 560, 613, 865, 966, 1001, 1268, 1353, 1442, 1462, and 1514 cm\"1; these frequencies were assigned as fundamentals. With the a i d of the i n f r a r e d s o l u t i o n spectrum, the r_-polarized bands at 496, 704, 1074, and 1314 cm - 1 were i d e n t i f i e d as separate l i n e s which were not f a c t o r group components of Aj or B^ l i n e s , and were a l s o assigned as B^ fundamentals. The p r e v i o u s l y d i s c u s s e d 1156 cm 1 i n f r a r e d band was favoured as the f i n a l fundamental sought s i n c e i t brings the B 2 fundamental block of frequencies i n t o c l o s e correspondence with those of carbazole and fluorene (see Appendix I I ) . The comment on the need f o r b e t t e r i n f r a r e d s p e c t r a to l o c a t e the A^ CH s t r e t c h i n g frequencies a l s o a p p l i e s to the a s s i g n i n g the four B 2 CH s t r e t c h i n g fundamentals. The i n f r a r e d bands at 3025, 3055 and 3117 cm\"1 were thought to mark the presence of three B_ fundamentals. - 172 -Symmetry The assignment of the fundamentals must r e s t e n t i r e l y on the p o l a r i z e d i n f r a r e d spectrum. Seven fundamentals were l o c a t e d at 138, 226, 421, 744, 770, 859, and 940 cm 1. A f a i r l y weak band at 895 cm 1 which was w e l l separated from other bands was assigned as another fundamental. Of the two p o s s i b l e B^ fundamentals at 724 and 794 cm 1 , the l i n e at 724 cm 1 was a r b i t r a r i l y s e l e c t e d to make up the f u l l complement of B^ fundamentals. A 2 Symmetry None of the A 2 fundamentals were i d e n t i f i e d i n the Raman spectrum. As p r e v i o u s l y p ointed out, the l i n e at 287 cm 1 may be the analogue of the A 2 Raman a c t i v e fundamental i n flu o r e n e at 287 cm 1 and i n carbazole at 299 -1 cm G. I n f r a r e d and Raman Spectra of Dibenzothiophene-dg A complete study of the dibenzothiophene-d c r y s t a l r e q u i r e s a p o l a r i z e d 8 i n f r a r e d spectrum of the r s - c r y s t a l face i n order to d i f f e r e n t i a t e between A^ and B^ fr e q u e n c i e s , and a s o l u t i o n Raman spectrum to provide d e p o l a r i z a -t i o n r a t i o s to a s s i s t i n s o r t i n g the t o t a l l y symmetric from the n o n - t o t a l l y symmetric v i b r a t i o n s . The i n f r a r e d s p e c t r a w i t h p o l a r i z e d l i g h t i n c i d e n t on the r b - c r y s t a l face are shown i n F i g . 39 and 40, and the frequencies together w i t h t h e i r assignments are l i s t e d i n Table 39. An i n f r a r e d spectrum i n carbon t e t r a -c h l o r i d e , carbon d i s u l p h i d e , and t e t r a c h l o r o e t h y l e n e s o l u t i o n ( F i g . 41) was recorded i n the region 550-2400 cm 1 , and i n cyclohexane s o l u t i o n ( F i g . 42) i n the re g i o n 80-700 cm 1. The r e s u l t s are inc l u d e d i n Table 39, and were used to d i s t i n g u i s h between d i s t i n c t o p p o s i t e l y p o l a r i z e d bands and f a c t o r group components of a s i n g l e f r e e molecule band. i 1 1 1 1 n 1 1 1 ~ i 1 1 1 1 1 i i i r~~ 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0 2 5 0 0 W A V E N U M B E R (cm\" 1) F i g . 40 I n f r a r e d s p e c t r a of dibenzothiophene-d\u00E2\u0080\u009E, c r y s t a l t h i c k n e s s 0.32 mm; s o l i d l i n e //b, o \u00E2\u0080\u0094 broken l i n e / / r . 1 1 1 1 1 1 \u00E2\u0080\u00A2 1 1 1 I 6 0 0 8 0 0 IOOO I 2 0 0 I 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 2 4 0 0 wavenumber ( c m - ' ) F i g . 41 I n f r a r e d spectrum of dibenzothiophene-dg i n carbon d i s u l p h i d e , carbon t e t r a c h l o r i d e , and t e t r a c h l o r o e t h y l e n e s o l u t i o n s . 100 200 3 0 0 4 0 0 5 0 0 6 0 o c m _ F i g . 42 I n f r a r e d spectrum of dibenzothiophene-d_ i n cyclohexane s o l u t i o n , i n the low-frequency r e g i o n . - 177 -Table 39. The i n f r a r e d spectrum of dibenzothiophene-dg S o l u t i o n //b_ / / r _ Symmetry 52 m b u 71 w b u 87 m a u 101 vw b u 106 w a u 115 m 126 vs 130 m B 1 135 sh ? 150 mw ? 196 m 198 vs 201 w A 215 s 215 vs 219 m Ai\u00C2\u00BBBi 261 s 58 m366 vs 363 s 372 m V B i 286 w A i ' B l V B i 390 m 388 ms 391 vw A1 403 vw . 402 w B 2 417 vw 419 vw B, 435 sh ' 438 vw B, 449 s 445 s 453 mw B 1 475 mw 472 ms B 2 484 ms 480 sh A J 494 w 490 s B\u00E2\u0080\u009E 508 vw 508 vw A 1 ' B 1 516 vw B 2 522 mw i 542 vw B' 561 s 561 ms B 1 S 571 ms B 2 583 w B 2 593 w 592 s B 566 ms ,B^ 2 - 178 -S o l u t i o n //b //r Symmetry 614 w 660 m 713 m 750 m 806 ms 835 w 844 w 860 m 907 vvw 940 vvw 972 vvw 991 w 1015 mw 1040 vs 1051 vw 622 mw 631 vw 664 s 727 s. 749 vw 761 ms 777 w 791 vvw 804 w 828 m 840 sh 845 ras 866 ms 886 w 895 vvw 911 ms 956 vw 978 vw 1014 sh 1021 w 1045 vw 1056 s 598 sh 635 w 646 vw 672 vs 709 vw 723 mw 748 m 763 ms 775 s 791 vvw 815 ms 833 s 854 s 886 vw 895 vw 910 w 948 s 965 vw 978 w 993 w 1015 s 1040 vs 1063 sh B 2 1 A, ,B V B 1 A 1 B 1 V B i V B i V B 2 A r B i A; ,I A I B2 B2 V A l B2 B2 A r B i - 179 -S o l u t i o n //b / / r Symmetry 105S w 1154 w 1188 sh 1199 m 1215 w 1260 vw 1286 vs 1320 vs 1335 w 1357 ms 1379 w 1417 mw 1443 vvw 1458 vvw 1478 vw 1533 w 1537 w 1555 sh 1564 ms 1576 m 1078 w 1120 sh 1125 mw 1143 w 1158 w 1.175 mw 1198 vs 1221 sh 1230 vw 1243 w 1255 m 1280 s 1320 vs 1353 vw 1371 sh 1382 sh 1390 m 1416 ms 1447 vw 1465 w 1521 w 1063 sh 1121 vw 1132 vw 1143 vw 1160 s 1203 m 1218 mw 1229 w 1265 w 1286 v s ; 1330 vs 1355 s 1369 sh 1380 m 1390 sh 1417 vw 1435 vvw 1446 m 1465 sh 1478 ms 1505 w 1530 m 1560 vs V B i V B i B2 B2 A l A, V B i V B i V B i A 1 ' B 2 1568 vs 1606 w - ISO -S o l u t i o n //b Hi. Symmetry 1614 w 1615 s B2 1625 w V B 1 1688 mw B2 1713 mw V B 1 1736 w B2 1763 vw 1766 w B 9 1767 mw B2 1806 w B2 1803 sh 1804 vw B2 1821 mw V B 1 1836 mw V B 1 2170 vw B2 2205 vw B2 2220 vw A l 2275 s A l 2284 s 2280 vs B2 2306 w 2315 sh A l 2345 w B2 2365 sh B2 2380 vw A l The p o l a r i z e d Raman spectrum of dibenzothiophene-'d c r y s t a l i s repro-8 duced i n F i g . 43 and 44; the frequencies together with t h e i r assignments are c o l l e c t e d i n Tables 40 and 41. The assignment of a Raman band i s based on i t s r e l a t i v e i n t e n s i t y d i s t r i b u t i o n among the various p o l a r i z a t i o n arrangements as p r e d i c t e d by equation (8.1). 181 ^ (ss) V JV)JLA_ -A_ I 7/ 1 1 B O O 2 Z O O 2 4 O 0 2 0 O 2 Symmetry In the absence of a p o l a r i z e d f l u o r e s c e n c e spectrum and lack of c o r r o b o r a t i o n from the Raman spectrum, the only b a s i s f o r the s e l e c t i o n of 185-fundamentals was i t s i n t e n s i t y i n the _r-pola r i z e d i n f r a r e d spectrum. Reasonable confidence may be placed i n the f a c t that the strong i n f r a r e d bands at 472, 492, 592, 672, 775, 815, 833, 854, 948, 1160, 1286, 1335, 1446 and 1478 cm 1 represent 14 of the 15 B^ fundamentals expected to have frequencies l e s s than 2000 cm 1. The sample was f a r too t h i c k f o r the i n f r a r e d r a d i a t i o n t o adequately penetrate the r e g i o n 1000-1600 cm 1 so that the p r e c i s e l o c a t i o n of some of the bands must await the r e s u l t s of f u r t h e r experimentation. The remaining B^ fundamental was q u i t e a r b i t r a r i l y chosen as the 1380 cm 1 band. No B^ fundamentals were assigned i n the re g i o n beyond 1500 cm 1 s i n c e the highest B^ fundamental frequency i n the protonated compound was 1520 cm and the v i b r a t i o n s of the deuterated species are expected to be s h i f t e d to lower f r e q u e n c i e s . The 2000-2500 cm 1 r e g i o n was not s u f f i c i e n t l y r e s o l v e d i n order to l o c a t e the four CD s t r e t c h i n g frequencies w i t h B 2 symmetry. A 2 Symmetry The A 2 fundamentals are expected to be a c t i v e only i n the Raman spectrum (Table 41) with most of t h e i r i n t e n s i t y i n the (rs) spectrum (see equation 8.1). Since the Raman bands at 156, 251, 645, and 746 cm 1 f u l f i l these requirements, they are assigned as fo u r of the nine A^ fundamental v i b r a t i o n s . Raman A c t i v e L a t t i c e Modes of Dibenzothiophene-dg In the region near the e x c i t i n g l i n e , the c l o s e resemblance of the Raman spectrum of dibenzothiophene-d and -h\u00E2\u0080\u009E i s not s u r p r i s i n g . The o o simple e x p l a n a t i o n l i e s i n i n t e r p r e t i n g the observed modes as r o t a t i o n a l o s c i l l a t i o n s i n the harmonic approximation. Since the i n t e r m o l e c u l a r f o r c e constants are the same i n both compounds, the r a t i o of the frequencies of the deuterated and protonated species i n the harmonic approximation i s - 186 -VH D In s p e c t i o n o f Table 42, which l i s t s the moments of i n e r t i a of the two i s o t o p i c species about the p r i n c i p a l molecular axes, shows t h a t the r a t i o ( Vp/v^j ) i s very c l o s e to one i n a l l cases. This accounts f o r the s i m i l a r -i t y of the low frequency regions i n the two compounds. 3. Table 42. Moments of i n e r t i a about the molecular axes of dibenzothiophene. Compound I X T y \ dibenzothiophene-h o 1927 510 1417 dibenzothiophene-d o 2119 561 1558 Units x 10 gm-cm I. Normal Coordinate C a l c u l a t i o n The r e l e v a n t d e t a i l s p e r t a i n i n g to the c a l c u l a t i o n of the in-plane fundamental frequencies of dibenzothiophene-h Q and -d were p r e v i o u s l y o o o u t l i n e d f o r the carbazole problem i n chapter 3. The bond lengths and bond angles used f o r the c o n s t r u c t i o n o f the G-matrix are shown i n F i g . 10. The in- p l a n e i n t e r n a l coordinate d e f i n i t i o n s are i l l u s t r a t e d i n F i g . 45, and most of the values o f the f o r c e constants are obtained from Table 11 which i s a p p l i c a b l e i f N i s replaced by S. Those f o r c e constants which were changed s l i g h t l y are shown i n Table 43. There were no a v a i l a b l e estimates of the values f o r the CS s t r e t c h i n g and CSC angle bending f o r c e constants, - 187 -and the values f o r the CC s t r e t c h i n g and CCC angle bending f o r c e constants were used i n t h e i r p l a c e . F i g . 45 In-plane i n t e r n a l coordinates of dibenzothiophene Table 43. In-plane f o r c e constants of dibenzothiophene. Type Force Constant Value Hccc H n = H e a) 0.91 H = H X, n 0.73 Hcsc H E 0.94 HHCC H . = H . 0 A 0.51 H = H y IT 0.50 Force constants not l i s t e d i n t h i s t a b l e were given the same value as those used f o r carbazole i n Table 11. J . D i s c u s s i o n The assignments of the in-plane fundamental frequencies of dibenzo-thiophene-h and -dg are given i n Table 44 together with the r e s u l t s of the normal coordinate c a l c u l a t i o n . The c a l c u l a t i o n was i n the nature of - 188 -a p r e l i m i n a r y t r i a l , and the r e s u l t s were not used as a guide i n making assignments. No refinement of the f o r c e f i e l d was undertaken i n order to improve the agreement w i t h the observed frequencies. The f a c t that such an approximate c a l c u l a t i o n can give reasonable agreement between the observed and c a l c u l a t e d frequencies f o r both dibenzothiophene-h and -d e s t a b l i s h e s o o the e s s e n t i a l correctness of the f o r c e f i e l d . A comparison of the normal coordinate treatments of phenanthrene, c a r b a z o l e , and dibenzothiophene support the observation that a simple valence f o r c e f i e l d t r a n s f e r r e d among g e o m e t r i c a l l y s i m i l a r l a r g e organic molecule can give an adequate account of the normal f r e q u e n c i e s . I t can be f u r t h e r concluded that the values of the f o r c e constants a s s o c i a t e d w i t h the CSC angle bending and CS bond s t r e t c h i n g coordinates i n dibenzothiophene are very s i m i l a r to those f o r the CCC and angle bending and CC bond s t r e t c h i n g coordinates i n phenanthrene. A comparison o f the assignments o f the fundamental frequencies of dibenzothiophene w i t h carbazole and fl u o r e n e i s presented i n Appendix I I . Although the incompleteness of the i n f r a r e d data of dibenzothiophene-d has o been i n d i c a t e d p r e v i o u s l y , i t i s mentioned again to emphasize that the assignment o f the dibenzothiophene fundamentals must be considered t e n t a t i v e . 33 According t o the T e l l e r - R e d l i c h product r u l e , the p r e d i c t e d r a t i o of the product of the frequencies f o r dibenzothiophene-d compared w i t h dibenzo-o thiophene-h i s 0.064 f o r the A. symmetry block. The r a t i o c a l c u l a t e d o I from the observed frequencies i s 0.17. The poor agreement between the observed and p r e d i c t e d r a t i o s need not n e c e s s a r i l y i n d i c a t e gross d i s c r e p -ancies i n the assignments, but p o i n t s out the need f o r a more complete set of data. The r e s u l t s f o r the symmetry block are p r e d i c t e d r a t i o (0.27) and c a l c u l a t e d r a t i o (0.18). The assignments f o r the A^ and B^ symmetry blocks are not complete so that a product r u l e c a l c u l a t i o n i s not p o s s i b l e . - 189 -F i n a l l y , i t i s r a t h e r i n t e r e s t i n g that an a l t e r n a t i v e e x p l a nation (apart from misalignment of the c r y s t a l ) f o r the appearance of what i s apparently the same Raman a c t i v e l a t t i c e mode i n s e v e r a l p o l a r i z a t i o n s i s p o s s i b l e by t r e a t i n g the molecule as a l i b r a t i n g harmonic o s c i l l a t o r . This simple treatment accounts reasonably w e l l f o r the experimental observations. Table 44. A comparison of the observed and c a l c u l a t e d fundamentals f o r dibenzothiophene (DBT-h ) and 8 deuterodibenzothiophene (DBT-dg). Species DBT-hg DBT-dg Observed C a l c u l a t e d Observed C a l c u l a t e d A l . . . 3090 2309 2299 3055 3069 2284 2284 3025 3044 2271 2266 3000 3025 2225 2255 1593 1657 1568 1617 1557 1609 1521 1546 1475 1550 1417 1422 1421 1503 1320 1389 1318 1427 1283 1340 1233 1401 1201 1212 1203 1266 1175 1192 1133 1248 1125 1057 1068 1226 1059 924 1026 1136 911 902 934 1051 866 878 .85.4 808 828 779 712 743 664 705 494 505 484 494 403 386 390 372 215 223 199 209 - 190 -Species DBT-hg DBT-\" d8 Observed C a l c u l a t e d Observed C a l c u l a t e d B2 3117 3090 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 2300 3055 3069 2284 . . . 3044 2267 3025 3025 2255 1514 1643 1478 1599 1462 1618 1446 1566 1442 1546 1380 1430 1353 1530 1335 1363 1314 1441 1286 1338 1268 1367 1160 1151 1156 1259 948 1089 1074 1204 854 972 1001 1139 833 921 966 1067 815 899 865 997 775 870 704 781 672 751 613 623 592 606 560 469 492 452 496 458 472 441 Species DBT-hg DBT-d Species 8 DBT-h o DBT-d 8 A 2 156 B r 940 845 287? 251 895 762 385 859 720 645 770 622 746 744 561 724 421 424 366 226 217 138 128 Chapter 9 A Study of Some E l e c t r o n i c States of Dibenzothiophene. A. I n t r o d u c t i o n In the complex dibenzothiophene absorption spectrum i n s o l u t i o n , the o o two systems at 3260 A (f=0.0299) a n d 2870 A (f=0.0665) almost c e r t a i n l y represent separate t r a n s i t i o n s , but each of the more complicated regions o o beginning about 2675 A ( f = 0.222) and 2450 A ( f = 0.645) may represent more than a s i n g l e t r a n s i t i o n . P l a t t 1 ^ suggested that the assignments of these bands, i n order of decreasing wavelength, are s h o r t , long, long, and short a x i s p o l a r i z e d , r e s p e c t i v e l y , by comparison with the assignments o o f o r phenanthrene. The assignment of the 3260 A and 2870 A systems agrees 89 with S i e g e l and J u d e i k i s ' r e s u l t s obtained from broad-band magnetophoto-s e l e e t i o n experiments on dibenzothiophene i n d i e t h y l ether s o l u t i o n at' 123 77\u00C2\u00B0K, and i s supported by Dorr's p h o t o s e l e c t i o n experiments. In t h i s chapter, the work on dibenzothiophene i s confined to as s i g n i n g the symmetry of the lowest-energy s i n g l e t s t a t e , to i n v e s t i g a t i n g the e x c i t e d s t a t e v i b r a t i o n a l s t r u c t u r e from the absorption spectrum, and to studying the ground s t a t e v i b r a t i o n a l s t r u c t u r e from fluorescence and phosphorescence s p e c t r a . B. Absorption Spectrum Microdensitometer t r a c i n g s of dibenzothiophene absorption s p e c t r a i n n-heptane and f l u o r e n e matrices are reproduced i n F i g . 46, and an a n a l y s i s -192 -30000 30 500 31OOO cm-' 46 Absorption' s p e c t r a of dibenzothiophene i n n-heptane (above) and i n fluorene (below) at about 15\u00C2\u00B0K. The energy s c a l e r e f e r s only to dibenzothiophene i n fluorene.. - 193 -of the v i b r a t i o n a l s t r u c t u r e i n hexamethylbenzene and n-heptane i s set out i n Table 4 5 , The v i b r a t i o n a l s t r u c t u r e of dibenzothiophene i n fl u o r e n e m a t r i x i s broad even at the lowest temperatures a t t a i n a b l e so that v i b r o n i c symmetry assignments are not p o s s i b l e . In n-heptane, the most intense l i n e s i n the lea d i n g m u l t i p l e t were l o c a t e d as resonance l i n e s at 30 205, 30 341, 30 364 and 30 423 cm 1. For s i m p l i c i t y and c l a r i t y , the a n a l y s i s Table 45. Absorption s p e c t r a o f dibenzothiophene i n n-heptane and hexamethylbenzene (HMB) at about 15\u00C2\u00B0K. HMB Int n-heptane Int Remarks 30 760 s 30-423 s o r i g i n 215 m 213 s 213, F a 267 w 267 w ? 396 w 396,F 405 w 402 s 402,F 420 m 2 x 213 - 6 477 m 485 s 485,F 681 w ? 689 m 693 ms 693, F 870 m 881 mw 402 + 485 - 6 891 mw 891, F 995 mw 1001 ms 1001,F 1042 w 1083 w 1087 m 1087,F 1215 vw 1208 m 1208,F 1257 vw 1253 w ? 1320 mw 1308 m 1308,F 1417 w ? 1439 w 1432 w 1432,F 1486 mw 1486,F - 194 -HMB Int n-heptane Int Remarks 1521 mw 1521,F; 213 + 1308 .1569 mw 1569,F 1648 w ? 1729 w 1 1790 w 485 + 1308 - 3 1998 w 693 + 1308 - 3 F i n d i c a t e s that the i n t e r v a l may correspond to an e x c i t e d s t a t e fundamental. i n Table 4 5 i s based on those i n t e r v a l s a s s o c i a t e d w i t h the p r i n c i p a l o r i g i n at 30 423 cm 1 . Although the absorption spectrum i n hexamethyl-benzene matri x i s completely d e p o l a r i z e d , the bands axe q u i t e sharp, and are i n c l u d e d i n Table 45 f o r comparison with the n-heptane a n a l y s i s . 1 C. Fluorescence Spectra The fluorescence s p e c t r a i n f l u o r e n e and n-heptane m a t r i c e s , and the c r y s t a l are shown i n F i g . 47 and the v i b r a t i o n a l i n t e r v a l s are given i n Table 46. The assignments of the fluorescence bands i n n-heptane are based on the i n t e r v a l s a s s o c i a t e d with the p r i n c i p a l o r i g i n at 30 423 cm 1 . The flu o r e s c e n c e o r i g i n i n the fluorene matrix i s w e l l p o l a r i z e d along the b - a x i s , but the v i b r o n i c bands i n the r e s t of the spectrum 'are e s s e n t i a l l y d e p o l a r i z e d , and are too broad to be measured p r e c i s e l y enough f o r use i n a v i b r a t i o n a l a n a l y s i s . Dibenzothiophene was a l s o studied i n a biphenyl matrix. The r e s u l t s , apart from a solvent s h i f t , are s i m i l a r to fl u o r e n e i n that the absorption and fluorescence are a l s o broad. The c r y s t a l f l u o r e s e n c e i s q u i t e sharp with an average band width of 10 cm \ and has most - 195 -27500 28 000 28 500 29 OOO 29 500 30 000 in rO | i | | | | 27 500 28 OOO 28 500 29 OOO 29 500 30 OOO 28 OOO 28 500 29OOO 29500 30000 30 500 W A V E N U M B E R (cm-') 47 Dibenzothiophene fluorescence s p e c t r a i n (a) a fluo r e n e m a t r i x , (b) the c r y s t a l , and (c) an n-heptane s o l u t i o n at about 15\"K. - 196 -of i t s i n t e n s i t y along the s - a x i s . Table 46. Dibenzothiophene fluo r e s c e n c e i n a fl u o r e n e m a t r i x , the c r y s t a l , and an n-heptane s o l u t i o n at about 15\u00C2\u00B0K. Fluorene matrix C r y s t a l //b //c I n t //s I n t . n-heptane I n t . Remarks 30 048 29 923 16 w 30 423 reabsorbed 38 w 42 63 s s ' 74 m 79 91 101 w m w 138 ms 138 w 210 ms 219 s 214 ms 214,A^ 258 ms 284 m 305 m 310 vw 321 ms 351 ms 354 vw 378 m 416 m 409 w 404 vw 404,A : 446 w 468 w -500 s 499 m 498 s 498,Aj 567 ms 609 w 632 m 665 w 706 m 707 w 705 717 m m 705,A : 214 + 498 + 5 - 197 -Fluorene matrix C r y s t a l //b //c I n t . //s I n t . n-heptane I n t . Remarks 738 vw 756 m 796 vw 814 vw 850 vw 850 vw 8 5 0 ^ 875 vw 997 vw 2 x 498 + 1 1023 w 1027 mw 1023 m 1 0 2 3 ^ 1078 w 1079 w 1079 w 1079 1132 w 1132 w 1132 mw 1132,A x 1154 vw 1164 w 1170 vw 1174 vw 1204 vw 1205 vw 1199 vw 1199; 498 + 1226 vw 1226 1239 vw 1237 w 1240 w 1240 1269 vw 1287 vw 1311 s 1315 vs 1310 vs 1310,A : 1349 m 1373 vw 1373 1403 m 1397 vw 1397 1415 vw 2 x 705 + 5 1474 w 1477 mw 1479 w 1479 1524 w 1528 mw 1526 m 214 + 1310 1569 vw 1588 vw 1591 vw 1591 1602 ms 1602 s 1603 s 1603,A 1 1636 w 1659 w 1692 w - 198 -Fluorene matrix C r y s t a l //b In t . //s Int. n-heptane I n t . Remarks 1728 w 1758 w 1769 vw 1811 vw 1821 w 1809 m 498 + 1310 + 1 1816 vw 1842 vw 1904 vw 1942 vw \u00E2\u0080\u00A2 2100 vw 2098 vw 2103 w 498 + 1603 + 2 2190 vw 2297 vw 2335 vw 2331 vw 1023 + 1310 + 2 2376 vw 2437 vw 2625 w 2621 w 2 x 1310 + 1 D- Phosphorescence Spectra Microdensitometertracings of the phosphorescence s p e c t r a of dibenzo-thiophene i n n-heptane and hexamethylbenzene m a t r i c e s , and from the c r y s t a l at about 15\u00C2\u00B0K are reproduced i n F i g . 48, and the v i b r a t i o n a l i n t e r v a l s together w i t h t h e i r assignments based on the c r y s t a l spectrum are given i n Table 47. With c o r r o b o r a t i o n from i n f r a r e d and Raman data l i s t e d i n Table 44, the symmetry assignments of the v i b r o n i c bands were made on the assump-t i o n that only A^ or/ fundamentals have appreciable i n t e n s i t y i n the phosphorescence spectrum,. The phosphorescence spectrum i n n-heptane i s p a r t i c u l a r l y complex; the components of the leading m u l t i p l e t are loc a t e d - 199 -F i g . 48 Phosphorescence s p e c t r a of dibenzothiophene i n [a) hexainethyl-benzene, (b) the c r y s t a l , and (c) n-heptane s o l u t i o n at about 15\u00C2\u00B0K. . - 200 -at 24 476, 24 417, 24 433, 24 395, 24 367, 24 344, 24 327, and 24 297 cm\"1. Only the bands a s s o c i a t e d w i t h the p r i n c i p a l o r i g i n at 24 417 cm 1 are inc l u d e d i n Table 47. Table 47. Phosphorescence s p e c t r a of dibenzothiophene i n n-heptane s o l u t i o n , hexamethylbenzene (HMB) matrix and c r y s t a l at about 15\u00C2\u00B0K. n-heptane I n t . HMB I n t . C r y s t a l Int Remarks 24 417 vs 24 096 vs 24 292 vs o r i g i n 16 s 40 ms 65 w 84 vw 110 m 145 m 180 m 210 m 223 ms 216 s 2 1 6 ' A 1 221 m 237 m 237? 254 w 216 + 4 0 - 2 -417- ms 413 ms 425 m 425,A x 429 ms 438 m 2 x 216 + 6 499 s 495 ms 498 s 498,A 1 514 \u00E2\u0080\u00A2 w 498 .+ 16 543 vw 498 +40 + 5 560 s 556 s 564 s 564,A 1 579 w 564 + 1 6 - 1 607 vw 564 + 4 0 + 3 632 m 628 vw 643 vw 216 + 4 2 5 - 2 707 s 707 s 706 s 706,A 1 725 mw 706 + 16 f 3 0 740 s 734 ms 746 m 746? - 201 --heptane I n t . HMB In t . C r y s t a l I n t . Remarks 768 s 767 m 771 m 771? 779 m 851 vvs 849 vvs 855 vvs 855 3A 1 870 m 875 m 855 + 1 6 + 4 894 m 855 + 4 0 - 1 920 m 216 + 706 - 2 934 s 931 ms 937 s 937,A x 954 w 937 + 1 6 + 2 1007 w 1003 w 1003,A 1 1024 w 1028 w 1026 ms 1026,A^ 1053 w 1038 w 1026 + 1 6 - 4 1061 vw (212 + 851 - 2) 1072 mw 1072 m 1070 ms 1070,A : 1093 vs 1087 w 1070 + 1 6 + 1 1118 vw 1118 m 1118? 1134 mw 1136 m 1133 ms 1 1 3 3 ^ 1153 ms 1153,k ; 1133 + 1 6 + 4 1171 ms 1133 + 4 0 - 2 ? 1204 w 1200 w 1201 m 1201,kx 1227 w. 1234 m 1234,A : 1267 w 1261 w 1268 w 564 + 706 - 2;498 + 771 1293 w 1290 vw 1285 w 216 + 1070 - 1 1315 w 1316 s 1316,A x 1333 w 1316 + .16 + 1 1349 m 1345 ms 1352 s 1352,A 1 1416 w 1412 w 1412 w 2 x 706 1431 vw 1441 w 1429 w 2 x 706 + 6 + 1 1450 w 216 + 1234 1476 w 1467 m 1476 m 1476,A : (1498)? w 1491 m 1494 w 2 x 746 + 2 1527 m 498 + 1026 + 3 1556 m 1554 ms 1559 s 1 5 5 9 ^ ? ; 706 + 855 - 2 1582 m 1585 w 1581 w 1581,A, 3 1 - 202 -n-heptane I n t . H M B I n t . C r y s t a l I n t . Remarks 1602 vs 1612 vs 1601 vs 1601 3A 1 1618 s 1601 + 16 + 1 1640 m 1601 + 40 - 1 1694 vw 1693 w 216 + 1476 + 1 1710 w 2 x 855 1718 m (707 + 1007 + 4; 1729 w (707 1024 - 2; 1744 w 425 + 1316 + 3 1773 w 425 + 1352 - 4 1792 w 771 + 1026 - 5 1824 mw 1816 m 216 + 1601 - 1 1839 mw 1834 w 216 + 1601 + 16 1855 w 216 1601 + 40 1878 m 1876 mw 1879 m 855 + 1026 - 2 1910 V/ 706 + 1201 + 3 1925 w 1965? 1984 m 1988 mw 1988 mw 855 + 1133 2035 w 2020 mw 2021 mw 425 + 1601 - 5 2056 w 2 x 1026 + 4 2103 m 2101 mw 2097 m 498 + 1601 _ 2 2139 vw 2 x 1070 ,+ 1 2164 ms 2166 m 2167 m 564 1601 + 2 2187 m 2189 mw 2178 2271 w 2272 vw 2262 w 2 x 1133 - 4 2311 mw 2316 mw 2305 w 706 + 1601 - 2 2342 mw 2339 w 1026 i + \u00E2\u0080\u00A2 1316 2385 w 2382 w 1026 1 -r \u00E2\u0080\u00A2 1352 + 4 2426 w 2417 w 706 + 2 x 855 + 2456 ms 2462 m 2454 m 855 + 160.1 - 2 2482 \u00E2\u0080\u00A2 w 2472 vw 855 + 1601 + 16 2490 . vw 855 1601 + 40 2538 m 2543 w 2536 w 937 + 1601 - 2 - 203 -n-heptane I n t . HMB I n t . C r y s t a l I n t . Remarks 2627 mw 2625 w 1026 + 1601 - 2 2676 mw 2684 w 2670 w 1070 -r 1601 - 1 2737 mw 2730 vw 2733 vw 1133 H r 1601 - 1 2750 vw 2755 vw; 1153 + 1601 + 1 2808 w 2813 W W 2802 VW 1201 -i- 1601 2834 vw 1234 + 1601 - 1 2877 vw 706 + 855 + 1316 2911 w 1316 + 1601 2953 mw 2957 vw 2952 w 1352 + 1601 3124 vw 1527 + 1601 - 4? 3160 mw 3168 vw 3159 vw 706 + 855 '+ 1601 - 3 3205 m 3226 w 3200 w 2 x 1601 - 2 3414 vw 216 + 2 x 1601 - 4 3448 vw 3479 w 3487 vw 855 '+ 1026 + 1601 + 6 3766 w 3775 VW 3773 vw 564 + 2 x 1602 + 7 . 3898 ' vw 3978 w 3983 vw 771 + 2 x 1601 + 10 4057 w 4073 vw 4058 vw 855 + 2 x 1601 + 1 E. D i s c u s s i o n o The pure e l e c t r o n i c o r i g i n of the weak absorption system at 3260 A of dibenzothiophene i n a fl u o r e n e matrix i s p o l a r i z e d along the b-axis ( F i g . 47), and i s assigned as an ^A^ \u00E2\u0080\u0094 *A^ t r a n s i t i o n i n agreement with P i a t t ' s p r e d i c t i o n , a n d S i e g e l and J u d e i k i s ' experimental r e s u l t . ^ The r e a b s o r p t i o n of the fluorescence o r i g i n so c h a r a c t e r i s t i c of medium stre n g t h and strong t r a n s i t i o n s i s c l e a r l y d i s p l a y e d i n F i g . 47. The fluorescence p o l a r i z e d along t h e . r - a x i s of the c r y s t a l probably gives a f a i r l y accurate i n d i c a t i o n of the i n t e n s i t y o f the o r i g i n band r e l a t i v e to - 204 -the r e s t of the bands i n the spectrum. Apparently, the breadth of both the absorption and f l u o r e s c e n c e . spectrum of dibenzothiophene i n f l u o r e n e and b i p h e n y l i s a m a n i f e s t a t i o n of guest-host i n t e r a c t i o n s which at present have no e x p l a n a t i o n . C l e a r l y , the i n t e r a c t i o n depends on the matrix because the absorption and f l u o r e s -cence observed i n the c r y s t a l and n-heptane i s sharp. Since only T T - T T * t r a n s i t i o n s are p o s s i b l e i n f l u o r e n e and n-n* as w e l l as T T - T T * t r a n s i t i o n s may be p o s s i b l e i n dibenzothiophene, an important c o n c l u s i o n may be drawn from a comparison of the fluorescence and phosphorescence s p e c t r a of these compounds i n n-heptane. Two observations support the general T T - T T * nature of the lowest-energy s i n g l e t and t r i p l e t t r a n s i t i o n s : ( i ) both the fluorescence and phosphorescence s p e c t r a of f l u o r e n e i n n-heptane Fig.47 and 48 show th a t the general features of the v i b r a t i o n a l s t r u c t u r e are s i m i l a r , and ( i i ) i n a l c o h o l , the dibenzothiophene s p e c t r a do not show the blue s h i f t that i s c h a r a c t e r i s t i c of an N - T T * t r a n s i t i o n . 1 2 ^ T h e o r e t i c a l s t u d i e s of thiophene 1 2*' and dibenzothiophene 1 2*^ have been concerned w i t h the c o n t r i b u t i o n of the sulphur d o r b i t a l s i n TT e l e c t r o n i c MO c a l c u l a t i o n s , but the r e s u l t s o f these c a l c u l a t i o n s are i n -c o n c l u s i v e s i n c e d o r b i t a l p a r t i c i p a t i o n has not been demonstrated. The decreased f l u o r e s c e n c e y i e l d and increased phosphorescence y i e l d of dibenzothiophene compared w i t h f l u o r e n e i s i n accord w i t h the smaller S*-T* energy gap (9400 cm - 1 i n f l u o r e n e and 6000 cm\"1 i n dibenzothiophene) 123 and the increased s p i n - o r b i t c oupling due to the sulphur atom. Although phosphorescence from the c r y s t a l was observed at 77\u00C2\u00B0K, a temperature dependent study i s r e q u i r e d to determine whether the luminescence i s from the c r y s t a l bulk or from defect s i t e s i n the l a t t i c e . Phosphorescence i s not normally observed from p e r f e c t organic molecular c r y s t a l s because during the - 205 -long l i f e t i m e of the t r i p l e t exciton there i s a high p r o b a b i l i t y t h a t s e v e r a l 19 excitons i n the c r y s t a l may i n t e r a c t and a n n i h i l a t e simultaneously. The v i b r a t i o n a l a n a l y s i s of the phosphorescence s p e c t r a i s s t r a i g h t forward, and no s p e c i a l f e a t u r e s are noted. There are no p r o g r e s s i o n s , apart from the overtone of the 1600 cm 1 C-C bond s t r e t c h i n g v i b r a t i o n , so that the t r i p l e t s t a t e geometry must be very c l o s e to that of the ground s t a t e . The f l u o r e s c e n c e a n a l y s i s , although not very e x t e n s i v e , i s c h a r a c t e r i s t i c of a s i n g l e t t r a n s i t i o n i n an aromatic molecule that has undergone very l i t t l e shape change. Chapter 10 A Study of Biphenyl C r y s t a l Phosphorescence Induced by I m p u r i t i e s A. I n t r o d u c t i o n Very few examples of phosphorescence from c a r e f u l l y p u r i f i e d s i n g l e c r y s t a l s of aromatic compounds are known, and the reason f o r t h i s l i e s i n 127 the now w e l l - i n v e s t i g a t e d phenomenon of t r i p l e t - t r i p l e t a n n i h i l a t i o n . Although the mechanism of the energy t r a n s f e r i s not completely understood, the e s s e n t i a l steps can be described according to a simple scheme. T + T y S* + S (10.1) S \u00E2\u0080\u0094y S + hv During the r e l a t i v e l y long l i f e t i m e of the t r i p l e t s t a t e i n the c r y s t a l * two t r i p l e t e x citons (T) i n t e r a c t to create a s i n g l e t e x c i t o n (S ) wliich may subsequently decay to the ground s t a t e (S) by a r a d i a t i v e process. The r e s u l t i n g emission, which i s c a l l e d delayed f l u o r e s c e n c e , i s s p e c t r a l l y i d e n t i c a l to the normal S'\"'-y S f l u o r e s c e n c e , but e x h i b i t s a l i f e t i m e of the order of m i l l i s e c o n d s to seconds. Weak e x c i t o n phosphorescence has been reported f o r benzophenone, 1^ some p-dihalogenated benzenes, 1 2^ and 130-132 anthracene c r y s t a l s . 'When the emission o r i g i n a t e s at a t r a p , u s u a l l y taken to be some (unspecified) l a t t i c e i m p e r f e c t i o n , the phosphorescence 132 133 i n t e n s i f i e s at low temperatures. \" ' The purpose of the f o l l o w i n g work - 207 -i s t o show that phosphorescence may be induced from the matrix by r e s i d u a l i m p u r i t i e s that themselves cannot act as acceptors o f the t r i p l e t energy. B. Carbazole i n Biphenyl In a c r y s t a l of b i p h e n y l doped w i t h c a r b a z o l e , the absorption and f l u o r e s c e n c e s p e c t r a are c h a r a c t e r i s t i c of carbazole s i n c e the v i b r a t i o n a l analyses (see Appendix I) are e s s e n t i a l l y i d e n t i c a l to those already given w i t h f l u o r e n e as matrix. F o l l o w i n g arguments presented by Hochstrasser 82 and S m a l l , carbazole may occupy s e v e r a l d i f f e r e n t s i t e s i n the biphenyl l a t t i c e . For example, i n a bc_' f a c e , two prominent o r i g i n bands are a c t i v e both i n f l u o r e s c e n c e and absorption at 29 672 and 29 698 cm 1 as w e l l as a t h i r d weaker l i n e at 29 678 cm Complete s p e c t r a are b u i l t on each l i n e . In the (20l) secondary cleavage plane, only two o r i g i n s at 29 678 and 29 698 cm 1 are observed. This v a r i a t i o n i n i n t e n s i t y was rioted but not understood, although i t may have been r e l a t e d to the f a c t that the be' s e c t i o n s were prepared by s u r f a c e g r i n d i n g . The l i n e s were about 5 cm 1 wide at a temperature of about 10-15\u00C2\u00B0K corresponding t o c o n d i t i o n s of strong i r r a d i a t i o n . Pure bip h e n y l c r y s t a l s d i d not phosphoresce even at the lowest tempera-tures reached (about 6\u00C2\u00B0K). However, when biphenyl was doped with carbazole an i n t e n s e , blue phosphorescence was e a s i l y recorded. A study of the v i b r a t i o n a l i n t e r v a l s and of the r e l a t i v e l i n e strengths shows that the e m i t t i n g species i s biphenyl and not carbazole. The main fea t u r e s of the a n a l y s i s are shown on the densitometer t r a c i n g of the spectrum i n F i g . 49, and t h i s may be compared w i t h the phosphorescence spectrum of biphenyl i n p a r a f f i n s o l v e n t s . 1 0 9 ' 1 1 0 The spectrum d i s p l a y s the same type of m u l t i p l e t s t r u c t u r e that i s evident i n the fluorescence and absorption s p e c t r a of - 208 -21 O O O 2 1 5 0 0 2 2 O O O 2 2 5 0 0 2 3 O O O Wavenumber (cm - 1) F i g . 49 Phosphorescence s p e c t r a of biphenyl c r y s t a l s doped with (a) carbazole at about 6\u00C2\u00B0K, (b) carbazole at 17\u00C2\u00B0K showing a second / trap coming i n t o prominence, (c.) dibenzothiophene-hg at about 6\u00C2\u00B0K, and (d) diber.zothiophene-d R at about 6\u00C2\u00B0K. - 209 -carbazole at s h o r t e r wavelengths (see Appendix I ] . The be' s e c t i o n s show two phosphorescence o r i g i n bands at 22 939 and 22 970 cm and i n the four (20l) s e c t i o n s s t u d i e d , there are three at 22 934, 22 960, and 22 985 cm - 1. These observations i n d i c a t e that the s e v e r a l ways that carbazole may sub-s t i t u t e i n the host perturb the biphenyl molecules of the l a t t i c e adjacent to the i m p u r i t y s i t e so as to create corresponding energy minima i n the l o c a l t r i p l e t .. . band s t r u c t u r e . Although the t r i p l e t l e v e l of carbazole i s about 1700 cm 1 above the biphenyl t r i p l e t band, there i s no evidence from the temperature dependence s t u d i e s to suggest that the minima represent metastable l e v e l s ; on the c o n t r a r y , the s t u d i e s s u b s t a n t i a t e the f a c t that the minima are energy traps below the band. The traps mentioned so f a r are of one type s i n c e they show a s i m i l a r behaviour as the tempera-tur e i s r a i s e d . The set of o r i g i n bands grow weaker, broaden, and r a p i d l y merge i n t o a s i n g l e l i n e that appears to s h i f t s l o w l y to lower wavelengths presumably because the higher energy traps are the f i r s t to empty i n t o the band. These were t r e a t e d as though degenerate and are r e f e r r e d to c o l l e c t i v e l y as traps of type 1 (J ~). A second type of trap (T^) was l o c a t e d by the appearance of another biphenyl phosphorescence o r i g i n at 22 886 cm 1 t o the red of the f i r s t o r i g i n (see F i g . 49). These were detected only at higher temperatures where traps of type 2 became more a c t i v e at the expense of those of type 1. 7 probably occurs through the presence e i t h e r of a second unsuspected impurity w i t h l e v e l s above the corresponding ones of b i p h e n y l , or of two carbazole molecules i n c l o s e p r o x i m i t y . The observation of deeper traps that show an analogous behaviour i n biphenyl doped with dibenzothiophene suggests that the l a t t e r e x p l a n a t i o n i s c o r r e c t . - 210 -The k i n e t i c s of the t r i p l e t - t r i p l e t a n n i h i l a t i o n process i n a mixed c r y s t a l system i n v o l v i n g a number of traps cannot be t r e a t e d e a s i l y . The f o l l o w i n g scheme shows only the more s i g n i f i c a n t steps a f t e r the e x c i t a t i o n energy has t r a n s f e r r e d to a guest molecule. N o n - r a d i a t i v e quenching: S -> S k l Guest f l u o r e s c e n c e : * S -> S + h v c r * k l I ntersystem c r o s s i n g : * s - T l k 2 Intersystem c r o s s i n g : * s k 3 Non - r a d i a t i v e quenching at tr a p T : \u00E2\u0080\u00A2 T -* S k4 Phosphorescence from t r a p T 1 \u00E2\u0080\u00A2 T 1 1 S + h v p * k 4 Non - r a d i a t i v e quenching at tr a p T 2 : T 2 -+ S K 5 Phosphorescence from t r a p T ' \u00E2\u0080\u00A2 T 2 2 S + h v p * K 5 Tj a n n i h i l a t i n g at s i t e T^: T l + T l * ->- s + s K l T j a n n i h i l a t i n g at s i t e T 2 '\u00E2\u0080\u00A2 T l + T 2 s* + s K 2 T^ a n n i h i l a t i n g at s i t e T^: T 2 + T 2 * -> s + s K 3 The s u b s c r i p t s F and P denote fluorescence and phosphorescence r e s p e c t i v e l y . The u n s t a r r e d constants represent n o n - r a d i a t i v e processes, and the s t a r r e d constants r e f e r to r a d i a t i v e processes. The v a r i a t i o n o f the i n t e n s i t y of the phosphorescence from and T 2 (I and I , r e s p e c t i v e l y ) and the delayed f l u o r e s c e n c e from carbazole p l p 2 ' (I n\u00E2\u0080\u009E) w i t h temperature i s shown i n F i g . 50. The maximum i n I i n d i c a t e d t h a t t r a p s of one type were emptying i n t o the band, with t r i p l e t - t r i p l e t a n n i h i l a t i o n subsequently o c c u r r i n g . The emptying t r a p was i d e n t i f i e d by observing a r a p i d decrease i n the corresponding I , i n t h i s case I . The P recombination process may be d e s c r i b e d 1 2 ' ' ' i n terms of the concentrations * of s i n g l e t e x c i t e d carbazole molecules [S ] and of the occupied traps of types 1 and 2 [T ] and [T^], r e s p e c t i v e l y , by the equation 4 I \u00C2\u00A7 J - = K l [ T l ] 2 + K 2 [ T l ] [ T 2 ] + K 3 [ T 2 ] 2 - K 4[S*] (10.2) CO t 1600-D y CC 140 < cc t CD 1 2 0 cc < ^_ lOO b CO 2 so LU I-Z o-t-O Ip F i g . 50 Temperature v a r i a t i o n of the i n t e n s i t y of the phos-phorescence ( I n and 1^ ) and P i V2 of the delayed f l u o r e s c e n c e I from a biphenyl c r y s t a l doped with \u00E2\u0080\u00A2 c a r b a z o l e . DF' lO 15 20 \u00C2\u00A35 T E M P E R A T U R E \u00C2\u00B0 K The temperature dependent a n n i h i l a t i o n r a t e constants and -X. i n v o l v e the s m a l l e r trap depth and K , the l a r g e r t r a p depth, e x p o n e n t i a l l y . The r a t e constant K i s the sum of, the l e s s temperature-dependent terms and k, . The f i r s t term on the r i g h t hand s i d e of the r a t e equation r e f e r s to the a n n i h i l a t i o n of T. at another T s i t e , the second term to T, a n n i h i l a t i n g - 212 -at T , and the t h i r d to 1'^ a n n i h i l a t i n g at another T'2. Each of these terms might be expected to dominate i n d i f f e r e n t temperature r e g i o n s . In p a r t i c u l a r at low temperature where was observed to be very small the f i r s t should be the important second-order term. Then the s t a t i o n a r y - s t a t e approximation * may be a p p l i e d to [S ] to give the r e l a t i o n l o g C i n e / 1 2 ) = constant (-AE 1/kT) (10.3) Ub p 1 J The slope of the l i n e a r p o r t i o n of such a p l o t y i e l d e d a depth f o r the set of shallow traps of about 250 cm 1 (see F i g . 51). This may be an over-estimate o ~i *r \u00E2\u0080\u0094 1 l/T F i g . 51 P l o t s o f l o g ( i n t e n s i t y r a t i o s ) vs. 1/T f o r the carbazole/biphenyl system. - 213 -of the t r a p depth s i n c e there may be s i g n i f i c a n t c o n t r i b u t i o n s to 1^ a r i s i n g from other processes i n the higher temperature regio n . In any event the percentage e r r o r (^20%) i n t h i s energy i s l a r g e s i n c e i t was estimated over a small temperature range using a r e l a t i v e l y crude thermometer. The mean phosphorescence energy from the set of traps T^ i s 22 960 cm 1 . I f t h i s value f o r the trap depth i s accepted, the corresponding estimate of the energy gap to the t r i p l e t band i s about 23 210 cm This estimate assumes that any p e r t u r b a t i o n i n the ground s t a t e of biphenyl i s small compared w i t h those i n the t r i p l e t s t a t e . In a more d i r e c t measurement, 135 -1 H i r o t a found the band gap at 78\u00C2\u00B0K to be about 23 200 cm . This r e s u l t suggests that the net r e p u l s i v e e f f e c t on the ground e l e c t r o n i c s t a t e of a biphenyl molecule caused by the i n s e r t i o n i n t o an adjacent l a t t i c e s i t e of a carbazole molecule (having a d i f f e r e n t shape) must be a p p r o p r i a t e l y s m a l l . 2 2 P l o t s formed by r e p l a c i n g I by I I or I i n the s i m p l i f i e d expression p l P l P 2 p 2 above d i d not y i e l d s t r a i g h t l i n e s . Other ways that the t r i p l e t band may become d i s t o r t e d may be p o s t u l a t e d . For example, i f the s o l u t e c o n c e n t r a t i o n were made high enough, the traps could become connected t h e r m a l l y through c l u s t e r i n g without i n v o l v i n g the t r i p l e t e x c i t o n band d i r e c t l y . C l u s t e r i n g should be unimportant i n our -4 -3 samples si n c e the nominal concentrations ranged from 5 x 10 to 4 x 10 M/M and the a c t u a l concentrations may w e l l have been l e s s . C. Dibenzothiophene i n Biphenyl The same general behaviour was observed when bi p h e n y l was doped w i t h dibenzothiophene. In t h i s case, the biphenyl phosphorescence bands (as w e l l as the dibenzothiophene f l u o r e s c e n c e and absorption bands) are unu s u a l l y broad - 214 - -(about 250 cm *) and no m u l t i p l e t s could be found (see F i g . 49). P o s s i b l y dibenzothiophene occupies a large number of d i f f e r e n t s i t e s i n the host l a t t i c e , or even some form of hindered r o t a t i o n may occur i f two biphenyl molecules are re p l a c e d . The change of s o l u t e from carbazole t o dibenzothiophene caused the highest energy phosphorescence band of the bc_' s e c t i o n to r e d - s h i f t by 80 cm Traps of type 2 were r e a d i l y observed above about 20\u00C2\u00B0K when another broad phosphorescence o r i g i n , about 320 cm 1 to the red of the f i r s t , g r a d u a l l y i ncreased i n st r e n g t h (see F i g . 52). The double maxima i n I^p i s apparently a consequence o f the traps T^ emptying i n the temperature range 20-30\u00C2\u00B0K and- T 2 i n the range 40-50\u00C2\u00B0K. From the va r i o u s p l o t s shown i n F i g . 53, the only s a t i s f a c t o r y t r a p depth was f o r T^ which was found t o be 670 cm 1. The corresponding (broad) phosphorescence o r i g i n band at 22 580 cm 1 gives an independent estimate f o r the band gap of 23 250 cm Because of the extreme width of the phos-phorescence bands, the assumption that a l l the traps l i e at the same energy below theband i s probably u n s a t i s f a c t o r y . This may also account f o r the n o n - l i n e a r i t y of the other p l o t s i n F i g . 51. T r i p l e t - t r i p l e t a bsorption was observed i n the biphenyl c r y s t a l doped w i t h dibenzothiophene, and the spec t r a (at about 10\u00C2\u00B0K) are shown i n F i g . 54. 136 For comparison, the spectrum observed by Ramsay and Munro of t r i p l e t -t r i p l e t ' a b sorption i n a r i g i d g l a s s at 77\u00C2\u00B0K i s a l s o given. Although the bands are broad, they are s p l i t s u f f i c i e n t l y t o i n d i c a t e the presence o f two e l e c t r o n i c t r a n s i t i o n s of opposite p o l a r i z a t i o n . No r e l i a b l e a s s i g n -ment of the symmetry of the lowest t r i p l e t ' s t a t e has been made. The 137 r e s u l t s of Stewart's c a l c u l a t i o n f o r planar biphenyl i n d i c a t e that the 215 2800 -*'DF F i g . 52 Temperature v a r i a t i o n of the i n t e n s i t y of the phos-phorescence (Ip^ and Ip ) and of the delayed fluorescence ( I n p ) from a biphenyl c r y s t a l doped with dibenzothiophene-h o T E M P E R A T U R E \u00C2\u00B0 K the o r b i t a l symmetry of the l o w e s t - t r i p l e t s t a t e may be B 2 u which, i n t h i s case, transforms l i k e an in-plane v e c t o r p a r a l l e l to the y_ axi s (see F i g . 5). I f t h i s assignment i s accepted, then the v i b r o n i c t r a n s i t i o n s are: 3 3 3 3 Ag\u00C2\u00AB<\u00E2\u0080\u0094 B 2 u 5 P a r a H e l to the short in-plane a x i s at low energy, and \u00E2\u0080\u0094 B 2 u ' p a r a l l e l to the long a x i s at higher energy. The experiments c a r r i e d out on biphenyl doped with dibenzothiophene-d o show no measureable s h i f t i n the wavelength of the induced phosphorescence C H w -1 .Q H \u00E2\u0080\u0094 o Q: >, CO - 2 c H\u00E2\u0080\u0094 c C P O -3 -4H AS A A O A ( A E = 6 7 0 c m - ' A 3 o o o o o o o o o o o o O O O O O o o o + D F OP) 2 ' D F .02 . 0 3 . 0 4 . 0 7 . 0 8 . 0 9 . 0 5 . 0 6 1 / T F i g . 53 P l o t s of log ( i n t e n s i t y r a t i o s ) vs 1/T f o r the dibenzothiophene-h g/biphenyl system, - 217 -: L J \u00C2\u00A3 i i i l J 24000 26 000 28 000 Cm~' F i g . 54 S o l i d l i n e t r i p l e t - t r i p l e t a b sorption i n a biphenyl c r y s t a l doped w i t h dibenzothiophene at about 10\u00C2\u00B0K. Broken l i n e : t r i p l e t - t r i p l e t a bsorption of biphenyl i n a r i g i d glass at 77\u00C2\u00B0K (according to 136 Ramsay and Munro ) . from that observed with dibenzothiophene-h as guest. The temperature o dependence of the i n t e n s i t y of the l i g h t emission are q u a l i t a t i v e l y s i m i l a r (compare F i g . 52 and 55). A pronounced delayed fluorescence from phenanthrene and to a lesser extent from anthracence became evident above about 25\u00C2\u00B0K and the system was not stu d i e d f u r t h e r . The phenanthrene and anthracene were present i n the commercial biphenyl-d^Q, and there was i n s u f f i c i e n t s t a r t i n g m a t e r i a l f o r complete p u r i f i c a t i o n and s y n t h e s i s . - 21 2800 2600 2400 2 2 C O H 2000 I S O O 1 6 0 0 1400 1200 I O O O 800H 6 0 0 H 400 200 A l D F ( D B T - d 8 ) A I d f (Phenanthrene) o Ip 40 50 60 T E M P E R A T U R E \u00C2\u00B0 K : of the i n t e n s i t y of the , and of \"the'delayed fluorescence ( I n p ) from a biphenyl c r v s t a l doped with dibenzothiophene-dg- . . Temperature v a r i a t i o n of the i n t e n s i t y of the phosphorescence fl\u00E2\u0080\u009E and I ) v,^^^^-^-l,\u00E2\u0080\u009E,1\u00E2\u0080\u009ET 1,-1 r- . K . * \" 1 ^2 - 219 -Appendix I The Absorption and Fluorescence Spectrum of Carbazole i n a Biphenyl M a t r i x Table 48. The absorption spectrum of carbazole i n biphenyl at about 15\u00C2\u00B0K. //b //c I n t . Remarks 29-698 o r i g i n 27 l a t t i c e 218 vs 218, A 1 420 s 420, A. 432 ms 2 x 218 - 4 505 m 505? 515 m 515, B 2 577 mw 577, B 2 650 vs 650, A 720 ms 720, A1 729 vw 218 + 5 0 5 + 6 793 vw 218 + 5 7 7 - 2 864 s 218 + 6 5 0 - 4 928 vs 928, B 2 936 m 218 + 7 2 0 + 2 985 991 vs 985, A 991, B 2 1006 s 985 + 2 7 + 4 1034 s 1034, B 2 1119 vs 1119, B 2 1141 w 420 + 720 + 1 1161 m 515 + 6 5 0 - 4 1201 m 48 + 9 8 5 - 2 1204 m 218 + 9 9 1 - 5 1238 vs 1238, A1 1299 s 2 x 650 - 1 - 220 -//b //c Int. Remarks 1339 s 218 + 1119 + 2 1451 ms 218 + 1238 - 5 1486 mw 1486, B2 1512 m 218 + 2 x 650 - i 1543 ms 1543, B2 1545 m 1545, A l 1573 650 + 928 - 5 1596 m 1596, A l 1630 mw 650 + 985 - 5 1637 m 650 + 991 - 4 1652 w 420 + 1238 - 6 1680 mw 650 + 1034 - 4 1705 w 720 + 985 1767 m 650 + 1119 - 2 1810 w 218 + 1596 -\u00E2\u0080\u00A2 4 1811 w 577 + 1238 - 4 \u00E2\u0080\u00A2 1848 mw 218 + 650 + 985 1882 m 650 + 1238 - 6 1945 m 3 x 650 - 5 1959 mw 720 + 1238 4 1 1981 w 2 x 218 + 1545 1986 w 218 + 650 + 1119 2018 w 2 x 650 + 720 -2096 w 218 + 650 + 1238 2163 w 928 + 1238 - 3 2193 w 650 + 1543 2219 m 985 + 1238 -\u00E2\u0080\u00A2 4 2226 w 991 + 1258 -\u00E2\u0080\u00A2 3 2242 m 650 + 1596 -\u00E2\u0080\u00A2 4 2281 m 2 x 650 + 985 -2309 mw 720 + 1596 -\u00E2\u0080\u00A2 7 2321 mw 985 + 1339 -\u00E2\u0080\u00A2 1 2353 mw 1119 + 1238 - 4 - 221 -//b / / \u00C2\u00A3 I n t . Remarks 2436 mw 218 + 985 + 1238 - 5 2470 mw 2 x 1238 - 6 2535 mw 2 x 650 + 1238 - 3 2781 vw 1238 + 1543 2866 vw 650 + 985 + 1238 - 7 The p o s i t i o n of the orij; . . . -1 ;in i s given cm and a l l other e n t r i e s show d i f f e r e n c e s from the o r i g i n . The spectrum exhibite 193 (1963). . ~ 117. K. W i t t , Spectrochim. Acta 24A, 1115 (1968). 118. R. Manzoni-Ansidei, Redd. Accad. L i n c e i , 2j5, 266 (1937). 119. T.A. Hariharan, J . Ind. I n s t . S c i . 36A, 215 (1954). 120. J . Behringer and J . Brandmuller, Z. Angew. Phys. 1_4, 674 (1962). 121. G.W.H. Scherf and R.K. Brown, Can. J . Chem. 38, 697 (1960). 122. See, f o r example, equation (28) i n reference 12. 123. F. Dorr, Agnew. Chem. i n t e r n a t . E d i t . 5_, 478 (1966). 124. R.N. Nurmukhametov and G.B. Gobov, Opt. Spektrosk. 1_8, 227 (1965).; [Opt. Spectrosc. L8, 126 (1965)]. 125. M. B i e l e f i e l d and D. F i t t s , J . Am. Chem. Soc. 8_8, 4804 (1966) and references c i t e d t h e r e i n . 126. J . Koutecky, R. Zahradnik and J . Paldus, J . Chim. Phys. 56_, 455 (1959). 127. H. S t e r n l i c h t , G.C. Nieman, and G.W. Robinson, J . Chem. Phys. 38, 1326 (1963); i b i d 39, 1610 (1963). 128. R.M. Hochstrasser, J . Chem. Phys. 39, 3153 (1963). \u00E2\u0080\u00A2129. G. Castro and R.M. Hochstrasser, J . Chem. Phys. 46, 3617 (1967). 130. D.F. W i l l i a m s , J . Chem. Phys. 47_, 344 (1967). 131. H.P. M u l l e r , P. Thoma, and G. Vaubel, Phys. S t a t . S o l . 23_, 253 (1967). 132. G.C. Smith, Phys. Rev. 166, 839 (1968). 133. H. Port and H.C Wolf, Z. Nat u r f o r s c h . 23a, 315 (1968). 134. T.N. M i s r a and S.P. McGlynn, J . Chem. Phys. 44, 3816 (1966). 135. N.J. H i r o t a , J . Chem. Phys. 44, 2199 (1966). - 236 -136. I .A. Ramsay and l.H. Munro. The T r i p l e t S t a t e , ed. A.B. Zahlan (Cambridge U n i v e r s i t y Press, 1967) p. 415. 137. E.T. Stewart, J . Chem. Soc. 1958, 4016. "@en . "Thesis/Dissertation"@en . "10.14288/1.0059930"@en . "eng"@en . "Chemistry"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "A Study of the spectra of some organic compounds"@en . "Text"@en . "http://hdl.handle.net/2429/35750"@en .