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Primary photochemical processes in hexafluorobiacetyl Reid, William John 1972

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PRIMARY PHOTOCHEMICAL PROCESSES IN HEXAFLUOROBIACETYL by William John Reid B.A. (Moderatorship), T r i n i t y College, University of Dublin, Ireland, 1967 M.Sc, University of B r i t i s h Columbia, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA September, 1972 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the 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 fo r reference and s tudy . I f u r t h e r agree t h a t pe rmiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or 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 ga in s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . Department of Chemistry The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date 22nd September, 1972 i i ABSTRACT Absolute photochemical quantum y i e l d s of hexafluoro-b i a c e t y l vapour have been obtained at various e x c i t i n g wave-lengths between 250 and 440 nm over the range 0.5 - 400 t o r r . The y i e l d s are strongly dependent on pressure demonstrating that v i b r a t i o n a l relaxation i s the dominant process competing with unimolecular d i s s o c i a t i o n . I t i s found that two d i f f e r e n t states contribute to d i s s o c i a t i o n . One i s i d e n t i f i e d as the excited s i n g l e t l e v e l reached on e x c i t a t i o n . The other i s attributed to the vibronic l e v e l reached on intersystem crossing from that i n i t i a l l y formed v i b r o n i c state. The intersystem crossing rate constant has been shown to be a strong function of e x c i t a t i o n energy. I t i s postulated that the f i r s t excited s i n g l e t state of hexafluorobiacetyl i s photochemically i n e r t unless i t has at l e a s t 70 kc a l of v i b r o n i c energy. This accounts for no de-composition being observed at the higher wavelengths. Temper-ature and quenching studies have shown that the e q u i l i b r a t e d t r i p l e t state i s unreactive photochemically. Phosphorescence l i f e t i m e measurements at very low pressures have confirmed i i i that wall-deactivation for the r e l a t i v e l y long l i v e d e q u i l i -brated t r i p l e t species i s important when the average distance which the t r i p l e t molecule can d i f f u s e i s of the same magni-tude as the c e l l radius. The data from the various investigations are combined to give a description of the primary photochemical and photo-ph y s i c a l events. From t h i s information a mechanism for the primary process i n hexafluorobiacetyl i s proposed and c r i t i -c a l l y evaluated. Estimates of the s p e c i f i c rate constants for the photochemical processes are given and discussed. i v TABLE OF CONTENTS Page T i t l e Page i Abstract. i i Table of Contents i v L i s t of Figures v i i L i s t of Tables i x Acknowledgements x CHAPTER I. INTRODUCTION 1 A. The Primary Process 1 B. Fluorinated Ketones 9 C. Previous Work on Hexaf luor o b i a c e t y l 11 D. Purpose of t h i s Investigation 16 CHAPTER I I . EXPERIMENTAL ARRANGEMENT AND PROCEDURE.. 18 A. B r i e f Synopsis of Experimental Procedure 18 B. Vacuum System 20 C. Preparation and P u r i f i c a t i o n of Chemicals 21 D. Op t i c a l Arrangements 22 E. Reaction C e l l s 25 F. Temperature Control Systems 27 G. Gas Analysis 28 H. Actinometry 31 I. Measurement of the Fraction of Light Absorbed by the Sample 32 V Page J. Emission Spectroscopy 33 K. Lifetime Measurements 33 L. Errors 35 CHAPTER I I I . PHOTOCHEMISTRY OF HFB - RESULTS 36 A. Absorption 36 B. Ratio Experiments 36 C. R e l i a b i l i t y of the Photolysis Data 38 D. Photolysis of HFB at 25° C 40 E. HFB Photolysis at Di f f e r e n t Temperatures 50 F. HFB - HFAM System 51 (i) Quenching Study 51 ( i i ) Photolysis of HFB - HFAM Mixtures 53 CHAPTER IV. DISSOCIATION AND COLLISIONAL DEACTIVATION 58 A. Graphical Presentation of Results . ... 58 B. General Observations 59 C. Proposed Mechanism 60 D. Discussion 75 (i) C o l l i s i o n a l Deactivation 75 ( i i ) Mechanistic Considerations 76 ( i i i ) Photochemical Inertness of Eq u i l i b r a t e d T r i p l e t 78 (iv) T r i p l e t D issociation 79 (v) Light Intensity 79 v i Page (vi) Evaluation of Rate Constants 81 (v i i ) Simulation of Quantum Y i e l d Results 84 ( v i i i ) V a r i a t i o n of k^, k2 and k,. with Wavelength.. 85 (ix) Other Possible Mechanisms 90 (a) Involvement of the V i b r a t i o n a l l y Excited Ground State 90 (b) V i b r a t i o n a l Energy D i s t r i b u t i o n Function.. 91 (x) Wall-Deactivation of the E q u i l i b r a t e d T r i p l e t State . 91 CHAPTER V. THE PRIMARY PROCESS . 97 A. Detailed Mechanism . 97 B. Independent Evaluation of (k^ + k£) 100 C. Evaluation of ^ 2 from the F u l l Mechanism 101 (i) Fluorescence/Photochemistry Ratio 101 ( i i ) Phosphorescence/Photochemistry Ratio... . . 103 ( i i i ) Discussion 105 D. Complementary Aspects of thi s and Previous Work.... 105 (i) Fluorescence and Intersystem Crossing 108 ( i i ) Limitations on Available Data . 112 E. Concluding Remarks - Suggestions for Further Work.. 115 BIBLIOGRAPHY 118 APPENDIX 123 A. Window Corrections 124 B. Wall Deactivation 126 v i i LIST OF FIGURES Figure Page 1. Jablonski diagram of hexafluorobiacetyl 4 2. Photolysis system 19 3. Photolysis l i n e shapes 23 4. 5-cm reaction c e l l and mixer 26 5. Gas analysis system 29 6. Absorption spectra of hexafluorobiacetyl at 25° C 37 7. Stern-Volmer quenching of hexafluorobiacetyl phosphorescence 57 8. Reciprocal quantum y i e l d s versus pressure - 254 nm, 25° C. High pressure region 61 9. Reciprocal quantum y i e l d s versus pressure - 254 nm, 25° C. Low pressure region 62 10. Reciprocal quantum y i e l d s versus pressure - 297 nm, 25° C. High pressure region 63 11. Reciprocal quantum y i e l d s versus pressure - 297 nm, 25° C. Low pressure region 64 12. Reciprocal quantum y i e l d s versus pressure - 313 nm, 25° C. High pressure region 65 13. Reciprocal quantum y i e l d s versus pressure - 313 nm, 25° C. Low pressure region 66 14. Reciprocal quantum y i e l d s versus pressure - 313 nm, 25° C. Low pressure region 67 15. Reciprocal quantum y i e l d s versus pressure - 334 nm, 25° C. High pressure region 68 16. Reciprocal quantum y i e l d s versus pressure - 334 nm, 25° C. Low pressure region 69 v i i i Figure Page 17. Reciprocal quantum y i e l d s versus pressure - 366 nm, 25° C. High pressure region 70 18. Reciprocal quantum y i e l d s versus pressure - 297 nm, 25° C and -20° C . 71 19. Reciprocal quantum y i e l d s versus pressure - 313 nm, various temperatures «. 72 20. Quantum y i e l d s versus r e c i p r o c a l pressures - High pressure region 80 21. Log of the various rate constants versus e x c i t a t i o n energy 89 22. Difference (y) between the rec i p r o c a l s of the observed l i f e t i m e (T) at low pressures and the constant high pressure t r i p l e t l i f e t i m e (t^ ) versus HFB pressure 94 23. Difference (y) between the reciprocals of the observed l i f e t i m e (T) at low pressures and the constant high pressure t r i p l e t l i f e t i m e (T0 ) versus r e c i p r o c a l HFB pressure 95 24. Ratio of fluorescence y i e l d to photochemical y i e l d versus HFB pressure at 313 nm. High pressure region 107 25. Ratio of phosphorescence y i e l d to photochemical y i e l d versus HFB pressure at 313 nm. High pressure region 107 26. Ratio of fluorescence y i e l d to photochemical y i e l d versus HFB pressure at 297 nm 110 27. Ratio of fluorescence y i e l d to photochemical y i e l d versus HFB pressure at 313 nm I l l 28. Fluorescence quantum y i e l d versus HFB pressure at 313 nm 113 29. Phosphorescence quantum y i e l d versus HFB pressure at 313 nm 114 i x LIST OF TABLES T a b l e Page 1. Mean m o l a r a b s o r p t i o n c o e f f i c i e n t s o f HFB a t the w a v e l e n g t h s used i n p h o t o l y s i s a t 25° C 39 2. R a t i o e x p e r i m e n t s 39 3. P h o t o l y s i s o f HFA a t 313 nm and 25° C 41 4. P h o t o l y s i s o f HFB a t 254 nm and 25° C 41 5. P h o t o l y s i s o f HFB a t 297 nm and 25° C 42 6 . P h o t o l y s i s o f HFB a t 313 nm and 25° C 44 7. P h o t o l y s i s o f HFB a t 334 nm and 25° C 45 8. P h o t o l y s i s o f HFB a t 366 nm and 25° C 46 9. P h o t o l y s i s o f HFB a t 405 nm and 25° C 47 10. P h o t o l y s i s o f HFB a t 436 nm and 25° C 47 11. P h o t o l y s i s o f HFB a t 297 nm and -20° C 48 12. P h o t o l y s i s o f HFB a t 313 nm and 50° C 48 13. P h o t o l y s i s o f HFB a t 313 nm and 76° C 49 14. P h o t o l y s i s o f HFB/HFAM m i x t u r e s a t 25° C 55 15. Rate c o n s t a n t s f o r d i s s o c i a t i o n and i n t e r s y s t e m c r o s s i n g a t 25° C 81 16. Rate c o n s t a n t s f o r d i s s o c i a t i o n and i n t e r s y s t e m c r o s s i n g a t v a r i o u s t e m p e r a t u r e s 82 17. Rate c o n s t a n t s f o r t h e sum o f s i n g l e t d i s s o c i a -t i o n and i n t e r s y s t e m c r o s s i n g from the i n i t i a l l y p o p u l a t e d v i b r o n i c s i n g l e t s t a t e 102 18. Rate c o n s t a n t s f o r s i n g l e t d i s s o c i a t i o n . 106 ACKNOWLEDGEMENTS I wish to sin c e r e l y thank Dr. Gerald B. Porter for his guidance and understanding during the course of this work. His encouragement not only aided i n the progress of t h i s work but also i n the development of th i s researcher. I am indebted to Dr. N. Basco, Mr. J.E. Hunt, Dr. J.S.E. Mcintosh and Dr. R. May for many h e l p f u l discussions r e l a t i n g to t h i s research problem. In addition the excellent craftmanship of Mr. S. Rak and Mr. J. Molnar of the Glassblowing Shop and of Mr. B. Powell and his colleagues of the Mechanical Shop i s warmly appreciated. F i n a l l y , I wish to thank my wife, Pamela, whose i n f i n i t e patience and constant encouragement made th i s thesis p o s s i b l e . to the REIDS and DILLS CHAPTER ONE INTRODUCTION A. The Primary Process Carbonyl compounds have probably attracted more attention i n the photochemical f i e l d than any other class of compound. The production of r a d i c a l s during photolysis has been a major i n t e r e s t . A c c e s s i b i l i t y of the long-wavelength absorption band i n the near UV region of the spectrum has enabled d e t a i l e d studies to be made. In par-t i c u l a r , the photochemical and photophysical behavior of a l i p h a t i c ketones i n the gas phase has been studied exten-s i v e l y . I t has become obvious that the systems have con-siderable k i n e t i c complexity. The Primary Process comprises the i n i t i a l act of absorption of a photon by a molecule to produce an excited e l e c t r o n i c state, and a l l subsequent events which lead e i t h e r to disappearance of the molecule (primary photochemical process), or to the return of the molecule to i t s thermally e q u i l i b r a t e d ground e l e c t r o n i c state (primary photophysical processes). 2 In the primary photochemical process there i s usually a va r i e t y of paths for degradation of the e l e c t r o n i c energy of e x c i t a t i o n . Chemical paths include intramolecular rearrangements (e.g. cis-t r a n s isomerization of ol e f i n s ) and the formation of free r a d i c a l s which combine with each other, (e.g. CF^ + CF^ •* C 2 F 6 ^ n ^- e x a^luoroacetone) or which combine with other molecules i n secondary processes to form new products. Frequently the unknown nature and magnitude of these l a t t e r processes tend to obscure the primary process. The f i r s t absorption band of ketones, which generally has a maximum around 280 nm, r e s u l t s from e x c i t a -t i o n of one of the non-bonding electrons of the carbonyl oxygen int o an antibonding o r b i t a l i . e . an i r * -«- n t r a n s i t i o n . The excited s i n g l e t state so produced i s designated S ^ . Other e l e c t r o n i c states are abbreviated thus: S Q (ground); S 2 ' S 3 " * " ( e x c i t e < ^ s i n g l e t s ) ; T^, T_ ... (excited t r i p l e t s ) . Absorption of l i g h t produces a species containing part of the e x c i t a t i o n energy as excess v i b r a t i o n a l energy. The stationary state concentration of excited species i s , under normal circumstances, so small that they are e s s e n t i a l l y 3 surrounded by a heat bath of unexcited molecules. In p r i n c i p l e , the v i b r a t i o n a l l y excited molecule i n an upper e l e c t r o n i c state can a t t a i n v i b r a t i o n a l equilibrium by two mechanisms. Either t h i s degradation of v i b r a t i o n a l energy i s accomplished by a single-step deactivation from high to low v i b r a t i o n a l l e v e l s or by a multistep cascade from one group of v i b r a t i o n a l l e v e l s 1—6 to the next lower group. Recent studies on hexafluoroacetone 8 9 and also on the isomerization of 1,3,5-cycloheptatriene ' support a multistage process. In the l a t t e r study the photo-chemical isomerization to toluene occurs v i a the v i b r a t i o n a l l y 8 9 excited ground state molecule. Atkinson and Thrush ' obtained the amount of v i b r a t i o n a l energy removed per c o l l i s i o n when quenching of isomerization by various added gases was studied at a va r i e t y of wavelengths. Nevertheless, much of the contem-porary k i n e t i c data lends support to the concept of strong c o l l i s i o n a l t ransfer. The paths that e x i s t for energy d i s s i p a t i o n from the photoexcited molecule are i l l u s t r a t e d schematically by the Jablonski diagram for hexafluorobiacetyl (Figure 1). FIG.1 I JABLONSKI DIAGRAM OF HEXAFLUOROBIAGETYL i I 5 Processes ( i i ) and ( i x ) , p h o t o d i s s o c i a t i o n , w i l l both o c c u r i n the g e n e r a l case. They must take p l a c e r a p i d l y t o compete w i t h i n t e r n a l c o n v e r s i o n and l o s s of v i b r a t i o n a l energy by c o l l i s i o n s . D i s s o c i a t i o n from the t r i p l e t w i l l o c c u r from the v i b r a t i o n a l l e v e l reached from i n t e r s y s t e m c r o s s i n g . Although i t seems t h a t T° (lowest v i b r a t i o n a l l e v e l of f i r s t e x c i t e d t r i p l e t s t a t e ) may be l o n g - l i v e d enough to be r e -e n e r g i z e d by c o l l i s i o n and d i s s o c i a t e i n c e r t a i n molecules (e.g. h e x a f l u o r o a c e t o n e ) , t h i s thermal d i s s o c i a t i o n s h o u l d be absent i n HFB as the phosphorescence l i f e t i m e i s both temper-a t u r e independent and p r e s s u r e independent near room tempera-t u r e . P r o c e s s ( i i ) w i l l have a v e r y s m a l l temperature depen-dence due t o the Boltzmann v i b r a t i o n a l energy d i s t r i b u t i o n i n the ground s t a t e . Both ( i i ) and (ix) should be wavelength dependent. From l e v e l 0 o f S^, the molecule can r e t u r n to any one o f the v i b r a t i o n - r o t a t i o n l e v e l s o f the ground s t a t e w i t h the e m i s s i o n of f l u o r e s c e n c e . F l u o r e s c e n c e i s a r a d i a t i v e t r a n s i t i o n between s t a t e s of l i k e m u l t i p l i c i t y . With some molecules i t seems t h a t f l u o r e s c e n c e i s coming a l s o from non-e q u i l i b r a t e d l e v e l s of S, (e.g. HFB) 1. 6 In the absence of quenchers the fluorescence quantum y i e l d depends on the r e l a t i v e rates of the ra d i a t i v e process on the one hand, and the ra d i a t i o n l e s s processes of intersystem crossing (process (vi)) and i n t e r n a l conversion (process ( v i i ) ) on the other. These r a d i a t i o n l e s s processes between e l e c t r o n i c states are poorly understood although they are receiving much t h e o r e t i c a l i n t e r e s t at p r e s e n t . ^ Internal conversion (a spin allowed t r a n s i t i o n ) process ( v i i ) i s an adiabatic crossing from the lowest excited s i n g l e t state to high v i b r a -t i o n a l l e v e l s of the ground state, followed by v i b r a t i o n a l r e l a xation. This appears to be an important route i n some systems, such as large dye molecules.*"'' whereas i n simple 12 13 aromatics and a l i p h a t i c carbonyl systems i t s contribution i s r e l a t i v e l y minor. The s i n g l e t - t r i p l e t intersystem crossing processes, (i) and ( v i ) , although spin forbidden, occur with rates comparable to that of an allowed r a d i a t i v e t r a n s i t i o n (k^s C = 2.4 ± 0.4 x 10 8 s e c - 1 i n b i a c e t y l ) . 1 4 As with i n t e r n a l conversion to the ground state, the rate of intersystem crossing increases rapi d l y as the separation of the level s decreases and, i n a d d i t i o n , i t depends on the degree of mixing 7 of the states i . e . the degree of t r i p l e t character i n the s i n g l e t state and v i c e - v e r s a . ^ In IT* •*- n t r a n s i t i o n s , intersystem crossing i s rapid, the energy separation of S^ and being small owing to the small overlap of the o r b i -t a l s . Since the competing r a d i a t i v e t r a n s i t i o n from upper s i n g l e t to ground state i s symmetry forbidden, the quantum e f f i c i e n c y of t r i p l e t formation often approaches unity (e.g. b i a c e t y l , 1 ^ ^ acetone, 1^ hexafluoroacetone^). Moreover, 14 Calvert and co-workers have recently confirmed Parmenter 18 and Poland's contention that an i s o l a t e d excited s i n g l e t molecule (for b i a c e t y l at least) undergoes a t r u l y unimole-cular intersystem crossing reaction with the same e f f i c i e n c y as i n the c o l l i s i o n a l l y perturbed system at high pressures. Radiative t r a n s i t i o n s between states of d i f f e r e n t m u l t i p l i c i t y (e.g. t r i p l e t - s i n g l e t ) can take place. This luminescence i s c a l l e d phosphorescence. In addition, however, a molecule i n a v i b r a t i o n -a l l y and e l e c t r o n i c a l l y excited state has a small but f i n i t e r a d i a t i v e t r a n s i t i o n p r o b a b i l i t y to other v i b r a t i o n a l states of the same e l e c t r o n i c state. A system of such molecules should emit the i n f r a r e d photons corresponding to these t r a n s i t i o n s . This e f f e c t has been reported for the t r i p l e t 8 state of b i a c e t y l . The importance of fluorescence and phosphorescence l i e s not i n t h e i r absolute magnitude (usually less than 20% of excited molecules lose energy v i a emission i n carbonyl compounds) but i n t h e i r diagnostic value. For example, i f intersystem crossing to the t r i p l e t manifold i s only v i a process (vi) which competes with fluorescence from the e q u i l -ibrated s i n g l e t state, pressure dependencies i n the fluorescence y i e l d s should be r e f l e c t e d by the phosphorescence y i e l d s too. 17 This was found to be the case i n b i a c e t y l . An increase xn the fluorescence y i e l d as the pressure i s raised indicates that photochemistry i s quenched from a higher v i b r a t i o n a l l e v e l . Although i t i s a spin forbidden t r a n s i t i o n , conver-sion from the lowest t r i p l e t to ground state, process ( x i ) , seems to be more important than conversion from the lowest s i n g l e t l e v e l . This i s a r e s u l t of the competing r a d i a t i v e process being very much slower. Radiationless conversion from the lowest t r i p l e t to ground state appears to be a major pathway for energy d i s s i p a t i o n i n ketones unless the primary 15 photochemical y i e l d xs large. 9 B. Fluorinated Ketones A l i p h a t i c ketones have been the subject of innumer-able gas phase photochemical i n v e s t i g a t i o n s . ^ ' ^ There are p r i n c i p a l l y two reasons for t h i s attention. F i r s t l y , these ketones are s t r u c t u r a l l y simple and therefore would seem to o f f e r the easiest examples for quantitative study of the primary process. Secondly, t h e i r longest wavelength absorp-t i o n (IT* n) l i e s at about 300 nm and i s re a d i l y accessible both i n terms of e x c i t a t i o n sources and o p t i c a l instrumentation. Unfortunately, however, the photochemistry of a l i -phatic ketones can be complicated by the large number of 20 products formed. The simplest member of the s e r i e s , acetone, can give as products carbon monoxide, ethane, methane, b i a c e t y l , methyl ethyl ketone, ketene and acetaldehyde depending on the 13 conditions of photolysis. B i a c e t y l (butane -2,3-dione), the f i r s t member of the homologous series of the diketones, i s equally complex. These compounds f a i l to be photochemically simple due to the r e l a t i v e weakness of the C-H bond (82 kcal/mole). Radicals formed i n the primary photochemical act re a d i l y abstract hydrogen atoms from the parent ketone, producing many f i n a l products. Acetone has the added disadvantage that one of i t s 10 products, b i a c e t y l , quenches i t s emission. Other members of the acetone and b i a c e t y l homologous series are plagued by s i m i l a r complications. The s i t u a t i o n i s not much improved i n f u l l y chlor-13 21 22 inated or p a r t i a l l y chlorinated ketones. ' For f u l l y f l u o r i n a t e d ketones, however, the strong C-F bond (114 k c a l / mole) r e s u l t s i n a suppression of secondary reactions over a wide range of conditions. For example, the primary photo-chemical act i n hexafluoroacetone r e s u l t s s o l e l y i n the forma-ti o n of CF^ and CO i n a 2:1 molar r a t i o at temperatures up to 370° C. 2 3 A great deal of photochemical as well as photo-2 4 physi c a l studies have been done on HFA. ' I t both fluoresces and phosphoresces, so i t would seem to be an i d e a l molecule i n which to observe d e t a i l s of the primary process. However i t has one drawback: i t s e q u i l i b r a t e d t r i p l e t state i s long-l i v e d enough to dissociate thermally at room temperature. This complicates the diagnosis of the primary process as i t i s a r e l a t i v e l y large e f f e c t . Any photochemistry o r i g i n a t i n g from an excited v i b r o n i c t r i p l e t state would probably be obscured by i t s magnitude. I t appears that i n HFB, t h i s pathway (anal-ogous to thermal dissociation) i s absent at a s i m i l a r temperature. 11 C. Previous Work on Hexafluorobiacetyl 24 Whittemore and Szwarc i n 1963 published a short note on the gas phase photolysis of HFB at 25° C and 150° C. They found carbon monoxide and hexafluoroethane produced i n a 2:1 molar r a t i o . Subsequently some preliminary photochem-25 1 i c a l quantum y i e l d s were obtained at 313 nm. Mcintosh studied the phosphorescence and fluorescence quantum y i e l d s of HFB vapour at various e x c i t i n g wavelengths between 29 0 and 440 nm, thereby showing, i n a quantitative manner, the importance of emission processes i n energy d i s s i p a t i o n . Furthermore, he showed the importance of v i b r a t i o n a l relax-ation processes and how they a f f e c t other parameters. He was also able to make assignments regarding the observed e l e c t r o n i c t r a n s i t i o n s from the absorption and emission spectra. In t e r e s t i n g l y , he observed that the phosphorescence l i f e t i m e of HFB vapour was independent of temperature from 27° to -57° C. within experimental error. Details of Mcintosh's work: to explain his emission re s u l t s q u a n t i t a t i v e l y he assumed only three species were of importance, namely, the v i b r a t i o n a l l y hot s i n g l e t state reached on e x c i t a t i o n and the thermally e q u i l i b r a t e d excited s i n g l e t 12 and t r i p l e t states which r e s u l t a f t e r v i b r a t i o n a l relaxation. Such a "strong" c o l l i s i o n a l mechanism i s not altogether 4 r e a l i s t i c i n view of recent work by Kutschke and co-workers 7 and Halpern and Ware. A k i n e t i c d e s c r i p t i o n of the primary process, which includes a complete multistage v i b r a t i o n a l degradation, becomes a l g e b r a i c a l l y unmanageable i n the sense of an actual evaluation of rate-constants, or even i n terms of a quantita-t i v e t e s t of the mechanism v i a various graphical p l o t s . A c t u a l l y , the strong c o l l i s i o n approach i s already complicated even though i t i s a gross s i m p l i f i c a t i o n . The processes necessary to account for the observed r e s u l t s are as follows: B + hv »- 1B* i B * k l 3B* (i) 1B* k2 2C0 + C 2 F g ( i i ) i B * k f B + hv f ( i i i ) 1B* + M co 1B°+ M* (iv) i B o k f ^ B + hv.-, (v) f l u o r i B o k i s c *- 3B* (vi) i B o = k g B ( v i i ) 3B* + M to *- 3B°+ M ( v i i i ) 13 3B* 3 B o 3 B 0 3 B 0 + 3 g 0 k l l '12 2C0 + C_Fg (ix) B + hv , (x) phos B (xi) ? ( x i i ) with B a ground state hexafluorobiacetyl molecule, and M, any molecule which causes v i b r a t i o n a l e q u i l i b r a t i o n . The super-s c r i p t s 1 and 3 r e f e r to the m u l t i p l i c i t y of the excited molecules and the superscripts 0 and an asterisk denote mole-cules i n equilibrium and non-equilibrium v i b r a t i o n a l states r e s p e c t i v e l y . * I t should be noted that Mcintosh assumed that the state involved for processes ( v i i i ) and (ix) was a v i b r a t i o n a l l y excited t r i p l e t . From a steady-state treatment of the mechanism, the emission y i e l d s were given by the expressions k, uM k* f r * f = (k f+ k i g c + k 6) (k* + k x + k_ + coM) (k* + k± + k_ + coM) (1) 6 coM k. ., _ _ i s c t_M ^p ~ (toM + k c) " (k. + k. + k_) (k* + k, + k 0 + coM) D r i s c D I 1 £. *The notation and numbering scheme used here w i l l be continued throughout the t h e s i s . 14 + B coM ^1 ( 2) (coM + k 5 ) (k* + k± + k 2 + coM) with 6 = k p k p + k l l I t follows from equations (1) and (2) that when M = 0 , • f " * f * (k* + \ + k 2 ) <3> a constant,and <f>p = 0. At i n f i n i t e pressure the l i m i t i n g y i e l d s become: k k ,°° _ f = , .«» _ B i s c fc. <j> = (4) and A = jr—^—j- . ) (5 1 (k. + k. + k J p u f + i s c + V x f i s c 6 i . e . and <$F are constant, independent of pressure and wavelength. 15 In the intermediate pressure range the pattern i s more complex. However, the shape the emission y i e l d s should take are shown below. Mcintosh's r e s u l t s showed these general trends. PREDICTED FORM OF THE EMISSION YIELDS 1 6 H e r a t i o n a l i z e d t h e d i f f e r e n t p r e s s u r e d e p e n d e n c i e s p h o s o f ^ P ^ Q * a n d < f ) f 1 . u o * i n t h e l o w e r p r e s s u r e r e g i o n a s e v i d e n c e f o r : ( a ) F l u o r e s c e n c e f r o m n o n - e q u i l i b r a t e d 1B* l e v e l s ( p r o c e s s ( i i i ) ) . ( b ) I n t e r s y s t e m c r o s s i n g t o t h e t r i p l e t m a n i f o l d f r o m e x c i t e d s i n g l e t l e v e l s ( p r o c e s s ( i ) ) a s w e l l a s f r o m e q u i l i b r a t e d l e v e l s ( p r o c e s s ( v i ) ) . ( c ) D i s s o c i a t i o n f r o m t h e v i b r o n i c t r i p l e t s t a t e r e a c h e d b y i n t e r s y s t e m c r o s s i n g ( p r o c e s s ( i x ) ) . M o r e o v e r , h e f o u n d t h a t t h e 4 > f i u o r e x t r a p o l a t e s s m o o t h l y t o a f i n i t e l i m i t a t z e r o p r e s s u r e . T h i s r e s i d u a l f l u o r e s c e n c e y i e l d d e c r e a s e s w i t h i n c r e a s i n g e x c i t a t i o n e n e r g y a s p r o c e s s ( i i ) p r e s u m a b l y b e c o m e s d o m i n a n t . K u t s c h k e a n d c o -4 w o r k e r s h a v e o b s e r v e d t h i s e f f e c t i n H F A . D . P u r p o s e o f t h i s I n v e s t i g a t i o n Q u a n t i t a t i v e d o c u m e n t a t i o n o f t h e p r i m a r y p r o c e s s i s v i r t u a l l y u n k n o w n i n a l i p h a t i c s y s t e m s . B i a c e t y l a n d H F A a r e p o s s i b l y t h e o n l y c o m p o u n d s f o r w h i c h t h e n e c e s s a r y e v i d e n c e e x i s t s . I n b o t h c a s e s h o w e v e r , c o m p l i c a t i o n s a r i s e w h i c h a r e * ^ p h o s a n c ^ ^p a s a s ^ f i u o r a n ( ^ ^ f a r e u s e c ^ i n t e r c h a n g e a b l y t h r o u g h o u t t h i s t h e s i s . 17 s p e c i f i c to that p a r t i c u l a r system: t r i p l e t - t r i p l e t i n t e r a c t i o n 2 g to give products i n b i a c e t y l and thermal-like d i s s o c i a t i o n for HFA.4 I t appears that these p e c u l i a r i t i e s are absent i n HFB. Moreover, Mcintosh 1 has recently determined the absolute emission quantum y i e l d at 250 t o r r of HFB and has put his y i e l d s , obtained as a function of pressure and e x c i t a t i o n energy, on an absolute basis. I t i s now necessary to assess the role of d i s s o c i a t i o n i n energy d i s s i p a t i o n i n the primary.process. By determining absolute photochemical quantum y i e l d s of HFB over a range of conditions information regarding the roles of s i n g l e t and t r i p l e t d i s s o c i a t i o n could be obtained. The aims of t h i s work have been to provide such a study. Absolute photochemical quantum yi e l d s of HFB are obtained at various e x c i t i n g wavelengths between 250 and 440 nm over a wide range of pressures. The e f f e c t s of temperature and of phosphorescence quenching on photochemistry are also ascertained. These r e s u l t s elucidate the importance of disso-c i a t i v e processes and t h e i r r e l a t i o n s h i p to v i b r a t i o n a l relax-ation i n the primary process. F i n a l l y the photochemical and photophysical data are combined and a k i n e t i c description of the primary process i s formulated. 18 CHAPTER II  Experimental Arrangement and Procedure A. B r i e f Synopsis of Experimental Procedure A block diagram of the setup for photolysis i s shown i n Figure 2 . B a s i c a l l y three quantities are necessary for the evaluation of a product quantum y i e l d - the f r a c t i o n of l i g h t absorbed by the sample, the i n t e n s i t y of l i g h t and the amount of product formed i n a s p e c i f i e d time. A f t e r f i l l i n g the c e l l at the desired pressure of gas, the f r a c t i o n of l i g h t absorbed was determined by a photo-tube (PT2). During the photolysis run the i n t e n s i t y of l i g h t was continuously monitored by another phototube (PT1) which had previously been c a l i b r a t e d absolutely. A f t e r photolysis the non-condensable product (carbon monoxide) was transferred q u a n t i t a t i v e l y from the c e l l to a small volume (V). The l a t t e r could then form part of the gas chromatographic c i r c u i t (GC). Emission spectra could also be monitored during a photolysis run. FIGURE 2 PHOTOLYSIS SYSTEM P T 1 A. B, C. D. E. F. G. H. J. PHOTOLYSIS SYSTEM Lamp and Housing . 1P21 P h o t o m u l t i p l i e r and Housing. C e l l and mixer . Corning g l a s s f i l t e r . Quartz p lano-convex l e n s . Leeds and Northrup m i c r o v o l t a m p l i f i e r and 10 mv s t r i p r e c o r d e r . B . & L . U V - V i s i b l e g r a t i n g monochromator. L i g h t s tops . L i g h t beam s p l i t t e r . PT1 and PT2: Phototubes (RCA 935 vacuum photodiode , S-5 response) . 20 B. Vacuum System In view of the fact that mercury i s an e f f i c i e n t 4 27 quencher of hexafluoroacetone t r i p l e t state ' i t was thought advisable to b u i l d a mercury-free system to handle hexafluoro-b i a c e t y l . The vacuum system was of standard, a l l glass construc-ti o n consisting of a g a l l e r y of storage globes, a section leading to the photolysis c e l l and a manifold with a series of traps for p u r i f i c a t i o n together with a Le Roy-Ward s t i l l . The temperature of the s t i l l was measured with Cu-constantan thermocouples placed at the top and bottom and two intermediate p o s i t i o n s . Apiezon N grease was used on the greaseable stop-cocks. High Vacuum Teflon stopcocks (Kontes, Vineland, New Jersey) were used, however, to connect the c e l l to the vacuum system. This type of greaseless stopcock was also used throughout the a n a l y t i c a l section of the apparatus. The pumping system consisted of a standard rotary o i l pump and a metal two-stage d i f f u s i o n pump (Edwards E01) operated with S i l i c o n e 704 o i l (Dow Corning). The system could be — fi evacuated to 3 x 10 t o r r with a r e l i a b l e working vacuum of -5 4 x 10 t o r r a f t e r i s o l a t i o n from the pump. Pressures were 21 measured with an NRC thermocouple, an NRC 53 8P i o n i z a t i o n gauge, and a pyrex s p i r a l gauge accurate to ± 0.5 t o r r . C. Preparation and P u r i f i c a t i o n of Chemicals (i) Hexafluorobiacetyl (HFB) HFB was prepared by the chromic acid oxidation of 2,3-dichloro-l,1,1,4,4,4-hexafluoro-2-butene following the 1 2 8 modification by Mcintosh of Moore and Clark's method. The crude condensate containing the HFB was p a r t i a l l y p u r i f i e d by trap-to-trap d i s t i l l a t i o n i n vacuo from -78° C (methanol/C0 2) to -96° C (toluene/liquid N 2) slush baths. This procedure was repeated several times. The majority of th i s semi-pure material was stored i n break-seals at -78° C. The portion of HFB to be used i n the series of experiments was further p u r i f i e d by d i s t i l l a t i o n through a Le Roy-Ward s t i l l set at -65° C and then stored i n the side arm of a blackened 1 - l i t r e globe at -196° C. Immediately p r i o r to use the sample was degassed at -196° C. A phosphorescence l i f e t i m e determination served as a c r i t e r i o n of adequate degassing. ( i i ) Hexafluoroacetone (HFA) HFA, obtained from A l l i e d Chemical was p u r i f i e d by outgassing from the l i q u i d held at the temperature of an ether 22 mush; the m.p. of HFA i s several degrees lower than that of ether. A phosphorescence l i f e t i m e determination again served as a c r i t e r i o n of p u r i t y . A l l HFA samples used had a T >3.1 msec. P " ( i i i ) Hexafluoroethane (HFE) HFE, supplied by Matheson Co. (Freon -116) was degassed at -196° C by trap-to-trap d i s t i l l a t i o n . (iv) Hexafluoroazomethane (HFAM) HFAM, supplied by Merck, Sharp & Dohme (Isotopic Products D i v i s i o n , Montreal) was degassed at -196° C by trap-to-trap d i s t i l l a t i o n . (v) Carbon Monoxide Carbon monoxide, supplied by Matheson Co. (CP. Grade) was used without further p u r i f i c a t i o n . D. O p t i c a l Arrangement The e x c i t a t i o n source for the experiments consisted of a PEK 110 mercury arc lamp operated at 10 0 watts from a s t a b i l i z e d DC power supply (PEK Model 401). A Bausch and Lomb 254 297 23 313 FIGURE 3 366 PHOTOLYSIS LINE SHAPES 334 (Relative i n t e n s i t i e s j| have no signif i c a n c e ) H * 1 i * *:-:-:>x f 2 5 0 2 9 0 3 3 0 3 7 0 24 Monochromator(33-86-25) was used for i s o l a t i o n of the e x c i t i n g wavelengths. They were the 436, 405, 366, 334, 313, 297 and 254 nm Hg l i n e s . A UV-visible grating (33-86-07) 1200 grooves/mm blazed at 250 nm, r e c i p r o c a l l i n e a r dispersion of 7.2 nm/mm was used i n conjunction with a p a i r of variable s l i t s . With 254 nm ra d i a t i o n the e x i t s l i t was set at 3.0 ram, while a 1.0 mm e x i t s l i t was used at a l l other wavelengths. The divergent beam was passed through a combination of two quartz plano-convex lenses which produced a p a r a l l e l beam of r a d i a t i o n . A Corning 7-54 v i s i b l e absorbing f i l t e r was used with these lenses at 254, 297, 313, 334 and 366 nm. At 405 nm and 436 nm Corning 3-75 and Corning 3-73 f i l t e r s were used respectively. This p a r a l l e l , f i l t e r e d beam then entered the reaction c e l l . The spectral c h a r a c t e r i s t i c s of the "monochromatic" ra d i a t i o n used i n the photolysis were ascertained using a second (Bausch and Lomb) grating monochromator, placed at the far end of the c e l l . Emergent l i g h t from t h i s analyser (lOOy e x i t s l i t ) f e l l on to a RCA 1P28 photomultiplier coupled with a Leeds and Northrup microvolt amplifier and a 10 mV recorder. 25 The l i n e shapes are summarized i n Figure 3. The half-widths could only be bettered at the expense of considerable loss i n i n t e n s i t y , and t h i s l a s t factor determined the p r a c t i c a -b i l i t y of quantitative photolysis under most conditions. The i n t e n s i t y of l i g h t was measured before i t entered the c e l l by having a quartz window i n the o p t i c a l t r a i n set at a s l i g h t angle so as to r e f l e c t a small propor-t i o n of the l i g h t onto an RCA 935 photocell (PT1). . The l a t t e r was used with a Leeds and Northrup microvolt amplifier and a 10 mV recorder. I t was c a l i b r a t e d absolutely by a c t i n -ometry (section H). The l i g h t i n t e n s i t y was also measured by a d i f f e r e n t photocell (PT2) af t e r i t traversed the c e l l . This detector was used primarily to determine the f r a c t i o n of l i g h t absorbed (in situ) for each photolysis run (section I ) . E. Reaction C e l l s C y l i n d r i c a l reaction vessels were of pyrex, with quartz windows whose transmission properties at various wave-lengths had been measured before attachment to the c e l l with "Ar a l d i t e " r e s i n cement. 26 FIGURE 4 g r e a s e l e s s va l ves m i x i n g c h a m b e r e m i s s i o n w i n d o w 5-cm reaction cell and mixer 27 Photochemical experiments were done i n either of two c e l l s , both of which had an i n t e r n a l diameter of 2 cms. One of the c e l l s was 30 cm long while the other was 5 cm long. Both were attached by CAJON (Cajon Company, Solon, Ohio 44139) s t a i n l e s s s t e e l f l e x i b l e tubing ( V O.D.) to the vacuum system at one end and the analysis system at the other end. This allowed for adjustments to be made i n the p o s i t i o n of the c e l l on the o p t i c a l t r a i n without having to cut any of the connecting glass tubing. The 30 cm. c e l l was f i t t e d with a thermostatic jacket for temperature con t r o l l e d runs. The 5 cm c e l l was attached to a glass magnetically-driven mixing chamber (Figure 4). A quartz window was f i t t e d to one side of t h i s c e l l with " A r a l d i t e " for emission measurements. F. Temperature Control Systems A Lauda Constant Temperature Bath and C i r c u l a t o r , Series N (Brinkmann Instruments Inc., N.Y.) was used with the 30 cm c e l l . For temperatures below 25° a dry ice heat ex-changer was connected "in-series"with the c i r c u l a t o r . Above room temperature, tap-water was c i r c u l a t e d through the b u i l t -28 i n cooling c o i l . The c e l l was well insulated with cotton batting. The temperature gradient along i t s length was measured with Cu-constantan thermocouples and was found to be constant ± 1° C. G. Gas Analysis The analysis system i s shown schematically i n Figure 5. (i) Normal Procedure A f t e r photolysis the contents of the reaction c e l l were allowed to flow through two successive traps at -190° ( l i q u i d N 2). These condensed out C^F^and unchanged HFB, and carbon monoxide was measured as the only non-condensable product. One re-evaporation of the condensed products was necessary to release occluded CO. The l a t t e r was then trans-29 ferred by a Delmar-Urry Automatic Topler Pump (Delmar S c i e n t i f i c Laboratories) to a small i s o l a t e a b l e volume. The c a r r i e r gas of the chromatograph was then diverted so that t h i s volume formed part of i t s c i r c u i t . Subsequently the CO was flushed onto the chromatographic column. The l a t t e r was a 5', V molecular sieve column (13X) operated at 100° C and Icelt | TOPLER PUMP V fiAS ANALYSIS SYSTEM (schematic) metal sample 'oop T T hel mm flow •greaseless \ stopcocks J / I G.C. column r o^hductivitx MfectGr^  , r e c o r d e r 30 at a flow of 75 ml/min. The detector was a Varian Aerograph Thermal Conductivity Detector (No. 01-000334-00) with two matched pairs of 30 ohm. tungsten rhenium (WX) filaments. They were operated at 150 mA and at ambient temperature. The power supply was a Kepco regulated DC supply (Model PAT 21-IT). Great care was taken to s h i e l d the various components from external noise. The detector block was housed i n a copper box which was then f i l l e d up with mica chips to keep temperature fluctuations at a minimum. The reference and sensing resistance elements were incorporated into a Wheat-stone bridge, and the out-of-balance sig n a l was applied to a Leeds and Northrup microvolt ampl i f i e r coupled to a Brown (MH) Recorder (Model 143 x 58) equipped with a Disc Chart Integrator (Model 201). The chromatograph was c a l i b r a t e d using known samples of CO a f t e r every run. ( i i ) Procedure for Ratio-Determining Runs Aft e r photolysis, the contents of the c e l l were expanded into the evacuated glass tubing and metal sample loop. The loop was then removed and placed i n p o s i t i o n , by means of B10 glass sockets, i n the helium stream. The sample was subsequently flushed onto a-51,h" Porapak Q (50/80 mesh) column 31 operated at 0° C and a flow of 75 ml/min. The chromatograph was c a l i b r a t e d using known synthetic mixtures of C_F,and CO. 2. D H. Actinometry The i n t e n s i t y of the absorbed r a d i a t i o n at the various pressures was monitored by means of a phototube (PT1) (RCA 935 vacuum photodiode, S-5 response) operated at 90 v o l t s . I t was c a l i b r a t e d against the potassium f e r r i o x a l a t e 30-31 actmometer of Hatchard and Parker. The quartz actino-meter c e l l , diameter 2.5 cm, depth 1.0 cm, was placed imme-d i a t e l y behind the reaction v e s s e l . O p t i c a l densities of exposed and developed solutions were determined on a Cary 14 spectrophotometer at 510 nm. The usual blank correction was made with an unexposed s o l u t i o n . Because of the large transmission losses from the s i l i c a - a i r interfaces of the reaction vessel and the actino-meter c e l l , i t was necessary to apply a correction factor to 32-33 obtain the absolute i n t e n s i t y of the absorbed r a d i a t i o n . For convenience a l l windows were assumed to have i d e n t i c a l losses from r e f l e c t i o n (Appendix 1). The phototube, i n conjunction with a Leeds and Northrup microvolt amplifier and 10 mV s t r i p chart recorder 32 was found to have a l i n e a r response over the range of inten-s i t i e s required for the series of experiments. This was done using a set of ca l i b r a t e d neutral density f i l t e r s ( O r i e l Optics Corporation). Actinometry was frequently done to check the photo-tube (PT1) c a l i b r a t i o n . No change was found for the l a t t e r over a period of 9 months. I. Measurement of the Fraction of Light Absorbed by the Sample The f r a c t i o n of l i g h t absorbed (FLA) by the sample was measured by phototube (PT2). Before each run, with the c e l l empty, PT1 and PT2 were simultaneously read. When the run had begun PT1 and PT2 were again read. This way, i f the absolute i n t e n s i t y of the l i g h t had changed then the PT2 reading could be corrected for i t . This procedure was f o l l -owed for every run where the FLA was between c i r c a 0.85 and 0.30. When i t lay outside th i s range (normally at pressures below 5 mm i n the 30 cm c e l l ) the value of the ex t i n c t i o n c o e f f i c i e n t (e) from the higher pressure runs was used and the FLA was computed from t h i s . This procedure was j u s t i f i e d by the f a c t that at intermediate pressures, where measure-ments could be made with both c e l l s , Lambert's Law was found 33 to hold exactly. Beer's law was obeyed at a l l wavelengths i n e i t h e r c e l l over the complete range of pressures used. J. Emission Spectroscopy Figure 2 shows the arrangement used for observing emission spectra with the 5 cm c e l l . The entrance s l i t of the analysing Bausch and Lomb monochromator (33-86-25) was placed as near to the observing window as possible. A v i s i b l e grating (33-86-02) 1350 grooves/mm, blazed at 250 nm, re c i p r o c a l l i n e a r dispersion of 6.4 nm/mm was used. The emission was recorded using a 1P21 photomultiplier operated at approximately 1000 v o l t s . K. Lifetime Measurements (i) Flash lamp Phosphorescence l i f e t i m e measurements were made with an argon f i l l e d coaxial c a p i l l a r y f l a s h lamp operated at 4-5 kv. This lamp dissipates 16 to 25 J and has a peak 34 r i s e time of 3 usee and a half-width of 8 usee. The argon discharge produces a continuum throughout the v i s i b l e and » u l t r a v i o l e t , down to the s i l i c a cut-off. The emission 34 emerging from one end of the c a p i l l a r y was passed through a Corning C.S. 7-54 (3 mm thick) f i l t e r and was focused with a s i l i c a lens i n t o the c e l l . Generally due to the long l i f e t i m e of the emission being studied and to the c e l l geometry, scattered e x c i t i n g r a d i a t i o n presented no d i f f i -c u l t i e s . For low pressures ( < 5 t o r r ) , however, considerable care had to be taken. Correction for scattered l i g h t was made by condensing the contents of the c e l l i n a side arm or by evacuating the c e l l completely. ( i i ) Lifetime C e l l A conventional T-shaped c e l l with a Wood's horn for a l i g h t trap was used. The main body of the c e l l (5.0 cm long) was constructed from 30 mm Pyrex tubing, with 20 mm tubing used for the l i g h t trap and viewing port which were at r i g h t angles to the main c e l l arm. S i l i c a windows were attached using epoxy r e s i n and the outside surfaces of the c e l l s were covered with a f l a t black paint. ( i i i ) Detection System The ketone emission was monitored perpendicular to the lamp discharge. Emitted l i g h t was recorded with a 35 1P28 photomultiplier (using a 10 k load re s i s t o r ) on the e x i t s l i t of one of the Bausch & Lomb v i s i b l e monochromators. A glass c o l l e c t i v e lens (33-85-33) was used on the entrance s l i t . S l i t widths used were: e x i t 3.00 mm; entrance 5.36 mm, ( i . e . , band pass 19.2 nm). The voltage output from the photo-m u l t i p l i e r was displayed eith e r on a Tektronix 542B or 547 oscilloscope and photographed with a Tektronix C-27 Trace-Recording Camera with Polaroid Type 47 (ASA 3000) f i l m . A 0.003 uf capacitor was placed i n p a r a l l e l with the o s c i l l o -scope and photomultiplier as a noise f i l t e r . This f i l t e r had a n e g l i g i b l e e f f e c t on the measured emission decay times. To improve accuracy of the analysis the decay curves were enlarged by approximately four times with a delineascope. The l i f e t i m e s obtained i n t h i s way were reproducible to within 5%. L. Errors The error l i m i t s are given as ± one standard deviation. 36 CHAPTER III Photochemistry of HFB RESULTS The majority of re s u l t s of the in v e s t i g a t i o n are given i n t h i s chapter. Where appropriate, they are d i s -cussed b r i e f l y . A. Absorption The mean molar absorption c o e f f i c i e n t s of HFB found by measuring the percent transmission i n s i t u (Page 32) are l i s t e d i n Table 1. The absorption spectrum obtained 35 on a Cary 14 recording spectrophotometer i s reproduced i n Figure 6. B. Ratio Experiments 24 Although Whittemore and Szwarc reported xn a short note that the gas phase photolysis of HFB at 25° snd 150° C produced only CO and C 2Fg i n a 2:1 molar r a t i o they gave no fig. 6 X(nm) Absorption spectra of hexafluorobiacetyl vapour at 25 °C 38 error l i m i t s for t h e i r r e s u l t s . If t h e i r observations were q u a n t i t a t i v e l y correct, then our analysis would be consid-erably s i m p l i f i e d . Table 2 shows the conditions used and r e s u l t s obtained i n t h i s work. CF^, a possible photochemical product was not detected under any of our conditions. Discussion: Although extreme conditions were used i t was found that CO and C^F^were indeed produced i n 2:1 molar z o r a t i o within our experimental error. On t h i s basis, there-fore, carbon monoxide only was determined thereafter for each photolysis run. C. R e l i a b i l i t y of the Photolysis Data Experimental quantum y i e l d s are notoriously prone to large random and systematic e r r o r s . Chief sources of random error are i n measuring the f r a c t i o n of l i g h t absorbed 2 and i n reading the s p i r a l gauge. Unlike the case of HFA ,small temperature fluctuations (± 2° C )would not contribute a noticeable random error. These random errors are f a i r l y 39 Table 1. Mean molar absorption c o e f f i c i e n t s of HFB at the wavelengths used i n photolysis, at 25° C. (nm) (M cm ) 254 6.60 297 11.9 313 5.26 334 2.42 366 4.89 405 12.9 436 12.2 Table 2. Ratio experiments Pressure Irrad.Time CO C F U2*6 Ratio Decompos i t i o n 297 5 mm 1200 sees 0.628 0. 326 1.93 ±0.20 3% 297 10 mm 18 hours 0.604 0.286 2.12 25% 297 20 mm 4.75 hours 0. 530 0.265 2.00 2.5% 313 1 mm 2 0 hours 2.17 35% F u l l Hg arc 5 mm 5 hours 1. 94 25% 40 r e f l e c t e d i n the scatter of points on l/<f>pl3 v s - (HFB) plots (e.g. Figure 10) and normally amount to about ±10%. Syste-matic errors were harder to point to. To t h i s end HFA was put i n t o the system and the photochemical quantum y i e l d s 4 obtained were compared with Kutschke's published data (obtained by i n t e r p o l a t i o n of his graphical data). The re s u l t s are given i n Table 3. There i s seen to be reasonably good agreement between the two studies. He used acetone as an actinometer. I t seemed therefore that the system was free of major systematic errors although the p o s s i b i l i t y of both studies being i n c o r r e c t could not be ruled out. F i n a l l y i t should be noted that the "absolute" value of a p a r t i c u l a r quantum y i e l d i s r a r e l y as important as i t s r e l a t i v e value with respect to other quantum y i e l d s . D. Photolysis of HFB at 25° C. The exposure time for a run at room temperature varied from 30 minutes to 36 hours, and was generally around 2 hours. At low absorptions (e.g. with 334 nm ra d i a t i o n and low pressure) or with low quantum y i e l d s (e.g. with 366 nm ra d i a t i o n and high pressure) t h i s time was determined by the 41 Table 3 Photolysis of HFA at 313 nm C e l l length: 30 cm Temperature: 25° C HFA Concentration (mm Hg) * (a) CO * (b) CO 50 0.32Q 0.33Q 20 0.371 0. 40 ? (a) This 4 work (b) Kutschke Table 4 Photolysis of HFB at 254 nm Temperature: 25° C HFB Cone (torr) 5 10 20 50 100 200 300 400 t ' c o 1.154 0.869 0.732 0.608 0.470 0.395 0.466 0. 378 P. P. 1/<J> P.P. 0.577 0. 435 0.366 0. 304 0.235 0.197 0.233 0.189 1.73 2.30 2.73 3.29 4.26 5.07 4.30 5.30 42 Table 5 Photolysis of HFB at 297 nm Temperature 25° C nrD tunc (torr) *co <t> P.P. l/4>p 0.5 1.280 0.640 1.56 0.5 1.226 0.613 1.63 0.5 1.226 0.613 1.63 1.0 1.130 0.565 1.77 1.47 1.035 0.518 1.93 2.0 0. 860 0.430 2.33 2.0 0.880 0.440 2.27 2.79 0.790 0.395 2.53 4.0 0.777 0.388 2.58 5.0 0.742 0.371 2.69 5.0 0.742 0.371 2.69 7.5 0.642 0.321 3.12 10.0 0.620 0.310 3.23 15.0 0.589 0.295 3.39 20.0 0.541 0.271 3.70 30.0 0.440 0.220 4.55 40.0 0.418 0.209 4.79 • P. 43 Table 5 (continued) Temperature 25°C (torr) yC0 "b.p. *""P.P. 50.0 0.402 0.201 4.97 50.0 0.394 0.197 5.07 100.0 0.306 0.153 6.55 200.0 0.217 0.109 9.22 400.0 0.137 0.068 14.6 44 Table 6 Photolysis of HFB at 313 nm Temperature 25 0 C HFB Cone (torr) *CO P.P. 0.5 1.091 0.546 1.83 1.0 0.657 0.329 3.04 2.0 0. 813 0.407 2.46 3.5 0.824 0.412 2.43 5.0 0.652 0.326 3.07 10.0* 0.569 0.284 3.52 10.0** 0.578 0.289 3.46 15.0 0.580 0.290 3.45 20.0 0.462 0.231 4.33 20.0 0.474 0.237 4.22 50.0 0.333 0.166 6.01 50.0 0.350 0.175 5.71 75.0 0.284 0.142 7.04 100.0 0.223 0.112 8.98 100.0 0.216 0.108 9.26 125.0 0.186 0.093 10.8 125.0 0.192 0.096 10.4 200.0 0.152 0.076 13.1 340.0 0.101 0.051 19.8 15 2 •Intensity of e x c i t i n g l i g h t , 0.48 x 10 photons/cm 2/sec. ••Intensity of e x c i t i n g l i g h t , 1.45 x 10 photons/cm /sec. 45 Table 7 Photolysis of HFB at 334 nm Temperature 25° C HFB Cone , , ... (torr) *C0 ^P.P. 1/<{)P.P, 2.0 3.0 4.0 5.0 5.0 5.0 7.5 10.0 15.0 20.0 50.0 100.0 100.0 150.0 200.0 200.0 300.0 400.0 1.035 0.900 0. 811 0.999 0. 824 0.803 0.621 0.544 0.511 0.413 0.211 0.121 0.125 0. 095 0.0692 0.0904 0.0527 0.0386 0.517 0.450 0.406 0.499 0.412 0.402 0. 311 0.272 0.256 0.207 0.106 0.061 0.062 0.048 0.0346 0.0452 0.0264 0.0193 1.93 2.22 2.46 2.00 2.43 2.49 3.22 3.68 3.92 4 . 83 9.49 16.5 16.0 21.0 28.9 22.1 37.9 51.8 46 Table 8 Photolysis of HFB at 366 nm Temperature 25 0 C HFB Cone (torr) ^CO *P.P. ^ P . P . 0.5 1.028 0.514 1.95 1.0 0.794 0.397 2.52 2.0 0.504 0.252 3.97 5.0 0.255 0.128 7.84 5.0 0.261 0.131 7.65 10.0 0.128 0.064 15.7 15.0 0.0904 0.0452 22.1 20.0 0.0612 0.0306 32.7 30.0 0.0384 0.0192 52 40.0 0.0285 0.0142 70 50.0 0.0277 0.0139 72 50.0 0.0240 0.0120 83 75.0 0.0186 0.0093 10 8 100. 0 0.0153 0.0077 .131 100.0 0.0185 0.0092 108 200.0 0.0126 0.0063 160 400.0 0.0081 0.0041 246 47 Table 9 Photolysis of HFB at 405 nmj Temperature 25° C HFB Cone (torr) "CO 'P.P. l/4> P.P. 1.0 2.0 < 0.0038 < 0.0032 < 0.0019 < 0.0016 > 530 > 630 Table 10 Photolysis of HFB at 436 nm* Temperature 2 5° C HFB Cone (torr) 'CO 'P.P. 1/4) P.P. 0.5 0.5 2.0 5.0 < 0.050 < 0.028 < 0.0014 < 0.00064 < 0.025 < 0.0014 < 0.00072 < 0.00032 > 40 > 71 > 1400 > 3100 *No product detected at these wavelengths under any conditions. 48 Table 11 Photolysis of HFB at 297 nm Temperature -20° C HFB Cone . . , ,, (torr) *CO *P.P. 1 / ( | )P.P, 42.5 0.302 0.151 6.62 34.0 0.326 0.163 6.13 25.5 0.364 0.182 5.49 Table 12 Photolysis of HFB at 313 nm Temperature 50° C HFB Cone . , , .. (torr) ^CO *P.P. ^ P . P . 10.8 0.598 0.299 3.34 21.7 0.522 0.261 3.83 32.5 0.441 0.220 4.54 54.2 0.377 0.189 5.29 108.4 0.266 0.133 7.51 4 9 Table 1 3 Photolysis of H F B at 3 1 3 nm Temperature 7 6 ° C H F B Cone , , ... (torr) ^CO V P . 1 / < { , P . P . 5 . 8 0 . 7 6 7 0 . 3 8 4 2 . 6 0 1 1 . 7 0 . 6 7 8 0 . 3 3 9 2 . 9 5 2 3 . 0 0 . 5 6 9 0 . 2 8 5 3 . 5 1 4 1 . 2 0 . 4 9 1 0 . 2 4 5 4 . 0 8 5 8 . 6 0 . 4 1 3 0 . 2 0 7 4 . 8 3 8 7 . 8 0 . 3 8 3 0 . 1 9 2 5 . 2 2 1 1 7 . 1 0 . 3 3 4 0 . 1 6 7 5 c 9 9 1 4 6 . 4 0 . 3 0 4 0 . 1 5 2 6 . 5 8 50 minimum amount of CO which could be measured to within ±1% -7 (2 x 10 . moles). Normally less than 1% of the HFB was decomposed. On rare occasions (e.g. 0.5 and 1.0 mm experi-ments) t h i s percentage was larger and corrections to the quantum y i e l d s were applied to allow for t h i s . The parameters measured i n each run are related by, , h (Moles of CO produced) Primary = process Einsteins absorbed (P.P.) where "Einsteins absorbed" = F x I x t and F = Fraction of l i g h t absorbed I = Intensity of l i g h t t = I r r a d i a t i o n time The re s u l t s for a l l seven wavelengths are presented i n Tables 4 through 10. E. HFB Photolysis at D i f f e r e n t Temperatures A measured pressure of HFB was dosed i n t o the 30 cm c e l l before i t was heated or cooled. Half an hour was allowed for the gas to e q u i l i b r a t e to the new temperature. The results 51 for the two wavelengths (297 nm at -20° C and 313 nm at 50° and 76° C) are given i n Tables 11 through 13. F. HFB - HFAM System When i t became clear that the d i s s o c i a t i o n of HFB was due to two or more states an attempt was made to d i f f e r -entiate between them. Although the "temperature runs" i n d i -cated that no thermalised t r i p l e t state was involved i n 4 d i s s o c i a t i o n ( c f . as i n HFA ), conclusive evidence had to await photochemical runs i n which the phosphorescence was completely quenched. Available evidence on a s i m i l a r system, 36 biacetyl-azomethane, indicated that hexafluoroazomethane (HFAM) should have a t r i p l e t state lower than HFB. (i) Quenching study I t was found that HFAM was a very e f f i c i e n t quencher of HFB phosphorescence. 50 t o r r HFB were dosed in t o the 5 cm c e l l and the emission spectrum recorded. Known amounts of HFAM were subsequently added by expansion from known volumes and the spectrum again recorded a f t e r each addition. No se n s i t i z e d HFAM phosphorescence was observed at any pressure within the spe c t r a l range of the photomultiplier (300 nm to 650 nm). 52 With the HFAM pressures used here (0 - 35 t o r r ) , only the HFB phosphorescence i s quenched. I f one considers the t r i p l e t state only, the processes of importance are 3HFB° k p »• HFB* + hv phos 3 fl k HFB 11 »• HFB* 3HFB° + HFAM kET ^ HFB* + 3HFAM* i n which the superscript 3 refers to the m u l t i p l i c i t y of the excited state molecule, the superscript 0 and an asterisk denote molecules i n equilibrium and non-equilibrium v i b r a -t i o n a l states, respectively. A steady-state treatment of the mechanism y i e l d s the f a m i l i a r Stern-Volmer r e l a t i o n s h i p p h o s = l + T . k [HFAM] phos ET L —1 ^phos with 4>pnos being the HFB phosphorescence y i e l d i n the absence of HFAM, and the mean t r i p l e t l i f e t i m e T , (2.1 msec) equal phos to the r e c i p r o c a l of the sum k , + k,,. The data so c phos 11 53 p l o t t e d are shown i n F i g u r e 7. The s l o p e i s 320 t o r r or 6.0 x 10^ l i t e r / m o l e , hence the b i m o l e c u l a r r a t e c o n s t a n t 9 k £ T = 3.52 x 10 l i t e r / m o l e s e c . The p r e s s u r e o f HFAM r e q u i r e f o r h a l f quenching i s 0.0025 t o r r , and about 0.2 t o r r are needed to completely remove the HFB phosphorescence. HFAM i s t h e r e f o r e an extremely e f f i c i e n t quencher o f HFB phosphor-escence. The f l u o r e s c e n c e o f HFB remained unchanged even at the h i g h e s t p r e s s u r e o f HFAM used (35 t o r r ) . ( i i ) P h o t o l y s i s o f HFB - HFAM mixtures A measured p r e s s u r e o f HFAM wa*s expanded from c a l i b r a t e d volumes i n t o the 5 cm c e l l which was then i s o l a t e d from the mixer. The mixer was f i l l e d w i t h the d e s i r e d HFB p r e s s u r e . The two gases were then allowed to mix, a i d e d by the m a g n e t i c a l l y d r i v e n s t i r r e r , f o r at l e a s t 30 minutes. The s t i r r e r was a l s o kept i n o p e r a t i o n and the HFB em i s s i o n was monitored throughout the e n t i r e run. The r e s u l t s f o r the t h r e e wavelengths used are g i v e n i n Tabl e 14. P r e v i o u s r e s u l t s f o r the same t o t a l p r e s s u r e s , but of pure HFB, are a l s o g i v e n f o r comparison. 54 Direct photolysis of HFAM was observed when high pressures of the quencher were used. For example, tfj-j was N2 found to be 0.7 (based on the quanta absorbed by HFAM only) with 313 nm radiatio n and a t o t a l pressure of 60 t o r r ( i . e . 25 t o r r HFB and 35 t o r r HFAM). Wu and R i c e 3 7 have found <f> to be 0.72 with 40 t o r r HFAM and 366 nm ra d i a t i o n . W2 In experiments where d i r e c t absorption by HFAM was n e g l i g i b l e , l i t t l e or no nitrogen product was observed. This observation implies that i f the quenching mechanism i s elec-t r o n i c energy transfer from the t r i p l e t state of HFB to the t r i p l e t state of HFAM then the l a t t e r i s unreactive photo-chemically. For example, the s e n s i t i z e d photochemical quantum y i e l d of HFAM can be calculated i f i t i s assumed that the energy transfer process i s 100% e f f i c i e n t . The ISC y i e l d i n HFB i s known to be 0.9.1 For 10 t o r r HFB and 0.2 to r r HFAM with 297 nm rad i a t i o n at 25° C the <}>„ (sensitized) was N2 found to be 0.02 ± 0.02. In the d i r e c t photolysis of 20 to r r HFAM, with 366 nm rad i a t i o n at 28.7° C, Wu and R i c e 3 7 determined to be 0.830. N2 Table 14 Photolysis of HFB/HFAM Mixtures Temperature 25° C C e l l Length 5.0 cm X HFB HFAM Total (nm) (mmHg) (mmHg) pressure ^CO ' P . P . / < F > P . P , (mmHg) 297 10.0 0.201 10.2 .586 .293 3.41 10.0 - 10.0 .620 .310 3.23 45.7 0.201 45.9 .371 .186 5.39 50.0 - 50.0 .398 .199 5.00 100.0 0.201 100.2 .344 .172 5.82 100.0 - 100.0 .306 .153 6.55 0.5 1.5 2.0 1.04 .520 1.93 0.5 1.5 2.0 0.875 .438 2.28 2.0 - 2.0 0.880 .440 2.27 2.0 - 2.0 0.860 .430 2.33 2.0 - 2.0 0.874 .422 2.28 Table 14(Continued) X HFB HFAM Total (nm) (mmHg) (mmHg) pressure , . , (mmHg) C^O *P.P. 1 / < ( ,P.P, 313 25.0 35.0 60.0 0.342 .171 5.85 60.0 - 60.0 0.304 .152 6.6 366 50.0 0.201 50.2 .0233 .0117 85.5 50.0 - 50.0 .0240 .0120 83.3 50.0 - 50.0 .0277 .0139 71.9 fig. 7 Stern-Volmer quenching hexafluorobiacetyl phosphorescence 0'.0C6 0!010 0'.015 O020 (Hexafluoroazomethane) t o r r 58 CHAPTER IV Dissociation and C o l l i s i o n a l Deactivation In t h i s chapter a mechanism i s presented which successfully accounts for the photochemical r e s u l t s . Each step i s discussed c r i t i c a l l y and i t s implications f u l l y r a t i o n a l i z e d . The various rate constants are then evaluated numerically so that the quantum y i e l d s can be simulated. Various a l t e r n a t i v e mechanisms to explain the photochemical r e s u l t s are considered. F i n a l l y experimental evidence for wall deactivation of the e q u i l i b r a t e d t r i p l e t state at low pressures i s presented and discussed. A. Graphical Presentation of Results The r e c i p r o c a l quantum y i e l d of the primary process as a function of HFB concentration i s plotted for a l l wave-lengths i n Figures 8 to 17. The low pressure region i s also shown where appropriate. Figures 18 and 19 show the data acquired at temperatures other than 25° C. 59 B. General Observations (i) At equivalent pressures the decomposition e f f i c i e n c y increases d r a s t i c a l l y with decrease i n wave-length . ( i i ) At a l l f i v e wavelengths where photochemistry takes place, the r e c i p r o c a l quantum y i e l d at high pressure i s a l i n e a r function of HFB concentration. ( i i i ) There i s no evidence of the quantum y i e l d s e i t h e r l e v e l l i n g o f f at high pressures at any wavelength or showing a large v a r i a t i o n with temperature. (iv) When the phosphorescence of HFB i s completely quenched, the quantum y i e l d s remain unchanged. (v) No photochemistry was observed with 40 5 nm or 436 nm r a d i a t i o n . (vi) At lower pressures the plots become curved downwards. Futhermore there i s no evidence to indicate that the slopes tend to zero at the pressures available here. ( v i i ) No quantum y i e l d greater than 0.64 was recorded although HFB was i r r a d i a t e d at pressures as low as 500y. 60 C. Proposed Mechanism One set of processes that accounts s a t i s f a c t o r i l y f o r the observed re s u l t s i s B + hv ^ 1B* 2 i f i * ' k l •* 3B* (i) 2 O lB* k2 2CO + C-F £ — ( i i ) 2 Z D i B * + M to \B° + M *- (iv) 3B*+ M OJ ?B° + M *- ( v i i i ) l 3 B * k 5 2CO + C 2 F g — (ix) with B a ground state hexafluorobiacetyl molecule and M any molecule which causes v i b r a t i o n a l e q u i l i b r a t i o n . The super-and subscripts at the lefthand side of B r e f e r to i t s m u l t i p l i c i t y and i t s state within that m u l t i p l i c i t y respec-t i v e l y . The r i g h t hand side superscripts, 0 and an asterisk denote molecules i n equilibrium and non-equilibrium v i b r a -t i o n a l states, r e s p e c t i v e l y . 1.0 o Parameters used for calculating curve k1 3.85 x 10 sec" 1 k 2 1.51 x 10 sec-1 k K 1.19 x 10 7 sec- 1 5 100 4 0 0 500 200 300 [HFB] torr Reciprocal quantum yields versus pressure— 254 nm, 25°C Hinh nrp^\irf> rpnion R e c i p r o c a l q u a n t u m y i e l d s v e r s u s p r e s s u r e — 2 5 4 n m , 2 5 ° C L o w p r e s s u r e r e g i o n 1 1 1 1 1 1 1 1 1 1 O 100 200 300 400 500 [ H F B ] t o r r R e c i p r o c a l q u a n t u m y i e l d s v e r s u s p r e s s u r e — 2 9 7 n m , 2 5 ° C High pressure r eg ion FIG. 11 P a r a m e t e r s u s e d for c a l c u l a t i n g c u r v e k1 6.34 x 10 sec-1 2.34 x 10 sec-1 k= 5.60 x IO sec-1 16 6 4 8 0 3 2 4 8 [ H F B ] t o r r R e c i p r o c a l q u a n t u m y ie lds v e r s u s p r e s s u r e —297 n m , 25 ° C L o w p r e s s u r e r e g i o n O 100 200 3 0 0 4 0 0 500 [HFB | t o r r Ln R e c i p r o c a l q u a n t u m y ie lds v e r s u s p r e s s u r e — 313 n m , 2 5 ° C Hiah n r e s s u r e r e a i o n 0.81 1 1 1 I 1 1 1 1 1 1 O 16 3 2 4 8 6 4 8 0 [ H F B ] to r r « R e c i p r o c a l q u a n t u m y i e l d s v e r s u s p r e s s u r e — 313 n m , 25 ° C L o w p r e s s u r e r e g i o n R e c i p r o c a l q u a n t u m yie lds v e r s u s pressure—313 n m , 25°C L o w p r e s s u r e r e g i o n P a r a m e t e r s u s e d fo r c a l c u l a t i n g c u r v e 8 k, 0.98 x TO sec"1 8 k2 .536 x IO sec"1 k l.OO x lO^ec 1 5 FIG. 15 o 100 4 0 0 2 0 0 3 0 0 [ H F B ] t o r r R e c i p r o c a l q u a n t u m y i e l d s v e r s u s p r e s s u r e — 3 3 4 n m , 2 5 ° C H i g h p r e s s u r e r e g i o n 5 0 0 CTl 00 12-8 -F I G . I 6 R R 4 -o-o i 1 r 16 P a r a m e t e r s u s e d f o r c a l c u l a t i n g c u r v e 8 k1 .981 x 10 sec-1 k2 .536 x IO sec-* k= 1.00 x IO sec-1 5 3 2 H F B 4 8 to r r 6 4 8 0 R e c i p r o c a l q u a n t u m y i e ld s v e r s u s p r e s s u r e — 334 n m , 25 ° C £ L o w p r e s s u r e r e g i o n 3 0 0 Reciprocal quantum yields versus pressure — 3 6 6 n m , 25°C High pressure region R e c i p r o c a l q u a n t u m y ie lds v e r s u s p r e s s u r e - - 2 9 7 n m . 0 2 5 ° C , • - 2 0 ° C . 1 1 1 1 1 10 20 30 4 0 50 [HFB] torr 73 T h e e x c i t e d s t a t e i n v o l v e d i n p r o c e s s e s ( v i i i ) a n d ( i x ) i s a s s u m e d t o b e t h e v i b r o n i c t r i p l e t f o r m e d o n i n t e r s y s t e m c r o s s i n g f r o m t h e v i b r o n i c l e v e l r e a c h e d o n e x c i t a t i o n . A l t h o u g h t h e d i s s o c i a t i n g m o l e c u l e ( p r o c e s s ( i x ) ) c o u l d b e i n a n y m e t a s t a b l e s t a t e a s f a r a s t h e e x p e r i -m e n t a l e v i d e n c e i s c o n c e r n e d , i t w i l l b e t a k e n t o b e i n t h e a b o v e m e n t i o n e d v i b r o n i c t r i p l e t s t a t e i n t h e d i s c u s s i o n t h a t f o l l o w s . F r o m a s t e a d y - s t a t e t r e a t m e n t o f t h e m e c h a n i s m , t h e p r i m a r y q u a n t u m y i e l d f o r d i s s o c i a t i o n i s g i v e n b y : P r i m a r y P r o c e s s k , + k „ + cuM M o l e c u l e s D e c o m p o s i n g f r o m N o n - e q u i l i b r a t e d S i n g l e t S t a t e + O J M X (6) M o l e c u l e s D e c o m p o s i n g f r o m T r i p l e t S t a t e H a v i n g O r i g i n a t e d f r o m N o n - V i b r a t i o n a l l y E q u i l i b r a t e d S i n g l e t S t a t e 74 Rearranging we f i n d ' P . P . k 1 + k 2 + coM k 2 + k l k 5 k 5 + OJM k l + k 2 COM 1 + k 2 ( 1 + TZ > (7) At high pressure ^P.P„ M -* » 1 + toM (8) Therefore, 1/<J>P p vs. HFB cone, w i l l be l i n e a r at high pressures with a slope and intercept given by w/k2 and (1 + k^ / k 2) respectively. 75 At low pressure k, w ( P.P. M ->• 0 k 2 ) M- (9) Therefore at low enough pressures the r e c i p r o c a l quantum y i e l d s w i l l tend toward unity. D. Discussion (i) C o l l i s i o n a l Deactivation I t can be seen that the mechanism accounts for the e s s e n t i a l features of the photochemical r e s u l t s . I t assumes a single-step deactivation. This does not necessarily mean that the v i b r a t i o n a l l y e q u i l i b r a t e d species (JB°) i s formed d i r e c t l y from *B* with unit e f f i c i e n c y , but simply that the f i r s t c o l l i s i o n reduces the p r o b a b i l i t y of d i s s o c i a t i o n to a n e g l i g i b l e value. However ,the recent work of Kutschke et a l on the pressure dependence of HFA photochemistry appears to give d e f i n i t e evidence that excess v i b r a t i o n a l energy i s 76 dissipated v i a a multistage cascade ("weak" c o l l i s i o n a l 7 3 8 process). Ware's l i f e t i m e measurements on HFA and P i t t s ' photochemical data on trans-Crotonaldehyde support a s i m i l a r conclusion. Nevertheless, a k i n e t i c d e s c r i p t i o n of the primary process which includes a complete multistage v i b r a t i o n a l degradation becomes a l g e b r a i c a l l y unmanagable with regard to evaluation of rate constants and diagnostic p l o t s . Therefore, the strong c o l l i s i o n approximation w i l l be taken as a basis for discussion, bearing i n mind the oversimpli-f i c a t i o n so produced. ( i i ) Mechanistic Considerations The data are consistent with a mechanism that has photochemistry o r i g i n a t i n g from two d i f f e r e n t states. One, the excited s i n g l e t state reached on e x c i t a t i o n , i s v i b r a t i o n -a l l y excited while the other i s postulated to be the v i b r a -t i o n a l l e v e l of the excited t r i p l e t reached on intersystem crossing (ISC) from the vi b r o n i c state reached on e x c i t a t i o n . At high pressures therefore, d i s s o c i a t i o n (process ( i i ) ) competes d i r e c t l y with c o l l i s i o n a l degradation from the s i n g l e t state. Once v i b r a t i o n a l l y e q u i l i b r a t e d , the 77 excited molecule can fluoresce (process (v)) , cross over to the t r i p l e t manifold (process (vi)) or return to i t s ground state by some other r a d i a t i o n l e s s process (process ( v i i ) ) . As the pressure i s lowered ISC (process (i)) as we l l as d i s s o c i a t i o n of *B* become more competitive with c o l l i s i o n a l deactivation. Consequently/the t r i p l e t state must be populated from the state reached on e x c i t a t i o n . The pressure at which t h i s competition becomes apparent depends on wavelength. The v i b r a t i o n a l l y excited t r i p l e t state molecule can then di s s o c i a t e (process (ix)) or be c o l l i s i o n -a l l y degraded to the v i b r a t i o n a l l y e q u i l i b r a t e d t r i p l e t . In t h i s low pressure region ( i . e . less than ca. 30 t o r r , but dependent on wavelength) photochemistry w i l l take place from both states. The f a c t that no photochemical products were detected with 405 nm or 436 nm r a d i a t i o n i s i n t e r e s t i n g . The S 1 S Q o r i g i n has been t e n t a t i v e l y placed at 450 ± 30 nm.1 E x c i t a t i o n with 436 nm and 405 nm radiation therefore, popu-lates HFB i n S^ with l i t t l e or no excess v i b r a t i o n a l energy and with 7.1 kcal/mole, resp e c t i v e l y . Consequently, i t appears that the f i r s t excited state of HFB does not dissociate unless 78 i t has at least 70 kcals of vibro n i c energy. The t r i p l e t formed by the intersystem crossing process from would also seem to be unreactive i n t h i s regard. ( i i i ) Photochemical Inertness of Eq u i l i b r a t e d  T r i p l e t No provision has been made here for d i s s o c i a t i o n following intersystem crossing from the eq u i l i b r a t e d s i n g l e t 4 state by analogy with HFA. This omission follows from the negative quenching re s u l t s with HFAM and the negative 436 nm photochemical r e s u l t s . The former suggests that the t r i p l e t mode responsible for photodecomposition i s an excited v i b r a -t i o n a l state that has not yet e q u i l i b r a t e d . Furthermore, the only s l i g h t change of gas phase emission i n t e n s i t y , 4>pnos / 39 4>£. r a t i o , and of T , with temperature confirms the r f l u o r phos c absence of thermal-like decomposition of the equ i l i b r a t e d t r i p l e t state (at lea s t at temperatures below 100° C). One CO would also expect a high pressure l i m i t i n g y i e l d (<J)p p ) independent of wavelength and strongly dependent on temper-ature i f d i s s o c i a t i o n of the eq u i l i b r a t e d t r i p l e t state was involved. Plots of <fp D versus r e c i p r o c a l HFB concentration (Figure 20) show that the high pressure quantum y i e l d s curve 79 towards zero at a l l wavelengths except 254 nm. Moreover the intercepts are d i f f e r e n t for each wavelength. I t i s not c l e a r whether thermal-like d i s s o c i a t i o n i s important at 254 nm or whether the y i e l d s would curve slowly to zero at higher pressures. F i n a l l y the temperature dependence of the quantum y i e l d s (Figures 18 and 19) i s very s l i g h t (-20° C to +80° C). (iv) T r i p l e t D i s s o c i a t i o n Inspection of the r e c i p r o c a l quantum y i e l d plots (Figures 9, 11, 13 and 16) shows that curvature, at t r i b u t e d to t r i p l e t d i s s o c i a t i o n , i s less pronounced with increasing wavelength. At 334 nm, for example, the curvature i s s l i g h t (Figure 16). The data at 366 nm (Figure 17) suggest that t r i p l e t d i s s o c i a t i o n becomes noticeable at a r e l a t i v e l y high pressure ( i . e . ca. 50 t o r r ) . (v) Light Intensity The fact that the primary process i s not a function of l i g h t i n t e n s i t y rules out the p o s s i b i l i t y of two t r i p l e t 2 state molecules reacting to give the products as i n b i a c e t y l 14 — at l e a s t i n the i n t e n s i t y range used here (5 x 10 to 3 x 15 2 10 photons/cm /sec). FIGURE 20 81 (vi) Evaluation of Rate Constants I t has been shown that ( P . P . 1 M -*• 0° 1 + + 10 M- (8) so that at high pressure a p l o t of r e c i p r o c a l quantum y i e l d s versus pressure w i l l be l i n e a r with a slope and intercept given by to/ko and (1 + k, / k 9) respectively. to the data (above approx. 40 torr) at each wavelength. The slopes and the intercepts are given i n Tables 15 and 16. computed from k i n e t i c theory as co = 1.2 x 10 liter/mole/sec (6.42 x 10^ t o r r ^sec ^ ) . k 2 and k^ could therefore be calculated. Subsequently k^ was computed using Equation (6) for each pressure (below approx. 20 torr) where an experimen-t a l quantum y i e l d was a v a i l a b l e . An averaged k^ was thus obtained for each wavelength. Some of the low pressure data at 313 nm (e.g. 1 t o r r and 15 torr) gave negative values for k r , because of large experimental uncertainty i n the quantum The method of l e a s t squares was therefore applied Taking the gas-kinetic c o l l i s i o n diameter as 7.o£ , co can be 11 TABLE 15 Rate Constants for Dissociation and Intersystem Crossing at 25° C Slope . m"8 i K . + - Q V O ^ + - v- v i n " ^ v v i n " ^ . k 0 x 10 intercept . k , x 10 k c x 10 Wavelength = c o / k 2 2 = * k 1 A 2 1 5 , nm tor r (sec ) 1' 2 (sec ) (sec ) 254 0.0043 15.1 3.55 2.55 38.5 11.9 ±0.0021 ±0.50 ±4.0 297 0.0274 2.34 3.71 2.71 6.34 5.6 ±0.0002 ±0.04 ±1.5 313 0.049 1.32 3.73 2.73 3.60 3.0 ±0.002 ±0.28 ±1.1 334 0.112 0.536 2.83 1.83 0.98 <1.0 ±0.005 ±0.90 366 0. 32 0. 20 65.5 64.5 24 1.9 ±0.04 ±7.2 .-. /.> = fi.d? v 1 0 ^ t o r r " * sec * TABLE 16 Rate Constants for Dissociation and Intersystem Crossing at Various Temperatures Slope 3 Wavelength Temp = w / k 2 n^T'Tv , kn / k , *2 . k l nm °C t o r r " 1 ( 1 + k l ' K2> 1 2 (sec - 1) (sec"*) ±0.002 313 +25 0.049 3.73 2.73 1.32 3 - 6 0 o ±0.002 ±.28 x 10 8 x 10 8 313 +50 0.041 3.06 2.06 1«56 Q 3.21 ±.13 x 10 8 x 10 8 313 +76 0.026 2.87 1.87 2 - 4 5 p 4 > 5 9 n ±0.003 ±.27 x 10 a x 10° 297 -20 0.066 3.82 2.82 0.96 2 - 7 2 8 +0.005 ±.18 x 10° x 10 297 +25 0.0274 3.71 2.71 2 ' 3 4 a 6 , 3 4 8 ±0.0002 ± .04 x 10 x 10 84 y i e l d s , when treated i n t h i s way. The l a t t e r values for k^ were ignored and so the quoted error l i m i t s are rather o p t i -m i s t i c for t h i s wavelength. The 334 nm data necessitate that only an upper l i m i t on k,. be given. Table 15 summarizes these r e s u l t s . ( v i i ) Simulation of Quantum Y i e l d Results The parameters given i n Table 15 were used to evaluate r e c i p r o c a l quantum y i e l d s at various pressures using Equation (6). The r e s u l t s , p l o t t e d as a smooth curve at each wavelength on the same Figure as the experimental data, are given i n Figures 8 to 17. To determine what e f f e c t each parameter had on the o v e r a l l quantum y i e l d , parameters at one wavelength (313 nm) were varied within t h e i r quoted error l i m i t s . Very l i t t l e change i s noticed i n the high pressure region (Figure 12). However i n the low pressure region changes i n k^ (Figure 14) appear to have a larger e f f e c t on the quantum y i e l d than changes i n k^ (Figure 13). This apparently larger e f f e c t may be due to using error l i m i t s for k c which are too small ( c f . section (vi) , t h i s chapter) . 85 ( v i i i ) V a r i a t i o n of k^, and with Wavelength The experimental data indicate that the rates of d i s s o c i a t i o n of both the s i n g l e t v i b r o n i c state molecules (k 2) and the t r i p l e t v i b r o n i c state molecules (k^) are a function of t h e i r energy content. These rel a t i o n s h i p s are shown i n Figure 21. The minimum energy necessary for s i n g l e t state d i s s o c i a t i o n l i e s between 405 nm (70.0 kcal) and 366 nm (78.0 k c a l ) . The minimum e x c i t i n g r a d i a t i o n necessary for t r i p l e t state d i s s o c i a t i o n l i e s between 405 nm and 334 nm (85.0 k c a l ) . The i n c l u s i o n of k^ (rate of intersystem crossing from the vibronic s i n g l e t l e v e l reached on excitation) was necessitated i n Mcintosh's work. 1 I f intersystem crossing to the t r i p l e t i s only v i a ( v i ) , which competes with fluorescence from *B°, pressure dependencies within the s i n g l e t manifold should be duplicated by the phosphorescence as we l l . This was not found to be the case. 1 Also, r a t i o s of ct> . /be are observed which are larger than the vphos T f i u o r 3 l i m i t i n g high pressure value and which are functions of pressure and energy. The photochemical quantum y i e l d s found here are also consistent with a mechanism i n which *B* i s 86 depleted by both intersystem crossing and d i s s o c i a t i o n i n competition with c o l l i s i o n a l degradation. The r e l a t i v e importance of k 2 and has been assessed from the extra-polated intercept of the high pressure data i n Figures 8, 10, 12, 15 and 17 (see Equation (8)). The Rate Constants at 366 nm: A few comments concerning the large k^/k 2 r a t i o at 366 nm (Table 15) are necessary. The point representing k^ at 366 nm i n Figure 21 i s incon-s i s t e n t with the values of k^ at other wavelengths i n the sense that i t i s an order of magnitude larger than i s expected by extrapolation. Mcintosh, i n f a c t , found k^ 7 -1 to be 6.5 x 10 sec , which i s i n l i n e with the other res u l t s (see Figure 21). The p l o t of log k 2 against energy should i n p r i n c i p l e be sharply concave down at the low energy side, as the threshold i s approached. For k^ on the other hand,the intersystem crossing rate i s probably not governed by a minimum energy. The value of k^ for the eq u i l i b r a t e d 1 7 - 1 s i n g l e t i s known to be 2.8 x 10 sec. Thus the values of log k^ approach t h i s l i m i t i n g value at low energy (see Figure 21). 8 7 The value of k^ has been determined here i n d i r e c t l y so that the experimental uncertainty i s quite large. Accor-ding to Equation (8) the slope of high pressure data represents oj/k 2, and k^ i s found by k^ - (Intercept -1) — - — slope The errors are thereby greatly magnified. For example, within error l i m i t s for each point of ±20%, one finds k 2 to be ± 30% but k^ i s uncertain by a factor of two I There i s every reason therefore to suspect that k^ at 366 nm i s not anomalous but that the value of k^ shown i n Figure 21 i s simply subject to large uncertainty. The point representing k^ at 366 nm i n Figure 21 i s likewise inconsistent with the values of k^ at the other wavelengths insofar that i t i s expectedly too large. The value of k^ has been computed using Equation (6) for each pressure below 30 t o r r where an experimental quantum y i e l d i s a v a i l a b l e . This procedure necessitates using the previously determined values of k^ and k 2 so that the large errors i n these parameters are r e f l e c t e d i n the value of k^. Conse-quently one finds k^ varying by a factor of three. The value of k^ shown in Figure 21 at 366 nm i s therefore also subject to large uncertainty. 88 The Intersystem Crossing Rate Constant: As aforementioned the rate of intersystem crossing from the v i b r o n i c s i n g l e t l e v e l reached on e x c i t a t i o n increases as the wavelength decreases (Figure 21). This behaviour of i s consistent with the idea that as the molecules are raised to upper v i b r a t i o n a l l e v e l s of the excited s i n g l e t state the hypervolume associated with the v i b r o n i c states increases and so the p r o b a b i l i t y of contacting p o t e n t i a l surface intersections increases. This i s the f i r s t reported instance of the intersystem crossing rate constant being a strong function of the energy of the e x c i t i n g r a d i a t i o n . A s i m i l a r but smaller e f f e c t i s however 40 known for benzene. Ware and co-workers have found that the nonradiative rate constant (assumed to be e n t i r e l y due to intersystem crossing) i s a function of the v i b r o n i c l e v e l i n the 1B2 U state i n t o which the molecule i s excited. Over an energy range of 1450 cm * the intersystem crossing rate constant varied by a factor of 1.3. For HFB the intersystem crossing rate constant i s found to vary by a factor of about 70 over an energy range of 9000 cm *. In cases where t r i p l e t 4 17 d i s s o c i a t i o n has been established ' the intersystem crossing process appears to originate mainly from the v i b r a t i o n a l l y e q u i l i b r a t e d s i n g l e t state. Consequently the wavelength of 10.0 LOG K; 254 40 nm 297 313 334 35 30 ENERGY 366 ^cm1xib3^ 405 436 fi9. 21 Log. of the various rate constants versus excitation energy. A from Mcintosh! 9 0 the i r r a d i a t i n g l i g h t has no e f f e c t on the intersystem crossing process for these systems. (ix) Other Possible Mechanisms (a) Involvement of the V i b r a t i o n a l l y Excited Ground State: I t i s possible that the excited s i n g l e t state, instead of crossing to the t r i p l e t manifold, crosses isoener-g e t i c a l l y to a highly v i b r a t i o n a l l y excited l e v e l of the ground state. The wavelength dependence found here would then be observed i f the rate of i n t e r n a l conversion depended on the v i b r a t i o n a l l e v e l of the excited state *B* or i f the rate of d i s s o c i a t i o n depended on the v i b r a t i o n a l l e v e l of 41 the ground state. Experimentally, involvement of the ground state ( l i k e involvement of the v i b r o n i c t r i p l e t state) i s d i f f i -c u l t to substantiate as a path for photodecomposition. I t can only be i n f e r r e d by the elimination of every other possi-b i l i t y . Even so, quantitative documentation i s sparse. Cycio | l . 3 . 5 J h e p t a t r i e n e ^ 9 ' 4 2 4 3 appears to be the only system for which the data are reasonably convincing. No carbonyl compound i s known to photodissociate from the ground state. 91 The evidence for ground state p a r t i c i p a t i o n i n the photolysis of g l y o x a l * 3 ' 4 4 i s , at best, i n d e c i s i v e . (b) V i b r a t i o n a l Energy D i s t r i b u t i o n Function: I t has also been suggested that curvature i n r e c i -procal quantum y i e l d versus pressure plots can be explained on the basis of the width of the v i b r a t i o n a l energy d i s t r i -45 bution function. This function embodies both the thermal energy d i s t r i b u t i o n factor of the ground e l e c t r o n i c state 46 and the energy p r o f i l e of the absorbed l i g h t . Bowers found for HFAM at 366 nm that the predicted " f a l l o f f " i n l/<j>p p occurred mostly at much lower pressures than had 37 been found experimentally by Wu and Rice. He concluded that more than one e l e c t r o n i c state might be involved. Further discussion of t h i s type of t h e o r e t i c a l approach to HFB photochemistry must await evaluation of the normal mode frequencies of the ground state of HFB and an i n d i c a t i o n as to how they may change i n the excited state. (x) Wall Deactivation of the Eq u i l i b r a t e d T r i p l e t State F i n a l l y , experimental evidence for another type of c o l l i s i o n a l deactivation i s presented and discussed. This 92 deactivation involves the e q u i l i b r a t e d t r i p l e t state of HFB at very low pressures (approximately one torr) i n t e r a c t i n g with the reaction c e l l w a l l . 1 8 Parmenter and Poland found that there was a marked decrease i n the quantum y i e l d of b i a c e t y l phosphor-escence emission and the l i f e t i m e of the t r i p l e t i n experi-ments at low pressures. They att r i b u t e d these e f f e c t s to the increasing importance of b i a c e t y l t r i p l e t d i f f u s i o n to and deactivation at the c e l l w a l l at low pressures. More recently Mcintosh 1 has observed that phosphor-escence quantum y i e l d s of HFB decrease rapidly i n the pressure region below 2 t o r r at 436 nm. Low pressure t r i p l e t l i f e -times were thus required to confirm the e f f e c t for HFB. Results and Discussion: The experimental set-up and apparatus for determining phosphorescence l i f e t i m e s of HFB i s described i n Chapter I I . Figure 22 shows the difference (y) between the reciprocals of the observed l i f e t i m e s ( 1 /T) at low pressure and the l i f e t i m e extrapolated at high pressure ( I /T^) against HFB concentration. There i s a marked deviation from the high pressure value for experiments in the region below 93 1 t o r r . Furthermore y i s found to be (within experimental error) a l i n e a r function of the r e c i p r o c a l of the HFB pressuree (Figure 23). The results shown i n Figures 22 and 23 are expected i f wall deactivation of t r i p l e t s i s important at low pressures. The rate of d i f f u s i o n i s inversely propor-47 t i o n a l to the gas concentration so that deactivation at the wall should also be inversely proportional to the pressure of the gas through which the t r i p l e t s must d i f f u s e to react at the w a l l . I t i s also expected that this e f f e c t w i l l occur under the conditions used here somewhat below 1 t o r r . At t h i s pressure the average distance which the t r i p l e t mole-cules can d i f f u s e during t h e i r l i f e t i m e approaches the aver-age distance from the c e l l wall at which the molecules are excited (approximately 1.5 cm).* These re s u l t s p a r a l l e l 18 those obtained for b i a c e t y l by Parmenter and Poland and 14 confirmed recently by Calvert and co-workers. The l a t t e r workers have also shown that the i s o l a t e d excited s i n g l e t state molecule undergoes a t r u l y unimolecular ISC process with the same e f f i c i e n c y as i n the c o l l i s i o n a l l y perturbed system at high pressures. They found that the r e l a t i v e *The time i n seconds for the t r i p l e t molecule to d i f f u s e a distance x cms. i s given approximately by x 2/D, where D i s the d i f f u s i o n c o e f f i c i e n t ; D = D Q/pressure(Torr)? 8 For HFB D0 = 1.1 x 10~ 2cm 2 /sec. (Appendix B). DIFFERENCE (y) BETWEEN THE RECIPROCALS OF THE OBSERVED LIFETIME (T) AT LOW PRESSURE AND THE CONSTANT HIGH PRESSURE TRIPLET LIFETIME ( T Q ) VERSUS HFB PRESSURE DIFFERENCE (y) BETWEEN THE RECIPROCALS OF THE OBSERVED LIFETIME (x) AT LOW PRESSURES AND THE CONSTANT HIGH PRESSURE TRIPLET LIFETIME ( T f l ) VERSUS RECIPROCAL HFB PRESSURE 96 phosphorescence quantum y i e l d of b i a c e t y l at 436.5 nm i s i n v a r i a n t of pressure between 12.5 t o r r and 11.4u . C o l l i -s i o n a l deactivation i s n e g l i g i b l e for s i n g l e t state b i a c e t y l 49 molecules below about 10 t o r r . There i s every reason to expect that the intersystem crossing reaction for HFB w i l l also be t r u l y unimolecular. 97 CHAPTER V  The Primary Process In t h i s chapter a de t a i l e d mechanism, incorporating a l l the known photochemical and photophysical data, i s presented. Its scope and l i m i t a t i o n s are c r i t i c a l l y discussed. Various rate constants for photochemical and photophysical processes are evaluated and compared with those obtained previously. The fluorescence and photochemical y i e l d s are combined to demonstrate decomposition competing with c o l l i -s i o n a l deactivation i n the t r i p l e t manifold. A. Detailed Mechanism The processes necessary to account for the observed photochemical and photophysical r e s u l t s are as follows: B + hv \B* 2 1 B * k l 3 B * (i) i B * k 2 - 2C0 + C„F, ( i i ) 2 Z D i B * k f B + hv f l ( i i i ) 2 rluor i B * + M cu *• JB° + M* (iv) i B o k f B + hv., (v) fluor i B o k i s c " 3B* (vi) 98 3 B * + M u ^ 3 Bo + M ( v i i i ) ? 3 B * k 5 " 2C0 + C 2 F g (ix) 3 Bo k „ B + hv , (x) 1 ^ phos 3 Bo k n B (xi) jB° + Wall k w a l l *~ B + wall ( x i i i ) A few comments concerning the i n d i v i d u a l processes seem to be i n order: intersystem crossing to the t r i p l e t manifold from the v i b r o n i c l e v e l reached on e x c i t a t i o n (process (i)) i s necessitated by Mcintosh's emission r e s u l t s as well as the photochemical r e s u l t s reported i n t h i s work. As, i f ISC to the t r i p l e t i s only v i a process ( v i ) , which competes with fluorescence from *B? pressure dependencies within the s i n g l e t manifold should be r e f l e c t e d by the phosphorescence as w e l l . However d» , with 40 5 nm and 366 nm e x c i t a t i o n phos has become pressure independent at 20 t o r r HFB, yet ^f^uor i s s t i l l increasing i n t h i s r e g i o n . 1 A l s o , ^ p n o s / ^ f i u o r r a t i o s are observed which are larger than the l i m i t i n g high pressure value, and which are functions of pressure and energy. 1 The intercepts i n the r e c i p r o c a l quantum y i e l d 99 versus p r e s s u r e p l o t s n e c e s s i t a t e s a mechanism which i n c l u d e s k^ as a means o f d e p l e t i n g (with k 2 ) the v i b r o n i c s i n g l e t s t a t e . The data r e q u i r e a mechanism t h a t has photochemistry o r i g i n a t i n g from two d i f f e r e n t s t a t e s - the v i b r o n i c s i n g l e t s t a t e (process ( i i ) ) and the v i b r o n i c t r i p l e t s t a t e (process ( i x ) ) . A t h i g h p r e s s u r e s p r o c e s s ( i i ) competes d i r e c t l y w i t h c o l l i s i o n a l d e g r a d a t i o n (process ( i v ) ) from the s i n g l e t s t a t e . Once v i b r a t i o n a l l y e q u i l i b r a t e d , the e x c i t e d molecule can f l u o r e s c e (process (v)) o r c r o s s t o the t r i p l e t m a n i f o l d (process ( v i ) ) . As the p r e s s u r e i s lowered ISC (process ( i ) ) and d i s s o c i a t i o n from the v i b r o n i c s i n g l e t s t a t e become more c o m p e t i t i v e w i t h c o l l i s i o n a l d e a c t i v a t i o n and conse-q u e n t l y the t r i p l e t s t a t e i s p o p u l a t e d from the l e v e l reached on e x c i t a t i o n . T h i s v i b r a t i o n a l l y e x c i t e d t r i p l e t s t a t e molecule can d i s s o c i a t e (process ( i x ) ) or be c o l l i s i o n a l l y degraded t o the v i b r a t i o n a l l y e q u i l i b r a t e d t r i p l e t (process ( v i i i ) ) . The e q u i l i b r a t e d t r i p l e t s t a t e can then e i t h e r phosphoresce (process (x)) o r r e t u r n t o the ground s t a t e by some r a d i a t i o n l e s s c o n v e r s i o n process (process ( x i ) ) . 100 Fluorescence must be possible from l e v e l s near those reached on e x c i t a t i o n (process ( i i i ) ) as well as from an e q u i l i b r a t e d state (process (v)) as Mcintosh 1 found that at low pressures <p n o s decreases much faster than <j>^^uori phos a P P a r e n t l y rapi d l y decreases to zero as 4>f i u o r approaches a f i n i t e non zero value. Although wall deactivation ( x i i i ) becomes a major consideration i n the very low pressure region, i t i s u n l i k e l y to change these general conclusions. The cb , r e s u l t s at shorter wavelengths show the same features Yphos 3 (as longer wavelengths) at higher pressures where w a l l -deactivation of the e q u i l i b r a t e d t r i p l e t would be n e g l i g i b l e . B. Independent Evaluation of (k^ + k^) Mcintosh 1 has evaluated ( + k 2) at each wavelength using his fluorescence data. Table 17 reproduces his e s t i -mates together with the values found i n the present work from the photochemical experiments. There i s good agreement between the two values of the rate constants. This agreement substantiates the proposed mechanism; i n p a r t i c u l a r the feature of the intersystem crossing process from the v i b r o n i c state reached on e x c i t a t i o n (process ( i ) ) . The apparent discrepancy 101 i n the 366 nm res u l t s i s discussed i n Chapter IV. I t should be noted however that Mcintosh has estimated k 2 at 366 nm 7 - 1 7 - 1 to be 1.9 x 10 sec while a value of 1.4 x 10 sec i s found i n t h i s work. C. Evaluation of from the F u l l Mechanism (i) Fluorescence/Photochemistry Ratio From a steady-state treatment of the mechanism, the primary photochemical y i e l d and the fluorescence y i e l d are given by the expressions A = i + ± x 2 _ (11) • ' k-j+ k 2+ OJM + k* k x+ k 2+ uM + k* k 5+ coM and Equation (1) respectively. The r a t i o i s • W _ a W M + k f - — ( 1 2 ) *P.P. k 5 k2 + k r + coM " k l Where a = *f + ilc 5 102 Table 17 Rate Constants for the Sum of Si n g l e t Dissociation and Inter-system Crossing from the I n i t i a l l y Populated Vibronic Singlet State. Wavelength (k x + k 2) x I O - 8 a (k± + k 2) x I O - 8 sec ~ 1 sec _ 1 254 53.6 297 8.68 13.0 313 4.92 3.7 334 1.52 2.2 366 24.2 0.84 405 — 0.61 a This work b Mcintosh 1 103 At high pressures (12) becomes • f l u o r _ a coM --(13) VP.P. K2 Combining the known value of a with the slope of the r a t i o versus pressure p l o t gives k 2« This was done for each wave-length where appropriate data were available. Figure 24 shows a t y p i c a l example. Table 18 gives the values of k 2 obtained by th i s method. ( i i ) Phosphorescence/Photochemistry Ratio The phosphorescence y i e l d , from a steady-state treatment of the mechanism, i s given by equation (2). Let ISC "ISC + k4 ISC and k + k,, p 11 The r a t i o of the phosphorescence y i e l d to the photochemical quantum y i e l d i s given by 104 • „ . , x rISC x 1 0 M + „ , .— x k.. cb , coM + k c toM+k- 1 pnos _ b b •P.P. " k 5 k_ + k, x '2 1 toM + k, B coM { <j)Jsc coM + k x } k 2 {coM + k 5> + k1 k 5 Now at high pressures coM >> k 5 and k 2 coM >> k^ k,-•phos = B ( * I S C " M + k l } *P.P. k2 (14) e • i s c u k l l b <- M + 8 i k2 * k2 (15) 105 A p l o t of this r a t i o against HFB pressure gives a slope equal •isc U 1 to 8 £ . Mcintosh gives values of 8 and <J>jSC (0.0 86 and 0.9 respectively) so that k 2 can be determined. This was done for each wavelength where appropriate data were av a i l a b l e . Figure 25 shows a t y p i c a l example. Table 18 gives the values of k 2 obtained by t h i s method. ( i i i ) Discussion I t can be seen from Table 18 that there i s a good agreement between the various methods used i n obtaining k 2. In p a r t i c u l a r , the absolute photochemical and photophysical quantum y i e l d s at high pressures (at least) are i n t e r n a l l y consistent. This lends credence to the proposed mechanism. D. Complementary Aspects of this and Previous Work Mcintosh's quantum y i e l d measurements were designed for a s p e c i f i c purpose - the absolute emission quantum y i e l d of HFB at high pressure (250 torr) at various e x c i t a t i o n 50 wavelengths. He therefore optimized his experimental arrangement with this i n mind. In p a r t i c u l a r , the ill u m i n a -t i n g beam passed through the cm square c e l l within 1 mm of the side observation window. The t r i p l e t l i f e t i m e r e s u l t s Table 18 Rate Constants for Singlet Dissociation Wavelength k 2 (Photochemistry nm only) J<2 (Fluorescence & photochemistry) k 2(Phosphorescence & photochemistry) 297 2.34 x 10 8 sec" 1 2.10 x 10 8 s e c " 1 2.58 x 10 8 sec" 1 313 1.32 x 10 8 sec" 1 1.14 x 10 8 s e c " 1 1.24 x 10 8 sec" 1 334 0.54 x 10 8 sec" 1 0.52 x 10 8 s e c " 1 8 —1 0.56 x 10 sec 366 0.20 x 10 8 s e c " 1 0.053 x 10 8 s e c " 1 8 —1 0.050 x 10 sec 107 F I G . 24 RATIO OF FLUORESCENCE YIELD TO PHOTOCHEMICAL YIELD VERSUS HFB PRESSURE AT 313 NM. HIGH PRESSURE REGION FIG.25 RATIO OF PHOSPHORESCENCE YIELD TO PHOTOCHEMICAL YIELD VERSUS HFB PRESSURE AT 313 NM. HIGH PRESSURE REGION 108 of the present work would indicate that his phosphorescence y i e l d s below about 5 or 10 t o r r are q u a n t i t a t i v e l y open to question. His observation that <j> , decreases as the phos pressure decreases i s undoubtedly r e a l as the shorter wave-length r e s u l t s display t h i s same pressure dependence (as longer wavelengths) at higher pressures where wall-deactiva-t i o n of the e q u i l i b r a t e d t r i p l e t would be n e g l i g i b l e . These pressure dependencies are caused by t r i p l e t d i s s o c i a t i o n competing favorably with c o l l i s i o n a l degradation. I t i s possible that i f d i s s o c i a t i o n i s absent (as at 436 nm) the l i m i t i n g high pressure <J>pnos w i l l be maintained to very low • w • 4- i 14 pressures as i n b i a c e t y l . The fluorescence y i e l d s and the s i n g l e t d i s s o c i a -t i o n y i e l d s are unaffected by the c e l l geometry because of the short l i f e t i m e of the s i n g l e t state ( ^ 50 nsec). The t r i p l e t d i s s o c i a t i o n y i e l d s are s i m i l a r l y unaffected. The fluorescence and photochemical data can be combined to i l l u s t r a t e t h i s competition i n the t r i p l e t mani-f o l d : (i) Fluorescence and Intersystem Crossing The f u l l expression for the fluorescence/photochem-i s t r y r a t i o , Equation (12), can be written i n the form 109 a (i)M + k* zn^or = f_ { 1 6 ) 9P.P. k2 + fikl k 5 where Q = k c + coM At high pressures i t i s found that t h i s r a t i o i s l i n e a r with pressure (Page 107). As the pressure i s lowered however, ft changes from 0 to 1. Figure 26 shows the experi-mental data at 29 7 nm confirming t h i s p r e d i c t i o n . The difference between the extrapolated value and the experimen-t a l l y determined value of 4 > f i u o r / 4>p p decreases below 30 t o r r . I t appears that these two l i n e s would converge at zero pressure as Equation (16) demands. The data at 313 nm show the same pressure dependence (Figure 27) beginning at a lower pressure and much less pronounced. No deviation from l i n e a r i t y i s noticeable with 334 nm or 366 nm rad i a t i o n even at the lowest pressures of HFB i r r a d i a t e d . Fluorescence data are unavailable with 254 nm e x c i t i n g r a d i a t i o n . RATIO OF FLUORESCENCE YIELD TO PHOTOCHEMICAL YIELD VERSUS HFB PRESSURE AT 297 RATIO OF FLUORESCENCE YIELD TO PHOTOCHEMICAL YIELD VERSUS HFB PRESSURE AT 313 NM 112 Figure 26 shows that decomposition can compete with c o l l i s i o n a l deactivation i n the t r i p l e t manifold below about 100 t o r r at 297 nm. As the wavelength i s increased the v i b r a t i o n a l energy of the t r i p l e t molecules formed decreases. The pressure at which th i s competition i s evident therefore, decreases. ( i i ) Limitations on Available Data A question must now be answered: are the two independent sets of data (photochemistry and emission) complementary? The absolute uncertainty i n the emission y i e l d s i s given as 17%. Of course t h i s would vary s l i g h t l y depending on wavelength and pressure region being considered. The r e l a t i v e error for the photochemical y i e l d s i s about 10% although t h i s r i s e s sharply under extreme conditions (e.g. very high pressures at 366 nm). A f a i r r e f l e c t i o n of these errors i s achieved i f the fluorescence and phosphorescence quantum y i e l d s are calculated using Equations (1) and (2) and pl o t t e d together with Mcintosh's experimental data. The calculations involve using his values of 4 > f i u o r •phos a n ^ v a x u e s °f k^, and k_ taken from t h i s work. A problem arises for <f> h at FLUORESCENCE QUANTUM YIELD VERSUS HFB PRESSURE AT 313 NM EXPERIMENTAL POINTS FROM McINTOSH'. SOLID L I N E CALCULATED FROM PARAMETERS INDICATED & PHOS I 0 0 0.0049 0.078 3.05 0.0049 0.078 r cm 0.05 0.5 PHOSPHORESCENCE QUANTUM YIELD VERSUS HFB PRESSURE AT 313 NM EXPERIMENTAL POINTS FROM McINTOSH". SOLID LINES CALCULATED FROM PARAMETERS INDICATED 200 4 0 0 6 0 0 [HFB] in t o r r 8 0 0 OS. 115 low pressures where wall-deactivation becomes important (process ( x i i i ) ) as the e f f e c t i v e c e l l radius (r) for Mcintosh's experiment can only be estimated. Therefore t h i s c a l c u l a t i o n was ca r r i e d out with what are considered to be extreme values of r: 0.5 cm and 0.05 cm respectively (Appendix B). Gross assumptions are also made regarding the d i f f u s i o n c o e f f i c i e n t and the p r o b a b i l i t y for wall r e f l e c t i o n without deactivation. The res u l t s at 313 nm are given i n Figures 28 and 29. I t i s evident that even at a pressure of 200 t o r r HFB a c e l l radius of 0.05 cm would necessitate wall deactivation being considered when report-ing phosphorescence quantum y i e l d s . Plots at 297, 334 and 366 nm show the same trends. No emission y i e l d s are a v a i l -able at 254 nm. Within the quoted experimental errors and l i m i t a -tions imposed on the calculated curves the two studies are seen to be e n t i r e l y complementary. E. Concluding Remarks - Suggestions for Further Work This i n v e s t i g a t i o n has helped to e s t a b l i s h the main features of the primary process i n the photolysis of hexa-116 f l u o r o b i a c e t y l . Its p o t e n t i a l as a model system i s obvious -predictable photochemical products as well as emission from two d i f f e r e n t e l e c t r o n i c states. Much remains to be done however, p a r t i c u l a r l y i n the low pressure region. 51 The technique of transient fluorometry o f f e r s a very powerful t o o l to observe the i n i t i a l l y - f o r m e d s i n g l e t s t a t e . By examining fluorescence decay as a function of e x c i t i n g wavelengths at very low pressures information could be obtained regarding the non-radiative processes (primary d i s s o c i a t i o n and intersystem crossing) depleting the excited s i n g l e t state. Emission data at these same wavelengths and low pressures would also be very useful. The experimental arrangement w i l l have to be such that wall-deactivation can be q u a n t i t a t i v e l y eliminated. The v a r i a t i o n of emission y i e l d s with pressure at very low pressures would indicate whether intersystem crossing i s t r u l y unimolecular and i f HFB i s i n the "large molecule" l i m i t discussed i n the theories of unimolecular t r a n s i t i o n s . 1 ^ An i n t e r e s t i n g phenomenon has been noted for the HFB-HFAM energy transfer system. 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Noyes, G.S. Hammond and J.N. P i t t s , Interscience Publishers, 1966 44. C S . Parmenter, J . Chem. Phys., 4_1, 658, (1964) 45. P.G. Bowers, J . Chem. S o c . (A), 466, (1967); i b i d . , Can. J. Chem., 4_6, 307, (1968) 46. Ibid., J. Phys. Chem., 74 , 952, (1970) 47. J.O. Hirschfelder, C F . Curtiss and R.B. B i r d , Molecular Theory of Gases and Liquids, Wiley, New York, 19 64. 48. S.W. Benson, The Foundations of Chemical K i n e t i c s , McGraw-H i l l Book Co., New York, 1960. 49. H.M. Poland, Fluorescence and Phosphorescence from B i a c e t y l and Glyoxal, Ph.D. Thesis, Indiana University, 1969. 50. J.S.E. Mcintosh, Ph.D. Thesis, University of B r i t i s h Columbia, Chapter VI, 1969. 51. E.W. Schlag, S. Schneider, and S.F. Fisher, Ann. Rev. Phys. Chem., 2j2, 465, (1971); W.R. Ware, Transient Luminescence Measurements, i n Creation and Detection  of Excited States, v o l . 1, ed. A. Lamola, Marcel Dekker, New York, 1971. 52. M. Kovacs, D.R. Rao and A. Javen, J. Chem. Phys., 4_8, 3339, (1968) 53. H.M. Poland, Ph.D. Thesis, Indiana University, Page 30, 1969. 123 A P P E N D I X 124 A. Window Corrections Because of the large transmission losses from the s i l i c a - a i r interfaces of the reaction vessel and the actino-meter c e l l , i t was necessary to apply a correction factor to 32-3 obtain the absolute i n t e n s i t y of the absorbed r a d i a t i o n . For convenience a l l windows were assumed to have i d e n t i c a l losses from r e f l e c t i o n . Let a be the f r a c t i o n of l i g h t l o s t i n passing through a window. Evacuated c e l l : -ractinometer measured I Q - 2 a I Q + a I Q 2 - ct(I 0 - 2 a l 0 + a Ip) ACTINOMETER CELL W 3 xo ~ a I 0 - a ( I Q - OCIQ) PHOTOLYSIS CELL (evacuated) 125 I Q - 2cxl 0 + c x 2 I 0 - a I Q + 2 a 2 I Q - a I Q - 3 a l 0 + 3 a 2 I Q I Q (1 - 3a + 3a 2) I Q (1 - 3 {a - a 2}) jactinometer _ _ measured (no gas) 1 - 3 ( a - a ) However, we want to measure the i n t e n s i t y of radiati o n j u s t inside the front window (W,). actinometer measured (no gas) i . e . I Q - a I Q = I I = I Q - a l 0 = I Q (1 - a) jactinometer I = measured (no gas) x ( i _ a) 1 - 3(a - a 2) I t was found that the transmission properties of the quartz windows (Chapter 2, Section E) at the various wave-lengths were i n excellent agreement with the v a r i a t i o n i n 126 r e f l e c t i o n as calculated using the index of r e f r a c t i o n of fused quartz, assuming Fresnel's Law and an a i r - SiC^ - a i r i n t e r -face . B. Wall Deactivation I t i s assumed that wall-deactivation i s responsible for a l l low pressure e f f e c t s on the e q u i l i b r a t e d t r i p l e t state. 52 The d i f f u s i o n equations developed i n i t i a l l y by Javen et a l 49 and subsequently by Poland for a s i m i l a r problem concerning wall-deactivation of t r i p l e t b i a c e t y l are used. The geometry of her experiments are s i m i l a r to ours. In both cases d i f f u -sion to the walls occurs i n a c y l i n d r i c a l c e l l uniformly f i l l e d with the excited d i f f u s i n g species. The rate constant for deactivation on the wall, k w a l l ' i s g i v e n b y k U 2 D wall r2 where k n = T w a l l a n <* T w a l l ^ s t* i e ^ i ^ e t ; " - m e that the excited species would have i f i t decayed only by d i f f u s i o n to the w a l l . D i s the usual d i f f u s i o n c o e f f i c i e n t , r i s the 127 c e l l radius and y i s given by the solution of the boundary condition y J ( y ) _ _ ( | r ) _ il - B ) J n (P) . o 1 ^ U (1 + 3) 49 J Q and are zero and f i r s t order Bessel functions. 47 D was calculated from D = 1 / 3 x v (MFP) where MFP = Free Mean Path and v and 3 are the mean speed and p r o b a b i l i t y for wall r e f l e c t i o n with deactivation res-2 -1 pec t i v e l y . D was calculated to be 0.011 cm sec . 3 was 53 chosen to be 0.1. 

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