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UBC Theses and Dissertations

The primary photoprocesses of chromium (III) complexes Chen, Schoen-Nan 1970

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THE PRIMARY PHOTOPROCESSES OF CHROMIUM(III) COMPLEXE by SCHOEN-NAN CHEN B . S c , E n g i n e e r i n g , N a t i o n a l T a i w a n U n i v e r s i t y , 1962 M . S c , N a t i o n a l T a i w a n U n i v e r s i t y , 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e D e p a r t m e n t I o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A u g u s t , 1970 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 r e q u i r e m e n t s f o r an advanced d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t 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 f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g 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 g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h Co lumbia V a n c o u v e r 8, Canada Department Date Svfj, 3 , tfnO ABSTRACT E n e r g y t r a n s f e r b e t w e e n R e i n e c k a t e i o n (donor) and h e x a c y a n o c h r o m a t e ( I I I ) i o n ( a c c e p t o r ) has b e e n s t u d i e d t h r o u g h q u e n c h i n g o f d o n o r p h o s p h o r e s c e n c e - ( l i f e t i m e and i n t e n s i t y ) and s e n s i t i z a t i o n o f a c c e p t o r p h o s p h o r e s c e n c e . R e s u l t s f r o m a l l m easurements f i t t h e e x p e c t e d S t e r n - V o l m e r r e l a t i o n s h i p 5 -1 -1 • w i t h a q u e n c h i n g c o n s t a n t = 7.2 x 10 M s e c a t - 6 5 ° C . The p r e - e x p o n e n t i a l f a c t o r and a c t i v a t i o n e n e r g y o f a r e 6.6 x 10"^ M '"sec 1 and 4.8 K c a l / m o l r e s p e c t i v e l y . The c o n s t a n t , k g H / i s a t t r i b u t e d e n t i r e l y ' t o an e n e r g y t r a n s f e r r a t h e r t h a n a q u e n c h i n g p r o c e s s . The e l e c t r o n i c s t a t e s d i r -2 2 ' e c t l y i n v o l v e d a r e t h e E ^ ( a n d / o r T i g ) s t a t e s o f b o t h d o n o r and a c c e p t o r . E n e r g y t r a n s f e r i s a d i f f u s i o n - c o n t r o l l e d ( c o l l i s i o n a l ) p r o c e s s . H e x a c y a n o c h r o m a t e ( I I I ) i o n i s i t s e l f q u e n c h e d i n t h e p r e s e n c e o f R e i n e c k a t e i o n . The q u e n c h i n g c o n s t a n t , k ' , w h i c h may be a t t r i b u t e d t o b a c k e n e r g y t r a n s f e r QH f r o m a c c e p t o r t o d o n o r , has a p r e - e x p o n e n t i a l f a c t o r o f 2 x 12 -1 -1 10 M s e c and an a c t i v a t i o n e n e r g y o f 7.6 K c a l / m o l . I n t h e same s y s t e m , q u e n c h i n g o f p h o t o a q u a t i o n h a s a l s o b e e n s t u d i e d a t - 6 5 ° C . The p h o t o a q u a t i o n quantum y i e l d o f -2 R e i n e c k a t e i o n i s 1.02 x 10 I t . i s r e d u c e d i n t h e p r e s e n c e o f h e x a c y a n o c h r o m a t e ( I I I ) i o n , b u t n o t as much as t h e p h o s -p h o r e s c e n c e o f R e i n e c k a t e i o n i s r e d u c e d . The l i m i t i n g oo 4 u n q u e n c h a b l e p a r t , ^ ^ g ^ o c c u r s v i a t h e Tp^ s t a t e , w h i l e 2 t h e q u e n c h a b l e p a r t must o c c u r t h r o u g h t h e s t a t e as an i n t e r m e d i a t e . The a c t u a l p a t h f o r t h e q u e n c h a b l e p a r t p r o -2 4 p o s e d i s b a c k -inter-system c r o s s i n g f r o m t h e E t o t h e s t a t e , w h i c h t h e n u n d e r g o e s a q u a t i o n . 2 The p r i m a r y p r o c e s s e s o f E s t a t e m o l e c u l e s have b e e n . g i n v e s t i g a t e d t h r o u g h t h e t e m p e r a t u r e d e p e n d e n c e o f p h o s p h o r -e s c e n c e l i f e t i m e s o f some C r ( I I I ) c o m p l e x e s . A l l t h e a v a i l a b l e e v i d e n c e s u p p o r t s t h e i d e a o f .the t h e r m a l l y a c t i v a t e d b a c k i n t e r s y s t e m c r o s s i n g . A c c o r d i n g t o t h i s mechanism, t h e o r i g i n s 4 o f t h e s t a t e s o f C r (III)complexes r e a c h e d by c r o s s i n g a r e f a r l o w e r i n e n e r g y t h a n has b e e n e x p e c t e d . A s s u m i n g t h e o c c u r r e n c e o f b a c k i n t e r s y s t e m c r o s s i n g , t h e a p p l i c a t i o n o f e n e r g y t r a n s f e r t o t h e d e t e r m i n a t i o n o f i n t e r s y s t e m c r o s s i n g quantum y i e l d , <|>^  , .has b e e n d e m o n s t r a t e d , The v a l u e s o f §j_sc f ° r R e i n e c k a t e and h e x a c y a n o c h r o m a t e ( I I I ) i o n s a r e e s t i m a t e d t o be 0.52 and 0.35, r e s p e c t i v e l y . The v a r i a t i o n o f cj>^  -with- t e m p e r a t u r e f o r t h e s e C r ( I I I ) c o m p l e x e s has a l s o b e e n m e a s u r e d , w h i c h s u g g e s t s t h a t i n g e n e r a l , i n -t e r n a l c o n v e r s i o n has a s t r o n g t e m p e r a t u r e d e p e n d e n c e . From t h e r i s e o f p h o s p h o r e s c e n c e w i t h t i m e a f t e r p u l s e e x c i t a t i o n , a new p a r a m e t e r , T , has b e e n o b t a i n e d w h i c h r e p r e s e n t s p o p u l a t i o n o f t h e p h o s p h o r e s c i n g s t a t e and i s 4 2 b e l i e v e d t o be t h e l i f e t i m e o f t h e T ( o r l e s s l i k e l y T i g ) s t a t e . E f f o r t s have b e e n made t o c o n f i r m and i d e n t i f y t h i s p a r a m e t e r . S t u d i e s o f x h a v e b e e n c a r r i e d o u t as a f u n c t i o n ^ x i v o f t e m p e r a t u r e . . M e c h a n i s m s ' b a s e d on d i f f e r e n t t e n t a t i v e a s s i g n m e n t s o f T a r e p r o p o s e d and t h e i r i m p l i c a t i o n s examine A l l p r i m a r y p r o c e s s e s , e x c e p t t h e i n t r i n s i c r a d i a t i v e t r a n s i -t i o n s , seem t o c o n s i s t o f a t l e a s t two components, w h i c h t a k e d i f f e r e n t p a t h w a y s and a r e d i f f e r e n t f u n c t i o n s o f t e m p e r a t u r e TABLE OF CONTENTS Page A b s t r a c t i i T a b l e o f C o n t e n t s v L i s t o f T a b l e s v i i i L i s t o f F i g u r e s i x A c k n o w l e d g e m e n t s x i i CHAPTER I . INTRODUCTION 1 1. T h e r m a l R e a c t i o n s 2 2. E l e c t r o n i c S t a t e s 2 3. S p e c t r a l P r o p e r t i e s • 6 4. P r i m a r y P h o t o p h y s i c a l P r o c e s s e s 7 5. P h o t o c h e m i c a l P r o c e s s e s . . . . 10 6. A B r i e f D e s c r i p t i o n o f t h i s Work 12 CHAPTER I I . GENERAL EXPERIMENTAL PART 14 1. E m i s s i o n M e a s u r e m e n t s 14 2. L i f e t i m e M e a s u r e m e n t s 19 3. P h o t o l y s i s M e a s u r e m e n t s 21 4. A b s o r p t i o n M e a s u r e m e n t s 24 5. D e o x y g e n a t i o n T e c h n i q u e s 24 6. C h e m i c a l s 25 CHAPTER I I I . ENERGY TRANSFER AND QUENCHING STUDIES . . . 28 E x p e r i m e n t a l S e c t i o n 29 R e s u l t s 30 D i s c u s s i o n 43 v i P a g e CHAPTER I V . PHOTOCHEMICAL STUDIES 51 G e n e r a l P r i n c i p l e s 52 E x p e r i m e n t a l a n d R e s u l t s . 54 D i s c u s s i o n 55 K i n e t i c T r e a t m e n t 59 CHAPTER V. TEMPERATURE DEPENDENCE OF THE PHOSPHOR-ESCENCE L I F E T I M E S 6 3 R e s u l t s 66 D i s c u s s i o n . 71 A F u r t h e r Comment 79 CHAPTER V I . QUANTUM Y I E L D OF INTERSYSTEM CROSSING AS A FUNCTION OF TEMPERATURE 81 E x p e r i m e n t a l and R e s u l t s 82 D i s c u s s i o n 89 4 2 CHAPTER V I I . THE L I F E T I M E OF THE T„ OR T, STATE . . 95 2g l g R e s u l t s 98 D i s c u s s i o n 103 CHAPTER V I I I . THE PRIMARY PROCESSES I l l M e c h a n i s m I 117 r M e c h a n i s m I I 121 M e c h a n i s m I I I 124 v i i Page CHAPTER IX. SOME FINAL REMARKS 130 1. The O r i g i n of the Lowest Quartet S t a t e 130 2 . Tunneling Mechanism and T e l l e r C r o s s i n g . . . . 133 3 . Primary Processes and Ligand F i e l d Strength . . 134 4 . Suggestions f o r F u r t h e r v-Iork. 135 BIBLIOGRAPHY 139 APPENDIX 145 v i i i LIST OF TABLES TABLE Page I. Photoaquation Quantum Y i e l d s of [ C r ( N H 3 ) 2 (NCS)^] as Donor 55 I I . Frequency f a c t o r s and a c t i v a t i o n e n e r g i e s of the temperature-dependent processes of the 2pg s t a t e . 70 I I I . I n t r i n s i c phosphorescence r a t e constants and low-temperature l i m i t i n g phosphorescence quantum y i e l d s 83 IV. Rate constants (sec "'") i n luminescence decay of C r ( I I I ) complexes 99 V. F l u o r e s c e n c e l i f e t i m e s a t l i q u i d n i t r o g e n temperature . 109 VI. The A r r h e n i u s parameters of k i s c based on Mechanism I and II 116 4 9 V I I . The p r e d i c t e d l o c a t i o n s of the T2g and T2g s t a t e s a c c o r d i n g to Mechanism I ana I I . . . . . . 120 V I I I . The A r r h e n i u s parameters of 1 / T 127 i x LIST OF FIGURES FIGURE Page 1. M o l e c u l a r o r b i t a l diagram of t r a n s i t i o n metal complex of 0^ symmetry 2. S i m p l i f i e d energy l e v e l diagram f o r the o c t a -h e d r a l d3 c o n f i g u r a t i o n 3. Primary processes i n v o l v i n g the lowest e x c i t e d s t a t e s 4. Schematic of the setup f o r emission measurements. 15 5. Schematic of the setup f o r l i f e t i m e measurements. 19 6. Schematic of the setup f o r p h o t o l y s i s measure-ments 22 7. The a b s o r p t i o n s p e c t r a of the [Cr (CN) ,. ] ~3 and trans-[Cr"(NH3) 2 (NCS) 4] - system. . . 31 8. The emission s p e c t r a of the [Cr(CN),]~3 and t r a n s - [ C r ( N H 3 ) 2 ( N C S ) 4 ] " system. 33 9. Stern-Voliner quenching of donor phosphorescence i n t e n s i t y a t -65°C . . 34 10. S e n s i t i z a t i o n of acceptor phosphorescence monitored a t 806 nm and a t 825 nm 35 11. The donor phosphorescence decay r a t e constants a t v a r i o u s acceptor c o n c e n t r a t i o n s as f u n c t i o n s of temperature . 36 12. Stern-Volmer cruenching of donor phosphorescence l i f e t i m e at -65°C . 37 13. A r r h e n i u s p l o t of k,_„ 39 OH 14. Quenching of the phosphorescence l i f e t i m e of [Cr(CN)g]-3 i n the presence of [ C r ( N H 3 ) 2 (NCS)4]~. 40 15. A r r h e n i u s p l o t of 41 16. The r i s e and decay t r a c e of the phosphorescence of the acceptor 42 3 5 9 X FIGURE Page 17. N o n - e x p o n e n t i a l d e c a y o f t h e t r a n s - [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ i n t h e e n e r g y t r a n s f e r s y s t e m :. 49 18. S t e r n - V o l m e r p l o t s f o r I ^ / I D and; f o r T"/TD . . . . . . . . . ? 57 o 19. The l i f e t i m e s o f t h e s t a t e s o f t r a n s -f e r ( N H 3 ) 2 (NCS)4]~ and [ C r ( e n ) 3 ] + 3 as f u n c t i o n s o f t e m p e r a t u r e 67 20. The l i f e t i m e o f t h e "'Eg s t a t e o f C r ( a c a c ) 3 as a f u n c t i o n o f t e m p e r a t u r e 6 8 21. The l i f e t i m e s o f t h e 2 E a s t a t e s o f [ C r ( N C S ) g ] - 3 and [ C r ( C N ) g ] ~ 3 as f u n c t i o n s o f t e m p e r a t u r e . . . 69 22. The p h o s p h o r e s c e n c e quantum y i e l d o f R e i n e c k a t e i o n as a f u n c t i o n o f t e m p e r a t u r e . 8 4 23. P h o s p h o r e s c e n c e quantum y i e l d s as f u n c t i o n s o f t e m p e r a t u r e 85 24. The p h o s p h o r e s c e n c e quantum y i e l d o f C r t a c a c ) ^ as a f u n c t i o n o f t e m p e r a t u r e . . . . . 86 25. Quantum, y i e l d s o f i n t e r s v s t e m c r o s s i n g f o r [ C r ( N C S ) 6 ] - 3 , t r a n s - [ C r ( N H 3 ) 2 ( N C S ) 4 ] a n d [ C r ( C N ) - ] - 3 . • 87 26. Quantum y i e l d s o f i n t e r s v s t e m c r o s s i n g f o r (1) [ C r ( e n ) 3 ] + 3 - (2) C r ( a c a c ) 3 ; (3) t r a n s -[ C r ( N H 3 ) 2(NCS) 4 ] - ; (4) C r ( C N ) 6 ] " 3 88 27. P l o t s o f l o g ( l / ( j ) i s c - l ) v s 1/T 94 28. O s c i l l o s c o p e t r a c e s o f l u m i n e s c e n c e d e c a y : (A) C r ( a c a c ) 3 , (B) [ C r ( C N ) g ] - 3 96 29. x as f u n c t i o n s o f t e m p e r a t u r e f o r t r a n s -f e r (NH 3) 2 (NCS) 4 ] -, and C r ( a c a c ) 3 . . . 100 30. T x as a f u n c t i o n o f t e m p e r a t u r e f o r [ C r ( e n ) 3 ] + 3 . 101 _3 31. Tx o f [ C r ( N C S ) , ] as a f u n c t i o n o f t e m p e r a t u r e . 102 x i FIGURE Page 32. L u m i n e s c e n c e s p e c t r a o b t a i n e d f r o m d e c a y c u r v e s f o r [ C r ( C N ) g ] - 3 i n r i g i d g l a s s s o l u t i o n a t 77°K . 106 33. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f C r ( a c a c ) 3 as a f u n c t i o n o f t e m p e r a t u r e 112 34. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f [ C r ( e n ) 3 ] + 3 as a f u n c t i o n o f t e m p e r a t u r e 113 35. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f [ C r ( N C S ) g ] ~ 3 as a f u n c t i o n o f t e m p e r a t u r e . . . . 114 36. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f t r a n s -f e r ( N H 3 ) 2 ( N C S ) 4 ] ~ as a f u n c t i o n o f t e m p e r a t u r e . . 115 37. S c h e m a t i c o f M e c h a n i s m I . . . 118 38. S c h e m a t i c o f M e c h a n i s m I I 123 39. S c h e m a t i c o f M e c h a n i s m I I I 125 4 40. The o r i g i n o f t h e T 2_ s t a t e o f C r ( I I I ) c o m p l e x e s as f u n c t i o n o f l i g a n a f i e l d s t r e n g t h 132 ACKNOWLEDGEMENTS I wish to express s i n c e r e thanks to Dr. G.B. P o r t e r f o r i n s p i r a t i o n , encouragement, a d v i c e , and c r i t i c i s m throughout the course of t h i s work. I would a l s o l i k e t o thank Dr. J.S.E. Mcintosh and Mr. A. P f e i l f o r i n d i s p e n s a b l e t e c h n i c a l a i d s and h e l p f u l d i s c u s s i o n s . I am indebted to my wife f o r a s s i s t a n c e i n some of the numerical treatments. F i n a n c i a l support from the N a t i o n a l Research C o u n c i l of Canada i s g r a t e f u l l y acknowledged. CHAPTER I INTRODUCTION Stimulated by the g r e a t s t r i d e s taken i n the s t u d i e s of the chemical and p h y s i c a l nature of e x c i t e d molecules i n g e n e r a l , the photochemistry of c o o r d i n a t i o n compounds has r e -c e n t l y quickened i t s pace and emerged as an i n t e r e s t i n g new branch of photochemistry. Numerous reviews have been p u b l i s h e d i n the l a s t few years,"'" "^ and the importance of t h i s new f i e l d i s now being g r a d u a l l y acknowledged. However, as i n any other f i e l d i n i t s i n f a n c y , most of the e x i s t i n g data and r e s u l t s of the photochemistry of c o o r d i n a t i o n compounds are only phenomenological and are r a t h e r s c a t t e r e d and i s o -l a t e d . I t i s thus e s s e n t i a l to have more sy s t e m a t i c mechan-i s t i c s t u d i e s . The f a m i l y of chromium(III) complexes i s one of the most s u i t a b l e systems f o r both i n t e n s i v e and e x t e n s i v e photochemical i n v e s t i g a t i o n . A l a r g e number of C r ( I I I ) com-plexes with widely v a r y i n g l i g a n d f i e l d s t r e n g t h have been prepared, t h e i r thermal s t a b i l i t i e s are u s u a l l y moderately s a t i s f a c t o r y , and t h e i r ground s t a t e chemical r e a c t i o n s have been w e l l c h a r a c t e r i z e d and studied."'"'"'"''' The bonding p r o p e r t i e s of the e l e c t r o n i c s t a t e s are r e l a t i v e l y c l e a r 12 — 18 and the s p e c t r o s c o p i c bands have been w e l l assigned. U n l i k e Co(III) complexes, C r ( I I I ) complexes only undergo 2 p h o t o s u b s t i t u t i o n , with no known photoredox r e a c t i o n s , hence t h i s l a r g e l y s i m p l i f i e s the problem. In a d d i t i o n , q u i t e a few C r ( I I I ) complexes phosphoresce i n f l u i d s o l u t i o n s a t low temperatures, or even at room temperature, f o r example, + 3 [Cr(en)^] . Emission s t u d i e s where p o s s i b l e are an e s s e n t i a l adjunct to photochemical s t u d i e s . In the f o l l o w i n g s e c t i o n s the t h e o r e t i c a l and e m p i r i c a l f a c t s which are important f o r t h i s work are b r i e f l y o u t l i n e d . More d e t a i l e d d e s c r i p t i o n s can be found i n the r e f e r e n c e s c i t e d i n t h i s chapter. 1. Thermal Reactions'^' ^ C r ( I I I ) complexes have h a l f - f i l l e d t ~ o r b i t a l s and - 2g are t h e r e f o r e c o n s i d e r e d to be s u b s t i t u t i o n a l l y i n e r t . Ligand exchange r e a c t i o n s i n the absence of l i g h t take p l a c e o n l y s l o w l y . They c o n s i s t of aquation, a n a t i o n , i s o m e r i z a t i o n , and r a c e m i z a t i o n . T h e i r a c t i v a t i o n e n e r g i e s range from 15 to 17 Kcal/mol. 12-18 2. E l e c t r o n i c States The molecular o r b i t a l diagram f o r a t r a n s i t i o n metal complex of 0^ symmetry i s shown i n F i g u r e 1. The d e l e c t r o n s of an o c t a h e d r a l complex i o n occupy the o r b i t a l s t 2 q . a n < ^ e g ' which are non-bonding or ir antibonding (t 2 q.) and a a n t i -* 3 bonding ( eg) • I n the d. system, to which chromium (III) belongs, the ground s t a t e o r b i t a l c o n f i g u r a t i o n i s t 2 ^. The 3 s p l i t t i n g , lODq, of the two sets of o r b i t a l s arises primarily from the anti-bonding character of the eg o r b i t a l s . However, i t may also be affected by TT bonding of the t 2 g o r b i t a l s . Thus a large value of lODq for a given complex means that a complex 4 2 i n the T 2 g^ t 2g e c ^ state i s expected to have considerably larger chromium-ligand separations than the ground state mole-cule because of the decrease i n the o v e r a l l (a and TT) bonding force when a t^^ electron i s promoted to an anti-bonding eg o r b i t a l . In other words, i t i s expected that the value of lODq 4 4 (obtained from the absorption maximum of T_ A„ and thus 2g 2g corresponding to the vibronic t r a n s i t i o n with maximum Franck-Condon overlap) w i l l be determined not only by the separation of the origins of the el e c t r o n i c states themselves, but also by the change i n the equilibrium nuclear configuration. Therefore, phenomenologically, the Stokes 1 s h i f t between the absorption maximum and the fluorescence maximum should increase markedly as lODq increases for a series of complexes. The energy l e v e l diagram from the s t r o n g - f i e l d c a l c u l a -tions of Tanabe and Sugano i n the s i m p l i f i e d form i s shown i n 3 Figure 2. Also a r i s i n g from the configuration t 2 are three 2 2 2 doublet states E^, T i g ' a n <^ T 2g* T h e degeneracy of the 2 2 E^ and states i s removed by s p m - o r b i t a l coupling and by deviation of the ligand f i e l d from octahedral symmetry. 5 lODq (kK) F i c r a r e 2. S i m p l i f i e d e n e r g y l e v e l d i a g r a m f o r t h e o c t a h e d r a l 3 d c o n f i g u r a t i o n . 2 The n e x t l o w e s t e l e c t r o n i c c o n f i g u r a t i o n , t„ e , g i v e s r i s e t o zg g 4 4 t h e q u a r t e t s t a t e s and T 1 ^ ( F ) . The o t h e r q u a r t e t s t a t e 4 (P) v/hich i s n o t shown i n t h e f i g u r e a r i s e s f r o m t h e t _ e c o n f i g u r a t i o n . 2 g g 3 . S p e c t r a l P r o p e r t i e s " * ' ^ ^ The main f e a t u r e s o f t h e a b s o r p t i o n s p e c t r a o f C r ( I I I ) c omplexes a r e : i n t h e r e d r e g i o n (700 t o 820 nm) a s e t o f s h a r p l i n e s o f low i n t e n s i t y a s s i g n e d t o t h e t r a n s i t i o n 2 4 + + E A_ and o t h e r s p i n - f o r b i d d e n t r a n s i t i o n s ; i n t h e v i s -g 2<? i b l e r e g i o n two ( o r sometimes t h r e e ) b r o a d s t r u c t u r e l e s s bands o f r e l a t i v e l y low i n t e n s i t y (e o f o r d e r o f 50 M ^ cm ^ ) , u max 4 4 r e p r e s e n t i n g t h e L a p o r t e f o r b i d d e n t r a n s i t i o n s ^2q «- A 2 g and 4 4 T^ A 2 g ' a n c * u s u a l l y i n t h e UV and n e a r UV r e g i o n o f t h e s p e c t r u m , a number o f i n t e n s e bands w h i c h a r e i n t r a l i g a n d a n d / o r c h a r g e t r a n s f e r b a n d s . I n e m i s s i o n s p e c t r a , two t y p e s o f t r a n s i t i o n s have b e e n 2 4 o b s e r v e d : p h o s p h o r e s c e n c e E -> A„ , and f l u o r e s c e n c e , L * g 2g ' 4 4 T_ -> A„ . Here f l u o r e s c e n c e i s t h e r a d i a t i o n t r a n s i t i o n b e -2g 2g tween two e l e c t r o n i c s t a t e s o f t h e same s p i n m u l t i p l i c i t y , w h i l e p h o s p h o r e s c e n c e i s o f d i f f e r e n t s p i n m u l t i p l i c i t i e s . U s u a l l y , p h o s p h o r e s c e n c e i s t h e o n l y e m i s s i o n w h i c h c a n be r e a d i l y ob-s e r v e d i n C r ( I I I ) c o m p l e x e s , b u t t h e r e a r e some c a s e s f o r The 0^ symmetry d e s i g n a t i o n i s u s e d t h r o u g h o u t t h i s t h e s i s f o r s i m p l i c i t y , e v e n t h o u g h some com p l e x e s a r e n o t o c t a h e d r a l , f o r example, t r a n s - [ C r (NH3) 2 (NCS) 4] ~ i s o f symmetry. 7 which f l u o r e s c e n c e emission i s comparable i n i n t e n s i t y with the phosphorescence and s t i l l others which show only f l u o -19 rescence. Which of these occur has been c o r r e l a t e d w i t h 2 4 20 the r e l a t i v e energies of the E and T„ s t a t e s . g 2g Phosphorescence s p e c t r a , which are u s u a l l y the m i r r o r images of the corresponding s p i n - f o r b i d d e n a b s o r p t i o n s p e c t r a , c o n s i s t of a s e t of sharp l i n e s , one of which occurs d i s t i n c t l y i n both a b s o r p t i o n and emission and r e p r e s e n t s the o r i g i n of the band or the zero-zero t r a n s i t i o n . T h i s i n d i c a t e s t h a t 4 the e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n s of the " A2g a n < ^ ^ e 2 E s t a t e s must be n e a r l y i d e n t i c a l . The f l u o r e s c e n c e s p e c t r a , g -- ' on the other hand, are i n v a r i a b l y broad and s t r u c t u r e l e s s . There i s m i r r o r image r e l a t i o n s h i p of the f l u o r e s c e n c e band 4 4 with the corresponding ^2q A 2 g ^ s o r p t i o n , but the Stokes' s h i f t i s r e l a t i v e l y l a r g e . T h i s i n d i c a t e s t h a t the 4 4 e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n s of the A~ and the T_ ^ 2g 2g s t a t e s must be cmite d i f f e r e n t . 4. Primary P h o t o p h y s i c a l Processes The J a b l o n s k i diagram f o r chromium(III) complexes 4 2 2 2 4 i n v o l v i n g the T„ , T„ , T, , E , and A 0 s t a t e s i s ^ 2g 2g l g cr 2g shown i n F i g u r e 3. For s i m p l i c i t y , the e l e c t r o n i c s t a t e s are d e s c r i b e d i n 0^ symmetry f o r a l l the complexes s t u d i e d 2 2 i n t h i s work, and the T, and E s t a t e s , because of s m a l l i g g energy s e p a r a t i o n between, them, are assumed to be i n r a p i d 8 e q u i l i b r i u m and are c o n s i d e r e d an e q u i l i b r a t e d s t a t e u n l e s s otherwise s p e c i f i e d . I t i s g e n e r a l l y b e l i e v e d t h a t a f t e r e x c i t a t i o n t o the Frank-Condon l e v e l s of the e x c i t e d e l e c t r o n i c s t a t e s , because the v i b r a t i o n a l r e l a x a t i o n and i n t e r n a l c onversions between high e r e x c i t e d s t a t e s are very f a s t , the molecule must reach -12 i t s lowest e x c i t e d e l e c t r o n i c s t a t e s w i t h i n 10 sec. The 4 l i f e t i m e of the T„ s t a t e has been estimated from o s c i l l a t o r 2g _7 s t r e n g t h and f l u o r e s c e n c e quantum y i e l d t o be l e s s than 10 sec. However, the observed values f o r complexes which f l u o -— 6 resce are i n the order of 10 sec or a l i t t l e l e s s . I n t e r -system c r o s s i n g , k„,completes f a v o r a b l y with the other p r o c e s s e s , and i n t e r n a l c o n v e r s i o n , k 2 (and/or i n t e r s y s t e m c r o s s i n g , kg), should be s i g n i f i c a n t as judged from the sum of photochemical and emission quantum y i e l d s which i s u s u a l l y f a r s m a l l e r than u n i t y . 2 Primarv processes from the E s t a t e have been b e t t e r g s t u d i e d . The phosphorescence r a d i a t i v e r a t e c o n s t a n t s , k^, have been approximately e v a l u a t e d through a simple s p i n -18 o r b i t - c o u p l i n g model. Although phosphorescence emissions are r e a d i l y d e t e c t a b l e a t l i q u i d n i t r o g e n temperature, the y i e l d s are u s u a l l y l e s s than a few p e r c e n t . Intersystem c r o s s i n g to the ground s t a t e , kg, i s the dominant degrading pathway at low temperatures, and t h e r m a l l y a c t i v a t e d i n t e r -system c r o s s i n g , k . , may be important at h i g h e r temperatures. 9 \ N 4 \ k _ 4 \ \ 2g *7 ^ 2g F i g u r e 3. P r i m a r y p r o c e s s e s i n v o l v i n g t h e l o w e s t e x c i t e d s t a t e s . k^, i n t r i n s i c f l u o r e s c e n c e e m i s s i o n ; k 2 , i n t e r n a l A c o n v e r s i o n ; k^, p h o t o c h e m i c a l r e a c t i o n f r o m t h e ^pg s 1 : a t e ; k^, i n t e r s y s t e m c r o s s i n g ; t h e r m a l l y a c t i v a t e d r e v e r s e i n t e r s y s t e m c r o s s i n g ; k^, i n t r i n s i c p h o s p h o r e s c e n c e e m i s s i o n ; k g , i n t e r s y s t e m c r o s s i n g t o t h e g r o u n d s t a t e ; k^, p h o t o -2 c h e m i c a l r e a c t i o n f r o m t h e E s t a t e . q 10 5. P h o t o c h e m i c a l P r o c e s s e s A q u a t i o n i s t h e p r i n c i p a l p h o t o r e a c t i o n t h a t h a s b e e n s t u d i e d f o r c h r o m i u m ( I I I ) c o m p l e x e s i n a q u e o u s s o l u t i o n . The p h o t o a q u a t i o n quantum y i e l d s r a n g e f r o m l e s s t h a n 0.1 t o 0.5. The l o w quantum y i e l d s a r e n o t due t o c a g e e f f e c t s as i s e v i d e n t f r o m t h e s t u d i e s o f t h e p h o t o s u b s t i t u t i o n o f [ C r ( H 2 0 ) g ] + 3 by o x y g e n - 1 8 e n r i c h e d w a t e r — a l t h o u g h t h e s o l v e n t 21 c a g e i s one o f t h e r e a c t a n t s , t h e q u antum y i e l d i s s t i l l l o w . A l l i n v e s t i g a t i o n s o f c o m p l e x e s w i t h o n l y one l i g a n d t y p e p r e s e n t r e p o r t t h e quantum y i e l d t o be i n d e p e n d e n t o f t h e w a v e l e n g t h o f e x c i t a t i o n . T h e s e r e s u l t s c a n be b e s t i n t e r -p r e t e d as m e a n i n g t h a t p h o t o a q u a t i o n d o e s n o t o c c u r d i r e c t l y 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 i n t h e a b s o r p t i o n p r o c e s s n o r f r o m o t h e r t h a n t h e l o w e s t e x c i t e d s t a t e o f one m u l t i -p l i c i t y . The i m m e d i a t e p r e c u r s o r t o t h e p h o t o c h e m i c a l r e a c -t i o n s o f C r ( I I I ) c o m p l e x e s has b e e n s o u g h t f o r y e a r s . I t 4 h a s b e e n s u g g e s t e d t o be t h e ^2g s t a t e , b a s e d on t h e b o n d -2 2 i n g c h a r a c t e r and e l e c t r o n i c s t r u c t u r e . H o wever, i t h a s a l s o b e e n h e l d t h a t t h e l i f e t i m e o f t h e l o w e s t e x c i t e d 4 -7 q u a r t e t , T 2 q ' ^ s n o g r e a t e r t h a n 10 s e c , w h i c h i s t o o b r i e f t o e n a b l e a q u a t i o n t o c o m p e t e e f f e c t i v e l y w i t h o t h e r 23 2 d e c a v p r o c e s s e s . The l o w e s t e x c i t e d d o u b l e t s t a t e , E , g i s r e l a t i v e l y l o n g - l i v e d s o c h e m i c a l r e a c t i o n may w e l l com-22 24 p e t e w i t h i n t e r s y s t e m c r o s s i n g s and p h o s p h o r e s c e n c e . " ' 11 C o n s i d e r a b l e e f f o r t has been d i r e c t e d a t measurement of the quantum y i e l d on i r r a d i a t i o n i n the r e g i o n of the sharp a b s o r p t i o n bands, i n order to populate the doublet 25 ? 8 s t a t e s d i r e c t l y r a t h e r than through i n t e r s y s t e m c r o s s i n g . 4 The i d e a i s to bypass the T2g s t a t e s o t h a t i f i t were r e s -p o n s i b l e f o r the photochemical r e a c t i o n , the y i e l d would be 2 decreased. I f , on the other hand, the E s t a t e were the g immediate p r e c u r s o r to photoaquation, the y i e l d might be l a r g e r , on the grounds t h a t i n t e r s y s t e m c r o s s i n g i s not 100% e f f i c i e n t . However, these experiments are d i f f i c u l t t o c a r r y out. .So f a r there i s no f i r m evidence t h a t the quantum y i e l d s are any d i f f e r e n t f o r e x c i t a t i o n i n the r e g i o n of the doublet bands. I t has a l s o been proposed t h a t the r e a c t i n g s p e c i e s may be a v i b r a t i o n a l l y e x c i t e d or hot ground-state molecule which i s formed through i s o e n e r g e t i c t r a n s i t i o n s from the 2 6 e l e c t r o n i c a l l y e x c i t e d s t a t e s . However, the r e a c t i o n p a t t e r n s and a c t i v a t i o n parameters are q u i t e d i f f e r e n t between 27 photochemical and thermal r e a c t i o n s . And r e c e n t evidence has shown t h a t while p h o t o s u b s t i t u t i o n does not depend on 29 the s o l v e n t composition thermal s u b s t i t u t i o n does. More-over, c o n s i d e r i n g the f a c t t h a t the l i f e t i m e s of the non-e q u i l i b r a t e d , h i g h l y e x c i t e d v i b r a t i o n a l l e v e l s are extremely -12 s h o r t (about 10 ' s e c ) , t h i s s u p p o s i t i o n i s g e n e r a l l y not f a v o r e d . 12 6. A B r i e f D e s c r i p t i o n o f t h i s Work The m a i n r a t i o n a l e o f t h i s work was t o i n v e s t i g a t e t h e 4 2 r o l e p l a y e d by t h e T_ and E s t a t e s i n t h e p h o t o a q u a t i o n o f C r ( I I I ) c o m p l e x e s . I n s t e a d o f p o p u l a t i n g t h e d o u b l e t s t a t e by d i r e c t e x c i t a t i o n , t h e more e f f i c i e n t e n e r g y t r a n s f e r t e c h n i q u e was u s e d . The f i r s t s t e p was t o s e a r c h f o r t h e s u i t a b l e e n e r g y t r a n s f e r s y s t e m s . One o f t h e s y s t e m s , t r a n s - [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ as a d o n o r and [ C r ( C N ) g ] 3 as an a c c e p t o r , i s c o n s i d e r e d i n C h a p t e r I I I . The emphasis t h e r e i s n o t o n l y on t h e a p p l i c a -t i o n o f e n e r g y t r a n s f e r t o p h o t o c h e m i c a l s t u d i e s b u t a l s o on t h e e n e r g y t r a n s f e r i t s e l f as a p r i m a r y p r o c e s s . The a p p l i c a t i o n o f e n e r g y t r a n s f e r t o t h e s t u d i e s o f t h e p h o t o c h e m i s t r y o f t r a n s - [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ i n t h e e n e r g y t r a n s f e r s y s t e m i s d e s c r i b e d i n C h a p t e r IV. I t was c l e a r a t t h i s s t a g e t h a t d e f i n i t i v e i n f o r m a t i o n a b o u t t h e r o l e o f e x c i t e d s t a t e s c a n n o t be o b t a i n e d f r o m s t u d i e s o f t h e p h o t o c h e m i s t r y a l o n e . S t u d i e s o f t h e o t h e r p r i m a r y p r o c e s s e s a r e n e e d e d . In o r d e r t o i d e n t i f y t h e q u e n c h a b l e p a r t o f t h e p h o t o -2 a q u a t x o n , t h e p r i m a r y p r o c e s s e s f r o m t h e E ^ s t a t e were i n -v e s t i g a t e d as d e s c r i b e d i n C h a p t e r V. The m a i n e f f o r t was t o c o n f i r m t h a t t h e t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g , k_4/ o c c u r s . I n C h a p t e r V I , t h e p o s s i b l e m e t h o d s ' a v a i l a b l e t o d e t e r m i n e i n t e r s y s t e m c r o s s i n g quantum y i e l d s f r o m e n e r g y t r a n s f e r and q u e n c h i n g a r e c o n s i d e r e d . I n t e r s y s t e m c r o s s i n g 13 quantum y i e l d s w e r e m e a s u r e d as f u n c t i o n s o f t e m p e r a t u r e i n t h e hope t h a t t h i s w o u l d p r o v i d e some i n f o r m a t i o n a b o u t t h e 4 T~ s t a t e . 2g I n C h a p t e r V I I , a t r a n s i e n t s p e c i e s o r s t a t e , b e l i e v e d 4 2 t o be e i t h e r o f t h e T„ s t a t e o r o f t h e T, s t a t e , was 2g l g c h a r a c t e r i z e d b y t h e r i s e o f t h e p h o s p h o r e s c e n c e i n t e n s i t y w i t h t i m e a f t e r p u l s e e x c i t a t i o n . I n C h a p t e r V I I I , a s s u m i n g t h e n e w l y o b s e r v e d l i f e t i m e 4 t o r e p r e s e n t t h a t o f , f i r s t , t h e T^ s t a t e a n d t h e n t h a t o f 2 t h e s t a t e , t h r e e m e c h a n i s m s a r e p r o p o s e d and e x a m i n e d . F i n a l l y , i n C h a p t e r I X , t h e s p e c t r o s c o p i c o r i g i n o f 4 t h e T„ s t a t e i s s t u d i e d . Some s p e c u l a t i o n s a r e d e s c r i b e d 2g and s u g g e s t i o n s f o r f u r t h e r i n v e s t i g a t i o n s a r e made. CHAPTER I I GENERAL EXPERIMENTAL PART Experimental i n f o r m a t i o n common to the e n t i r e t h e s i s i s o u t l i n e d i n t h i s chapter. T h i s i n c l u d e s d e t a i l s of the setup of equipment, experimental techniques, and the sources and p u r i f i c a t i o n procedures of the chemicals used. S p e c i f i c i n f o r m a t i o n and minor m o d i f i c a t i o n s l i m i t e d o n l y to a c e r t a i n i n v e s t i g a t i o n w i l l be mentioned i n the chapter where i t i s d e s c r i b e d . 1. Emission Measurements A block diagram of the setup f o r the s t e a d y - s t a t e emission measurements i s shown i n F i g u r e 4 . A l l measurements were c a r r i e d out i n a dark chamber. The r i g h t angle o p t i c a l arrangement was adopted mainly to a v o i d s c a t t e r e d l i g h t . F o r t u n a t e l y f o r a l l the systems s t u d i e d the i n n e r f i l t e r e f f e c t i s n e g l i g i b l e . 1.1 E x c i t a t i o n L i g h t Source The l i g h t source was a 100-watt, p o i n t - s o u r c e mercury arc lamp (PEK 110) powered by a s t a b i l i z e d DC power supply (PEK model 401). A s h o r t f o c a l - l e n g t h quartz lens (1" diam. 27 mm F.L.) was p l a c e d r i g h t b e f o r e the lamp i n order to c o l l e c t the maximal amount of l i g h t from the a r c . The lamp 30 housing was s i m i l a r to t h a t d e s c r i b e d by C a l v e r t and P i t t s . The l i g h t beam was so focused t h a t i t passed through the sample with n e a r l y constant c r o s s - s e c t i o n . 15 M CRS& PMT PSD PA F i g u r e 4. Schematic o f the s e t u p f o r e m i s s i o n measurements LS = l i g h t s o u r c e ; M = monochromator; S = s h u t t e r , F = g l a s s f i l t e r ; L = l e n s ; BS = beam s p l i t t e r ; CRS = c r y o s t a t ; C = sample c u v e t t e ; PT = p h o t o t u b e ; CHP = l i g h t chopper f o r the l o c k - i n a m p l i f i e d ; PMT = p h o t o m u l t i p l i e r t ube; PA = p r e a m p l i f i e r ; PSD = phase s e n s i t i v e d e t e c t o r ; R = r e c o r d e r . 16 E a c h o f t h e m e r c u r y l i n e s a t 456, 436, o r 366 nm, i s o l a t e d f r o m t h e t o t a l o u t p u t o f t h e a r c by a m o n o c h r o m a t o r , was e m p l o y e d as t h e m o n o c h r o m a t i c e x c i t a t i o n l i g h t . F o r t h e 546 nm l i n e , a y e l l o w g l a s s f i l t e r ( C o r n i n g 3-74) was a d d e d t o e l i m i n a t e t h e s e c o n d o r d e r u v r a d i a t i o n . The m o n o c h r o m a t o r u s e d was a B a u s c h & Lomb M o n o c h r o m a t o r (33-86-25) w i t h a v i s i b l e g r a t i n g (33-86-02) o f 1350 groove/mm a n d 500 nm b l a z e . The r e c i p r o c a l l i n e a r d i s p e r s i o n i s 6.4 nm/mm. The e n t r a n c e and e x i t s l i t s w e r e 5.36 a n d 3.00 mm r e s p e c t i v e l y . The o u t p u t o f t h e lamp a t 546 nm was a b o u t 12 mw. A l t h o u g h i t s s h o r t - t e r m s t a b i l i t y was b e t t e r t h a n 1%, some-t i m e s i t c o u l d d r i f t a s much as 5% o v e r 10 h o u r s . T h e r e f o r e t h e r e l a t i v e i n t e n s i t y o f t h e e x c i t a t i o n l i g h t was m o n i t o r e d c o n t i n u o u s l y t h r o u g h a beam s p l i t t e r , a p h o t o t u b e (RCA 9 3 5 ) , and a 10-mv r e c o r d e r . The f l u c t u a t i o n i n t h e e x c i t a t i o n l i g h t was p r o p e r l y c o r r e c t e d f o r i n t h e c a l c u l a t i o n s . 1. 2 Sample C r y o s t a t The c y l i n d r i c a l s a m p l e c r y o s t a t was b u i l t o f P l e x i g l a s s and i n s u l a t e d t h e r m a l l y w i t h t h i c k S t y r o f o a m . T h r e e d o u b l e -l a y e r e d q u a r t z windows were made f o r t h e e x c i t i n g , t r a n s m i t t i n g and e m i t t i n g . l i g h t beams r e s p e c t i v e l y . The o u t e r w indows w e r e b l a n k e t e d w i t h s t r e a m s o f warm a i r t o p r e v e n t them f r o m f r o s -t i n g when n e c e s s a r y . The s a m p l e c e l l - - a 1-cm q u a r t z l u m i n e s c e n c e c u v e t t e - -c o u l d be p o s i t i o n e d r e p r o d u c i b l y on a c o p p e r b l o c k . A g l a s s t u b i n g c o n n e c t e d t o t h e c e l l and p r o t r u d i n g o u t o f t h e c r y o -17 s t a t served as the c o n d u i t to i n t r o d u c e and withdraw the sample, to c a r r y a thermocouple l e a d , and to m a i n t a i n a con-tinuous flow of n i t r o g e n over the s o l u t i o n . A s h o r t f o c a l -l e ngth quartz lens was p l a c e d very c l o s e to the c e l l i n order to c o l l e c t the maximal amount of e m i s s i o n . The c o l d n i t r o g e n used to c o o l the sample was spread evenly onto the c e l l w a l l s through the holes on the i n l e t copper tubing which was extended to e n c i r c l e the c e l l base. In order to ensure a reasonably homogeneous temperature, a t i n y T e f l o n - c l a d magnetic bar was p l a c e d i n the c e l l t o s t i r the s o l u t i o n . F o r t u n a t e l y , when the s o l u t i o n becomes too v i s c o u s to be s t i r r e d , the emission becomes i n s e n s i t i v e to the temperature, thus s t i r r i n g i s not c r i t i c a l l y r e q u i r e d . 1.3 Temperature C o n t r o l and Measurement Temperature was c o n t r o l l e d manually by c o n t r o l l i n g the flow r a t e of c o l d n i t r o g e n gas, evaporated from l i q u i d n i t r o g e n , t h a t was i n t u r n c o n t r o l l e d by the c u r r e n t to the heater i n a 50 1 Dewar c o n t a i n i n g l i q u i d n i t r o g e n . Tempera-ture was probed with a g l a s s - c l a d copper-constantan thermo-couple which was immersed d i r e c t l y i n t o the s o l u t i o n with i t s j u n c t i o n c l o s e to but out of the e x c i t a t i o n l i g h t . The emf developed was measured with a Rubicon potentiometer. 1.4 D e t e c t i n g System The d e t e c t i n g system c o n s i s t e d of a sharp-cut red f i l t e r , l e n s e s , a chopper f o r the l o c k - i n a m p l i f i e r , an ana-18 l y z i n g monochromator, and a P h i l l i p s 150 CVP p h o t o m u l t i p l i e r . The red f i l t e r (Corning CS 2-58) was used to e l i m i n a t e f u r t h e r the s c a t t e r e d e x c i t a t i o n l i g h t . The a n a l y z i n g monochromator was a J a r r e l l - A s h 0.25 meter E b e r t monochromator (Model 82-400) with two g r a t i n g s of 1180 groove/mm and b l a z e d a t 300 and 600 nm r e s p e c t i v e l y . The l i n e a r d i s p e r s i o n i s 33 nm/mm; The entrance and e x i t s l i t s used were both 100 microns. The scan-ning speed was 25 nm/min. The monochromator was l a i d on i t s s i d e w a l l to make the entrance s l i t h o r i z o n t a l i n order to allow the image of the emission, which has been c a r e f u l l y focused, to f a l l e n t i r e l y onto i t . Before r e a c h i n g the mono-chromator, the emission l i g h t was chopped by a PAR Model BZ-1 chopper which was operated a t 100 Hz and p l a c e d r i g h t b e f o r e the entrance s l i t of the monochromator. The red s e n s i t i v e ( S - l s p e c t r a l response) P h i l l i p s 150 CVP p h o t o m u l t i p l i e r was p l a c e d i n a c r y o s t a t and c o o l e d with l i q u i d n i t r o g e n . I t was powered by a Kepco ABC 1500 DC high v o l t a g e supply. The v o l t a g e a p p l i e d to the p h o t o m u l t i p l i e r was 1.3 kv, the o p t i m a l v a l u e . The s i g n a l v o l t a g e across the load r e s i s t o r of the p h o t o m u l t i p l i e r was a m p l i f i e d by a Brookdeal LA350 Low-Noise A m p l i f i e r . The a m p l i f i e d s i g n a l from i t as w e l l as the r e f e r e n c e s i g n a l from the chopper was again fed i n t o a Brookdeal Phase S e n s i t i v e Detector/Meter U n i t PM322 which c a r r i e d out the l o c k - i n d e t e c t i o n . The r e s u l t i n g s i g n a l was then recorded on a Leeds and Northrup 10-mv r e c o r -der. The l i n e a r i t y of the system has been checked to be s a t i s f a c t o r y v/ith a s e r i e s of n e u t r a l d e n s i t y f i l t e r s . 19 F i g u r e 5. S c h e m a t i c o f t h e s e t u p f o r l i f e t i m e m e a s u r e m e n t s . OS = o s c i l l o s c o p e ; PMT = p h o t o m u l t i p l i e r t u b e ; CRS = c r y o s t a t ; M = m o n o c h r o m a t o r ; F = g l a s s f i l t e r ; F l = CuSO^ s o l u t i o n f i l t e r ; F2 = K 2 C r 2 0 y s o l u t i o n f i l t e r ; L = l e n s ; C = s a m p l e c u v e t t e ; F L = f l a s h l a m p; PT = p h o t o t u b e . 2. L i f e t i m e M e a s u r e m e n t s The b l o c k d i a g r a m o f t h e s e t u p f o r l i f e t i m e m e a s u r e m e n t s i s shown i n F i g u r e 5. The e m i s s i o n was d e t e c t e d a t an a n g l e 45° t o t h e e x c i t a t i o n l i g h t . 2 .1 F l a s h Lamp The n i t r o g e n f l a s h lamp was made i n t h i s l a b o r a t o r y . O p e r a t e d a t 5 atm and 15 k v , i t h a s a r i s e t i m e o f 60 n s e c and 14 h a l f - h e i g h t w i d t h o f 20 0 n s e c w i t h 5 x 10 p h o t o n s / p u l s e . The d e t a i l s o f i t s c o n s t r u c t i o n and c h a r a c t e r i s t i c s w i l l be p u b -31 l i s h e d by P f e i l and P o r t e r . The f l a s h lamp was f i l t e r e d w i t h a 5-cm s a t u r a t e d CuSO^ s o l u t i o n t o p r o v i d e t h e e x c i t a t i o n l i g h t f r o m a b o u t 300 t o 500 nm. I t was f o c u s e d t o a s m a l l s p o t o n t o t h e s a m p l e c u v e t t e . 20 2 . 2 Sample C r y o s t a t The c r y o s t a t c o n s i s t e d m a i n l y o f two c y l i n d r i c a l c a n s . The i n n e r one c a n be f i l l e d w i t h l i q u i d n i t r o g e n o r c i r c u l a t e d w i t h c o l d n i t r o g e n g a s . A c o p p e r b l o c k a t t a c h e d t o t h e l o w e r end o f t h i s c o o l i n g c a n s e r v e d as t h e c e l l h o l d e r . The sample c e l l was a s p e c i a l l y b lown P y r e x c u v e t t e w i t h a b o u t 1 ml c a p -a c i t y and 2 mm p a t h l e n g t h . The s p a c e between t h e i n n e r and o u t e r c a n s , where t h e c u v e t t e was, was e v a c u a t e d b e f o r e c o o l -i n g . The c u v e t t e was t i g h t l y c a p p e d , however i t a l l o w e d a s m a l l q u a r t z - c l a d t h e r m o c o u p l e t o be i n s e r t e d t h r o u g h t h e c a p and immersed d i r e c t l y i n t o t h e s o l u t i o n . The j u n c t i o n p o i n t o f t h e t h e r m o c o u p l e was s i t u a t e d a t t h e p o i n t o f f o c u s o f t h e f l a s h l i g h t t o m o n i t o r t h e t r u e t e m p e r a t u r e . T h e r e were two q u a r t z windows a t 45° t o e a c h o t h e r on t h e o u t e r c y l i n d e r f o r t h e e x c i t i n g and e m i t t i n g l i g h t s r e s p e c t i v e l y . T e m p e r a t u r e c o n t r o l and measurements were s i m i l a r t o t h o s e d e s c r i b e d i n t h e e m i s s i o n measurements. T e m p e r a t u r e was c h anged v e r y s l o w l y and measurements were t a k e n o n l y a f t e r t h e t e m p e r a t u r e became a p p a r e n t l y s t e a d y . 2 . 3 D e t e c t i n g System The d e t e c t i n g s y s t e m c o n s i s t e d o f f i l t e r s , a monochro-mator, and a p h o t o m u l t i p l i e r . A r e d g l a s s f i l t e r ( C o r n i n g CS 2-64) and a 2-cm s a t u r a t e d I<2Cr207 s o l u t i o n were u s e d t o e l i m i n a t e t h e s c a t t e r e d e x c i t a t i o n l i g h t . The a n a l y z i n g monochromator. was a B a u s c h & Lomb Monochromator (33-86-25) w i t h an i n f r a r e d g r a t i n g (33-86-03) o f 675 groove/mm and 1.0 21 m i c r o n b l a z e . The r e c i p r o c a l l i n e a r d i s p e r s i o n i s 12.8 nm/mm. The e n t r a n c e and e x i t s l i t s w e r e s e t a t 3.4 and 6 mm r e s p e c -t i v e l y . H owever, t h e m o n o c h r o m a t o r was u s e d i n s u c h a way t h a t t h e e x i t s e r v e d as an e n t r a n c e and t h e e n t r a n c e an e x i t . The p h o t o m u l t i p l i e r , i t s c o o l i n g s y s t e m a nd power s u p p l y w e r e t h e same t y p e as t h o s e u s e d i n e m i s s i o n m e a s u r e m e n t s e x c e p t t h e b l e e d e r n e t w o r k o f t h e p h o t o m u l t i p l i e r was w i r e d s p e c i a l l y f o r f a s t r e s p o n s e a p p l i c a t i o n s . The v o l t a g e a p p l i e d t o t h e p h o t o m u l t i p l i e r was b e t w e e n 0.9 and 1.5 k v . The s i g n a l , m e a s u r e d a c r o s s a 1,000-ohm l o a d r e s i s t o r , was p r e a m p l i f i e d w i t h a T e k t r o n i x Type L P l u g - I n and d i s p l a y e d on a Type 543B O s c i l l o s c o p e . The c o m b i n e d u n i t h a s a r i s e t i m e o f 15 n s e c and s e n s i t i v i t y o f 5 mv/cm. I t was t r i g g e r e d e i t h e r i n t e r -n a l l y o r e x t e r n a l l y w i t h s i g n a l f r o m a p h o t o t u b e . The d e c a y c u r v e s w e r e r e c o r d e d w i t h a Du Mont O s c i l l o g r a p h R e c o r d Camera Type 302 and P o l a r o i d Type 410 f i l m . The p i c t u r e s o b -t a i n e d w e r e t h e n e n l a r g e d w i t h a D e l i n e a s c o p e ( A m e r i c a n O p t i c a l C o . ) . 3. P h o t o l y s i s M e a s u r e m e n t s A b l o c k d i a g r a m o f t h e s e t u p f o r p h o t o l y s i s s t u d i e s i s shown i n F i g u r e 6. The l i g h t s o u r c e was e x a c t l y t h e same as t h a t d e s c r i b e d i n t h e e m i s s i o n m e a s u r e m e n t s e x c e p t t h a t t h e l i g h t beam was s o f o c u s e d t h a t i t f i l l e d m o s t o f t h e p h o t o l y s i s c e l l . 22 F i g u r e 6. Schematic of the setup f o r p h o t o l y s i s measurements. LS = l i g h t source; M = monochromator; S = s h u t t e r ; F = g l a s s f i l t e r ; L = l e n s ; BS = beam s p l i t t e r ; P T = phototube; GT = guard tube; CRS = c r y o s t a t ; C = sample cu v e t t e ; T = t h e r m i s t o r ; H = heater; HET = heat-exchange tube; TC = temperature con-t r o l l e r ; R & I = r e c o r d e r and i n t e g r a t o r . 3 .1 P h o t o l y s i s C e l l The p h o t o l y s i s c e l l was a 1-cm c y l i n d r i c a l q u artz c u v e t t e . I t was extended a t both ends w i t h long, evacuated guard tubes. The opening of the cuv e t t e was connected to a long g l a s s tube which served as the c o n d u i t t o i n t r o d u c e and withdraw samples, to p l a c e a thermocouple, and to m a i n t a i n a flow of n i t r o g e n above the s o l u t i o n d u r i n g p h o t o l y s i s . The p h o t o l y s i s c e l l was immersed i n a low temperature bath with the guard tubes protruded p a r t l y o u t s i d e i t . 3.2 Temperature C o n t r o l and Measurements The low temperature i s o t h e r m a l bath was b u i l t of a square copper v e s s e l i n s u l a t e d w i t h t h i c k Styrofoam, and con-t a i n i n g methanol as the c o o l a n t . C o o l i n g was achieved by passing c o l d n i t r o g e n gas through a copper t u b i n g heat ex-changer immersed i n the c o o l a n t . Temperature was kept w i t h i n 0.2°C of the d e s i r e d temperature by a heater which was con-t r o l l e d by a Cole Parmer Versatherm E l e c t r o n i c C o n t r o l l e r and a low temperature t h e r m i s t o r probe. The s o l u t i o n i n t r o d u c e d was allowed to reach e q u i l i b r i u m with the o u t s i d e c o o l a n t before p h o t o l y z i n g . Temperature was measured with a copper-constantan thermocouple immersed d i r e c t l y i n t o the s o l u t i o n . 3.3 Photon Counting System The i n t e n s i t y of the i n c i d e n t l i g h t was measured with the beam s p l i t t i n g technique as shown i n F i g u r e 6. A smal l p a r t of t h e . r a d i a t i o n was r e f l e c t e d from the beam s p l i t t e r and detected by a RCA 935 phototube. The s i g n a l from the 24 phototube was recorded and i n t e g r a t e d w i t h a Brown (MH) Recor-der (Model 143x58) equipped w i t h a D i s c Chart I n t e g r a t o r (Model 201). The r a t i o of photons r e a c h i n g the p h o t o l y s i s c e l l to the i n t e g r a t o r counts was determined with a Reineckate 2 8 actinometer i n the same p h o t o l y s i s c e l l . 4. A b s o r p t i o n Measurements Ab s o r p t i o n s p e c t r a and absorbances were measured wi t h a Cary 14 Spectrophotometer a t room temperature. 5. Deoxygenation Techniques A l l s o l u t i o n p r e p a r a t i o n s were c a r r i e d out i n a n i t r o -gen box. S o l u t i o n s were deoxygenated by b u b b l i n g pure n i t r o -gen v i a a f r i t t e d g l a s s gas d i s p e n s e r through them. Canadian L i q u i d A i r Co. L grade n i t r o g e n (oxygen content 20 ppm max.) was washed s u c c e s s i v e l y through two b o t t l e s of vanadous s o l u -32 t i o n and one of d i l u t e NaOH s o l u t i o n b e f o r e use. The com-ponent s o l v e n t s had been purged with n i t r o g e n f o r a long time (days) before they were used to make, up the mixed s o l v e n t . A f t e r adding the s o l u t e , the r e s u l t i n g s o l u t i o n was again purged with n i t r o g e n f o r a t l e a s t ten minutes be f o r e measure-ment. A l l s o l u t i o n s under measurement, i f open t o the atmosphere, were covered under a continuous flow of n i t r o g e n to keep o f f the oxygen. The s o l u t i o n s f o r l i f e t i m e measure-ments were prepared and i n t r o d u c e d i n t o the sample cu v e t t e i n the n i t r o g e n box and were t i g h t l y capped before being t r a n s -ported to the sample c r y o s t a t . The s o l u t i o n s f o r emission 25 and p h o t o l y s i s m e a s u r e m e n t s w e r e t r a n s p o r t e d w i t h a 10-ml s y r i n g e w i t h a l o n g f l e x i b l e T e f l o n n e e d l e t o t h e i r c e l l s , w h i c h w e r e a l r e a d y u n d e r f l o w s o f n i t r o g e n g a s . 6. C h e m i c a l s 6.1 P o t a s s i u m H e x a t h i o c y a n a t o c h r o m a t e ( I I I ) A n h y d r o u s p o t a s s i u m h e x a t h i o c y a n a t o c h r o m a t e ( I I I ) , [ C r ( N C S ) g ] , o b t a i n e d f r o m A l f a I n o r g a n i c s I n c . , was r e c r y s -t a l l i z e d more t h a n t h r e e t i m e s f r o m c o l d 95% a l c o h o l u n d e r e v a c u a t i o n . I t was t h e n d r i e d o v e r P 2 ° 5 -*-n a d r y i n g p i s t o l and s t o r e d o v e r P ~ 0 C i n a vacuum d e s i c c a t o r . The m o l a r f r a c -t i o n o f t h e f r e e t h i o c y a n a t e i o n i s l e s s t h a n 0.5%. 6.2 P o t a s s i u m H e x a c y a n o c h r o m a t e ( I I I ) E l e c t r o n i c g r a d e o o t a s s i u m c h r o m i c v a n i d e , K_. [Cr (CN) ] , - 3 D f r o m C i t y C h e m i c a l Co. was r e c r y s t a l l i z e d t w i c e f r o m w a t e r and washed w i t h e t h a n o l a nd t h e n w i t h e t h e r . I t was s t o r e d o v e r P 2 O t - i n a vacuum d e s i c c a t o r . The m o l a r f r a c t i o n o f t h e f r e e c y a n i d e i o n i s l e s s t h a n 0.5%. 6.3 P o t a s s i u m T e t r a t h i o c y a n a t o d i a m m i n e c h r o m a t e ( I I I ) P o t a s s i u m t e t r a t h i o c y a n a t o d i a m m i n e c h r o m a t e ( I I I ) , t r a n s -K [ C r ( N H ^ ) 2 ( N C S ) ^ ] , was p r e p a r e d f r o m F i s h e r C e r t i f i e d g r a d e 2 8 R e i n e c k e ' s s a l t as d e s c r i b e d b y Wegner a n d Adamson. The s a m p l e was f u r t h e r r e c r y s t a l l i z e d f r o m c o l d a l c o h o l a n d d r i e d o v e r P2°5 ""-n a v a c u u m d e s i c c a t o r . The m o l a r f r a c t i o n o f t h e f r e e t h i o c y a n a t e i o n i s l e s s t h a n 0.3%. 26 6.4 T r i s ( e t h y l e n e d i a m i n e ) c h r o m a t e ( I I I ) P e r c h l o r a t e T r i s ( e t h y l e n e d i a m i n e ) c h r o m a t e ( I I I ) p e r c h l o r a t e was prepared from i t s c h l o r i d e s a l t , [Cr (HpNCR^CHpNHp) 3 ] CI * 3$ R^O, which was a v a i l a b l e from A l f a Inorganics Inc. I t was r e c r y s -t a l l i z e d twice from water and then d r i e d and s t o r e d over &2(->5 i n a vacuum d e s i c c a t o r . 6.5 Chromium(III) A c e t y l a c e t o n a t e Chromium(III) a c e t y l a c e t o n a t e , Cr(CH3COCHCOCH3)3, obtained from A l f a I norganics Inc., was r e c r y s t a l l i z e d t hree times from benzene under e v a c u a t i o n . A l l the above compounds are l i g h t s e n s i t i v e , and t h e r e -f o r e were p u r i f i e d under dim red l i g h t and s t o r e d i n a vacuum d e s i c c a t o r i n the dark. 6.6 Methanol Eastman Kodak Spectro grade or F i s h e r C e r t i f i e d A.C.S. Spectroanalyzed methanol was used without f u r t h e r p u r i f i c a t i o n . 6.7 Ethylene G l y c o l Eastman Kodak reagent grade or F i s h e r C e r t i f i e d grade ethylene g l y c o l was used without f u r t h e r p u r i f i c a t i o n . 6.8 Water Water used was d i s t i l l e d water. 27 6 . 9 The Mixed Sol v e n t The s o l v e n t used t o make up the C r ( I I I ) complex s o l u -t i o n s c o n s i s t e d of two p a r t s ( i n volume) of methanol, one p a r t of ethylene g l y c o l , and one p a r t of water. I t forms c l e a r g l a s s below about -100°C. The g l a s s cracks a t about -150°C. The s o l u t i o n remains c l e a r whether i t i s coo l e d down or warmed up s l o w l y . CHAPTER I I I ENERGY TRANSFER AND QUENCHING STUDIES I n o r d e r t o e l u c i d a t e t h e k i n e t i c s and mechanisms o f t h e p h o t o p h y s i c a l and p h o t o c h e m i c a l p r i m a r y p r o c e s s e s , i t i s i m p o r t a n t t o be a b l e t o s e l e c t i v e l y p o p u l a t e , d e p o p u l a t e , o r b y p a s s a c e r t a i n e x c i t e d s t a t e o f a m o l e c u l e . I n o r g a n i c p h o t o c h e m i s t r y , e n e r g y t r a n s f e r has been p r o v e n t o be t h e most e f f i c i e n t and v e r s a t i l e t e c h n i q u e i n t h i s a s p e c t . 3 3 , 3 ^ ' 3 F o r C r ( I I I ) c o m p l e x e s , t h e d i r e c t e x c i t a t i o n t o t h e 25 28 d o u b l e t s t a t e has o f t e n b e en a t t e m p t e d , ' b u t owing t o t e c h n i c a l d i f f i c u l t i e s and d i v e r s e r e s u l t s , more a t t e n t i o n ha been d i r e c t e d r e c e n t l y t o e x c i t a t i o n e n e r g y t r a n s f e r . Q u e n c h i n g o f o r g a n i c t r i p l e t s by C r f a c a c ) ^ has been 36 o b s e r v e d , b u t i t was n o t c e r t a i n t h e n t h a t e n e r g y t r a n s f e r was i n v o l v e d . O n l y r e c e n t l y has e n e r g y t r a n s f e r f r o m e x c i t e d o r g a n i c m o l e c u l e s t o C r ( I I I ) c o m p l e x e s been d e m o n s t r a t e d 37 38 c l e a r l y t h r o u g h s e n s i t i z e d e m i s s i o n and p h o t o a q u a t i o n . U n f o r t u n a t e l y , t h e r e i s s t i l l some a m b i g u i t y a b o u t t h e s t a t e o f t h e i n o r a a n i c a c c e p t o r r e a c h e d . I t i s i m p e r a t i v e t o i d e n -t i f y t h e d o n a t i n g and a c c e p t i n g e l e c t r o n i c s t a t e s f o r b o t h t h e o r e t i c a l and p r a c t i c a l r e a s o n s . The s i t u a t i o n w i l l be c l e a r e r i f a s y s t e m i s so c h o s e n t h a t t h e l o w e s t s p i n - a l l o w e d and s p i n - f o r b i d d e n bands o f t h e d o n o r a r e , r e s p e c t i v e l y , lowe and h i g h e r t h a n t h o s e o f t h e a c c e p t o r . A c c o r d i n g l y [ C r ( C N ) g ] i s the bes t among the C r ( I I I ) complexes as an acceptor. In f a c t , energy t r a n s f e r from a s e r i e s of C r ( I I I ) double s a l t s _3 c o n t a i n i n g [Cr(CN)g] i n the c r y s t a l l i n e s t a t e has been 39 s t u d i e d . However, there i s evidence t h a t the observed e f f e c t i s not caused by energy t r a n s f e r but by a c r y s t a l per-t u r b a t i o n . The energy t r a n s f e r between the potassium s a l t s of the Reineckate i o n , t r a n s - [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ , and the hexacyano-_3 chromate(III) i o n , [Cr(CN) g] , was i n v e s t i g a t e d i n t h i s work. Experimental S e c t i o n The c o n c e n t r a t i o n of trans-[Cr(NH^)^(NCS)^] was kept _3 constant a t 0.05 M; while t h a t of [Cr(CN) /.] v a r i e d from 0 o to 0.07 M. A l l the s o l u t i o n s were deoxygenated. The absorp-t i o n s p e c t r a were measured i n 1 mm c e l l s at room temperature. Quantum y i e l d measurements were made with 546 nm r a d i -a t i o n a t -6 5°C. Since the a b s o r p t i o n s p e c t r a of the two ions are s u f f i c i e n t l y d i f f e r e n t (see F i g u r e 7), t h i s wavelength i s absorbed o n l y by the Reineckate i o n i n s o l u t i o n s c o n t a i n i n g both i o n s . Both donor and acceptor emission s p e c t r a were scanned. Since the phosphorescence i n t e n s i t y i s very s e n s i -t i v e to temperature, the average value was taken from a t l e a s t three separate measurements. L i f e t i m e measurements were made from -110° to -40°C. The flashlamp was f i l t e r e d w i t h a ye l l o w g l a s s f i l t e r t o pro-vide r a d i a t i o n near 500 nm so t h a t again only Reineckate i o n was e x c i t e d . The phosphorescence decays of the donor and 30 a c c e p t o r were measured s e p a r a t e l y a t 751 and 840 nm r e s p e c -t i v e l y . R e s u l t s The a b s o r p t i o n s p e c t r u m o f t h e s o l u t i o n c o n t a i n i n g b o t h i o n s , as shown i n F i g u r e 7, i s e x a c t l y t h e sum o f t h e s e p a r a t e -3 _ a b s o r p t i o n s p e c t r a o f t h e [ C r ( C N ) g ] and [ C r ( N H ^ ) 2 ( N C S ) ^ ] s o l u t i o n s . T h e r e f o r e , t h e two i o n s i n t h e same s o l u t i o n do n o t i n t e r a c t w i t h e a c h o t h e r n o t i c e a b l y i n t h e s e n s e o f i o n -p a i r i n g . E x c i t a t i o n o f [Cr(NH3)2(NCS)4]~ o n l y w i t h 546 nm r a d i -a t i o n i n t h e p r e s e n c e o f [ C r ( C N ) g ] - 3 r e s u l t s i n t h e a p p e a r a n c e o f p h o s p h o r e s c e n c e o f t h e l a t t e r and t h e d e c r e m e n t o f t h e p h o s p h o r e s c e n c e i n t e n s i t y o f t h e f o r m e r . T h a t t h e o b s e r v e d e m i s s i o n i s s e n s i t i z e d p h o s p h o r e s c e n c e i s e s t a b l i s h e d by t h e f a c t t h a t a t t h e same c o n d i t i o n s , b u t i n t h e a b s e n c e o f [ C r ( N H 3 ) 2 ( N C S ) 4 ] n o e m i s s i o n f r o m [ C r ( C N ) 6 ] ~ 3 i s d e t e c t a b l e . T h e r e f o r e , on t h e whole [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ a c t s as d o n o r and [ C r ( C N ) g ] ~ 3 as a c c e p t o r i n t h i s s y s t e m . The two p h o s p h o r e -s c e n c e s p e c t r a a r e e a s i l y s e p a r a b l e as shown i n F i g u r e 8. The r e s u l t s o f p h o s p h o r e s c e n c e q u e n c h i n g and s e n s i t i -z a t i o n f r o m quantum y i e l d measurements a t v a r i o u s a c c e p t o r c o n c e n t r a t i o n s a r e p r e s e n t e d i n F i g u r e s 9 and 10. The i n -t e n s i t i e s o f t h e d o n o r and a c c e p t o r e m i s s i o n s were t a k e n a t t h e p h o s p h o r e s c e n c e maxima, t h a t i s , 751 nm f o r [Cr(NH3) 2NCS)4]~ 650 600 550 500 X (nm) 450 400 350 F i g u r e 7. The a b s o r p t i o n s p e c t r a o f t h e [ C r ( C N ) g ] 3 and t r a n s - [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ s y s t e m . 0.05 M [ C r ( C N ) g ] " 3 1 mm p a t h - l e n g t h ; 0.05 M [Cr ( N H 3 ) 2 ( N C S ) 4 ] " 1 mm p a t h - l e n g t h ; 0.05 M [ C r ( C N ) 6 ] - 3 1 mm + 0.05 M [ C r ( N H 3 ) 2 ( N C S ) 1 mm p a t h - l e n g t h . _3 and 806 and 825 nm f o r [Cr(CN)g] . The i n t e n s i t i e s of the acceptor emission at both wavelengths have been c o r r e c t e d f o r the s m a l l donor emission (see F i g u r e 8). For quenching of the donor, a c c o r d i n g to Stern-Volmer mechanism, 1° D = 1 + k N „ T 0 [ A ] (3.1) where I D and 1^ are the i n t e n s i t i e s of the quenched and un-quenched emission, k g H , T Q , and [A] are, r e s p e c t i v e l y , the t o t a l quenching constant, the l i f e t i m e of the donor i n the absence of the acceptor, and the c o n c e n t r a t i o n of the acceptor, F i g u r e 9 shows t h a t the p l o t of ( I Q / I D ) a g a i n s t the concen-t r a t i o n of [Cr(CN)g]~3 y i e l d s a s t r a i g h t l i n e . I t s s l o p e , equal to k ^ T ^ , i s estimated to be 24.3 M A . Since T 0 i s 33 5 -1 -1 ysec, k^ ,^  i s 7.7 x 10 M sec (J ri For the s e n s i t i z e d emission of the acceptor, i t can be shown t h a t QH + (3.2) l A ' K k , K k e t T c ™ et A where I i s the i n t e n s i t y of the s e n s i t i z e d acceptor emission, k ^ the energy t r a n s f e r r a t e constant, and K an experimental p r o p o r t i o n a l i t y c onstant. F i g u r e 10 shows t h a t the p l o t of the r e c i p r o c a l i n t e n s i t y , 1 / I A , a g a i n s t the r e c i p r o c a l con-c e n t r a t i o n of acceptor y i e l d s a s t r a i g h t l i n e . I t can be seen t h a t k_ _ T T T D i s equal to the r a t i o of the i n t e r c e p t to the QH o ^ ^ slope of the l i n e of Equation 3.2. The values estimated f o r 33 ^QH To a r e 20.7 and 21.8 a t 806 and 825 nm r e s p e c t i v e l y . Con-s e q u e n t l y , k ^ H i s 6.8 and 7.2 x 10^ M s e c f r o m t h e s e d a t a . F o r t h e l i f e t i m e measurements, t h e p h o s p h o r e s c e n c e d e c a y r a t e c o n s t a n t k D , o r 1/T D, o f [ C r ( N H ^ ) ^ ( N C S ) ^ ] a t v a r -i o u s a c c e p t o r c o n c e n t r a t i o n s as f u n c t i o n s o f t e m p e r a t u r e i s p r e s e n t e d i n F i g u r e 11. I t i s s t r a i g h t f o r w a r d t o d e r i v e t h e e x p r e s s i o n K D = 1 / T D = 1 / T D + k Q R [ A ] (3.3) F i g u r e 12 shows t h a t t h e p l o t o f k D a t -65°C a g a i n s t t h e a c c e p t o r c o n c e n t r a t i o n y i e l d a s t r a i g h t l i n e . The s l o p e o f J I 1 I [ C r ( C N ) g ] 3 x 100 (M) g u r e 9.. S t e r n - V o l m e r q u e n c h i n g o f d o n o r p h o s p h o r e s c e n c e i n t e n s i t y a t -65°C. — I 1 1 1 1 1 l _ 4.4 4.6 4.8 5.0 5.2 5.4 5.6 1 , 0 0 0/T ( ° K _ 1 ) F i g u r e 1 1 . . The d o n o r p h o s p h o r e s c e n c e d e c a y r a t e c o n s t a n t s a t v a r i o u s a c c e p t o r c o n c e n t r a t i o n s as f u n c t i o n s o f t e m p e r a t u r e . 38 5 -1 t h e l i n e g i v e s d i r e c t l y t h e v a l u e o f k-„ as 7.2 x 10 M s e c ^, i n good a g r e e m e n t w i t h t h o s e f o u n d f r o m t h e quantum y i e l d measurements. F i g u r e 13 shows t h e A r r h e n i u s p l o t o f l o g ( k ) a g a i n s t r e c i p r o c a l t e m p e r a t u r e as a s t r a i g h t l i n e . Q H The f r e q u e n c y f a c t o r and a c t i v a t i o n e n e r g y o b t a i n e d f o r k_„ a r e 6.6 x 1 0 ^ M s e c ^ and 4.8 K c a l / m o l r e s p e c t i v e l y . _ 3 The r e s u l t s o f the. l i f e t i m e measurements o f [ C r ( C N ) g ] i n t h e e n e r g y t r a n s f e r s y s t e m a r e e s p e c i a l l y i n t e r e s t i n g . F i r s t l y , t h e p h o s p h o r e s c e n c e l i f e t i m e o f t h e a c c e p t o r , x , was f o u n d t o be s h o r t e n e d c o n s i d e r a b l y i n t h e p r e s e n c e o f -3 [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ as shown i n F i g u r e 14. A p p a r e n t l y [ C r ( C N ) g ] i s q u e n c h e d by [ C r ( N H ^ ) ^ ( N C S ) ^ ] ~ t o o . A l t h o u g h t h e r e a r e n o t s u f f i c i e n t e x p e r i m e n t a l d a t a t o c h e c k t h e S t e r n - V o l m e r r e l a -t i o n s h i p , t h e q u e n c h i n g c o n s t a n t , k' has been e s t i m a t e d from O n two p o i n t s . The A r r h e n i u s p l o t o f k' a g a i n s t r e c i p r o c a l On t e m p e r a t u r e i s shown i n F i g u r e 15. The f r e q u e n c y f a c t o r and i 1 2 — 1 —1 a c t i v a t i o n e n e r g y o b t a i n e d f o r k' a r e 2.2 x 10 M s e c • - • QH and 7.6 K c a l / m o l r e s p e c t i v e l y , a l t h o u g h t h e r e i s a l a r g e ex-p e r i m e n t a l u n c e r t a i n t y i n t h e s e v a l u e s . _ 3 S e c o n d l y , when t h e e m i s s i o n o f [ C r ( C N ) g ] a t 840 nm was f o l l o w e d w i t h an o s c i l l o s c o p e , t h e t r a c e s o f t h e e m i s s i o n showed an i n i t i a l i n c r e a s e i n i n t e n s i t y w i t h t i m e and f o l l o w e d by a l o n g e r d e c a y , as shown i n F i g u r e 16. As d e r i v e d i n t h e A p p e n d i x , t h e dependence o f p h o s p h o r e s c e n c e i n t e n s i t i e s o f t h e d o n o r and a c c e p t o r on t i m e , a f t e r an i d e a l p u l s e e x c i t a t i o n o f t h e d o n o r o n l y c a n be e x p r e s s e d a s : 39 4.2 4.6 5.0 5.4 5.8 1,000/T ( 0 K _ 1 ) F i g u r e 14. Q u e n c h i n g o f t h e p h o s p h o r e s c e n c e l i f e t i m e o f [ C r ( C N ) , ] " " 3 i n t h e p r e s e n c e o f [ C r (NH,) 0 (NCS) .] ~ . 41 Time Figu r e 16. The r i s e and decay t r a c e of the phosphorescence of the acceptor. Donor, 0.05 M and acceptor, 0.05 M, detected at 840 nm, a t -70.7°C. Time s c a l e : 50 ysec per d i v i s i o n . I D = K i e x p ( - t / T D ) ( 3 . 4 ) I A = K 2 . [ e x p ( - t / T A ) ~ e x p ( - t / x D ) ] ( 3 . 5 ) where and K 2 are a d j u s t a b l e paraments. Because the i n -stantaneous i n t e n s i t y of the donor emission i s much g r e a t e r than t h a t of the acceptor and t h a t the s l i t s of the mono-chromator used were l a r g e a t the s e t t i n g of 8 40 nm a smal l p a r t of the Reineckate emission was s t i l l d e t e c t a b l e . There-f o r e , the t o t a l i n t e n s i t y has the f o l l o w i n g form: 43 ^ o t a l = l A + K 3 l D ( 3 ' 6 ) = K 2 e x p ( - t / x A ) + ( K 3 K 1 + K 2 ) e x p ( - t / T D ) (3.7) wh e r e i s a w e i g h t i n g f a c t o r . The v a l u e s o f t h e d o n o r l i f e t i m e , T D , a t v a r i o u s tem-p e r a t u r e s o b t a i n e d f r o m t h e i n t e n s i t y t r a c e s o f e m i s s i o n a t 840 nm a c c o r d i n g t o E q u a t i o n 3.7 a r e shown as s t a r r e d p o i n t s i n F i g u r e 1 1. The v a l u e s s o o b t a i n e d a g r e e w i t h t h o s e f o u n d d i r e c t l y f r o m t h e p h o s p h o r e s c e n c e d e c a y o f [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ i n t h e same s y s t e m . T h i s c l e a r l y i n d i c a t e s t h a t t h e d o n a t i n g e l e c t r o n i c s t a t e o f t h e R e i n e c k a t e i o n i s t h e p h o s p h o r e s c e n t 2 s t a t e , E . g D i s c u s s i o n 4 W i t h 546 nm r a d i a t i o n , t h e s t a t e o f t h e R e i n e c k a t e i o n i s p o p u l a t e d d i r e c t l y , f o l l o w e d , b y i n t e r s y s t e m c r o s s i n g 2 41 t o t h e E s t a t e w h i c h e m i t s p h o s p h o r e s c e n c e . The r e s u l t s on q u e n c h i n g , s e n s i t i z a t i o n , a n d e s p e c i a l l y l i f e t i m e m e a s u r e -2 ments e s t a b l i s h u n e q u i v o c a l l y t h a t t h e F^ s t a t e o f t h e R e i n -e c k a t e i o n i s t h e d o n a t i n g s t a t e , o r t h e s t a t e b e i n g q u e n c h e d . However, i t i s c o n c e i v a b l e t h a t , i n s t e a d o f t h e p h o s -2 p h o r e s c e n t s t a t e , E , some t h e r m a l l y a c t i v a t e d e l e c t r o n i c g e x c i t e d s t a t e ( S ) " , o f t h e R e i n e c k a t e i o n i n e q u i l i b r i u m 2 w i t h t h e E s t a t e , c a n be t h e i m m e d i a t e p r e c u r s o r t o q u e n -9J 1 . 42 c h m g . E t ( S ) ' ( i n e q u i l i b r i u m ) g -? 4 (S) • + a c c e p t o r -> A» + a c c e p t o r k^ I T 2g QH 2 I t i s k m e t i c a l l y i n d i s t i n g u i s h a b l e w h e t h e r t h e E^ o r some 2 o t h e r d o u b l e t s t a t e , e . g . , t h e s t a t e i s d i r e c t l y i n -v o l v e d . H owever, b a s e d on t h e f o l l o w i n g a r g u m e n t s , i t c a n be e s t i m a t e d t h a t t h e t h e r m a l l y a c t i v a t e d s t a t e , ( S ) ? , i f a n y , 4 4 c a n n o t be t h e T» s t a t e . F i r s t l y , t h e T_ s t a t e u n d e r g o e s 2g - 2g ^ many r e l a x a t i o n p r o c e s s e s and h a s a d e c a v c o n s t a n t , 1 / k . j . , a t 2 l e a s t an o r d e r o f m a g n i t u d e b i g a e r t h a n t h a t o f t h e E s t a t e g -4 ( s e e C h a p t e r V I I ) . T h e r e f o r e , t h e s t a t e i s n o t i n e q u i -2 l i b r i u m w i t h t h e E s t a t e . I n f a c t , t h e t h e r m a l l y a c t i v a t e d . . 2 4 t r a n s i t i o n E »• T» i s t h e r a t e - d e t e r m i n i n a s t e p o f d e -g 2g - -2 4 g r a d m c f t h e e x c i t a t i o n e n e r g y o f t h e E s t a t e v i a t h e T_ 9 2g s t a t e ( s e e C h a p t e r V) . E v e n i f t h e ^ T 2 c f s t a t e i s q u e n c h e d , 2 ^ t h e E s t a t e w i l l n o t be a f f e c t e d . S e c o n d l v , t h e T„ s t a t e , g 2g --1 2 l y i n g a b o u t 3,000 cm ab o v e t h e E^ s t a t e ( s e e C h a p t e r V ) , i s much t o o h i g h i n e n e r g y t o be i n v o l v e d i n e n e r g y t r a n s f e r — t h e a c t i v a t i o n e n e r g y o f i s o n l y a b o u t 1,700 cm . The v a l u e s o b t a i n e d f o r t h e t o t a l q u e n c h i n g c o n s t a n t , k Q H , f r o m b o t h t h e quantum y i e l d and t h e l i f e t i m e m e a s u r e m e n t s a r e t h e same w i t h i n e x p e r i m e n t a l e r r o r s . T h i s i n d i c a t e s t h a t t h e i n t e r s y s t e m c r o s s i n g quantum y i e l d o f t h e d o n o r , [Cr ( N H 3 ) 2 (NCS)^] , i s n o t c h a n g e d by t h e p r e s e n c e o f [ C r ( C N ) g ] ~ 3 . I n f a c t , i t i s h i g h l y p r o b a b l e t h a t t h e 4 T 2 g s t a t e o f t h e d o n o r i s n o t p e r t u r b e d a t a l l i n t h e e n e r g y t r a n s -f e r s y s t e m . s t a t e . 2 T h e r e a r e two modes f o r q u e n c h i n g o f t h e d o n o r E g (Sg ) D + (Sg ) A (3-8> ( 2 E g ) D + ( 4 A 2 g ) A ( A 2 g ) + ( E g o r T 2 g ) " (3.9) The f i r s t p r o c e s s i s e q u i v a l e n t t o an i n t r a m o l e c u l a r r a d i a -t i o n l e s s t r a n s i t i o n f a c i l i t a t e d o r e n h a n c e d b y i n t e r a c t i o n w i t h t h e q u e n c h e r ; w h i l e t h e s e c o n d p r o c e s s i s an i n t e r m o l e c u l a e n e r g y t r a n s f e r . As t o t h e r e l a t i v e i m p o r t a n c e o f e n e r g y t r a n s f e r , E q u a t i o n 3.9 and q u e n c h i n g , E q u a t i o n 3.8, no q u a n t i t a t i v e a s s e s s m e n t c a n be made f r o m t h e d a t a on p h o s p h o r e s c e n c e i n -t e n s i t y a n d l i f e t i m e s . E q u a t i o n s 3.1, 3.2, and 3.3 i n c o r p o r a t e o n l y t h e sum o f t h e i r r a t e c o n s t a n t s : kQH = k q + k e t ( 3 ' 1 0 ) n o t t h e i r i n d i v i d u a l v a l u e s o r t h e i r r a t i o . The p a r t i c i p a t i o n o f e n e r g y t r a n s f e r i s c l e a r l y demon-s t r a t e d by t h e s e n s i t i z e d e m i s s i o n o f t h e a c c e p t o r . I f q u e n c h i n g as i n E q u a t i o n 3.8 o c c u r r e d , w i t h o u t t h e i n v o l v e m e n t o f any e l e c t r o n i c a l l y e x c i t e d s t a t e s o f t h e a c c e p t o r m o l e c u l e , t h e r e i s no r e a s o n t o e x p e c t [ C r ( C N ) g ] t o be any d i f f e r e n t i n q u e n c h i n g t h a n t h e d o n o r i t s e l f . Y e t s u c h s e l f - q u e n c h i n g d o e s n o t o c c u r , as i s d e m o n s t r a t e d b y t h e f a c t t h a t t h e p h o s p h o r e s c e n c e l i f e t i m e o f t h e R e i n e c k a t e i o n i s i n d e p e n d e n t 46 of i t s c o n c e n t r a t i o n . T h e r e f o r e , quenching cannot be an impor-ta n t r e a c t i o n and as measured, r e p r e s e n t energy t r a n s f e r , k ,, o n l y . e t ' -z 2 Although energy t r a n s f e r from the E s t a t e of the g 2 4 donor to e i t h e r of the E or T_ s t a t e s of the acceptor are 9 2g 43 spin-allowed, the s t a t e of the acceptor i s , l i k e t h a t of the donor, probably too h i g h i n energy to be i n v o l v e d i n energy t r a n s f e r . I t i s assumed t h a t energy t r a n s f e r occurs only v i a the lowest doublet s t a t e s of donor and a c c e p t o r . Then the energy t r a n s f e r e f f i c i e n c y , k ,/(k . + k ) can be e t 7 e t q estimated from the f o l l o w i n g equations. i A . D k . [A] „ „ v s p * i s c p- - k x (3.11) U/T) + (k + k ) [A] p ° ° 4 e t <!> = p (3.12) p i s c ( i A ° ) + ( k q + k e t ) [A] From Equations 3.10, 3.11, and 3.12, one can d e r i v e the energy t r a n s f e r e f f i c i e n c y as k e t 1 k P *so k . + k x A k_„[A] k A (})D e t q o QH p T p (3.13) where cf)^, " ^ g ^ and k^ are the quantum y i e l d of the quenched phosphorescence, the quantum y i e l d of i n t e r s y s t e m c r o s s i n g , and the i n t r i n s i c r a d i a t i v e r a t e constant r e s p e c t i v e , of the A A d o n o r ; and <b and k a r e t h e quantum y i e l d o f t h e s e n s i t i z e d P P p h o s p h o r e s c e n c e and t h e i n t r i n s i c r a d i a t i v e r a t e c o n s t a n t o f t h e a c c e p t o r . A t -65°C.and [A] = 0.07 M, t h e r a t i o <f>Ap/<f>p i s e q u a l t o 1 . 6 - - e v a l u a t e d b y c o m p a r i n g t h e a r e a s u n d e r t h e q u e n c h e d a nd t h e s e n s i t i z e d p h o s p h o r e s c e n c e s p e c t r a . The s e n s i t i v i t y o f S - l p h o t o m u l t i p l i e r i s r e l a t i v e l y f l a t i n t h i s r e g i o n . k^ and k A a r e 200 and 16 s e c r e s p e c t i v e l y . The v a l u e o f t h e e n e r g y t r a n s f e r e f f i c i e n c y o b t a i n e d a c c o r d i n g t o E q u a t i o n 3.13 i s a b o u t 1.1. T h i s i n d i c a t e s t h a t k^ t T = k . , and a g r e e s v / i t h QH e t ^ t h e c o n c l u s i o n p r e v i o u s l y made. From t h i s i t c a n a l s o be A shown t h a t t h e T 2 g s t a t e c a n n o t be t h e a c c e p t i n g s t a t e b e c a u s e , i f s o , t h e n t h e e n e r g y t r a n s f e r e f f i c i e n c y o b t a i n e d ( t a k i n g <t>". = 0.1) w i l l be 11 w h i c h i s n o t p o s s i b l e . ISC E n e r g y t r a n s f e r d o e s n o t o c c u r a t t e m p e r a t u r e s b e l o w -130°C w h e r e t h e s o l v e n t becomes a r i g i d g l a s s . T h i s e l i m -i n a t e s l o n g - r a n g e e n e r g y t r a n s f e r , c o m p l e x f o r m a t i o n , a n d t r i v i a l p r o c e s s o f r e a b s o r p t i o n . The a c t i v a t i o n e n e r g y o f k^„ i s v e r y c l o s e t o t h a t o f s o l v e n t f l u i d i t y , 5± 0.5 K c a l / m o l , and t h e m a g n i t u d e o f k^.^ i s c l o s e t o t h e e s t i m a t e d r a t e o f y n 6 -1 — 1 d i f f u s i o n c o n t r o l l e d p r o c e s s , a b o u t 4 x 10 M s e c f o r t h i s s o l v e n t , e s t i m a t e d f r o m v i s c o s i t y m e a s u r e m e n t s . A l l t h e s e f a c t s s u g g e s t t h a t t h e e n e r g y t r a n s f e r i s e s s e n t i a l l y a d i f -f u s i o n c o n t r o l l e d p r o c e s s . 48 2 -3 F i g u r e 14 shows t h a t t h e E s t a t e o f [ C r ( C N ) , ] i s g 6 a l s o q u e n c h e d i n t h e p r e s e n c e o f R e i n e c k a t e i o n . S i m i l a r l y , two modes o f q u e n c h i n g a r e p o s s i b l e : t h e e n h a n c e d i n t r a m o l e c u l a r r a d i a t i o n l e s s t r a n s i t i o n k and t h e i n t e r m o l e c u l a r e n e r g y q ^-t r a n s f e r k t . A g a i n e n e r g y t r a n s f e r i s f a v o r e d . S i n c e t h e _3 s e l f - q u e n c h i n g o f [ C r ( C N ) g ] i s n o t n o t i c e a b l e a s i s e v i d e n c e d i f r o m F i g u r e 14, t h e p r o c e s s k , s i m i l a r t o s e l f - q u e n c h i n g , s h o u l d n o t be o p e r a t i v e . I n f a c t , t h e d e c a y c u r v e o f t h e [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ e m i s s i o n a t 740 nm ( s e e F i g u r e 17) becomes n o n - e x p o n e n t i a l and d e c a y s more s l o w l y i n t h e l a t e r p a r t . T h i s may be due t o t h e d e l a y e d p h o s p h o r e s c e n c e c a u s e d by t h e 2 -3 b a c k e n e r g y t r a n s f e r f r o m t h e E s t a t e o f [Cr(CN).,] t o <? c t h a t o f [ C r ( N H ^ ) 2 ( N C S ) 4 ] - . U n f o r t u n a t e l y , t h e i n s t a n t a n e o u s i n t e n s i t y o f t h e d e l a y e d p h o s p h o r e s c e n c e i s t o o s m a l l and t h e c u r v e a t t h a t p a r t t o o n o i s y t o a l l o w r i g o r o u s a n a l y s i s . I n t h e f o l l o w i n g e n e r g y t r a n s f e r s kOH ( 2 E ) D + ( V ) A * ( 4 A ? ) D + ( 2 E ) A g i<i . i 2g a QH k ^ H i s assumed, t o be e q u a l t o t h e d i f f u s i o n - c o n t r o l l e d r a t e c o n s t a n t g i v e n by Debye e q u a t i o n 8RT k n „ =• = s e x p ( - A E /RT) (3.14) - 3000 n - r j " • . 44 w here A E ^ i s t h e a c t i v a t i o n e n e r g y o f t h e s o l v e n t f l u i d i t y . I t i s f u r t h e r assumed t h a t t h e r e v e r s e e n e r g v t r a n s f e r , k ' , w h i c h i s e n d o t h e r m i c , r e q u i r e s an A r r h e n i u s a c t i v a t i o n e n e r g y 49 Time Fi g u r e 17. Non-exponential decay of the trans-[Cr(NH^)^(NCS)^] i n the energy t r a n s f e r system. Donor c o n c e n t r a t i o n : 0.0 5 M; acceptor c o n c e n t r a t i o n : 0.05 M, a t ~38°C. Time s c a l e : 1 ysec per d i v i s i o n . i n d i c a t e s the i d e a l e x p o n e n t i a l decay t r a c e . equal to the enerqv d i f f e r e n c e , A E , between the E s t a t e s of g 45 the donor and a c c e p t o r . T h e r e f o r e , k0H = • s 0 H e x P ( - A E f l / p T ) e x P ( - A E / R T ) = s 0 H e x p [ - ( A E F L + AE)/RT] (3.15) The a c t i v a t i o n enerqv of k. T T obtained does agree w i t h A E _ , Qn f l (4.8 vs 5 Kcal/mol). The a c t i v a t i o n enercry of k'„ o b t a i n e d , 7.6 Kcal/mol, i s about 2.6 Kcal/mol more than A E - , . The 50 d i f f e r e n c e i s e q u a l t o A E . T h e v a l u e o f AE e v a l u a t e d f r o m -3 t h e e m i s s i o n s p e c t r a o f [ C r ( C N ) g ] a n d [ C r ( N H ^ ) ^ ( N C S ) ^ ] i s 870 cm 1 ( 2 . 5 K c a l / m o l ) . The t w o v a l u e s f r o m d i f f e r e n t m e t h o d s a r e i n a g r e e m e n t w i t h e a c h o t h e r . CHAPTER IV PHOTOCHEMICAL STUDIES Energy t r a n s f e r has been used s u c c e s s f u l l y i n o r g a n i c photochemistry t o i n v e s t i g a t e the r o l e p l a y e d by s i n g l e t 33 34 35 and t r i p l e t s t a t e s . ' ' However the a p p l i c a t i o n of t h i s technique t o the study of i n o r g a n i c p h o t o r e a c t i o n s has onl y been r e p o r t e d i n a few cases. And almost a l l of the i n v e s -t i g a t i o n s were made on the p h o t o s e n s i t i z e d r e a c t i o n s of metal complexes by o r g a n i c donors, p a r t i c u l a r l y b i a c e t y l . V o g l e r 46 and Adamson have s t u d i e d the p h o t o s e n s i t i z e d r e d u c t i o n of 47 some Co(III) ammines. P o r t e r has s t u d i e d the p h o t o s e n s i -4 8 t i z e d aquation of c o l b a t i c y a n i d e i o n ; and S a s t r i and Langford the t e t r a c h l o r o p l a t i n a t e ( I I I ) i o n . As f o r C r ( I I I ) complexes, 3 8 Adamson, M a r t i n , and Camessei have i n v e s t i g a t e d the p h o t o s e n s i t i z e d aquation of [ C r ( N H 3 ) 5 ( N C S ) ] + 2 , [ C r ( N H 3 ) 2 ( N C S ) 4 ] " -3 and [Cr(NCS)g] . Recently, some work on the p h o t o s e n s i t i z e d + 3 aquation of [Cr(en)^] by b i a c e t y l has been r e p o r t e d by 49 B a l z a n i , B a l l a r d i n i , G a n d o l f i , and Moggi. However, because the a c c e p t i n g s t a t e s were not known f o r sure, a c l e a r - c u t c o n c l u s i o n on the r e a c t i v i t y of the v a r i o u s e x c i t e d e l e c t r o n i c s t a t e s i s s t i l l f a r from c e r t a i n . Furthermore, there i s the c o m p l i c a t i o n t h a t the e x c i t e d b i a c e t y l may r e a c t d i r e c t l y . 49 with the l i g a n d s of the complex i o n s . 52 G e n e r a l P r i n c i p l e s The g e n e r a l p r i n c i p l e s and c r i t e r i a o f m a k i n g u s e o f e n e r g y t r a n s f e r measurements t o o b t a i n i n f o r m a t i o n c o n c e r n i n g p h o t o r e a c t i v e s t a t e s have been d e s c r i b e d . 3 3 35,50 i d e a i s y s t e m s a r e t h o s e i n w h i c h o n l y t h e p h o s p h o r e s c e n t s t a t e s o f t h e d o n o r and a c c e p t o r a r e i n v o l v e d . A p h o t o r e a c t i v e m o l e c u l e u n d e r s t u d y may be u s e d as e i t h e r a d o n o r o r an a c c e p t o r . I f i t i s u s e d as an a c c e p t o r , p o p u l a t i n g i t s p h o s p h o r e s c e n t s t a t e and s i m u l t a n e o u s l y b y p a s s i n g i t s f l u o r e s c e n t s t a t e a l l o w a s t u d y t o be made o f t h e r e a c t i v i t y o f t h e f o r m e r . T h a t i s , t h e p h o s p h o r e s c e n t s t a t e c a n be d e t e r m i n e d t o be r e a c t i v e o r i n e r t d e p e n d i n g on w h e t h e r t h e p h o t o r e a c t i o n s t i l l o c c u r s o r n o t a t a l l . However, u s u a l l y b e c a u s e o f t h e u n c e r t a i n t i e s o f t h e e n e r g y t r a n s f e r e f f i c i e n c y and o f t h e i n t e r s y s t e m c r o s s i n g quantum y i e l d o f t h e d o n o r , t h e r e a c t i v i t y o f t h e p h o s p h o r e s c e n t s t a t e i s v e r y d i f f i c u l t t o e v a l u a t e q u a n t i -t a t i v e l y . I t i s r e l a t i v e l y e a s y t o o b t a i n q u a n t i t a t i v e r e -s u l t s , i f t h e m o l e c u l e i s u s e d as a d o n o r . By q u e n c h i n g t h e p h o s p h o r e s c e n t s t a t e , t h e m a x i m a l r e d u c t i o n i n t h e o v e r a l l p h o t o c h e m i c a l quantum y i e l d a t v e r v h i g h q u e n c h e r c o n c e n -t r a t i o n i s t a k e n as a measure o f t h e p h o t o c h e m i c a l quantum y i e l d from t h e p h o s p h o r e s c e n t s t a t e , w h i l e t h e u n a f f e c t e d p o r t i o n i s e c m a l t o t h e p h o t o c h e m i c a l quantum y i e l d f r o m t h e f l u o r e s c e n t s t a t e . 53 The a b o v e s t a t e m e n t s h a v e t o be c o n s i d e r a b l y m o d i f i e d i f b a c k i n t e r s y s t e m c r o s s i n g i s a p p r e c i a b l y i n v o l v e d . F o r C r ( I I I ) c o m p l e x e s , f o r e x a m p l e , t h e b a c k i n t e r s y s t e m c r o s s i n g , k_^, h a s b e e n c o n s i d e r e d t o be t h e m a i n p a t h w a y f o r d e p l e t i o n 2 o f t h e E s t a t e ( s e e C h a p t e r V ) . I f s o , t h e n d i r e c t P O P U -g 2 l a t i o n o f t h e E^ s t a t e by e n e r g y t r a n s f e r ( o r d i r e c t e x c i -4 t a t i o n ) d o e s n o t b y p a s s t h e T^ s t a t e . I n t h i s c a s e , i f t h e p h o t o c h e m i c a l quantum y i e l d o b t a i n e d f r o m s e n s i t i z a t i o n i s g r e a t e r t h a n t h a t f r o m d i r e c t p h o t o l y s i s , t h e p h o s p h o r e s c e n t s t a t e i s r e a c t i v e ; i f s m a l l e r , t h e f l u o r e s c e n t s t a t e i s r e -a c t i v e . As p o i n t e d o u t q u a n t i t a t i v e d a t a a r e d i f f i c u l t t o o b t a i n f r o m s e n s i t i z a t i o n , a q u a l i t a t i v e r e s u l t o f m e r e l y o b s e r v i n g p h o t o s e n s i t i z e d r e a c t i o n c a n n o t be u s e d t o c o n c l u d e u n a m b i g u o u s l y t h a t t h e p h o s p h o r e s c e n t s t a t e i s r e a c t i v e . As f o r t h e q u e n c h i n g m e t h o d , t h e u n q u e n c h a b l e p a r t o f .the p h o t o -c h e m i c a l quantum y i e l d s t i l l i s t h e p h o t o c h e m i c a l q uantum y i e l d f r o m t h e f l u o r e s c e n t s t a t e . H o w e v e r , t h e m a x i m a l q u e n -c h a b l e p a r t may n o t be t h e q uantum y i e l d d i r e c t l y f r o m t h e p h o s p h o r e s c e n t s t a t e now. C a r e f u l a n a l y s e s h a v e t o be made t o d e t e r m i n e w h e t h e r q u e n c h i n g o f p h o t o r e a c t i o n o r i n t e r s y s t e m c r o s s i n g o r b o t h f r o m t h e p h o s p h o r e s c e n t s t a t e i s r e s p o n s i b l e f o r t h e d e c r e a s e i n o v e r a l l p h o t o c h e m i c a l q uantum y i e l d . I n C h a p t e r I I I e n e r g y t r a n s f e r b e t w e e n [ C r ( N H ^ ) 2 ( N C S ) ^ ] " _3 and [ C r ( C N ) g ] h a s b e e n shown c l e a r l y t o t a k e p l a c e v i a d o u b l e t s t a t e s o f e a c h i o n . I n t h i s c h a p t e r q u e n c h i n g s t u d i e s o f t h e p h o t o a q u a t i o n o f [Cr (NH3) 2(NCS)4]~ a r e d e s c r i b e d . 54 E x p e r i m e n t a l and R e s u l t s P h o t o l y s i s o f t h e R e i n e c k a t e i o n was c a r r i e d o u t a t -65.0 - 0.2°C i n t h e d e o x y g e n a t e d s o l u t i o n o f m e t h a n o l , w a t e r , and e t h y l e n e g l y c o l ( 2 : 1 : 1 ) . I r r a d i a t i o n was a t 546 nm and l a s t e d t y n i c a l l y one h o u r . D u r i n g i r r a d i a t i o n t h e s o l u t i o n was k e p t u n d e r a f l o w o f p u r e n i t r o g e n gas i n o r d e r t o k e e p o f f o x y g e n . The c o n c e n t r a t i o n o f t h e R e i n e c k a t e _ 3 i o n was 0.03 M, w h i l e t h a t o f [ C r ( C N ) g ] v a r i e d f r o m 0 t o 0.07 M. F o r a l l t h e s o l u t i o n s t h e r a d i a t i o n was o n l y a b s o r b e d by t h e R e i n e c k a t e i o n . The p h o t o c h e m i c a l quantum y i e l d was m e asured a c c o r d i n g t o t h e t h i o c y a n a t e i o n p r o d u c e d . A s o l u -t i o n o f t h e same amount and t h r o u g h t h e same o p e r a t i o n s e x c e p t i r r a d i a t i o n s e r v e d as t h e b l a n k . T h i o c y a n a t e i o n was 2 8 a n a l y z e d by t h e t e c h n i q u e o f Adamson and Wegner, e x c e p t t h a t e t h a n o l was added w i t h t h e [ ( C H ^ J ^ N j C l t o p r e c i p i t a t e _3 [ C r ( C N ) g ] as w e l l as t h e u n r e a c t e d R e i n e c k a t e i o n . The f i l t e r e d s o l u t i o n was t h e n made a l k a l i n e and a l l o w e d t o s t a n d o v e r n i g h t b e f o r e a d d i n g f e r r i c p e r c h l o r a t e s o l u t i o n and m e a s u r i n g t h e a b s o r b a n c e o f t h e f e r r i c - t h i o c y a n a t e com-p l e x . The m o l a r e x t i n c t i o n c o e f f i c i e n t t a k e n f o r t h e f e r r i c - t h i o c y a n a t e complex was 4.30 x 1 0 3 M ^ s e c - 1-. 2 8 Quantum y i e l d s were d e t e r m i n e d by r e f e r e n c e t o t h e room tem-2 8 p e r a t u r e R e i n e c k a t e a c t i n o m e t e r , ' t a k i n g a c c o u n t o f t h e f a c t t h a t t h e a n a l y s i s r e l e a s e s f o u r t h i o c y a n a t e i o n s p e r a q u a t e d complex. The p h o t o a q u a t i o n quantum y i e l d s o f t h e R e i n e c k a t e i o n a t s e v e r a l c o n c e n t r a t i o n s o f [ C r ( C N ) , ] 3 55 a r e l i s t e d i n T a b l e I , t o g e t h e r w i t h t h e r e l a t i v e p h o s p h o r -e s c e n c e quantum y i e l d o f t h e same i o n f o r c o m p a r i s o n . The v a l u e a t e a c h p o i n t i s a v e r a g e d f r o m a t l e a s t t h r e e r u n s . T a b l e I P h o t o a q u a t i o n Quantum Y i e l d s o f [Cr(NH3)2(NCS) 4 ] ~ as Donor C o n c e n t r a t i o n M./1 d> , x 10^ TD ,<» T c h e m I d> , Donor A c c e p t o r (donor) ( r e l a t i v e ) (donor) 0.03 0 1.02 + 0.02 1 —-0.03 :• 0.03 0.79 + 0.04 0.58 0. 49 X l O " 2 0.03 0 .05 0.76 + 0.01 0.45 0.55 X -2 10 0.03 0.07 0.69 + 0.01 0.37 0 .51 X l O " 2 _3 The a c c e p t o r i s [ C r ( C N ) g ] D i s c u s s i o n I t i s c l e a r f r o m t h e s e d a t a t h a t t h e p h o t o a q u a t i o n o f -3 t h e R e i n e c k a t e i o n i s q u e n c h e d i n t h e p r e s e n c e o f [ C r ( C N ) g ] , 4 b u t l e s s so t h a n t h e p h o s p h o r e s c e n c e . S i n c e t h e T„ s t a t e 2g o f t h e R e i n e c k a t e i o n i s n o t q u e n c h e d i n t h i s s y s t e m , and t h e 2 e x t e n t o f q u e n c h i n g o f t h e "E s t a t e c a n be e s t i m a t e d f r o m g q u e n c h i n g o f p h o s p h o r e s c e n c e , t h e m a x i m a l g u e n c h a b l e p o r t i o n o f t h e p h o t o a q u a t i o n c a n be c a l c u l a t e d a c c o r d i n g l y t o t h e f o l l o w i n g e q u a t i o n : (*chem - *chem} = (<f,chem " <^chem) ( l D ° D ) ( 4 ' 1 } o ^chem ^ s t ^ l e l i f t i n g photoaquation quantum y i e l d a t h i g h - 3 •' ' c o n c e n t r a t i o n of [Cr(CN)g] when the phosphorescent s t a t e _ 3 i s t o t a l l y quenched- The s o l u b i l i t v of [Cr(CN)_] p r e c l u d e s b a d i r e c t measurement of ^hem' however, the v a l u e s obtained f o r i t from Equation 4.1, as l i s t e d i n Table I, are c o n s i s t e n t -3 and have an average value of 5.2 x 10 . By r e a r r a n g i n g Equation 4.1 and making use of Equations 3.1 and 3.3 the f o l -lowing equations can be d e r i v e d . ^chem ^chem o o , . . Dr,., „, = 1 + k _ „ T [A] (4.2) *chem " *chem 1 P l o t s o f t h e p h o t o a q u a t i o n s quantum y i e l d , p h o s p h o r e s c e n c e i n t e n s i t y , and p h o s p h o r e s c e n c e l i f e t i m e a g a i n s t t h e c o n c e n -_3 t r a t i o n o f [ C r ( C N ) g ] a c c o r d i n g t o E q u a t i o n 4.2 a r e shown i n F i g u r e 18. The f a c t t h a t a l l t h e l i n e s a r e p r a c t i c a l l y c o i n c i d e n t d e m o n s t r a t e s t h a t p a r t o f p h o t o a q u a t i o n , criven by (cj)°, - (j)00, ) , and t h e p h o s p h o r e s c e n c e o c c u r v i a t h e same cbem cnem s t a t e . Two c o n c l u s i o n s c a n be drawn f r o m t h e s e e x p e r i m e n t s : 1 (1) h a l f o f t h e t o t a l p h o t o a q u a t i o n a t -65°C ( w i t h <j> = 5 x -3 -3 10 ) i s n o t q u e n c h e d by [ C r ( C N ) g ] and. t h u s o c c u r s f r o m e x c i t e d m o l e c u l e s t h a t have n o t b e e n t h r o u g h e q u i l i b r a t e d d o u b l e t s t a t e s , and (2) t h e r e s t o f t h e p h o t o a q u a t i o n , a g a i n w i t h <f> = 5 x 10 , c a n be q u e n c h e d and t h e r e f o r e must o c c u r v i a e i t h e r t h e p h o s p h o r e s c e n t s t a t e o r some o t h e r s t a t e r e a c h a b l e by t h e r m a l a c t i v a t i o n f r o m t h e p h o s p h o r e s c e n t s t a t e The r e s u l t s a r e t h e r e f o r e i n t e r p r e t e d i n t h e f o l l o w -i n g way. A f t e r e x c i t a t i o n t o a h i g h v i b r a t i o n a l l e v e l o f t h e l o w e s t e x c i t e d q u a r t e t s t a t e , r a p i d d e g r a d a t i o n r e s u l t s i n a 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 e x c i t e d q u a r t e t m o l e c u l e . P h o t o -a q u a t i o n t h e n o c c u r s f r o m t h i s s t a t e , i n c o m p e t i t i o n w i t h i n t e r s y s t e m c r o s s i n g t o t h e d o u b l e t s t a t e m a n i f o l d and 4 i n t e r n a l c o n v e r s i o n t o t h e g r o u n d s t a t e , M o l e c u l e s i n t h e d o u b l e t m a n i f o l d must d e g r a d e t o t h e l o w e s t d o u b l e t 2 s t a t e , E , as t h e p h o s p h o r e s c e n c e l i f e t i m e i s s t i l l 33 y s e c a t 6 5 ° C . Such m o l e c u l e s u n d e r g o a q u a t i o n i n c o m p e t i t i o n w i t h p h o s p h o r e s c e n c e and i n t e r s y s t e m c r o s s i n g t o t h e g r o u n d s t a t e -a l l o f w h i c h c a n be qu e n c h e d by e n e r g y t r a n s f e r . T h e r e a r e two p a t h s t o be c o n s i d e r e d f o r t h i s l a t t e r p a r t o f t h e a q u a -t i o n r e a c t i o n : d i r e c t s u b s t i t u t i o n o f t h e complex i n t h e d o u b l e t s t a t e by w a t e r , o r back i n t e r s y s t e m c r o s s i n g t o t h e l o w e s t e x c i t e d q u a r t e t s t a t e , w i t h an a c t i v a t i o n e n e r g y e q u a l t o t h e e n e r g y d i f f e r e n c e between t h e two s t a t e s , f o l l o w e d by a q u a t i o n o f .the r e s u l t i n g q u a r t e t s t a t e m o l e c u l e . The s e c o n d e x p l a n a t i o n i s f a v o r e d f r o m t h e s t u d i e s o f t h e t e m p e r a t u r e d ependence .of t h e p h o s p h o r e s c e n c e l i f e t i m e . T h i s means t h a t 2 t h e Eg s t a t e i s e s s e n t i a l l y s u b s t i t u t i o n a l l y i n e r t . The d e t a i l e d arguments a r e p r e s e n t e d i n C h a p t e r V. However, a t -65°C, back i n t e r s y s t e m c r o s s i n g of the Reineckate i o n occurs with an e f f i c i e n c y of about 90%. A forward i n t e r s y s t e m c r o s s i n g e f f i c i e n c y of about 52% would then be r e q u i r e d i n order t h a t the aquation quantum y i e l d s v i a the two paths be equal (see Chapter V I ) . K i n e t i c Treatment Since the above c o n c l u s i o n s , as w e l l as those i n Chapter I I I , were deduced from the experimental r e s u l t s through the c o n v e n t i o n a l analyses which are i n f a c t based on a mechanism which does not i n c l u d e back i n t e r s y s t e m c r o s -s i n g , i t i s necessary to demonstrate t h a t even i f back i n t e r -system c r o s s i n g i s i n c l u d e d the data treatment made and the c o n c l u s i o n s drawn are s t i l l v a l i d . 4 D ( A The complete mechanism i s I 4 D( 4T D( 4T D( 4T D( 4T 2g 2g 2g 2g 2g + hv k -> k z -> k_ -> k r D ( T2g> D ( 4 A 2 g ) + hv Product D ( 4 A 2 g ) + hv' (4.3) (4.4) (4.5) (4.6) (4.7) (4.8) (4.9) 60 D ( 2 E q ) + 7 P r o d u c t (4.10) D ( 2 E g ) ->6 ° ( 4 A 2 g ) (4.11) k D ( 2 E ) + A ( 4 A ~ ) ^ Q H D ( 4 A _ ) + A ( 2 E J (4.12) 9 2g 2g g The r e v e r s e o f p r o c e s s 4.12, i . e . , r e v e r s e e n e r g y t r a n s f e r i s o m i t t e d f o r t h e sa k e o f c o n v e n i e n c e . I n t h e t r a n s i e n t s t u d i e s , t h e a p p a r e n t l i f e t i m e o f t h e d o n o r c a n be e x p r e s s e d , as shown i n t h e A p p e n d i x and C h a p t e r V, as = k D = k 5 + k 6 + k y + (1 - a) k _ 4 + k Q H [ A ] (4.13) D T = 1 / T ° + k n N ^ (4.14) t O QH 4 where a = (4.15) k± + k 2 + k 3 + k 4 . D 1  and T = o k 5 + k 6 + k ? + (1 - a ) k _ 4 (4.16) E q u a t i o n 4.14 i s t h e r e f o r e e x a c t l y t h e same as E q u a t i o n 3.3. In t h e p h o s p h o r e s c e n c e i n t e n s i t y and p h o t o a q u a t i o n quantum y i e l d measurements where t h e s t e a d y - s t a t e a p p r o x i -m a t i o n i s j u s t i f i e d , t h e s t e a d y - s t a t e c o n c e n t r a t i o n s o f t h e 2 4 R e i n e c k a t e i o n s m t h e E and T. s t a t e s c a n be d e r i v e d as g 4g k .1 2 D 4 a ( E )° = g ss ( k l + k 2 + k3 + V ( k-4 + k5 + k 6 + k 7 + kQH [ A&" k4 k-4 (4.17) 61 ( k - 4 + V V V k Q H C A ] I a ( 4T ) D = — ( 4.18) ( k 1 + k 2 + k 3 + k 4 ) ( k _ 4 + k 5 + k 6 + k 7 + k Q H [ A ] ) - k 4 k _ 4 T h e r e f o r e q u antum y i e l d s o f t h e p h o s p h o r e s c e n c e and p h o t o -a q u a t i o n c a n be e x p r e s s e d a s k 4 k 5 A = 1_5 (4.19) p n O S (k +k +k +k ) ( k +k +k +k +k [ A ] ) - k k • k 3 ( k - 4 + k 5 + k 6 + k 7 + k O H C A ] ) + k 4 K 7 ^chem : (4.20) ( k ] + k 2 + k 3 + k 4 ) ( k _ 4 + k 5 + k 6 + k ? + k Q H [ A ] ) - k 4 k _ 4 From E q u a t i o n 4.18, i t c a n be shown t h a t 1° P°v, kQH [A] ° = D h o s = 1 + (4.21) 1° P D h o s k 5 + k 6 + k 7 + ( l - a ) k _ 4 = 1 + k 0 H x ° [ A ] (4.22) T h u s , E q u a t i o n 4.22 i s a l s o e x a c t l y t h e same as E q u a t i o n 3.1. Now, f r o m E q u a t i o n 4.19 L i m k 3 ^chem [ A 3 * °° ^chem k, + k„ + k., + k (4.23) T h i s shows t h e u n q u e n c h a b l e p a r t o f t h e p h o t o a q u a t i o n i s s t i l l e q u a l t o t h e quantum y i e l d o f p h o t o a q u a t i o n f r o m t h e q u a r t e t s t a t e . I t c a n f u r t h e r be shown t h a t 62 *chem " *chem k ° H [ A ] _2£^JB £11^. = i + ( 4 . 2 4 ) ^chem ^chern 5 6 7 - 4 = 1 + k Q H x ° [ A ] ( 4 . 2 5 ) T h i s i s again e x a c t l y the same as Equation 4 . 2 . We t h e r e f o r e can conclude t h a t Equations 3 . 1 , 3 . 3 , and 4 . 2 are g e n e r a l l y v a l i d whether back i n t e r s y s t e m c r o s s i n g i s i n c l u d e d i n the mechanism or not. CHAPTER V TEMPERATURE-DEPENDENCE OF THE PHOSPHORESCENCE LIFETIMES I t has long been known t h a t the phosphorescence l i f e -times of C r ( I I I ) complex ions are s t r o n g l y dependent on tem-p e r a t u r e . A few i n v e s t i g a t i o n s have been made i n the attempt 51 to c l a r i f y the mechanisms. Targos and F o r s t e r found t h a t the phosphorescence decay constants c o n s i s t e d of a tempera-ture-independent, a s l i g h t l y temperature-dependent, and a s t r o n g l y temperature-dependent term. The second term, as w e l l as p a r t of the f i r s t , was a s s i g n e d t o i n t e r s y s t e m c r o s -2 4 s i n g from the E g to the ground A 2 g s t a t e . The t h i r d term, which i s overwhelmingly dominant a t r e l a t i v e l y high tempera-t u r e s , although not d e f i n i t e then, was p o s t u l a t e d t o r e p r e s e n t 2 the t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g from the E g ; 4 2 4 back to the T„ s t a t e . Such c r o s s i n g , E • T„ , c e r -2g g 2g +3 t a i n l y does occur i n those complexes, e.g. [Cr(urea)^] , 2 which have r a t h e r s m a l l energy s e p a r a t i o n between the E g and 4 52 53 the T 2 a s t a t e s . ' However, the f i r s t d e f i n i t e evidence f o r the process came from s t u d i e s of ruby and emerald a t high temperatures. In both c r y s t a l s the c a l c u l a t e d a c t i v a t e d energy of the temperature-dependent component agreed reasonably w e l l with the energy d i f f e r e n c e between the z e r o - v i b r a t i o n a l 4 2 l e v e l s of the T„ and E s t a t e s , and emission of delayed 2 a a f l u o r e s c e n c e was a l s o o b s e r v e d . " * " 8 ' ^ 4 R e c e n t work o f C a m a s s e i 55 and F o r s t e r have d e m o n s t r a t e d u n a m b i g u o u s l y t h e i n v o l v e m e n t 2 4 o f Eg • T 2 i n a g r o u p o f C r ( I I I ) complex i o n s i n v a r i o u s c r y s t a l l i n e h o s t s t h a t f l u o r e s c e o n l y a t s u f f i c i e n t l y h i g h t e m p e r a t u r e . U n f o r t u n a t e l y , a l l t h e a b o v e - m e n t i o n e d c o m p l e x e s have s m a l l lODq v a l u e s and a c c o r d i n g l y , p r o b a b l y have s m a l l 4 2 20 T 2 c r ( v = 0) - E c t ( V = 0) s e p a r a t i o n s . F o r t h o s e c o m p l e x e s w h i c h have l a r g e lODq v a l u e s and g e n e r a l l y do n o t e x h i b i t d e l a y e d f l u o r e s c e n c e , i t i s i n t e r e s t i n g t o i n q u i r e w h e t h e r t h e t h e r m a l 2 4 a c t i v a t e d E *• T„ s t i l l p r e v a i l s ( e s p e c i a l l v i n s o l u -g 2g ^ t i o n s where p h o t o r e a c t i o n s may be c o m p e t i t i v e ) . S c h l a f e r 56 . e t aJL. have s t u d i e d t h e t e m p e r a t u r e d e p e n d e n c e o f t h e p h o s p h o r e s c e n c e i n t e n s i t y o f C r ( I I I ) complex i o n s i n s o l u -57 t i o n s . Z a n d e r has e x t e n s i v e l y i n v e s t i g a t e d t h e t e m p e r a t u r e e f f e c t on t h e p h o s p h o r e s c e n c e l i f e t i m e s o f many C r ( I I I ) com-p l e x e s i n b o t h c r y s t a l l i n e s t a t e and s o l u t i o n . W i t h o u t a d d i t i o n a l s u p p o r t , t h e y s i m p l y c o n c l u d e d t h a t t h e s t r o n g t h e r m a l q u e n c h i n g o f t h e p h o s p h o r e s c e n t s t a t e was due t o t h e 2 4 t h e r m a l a c t i v a t e d E >• T_ . However, t h e i r s o l u t i o n s g 2g were n o t d e - o x y g e n a t e d , and t h e r e f o r e t h e i r d a t a a r e n o t r e l i a b l e . + + I n f a c t , f o r t h e C r ( I I I ) c o m p l e x e s w i t h a l a r g e Oxygen q u e n c h i n g o f t h e p h o s p h o r e s c e n t s t a t e o f some C r ( I I I ) c o mplexes i n s o l u t i o n s has been d e m o n s t r a t e d i n t h i s l a b o r a -t o r y . 58 65 lODq v a l u e , t h e i d e n t i f i c a t i o n o f t h e s t r o n g l y t e m p e r a t u r e -2 d e p e n d e n t d e p l e t i n g component o f E w i t h t h e t h e r m a l a c t i -2 4 v a t e d E y T„ t r a n s i t i o n r e m a i n e d t h u s , a t b e s t , m e r e l y g 2 g as a p l a u s i b l e s p e c u l a t i o n . A s i d e f r o m t h e i n t e r e s t i n t h e r m a l a c t i v a t e d b a c k i n t e r s y s t e m c r o s s i n g i t s e l f , t h e r e a r e i m p o r t a n t r e a s o n s f o r c l a r i f y i n g t h e s p e c u l a t i o n . F i r s t l y , a t room t e m p e r a t u r e , 2 t h e r e l a x a t i o n o f t h e E ^ s t a t e p r o c e e d s a l m o s t e n t i r e l y by t h e s t r o n g l y t e m p e r a t u r e - d e p e n d e n t component. W i t h o u t c o n -f i r m i n g o r r u l i n g o u t t h e p r o p o s e d mechanism, s t u d i e s o f t h e 2 r e a c t i v i t y o f t h e E s t a t e , even.by s o l e l y p o p u l a t i n g o r g d e p l e t i n g i t by d i r e c t e x c i t a t i o n o r e n e r g y t r a n s f e r , c o u l d h a r d l y be i n t e r p r e t e d u n a m b i g u o u s l y . S e c o n d l y , i f t h e p r o -p o s e d mechanism i s c o n f i r m e d by m aking u s e o f t h e a c t i v a t i o n e n e r g i e s i t o f f e r s us a way t o e s t i m a t e t h e z e r o - v i b r a t i o n a l 4 l e v e l o f t h e ^ 2q s t a t e w h i c h i s , i n g e n e r a l , n o t a v a i l a b l e f r o m s p e c t r o s c o p i c s t u d i e s . Knowledge o f t h e l o c a t i o n o f 4 t h e z e r o - v i b r a t i o n a l l e v e l o f t h e T^^ s t a t e i s v e r y i m p o r -t a n t f o r t h e u n d e r s t a n d i n g o f t h e p r i m a r y p h o t o p r o c e s s e s o r i -4 g i n a t i n g f r o m t h e T^^ s t a t e and o f i t s g e o m e t r i c a l d i s t o r t i o n , P r e s e n t e d i n t h i s c h a p t e r a r e t h e e x p e r i m e n t a l r e s u l t s f o r t h e t e m p e r a t u r e d e p e n d e n c e o f t h e p h o s p h o r e s c e n c e l i f e -t i m e o f some C r ( I I I ) c o m p l e x e s w i t h l a r g e lODq v a l u e s i n w e l l d e o x y g e n a t e d . s o l u t i o n s as w e l l as t h e arguments t o s u p p o r t t h e t h e r m a l a c t i v a t e d i n t e r s y s t e m c r o s s i n g . The o t h e r p h o t o -66 2 processes o r i g i n a t i n g from the E g s t a t e are a l s o d i s c u s s e d here. S t u d i e s extended t o the z e r o - v i b r a t i o n a l l e v e l of the 4 'T„ s t a t e w i l l be d i s c u s s e d i n Chapters V I I I and IX. 2g R e s u l t s -3 The l i f e t i m e s of the complex ions [Cr(CN)g] , [ C r ( e n ) 3 ] + 3 , [Cr (NCS) ] ~ 3 , [Cr (NCS) 4 (NH 3) 2 ] _ 1 , and [ C r ( a c a c ) 3 ] have been measured as f u n c t i o n s of temperature from about 0° to -140°C i n the deoxygenated s o l u t i o n of methanol, water, and ethylene g l y c o l (2:1:1). The c o n c e n t r a t i o n s of a l l the complex ions were 0.05 M except C r ( a c a c ) ^ which was 0.01 M. The emission was monitored a t the known phosphorescence maxima. The r e s u l t s , p l o t t e d as the l o g a r i t h m of the r e -c i p r o c a l l i f e t i m e vs the r e c i p r o c a l a b s o l u t e temperature, are shown i n F i g u r e s 19, 20 and 21. 5 1 According t o Targos and F o r s t e r the r e c i p r o c a l l i f e -time, or decay r a t e constant, can be expressed as + k b ~ E a -Eh kD = 1 / T D = k 0 + k a + k b = k n + s e RT + s K e ^ T - ( 5 > 2 ) 0 a b 2 4 where krt i n c l u d e s the r a d i a t i v e E -> A_ t r a n s i t i o n pro-0 g 2g b a b i l i t y and the temperature-independent n o n r a d i a t i v e r a t e c onstant. However, the f a c t t h a t a l l the experimental curves become q u i t e s t r a i g h t at bo t h ends may i n d i c a t e t h a t kg i s not important over t h i s temperature range and t h a t k^ and k^ each dominates at one end of the temperature range ( i n t h i s 6.0 2.0 I " 1 » 4.0 5.0 6.0 7. 1,000/T C K - 1 ) 2 - r> F i g u r e 21. The l i f e t i m e s o f t h e E_ s t a t e s o f [ C r ( N C S ) , ] y 6 _3 and [ C r ( C N ) , ] as f u n c t i o n s o f t e m p e r a t u r e . 70 work). The decay r a t e constants were t h e r e f o r e decomposed i n t o two temperature-dependent terms o n l y : -Eg. - E b k D = s a e "RT + s ^ " ^ (5.3) The estimated v a l u e s of s , E , s, , and E. are sum-a a P p marized i n Table I I where some of Targos and F o r s t e r ' s and Zander's data are a l s o i n c l u d e d f o r comparison. The smooth curves i n F i g u r e s 17, 18, and 19 are p l o t t e d a c c o r d i n g to Equation 5.3 with parameters from Table I I . Table I I Frequency f a c t o r s and a c t i v a t i o n e n e r g i e s of the 2 temperature-dependent processes of the E^ s t a t e Complex s a E a sj-, Ei-, (sec ) (Kcal/mol) (sec x) (Kcal/mol [Cr(CN) c] 3 : 6 -3 [Cr(NCS)g] t r a n s - [ C r ( N H 3 ) 2 ( N C S ) 4 ] [Cr (en) 3 ] + 3 [ C r ( e n ) 3 ] + 3 a [ C r ( e n ) 3 ] ( C 1 0 4 ) 3 c Cr ( a c a c ) 3 Cr ( a c a c ) 3 : A l ( a c a c ) 3 b 7 . 2 x l 0 2 0 .26 8.7X10 1 1 8 .2 4 . 6 x l 0 2 0 .19 2 . 1 x l 0 1 4 9 .2 4 . 3 x l 0 3 0 .076 13 7 . 2 x l O X J 9 .0 1 . 6 x l 0 4 0 .10 13 1 . 5 x l O X J 10 .2 2 . 5 x l 0 3 0 .08 12 9 x l 0 x z 9 .9 1.8xl0 3 0 .06 8.5X10 1 1 9 .2 6 . 2 x l 0 3 0 . 18 14 6.9x10 7 .7 6 xlO 0 .4 1? 6 xlO z 8 .0 a. From Zander's data, i n water and g l y c e r i n ( 1 : 1 ) w i t h o u t de-oxygenation. [Cr (en) ^ l " 1 " was known not to be quenched by oxygen.5 8 b. From Targos and F o r s t e r ' s d ata. ^ c. C a l c u l a t e d from Zander's data i n s o l i d s t a t e . The phos-phorescence decay i s e x p o n e n t i a l i n s o l i d s t a t e f o r t h i s complex. Estimated from three p o i n t s . 71 D i s c u s s i o n An o v e r a l l examination of the curves r e v e a l s t h a t the onset temperature, a t which the l i f e t i m e begins t o shorten d r a s t i c a l l y , i s a str o n g f u n c t i o n of the s o l u t e . T h i s i s not 22 compatible with the proposed e x p l a n a t i o n t h a t the onset temperature corresponds t o the temperature a t which the s o l u -t i o n s t a r t s t o f l u i d i f y or s o l i d i f y . C e r t a i n l y , the e n v i r o n -ment does a f f e c t the r e l a x a t i o n p r o c e s s e s , but i t i s appar-e n t l y t h a t the i n f l u e n c e i s r a t h e r continuous and d i f f e r e n t i -able over the temperature range. On the whole, i t i s common r a t h e r than e x c e p t i o n a l t h a t the l i f e t i m e depends very much on temperature i n s o l u -59 t i o n . In some or g a n i c compounds, t h i s has been shown to be due to the d i f f u s i o n c o n t r o l l e d quenching by i m p u r i t i e s s i n c e the a c t i v a t i o n energy of t h i s d e p l e t i n g process f o r d i f f e r e n t s o l u t e s was p r a c t i c a l l y the same as the a c t i v a t i o n energy of the s o l v e n t f l u i d i t y . In the cas 0e of C r ( I I I ) com-plex i o n s , there i s much evidence to r u l e out the i n t e r m o l e c u l a r 5 8 mechanisms. Quenching by oxygen and by the other C r ( I I I ) complex ions v i a energy t r a n s f e r (see Chapter I I I ) has been w e l l s t u d i e d . Even i f we assume the presence of a n o t i c e a b l e amount of oxygen or some other p o s s i b l e complex i o n s , the quenching e f f e c t i s s t i l l f a r too s m a l l to be adequately accounted f o r , not to mention t h a t the s o l u t i o n s have been v/ell deoxygenated and the C r ( I I I ) complexes c a r e f u l l y p u r i -f i e d . I m p u r i t i e s i n the or g a n i c s o l v e n t s are not i m p o s s i b l e . 2 However, t h a n k s t o t h e l o w l y i n g E s t a t e , o r g a n i c i m p u r i -t i e s w o u l d be v e r y u n l i k e l y t o meet t h e n e c e s s a r y c o n d i t i o n s t o q u e n c h i t . S e l f - q u e n c h i n g , i f a n y , i s n o t i m p o r t a n t a t t h i s c o n c e n t r a t i o n . ~*7 F u r t h e r m o r e , t h e a c t i v a t i o n e n e r g i e s , E 's and E, 1 s i n t h e same s o l v e n t a r e w e l l s p r e a d and a l l a o a r e q u i t e d i f f e r e n t f r o m t h e a c t i v a t i o n e n e r g y o f t h e s o l v e n t f l u i d i t y - - a b o u t 5 K c a l / m o l f o r t h e m i x e d s o l v e n t u s e d . A l l t h e s e f a c t s c l e a r l y r u l e o u t t h e p o s s i b i l i t y o f any d i f f u s i o n c o n t r o l l e d q u e n c h i n g . T h e r e i s a l s o some e v i d e n c e t o s u p p o r t t h e s u p p o s i t i o n t h a t s ^ , s ^ , E a , and E^ a r e a l l m a i n l y i n t r a -51 m o l e c u l a r p a r a m e t e r s . C o n c l u s i v e e v i d e n c e comes f r o m t h e f a c t t h a t a l t h o u c r h s„ and s, c h a n g e w i t h e n v i r o n m e n t , E and a D a E^ a r e p r a c t i c a l l y i n v a r i a n t i n d i f f e r e n t e n v i r o n m e n t s . F o r e x a m p l e , E and E, r e s p e c t i v e l y r e m a i n t h e same f o r a p + 3 [ C r ( e n ) ^ 1 i n t h e m e t h a n o l : w a t e r : e t h y l e n e g l y c o l (2:1:1) m i x t u r e , t h e w a t e r : g l y c e r i n (1:1) m i x t u r e , and i n s o l i d s t a t e . T h i s i s a l s o t r u e f o r C r ( a c a c ) ^ i n s o l u t i o n a nd i n c r y s t a l -l i n e h o s t Aliacac)^ ( s e e T a b l e I I ) . The p o s s i b l e i n t r a m o l e c u l a r p a t h w a y s f o r d e p o p u l a t i o n 2 o f t h e e x c i t e d E^ s t a t e , as shown i n F i g u r e 3, a r e : (1) phos-p h o r e s c e n c e E^ ->- &2of i n t e r s y s t e m c r o s s i n g t o t h e g r o u n d s t a t e 2 E > 4 A 0 , (3) p h o t o c h e m i c a l r e a c t i o n s , a n d (4) t h e r m a l g 2g' . 2 4 a c t i v a t e d i n t e r s y s t e m c r o s s i n g E >- T_ . The r a d i a t i v e g 2g E -* "A,, t r a n s i t i o n , assumed t o be t e m p e r a t u r e - i n d e p e n d e n t , g 2g ^ i s i n c l u d e d i n k^ i n E q u a t i o n 5.2. P h o s p h o r e s c e n c e q uantum 73 y i e l d s f o r the complex are u s u a l l y v ery s m a l l and thus not an important c o m p e t i t i v e process i n t h i s temperature range. Since E a i s too s m a l l to be due to the thermal e x c i -2 4 t a t i o n from E t o T_ , k has been assi g n e d t o a d i r e c t g 2g a 3 2 4 55 t r a n s i t i o n from E t o A~ . Although the r a d i a t i v e g 2g t r a n s i t i o n p r o b a b i l i t y k,- may be temperature dependent due to the f a c t t h a t i t i s v i b r o n i c a l l y induced, to i d e n t i f y k g with k,_ cannot be j u s t i f i e d on the grounds t h a t the phosphorescence quantum y i e l d i s g e n e r a l l y f a r too s m a l l . T h e r e f o r e , k i s ~ - ~ a 2 4 assigned t o the i n t e r s y s t e m c r o s s i n g from E t o A„ and g 2g thus i d e n t i f i e d with k,. o Modern t h e o r i e s of r a d i a t i o n l e s s t r a n s i t i o n by Robinson and F r o s c h , 6 ( ) S i e b r a n d , ^ 1 and L i n ^ 2 a l l i n d i c a t e t h a t there may be but a s m a l l temperature dependence f o r r a d i a t i o n l e s s d e a c t i v a t i o n s . L i n and Bersohn have d e r i v e d from L i n ' s theory an e x p l i c i t equation r e l a t i n g t o the temperature e f f e c t on t r i p l e t - s t a t e l i f e t i m e s . ^ At s u f f i c i e n t l y low temperature 1/T - 1 / T q + a / T Q e ~ e / T (5.4) where T i s the l i f e t i m e at T=0°K. In most aromatic compounds o _ T g 3 s t u d i e d , the value of 8 ranges from about 300 to 800 cm '. L i n suggested t h a t some of the lower-frequency i n t r a m o l e c u l a r v i b r a t i o n s (out-of-plane bends) may be r e s p o n s i b l e f o r the 6 3 temperature dependence. For C r ( I I I ) complexes, from. Table I I , s a i s g e n e r a l l y l e s s than 300 cm x . Taking the lower fr e q u e n c i e s of the m e t a l - l i g a n d v i b r a t i o n s i n t o account, these v a l u e s are reasonably s a t i s f a c t o r y . For the second temperature-dependent component k^, the l a s t one to be i d e n t i f i e d , i t can again be (1) i n t e r s y s t e m c r o s s i n g kg, (2) photochemical r e a c t i o n k^, or (3) t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g k_ 4. Evidence c o n f i r m i n g t h a t k^ i s a c t u a l l y k_ 4 comes mainly from photochemical s t u d i e s . In s t u d i e s of the energy t r a n s f e r between t r a n s -[ C r ( N H 3 ) 2 ( N C S ) 4 ] " and [ C r ( C N ) g ] - 3 , i t was w e l l demonstrated (see Chapter III) t h a t the phosphorescence s t a t e of the former was quenched by the l a t t e r . F u r t h e r photochemical s t u d i e s of the same system a t -65°C i n d i c a t e d (see Chapter IV) t h a t photoaquation of t r a n s - [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ was quenched too, but to a l e s s extent than phosphorescence. T h i s r e q u i r e s t h a t k^ i s a process l e a d i n g to photochemical r e a c t i o n and r u l e s out the f i r s t p o s s i b l e case, i . e . , k, = . Wegner ' b 6 2 8 and Adamson s t u d i e d the photoaquation y i e l d s of t r a n s -[Cr (NH3) 2 (NCS) 4 ] " and [Cr(NR" 3)g] + 3 at room temperature by d i r e c t e x c i t a t i o n i n t o the lowest doublet e x c i t e d s t a t e and found w i t h i n experimental e r r o r t h a t the photoaquation quan-tum y i e l d was e s s e n t i a l l y the same—about 0.3 f o r both complexes—whether the e x c i t a t i o n was i n t o the e x c i t e d q u a r t e t or doublet s t a t e s . Since the d e p l e t i o n process of 2 the E s t a t e at room temperature i s p r a c t i c a l l v e n t i r e l y caused by process k^, i f k^ = kg or k^ = k^, the photo-aquation quantum y i e l d should be 0 or 1, r e s p e c t i v e l y , upon 75 2 d i r e c t e x c i t a t i o n i n t o the E g s t a t e . A p p a r e n t l y , t h i s was not the o b s e r v a t i o n . T h e r e f o r e , both the f i r s t and second p o s s i b l e cases, i . e . , = k g and k^ = k^ have to be r u l e d out. I f the p o s s i b l e cases l i s t e d are ex h a u s t i v e , then k^ must be i d e n t i f i e d v/ith k_^ — t h e l a s t p o s s i b l e case. In f a c t , by assuming k^ = k_^, i t becomes p o s s i b l e to e x p l a i n c o n s i s t e n t l y a l l the experimental r e s u l t s . I t i s c e r t a i n l y t r u e t h a t the t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g k_^ i s not a p h o t o r e a c t i v e process i n i t s e l f , and to accept 2 k, = k „ i m p l i e s t h a t the E s t a t e i s s u b s t a n t i a l l y i n e r t , b -4 g y o However, process k . a c t i v a t e s complex ions i n the E s t a t e -4 g 4 up to the T„ s t a t e and from t h i s s t a t e p h o t o s u b s t i t u t i o n 2g as w e l l as the other primary processes (with p o s s i b l e r e -4 2 4 c y c l i n g s of T_ • E »• T„ ) o c c u r s . T h e r e f o r e , i t 2g g 2g ' i s c l e a r t h a t k_^ i s a process l e a d i n g to photoaquation. And the decrease i n photoaquation quantum y i e l d caused by 2 quenching the E g s t a t e a c t u a l l y r e s u l t s from a quenching of the t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g k ^. From the same mechanism, i t f o l l o w s t h a t the complex ions formed i n 2 the E s t a t e v i a d i r e c t e x c i t a t i o n at room temperature w i l l g 4 e n t i r e l y be a c t i v a t e d to the T_ s t a t e from where f u r t h e r 2g r e l a x a t i o n s proceed--a s i t u a t i o n , as f a r as photoaquation i s concerned, not d i f f e r e n t from d i r e c t e x c i t a t i o n i n t o the qu a r t e t bands. T h e r e f o r e , the f a c t t h a t the photoaquation quantum yield, was the same whether e x c i t a t i o n was i n t o the 2 E a s t a t e or the e x c i t e d q u a r t e t s t a t e s i s s e l f - e v i d e n t . 76 The above arguments and i n t e r p r e t a t i o n s may be c r i t i -c i z e d on the grounds t h a t due a t t e n t i o n has not been p a i d t o the p o s s i b l e a l t e r n a t i v e mechanisms other than = Phenomenologically, k^ may c o n s i s t of two r o u t e s : one leads to the photochemical r e a c t i o n and the other t o a n o n - r a d i a t i v e t r a n s i t i o n . From the s t r a i g h t n e s s of the A r r h e n i u s p l o t s of k ^ 1 s , i t r e q u i r e s t h a t the two routes e i t h e r undergo the same r a t e - d e t e r m i n i n g step or have q u i t e c l o s e a c t i v a t i o n e n e r g i e s . A n o n - r a d i a t i v e t r a n s i t i o n with such a l a r g e a c t i -v a t i o n energy i s hard to understand from t h e o r i e s . However, Hammond has i l l u s t r a t e d i n the case of the p h o t o i s o m e r i z a t i o n 6 4 of o l e f i n i c compounds " t h a t the photochemical process and the i n t e r n a l c o n v e r s i o n are v i r t u a l l y the same. An e q u i v a -l e n t case i n the p h o t o s u b s t i t u t i o n of C r ( I I I ) complexes can 41 be, as suggested i n r i g i d media by C h a t t e r j e e and F o r s t e r , t h a t k^ i s a process i n v o l v i n g bond breaking and immediate recombination can be o p e r a t i v e because of a cage e f f e c t . 27 A c t u a l l y , C r ( I I I ) photochemistry has been thought to be e s s e n t i a l l y t h a t of cage r e a c t i o n s . _3 Taking [CrAg] as an example, [CrA 5H 20] 2 (5.5) [Cr.7\ ] ~ 3 ( 2 E ) - •*• ( C r A r • • -A) " 3 6 g J J 10 • Q. • S • ^^ ^^  ^ * ^ [ C r A 6 ] ~ J ( 4 A 2 g ) ( 5 . 6 ) In t h i s i n t e r p r e t a t i o n , the bond breaking p r o c e s s — v i r t u a l l y ky---is the r a t e - d e t e r m i n i n g s t e p . I f i t i s f o l l o w e d by 77 d i f f u s i o n away of the l e a v i n g l i g a n d , then a net chemical r e a c t i o n occurs; i f i t i s f o l l o w e d by recombination, then a net i n t e r s y s t e m c r o s s i n g t o the ground s t a t e o c c u r s . Appar-e n t l y , a l a r g e a c t i v a t i o n energy f o r the n o n - r a d i a t i v e t r a n s -i t i o n can thus be r a t i o n a l i z e d with t h i s mechanism. But i f i t i s t r u e , then the photoaquation quantum y i e l d should be s t r o n g l y dependent on the cage s t r u c t u r e . Langford has s t u d i e d the s o l v o l y s i s of [Cr(NCS)g] i n a c e t o n i t r i l e - w a t e r ?9 mixtures and found t h a t , i n c o n t r a s t to the thermal aqua-t i o n , the photoaquation quantum y i e l d i s independent of the s o l v e n t c o m p o s i t i o n . + + T h i s i n d i c a t e s cage e f f e c t may not be important i n the photoaquation of C r ( I I I ) complexes and t h e r e f o r e makes t h i s mechanism improbable. The other i n t e r p r e t a t i o n i s t h a t k, i s e s s e n t i a l l v p kg, but a f t e r i s o e n e r g e t i c i n t e r s y s t e m c r o s s i n g t o the ground s t a t e , v i b r a t i o n a l r e l a x a t i o n and aquation from the h i g h l y 4 e x c i t e d v i b r a t i o n a l l e v e l s of the A_ s t a t e can be cotnpe-2g -3 t i t i v e . Taking [CrAg] as an example, .[CrA,H O ] " 2 - 3 2 ^6 - 3 4 ' [CrA f i] ( E ) - v[CrA,] J ( A_ ;V=n) 6 g r d s 6 2 g \_ -3 . r - a - 5 ' ^ [CrA c ] (thermal e q u i l i b r a t e d ) I t has to be noted t h a t l a r g e r p a r t of the photoaquation may come from the lowest e x c i t e d q u a r t e t s t a t e s and the r e s u l t s may l a r g e l y r e f l e c t the p r o p e r t i e s of those s t a t e s , However,, i f caqe e f f e c t i s not important f o r the lowest e x c i t e d q u a r t e t s t a t e s , there are no reasons why i t should be important f o r the l o n g e r - l i v e d lowest e x c i t e d d oublet s t a t e s . In t h i s mechanism, the l a r g e a c t i v a t i o n energy f o r the non-r a d i a t i v e t r a n s i t i o n kg i s hard to e x p l a i n . + + Moreover, f o r the photochemical r o u t e , the mechanism i s e s s e n t i a l l y t h a t of the thermal a c c e l e r a t e d model which has long been thought to be improbable based on the s h o r t , l i f e t i m e of the n o n - e q u i l i b r a t e d v i b r a t i o n a l e x c i t e d s t a t e s and on the d i f -f e r e n t r e a c t i o n p a t t e r n s and c h a r a c t e r i s t i c s between thermal 27 29 and photochemical r e a c t i o n s . ' I t i s not i m p o s s i b l e t h a t k^ may c o n s i s t of independent chemical process and i n t e r s y s t e m c r o s s i n g to the ground s t a t e . But the l a r g e a c t i v a t i o n energy f o r the n o n - r a d i a t i v e t r a n s -i t i o n i s again hard to r a t i o n a l i z e . + + U n f o r t u n a t e l y , to r u l e out the i n t e r p r e t a t i o n with experimental evidence seems extremely d i f f i c u l t . However, the f a c t t h a t the a c t i v a t i o n e nergies f o r those two independent processes are the same or n e a r l y the same f o r a l l the Cr (III) complexes s t u d i e d seems f a r too c a s u a l to be p o s s i b l e . From the above evidence and arguments, i t i s c l e a r t h a t the i d e n t i f i c a t i o n of the s t r o n g l y temperature-dependent term k^ with the t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g . 2 4 k . from the F to T n s t a t e i s overwhelmincr.lv f a v o r e d . -4 g 2g - -A d d i t i o n a l support comes from the l o c a t i o n s of the 4 z e r o - v i b r a t i o n a l l e v e l of the T_ s t a t e , which can be 2g ++ Intersystem c r o s s i n g s through the i n t e r s e c t i o n p o i n t s of the p o t e n t i a l energy s u r f a c e s ( T e l l e r c r o s s i n g ) may account f o r a l a r g e a c t i v a t i o n .energy. However, to the author's knowledge, no precedent examples with such a l a r g e a c t i v a t i o n energy have been r e p o r t e d . 2 estimated bv adding E ' s to the known E (v=0) s t a t e s . For D g a l l the C r ( I I I ) complexes s t u d i e d , f l u o r e s c e n c e has been de-t e c t e d near and only near the phosphorescence maxima. There-f o r e the f l u o r e s c e n c e maximum should l i e f a i r l y c l o s e l y t o the phosphorescence maximum (see Chapter V I ) . I t i s found 4 that the estimated T„ (v=0) l e v e l s a l l l i e about i n the c e n t r a l 2g r e g i o n between the a b s o r p t i o n and f l u o r e s c e n c e maxima. Secondly, 4 s t u d i e s of the i n t e r s v s t e m c r o s s i n g from the T_ s t a t e to the e x c i t e d doublet s t a t e s (see Chapter VII) r e v e a l e d a t h e r m a l l y 4 2 a c t i v a t e d process assigned to ^2q *" T 2 g " t * i e c o r r e s P o n ~ 4 ding a c t i v a t i o n energies are added to the T^{v=0) l e v e l s 2 r e s p e c t i v e l y , the energy l e v e l s so obtained f o r the T^^ s t a t e agree very w e l l with those p r e d i c t e d t h e o r e t i c a l l y (see Chapter V I I I ) . These a l l support i n d i r e c t l y the t h e r m a l l y 2 a c t i v a t e back i n t e r s v s t e m c r o s s i n g from the E s t a t e to the g 4 T„ s t a t e . 2g A F u r t h e r Comment Since the phosphorescence decay r a t e constant k^ was determined by the c o n v e n t i o n a l method which t a c i t l y assumes no a p p r e c i a b l e back i n t e r s y s t e m c r o s s i n g , a c a r e f u l a n a l y s i s to i n v e s t i g a t e the l i m i t a t i o n s of the data treatments and the v a l i d i t y of the arguments i s i n d i s p e n s i b l e when the back i n t e r -system c r o s s i n g i s known to be g r e a t l y i n v o l v e d . A f t e r an i d e a l instantaneous e x c i t a t i o n , the phosphor-escence i n t e n s i t y as a f u n c t i o n of time can be expressed (see the Appendix) as where k a 80 -k t -k t I = A( e a - e 15 ) (5.7) i _ 1 *• ( k E + k T ) - * [ ( k T - k E ) z + 4 k 4 k _ 4 ] = k 6 = * ( k E + V + * [ ( k T " k E ) 2 + 4 k 4 k - 4 ] 1 and k E = k 5 + k g + k ? + k _ 4 k T = k x + k 2 + k 3 + k 4 I n t h e C r ( I I I ) c o m p l e x e s s t u d i e d i n t h i s work, k R g e n e r a l l y g r e a t e r t h a n k , t h e r e f o r e t h e s e c o n d t e r m d e c r e a s e s much f a s t e r t h a n t h e f i r s t t e r m . B e c a u s e t h e a p p a r e n t d e c a y r a t e c o n s t a n t k^ was d e t e r m i n e d f r o m t h e d e c a y i n g n a r t o f t h e c u r v e w e l l p a s t t h e maximum, i n p r a c t i c e -k t I = Ae a (5.8) T h e r e f o r e k^ = k , and t h i s p a r t o f t h e d e c a v c u r v e s h o u l d be D a e x p o n e n t i a l as o b s e r v e d . A s s u m i n g k m i s s u f f i c i e n t l v g r e a t e r t h a n k„ w h i c h i s c e r t a i n l y t h e c a s e o v e r t h e s t u d i e d t e m p e r a t u r e r a n g e , t h e n k c an be a p p r o x i m a t e d as (see t h e A p p e n d i x ) a - • k^ = k = k , + k c + k_ + (l-<f>. )k . (5.9) D a 5 6 7 T i s c -4 S i n c e <J>j_sc c a n o n l y be between 0 and 1, and i t i s n o t v e r y t e m p e r a t u r e d e p e n d e n t o v e r t h e s t u d i e d t e m p e r a t u r e r a n g e (see C h a o t e r V I ) , t h e f a c t o r (1 - d>. ) i s e s s e n t i a l l v c o n s t a n t i s c c o m p a r i n g t o t h e g r e a t l y v a r y i n g k _ 4 » T h e r e f o r e t h e p a r a -m e t e r s s a , s]-), E a , and Eb e s t i m a t e d a r e j u s t i f i e d e x c e p t t h a t sb has t o be c o r r e c t e d bv a f a c t o r o f (1 -6. ). i s c CHAPTER VI QUANTUM YIELD OF INTERSYSTEM CROSSING AS A FUNCTION OF TEMPERATURE E x c e p t f o r a few C r ( I I I ) c o m p l e x e s t h a t f l u o r e s c e , t h e 4 k i n e t i c p a r a m e t e r s o f t h e ^2q s^-a^-e a r e g e n e r a l l y i n a c c e s -2 s i b l e . The o n l y p r o b e o b t a i n a b l e f r o m 'E p a r a m e t e r s t h a t c a n g ^ 4 be u s e d t o p r o v i d e some i n f o r m a t i o n a b o u t t h e T„ s t a t e i s 2g t h e quantum y i e l d o f i n t e r s y s t e m c r o s s i n g o r t h e quantum y i e l d o f d o u b l e t f o r m a t i o n . From t h e o b s e r v a t i o n s o f p h o s p h o r e s c e n c e e m i s s i o n i n most C r ( I I I ) c o m p l e x e s , i t i s e v i d e n t t h a t i n t e r s y s t e m c r o s -4 2 s i n g f r o m t h e T„ t o E s t a t e c e r t a i n l y t a k e s p l a c e t o an 2g g i r a p p r e c i a b l e e x t e n t . However, F o r s t e r e t a l . have d e m o n s t r a t e d b o t h t h e o r e t i c a l l y and e x p e r i m e n t a l l y " ^ ' 4 x ' t h a t i t s quantum y i e l d i s g e n e r a l l y s u b s t a n t i a l l y l e s s t h a n u n i t y and t h e r e f o r e 4 4 c o n c l u d e d t h a t i n t e r n a l c o n v e r s i o n f r o m t h e T„ t o A 0 s t a t e 2g 2g i s i m p o r t a n t . The quantum y i e l d o f i n t e r s y s t e m c r o s s i n g c a n be ex-p r e s s e d as cj). = , P (6.1) 5 p I t i s i n t e r e s t i n g t o n o t e t h a t e v e n when back i n t e r s y s t e m c r o s s i n g o c c u r s , E q u a t i o n 6.1 s t i l l h o l d s . T h i s c a n be v e r i -f i e d t h r o u q h E q u a t i o n 4.19 and t h e d e f i n i t i o n 82 k 4  ^ i s c ~ k 1 + k 2 + k 3 + k 4 ( 6 * 2 ) The p h o s p h o r e s c e n c e l i f e t i m e s o f some C r ( I T I ) c o m p l e x e s as f u n c t i o n s o f t e m p e r a t u r e h a v e b e e n d i s c u s s e d i n C h a p t e r V. F u r t h e r , k,- h a s b e e n e s t i m a t e d t h e o r e t i c a l l y f o r many C r ( I I I ) complexes. 1**' 4"'" I f we assume t h a t k,- i s t e m p e r a t u r e i n v a r i -a n t , t h e n m e a s u r e m e n t s o f p h o s p h o r e s c e n c e q uantum y i e l d s as f u n c t i o n s o f t e m p e r a t u r e w i l l e n a b l e u s , t h r o u g h E q u a t i o n 6.1, t o d e t e r m i n e t h e t e m p e r a t u r e e f f e c t on t h e quantum y i e l d o f i n t e r s y s t e m c r o s s i n g . E x p e r i m e n t a l and R e s u l t s E x c e p t t h e C r ( a c a c ) ^ s o l u t i o n w h i c h was 0.01 M, a l l t h e c o m p l e x s o l u t i o n s u s e d w e re 0.05 M and a b s o r b e d t h e e x c i t a t i o n -3 l i g h t t o t a l l y . The i r r a d i a t i o n was a t 546 nm f o r [ C r ( N C S ) g ] , [ C r ( N H 3 ) 2 ( N C S ) 4 ] ~ , [ C r ( C N ) 6 ] ~ 3 , a nd C r ( a c a c > 3 ; a nd a t 436 nm + 3 -3 f o r [ C r ( e n ) . ] • and [ C r ( C N ) - ] . I n o r d e r t o m i n i m i z e t h e 3 6 p h o t o c h e m i c a l r e a c t i o n , i r r a d i a t i n g t i m e was k e p t as s h o r t as p o s s i b l e . T h e r e f o r e , f o r e a c h c o m p l e x , o n l y t h e i n t e n s i t y a t t h e e m i s s i o n maximum was u s e d as a m e a s u r e o f t h e r e l a t i v e p h o s p h o r e s c e n c e quantum y i e l d . H o wever, t h e w h o l e s p e c t r u m was s c a n n e d i n t e r m i t t e n t l y t o e n s u r e t h a t t h e s p e c t r a l d i s -t r i b u t i o n was r e a s o n a b l y c o n s t a n t o v e r t h e t e m p e r a t u r e r a n g e . No m e a s u r e m e n t s o f t h e a b s o l u t e quantum y i e l d w e r e 41 a t t e m p t e d i n t h i s w o r k . A c c o r d i n g t o C h a t t e r - | e e and F o r s t e r t h e p h o s p h o r e s c e n c e quantum y i e l d r e a c h e s a l i m i t i n g v a l u e a t l o w t e m p e r a t u r e and t h e v a l u e r e m a i n s t h e same i n d i f f e r e n t 83 s o l v e n t s . The a b s o l u t e p h o s p h o r e s c e n c e quantum y i e l d s u s e d i n t h i s c h a p t e r w e r e e s t i m a t e d b y f i x i n g t h e l o w - t e m p e r a t u r e l i m i t i n g i n t e n s i t i e s a t t h e v a l u e s o b t a i n e d by C h a t t e r j e e and F o r s t e r f o r t h e a b s o l u t e p h o s p h o r e s c e n c e quantum y i e l d s . The v a l u e s f o r k_ and l i m i t i n a l o w - t e m p e r a t u r e <f> a r e l i s t e d i n 5 P T a b l e I I I . T a b l e I I I I n t r i n s i c P h o s p h o r e s c e n c e R a t e C o n s t a n t s and L o w - T e m p e r a t u r e L i m i t i n g P h o s p h o r e s c e n c e Quantum Y i e l d s C o m p l e x k 5 ( s e c - 1 x 1 0 3 ) , l i m <P P [ C r ( N H 3 ) 2 ( N C S ) 4 ] - 0.20 0.011 [ C r ( N C S ) 6 ] ~ 3 . 0 . 2 3 + + 0.23 [ C r ( C N ) 6 ] ~ 3 0 .016 0.0042 [C r (en) 3 ] + 3 0.15 0.0090 [ C r ( a c a c ) 3 ] 0.13 0.021 V a l u e t a k e n f r o m p h o s p h o r e s c e n c e d e c a y c o n s t a n t a t l i q u i d n i t r o g e n t e m p e r a t u r e . The p h o s p h o r e s c e n c e quantum y i e l d s w e r e p l o t t e d as f u n c t i o n s o f t e m p e r a t u r e f o r t h e s e f i v e C r ( I I I ) c o m p l e x e s , as s h o w n . i n F i g u r e s 22, 23, 24. The quantum y i e l d s o f i n t e r s y s t e m c r o s s i n g w ere t h e n e v a l u a t e d v i a E q u a t i o n 6.1 and a r e shown as f u n c t i o n s o f t e m p e r a t u r e i n F i g u r e 25,26. 0 86 0 . 7 89 D i s c u s s i o n On the whole, the quantum y i e l d of i n t e r s y s t e m c r o s s i n g ( J K s c , does not change much with temperature f o r the C r ( I I I ) complexes s t u d i e d . The v a l u e s c a l c u l a t e d f o r 4>^ s c range from 0.1 f o r [ C r ( C N ) 6 ] ~ 3 to 0.7 f o r [ C r ( e n ) 3 ] + 3 . The s m a l l o s c i l -l a t i o n s i n the curves can probably be a t t r i b u t e d t o experimen-t a l e r r o r s , but, the steady d e c r e a s i n g of the curves a t the high temperature ends must be c o n s i d e r e d to be r e a l . Since there are u n c e r t a i n t i e s i n both the e v a l u a t i o n s of k c and the determinations of cb , the a b s o l u t e v a l u e s e s t i -5 p mated f o r d>. are not very r e l i a b l e . However, 6. f o r i s c r i s c [Cr(NH^)2(NCS)^] has been estimated from photochemical s t u d i e and the same method can be extended to other C r ( I I I ) complexes Assuming k^ = 0 and [A]=0, i t can be shown through Equation 4.20 t h a t 6 °° = 1 - <J>. (6.3) *chem 1 8 C 1 8 C k - 4 where <\>. = (6.4) k . + k c + k r -4 5 6 cb. i s not r e a d i l v a v a i l a b l e , but i t can be estimated from i s c s t u d i e s of the temperature-dependence of phosphorescence l i f e t i m e s . From r e s u l t s i n Chapter V, ( 1 ~ ^ i s c ) k - 4 k b = „ (6.5) (1 - cJ). )k . + k c + k, k_ Y i s c -4 5 6 D 90 — oo For [ C r ( N H 3 ) 2 ( N C S ) 4 ] at - 6 5 ° C , < ! > c h e i T / 4 > c h e m = 0.0052/0.0102 = 0.51 from Chanter IV and k^/k^ = 0.89 from Chapter V. Equations 6.3, 6.4, and 6.5 can be s o l v e d t o g i v e a value of 0.52 f o r the quantum y i e l d of i n t e r s y s t e m c r o s s i n g i n [Cr (NH3) 2 (NCS) 4 ] " " , and 0.9 4 f o r ^  . The quantum y i e l d of i n t e r s y s t e m c r o s s i n g can a l s o be estimated throuqh the s t u d i e s on the s e n s i t i z e d phosphorescence. In the energy t r a n s f e r system of [ C r ( N H 3 ) 2 ( N C S ) ^ ] ~ and - 3 2 [Cr(CN)g] , the energy t r a n s f e r e f f i c i e n c y i s u n i t y and E g i s the a c c e p t i n g s t a t e (see Chapter I I ) . The mechanism p r e d i c t s : * A = ' *? ( n k Q H ^ ] ) ( ^ ) (3.12) S P 1 S C k D + k n H [ A ] k A 0 QH 1 A kp <f> = (—* ) (6.6) V P T i s c ,A k From the above two equations * i s c = ( * p ) )<{,D (6.7) 7A" k D + k [A] ^ 1 S C *sp ° At -65°C and [A] = 0.07 M, 4>A = 6.3 x 1 0 ~ 4 (Figure 2 3 ) , cf>Ap = 5.9 x 1 0 ~ 4 , k^„[A]/(k D + k r A ]) = 0.63 (ChaDter I I I ) , and Qri O yn <})?sc = 0.52. S u b s t i t u t i n g these v a l u e s i n t o Equation 6.7, one obtains 4>A = 0 . 3 5 f o r [ C r ( C N ) , ] ~ 3 . i s c 6 The quantum y i e l d s of i n t e r s y s t e m c r o s s i n g f o r [Cr (NH-.) (NCS) . ] ~ and [Cr(CN),]"" 3 obtained i n t h i s wav are 3 Z 4 6 three times g r e a t e r than those estimated from Equation 6.1 (0.52 and 0.35 vs 0.17 and 0 . 1 1 ) . S i n c e l i f e t i m e measurements a r e c o m p a r a t i v e l y c e r t a i n , t h i s c o n s t a n t f a c t o r c o u l d a r i s e f r o m a s y s t e m a t i c e r r o r e i t h e r i n t h e e v a l u a t i o n s o f kg o r i n t h e a b s o l u t e d e t e r m i n a t i o n s o f <f> . However, a p p l y i n g t h i s + 3 f a c t o r t o [ C r f e n ) ^ ] w o u l d r e s u l t i n a quantum y i e l d o f i n t e r -s y s t e m c r o s s i n g f a r g r e a t e r t h a n u n i t y t h a t , o f c o u r s e , i s n o t a c c e p t a b l e . However, a b s o l u t e e m i s s i o n quantum y i e l d s a r e n o t o r i o u s l y d i f f i c u l t t o measure and i t i s l i k e l y t h a t t h e s e v a l u e s may be i n e r r o r . A l l t h e quantum y i e l d s o f i n t e r s y s t e m c r o s s i n g o b t a i n e d become c o n s t a n t below - 7 0 ° C . T h i s i n d i c a t e s t h a t i n t e r n a l 4 c o n v e r s i o n and i n t e r s y s t e m c r o s s i n g y i e l d s f r o m t h e ^2g s t a t e a r e t e m p e r a t u r e i n d e p e n d e n t o r o n l y s l i g h t l y t e m p e r a t u r e d e -p e n d e n t i n t h i s t e m p e r a t u r e r a n g e . However, t h e v do m o n o t o n i -c a l l y d e c r e a s e a t h i g h e r t e m p e r a t u r e s . F o r t h i s , two o t h e r f a c t o r s must f i r s t be c o n s i d e r e d : p h o t o c h e m i c a l r e a c t i o n and t h e v a r i a t i o n o f kg w i t h t e m p e r a t u r e . P h o t o c h e m i s t r y , as 4 e s t a b l i s h e d i n t h e p r e v i o u s c h a p t e r s , o c c u r s f r o m t h e s t a t e , and i t s y i e l d i n c r e a s e s w i t h t e m p e r a t u r e , b u t i t s q u a n -tum y i e l d r a r e l y e x c e e d s 0.1 b e l o w - 3 0 ° C . I t t h e r e f o r e c a n n o t c a u s e t h e quantum y i e l d o f i n t e r s y s t e m c r o s s i n g t o d e c r e a s e a p p r e c i a b l y . Change o f kg w i t h t e m p e r a t u r e has b e en n o t i c e d i n r u b y (213 and 244 s e c - 1 a t 77° and 3 0 0 ° K ) . 6 5 B e c a u s e t h e 2 4 t r a n s i t i o n E «- T 0 i s v i b r o n i c a l l y i n d u c e d , i t i s p o s s i b l e g 2g J -t h a t kg does have a t e m p e r a t u r e d e p e n d e n c e . I n f a c t , t h e _ 3 s p e c t r a l d i s t r i b u t i o n o f [ C r ( C N ) g ] e m i s s i o n c h a n g e s b e l o w ~ 1 3 0 ° C . However, t h e s p e c t r a o f t h e e m i s s i o n s s t u d i e d do n o t change a p p r e c i a b l y f r o m -120° t o - 3 0 ° C . I t i s b e l i e v e d t h a t t h e d e c r e a s e o f <f>. i s s t i l l t o o l a r c r e t o be a c c o u n t e d f o r T i s c by any v a r i a t i o n o f k,.. From E q u a t i o n 6.2, by n e g l e c t i n g f l u o r e s c e n c e p r o c e s s , k^, and p h o t o c h e m i c a l r e a c t i o n , k^, t h e f o l l o w i n g e x p r e s s i o n c a n be o b t a i n e d . 1 k 2 x = 1 + —T-= (6.8) Y i s c 4 B o t h i n t e r n a l c o n v e r s i o n , k^, and i n t e r s y s t e m c r o s s i n g , k^, may c o n s i s t o f a t e m p e r a t u r e - i n d e p e n d e n t (or o n l y s l i g h t l y d e p e n d e n t ) component and a t e m p e r a t u r e - d e p e n d e n t component. B e l o w ' - 7 0 ° C , t h e t e m p e r a t u r e - i n d e p e n d e n t component p a r t s o f ea c h a r e d o m i n a n t , t h e r e f o r e d>. does n o t change much w i t h Y i s c t e m p e r a t u r e . Above - 7 0 ° C , t h e t e m p e r a t u r e - d e p e n d e n t p a r t s b e -come i m p o r t a n t and tf>. s t a r t s t o d e c r e a s e . S i n c e k„ i s r i s c 4 u n l i k e l y t o d e c r e a s e w i t h t e m p e r a t u r e , t h e f a c t t h a t $ ^ s c d e c r e a s e s w i t h r i s i n g t e m p e r a t u r e i m p l i e s t h a t a t l e a s t i n t e r n a l c o n v e r s i o n i s t e m p e r a t u r e - d e p e n d e n t . I f t h e above i n t e r p r e t a t i o n i s r i g h t , t h e n an A r r h e n i u s p l o t o f l o g ( l / c } K -1) a g a i n s t 1/T w i l l y i e l d t h e d i f f e r e n c e i n t h e a c t i v a t i o n e n e r -g i e s o f k^ and k^, p r o v i d e t h e t e m p e r a t u r e r a n g e i s s u f f i c i e n t l y h i a h . 93 The A r r h e n i u s p l o t has been done f o r [Cr(NH^) 2(NCS)^] -3 ; and [ C r ( C N ) r l with the c o r r e c t e d d>. from F i g u r e 25. The 6' i s c r e s u l t i n g l i n e s are s l i g h t l y curved as shown i n F i g u r e 27. The a c t i v a t i o n energy d i f f e r e n c e s estimated from the h i g h -temperature end are about 5 Kcal/mol f o r [ C r ( N H ^ ) 2 ( N C S ) ^ ] ~ and -3 2 Kcal/mol f o r [Cr(CN)^] . T h e r e f o r e , the a c t i v a t i o n e n e r c i e s f o r t h e i r i n t e r n a l conversions must be g r e a t e r than 5 and 2 Kcal/mol r e s p e c t i v e l y . They are s u r p r i s i n g l y l a r g e values f o r i n t e r n a l c o n v e r s i o n s , s i n c e the p r e s e n t t h e o r i e s a l l p r e d i c t ^ " ' ^ ' '^" t h a t the temperature dependence of n o n - r a d i a t i v e t r a n s i t i o n s should be s m a l l or none at a l l . However, i n t e r n a l c o n v e r s i o n has not r e a l l y been measured i n d e t a i l f o r any molecule. Con-4 f i r m a t i o n at t h i s p o i n t w i l l r e q u i r e d i r e c t study of the T 2 g s t a t e . 0 . 5 igure 27. Plots of log(l /< j ) . - 1) v s 1/T. CHAPTER V I I THE LIFETIME OF THE 4 T OR STATE 2g I n t h e l i f e t i m e measurements o f e n e r g y t r a n s f e r between R e i n e c k a t e and h e x a c y a n o c h r o m a t e ( I I I ) i o n s , an i n i t i a l r i s e p r e c e d i n g a l o n g e r d e c a y o f t h e p h o s p h o r e s c e n c e o f t h e a c c e p -t o r was n o t i c e d . I t was i m m e d i a t e l y r e c o g n i z e d t o be t h e p o p u l a t i o n p r o c e s s o f t h e p h o s p h o r e s c e n t s t a t e o f t h e a c c e p t o r (see C h a p t e r I I I ) . Out o f c u r i o s i t y , t h e i n i t i a l p a r t o f t h e d e c a y c u r v e o f t h e d o n o r , R e i n e c k a t e i o n , was r e i n v e s t i g a t e d . I t was a s o r t o f s e r e n d i p i t y t o f i n d t h a t t h e r e was an i n i t i a l r i s e i n t h e d o n o r p h o s p h o r e s c e n c e t o o , a l t h o u g h i n a s h o r t e r p e r i o d . I t has b een e s t a b l i s h e d f r o m s u b s e q u e n t s t u d i e s t h a t an i n i t i a l r i s e o f p h o s p h o r e s c e n c e a f t e r a p u l s e e x c i t a t i o n i s a , g e n e r a l phenomenon among t h e i n d i v i d u a l C r ( I I I ) c o m p l e x e s . F i g u r e 28A shows, as a t y p i c a l example, t h e p o p u l a t i o n and d e c a y c u r v e o f C r ( a c a c ) ^ p h o s p h o r e s c e n c e . The o n l y e x c e p t i o n _3 o b s e r v e d i s [ C r ( C N ) g ] whose d e c a y c u r v e c o n s i s t s o f a f a s t e r and a much s l o w e r component as shown i n F i g u r e 2 8B. S i m i l a r t o t h e c a s e o f e n e r g y t r a n s f e r , t h e i n i t i a l r i s e o f p h o s p h o r e s c e n c e r e l a t e s t o t h e p o p u l a t i o n p r o c e s s 2 o f t h e E s t a t e . The i m p o r t a n c e o f b e i n g a b l e t o o b s e r v e 9" t h e p o p u l a t i o n p r o c e s s i s a p p a r e n t . However, f i r s t o f a l l , 97 we have to prove t h a t the phenomenon i s not due to instrumen-t a l f a c t o r s or from i m p u r i t i e s . The h a l f - h e i g h t width of the f l a s h lamp was l e s s than 200 nsec and i t s s c a t t e r e d l i g h t from the blank s o l u t i o n was not d e t e c t a b l e a t the s e n s i t i v i t y used. From these f a c t s one c o u l d conclude t h a t the lamp d i d not i n t e r f e r e w i t h the emis-_ 3 s i o n . In the case of [Cr(CN)g] t h i s i m p l i e d t h a t the i n i t i a l f a s t e r decay c o u l d not be caused by the t a i l of the lamp output. A more probable source of d i s t o r t i o n , i f any, might come from the d e t e c t i n g and r e c o r d i n g systems. However, the p h o t o m u l t i -p l i e r was wired s p e c i a l l y f o r f a s t response a p p l i c a t i o n s . In f a c t , the b l e e d e r r e s i s t o r s have been changed to achieve a • hi g h e r b l e e d e r c u r r e n t , but s t i l l no a p p r e c i a b l e change has thus been caused i n the p o p u l a t i o n and decay curve. The o s c i l -loscope has a r i s e time of 15 nsec a t the s e n s i t i v i t y used and i s t h e r e f o r e s u f f i c i e n t l y f a s t . D i f f e r e n t c a p a c i t o r s were used i n d i v i d u a l l y across the load r e s i s t o r to a l l e v i a t e the n o i s e , but the RC time constant was kept to be l e s s than t h a t of the r i s e time s t u d i e d by a f a c t o r of about 100. And the curves remained the same average f i g u r e upon d e c r e a s i n g the RC time constant or e l i m i n a t i n g the e x t e r n a l c a p a c i t o r a f t e r a l l . In f a c t the d e t e c t i n g system can e a s i l y f o l l o w the 50 nsec r i s e and 150 nsec decay of the f l a s h lamp. Moreover, the r i s e times of the phosphorescence does vary with temperature. A l l these r u l e out the p o s s i b i l i t y of having i n s t r u m e n t a l d i s -t o r t i o n s . 98 For i m p u r i t y t o cause a r i s e i n t h e decay c u r v e , t h e o n l y way can be thought about i s t h a t t h e i m p u r i t i e s absorb l a r g e p a r t o f t h e e x c i t a t i o n r a d i a t i o n and t r a n s f e r i t s l o w l y t o the C r ( I I I ) complex under s t u d y . J u d g i n g from the a b s o r -bances, i t i s h i g h l y i m p r o b a b l e t o have so much o f i m p u r i t i e s i n the sample. Of c o u r s e , i m p u r i t i e s w i t h f a s t decay and h i g h e m i s s i o n quantum y i e l d can cause a f a s t e r decay component b e f o r e the s l o w e r phosphorescence decay o f hexacyanochromate(ITI) i o n . However, c a l c u l a t i o n s based on some r e a s o n a b l e assumptions show t h a t the amount of i m p u r i t y needed i s s t i l l t o o h i g h t o be p r o b a b l e i n t h e compound used. I t i s t h e r e f o r e b e l i e v e d t h a t t h e i n i t i a l r i s e and the i n i t i a l f a s t component are i n t r i n s i c phenomena of t h e C r ( I I I ) complexes. R e s u l t s I t was found t h a t a l l t h e phosphorescence i n t e n s i t y -time c u r v e s c o u l d be f i t t e d s a t i s f a c t o r i l y t o t h e f o l l o w i n g e q u a t i o n I = A [ e x p ( - t / x ) + a e x p ( - t / x )] (7.1) p x 2 where x i s the l i f e t i m e o f the E s t a t e , x i s the l i f e t i m e P g ' x 2 of a s t a t e p r e c e d i n g the E s t a t e as w i l l be shown i n the next s e c t i o n . Both A and a are p r o p o r t i o n a l i t y c o n s t a n t s . A i s a r b i t r a r y . The v a l u e s of a depend on the w a v e l e n g t h m o n i t o r e d . The d a t a from, l i q u i d n i t r o g e n t e m p e r a t u r e measure-ments are c o l l e c t e d i n T a b l e IV. Temperature dependence of 99 T x has a l s o been s t u d i e d f o r [Cr(NCS) g] , [Cr(NHg) 2(NCS)^]~, + 3 [Cr(en)^] , and C r ( a c a c ) ^ ; and the r e s u l t s are presented i n Fi g u r e s 29, 30, and 31. Table IV Rate Constants (sec "*") i n Luminescence Decay of C r ( I I I ) Complexes Complex (1/T ) ' p x l 0 ~ 3 (1/T F) x l O - 4 k^ (calcd) x l O " 4 a + + 4> fxl0 4 [Cr (NH3) ? (NCS) 4 ] ~ 3.0 13 30 -0. 38 6 t C r ( N C S ) g ] " 3 0.23 12 20 -0.47 2 [ C r ( C N ) 6 ] ~ 3 0.29 2.0 10 + 1.6 3 C r ( a c a c ) 3 2 . 8 12 10 -0.40 2 [ C r ( t n ) 3 ] + 3 9.3 13 — -0.47 -[ C r ( e n ) 3 ) + 3 10.0 8.9 23 -0.38 3 The v a l u e s of a depend on the wavelength monitored. The values r e p o r t e d are at t h e i r phosphorescence maxima. The accuracy i n e v a l u a t i n g x x depends very much on the noise l e v e l of the curve. At l i q u i d n i t r o g e n temperature, the values c a l c u l a t e d f l u c t u a t e w i t h i n 40%, wh i l e at h i g h e r tem-p e r a t u r e s , they can f l u c t u a t e as much as 100%. U s u a l l y the average value of x x at each temperature was determined from three o s c i l l o g r a m s . : 102 log (1/T X) 4.0 5.0 6.0 7.0 -3 Figure 31. T o f [Cr(NCS),] as a function of temperature. 103 D i s c u s s i o n 4 S i n c e t h e c o m p l e x i s m a i n l y pumped i n t o t h e T a n d 4 T^ g s t a t e s i n t h e p u l s e e x c i t a t i o n , t h e p o s s i b l e known e l e c -2 4 4 2 t r o n i c s t a t e s p r e c e d i n g t h e E s t a t e a r e T, , T_ , T„ , 3 g l g ' 2g' 2g' 2 4 2 and T, s t a t e s . The T n and T„ s t a t e s c a n be c o n f i d e n t l y l g l g 2g r u l e d o u t as t h e c o m p a r a t i v e l y l o n g - l i v e d s t a t e ( c a . 10 y s e c ) . I f t h e y -were, t h e n e m i s s i o n s w o u l d h a v e o c c u r r e d a t w a v e l e n g t h s much s h o r t e r t h a n t h e p h o s p h o r e s c e n c e . The t r a n s i t i o n b e t w e e n 2 2 T, and E s t a t e s h a s b e e n assumed t o be v e r v f a s t . T h e r e -l g g f o r e t h e f o l l o w i n g d i s c u s s i o n w i l l be b a s e d on t h e a s s u m p t i o n 4 t h a t t h e T„ s t a t e i s t h e l o n e r - l i v e d p r e c u r s o r f r o m w h i c h t h e 2g 2 e x c i t a t i o n e n e r g y i s t r a n s f e r t o t h e E g s t a t e . H o wever, a 9 4 m e c h a n i s m c o n t a i n i n a T n i n s t e a d o f T„ as t h e l o n g - l i v e d l g 2g p r e c u r s o r w i l l be d i s c u s s e d i n C h a p t e r V I I I . 4 2 I f T„ i s t h e l o n e r - l i v e d p r e c u r s o r t o t h e E s t a t e , 2g - ' g t h e n a c c o r d i n g t o t h e d e r i v a t i o n s i n t h e A p p e n d i x , t h e i n t e n -s i t y o f f l u o r e s c e n c e and o f p h o s p h o r e s c e n c e a t any one wave-l e n g t h as a f u n c t i o n o f t i m e a f t e r a p u l s e e x c i t a t i o n c a n be e x p r e s s e d as I f = A f e x p ( - t / x f ) (7.2) I p . = A [ e x p ( - t / T ) - e x p ( - t / x f ) ] (7.3) E q u a t i o n 7.3 d o e s n o t a g r e e w i t h t h e e m p i r i c a l E q u a t i o n 7.1, b e c a u s e t h e v a l u e o f a i s n o t g e n e r a l l y -1. T h e r e a r e two p o s s i b l e e x p l a n a t i o n s : (1) b o t h f l u o r e s c e n c e and p h o s p h o r -e s c e n c e a r e i n f a c t d e t e c t e d a t t h e w a v e l e n g t h m o n i t o r e d , 1 0 4 t h e r e f o r e the t o t a l i n t e n s i t y i s a c t u a l l y I = A { e x p ( - t / T ) + [(A f/A ) - l ] e x p ( - t / x f ) } (7.4) thus a = (A f/A p) - 1 (7.5) Since A.F and A are p o s i t i v e p r o p o r t i o n a l i t y c o n s t a n t s , a must P 2 be equal to or g r e a t e r than - 1 , as observed. (2) The E g s t a t e i s populated i n p a r t through some very f a s t t r a n s i t i o n s 4 -y 2 and i r i p a r t through the slower d i r e c t T~ E t r a n s i t i o n . 2g g There f o r e , the instantaneous i n t e n s i t y can be expressed as I = A [ e x p ( - t / T ) - e x p ( - t / T ) ] + A ' ' e x p ( - t / T ) (7.6) y.j t-J J_ yj t*j A = (A' + A'') [exp(-t/x ) j P , , e x p ( - t / r f ) ] (7.7) P P P (A + A ) 1 p p t h e r e f o r e a = -A ' / ( A ' + A'') (7.8) P P P • Since both A' and A''are p o s i t i v e c o n s t a n t s , a can onl y be P P J between 0 and - 1 . U n f o r t u n a t e l y , t h i s e x p l a n a t i o n i s inade-_ 3 quate f o r [Cr(CN)g] which has a p o s i t i v e a. A s l i g h t m o d i f i e d scheme of t h i s e x p l a n a t i o n w i l l be d i s c u s s e d again i n Chapter IX. I f f l u o r e s c e n c e and phosphorescence are both d e t e c t e d at the same wavelength, by making use of Equations 7.2 and 7.3, the s t e a d y - s t a t e emission i n t e n s i t i e s can be r e l a t e d as I f V S S I f ( t ) d t T f A f I A / I ( t ) d t x A - A. p X o p P P f f 105 (7.9) = ( T F A f ) / ( T p A p) (,r p » T F ) (7.10) Since T , T F , and . (A f/A ) can be obtained through Equations 7.1 and 7.5, the r a t i o of the s t e a d y - s t a t e i n t e n s i t y of f l u o r e s c e n c e to t h a t of phosphorescence can be estimated from Equation 7.10. As a l l the emission s p e c t r a of C r ( I I I ) com-plexes are r e a d i l y a v a i l a b l e , the f l u o r e s c e n c e s p e c t r a can be roughly c o n s t r u c t e d . F i g u r e 32 shows the f l u o r e s c e n c e spec--3 trum of [Cr(CN) g] by t h i s technique. I f Equation 7.10 i s f u r t h e r i n t e g r a t e d with r e s p e c t to the wavelength, X, then 4> f T F A - / — — dX (7.11) P P A p Phosphorescence quantum y i e l d s of C r ( I I I ) complexes have been 18 given by F o r s t e r , t h e r e f o r e by making use of Equation 7.11, the f l u o r e s c e n c e quantum y i e l d can be very roughly estimated. The c a l c u l a t e d values are l i s t e d i n Table IV. They r e p r e s e n t minimum values because the f l u o r e s c e n c e s p e c t r a may be broader. For a l l the complexes s t u d i e d / the f l u o r e s c e n c e maximum occurs near the phosphorescence maximum; and t h e r e f o r e f a r _3 awav from the a b s o r p t i o n maximum. [Cr(CN)^] r e p r e s e n t s the extreme case to have the s e p a r a t i o n between the a b s o r p t i o n and f l u o r e s c e n c e maximum w e l l over 14,000 cm x . The e x t r a -o r d i n a r y Stokes 1 s h i f t i m p l i e s t h a t the e q u i l i b r i u m n u c l e a r 800 850 900 (nm) F i g u r e 32. L u m i n e s c e n c e s p e c t r a o b t a i n e d f r o m d e c a y c u r v e s f o r [ C r ( C N ) g ] -3 i n r i g i d g l a s s s o l u t i o n a t 77°K. The u p p e r c u r v e i s f l u o r e s c e n c e ; t h e l o w e r i s p h o s p h o r e s c e n c e w i t h d e t a i l s o f t h e s p e c t r u m s k e t c h i n from s t e a d v - s t a t e measurements o 107 4 c o n f i g u r a t i o n o f t h e s t a t e i s i n d e e d v e r y d i f f e r e n t f r o m t h a t o f t h e g r o u n d s t a t e . T h i s i s a l s o s u p p o r t e d by t h e e v i -d ence f r o m t h e s t u d i e s o f t h e t h e r m a l l y a c t i v a t e d back i n t e r -s y s t e m c r o s s i n g , w h i c h i n d i c a t e s t h a t t h e z e r o - v i b r a t i o n a l 4 l e v e l s a r e v e r y f a r b e l o w t h e a b s o r p t i o n maxima o f t h e T„ 2g s t a t e ( see C h a p t e r s V and V I I I ) . B e c a u s e t h e f l u o r e s c e n c e was n o t d e t e c t e d i n s t e a d y -s t a t e measurements f o r t h e s e c o m p l e x e s , i t has b e e n w i d e l y 4 h e l d t h a t t h e l i f e t i m e o f t h e T» s t a t e s h o u l d be s h o r t e r 2g _ Q 8 t h a n 10 s e c . From d a t a g i v e n by F o r s t e r , t h e i n t r i n s i c f l u o r e s c e n c e r a t e c o n s t a n t , k^, c a n be c a l c u l a t e d u s i n g t h e 4 4 o s c i l l a t o r s t r e n g t h s , f , f o r t h e t r a n s i t i o n T_ ->• T~ 2g 2g a c c o r d i n g t o k x . n 2 f v 2 / 1 . 5 (7 .12) a s s u m i n g v t o be a t t h e same w a v e l e n g t h as t h e p h o s p h o r e s c e n c e maximum and n = l . The c a l c u l a t e d v a l u e s o f k^ a r e i n c l u d e d i n T a b l e IV. I n e v e r y c a s e k^ i s l a r g e r t h a n t h e o b s e r v e d 1/T^, where i t s h o u l d i n f a c t be s m a l l e r , s i n c e k l = * f / T f (7 .13) The v a l u e s f o r t h e o b s e r v e d k^ ( a c c o r d i n g t o E q u a t i o n 7 .13) a r e a l s o i n c l u d e d i n T a b l e IV. I n g e n e r a l , t h e o b s e r v e d k^ . i s a b o u t f o u r o r d e r s o f m a g n i t u d e s m a l l e r than, t h e c a l c u l a t e d one ( a c c o r d i n g t o E q u a t i o n 7 . 1 2 ) . The l a r g e d i s c r e p a n c y no d o u b t r e f l e c t s t h e e r r o r s i n h e r e n t i n t h e a p p l i c a t i o n o f 22 E a u a t i o n 7 . 1 2 . Adamson has d e s c r i b e d i n d e t a i l t h e a r g u -108 ments f o r the d i s p a r i t y based on the expected d i f f e r e n c e i n 4 4 e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n s of the ^2q a n (^' A 2 g s t a t e s . S t r i c t l y speaking, Equation 7.12 i s a p p l i c a b l e o n l y to atomic systems, whose t r a n s i t i o n s are sharp. Although a m o d i f i e d equation of S t r i c k l e r and Berg has been proposed f o r broad 6 6 molecular bands when the t r a n s i t i o n i s s t r o n g l y allowed, there are s t i l l no equations d e r i v e d f o r the f l u o r e s c e n c e which i s symmetry-forbidden and has a l a r g e Stokes' s h i f t . B i r k s 6 7 and Dyson have shown i n the diphenylpolyene s e r i e s t h a t k ^ ( o b s . ) / k ^ ( c a l . ) p r o g r e s s i v e l y decreases as the Stokes' s h i f t becomes l a r g e r and l a r g e r , t h a t i s , the lowest e x c i t e d s t a t e becomes d i s t o r t e d more and more from the ground s t a t e . The e x t r a o r d i n a r i l y s m a l l v a l u e f o r k ^ ( o b s . ) / k ^ ( c a l . ) i n these C r ( I I I ) complexes seems c o n s i s t e n t with the l a r g e Stokes' s h i f t . The C r ( I I I ) complexes with r e l a t i v e l y s m a l l lODq v a l u e 20 have been known to f l u o r e s c e . The f l u o r e s c e n c e l i f e t i m e s 57 of them have r e c e n t l y been measured c o n v e n t i o n a l l y by Zander, the r e s u l t s a t l i q u i d n i t r o g e n temperature are reproduced i n Table V. The f l u o r e s c e n c e quantum y i e l d s f o r these complexes have not been r e p o r t e d , but i f we assume them to be about 0.01, then k ^ ( o b s . ) / k ^ ( c a l . ) s t i l l w i l l be one or two orders of magnitude l e s s than u n i t y , although not so much l e s s as i n the group of C r ( I I I ) complexes d e s c r i b e d i n t h i s work. I t i s known t h a t the Stokes' s h i f t i n c r e a s e s as lODq i n c r e a s e s , 109 T a b l e v F l u o r e s c e n c e L i f e t i m e s a t l i q u i d N i t r o g e n T e m p e r a t u r e Complex x 10 s e c [ C r ( a t p ) g ] ( C 1 0 3 ) 3 20 [ C r ( a t p ) g ] ( C 1 0 4 ) 3 6.5 [ C r ( a t p ) g ] I 3 6.0 [ C r ( u r e a ) g ] ( C 1 0 4 ) 3 0.36 C r C l 3 4.8 [CrFg] ( N H 4 ) 3 1.4 t h e r e f o r e , i t i s e x p e c t e d t h a t t h e v a l u e o f k ^ ( o b s . ) / k ^ ( c a l . ) f o r t h e complexes w i t h s m a l l e r lODq v a l u e s h o u l d be g r e a t e r t h a n t h a t f o r t h e one w i t h l a r g e r lODq v a l u e . T h i s c e r t a i n l y i s t h e o b s e r v a t i o n . A l t h o u g h we c a n n o t be c e r t a i n t h a t t h e new l i f e t i m e , 4 x , i s r e a l l y t h e l i f e t i m e o f t h e T_ s t a t e , t h e o b s e r v a t i o n s • x - 2g + 3 f r o m t h e s t u d i e s o f [ C r ( u r e a ) g ] a t v e r y low t e m p e r a t u r e s 52 (below 25°K) by D i n g l e do g i v e some s u p p o r t . The e m i s s i o n -1 4 from t h e 14,222 cm r e g i o n ( T 2 g ?) has a f a s t d e c a y compon-e n t (prompt f l u o r e s c e n c e ?) o f T ^ 50 y s e c and a s l o w compon-e n t ( d e l a y f l u o r e s c e n c e ?) o f T ^300-400 y s e c , w h i l e e m i s s i o n 110 -1 2 f r o m 14,196 cm r e g i o n ( E ) d o e s n o t r e a c h a maximum u n t i l 50 - 70 y s e c a f t e r t h e p u l s e . A l t h o u g h no d e t a i l e d q u a n t i t a -2 t i v e a n a l y s i s was g i v e n , t h e b u i l d - u p p r o c e s s o f t h e E g s t a t e seems t o c o r r e s p o n d v e r y w e l l w i t h t h e p r o m p t d e c a y o f t h e 4 T„ s t a t e . 2g A l t h o u g h t h e d e t a i l e d m e c h a n i s m s t i l l r e m a i n s unknoi^n, 4 t h e T_ s t a t e h a s b e e n shown t o be c h e m i c a l l v r e a c t i v e ( s e e 2g C h a p t e r I V ) . From t h i s t h e l i f e t i m e o f ^ T 2 g s t a t e w o u l d be -7 -1 e x p e c t e d t o be l o n g e r t h a n 10 s e c The i n t e r n a l c o n v e r s i o n r a t e c o n s t a n t , k^, h a s b e e n shown, w i t h o u t u s i n g any o f t h e q u a r t e t s t a t e p a r a m e t e r s , t o be s t r o n g l y d e p e n d e n t on t e m p e r a t u r e a t t h e h i g h e r t e m p e r a t u r e 4 r a n g e . T h e r e f o r e t h e l i f e t i m e o f t h e T„ s t a t e m t h i s 2g r e g i o n t o o s h o u l d be s t r o n g l y d e p e n d e n t on t e m p e r a t u r e . The r e s u l t s o b t a i n e d i n t h i s c h a p t e r s u p p o r t t h i s c o n c l u s i o n . CHAPTER VI I I THE PRIMARY PHOTOPROCESSES The i n t e r s y s t e m c r o s s i n g r a t e constant, k^, can be evalu a t e d a c c o r d i n g to the f o l l o w i n g equation: k 4 = W T f I f we assume t h a t the newly observed l i f e t i m e , T , i s the f l u o r e s c e n c e l i f e t i m e , then with the r e s u l t s f o r d>. ' f Y1 s c and r e p o r t e d i n Chapters VI and VII r e s p e c t i v e l y , k^ as a f u n c t i o n of temperature can be ob t a i n e d . The r e s u l t s are shown i n F i g u r e s 33,3 4,35, and 36. I t has to be noted t h a t the absolute v a l u e , due to the u n c e r t a i n t i e s i n A. and ^ i s c can only be ac c u r a t e , at b e s t , to w i t h i n 100% f o r [ C r ( e n ) 3 ] + 3 and C r ( a c a c > 3 and w i t h i n 50% f o r [Cr(NH^)^(NCS) 4 3 ~ and [Cr(CN),] . The curves so obtained f o r k. , i . e . , k. 6 i s c ' 4 as a f u n c t i o n of temperature are found to have the f o l l o w i n g form: k 4 = k c + k d (8.2) = s cexp(-E c/RT) + s^exp(-E d/RT) (8.3) The b e s t values of the parameters f o r the fou r complexes are c o l l e c t e d i n Table VI, where the Arr h e n i u s parameters f o r k^ are a l s o i n c l u d e d f o r convenience. The temperature dependence of i n t e r s y s t e m c r o s s i n g i n some aromatic compounds, e.g., anthracene, naphthalene, pyrene, 114 1,000/T (°K x ) I I 4.0 5.0 6.0 7.0 F i g u r e 35. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f [ C r f N C P ) ^ ] as a f u n c t i o n o f t e m p e r a t u r e . A c c o r d i n g t o Mechanism I . u Ul •H M 0-O 4.0 5.0 6.0 7.0 F i g u r e 36. I n t e r s y s t e m c r o s s i n g r a t e c o n s t a n t o f t r a n s - [ C r ( N H ^ ) ^ ( N C S ) ^ f u n c t i o n o f t e m p e r a t u r e . A c c o r d i n g t o M e c h a n i s m I . T a b l e VI The A r r h e n i u s P a r a m e t e r s o f k. b a s e d on Mechanisms I and I I . The i s c f r e q u e n c y f a c t o r s a r e i n s e c _ l and t h e a c t i v a t i o n e n e r g i e s i n K c a l / m o l . C o m p l e x s a E a S b - Eb s c F C s d d f C r ( N H 3 ) ? ( N C S ) 4 ] ~ 4. 3 x l 0 3 0 .08 7 13 .2x10 9.0 1. 2 x l 05 0 3 . I x l O 1 2 7.0 C r ( a c a c ) 3 6 . 2 x l 0 3 0 .18 6 14 . 9 x l 0 X 4 7.7 7. 4 1x10 0 .20 2 . 6 x l 0 1 5 7.8 [ C r ( N C S ) 6 ] " 3 4. 6x10 2 0 .19 2 . I x l O 1 4 7.9 7. 9 x l 0 3 0 .20 1 12 .0x10 7.5 [ C r ( e n ) 3 ] + 3 1. 4 6x10 0 .10 1 . 5 x l 0 1 3 10 .2 2. I x l O 5 0 .26 3 . 6 x l 0 6 3.3 117 and t h e i r d e r i v a t i v e s has been s t u d i e d by s e v e r a l r e s e a r c h 6 8 73 groups. I t was found t h a t t h e i r i n t e r s y s t e m c r o s s i n g r a t e constants c o n s i s t e d of both temperature independent and dependent components. The former was assigned to the d i r e c t i n t e r s y s t e m c r o s s i n g from the lowest e x c i t e d s i n g l e t s t a t e to the lowest t r i p l e t s t a t e w h i l e the l a t t e r from the same s t a t e to a h i g h e r t r i p l e t s t a t e . S i m i l a r l y , i n the C r ( I I I ) complexes, the process k c can be assigned to the i n t e r s y s t e m c r o s s i n g from the lowest 4 v i b r a t i o n a l l e v e l s of the T„ s t a t e d i r e c t l y to the l s o e n e r -2g 2 2 g e t i c v i b r o n i c l e v e l s of the T, and/or E s t a t e s . And the s t r o n g l y temperature dependent process may correspond to the i n t e r s y s t e m c r o s s i n g v i a h i g h v i b r a t i o n a l l e v e l s (higher than the z e r o - v i b r a t i o n a l l e v e l by an average of E^) 4 2 ' of the T» s t a t e to the T_ s t a t e . Zq Zq In the f o l l o w i n g s e c t i o n s , the o v e r a l l m e c h a n i s t i c schemes i n the C r ( I I I ) complex are d i s c u s s e d w i t h a l l the a v a i l a b l e i n f o r m a t i o n . Mechanism I ... In t h i s mechanism l e t us assume t h a t process k i s the cl 2 i n t e r s v s t e m c r o s s i n g from the E s t a t e to the around s t a t e g 4 A 2 c t , process k^ the t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g 2 4 from the E • to T„ s t a t e , process k the i n t e r s v s t e m c r o s s -g 2g - c • 4 2 m a from the T„ to E s t a t e , and nrocess k, i n t e r s v s t e m 2g a ' d -4 2 ' c r o s s i n a from the T~ to T„ s t a t e (see F i g u r e 37). A l l Zq Zq 118 Franck-Condon State s \ k 3 \ \ 2g / / • d f a s t •4— k \ ' \ k b k r •I 2g T ' 7 J ! H 4, l2g Figure 3 7 . Schematic of Mechanism I. 119 the f o u r processes are assumed t o proceed v i a a t u n n e l l i n g mechanism. 4 A c c o r d i n g l y , the z e r o - v i b r a t i o n a l l e v e l of the 1^^ s t a t e can be expressed as W 4 V = W 2 V + E b ( 8 - 4 ) 2 and the l o c a t i o n of the T« s t a t e can be estimated by 2g J v ( 2 T 2 g ) = v 0 _ 0 ( 4 T 2 g ) + E d (8.5) 2 The z e r o - v i b r a t i o n a l l e v e l s of the E s t a t e are mostlv g — 4 — 2 known. The values estimated f o r v r t n ( T_ ) and v( T„ ) are 0-0 2g 2g l i s t e d i n Table V I I . Since the z e r o - v i b r a t i o n a l l e v e l s of 4 the T_ s t a t e s of these C r ( I I I ) complexes have never been 2g observed s p e c t r o s c o p i c a l l y , a d i r e c t c o n f i r m a t i o n of t h i s 2 assignment i s out of the q u e s t i o n . However, the T 2 g band 74 — 2 has been observed i n some C r ( I I I ) complexes, v( T 2 g ) l i e s from 18,000 cm 1 to 21,000 cm x . Wi t h i n experimental e r r o r s , the values l i s t e d i n Table VII f a l l q u i t e w e l l w i t h i n the r e g i o n . In s p i t e of the s a t i s f a c t o r y argument based on ener-g e t i c c o n s i d e r a t i o n s f o r t h i s mechanism, there are some d i f -f i c u l t i e s with i t too. F i r s t l y , the frequency f a c t o r s , s c and s^, supposedly being the t r a n s i t i o n p r o b a b i l i t i e s between 4 2 the i s o e n e r a e t i c l e v e l s of the T n and E s t a t e s and of 2g • a 4 2 . the T„ and T„ s t a t e s , d i f f e r by as much as seven orders 2g 2g 1 2 0 of magnitude. In the aromatic compounds s t u d i e d , the frequency f a c t o r s of the two components of i n t e r s y s t e m c r o s s i n g are of 2 comparable magnitude. Since the T 2 g s t a t e l i e s not much c l o s e r to the 4 T „ (vn A) than the 2 E s t a t e (comparing E, and Zg U-U g 1 ct E, ) , the f a c t t h a t s, i s much more e f f i c i e n t than s i s not p a c 6 0 7 6 a p p l i c a b l e on the energy gap h y p o t h e s i s . ' Table VII 4 2 The p r e d i c t e d l o c a t i o n s of the T„ and T„ s t a t e s 2 g 2 g a c c o r d i n g to Mechanism I and I I . Wave number i n kK Complex V o ( 2 V Mechanism Vo^V I Mechanism,II t r a n s - [ C r ( N H ^ ) 2 ( N C S ) 4 ] ~ 1 3 . 3 3 1 6 . 5 1 8 . 9 1 4 . 0 [ C r ( C N ) g ] ~ 3 1 2 . 3 8 1 5 . 2 [ C r ( N C S ) g ] " 3 1 2 . 8 5 1 5 . 6 1 8 . 2 1 3 . 0 [ C r ( e n ) 3 ] + 3 1 4 . 9 8 1 8 . 6 1 9 . 7 1 7 . 4 Cr (acac)^ . 1 2 . 8 4 1 5 . 5 1 8 . 3 1 2 . 8 I t i s con s i d e r e d to be due to the f a c t t h a t the doublet 2 2 2 s t a t e s , E , T, , and T„ , have n e a r l y the same e q u i l i b r i u m ' a . l a 2 g ' 1 l 4 n u c l e a r c o n f i g u r a t i o n i ^ i t h the ground s t a t e A 2 a , while the 4 T 2 c t s t a t e i s s t r o n g l y d i s t o r t e d from the ground s t a t e . There-4 — 2 f o r e , the intersystem. c r o s s i n g from T„ {vn n) to the T, and ' - - 2a 0 - 0 l g 2 E g s t a t e s i s s t r o n g l y f o r b i d d e n by the Frank-Condon p r i n c i p l e , 2 On the other hand, because of the T_ s t a t e and the Frank-2g 4 Condon s t a t e ( a b s o r p t i o n maximum) of the T,, s t a t e are c l o s e together i n both energy and n u c l e a r geometry, the i n t e r a c t i o n between them i s expected to be l a r g e , consequently, the r a d i a t i o n l e s s t r a n s i t i o n p r o b a b i l i t y between them i s h i g h . U n f o r t u n a t e l y , more s e r i o u s d i f f i c u l t y comes from the other p a i r of frequency f a c t o r s , s c and s^, supposedly being the forward and r e v e r s e t r a n s i t i o n p r o b a b i l i t i e s of the i n t e r s y s t e m c r o s s i n g between the z e r o - v i b r a t i o n a l l e v e l of 4 2 the T_ s t a t e and the i s o e n e r g e t i c l e v e l s of the E s t a t e . 2g J cr They are expected t o be of the same magnitude, but they are i n f a c t d i f f e r e n t by from seven to ten orders of magnitude from each ot h e r . Although r a d i a t i o n l e s s t r a n s i t i o n s between two e l e c t r o n i c s t a t e s which d i f f e r very much i n e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n have never been explored t h e o r e t i c a l l y or e x p e r i m e n t a l l y , the extreme l a r g e d i s c r e p a n c y between the forward and r e v e r s e t r a n s i t i o n s observed on t h i s mechanism may be p o s s i b l e but can h a r d l y be reasonable. Mechanism II In t h i s mechanism, to o b v i a t e the d i f f i c u l t i e s i n Mechanism I, we assume (see F i g u r e 38) t h a t processes k^ and k. are the forward and r e v e r s e t r a n s i t i o n s of the t h e r m a l l y b a c t i v a t e d i n t e r s y s t e m c r o s s i n g and assume they do not pro-ceed v i a a t u n n e l i n g mechanism but i n s t e a d v i a T e l l e r c r o s s i n g 122 through the i n t e r s e c t i o n b o u n d a r i e s of t h e m u l t i - d i m e n s i o n e d 4 2 p o t e n t i a l energy s u r f a c e s of t h e and E s t a t e . Of 2g g c o u r s e , p r o c e s s k c i s s t i l l assumed t o be t h e t u n n e l i n g i n t e r -system c r o s s i n g . I t has t o be noted t h a t t h e t u n n e l i n g p r o -cess and T e l l e r c r o s s i n g have not been r e p o r t e d t o be compe-t i t i v e i n t h e same m o l e c u l e f o r any o t h e r compound s t u d i e d . But t h e o r e t i c a l l y t h e r e i s no r e a s o n why they s h o u l d n ot be. A c c o r d i n g t o t h i s mechanism, t h e z e r o - v i b r a t i o n a l l e v e l 4 o f the ^2q s t a i : e l S n o w e x p r e s s e d as V Q - C / V = W 2 E g ) + E b " E d ( 8 - 6 ) The v a l u e s so o b t a i n e d f o r vn „( 4T_ ) are a l s o l i s t e d i n 0-0 2g T a b l e V I I . The o n l y d i f f i c u l t y v / i t h t h i s mechanism i s t h a t i t r e -4 c m i r e s the l o c a t i o n o f t h e z e r o - v i b r a t i o n a l l e v e l o f the T„ 2g s t a t e t o be v e r y low i n energy. However, t h i s i s not i m p o s s i b l e , — 4 as a m a t t e r o f f a c t , t h e l o c a t i o n o f v . „( T_ ) has never been (J-U Zq d e t e r m i n e d , f o r t h e s e complexes. Now the f r e q u e n c y f a c t o r s s^ and s^ a r e o f comparable magnitude a s ' t h e y s h o u l d be a c c o r d i n g t o t h i s mechanism. I t i s i n t e r e s t i n g t o note t h a t t h e energy s e p a r a t i o n 4 2 55 between the T„ and E s t a t e s e s t i m a t e d from d e l a y e d zq g f l u o r e s c e n c e : i s the d i f f e r e n c e i n the a c t i v a t i o n e n e r g i e s o f the forv/ard and r e v e r s e i n t e r s y s t e m c r o s s i n g . The v a l u e s found f o r ruby and emerald agree w i t h t h e v a l u e s o b t a i n e d s p e c t r o s c o p i c a l l y (see Ref. 18) . I f the l i f e t i m e s o f t h e s e \ k 3 1 2g i n t e r s e c t i o n / I / \ s k k F i g u r e 38. Schematic of Mechanism II 'Eg 2g 124 compounds a r e s t u d i e d , t h e component c o r r e s p o n d i n g t o t h e t h e r m a l l y a c t i v a t e d i n t e r s y s t e m c r o s s i n g may have an a c t i -v a t i o n e n e r g y w h i c h i s E ^ h i g h e r t h a n t h e e n e r g y s e p a r a t i o n 4 2 between t h e T„ and E s t a t e p r e d i c t e d f r o m t h e d e l a y e d 2g . g f l u o r e s c e n c e s t u d i e s . Mechanism I I I I n t h i s mechanism (see F i g u r e 3 9 ) , we assume t h a t t h e 2 newlv f o u n d l i f e t i m e , T , t o be t h e l i f e t i m e o f t h e T, s t a t e , x l g Now l e t us decompose T i n t o two components - x 1/T = k + k (8.8) ' x y z = s e x p ( - E /RT) + s z e x p ( - E z / R T ) (8.9) The v a l u e s f o r s , E , s , and E a r e c o l l e c t e d i n T a b l e V I I I . y y z z B e c a u s e o f t h e d i f f i c u l t y i n t h e measurement o f T , s i s J- x y u n c e r t a i n by ± 50%, and s z i s known o n l y w i t h i n two o r d e r s o f m a g n i t u d e , E ^ i s ± 0.0 3 K c a l / m o l , and E z ± 1 K c a l / m o l . 2 2 S i n c e t h e E and T, s t a t e s a r e assumed t o be popu-g l g v • 2 2 l a t e d p r o m p t l y and t h e t r a n s i t i o n T ^ g • E g i s t h e s l o w r a t e - d e t e r m i n i n g s t e p , t h e mechanism a u t o m a t i c a l l y f i t s E q u a t i o n 7.1 v / i t h a between 0 and -1 ( r e f e r t o E q u a t i o n s 7.6, 7.7, and 7 . 8 ) . However, i n o r d e r t o e x p l a i n t h e p o s i t i v e a -3 f o r [ C r ( C N ) g ] and t h e f a c t t h a t a c h a n g e s w i t h w a v e l e n g t h , 2 i t i s n e c e s s a r y t o assume t h a t p h o s p h o r e s c e n c e f r o m t h e s t a t e i s a l s o d e t e c t e d . U n f o r t u n a t e l y , t h e s p e c t r a so a t t r i -2 b u t e d t o t h e p h o s p h o r e s c e n c e (see C h a p t e r V I I ) a r e Figure 39. Schematic of Mechanism I I I . 126 l o c a t e d near or even at longer wavelengths than those of the 2 2 E phosphorescence. A l a r g e Stokes' s h i f t i n the T, Dhos-g - ^ l g 2 phorescence i s u n l i k e l y (because t h a t of the E g phosphores-cence i s only about 100 cm "*") but i s not i m p o s s i b l e . 2 D e p l e t i o n of the T.^ s t a t e and f u r t h e r p o p u l a t i o n of 2 the E^ s t a t e are achieved by process k y a t lower temperatures 4 and by process k„ to the T„ s t a t e f o l l o w e d bv rar>id i n t e r -•z c z 2g 2 system c r o s s i n g t o the E g s t a t e a t h i g h e r temperatures. I f we assume process k, and k proceed v i a a t u n n e l i n g b z r mechanism, then v ( 2 T , ) - v ( 2 E ) = E, - E_ (8.10) l g g p z The values f o r ( E b - E z) are 0.4, 0.6, -0.9, and 6.6 Kcal/mol f o r [Cr(NH 3) 2 (NCS) 4 ] ~ , [Cr (NCS) g ] ~ 3 , C r ( a c a c ) 3 , and [ C r ( e n ) , 3 ] + 3 2 r e s p e c t i v e l y . However, the energy s e p a r a t i o n of the J E g and 2 74 T^ g i s known to be about 2 Kcal/mol. The values p r e d i c t e d f o r [Cr(NH_)„(NCS) . ] ~ and [ C r ( N C S ) , ] - mav be c o n s i d e r e d s a t -5 1 4 6 + 3 i s f a c t o r y w i t h i n experimental e r r o r , but those f o r [Cr(en)^] and C r ( a c a c ) 3 can not be. In t h i s mechanism, judging from s^ and s z , the l i f e -4 -10 time of the T„ s t a t e must be at the order of 10 sec or 2g l e s s . In f a c t , K i s l i u k and Moore estimated the l i f e t i m e of 4 -3 54 the T_ m rubv t o be l e s s than 10 sec. But comparing 2g to the f l u o r e s c e n c e - o n l y complexes, the valu e s s p e c u l a t e d are - 7 -10 much s h o r t e r (10 vs 10 s e c ) . Since the only apparent 127 T a b l e V I I I The A r r h e n i u s P a r a m e t e r s o f l / x x Complex S Y E y s z E z ( s e c ) ( K c a l / m o l ) ( s e c ) ( K c a l / m o l ) [ C r ( N H 3 ) 2 ( N C S ) 4 ] " 3 . 4 x l 0 5 0.15 14 1.2x10 8.6 C r ( a c a c ) 3 l . S x l O 5 0.14 17 1.5x10 ' 8.7 [Cr ( N C S ) 3 ] ~ 3 5 . 9 x l 0 4 0.18 14 5.9x10 8.6 [ C r ( e n ) 3 ] + 3 2 . 0 x l 0 5 0.12 1 . 2 x l 0 9 3.6 d i f f e r e n c e between t h e p h o s p h o r e s c e n c e and t h e f l u o r e s c e n c e -o n l y C r ( I I I ) c o m p l e x e s i s t h e p a r t i c i p a t i o n o f i n t e r s y s t e m c r o s s i n g i n t h e f o r m e r , i f we assume t h e s h o r t e n i n g o f t h e q u a r t e t l i f e t i m e f r o m t h e l a t t e r t o t h e f o r m e r i s c a u s e d m a i n l y by i n t e r s y s t e m c r o s s i n g , t h e n we w o u l d e x p e c t t h e d o u b l e t f o r m a t i o n quantum y i e l d t o be u n i t y . The <j>. JL S C o b t a i n e d i n C h a p t e r V I , a c c o r d i n g t o t h i s mechanism, s h o u l d 2 be t h e quantum y i e l d o f t h e E s t a t e f o r m a t i o n . S i n c e 6. 1 J g r i s c i s s u b s t a n t i a l l y l e s s t h a n u n i t y , we a r e l e d t o t h e c o n c l u s i o n 2 t h a t 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 s t a t e t o t h e g r o u n d 2 s t a t e i s v e r y e f f i c i e n t . T h i s i s d i f f e r e n t f r o m t h e E g s t a t e (10^ v s 1 0 4 s e c "*") . From c o n s i d e r a t i o n s o f n u c l e a r 2 2 c o n f i g u r a t i o n and e n e r g y l e v e l , t h e E and T n s t a t e s a r e ^ -— g l g 128 e x p e c t e d t o b e h a v e s i m i l a r l y w i t h r e s p e c t t o t h e g r o u n d - 2 2 s t a t e s . M o r e o v e r , t h e t r a n s i t i o n T.. • E i s t o o s l o w t o l g g b e r a t i o n a l i z e d , a s t h e s e p a r a t i o n b e t w e e n t h e t w o s t a t e s i s ++ s m a l l . I n o r d e r f o r t h e c h e m i c a l r e a c t i o n t o compete w i t h 4 o t h e r p r o c e s s e s i n t h e s t a t e , t h e r a t e c o n s t a n t o f c h e m -i c a l r e a c t i o n m u s t b e l a r g e r t h a n l O 1 ^ s e c 1 . T h e d i f f i c u l t y i s why a r e a c t i o n p r o c e e d i n g s o f a s t i n s o l u t i o n i s n o t s u b -j e c t e d t o t h e c a g e e f f e c t . + 3 2 I n [ C r ( u r e a ) 0 ] , i t i s known t h a t t h e T . s t a t e o l g ' 4 l i e s a b o v e t h e s t a t e i n e n e r g y . T h e r e f o r e , i t i s e x p e c -t e d t h a t a t e x t r e m e l y l o w t e m p e r a t u r e s , t h e i n t e r s y s t e m c r o s -4 2 . . 2 s i n g T 2 g •> T l g m u s t b e n e g l i g i b l e a n d t h e s t a t e t h u s n o t a p p r e c i a b l y p o p u l a t e d . I f s o , t h i s w o u l d r e s u l t i n a p h o s p h o r e s c e n c e c u r v e w i t h o u t a r i s e . I n f a c t , D i n g l e + 3 o b s e r v e d t h e p h o s p h o r e s c e n c e f r o m [ C r ( u r e a ) g ] ~ a t 25°K d i d 52 show a n i n i t i a l r i s e b e f o r e d e c a y . R e c e n t l y , a n e x p e r i m e n t a l u p p e r l i m i t o f 4 n s e c h a s 4 b e e n d e t e r m i n e d f o r t h e l i f e t i m e o f t h e T~ s t a t e o f r u b y . 2g 2 2 A n d i t a l s o h a s b e e n s h o w n t h a t t h e '7. and E s t a t e s a r e l g g p r a c t i c a l l y i n t h e r m a l e q u i l i b r i u m a s i s e v i d e n t f r o m t h e 2 f a c t t h a t t h e p h o s p h o r e s c e n c e o f t h e T ^ g s t a t e s i s o b s e r v e d 25 a t 300°K b u t n o t a t 7 7 ° K . T h i s c o n t r a d i c t s t h e a s s u m p t i o n made i n M e c h a n i s m I I I . H o w e v e r , t h e d i f f e r e n c e i n p r i m a r y p r o c e s s e s b e t w e e n t h e i o n i c a n d m o l e c u l a r C r ( I I I ) c o m p o u n d s s h o u l d n o t be o v e r l o o k e d . + + T h e e n e r g y gap h y p o t h e s i s s h o u l d be v a l i d i f t h e i n i t i a l and f i n a l s t a t e s o f a t r a n s i t i o n a r e n o t t o o d i f f e r e n t i n t h e e c r u i l i b r i u m n u c l e a r c o n f i g u r a t i o n . 129 From t h e above c o n s i d e r a t i o n s , a l t h o u g h i t i s s t i l l 4 f a v o r e d t h a t T i s t h e l i f e t i m e o f t h e T„ s t a t e , i t i s x 2g u n d e n i a b l e t h a t none o f t h e mechanisms p r o p o s e d has been u n a m b i g u o u s l y c o n f i r m e d o r n e g a t e d . Some more r e s e a r c h i s needed t o c l a r i f y t h i s p r o b l e m . -These w i l l be d i s c u s s e d i n C h a p t e r IX. No m a t t e r w h i c h mechanism i s t r u e , on t h e w h o l e , a l l t h e p r i m a r y p r o c e s s e s behave q u i t e " n o r m a l l y " a t low t e m p e r a -t u r e s . B u t as s o o n as t h e t e m p e r a t u r e r i s e s s u f f i c i e n t l y , t h e p r i m a r y p r o c e s s e s s t a r t t o change t h e i r c o u r s e s t o t h e t h e r m a l l y a c t i v a t e d r o u t e s and r e s u l t i n an " a b n o r m a l " and c o m p l i c a t e d m e c h a n i s t i c scheme. CHAPTER IX SOME FINAL REMARKS The primary photoprocesses i n C r ( I I I ) complexes are very d i f f e r e n t i n behavior from those known i n o r g a n i c com-pounds. For example, i n C r ( I I I ) complexes, the i n t r i n s i c f l u o r e s c e n c e l i f e t i m e s are much longer than p r e d i c t e d theore-t i c a l l y , the quantum y i e l d s of i n t e r n a l conversions are l a r g e , and the f l u o r e s c e n t s t a t e s i n s t e a d o f the phosphorescent s t a t e s are d i r e c t p r e c u r s o r s t o photochemical r e a c t i o n s . A l l these can be ex p l a i n e d c o n s i s t e n t l y by the f a c t t h a t the 4 e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n of the T^ s t a t e i s d i s -4 t o r t e d very much from t h a t of the ground s t a t e , A„ . Before i n v e s t i g a t i n g the p o s s i b l e r o l e s the d i s t o r t i o n p l a y s i n the primary processes, l e t us f i r s t study the d i s t o r t i o n as a f u n c t i o n of lODq, the l i g a n d f i e l d s t r e n g t h . The d i s t o r t i o n — 4 — 4 i s q u a l i t a t i v e l v measured by the q u a n t i t y v ( T_ ) - v r t _( T, ~ max Zq 0-0 ^ The arguments and d i s c u s s i o n s i n the f o l l o w i n g s e c t i o n s are based more on i n t u i t i o n than on s o l i d evidence, t h e r e f o r e they should be con s i d e r e d t o be t e n t a t i v e and s p e c u l a t i v e . 9.1 The O r i g i n of the Lowest Quartet S t a t e Three methods are used t o estimate roughly the l o c a t i o n of the z e r o - v i b r a t i o n a l l e v e l of the 4 T „ s t a t e : (1) f o r 2g 131 those complexes which f l u o r e s c e , the o r i g i n i s assumed to be a t the c e n t e r of a b s o r p t i o n and f l u o r e s c e n c e maxima. The necessary data are taken from the review by F l e i s c h a u e r and 17 F l e i s c h a u e r . (2) from the temperature-dependence s t u d i e s of delayed f l u o r e s c e n c e . The r e s u l t s are g i v e n by Camassei and 55 F o r s t e r . (3) From the a c t i v a t i o n e n e r g i e s of the t h e r m a l l y a c t i v a t e d primary processes (see Chapter V I I I ) . F i g u r e 40 4 shows the p l o t of the o r i g i n of the T^ g s t a t e a g a i n s t lODq. I t can be seen t h a t the l o c a t i o n of the o r i g i n i s very i n s e n s i t i v e to lODq. And to our s u r p r i s e , the o r i g i n of the 4 r?2q s t a t e i s lower i n energy f o r complexes with very l a r g e lODq value than f o r those with s m a l l e r lODq v a l u e . This i n -d i c a t e s the inadequacy of the p r e s e n t bonding t h e o r i e s f o r the e x c i t e d e l e c t r o n i c s t a t e s . Ligand F i e l d Theory may w e l l be j u s t i f i e d i n the p r e d i c t i o n of a b s o r p t i o n maxima i n metal complexes because the e x c i t e d v i b r o n i c s t a t e s (Frank-Condon s t a t e s ) reached by the v e r t i c a l t r a n s i t i o n have the same nucle a r c o n f i g u r a t i o n as the ground s t a t e . But i n g e n e r a l , the M o l e c u l a r O r b i t a l T heories a t p r e s e n t stage are r a t h e r impotent to p r e d i c t the o r i g i n s (not to mention the geometry) of the e x c i t e d s t a t e s , because the e q u i l i b r i u m n u c l e a r con-f i g u r a t i o n may be d i f f e r e n t i n d i f f e r e n t e l e c t r o n i c s t a t e s . T h i s i s c e r t a i n l y t r u e i n the case t h a t a bonding or non-bonding e l e c t r o n i s e x c i t e d to an a nti-bonding o r b i t a l . T r a n s i t i o n s from the ground s t a t e to the e x c i t e d q u a r t e t 132 14 16 18 20 22 24 26 lODq (kK) 4 F i g u r e 40. The o r i g i n of the T^^ s t a t e o f C r ( I I I ) complexes as f u n c t i o n of l i g a n c l f i e l d s t r e n g t h . • , Method 1; A , Method 2; and O Method 3, a c c o r d i n g t o Mechanism. I . The 57 broken c i r c l e s a re d a t a from Zanders. The o r i g i n s a re even lower a c c o r d i n g t o Mechanism I I or I I I . The heavy l i n e r e p r e s e n t s the l i m i t i n g c o n d i t i o n t h a t the i n i t i a l and f i n a l s t a t e s have i d e n t i c a l e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n . 133 s t a t e s i n C r ( I I I ) i n v o l v e e x c i t a t i o n of an e l e c t r o n from the bonding o r b i t a l ( t 2 g ^ t o a n a n t i - b o n d i n g o r b i t a l ( e g ) , s t r o n g d i s t o r t i o n of the q u a r t e t e x c i t e d s t a t e s i s thus expected. From F i g u r e 40 i t can a l s o be seen t h a t the d i s t o r t i o n of the 4 2q s * - a t e i n c r e a s e s as lODq i n c r e a s e s . \ 9.2 Tunneling Mechanism and T e l l e r C r o s s i n g The c o m p e t i t i o n between t u n n e l i n g and T e l l e r c r o s s i n g has not been observed f o r r a d i a t i o n l e s s t r a n s i t i o n s i n aro-matic compounds. I t i s b e l i e v e d t h a t t u n n e l i n g i s the more e f f i c i e n t route f o r t r a n s i t i o n s between two e l e c t r o n i c s t a t e s which have the same e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n so t h a t t h e i r p o t e n t i a l energy s u r f a c e s i n t e r s e c t at a very high energy l e v e l or do not i n t e r s e c t a t a l l . The T e l l e r c r o s s i n g can become co m p e t i t i v e with the t u n n e l i n g process i f the two e l e c t r o n i c s t a t e s are q u i t e d i f f e r e n t from each other i n the e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n ; and t h e r e -f o r e the i n t e r s e c t i o n l i n e s of t h e i r p o t e n t i a l energy s u r -faces l i e very low i n energy with r e f e r e n c e to the zero-v i b r a t i o n a l l e v e l of the i n i t i a l e l e c t r o n i c s t a t e . At low temperature, unless the i n t e r s e c t i o n l i n e s go r i g h t through the minimum i n the p o t e n t i a l energy s u r f a c e of the i n i t i a l s t a t e , . t h e t u n n e l i n g process i s always dominant; however, at high temperature, T e l l e r c r o s s i n g g r a d u a l l y stands out. At the i n t e r s e c t i o n boundaries, T e l l e r c r o s s i n g must take p l a c e 134 i n a few v i b r a t i o n s . T h e r e f o r e , the net process i s expected to have an a c t i v a t i o n energy equal to the s e p a r a t i o n between the z e r o - v i b r a t i o n a l l e v e l and the i n t e r s e c t i o n l i n e s ; and 12 13 -1 to have a p r e - e x p o n e n t i a l f a c t o r of about 10 t o 10 sec , th a t i s , about the r a t e constant of the v i b r a t i o n . ; For process k^ and k^, we cannot t e l l whether t r a n s i -t i o n s occur v i a t u n n e l i n g or T e l l e r c r o s s i n g , because h i g h e r e l e c t r o n i c s t a t e s are a v a i l a b l e . But a t l e a s t T e l l e r c r o s s i n g has t o be assumed to occur i n the i n t e r n a l c o n v e r s i o n which a l s o has a t h e r m a l l y a c t i v a t e d component with a l a r g e a c t i v a -t i o n energy. 9.3 Primary Processes and Ligand F i e l d S t rength In t h i s s e c t i o n the e f f i c i e n c i e s of the primary p r o -4 cesses from the s t a t e are d i s c u s s e d xn terms of energy g a p s ^ and b a r r i e r w i d t h s 7 ^ between the i n i t i a l and the f i n a l s t a t e s of the t r a n s i t i o n s when they take p l a c e v i a t u n n e l i n g mechanism; and i n terms of the energy s e p a r a t i o n s between the i n t e r s e c t i o n s and the o r i g i n s of the i n i t i a l s t a t e s when they take p l a c e v i a T e l l e r c r o s s i n g . 4 Since the o r i g i n of the T„ s t a t e does not change . 2g much with l i g a n d f i e l d s t r e n g t h , lODq, the more l i k e l y e f f e c t of lODq on the primary processes i s the change i t causes i n 4 the e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n of the T 0 g. s t a t e . 4 With i n c r e a s i n g lODq, the T s t a t e i s g e n e r a l l y d i s t o r t e d more and more from both the lowest doublet s t a t e s and the , 135 ground s t a t e . I t can be seen from the s i m p l i f i e d two-dimen-s i o n a l p o t e n t i a l energy s u r f a c e s t h a t t h i s r e s u l t s i n wider 4 2 2 b a r r i e r widths between the T„ s t a t e and the E and T.. 2g g l g , 4 s t a t e and a narrower b a r r i e r width between the T_ s t a t e 2g and the ground s t a t e . T h e r e f o r e , i n t e r s y s t e m c r o s s i n g s and i n t e r n a l c o n v e r s i o n are expected to decrease and i n c r e a s e r e s -p e c t i v e l y with i n c r e a s i n g lODq. At l i q u i d n i t r o g e n temperature, o n l y i n t e r s y s t e m c r o s -s i n g and i n t e r n a l c o n v e r s i o n are important i n d e p l e t i n g the 4 T^g s t a t e , the quantum y i e l d of i n t e r s y s t e m c r o s s i n g w i l l be s m a l l e r ; and the quantum y i e l d of i n t e r n a l c o n v e r s i o n l a r g e r as lODq i n c r e a s e s . In f a c t , the quantum y i e l d s of i n t e r s y s t e m c r o s s i n g f o r C r ( I I I ) complexes with low lODq v a l u e s are known to be c l o s e to u n i t y , but f o r those w i t h high lODq v a l u e s , they are s u b s t a n t i a l l y l e s s than u n i t y . 9.4 Suggestions f o r F u r t h e r Work Although many problems have been c l e a r e d up i n t h i s work, there are more new q u e s t i o n s i n t r o d u c e d , and the mech-anisms of the primary photoprocesses are f a r more complicated than were expected b e f o r e . Besides being comparatively l e s s 4 explored, the ^2a s * - a t e i s bound to be the focus of r e s e a r c h on C r ( I I I ) complexes i n the near f u t u r e . Three parameters w i l l be widely used: photochemical quantum y i e l d , d o ublet formation quantum y i e l d , and e s p e c i a l l y the l i f e t i m e of the 136 4, work, T^g s t a t e . The f o l l o w i n g are some suggestions f o r f u r t h e r 4 (1) The T^g s t a t e has been proven to be the immediate p r e -c u r s o r t o the p h o t o s u b s t i t u t i o n i n Reineckate i o n , but more C r ( I I l ) complexes have t o be s t u d i e d (by the same technique) to e s t a b l i s h the g e n e r a l i t y of the c o n c l u s i o n . E s p e c i a l l y i n t e r e s t i n g w i l l be the work extended t o those C r ( I I I ) com-+ 2 p l e x e s , f o r example, [Cr (NH^) (NCS) ] ", which e x h i b i t two modes of p h o t o s u b s t i t u t i o n and are thought t o have two d i f f e r 22 27 ent p h o t o r e a c t i v e i n t e r m e d i a t e s . ' (2) The temperature-dependence s t u d i e s of the photochemical quantum y i e l d s of the C r ( I I I ) complexes i n deoxygenated s o l u t i o n s are e s s e n t i a l to understand the r e a c t i v i t y of the 4 4 T_ s t a t e . In the case t h a t the l i f e t i m e of the T„ s t a t e 2g 2g i s a v a i l a b l e , the r a t e constant and the a c t i v a t i o n energy of. the photochemical r e a c t i o n can be e v a l u a t e d . C o r r e l a t i o n of the a c t i v a t i o n energy with the l i g a n d f i e l d s t r e n g t h w i l l 4 giv e us an i n s i g h t i n t o the d e t a i l e d mechanism of the T^^ s t a t e chemistry and w i l l p r o v i d e some c l u e to the r e l a t i o n s h i between s t r u c t u r e and r e a c t i v i t y . 4 In the case t h a t the l i f e t i m e of the T n s t a t e i s not 2g a v a i l a b l e , only the apparent a c t i v a t i o n energy can be e s t i -mated, and i t i s d i f f i c u l t to i n t e r p r e t unambiguously. However, at s u f f i c i e n t l y high temperature, r e p o p u l a t i o n of 4 the T^g s t a t e causes p r a c t i c a l l y complete d e p l e t i o n of the 2 Eg s t a t e ; there are only two e f f i c i e n t pathways to degrade 137 t h e e x c i t a t i o n e n e r g y , t h e r e f o r e , k 3 <f> h o T n = (9.1) C h e m k 2 + k 3 o r k (*chem " X ) = -T- ( 9 ' 2 ) k 3 (These c a n a l s o be d e r i v e d f r o m E q u a t i o n 4.20 by a s s u m i n g t h e 2 E ^ s t a t e i s s u b s t i t u t i o n a l l y i n e r t . ) The A r r h e n i u s p l o t o f l o g ( i / ^ h g j r j ~ 1) v s 1 / T w i l l y i e l d t h e d i f f e r e n c e between t h e a c t i v a t i o n e n e r g i e s o f t h e p h o t o c h e m i c a l r e a c t i o n and i n t e r n a l c o n v e r s i o n . (3) The c o n f i r m a t i o n o f t h e l i f e t i m e o f t h e 4 T _ s t a t e i s o f 2g 4 u t m o s t i m p o r t a n c e f o r s u b s e q u e n t s t u d i e s o f t h e T« s t a t e . Zg The r i s e o f p h o s p h o r e s c e n c e i n C r ( I I I ) c o m p l e x e s s h o u l d be s t u d i e d more e x t e n s i v e l y and s y s t e m a t i c a l l v . And t h e a c c u r -a c y o f t h e d a t a must be i m p r o v e d . T h i s c a n be a c h i e v e d w i t h a,more i n t e n s e f l a s h lamp and a more s e n s i t i v e b u t l e s s n o i s e d e t e c t i n g s y s t e m . D i r e c t o b s e r v a t i o n o f t h e e x c i t e d s t a t e whose l i f e t i m e i s T i s p o s s i b l e and p r i m i s i n g by u s i n g t h e s u b m i c r o s e c o n d f l a s h - k i n e t i c s p e c t r o p h o t o m e t r y method. The m i l l i s e c o n d f l a s h - k i n e t i c s t u d i e s o f t h e C r ( I I I ) c o m p l e x e s have r e c e n t l y "77 78 * 5 b e e n r e p o r t e d . ' However, f r o m t h e f a c t t h a t T = 1 0 x s e c , we know t h e r e a r e p r a c t i c a l l y no d e t e c t a b l e e x c i t e d 138 4 2 (or T i g ) s t a t e molecules remaining a f t e r m i l l i s e c o n d 2 delay times. T h e r e f o r e only the a b s o r p t i o n s p e c t r a of E g s t a t e s of C r ( I I I ) complexes can be observed. The submicro-second f l a s h - k i n e t i c method has the p o t e n t i a l to d e t e c t the e x c i t e d s t a t e having x and w i l l e v e n t u a l l v l e a d us to the r x i d e n t i f i c a t i o n of i t . T h i s can be achieved with a p u l s e d l a s e r . (4) The three mechanisms mentioned i n Chapter V I I I f o r the primary processes can be confirmed or negated once the l o c a -2 2 4 t i o n s of the T. and T„ s t a t e s and the o r i g i n of the T„ l g 2 g y 2 g 2 s t a t e are known. There are s c a t t e r e d data f o r the T, and l g :.Cf s t a t e s , but a more sy s t e m a t i c study i s d e s i r e d . To look 4 t o r the o r i g i n of the T_ s t a t e s p e c t r o s c o p i c a l l y i s a d i f -zg f i c u l t t a s k . However, c a r e f u l i n v e s t i g a t i o n s of the absorp-t i o n s p e c t r a of these C r ( I I I ) complexes at an extremely low temperature, say, a t 2°K, may r e v e a l the l o c a t i o n s of the 4 o r i g i n s of the T„ s t a t e . 2 g (5) I t i s c l e a r t h a t the environmental s t r u c t u r e p l a y s an important r o l e i n a l l the primary processes of C r ( I I I ) com-pl e x e s . However, f o r a f i x e d s o l v e n t , the v i s c o s i t y of the s o l v e n t does not seem to a f f e c t the primary processes a t a l l , Since the e f f e c t of v i s c o s i t y of the s o l v e n t on the primary 59 processes has been s t r e s s e d , i t i s i n t e r e s t i n g to have a systematic study by v a r y i n g the composition of the s o l v e n t g r a d u a l l y . B I B L I O G R A P H Y 140 1 . F . B a s o l o a n d R . G . P e a r s o n , M e c h a n i s m s o f I n o r g a n i c R e a c t i o n s , 2nd e d . , New Y o r k , W i l e y & S o n s I n c . , 1 9 6 7 . 2 . E . L . W e h r y , Q u a r t . R e v . , 2JL, 128 ( 1 9 6 7 ) . 3 . A . W . A d a m s o n , C o o r d . C h e m . R e v . , _3' 1 6 9 ( 1 9 6 8 ) . 4 . A . W . A d a m s o n , W . L . W a l t z , E . Z i n a t o , D .W. W a t t s , P.D. F l e i s c h a u e r , a n d R.D. L i n d h o l m , C h e m . R e v . , 6_8, 541 (1968) 5 . A . W . A d a m s o n , R e c o r d o f C h e m i c a l P r o g r e s s , 29_, 191 ( 1 9 6 8 ) . 6 . V . 3 a l z a n i , L . M o g g i , F . S c a n d o l a , a n d v . C a r a s s i t i , I n o r g a n i c C h e m i c a A c t a R e v i e w s , 1 , 7 ( 1 9 6 7 ) . 7 . v . B a l z a n i , L . M o g g i , a n d V . C a r a s s i t i , B e r . B u n s e n g e s . P h y s i k . C h e m . , 72^, 288 ( 1 9 6 8 ) . 8 . D. V a l e n t i n e , "Some P h o t o c h e m i c a l R e a c t i o n s o f C h r o m i u m ( I I I ) a n d C o b a l t ( I I I ) C o m p l e x e s i n S o l u t i o n , " i n A d v a n c e s i n P h o t o c h e m i s t r y , G . S . Hammond, W . A . N o y e s , J r . , a n d J . N . P i t t s , J r . , E d s . v o l . 6 , I n t e r s c i e n c e , New Y o r k , 1 9 6 8 . 9 . D. 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A P P E N D I X 146 A c c o r d i n g to the f o l l o w i n g m e c h a n i s t i c scheme (see 3) : 4 " T2g k l (1) 2g k 2 (2) 2g k 3 -y Photochemical products (3) S 2g k 4 k-4 2 E g (4) g k 5 2g hv P (5) 2 E g k 6 2g (6) g k 7 -> Photochemical products (7) a f t e r an i d e a l instantaneous e x c i t a t i o n , suppose t h a t o n l y the 4 T_ s t a t e i s immediately populated, the r a t e equations can be 2g expressed as d[Sa] 4 2 -f~ = (VVVVf4T ] - k _ 4 [ 2 E ] (8) ; d t d [ 2 E ] V __ / 1 _ .1- • 1 - .1- \ r ^  T-I 1 1 r ?r d t and (W k7 + k-4 } [ Eg] " V V ( 9 ) [ 4 T 2 g ] = 'SgV t2E ] = 0 (10) 147 The s o l u t i o n s are: r 2 n i k 4 f T 2 q ] o r -k t - k 0 t , .... [ E ] = { e a - e 8 } (11) [ 4 T 2 g ] = [ V o { ( k E _ ^ e - k a t + ( V k p ) e - k B t } (12) ke " k a where k a = * (k E + V - H ( k T - k E ) 2 + 4 k 4 k _ 4 ] * (13) k g = | ( k E + k T) + | [ ( k T - k E ) 2 + 4 k 4 k _ 4 ] * (14) and k p = k 5 + k 6 + k 7 + k_4 (15) k T = k± + k 2 + k 3 + k 4 (16) Since emission i n t e n s i t i e s are p r o p o r t i o n a l to the con-c e n t r a t i o n s of the e m i t t i n g s p e c i e s , t h e r e f o r e , I n = K (e - e - k 3 t ) (17) I f = K ' [ ( k E - k a ) e " k a t - ( k E - k p ) e " k B t ] (18) In the case t h a t r e v e r s e process 4 does not occur, t h a t i s , k_ 4 = 0, then the above equations can be s i m p l i f i e d to I p = K(e - e " k T t ) (19) I f = K' ' e - k T ^ : (20) I f processes k_ 4 do occur, but k^ i s much g r e a t e r than k„, then 148 k a = *< kE + V - • ( k T - V f 1 + f v v 1 2 ( k T " k E } " = A(k E + k T ) - > ( k T - k E ) - k 4 k _ 4 / ( k T - k E ) " k E " k 4 k - 4 A T = k^ - rp. k . E * i s c -4 = k c + k, + k_ + (1 - cp. )k . 5 6 7 T i s c -4 where cf>. = k . / ( k , + k~ + k_, + k.) ' i s c 4' 1 2 3 4 S i m i l a r l y , we c a n o b t a i n T h i s m e c h a n i s t i c scheme and t h e above d e r i v a t i o n s c a n be a p p l i e d e q u a l l y w e l l t o t h e e n e r g y t r a n s f e r s y s t e m w i t h t h e terms p r o -p e r l y c h a n g e d . 

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