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Some chiroptical effects on the photophysics and photochemistry of tris(bipyridine)ruthenium(II) ions… Sparks, Robert Henry 1979

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SOME CHIROPTICAL EFFECTS ON THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF TRIS(BIPYRIDINE)RUTHENIUM(II) IONS IN SOLUTION By ROBERT HENRY SPARKS A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept t h i s t h e s i s as conforming to "the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA AUGUST, 1979 © Robert Henry Sparks In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 E - S B P 75-5 1 1 E i i ABSTRACT: The photoracemization of Ru(bipy)^ i n aqueous s o l u t i o n was studied. 3 | | Quenching studies show the involvement of the (CT) Ru(bipy)^ i n the mech--4 anism of racemization and the low quantum y i e l d (2.9 x 10 ) shows that t h i s state i s asymmetric. Quenching studies show no increase of racemiza-t i o n rate for Ru(I) or (III) species. The temperature dependence gives evidence f o r a d i s s o c i a t i v e racemization mechanism. Quenching with Co(acac) shows c h i r o s e l e c t i v e electron transfer as measured by the r e s u l t i n g photo-chemistry. i i i TABLE OF CONTENTS PAGE I n t r o d u c t i o n 1 Background 2 Experimental 7 Res u l t s and D i s c u s s i o n • 14 Conclusions 28 Appendix A 30 Appendix B 33 References 34 LIST OF TABLES Table I 17 Table I I 19 Table I I I 19 Table IV 33 LIST OF FIGURES Fi g u r e 1 4 Fig u r e 2 9 Figure 3 13 Figure 4 20 Figure 5 21 Figure 6 22 Figure 7 25 Fig u r e 8 27 iv ACKNOWLEDGEMENTS: I would l ike to thank Professor G.B. Porter for his patient teaching, supervision, and guidance during this project. For his useful discussion, I would l ike to thank Professor L .D. Hayward as well as other members of the Faculty of Chemistry and their supporting staff. Also, I would l ike to thank those who have worked in lab 244: C . P . J . Bennington, K. Sarantidis, Dr. J . Van Houten. Most of a l l I wish to thank God for his creation which we a l l explore. INTRODUCTION: Over the l a s t decade there has been a r a p i d increase i n the amount of I | l i t e r a t u r e published on the t r i s ( b i p y r i d i n e ) r u t h e n i u m ( I I ) i o n (Ru(bipy)^ )• This i s p a r t l y due to i t s i n t e r e s t i n g i n t e r a c t i o n s w i t h l i g h t . The absorp-4 4 t i o n spectrum shows q u i t e intense bands ( e / r o = 1.29 x 10 , e „ 0 £ = 7.67 x 10 ) HOZ zoo M'crn) The symmetry l a b e l s and m u l t i p l i c i t i e s of the e x c i t e d s t a t e s have 2-9 been discussed at len g t h i n the l i t e r a t u r e . The intense absorption t o -I ) gether w i t h the l o c a t i o n of v a r i o u s e x c i t e d s t a t e s make Ru(bipy)^ a good candidate f o r i n c l u s i o n i n s o l a r energy research. This complex i o n a l s o emits s t r o n g l y w i t h a l i f e t i m e of approximation 0.6 us. ^  The l i f e t i m e i s of a convenient l e n g t h f o r studying the quenching 12-14 e f f e c t s of other molecules on the luminescence. I | The e x c i t e d s t a t e of Ru(bipy)^ a l s o e x h i b i t s c o n s i d e r a b l e photochemistry both of a permanent and short l i v e d or t r a n s i e n t nature. Examples of a t r a n s -3+ 15 le n t photochemistry are the o x i d a t i o n to Ru(bipy)^ and the r e d u c t i o n of ni t r o g e n s u b s t i t u t e d 4 , 4 - b i p y r i d i n e d e r i v a t i v e i n the absence of EDTA.^ 17 18 For permanent photochemistry, as w e l l as o x i d a t i o n s and reductions 19 20 there are p h o t o s e n s i t i z a t i o n , p h o t o l y s i s r e a c t i o n s , and photoanation 21 r e a c t i o n s . I j In the context of the above r e a c t i o n s the Ru(bipy)^ i s r e l a t i v e l y i n e r t 15 18 and can undergo e l e c t r o n t r a n s f e r s ' and e x c i t a t i o n s without l o s s of the bipy-r i d i n e l i g a n d s . The o p t i c a l a c t i v i t y has a l s o been shown to have great thermal 22 23 2 A s t a b i l i t y . ' O p t i c a l s t a b i l i t y during o x i d a t i o n has a l s o been shown. ++ 25 Recently i t was suggested that Ru(bipy)^ i s c h i r o p t i c a l l y s t a b l e t o e x c i t a t i o n . This i s i n f a c t not the case, as i s shown i n t h i s work on the photoracemiza-I [ t i o n of Ru(bipy)^ • - 2 -BACKGROUND R u ( b i p y ) 3 The abso r p t i o n near 450 nm i n the spectrum of Ru(bipy)^ has been sug-gested to represent d->-rr t r a n s i t i o n s . This would mean an e l e c t r o n which i s i n a mainly metal d o r b i t a l i s promoted to a l i g a n d IT o r b i t a l , which l o c a l i z e s t h i s e l e c t r o n mainly on the l i g a n d s . P r o t o n a t i o n experiments support t h i s view i n the increased b a s i c i t y of the l i g a n d s i n the e x c i t e d 26 s t a t e . A change i n the m e t a l - l i g a n d bond strengths and a greater l i g a n d -l i g a n d r e p u l s i o n i n the e x c i t e d s t a t e over the ground s t a t e i s evident. * 3 This d-Mr e x c i t e d s t a t e w i l l be r e f e r r e d to i n t h i s paper as (CT) Ru(bipy) i n s p i t e of the ambiguity of t h i s l a b e l l i n g . 3 | | (CT) Ru(bipy)^ can r e t u r n to the ground s t a t e through a number of routes: i t can luminesce; i t can t r a n s f e r energy and/or an e l e c t r o n to or from another molecule. Energy t r a n s f e r alone i s p r e f e r r e d i n organic s o l -27 vents. Energy t r a n s f e r processes can give r i s e to photochemistry of the 3 ++ quencher. When the quencher r e c e i v e s an e l e c t r o n from (CT) Ru(bipy)^ t h i s i s c a l l e d r e d u c t i v e quenching and ruthenium(III) i s generated. Examples 3- 3+ 3+ of such quenchers are: Fe(CN)^ Fe , Co(acac).j> Co(phen)^ , and E u ( I I I ) . 3 S i m i l a r l y (CT) Ru(bipy)^ can gain an e l e c t r o n , o x i d a t i v e quenching, and ruthenium w i l l be i n the o x i d a t i o n s t a t e ( I ) . Examples of t h i s type of quencher i n c l u d e Fe(CN) 4~ , Eu(II) and R u ( N H . ) t + . 1 2 C i s - t r a n s i s o m e r i z a t i o n D J O w i t h i n the l i g a n d has been shown to come about as a r e s u l t of e x c i t a t i o n 28 i n t o a s i m i l a r wavelength band of a r e l a t e d ruthenium b i p y r i d i n e complex. Racemization: Racemization i s the change from an o p t i c a l l y a c t i v e compound to a 29 i i racemic compound or mixture. In the case of Ru(bipy)^ a racemic mixture - 3 -29 i s the r e s u l t . The process i s a l s o defined as the opposite to r e s o l u t i o n . For metal c h e l a t e complexes, where the metal atom i s the only c h i r a l c e n t r e , the mechanisms f o r racemization can be d i v i d e d i n t o d i s s o c i a t i v e , non-dissoc-i a t i v e , and a s s o c i a t i v e . D i s s o c i a t i v e mechanisms i n c l u d e the s e p a r a t i o n of one l i g a n d , or i n the case of a b i d e n t a t e l i g a n d , at l e a s t one end of the l i g a n d from the metal centre. The remaining l i g a n d s rearrange without bond d i s s o c i a t i o n and a d i s s o c i a t e d l i g a n d or end returns to the c o o r d i n a t i o n sphere of the metal, the t o t a l process changes the o p t i c a l a c t i v i t y to the opposite sense to the i n i t i a l s t a t e . For a d e s c r i p t i o n of l i g a n d rearrangements of t h i s type, see reference 30. This mechanism can be c h a r a c t e r i z e d by an a c t i v a t i o n energy not l e s s than the energy required t o d i s s o c i a t e one metal to l i g a n d bond. A p o s i t i v e a c t i v a t i o n volume which a l s o c h a r a c t e r i z e the mechanism, i s p re-sumably due to the f r e e l i g a n d , when s o l v a t e d , t a k i n g up more space than 31 the coordinated l i g a n d . N o n - d i s s o c i a t i v e mechanisms maintain a l l l i g a n d s i n the c o o r d i n a t i o n sphere of the metal. There have been proposed 3 such rearrangements or 32 t w i s t s which i n v e r t the o p t i c a l a c t i v i t y . These are the B a i l a r t w i s t , 33 34 Ray and Dutt t w i s t , and the Springer and Sievers t w i s t . See f i g u r e 1 f o r a d e s c r i p t i o n of these mechanisms. The donor atoms are numbered the same f o r the isomers l a b e l l e d A . The transformations i n the f i g u r e show that the Ray and Dutt mechanism does r e s u l t i n a d i f f e r e n t arrangement of donor atoms about the metal centre than does the B a i l a r mechanism. N o t i c e the trans p a i r s i n the A isomer 1-3, 2-4, and 5-6. A f t e r a B a i l a r t w i s t they are 1-5, 2-6, and 3-4, but a f t e r a Ray and Dutt t w i s t they are 1-6, 2-5, and 3-4. By f o l l o w i n g these p a i r s the f i g u r e shows that the B a i l a r and Springer S i e v e r s mechanisms are e x a c t l y the same, not only i n the f i n a l a r r a y of donor atoms, but a l s o i n the a r r a y of donor atoms i n the D ^ Figure 1 Springer and Sievers - 5 -proposed intermediate. These t w i s t mechanisms are c h a r a c t e r i z e d by an a c t i v a t i o n energy which could be l e s s than the d i s s o c i a t i o n energy f o r one end of the l i g a n d and by a s m a l l a c t i v a t i o n volume. In some cases the a c t i v a t i o n volume, AV^ , can even be negative. This i s explained as a D^^ intermediate having a greater a b i l i t y to pack solvent molecules between the l i g a n d s than the p r o p e l l e r -shaped c o n f i g u r a t i o n s of the o p t i c a l isomers. A l s o n o t i c e that the i n t e r -mediates of both mechanisms are o p t i c a l l y i n a c t i v e ( i . e . have at l e a s t 1 plane of symmetry). An a s s o c i a t i v e mechanism has been i n v e s t i g a t e d t h e o r e t i c a l l y which 35 i n c l u d e s a seven coordinate species. This would mean p i c k i n g up a u n i -dentate i o n or solvent molecule. This mechanism w i l l not be discussed at I | l e n g t h here as i t i s most u n l i k e l y f o r Ru(bipy)^ • The p e r c h l o r a t e i o n i s known to a s s o c i a t e w i t h the t r i s ( b i p y r i d i n e ) chromium(III) i o n because the luminescence i s enhanced i n the presence of 36 p e r c h l o r a t e i o n s . Thus i t i s not s u r p r i s i n g that p e r c h l o r a t e a f f e c t s the r a t e of thermal racemization of other t r i s ( b i p y r i d i n e ) and t r i s ( l , 1 0 - p h e n a n -37 t h r o l i n e ) metal c h e l a t e i o n s . Photoracemizations of metal c h e l a t e complexes are not common. Four +3 30 such racemizations which are known are t r i s ( o x a l a t o ) c o b a l t ( I I I ) [Coiox)^ ] > 3+ 33 39 tris(l,10-phenanthroline)chromium(III)[Cr(phen),j ] ' , tris(1,10-phenanthro-l i n e ) c o b a l t ( I I I ) [ C o ( p h e n ) ^ + ] ^ , and tris(ethylenediamine)chromium(III) 3+ 41 [Cr(en)^ ] . Although luminescence, r a t e , and quenching s t u d i e s have been 3+ 38 39 done on the racemization of Cr(phen).j , ' the mechanism has not been 3+ | | e l u c i d a t e d . The Co(phen)^ becomes Co(phen)^ i n the presence of l i g h t and the l a b i l e Co(II) racemizes by a d i s s o c i a t i v e mechanism.^ As w i t h C r ( p h e n ) ^ + , 3+ 41 the mechanism f o r the racemization of C r ( e n ) ^ has not been e l u c i d a t e d . - 6 -Thus there i s l i t t l e background to suggest what mechanism the photo-chemical racemization of Ru(bipy) would follow. - 7 -EXPERIMENTAL: T r i s ( b i p y r i d i n e ) r u t h e n i u m ( I I ) d i c h l o r i d e hexahydrate (A) was from G.F. Smith and had been p r e v i o u s l y r e c r y s t a l l i z e d . R e s o l u t i o n of Ru(bipy)^ : 22 The method of B u r s t a l l gave low o p t i c a l y i e l d . A method s i m i l a r to 24 -4 Dwyer and Gayarfas aff o r d e d the best r e s u l t s . 0.2926 gm (3.957 x 10 -4 moles) A and 0.1317 gm (3.944 x 10 moles) potassium antimony t a r t r a t e were d i s s o l v e d i n 5 ml of hot water. On slow c o o l i n g to i c e temperature dark orange c r y s t a l s of ^ 1 mm dimensions formed. The s o l u t i o n was vacuum f i l t e r e d and the c r y s t a l s were r e c r y s t a l l i z e d i n the dark from 2.5 ml of water to g i v e 85.4 mg of [ (SbO) (C .H. 0,. ] „Ru (bipy) „. From t h i s p o i n t on every-4 4 b z j t h i n g was c a r r i e d out under red l i g h t or i n the dark. This product was d i s s o l v e d i n 20 ml of 10% /v NaOH. The Ru(bipy)^ was p r e c i p i t a t e d by a d d i t i o n of KBr. T h i s mixture was heated u n t i l d i s s o l u t i o n was complete and then cooled sl o w l y to g i v e f l a k e y orange c r y s t a l s which were f i l t e r e d by s u c t i o n . These were r e c r y s t a l l i z e d from a KBr s o l u t i o n . A s o l u t i o n of 5.47 x 10~ 4 M R u ( b i p y ) 3 B r 2 gave [ o ] ^ = -793 ± 8 o ! T r i s (acetylacetonato) c o b a l t ( I I I ) : 42 The p r e p a r a t i o n i s e s s e n t i a l l y the same as Bryant and F e r n e l i u s . A saturated s o l u t i o n of CoC£2*6 ^ 0 was poured i n t o a concentrated s o l u t i o n of potassium carbonate. The r e s u l t i n g purple c o b a l t carbonate was f i l t e r e d o f f . 0.6 gm of the purple s o l i d together w i t h 4 ml 2,5-pentandione were heated on a steam bath to ^90°C. A f t e r 6 ml of 10% H ^ were added, the r e a c t i o n was complete i n 2 min.. The s o l u t i o n was l e t stand f o r 3 days and the green s o l i d f i l t e r e d o f f . This s o l i d was r e c r y s t a l l i z e d from benzene/ 42 65-110°C pet ether as i n the reference. - 8 -T r i s (1,10 -phenanthroline) c o b a l t t r i c h l o r i d e : 43 The p r e p a r a t i o n i s s i m i l a r to V.P. P f e i f f e r and Br. Herdelmann. Cobalt (H)pe'yf* m " I ' l ^ 0.25 gm (1.1 mmoles) and 1,10-phenanthroline, .6 gm (3.1 mmoles) were heated together f o r 4 hours at 180°C, l e t c o o l to room temperature and d i s s o l v e d i n 15 ml of 30% methanol. The r e s u l t i n g mixture was g r a v i t y f i l t e r e d to g i v e a purple s o l i d and an orange s o l u t i o n . The s o l u t i o n was b o i l e d down to 2 ml and sodium c h l o r i d e was added u n t i l 2 phases were observed. The mixture was b o i l e d down to a t h i c k o i l and ex-t r a c t e d w i t h 15 ml of b o i l i n g acetone. A f t e r decanting the acetone, the s o l i d was heated on a steam bath to remove a l l t r a c e s of acetone. The s o l i d was r e c r y s t a l l i z e d from water. Equipment: I s o t r o p i c s p e c t r a were run on a Cary 11. C i r c u l a r dichroism s p e c t r a were run on a Jasco (J20) spectrometer. Luminescence l i f e t i m e measurements: Apparatus f o r l i f e t i m e measurements i s s i m i l a r to that of Demas and 43 Flynn. The gas l a s e r tube i s about 50 cm long. I t was run at a pres-sure between 110 and 125 t o r r w i t h a flow of n i t r o g e n c o n t r o l l e d by a needle v a l v e on the N 2 tank (grade G Canadian L i q u i d A i r ) and pumped from the other end of the l a s e r tube w i t h a r o t a r y vacuum pump. Discharge of the c a p a c i t o r s b u i l t onto the l a s e r tube was c o n t r o l l e d by an e x t e r n a l spark gap and t h y r a -t i o n . The N 2 plasma produces an emission at 337 nm. The c a p a c i t o r s were charged to 12 kV by a Spellman model PN-15 0-15 kV high v o l t a g e power supply. The l a s e r i t s e l f i s housed i n an aluminum screen cage and removed about 3 m from the d e t e c t i o n apparatus to reduce e l e c t r i c a l n o i s e . See f i g u r e 2 f o r a r e p r e s e n t a t i o n of the o p t i c a l paths. The l a s e r pulse was focused i n t o - 9 -10.5kV Power Supply P. M. Mono-ch ro meter {)—O Sample Oscilloscope L 12 kV A Power S F Sup ply *-* R Figure 2 - 10 -the sample c e l l w i t h a 5.5 cm diameter, 10.8 cm f o c a l l e n g t h lens . Perpen-d i c u l a r to the path of the l a s e r pulse was set the d e t e c t i o n system. A Nikon Auto 1:2 l e n s of f o c a l l e n g t h 50 mm was used to focus the emitted l i g h t onto the s l i t s of a Bausch and Lomb monochrometer (cat. no. 33-86-02). The monochrometer has 5.36 mm s l i t s and was always set at 600 nm. The l i g h t was detected by an RCA 8645 p h o t o m u l t i p l i e r (P.M.) mounted i n a bronze housing and powered by a Kepco model ABC 1500 M D.C. power supply. Voltage to the m u l t i p l i e r was about 10.5 kV. The s i g n a l was taken from the m u l t i p l i e r through a low capacitance c o a x i a l cable system w i t h a 93 r e s i s t o r at each end having an e f f e c t i v e r e s i s t a n c e of 46.5 fi. The s i g n a l was a m p l i f i e d through a plug i n u n i t 7A16 and d i s p l a y e d on a Tektronics model 7904 o s c i l -loscope. The t r a c e was photographed w i t h a Tektronics C-27 scope camera; time s e t t i n g B and f 1.9 lens u s i n g P o l a r o i d type 47 ASA 3000 high speed f i l m . The system was found adequate to f o l l o w the decay of Rhodamine 6G i n ethanol as l e s s than 10 ns and the time constant of the system i s too small to a f f e c t l i f e t i m e s over 100 ns. S o l u t i o n l i f e t i m e s were measured i n a quartz emission c e l l or by emission from the f r o n t surface of a c y l i n d r i c a l quartz spectrometer c e l l . The c e l l o r i e n t a t i o n f o r the second case was such that the r e f l e c t e d beam t r a v e l -l e d between the d e t e c t i o n system and the e x c i t a t i o n pulse path towards point P, see f i g u r e 2. A Corning CS3-71-1.53 mm U.V. c u t o f f f i l t e r was used between the sample c e l l and the Nikon lens . No d i f f e r e n c e was d e t e c t a b l e i n the l i f e t i m e of Rhodamine 6G by the two d i f f e r e n t arrangements. Actinometry: Potassium f e r r i o x a l a t e actinometry was c a r r i e d out as described i n "Photochemistry"by C a l v e r t and P i t t s . ^ In subdued l i g h t , to each of 2 - 11 -i d e n t i c a l quartz c e l l s was d e l i v e r e d 3 ml of the .006 M K^FeCC^O^)^ s o l u t i o n . Each c e l l was capped w i t h a small magnetic s t i r r e r i n s i d e . The c e l l s were t r e a t e d e x a c t l y the same except that one was set i n the o p t i c a l t r a i n i n place of a photoracemization s o l u t i o n f o r a measured length of time. The t r a n s m i t t e d l i g h t was measured by a power meter, w i t h and without the c e l l present. Each c e l l was then emptied i n t o a 25 ml v o l u m e t r i c actinometry f l a s k again i n subdued l i g h t . Each c e l l was r i n s e d w i t h 2 ml of the .1% 1,10-phenanthroline s o l u t i o n , 1.5 ml of the NaOAc/H^SO^ b u f f e r s o l u t i o n , and 3 ml of d i s t i l l e d water. The r i n s i n g s were s u c c e s s i v e l y added to the cor-responding v o l u m e t r i c f l a s k . The f l a s k s were f i l l e d to the mark w i t h d i s t i l l e d water and l e f t to develop f o r 1 hr. The absorption of the two s o l u t i o n s was measured on a Cary 17 at 510 nm. The a b s o r p t i o n of the f e r r i o x a l a t e s o l u t i o n was a l s o measured at 457.9 nm. Sample p r e p a r a t i o n and racemization run: D i s t i l l e d water or a s o l u t i o n of the appropriate quencher, w i t h i o n i c s t r e n g t h adjusted, was placed i n a 1 cm spectrometer c e l l and the s o l i d c h i r a l Ru(bipy> 3 as the dibromide s a l t was added and d i s s o l v e d u n t i l the o p t i c a l d e n s i t y of the s o l u t i o n was near but d i d not exceed 1.2 at 450 nm. The s o l u t i o n was then placed i n the degassing f l a s k of a Zwickle f l a s k (see f i g u r e 3) and degassed w i t h argon. The argon was p r e v i o u s l y p u r i f e d by pass-in g through Zn/ECH, Cr(N0 3> 2 and H 2S0^ towers, NaOH/calcium s u l f a t e d r y i n g tube, and f i n a l l y bubbled through d i s t i l l e d water to humidify the argon. The s o l u t i o n was d e l i v e r e d i n t o a spectrometer c e l l which had been swept out w i t h f l o w i n g argon and contained a small magnetic s t i r bar. The adapter to the spectrometer c e l l was s h o r t e r than that shown i n f i g u r e 3 so that i t could be accommodated i n the sample compartment of the Jasco (J-20). A f t e r c l o s i n g - 12 -as the lower stopcocks, the #5 o-ring j o i n t was dissembled and the i s o t r o p i c and CD spectra were recorded. The s o l u t i o n was s t i r r e d and placed i n the beam of an argon i o n l a s e r tuned to 457.9 nm. The time i n the beam was measured w i t h a stopwatch and the power of the beam was measured w i t h and without the c e l l present. The CD absorption at 470 nm was recorded. Successive values were taken to determine the r a t e of the racemization. A f t e r the race-m i z a t i o n had proceeded s i g n i f i c a n t l y the i s o t r o p i c spectrum was recorded again. Where p o s s i b l e the same s o l u t i o n was then used f o r l i f e t i m e s t u d i e s i n the f r o n t s urface arrangement. Temperature c o n t r o l l e d photoracemization: A sample, prepared as described before, was placed i n a d i s t i l l e d water thermostatic bath. The thermostatic bath was s t a i n l e s s s t e e l w i t h a vacuum j a c k e t and double quartz windows f r o n t and r e a r and on each s i d e . The water was c i r c u l a t e d by a Monostat Corporation p e r i s t a l t i c pump through a 10 f t . copper c o i l . A l l tygon l i n e s connecting pump, c o i l , and bath were i n s u l a t e d w i t h foam tubing. The copper c o i l was placed i n a l a r g e Dewar f l a s k w i t h an i c e bath f o r below room temperature runs and i n a thermostatted bath ( c a t . no. 66600) from P r e c i s i o n S c i e n t i f i c f o r above room temperature runs. The l a s e r was focused by a lens of f o c a l l ength 20 cm attached to the f r o n t window. The c e l l was s t i r r e d m a g n e t i c a l l y from underneath. Actinometry was c a r r i e d out i n s i d e the temperature c o n t r o l l e d bath to measure a c c u r a t e l y the l i g h t i n t e n s i t y . Figure 3 - 14 -RESULTS AND DISCUSSION: Photoracemization: While e x p l o r i n g the p o s s i b i l i t y of s t e r e o s p e c i f i c photochemistry of I | A-Ru(bipy) 3 , we have e s t a b l i s h e d that i t s CD spectrum decreases on exposure to l i g h t w i t h no corresponding r e d u c t i o n of the i s o t r o p i c spectrum. I t has been noted by Van Houten and Watts, that t h i s molecule i s extremely photo-—5 8 s t a b l e at room temperature i n degassed s o l u t i o n (<j> < 10 ) . The photorace-m i z a t i o n could be followed q u a n t i t a t i v e l y by the l i n e a r dependence of the logarithm of the d i f f e r e n t i a l a bsorption at 470 nm on i n t e g r a t e d absorbed i n t e n s i t y : l n -4- = k l t A a o The t h e o r e t i c a l b a s i s f o r t h i s f o r m u l a t i o n i s discussed i n a l a t e r s e c t i o n , and derived i n d e t a i l i n Appendix A, where the constant k i s r e l a t e d to the rac e m i z a t i o n quantum y i e l d . Since the quantum y i e l d f o r ra c e m i z a t i o n , < r r a c » i n water at room temp--4 erature i s very s m a l l (<J> = 2.9 x 10 ), i t f o l l o w s that the e l e c t r o n i c a l l y 1T3-C e x c i t e d species through which the racemization occurs must be a c h i r a l species i f formed i n high y i e l d , but could be a c h i r a l i f formed only i n the same y i e l d as the racemization i t s e l f . In a sense both of these statements are t r u e i n t h i s system, as we s h a l l i n d i c a t e i n the subsequent development. Quenching e f f e c t s : I | There i s a l o n g - l i v e d s t a t e f o r RuCbipy)^ , w i t h T = 0.6 ysec which i s r e s p o n s i b l e d i r e c t l y f o r the in t e n s e phosphorescence of t h i s molecule. The r o l e of that s t a t e i n the photoracemization was explored by s t u d i e s of both luminescence and racemization quenching under comparable c o n d i t i o n s . In - 15 -a d d i t i o n , i t was hoped to e s t a b l i s h whether or not Ru(I) or R u ( I I I ) species take part to any appreciable extent i n the racemization by using s u i t a b l e quenchers which act as e l e c t r o n donors or e l e c t r o n acceptors. The quenching of phosphorescence was followed by measuring l i f e t i m e s from o s c i l l o s c o p e d i s p l a y s of the phosphorescence decay. The data were f i t -ted t o : l n I - l n I = -t/x o In every case the data p l o t t e d as l n I ( i n t e n s i t y ) against time t gave s t r a i g h t l i n e s . The b a s e l i n e f o r the d i s p l a y s was c a r e f u l l y e s t a b l i s h e d , however, an e r r o r of only 5% i n the p o s i t i o n of the b a s e l i n e r e s u l t e d i n a n o t i c e a b l e curvature i n the p l o t . Racemization quantum y i e l d s i n the presence of v a r i o u s i o n i c quenchers were explored. The data w i t h FeC£„, K.Fe(CN), and Co(phen) 0C£ 0 are discussed 5 H D 5 5 here as examples of e l e c t r o n t r a n s f e r quenchers. In each of these cases, however, the i o n i c s t r e n g t h dependence i s enormous, so t h a t , even though we have t r i e d to use the same c o n d i t i o n s as previous work i n the l i t e r a t u r e , the agreement w i t h l i t e r a t u r e quenching data i s not very good. FeC£.j quenching ( e l e c t r o n acceptor) was c a r r i e d out i n 0.10 M-NaC&O^ and 0.01 M-HC&O^ to correspond w i t h that i n reference 14. Quencher concen--3 -4 t r a t i o n of 3.0 x 10 M gave: <j> = 3.0 x 10 and T = 250 nsec. A c o r -X"3 C r e c t i o n f o r the p e r c h l o r a t e i o n e f f e c t (see l a t e r d i s c u s s i o n ) reduces <l> £ -4 to 2.3 x 10 . The extent of quenching of the racemization i s somewhat l a r g e r than that of the luminescence l i f e t i m e , n e v e r t h e l e s s , the racemization quantum y i e l d i s l e s s than i n water. S i m i l a r r e s u l t s were obtained w i t h the 3+ -3 other e l e c t r o n acceptor quenching molecule, Co(phen)^ . With 4 x 10 3+ -4 M-Co(phen). and 0.5 M-NaC£, t}> = 1.5 x 10 and T = 236 nsec. In t h i s 3 rac case, the racemization has been quenched by ca. 50%, w h i l e the l i f e t i m e has - 16 -been reduced to 36% of i t s value i n water. Although there i s the complica-t i o n of a l a r g e i o n i c s t r e n g t h e f f e c t , i t i s apparent that there i s no e f f e c t 3+ of a c c e l e r a t i o n of the racemization v i a t r a n s i e n t formation of R u ( b i p y ) 3 , -3 4-With 2.0 x 10 M-Fe(CN),, , which i s an e l e c t r o n donor quencher, o and 0.5 M-NaCil, there i s not only the problem of i o n i c s t r e n g t h e f f e c t s , but 13 a l s o that considerable i o n p a i r formation may occur. The values of <J> = J rac -4 2.0 x 10 and x = 263 nsec i n d i c a t e comparable quenching of the two processes. This system i s a l s o complicated by a permanent thermal r e a c t i o n d e s t r o y i n g R u ( b i p y ) 3 . A l l of these data i n d i c a t e that n e i t h e r Ru(I) nor Ru(II I ) species p l a y an important part i n the racemization. From the i n t e n s i t y we have used and the quantum y i e l d of quenching, the t r a n s i e n t concentrations of R u ( I ) , pro-13 45 posed by Demas and Addington and of Ru(II I ) proposed by L i n and S u t i n , —8 are c a l c u l a t e d to be ca. 10 M and t h e r e f o r e would have a h a l f l i f e of 10 3+ msec (compare r e f . 13). I t i s known that Ru(bipy)^ i s k i n e t i c a l l y r e l a t i v e l y 24 i n e r t and i s u n l i k e l y to isomerize to any extent i n times of t h i s order. For a n e u t r a l quencher, both i o n i c s t r e n g t h and i o n - p a i r i n g problems are e l i m i n a t e d . C o ( a c a c ) 3 i s such a s u i t a b l e quencher, w i t h a quenching •^8 —1 —1 12 r a t e constant of 6.7 x 10 M sec . I t s a c t i o n i s s a i d to i n v o l v e 3+ e l e c t r o n t r a n s f e r to the co b a l t e n t i t y r e s u l t i n g i n Co(II) and Ru(bipy)^ 46 t r a n s i e n t s p e cies. We have confirmed the value of the quenching constant -3 by our l i f e t i m e measurements: 3.3 x 10 M-Co(acac) 3 reduces the luminescence g l i f e t i m e to 222 ns from i t s v a l u e of 657 ns i n water. Thus k = 9.0 x 10 q -1 -1 M s At the same co n c e n t r a t i o n of Co(acac)„, the value of d> i s reduced 3 rac -4 to 1.3 x 10 , 45% of i t s v a l u e i n the absence of quencher. T h i s corresponds 8 —1 —1 to = 5.7 x 10 M s . I t i s t h e r e f o r e reasonably w e l l e s t a b l i s h e d that the photoracemization occurs v i a the same e l e c t r o n i c s t a t e as that r e s p o n s i b l e - 17 -3 2+ f o r the luminescence, namely, the (CT) Ru(bipy)^ . This s t a t e i s known to be formed w i t h near u n i t quantum y i e l d , hence i t i s the s t a t e which r e t a i n s the c h i r a l i t y of the ground s t a t e through e x c i t a t i o n and d e e x c i t a t i o n , except f o r that small part which undergoes racemization. E f f e c t of D 20: S u b s t i t u t i o n of D 20 f o r H 20 i s known to increase both the quantum y i e l d 2+ and the l i f e t i m e of luminescence of Ru(bipy)^ • We have confirmed t h i s e f f e c t by l i f e t i m e measurements (see Table I) and have a l s o determined the quantum y i e l d s f o r racemization i n the two s o l v e n t s . The r a t i o of A ^ ^ / A ^ O = 1.83 rac rac i s the same, w i t h i n experimental e r r o r as the r a t i o T ^ ^ / T ^ ^ . Table I H 20 D 20 [Ru(bipy)^ +] 8.5x10" 5 7. 4x10 * rac 2 . 9 x l 0 ~ 4 5.3x10 x/ns 657 1166 */*o 1 1.83 x / t o 1 1.77 -5 -4 The l o g i c a l c o n c l u s i o n from these data i s again that the s t a t e l e a d i n g to phosphorescence and that i n v o l v e d i n the racemization are one and the same, the (CT) 3. E f f e c t of perchorate i o n : The quantum y i e l d f o r racemization i n the presence of 0.20 M NaCdO^ i s increased over that i n water by a f a c t o r of 1.46. U n l i k e the case of D 20 as s o l v e n t , however, the l i f e t i m e i s v i r t u a l l y u naffected by the presence ° f C*°4~' T C £ 0 4 - / T H 2 0 = °- 9 2' - 18 -A c c e l e r a t i o n of the racemization i s unusual. In thermal r a c e m i z a t i o n s , 9 CJIO^  r e t a r d s the i n v e r s i o n . In t h i s case, there i s very l i t t l e e f f e c t on the p h o t o p h y s i c a l processes, according t o x . Perhaps the only concrete c o n c l u s i o n to be reached i s that some a s s o c i a t i o n occurs between metal c h e l a t e complexes of b i p y r i d i n e and 1,10-phenanthroline w i t h CW^ i o n s . Temperature dependence: 3 2+ Since only one i n about 4000 (CT) Ru(bipy)^ molecules undergoes i n v e r -s i o n t o the opposite stereoisomer at room temperature, there must be a c o n s i d e r -able energy b a r r i e r to i n v e r s i o n i n the t r i p l e t s t a t e . We have measured t h i s energy b a r r i e r d i r e c t l y by f o l l o w i n g the temperature dependence of <j> T03.C Q u a l i t a t i v e l y , we note that the l i f e t i m e i t s e l f decreases w i t h i n c r e a s i n g g temperature, as has been examined i n d e t a i l by Van Houten and Watts and 9 by Watts, Harrington and Van Houten. However, the quantum y i e l d f o r race-m i z a t i o n i n c r e a s e s f a s t e r w i t h i n c r e a s i n g temperature than does the T " ' . I t i s not known whether the r e a c t i o n occurs v i a an a c h i r a l intermediate w i t h i n the t r i p l e t m anifold: or by a process that i n v o l v e s simultaneous d e e x c i t a t i o n to the ground s t a t e : 2+ 2+ ->- A-Ru(bipy) 2+ In e i t h e r case, the energy of a c t i v a t i o n f o r i n v e r s i o n w i l l g i v e u s e f u l i n f o r m a t i o n about the mechanism of racemization. - 19 -Table I I Temp/°C K/T x/ns l n ( - ) 37.0 3. 23x10" -3 507 14 .49 23.0 3. 36x10" -3 657 14 .24 15.6 3. 47x10* •3 677 14 .21 11.0 3. 52x10* -3 690 14 .19 To analyze the data, we have determined the apparent energy of a c t i v a t i o n a s s o c i a t e d w i t h the l i f e t i m e . The data i n Table I I , when f i t t e d to an Arrhenius equation of the form: - l n x = + l n A(x) g i v e , f o r our data: E(x) = 2.10 kcal/mole and A(x) = 5.71 x 10 ' s \ and, 8 7 f o r Van Houten and Watts data: E(x) = 1.81 kcal/mole and A(x) = 3.57 x 10 s , f o r the temperature range from 5° to 40°C, see f i g u r e 4. 9 In the paper by Watts, H a r r i n g t o n and Van Houten, the temperature dependence over a much l a r g e r range was f i t t e d to a more complicated equation. Since our racemization data extend only over a small range of temperature, t h i s d e t a i l e d a treatment i s unnecessary. However, E(x) has no r e a l s i g n i f i -cance i n terms of the mechanism; i t i s an apparent a c t i v a t i o n energy. A more complete treatment i s given i n Appendix B. When the racemization data are f i t t e d to a s i m i l a r equation, there r e s u l t s , see f i g u r e 5,: E(rac) = 13.9 kcal/mole and A(rac) = 5.0 x 10^ s ^. Table I I I Temp/°C K/T ^ I n * ^ 3.0 3.62xl0" 3 .0000521 -9.96 23.0 3 . 3 8 x l 0 _ 3 .000288 -8.15 40.0 3.19xl0~ 3 .000933 -6.98 F i g u r e 4 - 20 -m l v s . 1 r — : T 14.5 • from reference S 14.4 T * from Table IT m l 14.3 + 14.2 14.1 3-2 3.3 3.4 1 « - 22 -Again, t h i s i s an apparent a c t i v a t i o n energy whose meaning i s examined i n the f o l l o w i n g s e c t i o n . Mechanism: From the experiments done we know the mechanism must i n c l u d e every mo 3 3 c u l e passing through the (CT) s t a t e , the (CT) s t a t e must be c h i r a l , and a thermal a c t i v a t i o n energy must be a s s o c i a t e d w i t h the u l t i m a t e i n v e r s i o n Consider the f o l l o w i n g network: Figure 6 rac k„ + k„ + k. k 0 + k^ + k £• 5 H 1 j ' From the values of <j> we know k,<<k„ + k„. Therefore: rac 4 2 3 - 23 -and e l i m i n a t i n g + k^: rac 4 Both k^ and x have the temperature dependences: ~I4_ RT k, = A. e 4 4 - E ( T ) - 1 K t \ R T x - A(x) e Thus l n + - *n / V U E ( T ) " E4 (__^ RT The apparent E(rac) from the previous s e c t i o n i s t h e r e f o r e : E(rac) = E(x) - E. 4 and hence E. = 16.0 kcal/mole 4 S i m i l a r l y A 4 = 2.90 x 1 0 1 4 s e c " 1 . By applying t h i s model to our data we c a l c u l a t e a t r u e a c t i v a t i o n Cj energy f o r racemization of 16.0 kcal/mole. The work of Van Houten et a l 3 shows an a c t i v a t i o n energy of 16.02 kcal/mole f o r production from (CT) R u ( b i p y ) 3 of a photo product i n s t r o n g l y a c i d s o l u t i o n s at elevated tempera-tur e s . They suggest that the photo product i s a Ru(II) species w i t h a one-ended b i p y r i d i n e l i g a n d . I f t h i s i s the case, i t provides evidence that racemization proceeds through a d i s s o c i a t i v e mechanism. - 24 -S e l e c t i v e Photochemistry: S e l e c t i v e quenching due to o p t i c a l a c t i v i t y has been observed before i n 47 organic systems. We decided to study s e l e c t i v e photochemistry which could r e s u l t from c h i r o s e l e c t i v e luminescence quenching. A number of s p e c i f i c a -t i o n s are necessary f o r such a study. The quenching must be slower than d i f f u s i o n c o n t r o l l e d to a l l o w s e l e c t i v i t y . For the r e s u l t i n g o p t i c a l a c t i v i t y to be measurable the quencher or i t s photochemical product must be thermally and photochemically s t a b l e to racemization. The values of E and e must be s u f f i c i e n t l y d i f f e r e n t to a f f o r d measurement of enantiomeric excess p r e f e r -I [ ably at a wavelength where Ru(bipy)^ does not absorb. 3+ | | Co(phen)^ , although c h i r a l , gives Co(phen)^ i n the presence of l i g h t 40 which sets up a chain r e a c t i o n of r a c e m i z a t i o n . Co(acac)^ f i l l s a l l the necessary c r i t e r i a . I t has a l s o been shown that Co(acac)^, when s e n s i t i z e d by Ru(bipy)^ does undergo photochemical change. Indeed t h i s quenching by Co(acac)^ does show a s e l e c t i v i t y . During photoracemization i n the presence of t h i s quencher, the CD spectrum of Co(acac)^ appears. The spectra i n f i g u r e 7 show the growth w i t h i n c r e a s i n g i r r a d i a t i o n time of the. CD a b s o r p t i o n i n the r e g i o n of 550-720 nm. This CD spectrum i s c l e a r l y i d e n t i f i e d w i t h that of Co(acac)^ not only by i t s general shape, but more im p o r t a n t l y , by the p o i n t of zero d i f f e r e n t i a l a b s o r p t i o n at 48 about 622 nm given by Jonas and Norden. According to the absolute c o n f i g u r a t i o n assignment of McCaffery, Mason 49 ++ and Norman the Ru(bipy)^ we used i s A or r i g h t handed screw h e l i x . Sim-i l a r l y the r e s u l t i n g excess of the Co(acac)^ isomer i s a l s o A according to the assignment of Jonas and Norden."^ The i n t e r a c t i o n of the (CT)^RuCbipy)"^ w i t h Co(acac)^ could e i t h e r cause s e n s i t i z e d r a c e m i z a t i o n or d i s s o c i a t i o n of 3 | | the Co isomer which p r e f e r e n t i a l l y r e a c t s w i t h (CT) Ru(bipy)^. Meyerstein F i gu re 7 wavelength in nm 550 600 / I I 1 I I I 1 I 1/ baseiin / / CD spectrum of Co(acac).j from r e f . 48. Evolution of CD spectrum of Co(acac), photosensitized by A-Ru(bipy)^2+ # - 26 -46 et a l r e f e r to photoreduction and decomposition of the CoCacac)^. Their 3 <j)'s show that production of Co(II) i s p r i m a r i l y due to s e n s i t i z a t i o n by (CT) I [ Ru(bipy)^ r a t h e r than by d i r e c t a b s o r p t i o n . The production of a C o ( a c a c ) 3 3 CD spectrum at a l l , p o i n t s once again to r e t e n t i o n of c h i r a l i t y by the (CT) I j R u ( b i p y ) 3 . The generation of the spectrum of the A isomer of C o ( a c a c ) 3 I | shows a p r e f e r r e d i n t e r a c t i o n between A C o ( a c a c ) 3 and A R u ( b i p y ) 3 l e a d i n g to d e s t r u c t i o n of the former. Figure 8 shows q u a n t i t a t i v e l y the growth of the enantiomeric excess of the Co(acac).j. From i n t i a l r a t e s , an estimate of the o p t i c a l s e l e c t i v i t y of the quenching i n t e r a c t i o n shows the r a t e of growth of A C o ( a c a c ) 3 i s 14% of the r a t e of d e s t r u c t i o n of Co(acac).j. During the same f i r s t 2 minutes the A R u ( b i p y ) 3 l o s t 13% of i t s i n i t i a l o p t i c a l a c t i v i t y . Thus the s e l e c t i v i t y i n the quenching process i s greater than 14%. On a molar b a s i s , however, the I | a c t i v i t y generated i n the C o ( a c a c ) 3 frUP exceeds that l o s t by the Ru(bipy)^ because of the small i> of the l a t t e r . rac In a survey of thermal e l e c t r o n t r a n s f e r r e a c t i o n s , n o evidence could be found f o r any c h i r o s e l e c t i v e processes. P f f u r e 8 C o n c e n t r a t i o n vs. t i m e 5 t a [ C o ( a c a c ) 3 ] + [ARu(bipy) 3] - [ A R u ( b i p y ) 3 ] ® [ A C o ( a c a c ) 3 ] - [ A C o ( a c a c ) 3 ] - 28 -Conclusions: 1. From < r r a c a n a d i f f e r e n t i a l quenching s t u d i e s we have e s t a b l i s h e d that 3 4 + (CT) R u ( b i p y ) 3 i s c h i r a l . 2. There i s o p t i c a l s e l e c t i v i t y i n the quenching process f o r d e a c t i v a t i o n of ( C T ^ R u O j i p y ) ^ . 3. There i s evidence that the racemization occurs v i a a d i s s o c i a t i v e process. D i f f e r e n t i a l quenching e f f e c t s of o p t i c a l isomers should not have been an unexpected e f f e c t . N a t u r a l photochemical r e a c t i o n s have been shown to occur S 5 i w i t h o p t i c a l l y a c t i v e m a t e r i a l s such as rhodop^in pigments and c h l o r o -52 p h y l l . The mechanism by which e x c i t a t i o n energy of rhodopin i s t r a n s f e r r e d to the nervous system may i n v o l v e c h i r o s e l e c t i v e energy and/or e l e c t r o n t r a n s -f e r . S i m i l a r l y t h i s could be the case f o r antenna c h l o r o p h y l l . D i f f e r e n t i a l quenching and s e n s i t i z a t i o n of o p t i c a l isomers could be an e f f e c t i v e s y n t h e t i c t o o l . I f a c h i r a l cage s t r u c t u r e of p y r i d i n e l i g a n d could be made f o r Ru(II) i n such a way that the Ru(II) species could not racemize, a greater r e s o l u t i o n could be achieved f o r C o ( a c a c ) 3 . We showed 14% s e l e c t i v i t y where a recent r e s o l u t i o n by p a r t i t i o n i n g between c h i r a l 53 s o l v e n t s showed 5-8% o p t i c a l y i e l d . Another c h i r a l quencher Co(EDTA) would l i k e l y have greater s o l u b i l i t y than Co(acac),j. I n low i o n i c s t r e n g t h , i o n p a i r i n g favours increased I [ quenching and thus l e s s racemization of Ru(bipy)^ . This quencher could show even greater s e l e c t i v i t y i n photochemistry. Further i n v e s t i g a t i o n of the mechanism of racemization should i n c l u d e determination of the a c t i v a t i o n volume and pH e f f e c t s . The a c t i v a t i o n volume should be l a r g e and p o s i t i v e f o r a d i s s o c i a t i v e mechanism. In a c i d s o l u t i o n the racemization should be enhanced by lengthening the l i f e of the d i s s o c i a -t i v e intermediate to the racemization. - 29 -I f f l a s h p h o t o l y s i s u s i n g a c i r c u l a r l y p o l a r i z e d a n a l y s i n g beam could be p e r f e c t e d , observation of some intermediates i n the mechanism could be p o s s i b l e . This could d i s t i n g u i s h between the p o s s i b l e r e a c t i o n s : A ( C T ) 3 R u ( b i p y ) ^ + + A (CT) 3Ru ( b i p y ) ^ " or A ( C T ) 3 R u ( b i p y ) ^ + -> ARu(bipy)^ 1" ( l a b e l l e d i n f i g u r e 6). In g e n e r a l , the r o l e of o p t i c a l a c t i v i t y i n the photo p h y s i c a l processes of Ru(bipy)^ has been ignored. We have shown that t h i s can have s i g n i f i c a n t e f f e c t s . APPENDIX A - K i n e t i c A n a l y s i s of Racemization For each racemization run d i f f e r e n t i a l absorbances of c i r c u l a r l y polar-i z e d l i g h t were recorded corresponding to d i f f e r e n t times of i l l u m i n a t i o n . The f o l l o w i n g s i m p l i f i e d mechanism was u t i l i z e d for. data r e d u c t i o n : A R u C b i p y ) ^ + 1 A C R u b i p y ) ^ (A) A R u C b i p y ) ^ A R u C b i p y ) ^ (B) c^AA ' which corresponds to r a t e R^, i s defined i n t h i s case as: I | | | moles ofARuCbipy)^ produced from A Ru(bipy)^ A A moles of hv absorbed by A RuCbipy)*"1" ' ' c j>^ i s defined s i m i l a r l y f o r r e a c t i o n (B) . The r a t e s and tfi's are r e l a t e d : R l " « A A ^ ( 2 ) R 2 = * A A I A (3) where 1^  and 1^  are the i n t e n s i t i e s of l i g h t absorbed by the two isomers. The r a t e of formation and d e s t r u c t i o n of each isomer can be deduced as: . ^ r 4 - * A 4 I* (5, Since <|> ^ must by symmetry equal c t ^ d A R u C b i p y ) ^ = ^ ( I A _ r A } ( f i ) d A R u ( b i p y ) ^ m + ^ ( I A _ l A ) ( ? ) Due to the nature of the phenomena, only the d i f f e r e n c e i n c o n c e n t r a t i o n of A and A isomers i s measured i n CD spectroscopy. The q u a n t i t y of i n t e r e s t - 31 -i s then: d(ARu(bipyK - ARu(bipy)„ ) , A A 1^ and 1^ are defined i n a mixture as: 3-A = [ARu(bipy)^ ] l a ( g ) [ARu(bipy)^"] + tARuCbipy)^ 1"] -H-where l a i s the t o t a l l i g h t absorbed by both Ru(bipy)^ isomers. S u b s t i t u t -ing i n (8) g i v e s : d [ARu(bipy) 3 ] - [ARu(bipy) 3 ] = <fr,A2Ia( [ARu(bipyK ] - [ARu(bipyK ] dt [ARu(bipy) 3 ] + [ARu(bipy) 3 So as to a c e r t a i n <j>^ ^ from the observed time dependence the above expression (10) i s i n t e g r a t e d to g i v e : £ n ([ARu(bipy)t*l - [ A R u ( b i p y ) ^ ] ) t = ? ^ A A ^ (11) ([ARu(bipy)^ 4"] - [ARu(bipy)3 ] ) Q [ARu(bipy)^'] + [ARu(bipy) 3~ f] In our i n i t i a l c o n d i t i o n s we have at t = 0: [ A R " ( b i p y ) £ j = o ( 1 2 ) [ A R u t b i p y ) ^ ] + [ARu(bipy) 3 H'] and [ARu(bipy)^ +] = x ( 1 3 ) [ARu(bipy) 3^"] + [ARu(bipy)^ +] Thus the i n i t i a l CD absorption at 470 nm, A^ - A^, i s taken as ( [ A R u ( b i p y ) 3 ] • I | | j | | [ARu(bipy) 3 ] ) ^ . Successive values of ( [ A R u ( b i p y ) 3 ] - [ARu(bipy) 3 ] ) f c are then simply the successive CD abs o r p t i o n values a f t e r i r r a d i a t i o n time t , i . e . (A^ - A R ) J . . (J>^ was c a l c u l a t e d from the slope of: i n -^L ^ - t v s . t ( A L - Vo - 32 -(or lat i f la changed during a run). Total ruthenium species present was calculated using the volume of the c e l l and the isotropic extinction coef-I | f ic ient for Ru(bipy) 3 at 450 nm. The slope is then: m = H»AA 21a A450 / E450 k V where V = .0030 l i t r e s , the volume of the c e l l . Ia = absorbed intensity in Einsteins/sec, i . e . moles hv/sec. - 33 -APPENDIX B - Watts e t . a l . Analysis t / ° C 3.0 23.0 40.0 K/T 3.62x10 -3 3.38x10 -3 3.19x10 -3 Table IV rac .0000521 .000288 .000933 r / sec 7.48x10 -7 6.62x10 -7 5.27x10 -7 ^sec T 69.7 435 £n^-T 4.24 6.07 1.77xl0 J 7.47 x is calculated using the equation: x(T) = [ k l r + k l q + k 2 q e A E L F / k T ] 1 taken from reference 9. k„ is the rate constant of radiative decay for the lowest excited state, l r and k n is the rate constant for non-radiative decay from the same state. iq k 2 q i s the Arrhenius factor for non-radiative decay from a ligand f i e ld state A E T _ in energy above the lowest excited state, k and T are the Boltzmann con-stant and the temperature in K respectively. Plotting £n — vs. — where A / T = k, we find E , = 15.0 kcal/mole and the x t 4 4 13 -1 Arrhenius factor A^ = 4.78 x 10 sec . The small discrepancy in E ^ values is due to a systematic error in the T ' S calculated. - 34 -REFERENCES 1. J.D. M i l l e r and R.H. P r i n c e , J . Chem. Soc. (A), 1048 (1966). 2. G.B. 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