Electron Transfer, Energy Transfer, and Excited-State Annihilation in

14 Electron Transfer, Energy Transfer, and Excited-State Annihilation Downloaded by MONASH UNIV on September 21, 2015 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch014

in Binuclear Compounds of Ruthenium(II) Takeshi Ohno , Koichi Nozaki , Noriaki Ikeda , and Masa-aki H a g a 1

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Department of Chemistry, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan Department of Chemistry, Faculty of Education, M i e University, Tsu, M i e 514, Japan

Electronic excited states of binuclear compounds of ruthenium(II) bridged by bis-2,2' -(2"-pyridyl)bibenzimidazole (bpbimH ) were studied by means of laser photolysis kinetic spectroscopy. Excitation of RuL (bpbimH )RuL [Lis 2,2'-bipyridine (bpy), 4,4'-dimethyl2,2'-bipyridine (dmbpy), or 1,10-phenanthroline (phen)] into the metal-to-ligand charge-transfer (MLCT) triplet state gives rise to a transient absorption spectrum revealing electron occupation on both L and bpbimH in CH CN. In an asymmetric binuclear compound, excitation-energy transfer takes place from the higher energy site to the other site. Production of the MLCT triplet excited state in a symmetric binuclear compound, Ru(bpy) (bpbimH )Ru(bpy) , is compared with that of the corresponding mononuclear compound, Ru(bpy) (bpbimH ) . Smaller production of the excited triplet state in the binuclear compound is ascribed to a rapid triplet-triplet annihilation process. A decay rate of the excited Ru(dmbpy) (bpbimH ) linked to Rh(phen) in butyronitrile was obtained by extrapolation of rates measured at lower temperatures. Mechanisms of the intramolecular reaction are discussed. 2

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R A T E S O F M A N Y N O N A D I A B A T I C electron-transfer reactions are c o n -

t r o l l e d b y b o t h t h e r m a l l y averaged F r a n c k - C o n d o n integrals a n d e l e c t r o n i c 0065-2393/91/0228-0215$06.00/0 © 1991 American Chemical Society

In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1991.

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c o u p l i n g b e t w e e n reactants. T h e q u a n t u m t h e o r y o f e l e c t r o n transfer p r e dicts a b e l l - s h a p e d energy-gap d e p e n d e n c e o n t h e F r a n c k - C o n d o n i n t e g r a l (1,2). Because o f this expectation, the energy-gap d e p e n d e n c i e s o f e l e c t r o n transfer rates have b e e n a n a l y z e d mostly i n t e r m s o f the t h e r m a l l y averaged F r a n c k - C o n d o n i n t e g r a l (3-9). S o m e t i m e s electron-transfer rates are e i t h e r slower t h a n p r e d i c t e d b y the F r a n c k - C o n d o n i n t e g r a l o r w e a k l y d e p e n d e n t o n t h e e n e r g y gap o f the process (6,10-13). I n these cases weak electronic c o u p l i n g b e t w e e n reactants has b e e n r e g a r d e d as responsible for the slow electron-transfer rates. E l e c t r o n i c c o u p l i n g has b e e n assumed to b e weak for b o t h long-range a n d s p i n i n v e r t e d electron-transfer processes. H o w e v e r , the i n t e r m o l e c u l a r e l e c t r o n i c c o u p l i n g i n transition-state e l e c t r o n transfer has s e l d o m b e e n e s t i m a t e d quantitatively because distance a n d o r i e n t a t i o n b e t w e e n transition-state reactants are b o t h u n k n o w n (14). T o investigate t h e e l e c t r o n i c - c o u p l i n g t e r m i n d e p e n d e n t l y i n t h e e l e c tron-transfer rate, i n t r a m o l e c u l a r e l e c t r o n transfers o c c u r r i n g i n b i e h r o m o phoric compounds should be studied. Electronic coupling between the c h r o m o p h o r e s i n these c o m p o u n d s can b e e s t i m a t e d b y spectroscopic m e t h ods (15-20). A c c o r d i n g l y , e l e c t r o n i c c o u p l i n g b e t w e e n Ru(II) a n d Ru(III) i n m i x e d - v a l e n c e b i n u c l e a r complexes has b e e n e x a m i n e d (15-19) to d e t e r m i n e a c o r r e l a t i o n b e t w e e n t h e electron-transfer rate a n d e l e c t r o n i c c o u p l i n g . M e t a l - m e t a l i n t e r a c t i o n i n t h e photoexcited states o f b i n u c l e a r Ru(II) c o m p o u n d s has attracted m u c h attention i n recent years. I f the m e t a l - m e t a l i n t e r a c t i o n exceeds 10 c m " , rates o f e l e c t r o n transfer, e n e r g y transfer, a n d t r i p l e t - t r i p l e t ( T - T ) a n n i h i l a t i o n can b e s t u d i e d . E l e c t r o n transfer takes place b e t w e e n a n excited-state Ru(II) site a n d a ground-state m e t a l site i f i t is energetically feasible (21-23). T h e rate o f m e t a l - m e t a l energy transfer (24-27) depends o n t h e excitation-energy difference b e t w e e n t h e acceptor a n d donor. W h e n a laser p u l s e is strong e n o u g h to excite t h e Ru(II) sites o f a b i n u c l e a r c o m p o u n d , such a n excited site w i l l u n d e r g o a n a n n i h i l a t i o n process w i t h a n e i g h b o r i n g site. 1

I n t r a m o l e c u l a r reactions (electron transfer, energy transfer, a n d T - T annihilation) i n b i n u c l e a r R u ( I I ) - R u ( I I ) a n d R u ( I I ) - R h ( I I I ) c o m p o u n d s have b e e n e x a m i n e d b y means o f laser flash k i n e t i c spectroscopy. A n i n t e r v e n i n g tetradentate l i g a n d , b i s - 2 , 2 ' - ( 2 " - p y r i d y l ) b i b e n z i m i d a z o l e ( b p b i m H ) (28), a n d 2 , 2 ' - b i b e n z i m i d a z o l e (29, 30) have a strong σ - d o n o r a n d w e a k ττ-aeeeptor p r o p e r t y i n c o m p a r i s o n w i t h 2 , 2 ' - b i p y r i d i n e (bpy). R e c e n t l y a b i n u c l e a r c o m p o u n d , R u ( b p y ) ( b p b i m H ) R u ( b p y ) , has b e e n s h o w n to behave as a dibasic a c i d b y u s i n g stepwise deprotonation from t h e i m i n o N - H groups o n the b r i d g i n g b p b i m H . T h e pK a n d p K are 5.61 a n d 7.12, r e s p e c t i v e l y , i n C H C N buffer (1:1 v / v ) . T h e p K for t h e m i x e d - v a l e n c e c o m p o u n d s [Ru(bpy) (bpbimH )Ru(bpy) ] and [Ru(dmbpy) (bpbimH )Rh(phen) ] (where d m b p y is 4 , 4 ' - d i m e t h y l - 2 , 2 ' - b i p y r i d i n e a n d p h e n is 1,10-phenan throline) are c o n s i d e r a b l y r e d u c e d to 1.2 (28) a n d 2.89 (31), r e s p e c t i v e l y . R u t h e n i u m - r u t h e n i u m i n t e r a c t i o n i n the m i x e d - v a l e n c e R u ( I I ) - R u ( I I I ) c o m 2

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p o u n d was e s t i m a t e d o n the basis of a weak i n t e r v a l e n c e t r a n s i t i o n to b e as s m a l l as 0.01 e V (28). T h e structure o f b i s - 2 , 2 ' - ( 2 ' - p y r i d y l ) b i b e n z i m i d a z o l e ;

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bpbimH Downloaded by MONASH UNIV on September 21, 2015 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch014

2

Experimental Details Compounds. Mononuclear ruthenium(II) compounds, R u ( L ) ( b p b i m H ) (L is bpy, dmbpy, and phen), and binuclear ruthenium(II-II), Ru(bpy) (bpbimH )R u ( b p y ) , were prepared as described elsewhere (28). A n asymmetric binuclear ruthenium compound, [Ru(dmbpy) (bpbimH )Ru(phen) ](Cl0 )4*5H 0, was prepared from Ru(phen) Cl (0.15 g, 0.28 mmol) with [Ru(dmbpy) (bpbimH )](C10 ) (0.3 g, 0.28 mmol) i n ethylene glycol (30 mL). The solid sample obtained was purified by recrystallization from methanol-water (4:1 v/v). Yield, 0.28 g (54%). Anal. Calcd. for C H 5 6 N 0 C l 4 R u - 5 H 0 : C , 47.85%; H , 3.58%; N , 10.85%. Found: C , 47.68%; H , 3.50%; N , 10.66%. A heterobinuclear compound, [Ru(dmbpy) (bpbimH )Rh(phen) ](C10 )5» was prepared by heating Rh(phen) (bpbimH )Cl (0.47 g, 0.41 mmol) and Ru(dmbpy) Cl (0.26 g, 0.45 mmol) in ethanol-water. The purification was effected by column chromatography on cross-linked dextran polymer beads (Sephadex LH-20) with methanol as eluent. Yield, 0.52 g (70%). Anal. Calcd. for C T ^ e o N n O a a C l s R u R h ^ H a O : C, 46.63%; H , 3.26%; N , 10.57%. Found: C , 46.76%; H , 3.59%; N , 10.51%. 2

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Apparatus. A Hitachi spectrofluorometer (MPF-2A) was used for phosphorescence spectra at 77 K . The Q-switched N d - Y A G laser (Quantel YG580) and a transmittance-change acquisition system used have been described elsewhere (32). The laser energies for 532- and 355-nm pulses were less than 80 and 40 m j , respectively. A xenon arc lamp (150 W) was 30 X intensified for 2 ms to improve the signal-to-noise ratio of the transmittance changes of the sample solutions. Time evolution of sample-solution transmittance and phosphorescence were recorded on a transient digital (10 bits) memory (Electronica C o . , E L K - 5 1 2 0 , 10 M H z ) or a storagescope (Iwatu C o . , 8123, 200 M H z , 8 bits). Oxidation potentials of ruthenium(II) compounds were measured by means of differential-pulse voltammetry with a direct-current pulse polarograph ( H E C S - 3 1 2 B , Huso, Japan). A l l voltammograms were obtained at a platinum disc electrode (d. 0.5 mm) i n C H C N containing 0.1 M tetrabutylammonium perchlorate. A l l potentials are referred to the formal potential of the ferrocenium-ferrocene ( F c - F c ) system, which is -0.33 V against a saturated calomel electrode (SCE). 3 +

3

+

Measurements. The sample solutions of ruthenium(II) compounds dissolved in acetonitrile, in butyronitrile, and i n a mixed solvent of ethanol and methanol (4:1 v/v) were deaerated by bubbling with nitrogen more than 12 min. Either H C 1 0 or 4


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CF3COOH (1 raM) was added to suppress deprotonation of the intervening ligand, b p b i m H . Production of the excited states of ruthenium(II) compounds on exposure to the second harmonic pulse (532 nm) of the Y A G laser was measured by monitoring the absorbance change of the sample solution. Time profiles of the transmittance i n a 580-780-nm region were corrected for the strong phosphorescence of the sample. The temperature of the sample solutions (89-300 K) was controlled by using a cryostat (Oxford DN1704) and a controller (Oxford ITC4). The sample temperature was mon itored by putting a thermocouple on a copper sample holder. 2


Results and Discussion I n t r a m o l e c u l a r m e t a l - m e t a l i n t e r a c t i o n i n b i n u c l e a r Ru(II) c o m p o u n d s d e pends o n an i n t e r v e n i n g l i g a n d a n d the electronic states of the m e t a l sites. A n i n t e r v e n i n g l i g a n d , b p b i m H , has a b i p h e n y l structure i n w h i c h the ττ electrons are w e a k l y conjugated t h r o u g h o u t two moieties of 2 - p y r i d y l - 2 ' b e n z i m i d a z o l e ( p b i m H ) . T h e d ^ electrons of one m e t a l site are able to m i x w i t h those of the o t h e r m e t a l site through the conjugated ττ a n d IT* electrons of the l i g a n d . Photoexcitation of a b i n u c l e a r R u c o m p o u n d to its m e t a l - t o l i g a n d charge-transfer ( M L C T ) state is manifested t h r o u g h p h o t o p h y s i c a l a n d p h o t o c h e m i c a l processes such as energy transfer f r o m one Ru(II) site to another i n an a s y m m e t r i c b i n u c l e a r c o m p o u n d , e l e c t r o n transfer f r o m a Ru(II) site to a Rh(III) site i n a h e t e r o b i n u c l e a r c o m p o u n d , a n d T - T a n n i h i l a t i o n i n a s y m m e t r i c a l b i n u c l e a r Ru(II) c o m p o u n d . 2

M L C T E x c i t e d State a n d E n e r g y T r a n s f e r . L o w e s t e x c i t e d states of m a n y r u t h e n i u m ( I I ) p o l y p y r i d i n e c o m p o u n d s are d e s c r i b e d as a p h o s phorescent state of R u —» l i g a n d charge transfer (33, 34). A transferred e l e c t r o n i n this l o c a l i z e d m o d e l occupies an o r b i t a l o n the most easily r e d u c e d l i g a n d . W h e n the redox potentials o f the adjacent ligands are close to that of the most easily r e d u c e d l i g a n d , e l e c t r o n h o p p i n g takes place a m o n g the ligands (34-39). T h e most easily r e d u c e d l i g a n d can b e assigned o n the basis o f redox potentials [E° ( L / L ) ] of R u L , unless the difference i n the redox potentials b e t w e e n ligands is subtle (38). T h e assignment of the e l e c t r o n - o c c u p i e d o r b i t a l (ligand) is feasible b y u s i n g e m i s s i o n from a n d absorption o f e x c i t e d states. E m i s s i o n spectra at 77 K , i n this case, are of no use for the assignment o f the e l e c t r o n - o c c u p i e d o r b i t a l because the v i b r o n i c progressions of p h o s phorescences are not distinct. 3

2 +

Transient a b s o r p t i o n spectra f o l l o w i n g laser excitation of R u ( b p y ) ( b p b i m H ) , R u ( d m b p y ) ( b p b i m H ) , and R u ( p h e n ) ( b p b i m H ) exhibit the ΤΓ-ΤΓ* b a n d of a r e d u c e d l i g a n d as a c o m p o n e n t , a n d this b a n d is strong e v i d e n c e for the identification of a r e d u c e d l i g a n d . B a n d s i n the — 3 7 0 - 4 2 0 a n d — 5 0 0 - 6 0 0 - n m regions i n F i g u r e s l a a n d l b can be assigned to a ττ-ττ* transition o f b p b i m H ~ a n d / o r a r e d - s h i f t e d ττ-ττ* of b p b i m H c o o r d i n a t i n g to Ru(III). T h e s e bands w e r e not e v i d e n t i n the excited-state a b s o r p t i o n 2

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CO .Ω CO

p h e n > b p b i m H ~ d m b p y . T h e decreasing o r d e r of l i g a n d effectiveness as a n e l e c t r o n acceptor i n the M L C T state is consistent w i t h the decreasing o r d e r o f t h e redox p o t e n t i a l of R u ( b p y ) (-1.34 V vs. SCE), Ru(phen) ( - 1 . 3 5 V vs. S C E ) , a n d R u ( d m b p y ) ( - 1 . 4 5 V vs. S C E ) (46). 2

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Transient absorption spectra for the b i n u c l e a r c o m p o u n d s [ R u ( b p y ) ] (bpbimH ) and [Ru(dmbpy) ] (bpbimH ) are almost i d e n t i c a l to those o f the c o r r e s p o n d i n g m o n o n u c l e a r c o m p o u n d s . B i m e t a l a t i o n is a c c o m p a n i e d b y a s m a l l r e d shift of the b a n d peak from 3 8 0 - 3 8 5 to 390 n m ; this change 2

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wavelength/nm Figure 2. Transient difference absorption spectra in a mixed solvent of ethanol and methanol (4:1 ν/υ) containing 1 X 10^ M HC10 or CF COOH at 100 ns after laser excitation at 89 K. a: [Ru(bpy) ] (bpbimH ) . b: [Ru(dmbpy) ] (bpbimH ) . c: Ru(phen) (bpbimH ) . 4

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suggests that less b p y " is f o r m e d i n the M L C T state. Ru(III) i n t h e singly excited M L C T state of the b i n u c l e a r c o m p o u n d s p r o b a b l y interacts w i t h u n e x c i t e d Ru(II) to stabilize the phosphorescent state. T h e f o l l o w i n g list shows the phosphorescence energies (E i n r e c i p r o c a l centimeters) o f the R u c o m p o u n d s . Ru(bpy) (bpbimH ) 2

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T h e e n e r g y shift of e m i s s i o n o b s e r v e d for [Ru(bpy) ] ( b p b i m H ) (130 c m ) is as s m a l l as the R u ( I I I ) - R u ( I I ) i n t e r a c t i o n (—80 c m ) e s t i m a t e d f r o m the i n t e n s i t y of i n t e r v a l e n c e transition (28). T h e result of d i f f e r e n tial-pulse v o l t a m m e t r y o n [ R u ( b p y ) ] ( b p b i m H ) confirms that the Ru(III)-Ru(IÎ) i n t e r a c t i o n is less than 0.040 e V (320 c m " ) . T h e weak effect of d i m e t a l a t i o n i n the b p b i m H c o m p o u n d s is r e l a t e d to w e a k e l e c t r o n i c c o u p l i n g b e t w e e n the p b i m H moieties of b p b i m H , w h i c h are not coplanar o w i n g to p r o t o n - p r o t o n r e p u l s i o n . I n b p b i m H c o m p o u n d s the d i m e t a l a t i o n effects o n e m i s s i o n energy are m u c h s m a l l e r t h a n those o b t a i n e d for [ R u ( b p y ) ] ( b p y m ) (47-49) a n d [ R u ( b p y ) ] ( d p p ) (47-51), w h e r e b p y m a n d d p p are 2 , 2 ' - b i p y r i m i d i n e a n d 2,3-bis(pyridyl)pyrazine, r e s p e c t i v e l y . I n the latter cases, the l o w e r energy e m i s s i o n was a s c r i b e d to the r e d u c t i o n potentials of b p y m or d p p , w h i c h are 0.4 V less negative t h a n those of the c o r r e s p o n d i n g m o n o n u c l e a r c o m p o u n d s (50-52). To see w h e t h e r the weak m e t a l - m e t a l i n t e r a c t i o n allows h o p p i n g of M L C T b e t w e e n the Ru(II) sites, w e e x a m i n e d an a s y m m e t r i c b i n u c l e a r c o m p o u n d , R u ( d m b p y ) ( b p b i m H ) R u ( p h e n ) . E x c i t a t i o n - e n e r g y transfer f r o m the R u - p h e n site to the R u - d m b p y site is energetically possible; the assigned excitation energies are 16,700 c m " for R u ( p h e n ) ( b p b i m H ) and 1 6 , 3 0 0 c m " for R u ( d m b p y ) ( b p b i m H ) . T h e p h o s p h o r e s c e n c e o f Ru(dmbpy) (bpbimH )Ru(phen) o b s e r v e d at 77 Κ (16,100 c m " ) is e m i t t e d f r o m the R u - d m b p y site. Because the e m i s s i o n l i f e t i m e of R u ( p h e n ) (bpbimH ) is close to 4 μ8 at 90 K , the energy-transfer rate is estimated as larger than 3 Χ 1 0 s" . 2

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T h e E S A s p e c t r u m of R u ( d m b p y ) ( b p b i m H ) R u ( p h e n ) immediately after laser excitation at 300 Κ c o m p l e t e l y agrees w i t h that of R u ( d m b p y ) ( b p b i m H ) . T h e characteristic b l e a c h i n g of the excited R u - p h e n i n the 4 2 0 - 4 3 0 - n m r e g i o n was not o b s e r v e d at a l l d u r i n g the laser excitation, a result i n d i c a t i n g that the e n e r g y transfer at 300 Κ took place d u r i n g laser excitation (k > 1 0 s" ). T h e energy transfer f r o m the R u - p h e n site to the R u - d m b p y site occurs v i a a consecutive process, an electron-energy transfer "cascade" (22, 25, 47). E l e c t r o n transfer f r o m p h e n to b p b i m H generates R u —» b p b i m H C T i n one R u site, a n d hole transfer f r o m Ru(III) to Ru(II) generates R u —» b p b i m H C T , w h i c h is i n e q u i l i b r i u m w i t h R u —> d m b p y CT. E l e c t r o n transfer b e t w e e n adjacent ligands has b e e n i n t e r p r e t e d as exc i t o n h o p p i n g , w i t h an activation energy e q u a l to the e n e r g y difference 2

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b e t w e e n the M L C T states (38, 39). A s m a l l energy difference b e t w e e n R u —» p h e n C T a n d R u —» b p b i m H C T m a y not suppress the e l e c t r o n transfer rate i n the R u ( d m b p y ) ( b p b i m H ) R u ( p h e n ) . H o l e transfer, w h i c h is exergonic (~0.1 e V ) , is the r a t e - d e t e r m i n i n g process. T h e hole-transfer rate w i l l be discussed i n c o n j u n c t i o n w i t h a rate of an i n t r a m o l e c u l a r e l e c t r o n transfer o c c u r r i n g i n R u ( d m b p y ) ( b p b i m H ) R h ( p h e n ) . 2

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2

5 +

A n alternative m e c h a n i s m of energy transfer, d i p o l e - d i p o l e i n t e r a c t i o n m e c h a n i s m , is i m p r o b a b l e because the e n e r g y - m a t c h i n g r e q u i r e m e n t is not fulfilled b e t w e e n the R u - p h e n emission a n d the R u - d m b p y absorption. I f the s p i n - f o r b i d d e n M L C T absorption of the acceptor c h r o m o p h o r e w e r e i n a r e g i o n of the d o n o r e m i s s i o n , the d i p o l e - d i p o l e i n t e r a c t i o n m e c h a n i s m c o u l d b e c o m p e t i t i v e w i t h the exchange m e c h a n i s m (24-27).

Intramolecular

Electron Transfer in

Ru(dmbpy) (bpbimH )2

2

Rh(phen) . T h i s c o m p o u n d exhibits the s u m of the a b s o r p t i o n spectra of the c o m p o n e n t c o m p o u n d s . T h e phosphorescence a n d E S A of R u —> d m b p y C T w e r e c o m p l e t e l y q u e n c h e d i n the 1 m M H C 1 0 i n C H C N . B o t h the phosphorescence a n d the E S A of the R u - d m b p y site w e r e d e t e c t e d for several h u n d r e d nanoseconds i n the n e u t r a l m e d i u m , w h e r e d e p r o t o n a t i o n f r o m an i m i n o N - H group of p b i m H m o i e t y c o o r d i n a t i n g to Rh(III) was o b s e r v e d b y means of absorption spectroscopy. T h e d e p r o t o n a t i o n shifts the r e d u c t i o n p o t e n t i a l of the R h site negatively from - 1 . 1 5 to - 1 . 4 5 V vs. F c - F c , b u t does not change the oxidation p o t e n t i a l of the R u site (0.68 V vs. F c - F c ) i n R u ( b p y ) ( b p b i m H ) R h ( b p y ) . T h e e r g o n i c i t y of Ru(II) - > Rh(III) electron transfer i n the excited state of Ru(dmbpy) (bpbimH )Rh(phen) can b e r e g a r d e d as - 0 . 1 6 a n d 0.14 e V for the a c i d i c f o r m a n d the basic f o r m , r e s p e c t i v e l y , as it is for Ru(bpy) ( b p b i m H ) R h ( b p y ) . T h e r e f o r e , the phosphorescence q u e n c h i n g i n the acidic m e d i u m is a t t r i b u t e d to Ru(II) —» Rh(III) e l e c t r o n transfer. 2

5+

4

3

+

+

2

2

2

2

5 +

2

2

2

5 +

2

2

2

5 +

T h e rate of Ru(II) —» Rh(III) e l e c t r o n transfer was m e a s u r e d o n c o o l i n g the b i n u c l e a r c o m p o u n d i n b u t y r o n i t r i l e (mp 126 K ) . T h e decay rate of the e x c i t e d R u - d m b p y site m o n i t o r e d at 400 a n d 560 n m was c o n s i d e r a b l y d e p e n d e n t o n t e m p e r a t u r e . T h e rate constants of e l e c t r o n transfer (k ) w e r e o b t a i n e d f r o m the excited-state l i f e t i m e of the b i n u c l e a r c o m p o u n d b y s u b tracting that of R u ( d m b p y ) ( b p b i m H ) . T h e value of fc at 300 K , e s t i m a t e d b y extrapolation to b e 2 Χ 1 0 s" ( F i g u r e 3), is not v e r y different f r o m the Ru(II)-to-Ru(II) energy-transfer rate ( > 1 0 s" ) of R u ( d m b p y ) (bpbimH )Ru(phen) i n C H C N . I n Ru(II) Rh(III) e l e c t r o n transfer, the e l e c t r o n r e s i d i n g o n one p b i m H m o i e t y of b p b i m H moves to Rh(III) t h r o u g h the other p b i m H m o i e t y . I n the energy transfer f r o m the R u - p h e n site to the R u - d m b p y site, e l e c t r o n transfer from a p h e n to b p b i m H was f o l l o w e d b y hole transfer f r o m the R u ( I I I ) - p h e n to the R u ( I I ) - d m b p y , w h i c h ET

2

2 +

2

8

ET

1

8

2

2

4 +

1

2

3

2

2



224


T-'XIOVK"

1

Figure 3. Temperature dependence of electron-transfer rate constant (k r) of Ru(II) —» Rh(IH) in the excited MLCT state of Ru(dmbpy) (bpbimH )Rh(phen) . [HCl0 ] is 1 mM in butyronitrile. E

2

5+

2

2

4

takes place v i a b p b i m H . T h e r e f o r e , i t is reasonable that t h e rates o f the two processes a r e similar. A r a p i d r e c o v e r y o f the M L C T b a n d at 460 n m w i t h o u t delay after t h e decay o f E S A leads to t h e c o n c l u s i o n that back electron transfer (Rh(II) —» Ru(III)) is as r a p i d as f o r w a r d e l e c t r o n transfer. R a p i d back e l e c t r o n transfer is consistent w i t h t h e h i g h exergonicity (1.82 e V ) i n v o l v e d i n t h e process. T h e exergonicity o f 1.7 e V gives rise to the m a x i m u m rate for several e l e c tron-transfer reactions w i t h i n a cage f o l l o w i n g i n t e r m o l e c u l a r electron-transfer q u e n c h i n g (5, 6, 32). S p i n - f l i p , w h i c h is r e q u i r e d for t h e back e l e c t r o n transfer, [ R u ( I I I ) - R h ( I I ) ] - > V R ^ I I ) - ^ ! ! ! ) ] , may suppress the rate to 2

3

2

2


14.

O H N O ET AL.

225

Binuclear Compounds of RutheniumÇH)

some extent. T h e Ru(II) —» Rh(III) e l e c t r o n transfer i n R u ( d m b p y ) (bpbimH )Rh(phen) is as fast as that i n a m i x e d - v a l e n c e c o m p o u n d of R u ( I I ) - R u ( I I I ) b r i d g e d b y p y r a z i n e (21). 2


2

2

5 +

Intramolecular T r i p l e t - T r i p l e t Annihilation. Aromatic comp o u n d s i n the excited state u n d e r g o excited-state b i m o l e c u l a r a n n i h i l a t i o n r e s u l t i n g i n the formation o f a h i g h e r excited state a n d the g r o u n d state (53). T - T a n n i h i l a t i o n of some d y e m o l e c u l e s , s u c h as m e t a l - p o r p h y r i n (54) a n d l u m i f l a v i n e (55) i n the t r i p l e t excited state, efficiently o c c u r r e d to p r o d u c e a cation radical a n d an a n i o n radical i n p o l a r m e d i a . A s for the M L C T t r i p l e t excited state o f a Ru(II) c o m p o u n d s u c h as R u ( b p y ) , a c h a n n e l of T - T a n n i h i l a t i o n is energetically possible. E l e c t r o n transfer b e t w e e n excited-state m e t a l sites is strongly exergonic (1.6 eV) b e cause o f t h e i r excitation energies (2 X 2.1 e V ) , a l t h o u g h d i s p r o p o r t i o n a t i o n of two Ru(II) sites i n the g r o u n d state is e n d e r g o n i c (2.6 eV). T h e r e f o r e , w h e t h e r i n t r a m o l e c u l a r T - T a n n i h i l a t i o n takes place o r not is d e p e n d e n t o n the d e n s i t y o f the excited c h r o m o p h o r e s i n a b i n u c l e a r c o m p o u n d a n d R u ( I I ) - R u ( I I ) i n t e r a c t i o n t h r o u g h an i n t e r v e n i n g ligand. 3

2 +

T h e i n t e n s i t y of the 5 3 2 - n m laser was insufficient to c o n v e r t 40 μπιοί/ d m of R u ( b p y ) ( b p b i m H ) or [ R u ( b p y ) ] ( b p b i m H ) to the M L C T ex c i t e d state. To d e t e r m i n e the p r o d u c t i o n of the excited state, energy transfer from the r u t h e n i u m ( I I ) c o m p o u n d to anthracene was u s e d . A d d i t i o n o f anthracene (

Electron Transfer, Energy Transfer, and Excited-State Annihilation in

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