Electron Transfer in Inorganic, Organic, and ... - ACS Publications

8 Solvent-Dependent Photophysics of Fixed-Distance ChlorophyllDownloaded by COLUMBIA UNIV on March 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch008

Porphyrin Molecules The Possible Role of Low-Lying Charge-Transfer States Michael R. Wasielewski , Douglas G . Johnson , Mark P. Niemczyk , George L . Gaines III , Michael P. O'Neil , and Walter A. Svec 1

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Chemistry Division and Biological, Environmental, and Medical Research Division, Argonne National Laboratory, Argonne, IL 60439 2

The properties of a series of fixed-distance chlorophyll-porphyrin molecules are described. These molecules consist of a methyl pyrochlorophyllide a moiety that is directly bonded at its 2-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-(ptolyl)porphyrin. Steric hindrance between adjacent substituents rig idly positions the πsystems of both macrocycles perpendicular to each other. The macrocycles were selectively metalated with zinc to give the four possible derivatives, HCHP, ZCHP, HCZP, and ZCZP (where H, Z, C, and Ρ denote free base, Zn derivative, chlorin, and porphyrin, respectively). The lowest excited singlet states of HCHP and ZCHP, which are localized on HC and ZC, respectively, exhibit lifetimes and fluorescence quantum yields that are solvent-polarityindependent and do not differ significantly from those of chlorophyll itself. ZCZP and HCZP, however, display solvent-polarity-dependent photophysics. HCZP forms an ion-pair state following excitation in polar media, although ZCZP does not. Nevertheless, nonradiative decay is substantially enhanced in ZCZP as the solvent polarity in creases. These effects are discussed in terms of mixing low-lying charge-transfer states of ZCZP into its locally excited singlet state. Enhanced nonradiative decay of excited heterodimers within bacte rial reaction centers has recently been observed. 0065-2393/91/0228-0133$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.

134

E T IN INORGANIC, ORGANIC, A N D BIOLOGICAL SYSTEMS

IN PHOTOSYNTHETIC REACTION CENTERS

the e l e c t r o n donors a n d acceptors are p o s i t i o n e d at precise distances a n d orientations relative to one another to p r o m o t e efficient p h o t o i n d u c e d charge separation a n d to i m p e d e charge r e c o m b i n a t i o n (1-3). H o w e v e r , the nature of the m e d i u m that engulfs each d o n o r - a c c e p t o r p a i r is thought to have a large influence o n the o b s e r v e d rates o f e l e c t r o n transfer. F o r example, i n the b a c t e r i a l p h o t o s y n t h e t i c r e action c e n t e r a b a c t e r i o c h l o r o p h y l l ( B C h l ) m o l e c u l e lies b e t w e e n the d i m e r i c b a c t e r i o c h l o r o p h y l l d o n o r ( B C h l ) a n d the b a c t e r i o p h e o p h y t i n ( B P h ) a c c e p tor. T h e IT systems of these c h r o m o p h o r e s l i e at large angles relative to one another (about 70°) i n an approximate edge-to-edge configuration. It is t h o u g h t that superexchange, w h i c h mixes l o w - l y i n g , i o n i c , v i r t u a l states i n v o l v i n g the i n t e r m e d i a t e B C h l w i t h the locally e x c i t e d state of the d o n o r , m a y l e a d to a greatly increased rate of charge separation {4-7).

Downloaded by COLUMBIA UNIV on March 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch008

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R e c e n t l y , b o t h p h o t o c h e m i c a l h o l e - b u r n i n g e x p e r i m e n t s (8-11) a n d Stark effect spectroscopy (12, 13) o n reaction centers have b e e n r e p o r t e d . T h e results of these e x p e r i m e n t s suggest that excitation of B C h l p r o d u c e s an e x c i t e d state w i t h significant charge-transfer (CT) character. A r e c e n t report o n reaction centers f r o m genetically a l t e r e d Rhodobacter capsuhtus, i n w h i c h the B C h l d o n o r is transformed i n t o a B C h l - B P h h e t e r o d i m e r , shows that excitation of this species results i n a 5 0 % q u a n t u m y i e l d of n o n radiative decay to g r o u n d state w i t h a 30-ps t i m e constant (14). T h e m a j o r i t y of femtosecond spectroscopic e x p e r i m e n t s of native reaction centers show no e v i d e n c e for i n t r a d i m e r charge-separated i n t e r m e d i a t e s (15-17). H o w ever, recent femtosecond transient absorption measurements suggest that a distinct B C h l " i n t e r m e d i a t e m a y b e i n v o l v e d i n the p r i m a r y charge separation (18). 2

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H o w e v e r , the i m p a c t of l o w - l y i n g v i r t u a l states possessing C T character o n the photophysics of p o r p h y r i n a n d c h l o r o p h y l l d o n o r - a c c e p t o r m o l e c u l e s r e m a i n s unclear. W e r e c e n t l y d e m o n s t r a t e d that a c h l o r o p h y l l d i m e r can u n d e r g o s o l v e n t - i n d u c e d s y m m e t r y b r e a k i n g w i t h i n the e x c i t e d singlet m a n ifold (19). T h e result of this s y m m e t r y b r e a k i n g is e n h a n c e d n o n r a d i a t i v e decay of the e x c i t e d state w i t h o u t the appearance of distinct charge-separated i n t e r m e d i a t e s as m o n i t o r e d b y transient absorption spectroscopy. S t r i c t l y speaking, C T - s t a t e formation w i t h i n a s y m m e t r i c d i m e r is s y m m e t r y f o r b i d d e n . H o w e v e r , b y p r o v i d i n g different e n v i r o n m e n t s for each of the t w o c h r o m o p h o r e s , solvent molecules can break the s y m m e t r y a n d t h e r e b y a l l o w the formation of C T states. A w e l l - c h a r a c t e r i z e d example of a m o l e c u l e that undergoes s o l v e n t - i n d u c e d s y m m e t r y b r e a k i n g is the s y m m e t r i c b i c h r o m o p h o r e 9 , 9 ' - b i a n t h r y l (20, 21). Because solvation is adequate to destroy the s y m m e t r y a n d a l l o w the f o r m a t i o n of C T states i n m o d e l systems, i t is reasonable that the e n v i r o n m e n t of the reaction-center p r o t e i n s u r r o u n d i n g the d i m e r i c d o n o r is sufficiently a s y m m e t r i c to p r o m o t e m i x i n g of C T states w i t h the locally excited singlet state of the d i m e r (J). M a t a g a a n d co-workers have r e c e n t l y discussed several i n t e r e s t i n g features r e g a r d i n g the f o r m a t i o n of C T states i n s y m m e t r i c b i c h r o m o p h o r e s (22, 23).


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Chlorophyll-Porphyrin

Molecules

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T o b e t t e r u n d e r s t a n d the influence of l o w - l y i n g C T states o n the o b servable photophysies of c h l o r o p h y l l a n d p o r p h y r i n d o n o r - a c c e p t o r m o l e cules, w e p r e p a r e d a series of c h l o r o p h y l l - p o r p h y r i n molecules i n w h i c h the two macrocycles m a i n t a i n a fixed distance a n d o r i e n t a t i o n relative to one another. M o l e c u l e s H C H P , Z C H P , H C Z P , a n d Z C Z P (where H , Z , C , a n d Ρ d e n o t e free base, Z n d e r i v a t i v e , c h l o r i n , a n d p o r p h y r i n , respectively) possess a c h l o r o p h y l l m o l e c u l e r i g i d l y b o u n d to a p o r p h y r i n such that the IT systems of b o t h macrocycles are constrained b y steric interactions to b e p e r p e n d i c u l a r to each other. T h i s geometry was chosen to m i m i c a p p r o x i m a t e l y the g e o m e t r i c r e l a t i o n s h i p b e t w e e n the B C h l d o n o r a n d the i n t e r m e d i a r y B C h l w i t h i n photosynthetic reaction centers. Downloaded by COLUMBIA UNIV on March 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1991 | doi: 10.1021/ba-1991-0228.ch008

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Experimental Details Solvents for all spectroscopic experiments were dried and stored over 3-Â molecular sieves. H P L C - g r a d e toluene was distilled from L i A l H (LAH). Butyronitrile was refluxed over K M n 0 and N a C 0 , then twice distilled; we retained the middle portion each time. 2-Methyltetrahydrofuran ( M T H F ) was freshly distilled from L A H before each experiment. U V - v i s i b l e absorption spectra were taken on a Shimadzu UV-160 spectrometer. Fluorescence spectra were obtained by using a P e r k i n - E l m e r M P F - 2 A fluorometer interfaced to a P D P 11/34 computer. A l l samples for fluorescence were purified by preparative thin-layer chromatography (TLC) on Merck silica gel plates. Samples for fluorescence measurements were ΙΟ" M in 1-cm cuvettes. The emission was meas ured 90° to the excitation beam. Fluorescence quantum yields were determined by integrating the digitized emission spectra from 600 to 800 nm and referencing the integral to that for chlorophyll a in diethyl ether (24). Redox potentials for H C H P , H C Z P , Z C H P , and Z C Z P were determined i n butyronitrile containing 0.1 M tetra-n-butylammonium perchlorate by using a Pt disc electrode at 21 °C. These potentials were measured relative to a saturated calomel electrode using ac voltammetry (25). Both one-electron oxidations and reductions of these molecules exhibited good reversibility. The picosecond transient absorption and emission measurements were obtained by using apparatus described previously (26). The fluorescence lifetimes and error limits given i n Table I are the average of at least three measurements. Lifetimes >1 ns were measured with a Hamamatsu R1294U microchannel plate photomultip l i e r / T E K 7912 digitizer combination. The total system possessed a time response of 0.8 ns. Lifetimes 2 5 for Z C Z P a n d about 8 for H C Z P . I n a d d i t i o n , w e can use the d i s t i n c t spectroscopic signature of the Soret bands to detect the i n v o l v e m e n t of c h l o r i n or p o r p h y r i n states i n the o v e r a l l excited-state d e s c r i p t i o n of these m o l e c u l e s . F i g u r e 2 shows the e n e r g y levels o f the locally e x c i t e d states o f the c h r o m o p h o r e s w i t h i n H C H P , Z C H P , H C Z P , a n d Z C Z P a n d several h y p o thetical C T states. T h e energies of the e x c i t e d states are d e t e r m i n e d f r o m the p o s i t i o n of the (0,0) b a n d of the fluorescence e m i s s i o n . T h e h y p o t h e t i c a l C T state energies are estimated from a s i m p l e s u m of the o n e - e l e c t r o n


138


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Z

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HC

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2.2 — 2.1

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1

HC* HPZC*

1

2.5

HC- HP* zc- ZP*

2.0 — 1.9 —

HC* ZP*

1.8

—

1.7 — Figure 2. Energy-level diagrams for low-lying locally excited singlet states and charge-transfer states of HCHP, HCZP, ZCHP, and ZCZP in butyronitrile. oxidation a n d r e d u c t i o n potentials of the c h r o m o p h o r e s m e a s u r e d w i t h i n each m o l e c u l e i n b u t y r o n i t r i l e . T h e s e C T - s t a t e energies are reasonable estimates o n l y i n h i g h l y polar m e d i a , w h e r e the ions are strongly solvated a n d the c o u l o m b i c i n t e r a c t i o n b e t w e e n the ions is small. T w o i m p o r t a n t features of the energetics o f these molecules can b e n o t e d f r o m these data. F i r s t , most o f the p r e d i c t e d C T states are l o w - l y i n g (i.e., energetically close to S ^ . S e c o n d , o n l y H C ~ Z P is b e l o w the locally e x c i t e d singlet states o f the c h r o m o p h o r e s . T h e next l o w e s t - l y i n g state belongs to Z C ~ Z P . T h u s , one m i g h t expect H C Z P a n d Z C Z P to show the greatest p e r t u r b a t i o n o f t h e i r locally e x c i t e d singlet states b y m i x i n g w i t h the C T states. +

+

F i g u r e 3 gives the fluorescence q u a n t u m yields o f H C H P , Z C H P , H C Z P , a n d Z C Z P , a l o n g w i t h the z i n c a n d free-base m e t h y l p y r o p h e o p h o r b i d e a reference c o m p o u n d s . W e f o u n d that e n e r g y transfer from the p o r p h y r i n to the c h l o r i n i n each o f these molecules proceeds i n t i m e s < 1 ps; therefore, the locally e x c i t e d singlet state resides o n the c h l o r i n . T h e o n l y c o m p o u n d s i n the series that show s o l v e n t - p o l a r i t y - d e p e n d e n t fluorescence q u a n t u m yields are H C Z P a n d Z C Z P . T h e e m i s s i o n from b o t h molecules is strongly q u e n c h e d i n p o l a r m e d i a . T h e static d i e l e c t r i c constant r e q u i r e d to d i m i n i s h the e m i s s i o n y i e l d o f Z C Z P is somewhat h i g h e r t h a n that o f H C Z P . T a b l e I presents fluorescence l i f e t i m e data for Z C Z P a n d H C Z P i n the same solvents u s e d to o b t a i n the q u a n t u m y i e l d data. F i g u r e 3 a n d T a b l e I show that the r e d u c t i o n i n e m i s s i o n q u a n t u m y i e l d closely parallels the o b s e r v e d decrease i n fluorescence l i f e t i m e as the solvent p o l a r i t y increases. T h i s finding s u g gests that i n c r e a s i n g the solvent p o l a r i t y results i n a n e w n o n r a d i a t i v e c h a n -



8.

WASIELEWSKI ET AL.

Ο

Chlorophyll-Porphyrin

δ

10

139

Molecules

15

20

DIELECTRIC CONSTANT Figure 3. Dependence of fluorescence quantum yields on solvent constant for the indicated compounds.

dielectric

n e l that competes w i t h radiative decay. T h e nature of this c h a n n e l can b e e x a m i n e d b y l o o k i n g for transient spectral changes that arise f o l l o w i n g ex citation of H C Z P a n d Z C Z P . F i g u r e s 4 A a n d 4 B show the transient absorption difference s p e c t r u m of Z C Z P i n toluene (molar absorption € = 2.4) a n d i n b u t y r o n i t r i l e (e = 20) o b t a i n e d 20 ps after a 1-ps laser flash at 610 n m . I n b o t h solvents the Soret b a n d d u e to the c h l o r i n c h r o m o p h o r e at 430 n m bleaches w i t h i n the 1-ps t i m e of the laser flash. T h e r e is no e v i d e n c e for b l e a c h i n g of the p o r p h y r i n Soret b a n d at 414 n m . S i m i l a r l y , i n b o t h solvents ( F i g u r e 4B) the Q b a n d of the c h l o r i n bleaches i m m e d i a t e l y u p o n excitation. T h e r e is no e v i d e n c e of significant absorption i n the n e a r - i n f r a r e d r e g i o n of the s p e c t r u m , w h e r e the r a d i c a l i o n states of b o t h p o r p h y r i n s a n d c h l o r i n s have absorbances (28-30). F i g u r e 5 shows the r e c o v e r y of the Q b a n d b l e a c h as a f u n c t i o n o f t i m e . T h e data show that the excited state recovers m u c h faster i n polar m e d i a than i n n o n p o l a r m e d i a , 119 ps vs. 3.4 ns, respectively. T h e excited-state r e c o v e r y times o b t a i n e d v i a transient absorption agree w e l l w i t h those o b t a i n e d from fluorescence e m i s s i o n decays, as s h o w n i n T a b l e I. T h r o u g h o u t the r e c o v e r y t i m e of the absorption changes, the spectral features i l l u s t r a t e d i n F i g u r e s 4 A a n d 4 B o n l y d i m i n i s h . N o n e w features or changes i n b a n d shape or structure are o b s e r v e d . F i g u r e s 6 A a n d 6 B show the c o r r e s p o n d i n g transient absorption changes y

y


140



0.1-1

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1

380

r 400

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Electron Transfer in Inorganic, Organic, and ... - ACS Publications

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