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    MIMICKING PHOTOSYNTHETIC ELECTRON TRANSFER

    DEVENS GUST, THOMAS A. MOORE, AND ANA L. MOORE

    Department o f Chemistry an d Cente r for the Study o f Ear ly Eventsin P h o t o s y n t h e s i s , Ar izona S t a t e U n i v e r s i t y, Tempe, Ar izon a ,85287, USA.

    ABSTRACT

    The p h o t o s y n t h e t i c r eac t ion c e n t e r s o f p l a n t s an d b a c t e r i aare p h o t o v o l t a i c d e v i c e s on th e molecu l a r sca le which conver tl i g h t energy in to chemical p o t e n t i a l energy in th e form of long-l i ved , e n e rg e t i c charge separa ted s t a t e s . It is now p o s s i b l e toprepare syn the t i c mult icomponent molecu les which mimic importanta s p e c t s o f this p r o c e s s . F o r e x a m p l e , one o f th e k ey s toreaction center f u n c t i o n is a multistep electron transfers t r a t egy. In this paper, two genera l types o f mul t i s t ep e lec t ront r a n s f e r, s e q u e n t i a l an d p a r a l l e l , are descr ibed an d illustratedwith seve ra l s y n t h e t i c triad and pentad molecules .

    INTRODUCTION

    In p h o t o s y n t h e t i c organisms, th e convers ion o f l i g h t energyi n to u se fu l p o t e n t i a l energy t akes place in a s t ruc tu re known asth e reaction center. The reaction center is actually ap h o t o v o l t a i c device which ope ra t e s a t t he molecu l a r l e v e l . Itu ses th e energy o f a photon to transfer an e l e c t r o n a c r o s s th et h i ckness o f a lipid b i l aye r membrane an d genera te an ene rge t i c ,l o n g - l i v e d charge separa ted s t a t e . The p o t e n t i a l energy o f thisstate is t hen e x p l o i t e d by th e organism in a number o f ways.Pho to s yn the s i s is an extremely successfu l s o l a r energy ha rves t ingp r o c e s s , and as a result th e d e s i g n , s y n t h e s i s and s t u d y ofartificial photosynthe t ic systems which mimic some aspec ts o f th e

    n a t u r a l proces s is an ac t ive field o f r e sea rch . The knowledgegleaned from such s t u d i e s can n o t only tell us more a b o u t hown a t u r a l pho tosyn thes i s works, but a l so con t r ibu te to t he de s igno f man-made solar e n e rg y c o n v e r s i o n s y s t e m s and m o l e c u l a re lec t ron ic devices .

    Mimicry of th e photosynthe t ic r eac t ion center would a t firsta p p e a r to be a f o rmidab l e t a s k , as th e t y p i c a l r e a c t i o n cen t e rfrom a p h o t o s y n t h e t i c bac te r ium c o n t a i n s t h o u s a n d s o f atoms.However, most o f th e mass o f th e r e a c t i o n c e n t e r is a s s o c i a t e dwi th protein material, wher ea s th e b a s i c p h o t o c h e m i s t r y isca r r i ed out by a few r e l a t ive ly smal l organic co fac to r s . A majorr o l e o f th e p r o t e i n is to hold t h e o rg a n i c c o f a c t o r s in th es p a t i a l arrangement an d environment necessary fo r pho tosyn the t i celectron and ene rgy transfer. One a p p r o a c h to artificialp h o t o s y n t h e s i s is to use s y n t h e t i c organ ic pigments , electrondonors , an d acceptors s i m i l a r to those found in n a t u r a l r eac t ion

    Mat. Res. Soc. Symp. Proc. Vol. 218. 1991 Materials Research Society

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    c e n t e r s , bu t to r ep l ace th e structural ro le o f th e p r o t e i n withcovalent chemical l i nkages . This is th e approach which w i l l bediscussed below.

    NATURAL PHOTOSYNTHETIC ELECTRON TRANSFER

    The o rg a n i c c o f a c t o r s found in th e reaction c e n t e r s ofpho tosyn the t i c bac te r i a include two bac te r ioch lo rophy l l moleculesin a " s p e c i a l pair," tw o a c c e s s o r y b a c t e r i o c h l o r o p h y l l s , tw ob a c t e r i o p h e o p h y t i n s ( b a c t e r i o c h l o r o p h y l l s in which th e c e n t r a lmagnesium atom is rep laced by hydrogens), two quinone molecu les ,and a c a r o t e n o i d polyene . These molecules a re embedded wi th in

    th e p r o t e i n ,which

    in t u rnspans a

    lipid b i l a y e r membrane. Asm e n t i o n e d above, th e pu rpose o f th e r e a c t i o n c e n t e r is to uselight energy to sepa ra t e charge ac ros s t he membrane. However,th e t r an sm embrane charge s e p a r a t i o n is n o t carried o u t in as i n g l e l ong - r ange e l e c t r o n transfer event . Elec t ron t r a n s f e racross the e n t i r e t h i c k n e s s o f th e b i l a y e r would be to o slow tocompete w i th t h e o t h e r p r o c e s s e s which depopu la t e t he spec i a lp a i r b a c t e r i o c h l o r o p h y l l e x c i t e d s i n g l e t state, which func t ionsas the primary e l ec t ron donor. As a r e s u l t , th e r eac t ion centersemploy a mul t i s t ep e l ec t ron t r a n s f e r s t r a t egy whereby an e lec t ronis moved across th e t h i cknes s o f th e b i l a y e r membrane in a se r i e s

    o f s h o r t range , f a s t , and efficient s t e p s . The proces s beginswith t r a n s f e r o f an e lec t ron from th e s p e c i a l p a i r first exc i t edsinglet state to a b a c t e r i o p h e o p h y t i n , a i d e d by one of th eaccessory bac te r ioch lo rophy l l s . From the bac ter iopheophyt in , anelectron is d o n a t e d to one of th e q u i n o n e s . Th e resultingr a d i c a l anion in t u rn reduces t he o the r quinone , which r e s i d e snear one side of th e membrane. The pos i t ive charge which is l e f ton th e s p e c i a l p a i r is n e u t r a l i z e d by e l e c t r o n donat ion from ac y t o c h r o m e l o c a t e d on t h e o t h e r s i d e of th e membrane. Thiss e q u e n c e o f e v e n t s p r o d u c e s energetic, long-lived c h a r g esepara ted s t a t e s in high quantum y i e l d .

    ARTIFICIAL PHOTOSYNTHETIC MOLECULES

    It has been o f particular interest in our l a b o r a t o r i e s tod e v i s e s y n t h e t i c molecu l e s which can be used to e x p l o r e thism u l t i s t e p e l e c t r o n transfer s t r a t e g y. A c t u a l l y, t h e r e a re tw og e n e r a l c l a s s e s o f this phenomenon which can be i n v e s t i g a t e d .The first, which is illustrated in e q u a t i o n 1, is s e q u e n t i a lmul t i s t ep e l ec t ron t r a n s f e r.

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    1 2"D-AI"A2 "' D+'A ' -A 2 -- D+'A 1 -A 2 " (1)

    1j,3D-A1 -A2

    Equat ion 1 f ea tu re s a t h ree -pa r t , or triad molecule cons i s t ing ofan e lec t ron donor D cova len t ly l i nked to two accep to r mo ie t i e s .The exc i t ed state o f th e donor, *D, t r a n s f e r s an e lec t ron to th eprimary acceptor A1 to generate an initial charge separa ted stateDI+-AI - -A 2 . T h i s h i g h e n e rg y state will t e n d t o rapidlyrecombine v ia s t ep 3 to yield th e ground state. However, a

    s e c o n d electron transfer, f rom th e p r i m a r y acceptor to as e c o n d a r y a c c e p t o r A 2 ( s t e p 2), c o m p e t e s w i t h this c h a r g er e c o m b i n a t i o n and l eads to a final D+-AI-A2 state. Properm o lec u l a r des ign should ensure t h a t th e final state h as a longl if e t im e fo r charge sepa ra t ion du e to a l a rge s p a t i a l sepa ra t ionbetween th e charges. As mentioned above, pho tosyn the t i c r eac t ionc e n t e r s u se this s t r a t e g y to achieve th e t r ansmembrane chargesepara ted s t a t e .

    E q u a t i o n 2 illustrates a somewhat different strategy,p a r a l l e l mul t i s t ep e l ec t ron t r a n s f e r.

    1 D2 -D 1 +-A1 -A 2 --A D 2 -D 1 +-AI-A 2 D2 +-D 1 -A 1 -A 2

    3D 2 D2+'D"A1-AA 2 5 (2)

    D2-DD1-AA1(A2

    As was th e case in e q u a t i o n 1, th e e x c i t e d state o f d o n o r D1transfers an e l e c t r o n to th e a t t a c h e d a c c e p t o r A1 in s t ep 1 togene ra t e an initial c h a rg e - s e p a r a t e d state. This state can ofc o u r s e undergo charge r e c o m b i n a t i o n v ia s t ep 3 to yield th eground state. However, compet ing with charge recombina t ion a retw o e l e c t r o n transfer s t e p s , 2 and 4, which bo th compete withs t ep 3 and g e n e r a t e new charge s e p a r a t e d states. Subsequen te l e c t r o n transfers by s t e p s 5 and 6 bo th y i e l d th e same f i n a lD2+ -D 1 -A-A 2 - cha rge s e p a r a t e d state. Thus, s t e p s 2 an d 4o p e r a t e in parallel an d converge on th e same final state. Thequantum yie ld of th e f i n a l s t a t e ca n then be enhanced because twoe lec t ron t r a n s f e r s t eps are competing with charge recombina t ion ,rather t han one. Many o t h e r applications o f this g e n e r a ls t r a t egy ca n be imagined. For example, a t t ach ing tw o i d e n t i c a le lec t ron acceptors to th e same exc i t ed state donor would enhancecompet i t ion o f e l ec t ron t r a n s f e r with the photophy s ica l p rocessesdepopula t ing th e exci ted s t a t e .

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    Molecular Triads

    In o u r laboratories we h a v e prepared a variety of

    mult icomponent m o l e c u l a r spec ies which illustrate one or both ofthese s t r a t e g i e s . In 1983 we repor ted the p r e p a r a t i o n an d s tudyof triad 1, which cons i s t s o f a porphyr in moiety (P) cova len t ly

    HH3

    NIC

    N H\

    N N

    H3

    l i nked to a ca ro t eno id polyene (C) an d a quinone (Q) . [1 ,2 ] Thephotochemical events fo l lowing exc i t a t ion of 1 in dich loromethaneso lu t ion are diagrammed in Figure 1. Exc i t a t i on o f th e porphyr inmoiety l eads to th e formation o f C- 1 P-Q, which decays by e lec t ront r a n s f e r to th e a t tached quinone (s tep 2) to give C-P'+-Q*-. Thequantum y ie ld o f this process is essen t i a l ly un i ty. High quantumyields fo r photoinitiated electron transfer a re routinelyobta ined fo r p rope r ly designed P-Q dyad systems r e l a t e d to 1. 3]However, th e very s t r u c t u r a l an d e n e rg e t i c f ac to r s which favor ahigh quantum y i e l d o f P ' + - Q ' - a l so t end to f avo r r ap id charger ecombina t i on to form th e ground state. Indeed , most P'+-Q'-states have lifetimes o f a few hundred p icoseconds o r l e s s ins o l u t i o n . In triad 1, charge r ecombina t i on by s t ep 3 is a l s or a p i d . However, a second electron transfer s t e p ( s t e p 4)competes with charge recombina t ion to y i e l d a final C'+-P-Q-charge s e p a r a t e d state. This final state has th e p o s i t i v e andnega t ive charges w e l l sepa ra t ed in space, an d as a consequencethis state h as a lifetime of 298 ns in d i c h l o r o m e t h a n e . Thes ta te is formed with a quantum y ie ld of 4%, an d s to res about 1.1eV o f th e 1 .9 eV i nhe ren t in th e porphyr in first exc i t ed s ing le ts t a t e .

    Triad 1 p rov ide s a good example o f th e s e q u e n t i a l m u l ti s te pe lec t ron t r a n s f e r s t r a t egy discussed above. The biomimet ic two-step e l ec t ron t r a n s f e r sequence l eads to a ca . 1000-fold increasein th e l i fe t im e o f th e f i n a l charge sepa ra t ed state relative toP-Q dyads, an d this state s t o r e s a s u b s t a n t i a l f r a c t i o n o f th e

    exci ted s i n g l e t s ta te energy as chemical p o t e n t i a l .

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    Figure 12.0

    eV

    1.0 1-

    C- 1 P-Q

    1

    I 2

    C-P *-Q

    Ccp-P-Qe

    3

    0.0 1

    Molecular Pentads

    As n o t e d above , th e quantum yield o f th e final c h a rg esepara ted s t a t e in 1 was r a the r low in dich loromethane . Variousapproaches to increas ing t h i s y ie ld while r e t a in ing th e des i r ab lef ea tu re s of th e triad system have been e x p l o r e d . [ 4 , 5 , 6 ] Some ofthese involve employing th e p a r a l l e l mul t i s t ep e lec t ron t r ans fe rs t r a t egy d i scussed above. A r ecen t an d very s u c c e s s f u l exampleis prov ided by th e C-PZn-P-QA-QB pentad 2. [7] This structuref ea tu re s a covalent ly l inked diporphyrin moiety. One o f th e

    2

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    p o r p h y r i n s is present in th e free b a s e f o r m , an d b e a r s adiqu inone spec ies . The second porphyr in conta ins a zinc io n andb ea r s a ca ro t eno id polyene i d e n t i c a l to t h a t employed in 1. Thed i q u i n o n e moie ty is of special interest. It consists o f ab e n z o q u i n o n e rigidly l i n k e d to a n a p h t h o q u i n o n e t h r o u g h ab i c y c l i c br idge . This he lp s ensure t h a t th e benzoquinone, whichis th e better e l e c t r o n accep to r, is held f a r t h e r away from th ef r ee base porphyr in than is th e naphthoquinone . This in turnpredisposes the system toward a sequen t i a l e l ec t ron t r a n s f e r fromP to QA to QB" S i m i l a r l y, th e z inc p o r p h y r i n is more easilyoxid ized than th e free base, and th e caro tenoid is an even be t t e relectron donor. Thus, th e redox potentials in th e v a r i o u sm o i e t i e s have been ad jus t ed in o r d e r to f avo r transport o f an

    e l ec t ron toward th e benzoquinone and a pos i t ive charge toward th ecaro tenoid .The absorp t ion spectrum o f 2 in chloroform is e s s e n t i a l l y a

    linear combina t ion o f th e s p e c t r a o f th e i n d i v i d u a l u n l i n k e dchromophores. The f luorescence emiss ion spec t rum, on th e otherhand, c o n s i s t s mainly o f an emiss ion wi th maxima a t 655 an d 718nm , which is c h a r a c t e r i s t i c o f th e f ree base porphyr in . Only at r ace o f emiss ion from th e zinc p o rp h y r in moiety is d e t e c t e d at611 nm . This result sugges t s t h a t e x c i t a t i o n o f th e pentad isfo l lowed by efficient singlet-singlet energy transfer from th ez i n c p o r p h y r i n m o i e t y to th e free b a s e . T h i s p r o c e s s is

    des igna ted as step 1 in Figure 2, which shows th e ene rge t i c s and

    Figure 2C-1PZn-PQQ

    2.0 .2.% C 1P-z-.

    C-Pzn-P a-Q Q

    473

    eV CPPo CPzne+'P 6 C+.pzn-Q

    7 .91 8 C0+-p1.0 10 Zn-P-Q-Q

    0.0 L-

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    se lec ted e lec t ron an d energy t r a n s f e r pathways fo r th e p e n t a d inch loroform. Indeed, this is confirmed by f luorescence e x c i t a t i o ne x p e r i m e n t s , which indicate that th e singlet-singlet ene rgytransfer e f f i c i e n c y is nea r ly quantitative. In a d d i t i o n , th ef l u o r e s c e n c e e m i s s i o n of th e f r e e b a s e p o r p h y r i n moie ty iss t rong ly quenched. This sugges ts th e appearance o f a new pathwayfo r decay o f th e porphyr in s i n g l e t state: e lec t ron transfer toth e a t tached quinone (s tep 2 in the f igure) .

    Time r e s o l v e d f l u o r e s c e n c e spectroscopic studies wereu n d e r t a k e n in o r d e r to learn more a b o u t these q u e n c h i n gproces ses . Porphyr in f l uo re scence decay curves were determineda t 14 different wave leng ths , w i th excitation a t 590 nm. Thesingle photon t i m i n g t e c h n i q u e was us e d , and th e i n s t r u m e n tresponse was -35 ps. The 14 decays

    were analyzed s imu l t aneou s lyby a g l o b a l t e c h n i q u e to yield f o u r e x p o n e n t i a l components(F igure 3). Two o f t he se components are s m a l l in ampl i tude and

    probably r e p r e s e n t i m p u r i t i e s . The tw o components of interesthave l i f e t imes of 39 ps an d 1.2 ns.

    Th e 39 p s component o f th e d e c a y has a s t r o n g positiveemission in th e 600 nm region where th e zinc porphyr in emi t s , anda negat ive c o n t r i b u t i o n (growth o f f luorescence i n t e n s i t y ) in th e72 0 nm r e g i o n where th e f r e e base p o r p h y r i n e m i t s s t r o n g l y.Thus, this component o f the decay r e p r e s e n t s th e s i n g l e t - s i n g l e tenergy t r a n s f e r from th e zinc porphyr in to th e a t tached f r ee

    Figure 37

    A 0.039 ns6 9 1.2 ns

    5 * 4.6 nsv 0.25 ns

    cD 4= 34-

    E< 2

    1 -

    C 0

    -1

    -2

    -3 IIIIIIII

    600 620 640 660 680 700 720 740

    Wavelength (nm)

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    b a s e . The d e c a y o f th e z i n c porphyrin singlet state isaccompanied by a corresponding increase in th e amount o f th e f reebase singlet state. The singlet-singlet energy transfer rate

    cons t an t k, may be es t imated from equation 3,

    h = ( l iT) - (i /To) (3)

    where r is th e f l u o r e s c e n c e lifetime o f th e z inc p o r p h y r i ns i n g l e t state in th e pen t ad (39 ps), an d 7O is th e l i fe t i m e ofth e singlet state of a model porphyr in which l a c k s th e en e rgyt r a n s f e r possibility, b u t otherwise has th e same ph o t oph ys i c s asth e porphyr in moiety in th e pentad . A su i t ab le model compound, ac a r o t e n o i d - b e a r i n g z inc porphyr in , has an exc i t ed s i n g l e t statel i f e tim e of 370 ps. Thus, ki equals 2 .3 x 10 1 0 s - I . The quantumyie ld o f energy t r a n s f e r, 4i , is given by

    P = k * T (4)

    and equals 0.90. This is in good agreement with th e f luorescenceexc i t a t ion experiment mentioned above.

    The 1 .2 n s c o m p o n e n t o f th e d e c a y in F i g u r e 3 has th eemiss ion spectrum o f th e f ree base porphyr in . A model f ree basep o r p h y r i n l a c k i n g th e a t t a c h e d q u i n o n e s ( b u t b e a r i n g a z incporphyr in moie ty) has a first exc i t ed s i n g l e t state l if e t i m e of7 .8 ns. As men t ioned above, th e quench ing o f th e f r e e baseporphyr in emiss ion ca n be attributed to p h o t o i n i t i a t e d e l e c t r o ntransfer v i a s t e p 2 in F i g u r e 2. Equa t ion 3 may be used tode t e rmine k2 to be 7 .1 x 108 s - I . The c o r r e s p o n d i n g q uan tumy i e l d of C-PZn-P '+-QA' - -QB, as c a l c u l a t e d from e q u a t i o n 4, is0.85.

    Figure 2 sugges ts t h a t fo rmat ion o f C-Pzn-P'+-QA'--QB mightbe fo l lowed by e lec t ron transfer v ia s t eps 3 an d 4 to y i e l d newi n t e r m e d i a t e charge s e p a r a t e d states. These s t e p s would bothcompe te w i t h c h a r g e r e c o m b i n a t i o n via step 10. The newin te rmedia te s t a t e s might then go on by s t eps 5 - 9 to genera te af i n a l C'+-Pzn-P-QA-QB"- state. Trans i en t a b s o r p t i o n t e chn iqueswere used to d e t e c t f o rma t ion o f such a state. The ca ro teno idr a d i c a l ca t ion has a s t rong absorp t ion in th e 970 nm region whichcan s e r v e as a marke r fo r th e p r o d u c t i o n o f th e state inq u e s t i o n . Indeed , e x c i t a t i o n o f a chloroform s o l u t i o n of th epentad with a -15 ns l a s e r pulse a t 650 nm led to th e observa t ion

    o f a t r a n s i e n t abso rp t ion whose spectrum was identified as t ha to f a ca ro t eno id r ad ica l ca t ion . Thus, th e t r a n s i e n t decay arosefrom C'+-Pzn-P-QA-QB . The l if e t i m e o f this state was 55 ps ,an d th e quantum y i e l d , based on total light absorbed a t 650 nm ,was -0.83. Thus, the e ff i c i ency o f conversion of C-PZn-P+- QA"--

    QB to the f i n a l s t a t e is e s s e n t i a l l y un i ty.

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    The pentad 2 behaves in a s i m i l a r manner in dich loromethaneso lu t ion . The r a t e cons t an t fo r s i n g l e t - s i n g l e t energy t r a n s f e r(s tep 1 in Figure 2) is e s s e n t i a l l y unchanged a t 2 .5 x 1010 s-1 .

    The rate o f e l e c t r o n transfer by s t ep 2, however, h as slowedsomewhat (k 2 = 2 .9 x 108 S-1) . As a consequence , th e quantumyield o f C-Pzn -P -QA -QB is o n l y 0 .71 in this solvent.Trans i e n t absorp t ion s tud ie s again revea l t h a t a f i n a l C'+-Pzn-P-QA-QB' state is produced . The lifetime o f this state ha sincreased to about 200 us , b u t th e quantum y ie ld has decreased to0.60.

    Th e a n a l o g o f p e n t a d 2 in w h i c h th e z i n c io n has beenr e p l a c e d by h y d r o g e n a toms ( p e n t a d 3) was a l s o p r e p a r e d andi n v e s t i g a t e d . It to o g e n e r a t e s a l o n g - l i v e d charge s e p a r a t e d

    state upon e x c i t a t i o n , as illustrated in Figure 4. The processwas s t u d i e d u s i n g t ime r e s o l v e d f l u o r e s c e n c e s p e c t r o s c o p y asdescr ibed above fo r 2. Exc i t a t i on o f a dich loromethane s o l u t i o n

    Figure 42.0 c-

    1P-P-p-. C-P-

    1P-0-Q

    C-P-PO+-o.--o---.4"

    eV 3C~pPS+4tQo -,5-- 6V c-P-P'+' _

    7 \8 9 C9-P-P-Q -O

    co+-P-P-0-Qg01.0

    10

    C-P-P-0-C0.0

    of 3 a t 590 nm y i e l d s both porphyrin first exci ted s i n g l e t s t a t e sin essentially equa l amounts , because th e tw o p o r p h y r i n s havenear ly identical a b s o r p t i o n and emis s ion s p e c t r a . These tw ostates undergo r ap id singlet-singlet energy transfer, b u t thisp r o c e s s c a n n o t be

    o b s e r v e d directly in th e t i m e resolvedf luorescence experiment because of th e s im i la r ity in th e emiss ioncharacteristics o f th e tw o porphyr in m o i e t i e s . Th e p o r p h y r i nfirst exc i t ed s i n g l e t states then decay with a l if e t im e o f 3.1ns. Again, t he re is subs tan t i a l quenching of the porphyr in firstexc i t ed s i n g l e t states v ia e lec t ron transfer to gene ra t e a C-P-

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    P.+-QA'--QB state (s tep 2 in Figure 4). The r a t e c o n s t a n t fo rt h a t step was found to be 2 .3 x 108 s-1, which is very s imi l a r to

    t h a t found fo r th e comparable s tep in 2 in th e same so lvent . Thequantum y ie ld fo r s tep 2 is reduced to 0.35, however, because ofan i n c r e a s e d rate o f decay o f th e p o r p h y r i n singlet state byother pathways.

    F o r m a t i o n o f C-P-P '+ -QA' - -QB is f o l l o w e d by electront r a n s f e r v ia s t eps 2 - 9 to y ie ld a f i n a l C+-P-P-QA-QB- gesepara ted state, which was de tec t ed v i a the t r a n s i e n t abso rp t iono f th e c a r o t e n o i d r a d i c a l ca t ion as was th e ca se with 2. Thelifetime of th e final state was very long ( -340 us), bu t th equantum y ie ld was only 0 .15. P a r t of the reason fo r th e reducedquantum y ie ld was th e reduced quantum y ie ld o f th e initial C-P-P'+- QA--QB state, as d i scussed above. In add i t ion , with 3 th ein te rporphyr in e l ec t ron t r a n s f e r step 4 in Figure 4 competes lessefficiently with charge recombina t ion v ia s t ep 10 because th e C-P'+-P-QA'--QB state is about 0 .2 eV l e s s s t a b l e than C-PZn'+-P-

    QA'--QB. Step 4 has l e s s thermodynamic dr iv ing force in 3 t hanin 2, and is t h e r e f o r e s lower.

    The l i f e t imes for the f i n a l charge separa ted s t a t e s in 2 and3 are about 1000 t imes longer than t h a t found fo r triad 1 unders i m i l a r c o n d i t i o n s , an d about 1 ,000 ,000 t imes longer than thosefound fo r typical P-Q dyad charge s e p a r a t e d states. However,t h e y a r e still rather shorter t han one mi gh t e x p e c t fo r tw ocha rg es s e p a r a t e d by such d i s t a n c e s ( the pen tads a r e a b o u t 80Angstroms in length) . The mechanism o f charge recombina t ion hasnot y e t been determined, but may involve a mul t i s t ep process suchas was found fo r some C-P-Q triad molecules . (8]

    The pentad molecules are good examples o f the app l i ca t ion ofth e p a r a l l e l m u l t i s t e p e lec t ron t r a n s f e r discussed above. In 2,fo r example, th e high quantum y ie ld fo r product ion o f th e f ina lcha rge s e p a r a t e d state is due in part to th e fact that tw oelectron transfer s t e p s ( s t e p s 3 and 4) work in parallel tocompete with charge recombina t ion . Both o f these s teps l ead to a

    cascade of subsequen t e lec t ron transfer pathways, all o f whichconverge to th e same f i n a l C'+-Pzn-P-QA-QB"- s t a t e .

    CONCLUSIONS

    It is i n t e r e s t i n g to compare th e results fo r pentad 2 withe lec t ron t r a n s f e r in th e n a t u r a l b a c t e r i a l r eac t ion cen te r. Thepentad uses l i g h t energy to t r a n s f e r an e lec t ron over a d i s t anceof up to 80 Angstroms ( the length o f th e molecule) with a quantumy ie ld o f 0.83, an d conserves about 50% o f th e energy o f th e f reebase porphyr in first exc i t ed s i n g l e t state in th e final chargesepara ted spec ies . I so l a t ed b a c t e r i a l r eac t ion cen t e r s car ry outcharge sepa ra t ion over a d i s t a n c e o f about 30 Angstroms with aquantum y i e l d o f e s s e n t i a l l y un i ty, an d cap tu re about one thirdo f th e initial exc i t ed s ta te energy a t a comparable p o i n t in th eredox proces s . [9 ]

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    It is clear from th e above discussion that artificialpho tosyn the t i c systems can do a r a the r good jo b o f mimicking th eg r o s s f e a t u r e s o f natural p h o t o s y n t h e t i c electron transfer.There are, however, many impor tan t d e t a i l s o f th e n a t u r a l processw h i c h a re n o t m i m i c k e d by t h e s e mo de l s , and w h i c h a re no tcur ren t ly wel l unders tood . Thus, a g r e a t dea l more work w i l l ben e c e s s a r y in this a rea , bo th w i th n a t u r a l and wi th s y n t h e t i csystems.

    The examples d i s c u s s e d above demons t r a t e that m u l t is t e pe lec t ron t r a n s f e r s t r a t e g i e s ca n be used in syn the t i c molecu la rdevices to prepare long l i ved , ene rge t i c charge sepa ra t ed s t a t e sin good y ie ld . Simi l a r s t r a t e g i e s ca n o f course be employed inth e des ign o f o t h e r sys tems such as s y n t h e t i c antenna networkswhich func t ion v ia s i n g l e t - s i n g l e t energy t r a n s f e r an d molecularsys tems which channe l triplet energy to r e c e p t o r chromophores ,and in in format ion process ing a t th e molecular l eve l .

    ACKNOWLEDGMENTS

    This work was suppor ted by the Nat io na l Science Founda t ion(CHE-8903216 and INT-8701663) and th e D i v i s i o n o f ChemicalS c i e n c e s , O ff i c e of Bas i c Energy S c i e n c e s , Off i ce of EnergyResearch, U. S. Department o f Energy (DE-FG02-87ER13791) . This

    is p u b l i c a t i o n No. 40 from th e Arizona Sta t e Unive r s i ty Centerfor the Study o f Early Events in Pho tosyn thes i s . The Center isfunded by U. S. Department o f Energy grant No. DE-FG02-88ER13969as p a r t o f th e USDA/DOE/NSF Pl an t Sciences Center program.

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