Mimicking Photosynthetic Electron Transfer

of 12

  • date post

    29-May-2018
  • Category

    Documents

  • view

    214
  • download

    0

Embed Size (px)

Transcript of Mimicking Photosynthetic Electron Transfer

  • 8/8/2019 Mimicking Photosynthetic Electron Transfer

    1/12

    141

    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

  • 8/8/2019 Mimicking Photosynthetic Electron Transfer

    2/12

    142

    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.

  • 8/8/2019 Mimicking Photosynthetic Electron Transfer

    3/12

    143

    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 .

  • 8/8/2019 Mimicking Photosynthetic Electron Transfer

    4/12

    144

    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