PHOTOINDUCED CHARGE SEPARATION IN A CAROTENOID-PORPHYRIN-DIQUINONE TETRAD - ENHANCEMENT OF QUANTUM YIELDS VIA CONTROL OF ELECTRONIC COUPLING

A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q(A)-Q(B)) has been sy nthesized, and its photochemistry has been investigated using time-res olved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P.+-Q(A).--Q(B) state, which is formed with a r ate constant of 2.3 x 10(9) s-1 and a quantum yield of 0.87. In chloro form, the rate is 4.1 x 10(9) s-1 and the quantum yield is 0.94. Trans ient absorption studies show that this state evolves by subsequent ele ctron transfer pathways to a final C.+-P-Q(A)-Q(B).- charge-separated state whose lifetime is 7.4 mus in dichloromethane and 740 ns in chlor oform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P.+-Q(A).--Q(B) spec ies is lower. This fact is interpreted in terms of the rates of charge -separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules ma y be altered in a predictable fashion using the basic tenets of electr on transfer theory.