PHOTOINDUCED ELECTRON AND ENERGY-TRANSFER IN MOLECULAR PENTADS

A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q(A)-Q(B)), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-1P-Q(A)-Q(B) singlet state to yield the charge-separated state C-P-P.+-Q(A).--Q(B). Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C.+-P-P-Q(A)-Q(B).- state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C.+-P-P-Q(A)-Q(B).- state with a quantum yield of 0.83 and a lifetime of 55 mus. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds.