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 moiet y (Q(A)-Q(B)), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient ab sorption and emission spectroscopies. Each of the pentads undergoes ph otoinduced electron transfer from the C-P-1P-Q(A)-Q(B) singlet state t o 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 fin al C.+-P-P-Q(A)-Q(B).- state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensit ive functions of the state energies and the electronic coupling betwee n the porphyrin and diquinone moieties. One of the pentads undergoes p hotoinduced 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 exa mple of an artificial photosynthetic reaction center preserves about h alf of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred mic roseconds.