LONG-LIVED PHOTOINITIATED CHARGE SEPARATION IN CAROTENE DIPORPHYRIN TRIAD MOLECULES

A variety of molecular triads and dyads consisting of covalently linked carotenoid (C) and/or porphyrin (P) moieties have been prepared and studied with transient absorption and time-resolved fluorescence techniques. Diporphyrins of the type P(A)-P(B) and C-P(A)-P(B) triads demonstrate interporphyrin singlet-singlet energy transfer with rate constants ranging from 8.1 x 10(8) to 2.3 x 10(10) s-1. The energy-transfer rates are not in accord with those predicted by the Forster dipole-dipole theory, and it is suggested that energy transfer involves a contribution from an electron-exchange mechanism. Interporphyrin photoinitiated electron transfer is observed in molecules possessing sufficient thermodynamic driving force to produce P(A).+-P(B).- and C-P(A).+-P(B).- charge-separated states. The electron-transfer rate constant increases with increasing reaction free energy change for the molecules studied, and rate constants up to 3.5 x 10(9) s-1 and quantum yields up to 0.68 were measured. The carotenodiporphyrin triad systems undergo a subsequent electron-transfer step to give final C.+-P(A)-P(B).- states. These states are rather long lived (tau-almost-equal-to 250 ns), and the overall quantum yields range up to 0.32.