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 × 10⁸ to 2.3 × 10¹⁰ s⁻¹. The energy-transfer rates are not in accord with those predicted by the Förster 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^•+-Pb^•− 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 × 10′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 (τ ≈ 250 ns), and the overall quantum yields range up to 0.32.