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 PA-PB and C-PA-PB 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 Pa•+-Pb•− and C-PA•+-PB•− 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•+-PA-PB•− states. These states are rather long lived (τ ≈ 250 ns), and the overall quantum yields range up to 0.32.
ASJC Scopus subject areas
- Colloid and Surface Chemistry