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 × 109 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.
|Number of pages||10|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - May 8 1991|
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