Through bond mechanism versus exciplex formation in the photochemistry of fullerene/ferrocene donor-bridge-acceptor dyads

A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 ((Phi(rel) = 0.17 x 10(-4)), 3 (Phi(rel) = 0.78 x 10(-4)), 4 (Phi(rel) = 1.5 x 10(4)), 5 (Phi(rel) = 0.7 x 10(4)), and 6 (Phi(rel) = 2.9 x 10(4)) were substantially quenched, relative to N-methyl fulleropynolidine (1) (Phi(rel) = 6.0 x 10(-4)). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28 x 10(9) s(-1), 6.9 x 10(9) s(-1), and 3.4 x 10(9) s(-1), 14 x 10(9) s(-1), 2.3 x 10(9) s(-1) respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic pi-radical anion bands at lambda(max) = 400 and 1055 nm in degassed benzonitrile with tau(1/2) = 1.8 mu s (3) and 2.5 mu s(4).