Charge transfer kinetics in fullerene-oligomer-fullerene triads containing alkylpyrrole units

A photoinduced electron-transfer reaction has been observed in three fullerene-donor-fullerene triads containing an electron-rich pyrrole ring in the donor moiety. The kinetics of charge separation in solution has been investigated by photoluminescence and transient absorption spectroscopy. The polarity of solvent and the distance between donor and acceptor affect the forward electron-transfer reaction, in qualitative agreement with a semiempirical model for the Gibbs free energy for charge separation. Depending on the conditions, charge separation occurs at a rate of 10(9)-10(10) s(-1). The charge recombination rate is estimated to be faster than 2 x 10(10) s(-1). The relatively large contribution of the singlet-excited-state S-n <-- S-1 absorption of the fullerene moiety to the photoinduced absorption hampered a more accurate determination of the recombination rate.