Charge separation and recombination in small band gap oligomer-fullerene triads

Synthesis and photophysics of a series of thiophene−thienopyrazine small band gap oligomers, end-capped at both ends with C60, are presented. In these triads, a photoinduced electron transfer reaction occurs between the oligomer as a donor and the fullerene as an acceptor. Femtosecond photoinduced absorption has been used to determine the rates for charge separation and recombination. It was found that charge separation takes place within approximately 10 ps, and is situated close to the Marcus optimal region. Charge recombination is faster in o-dichlorobenzene (ODCB) (15−45 ps) than in toluene (90−730 ps), because in ODCB charge recombination takes place close to the optimal region. In toluene, the recombination is situated in the inverted region, with a much higher activation barrier. No signs of recombination into a triplet state were observed