Photoinduced singlet and triplet energy transfer in fullerene-oligothiophene-fullerene triads

Photophysical properties of fullerene-oligothiophene-fullerene (C60-nT-C60) triads with n = 3, 6, or 9 thiophene units have been investigated using photoinduced absorption (PIA) and (time-resolved) fluorescence spectroscopy in toluene and compared to mixtures of oligothiophenes (nT) with N-methylfulleropyrrolidine (MP-C60). Photoexcitation of the oligothiophene moiety of the C60-nT-C60 triads in toluene results in a very fast singlet-energy transfer reaction to the fullerene moiety with rates varying between 10e12 and 10e13 s-1. In the mixtures, an inter-molecular triplet-energy transfer reaction occurs. The preference for intra- and inter-molecular energy instead of electron transfer reactions in these triads in toluene is in full agreement with predictions that can be made for the change in free energy upon charge separation.