Two-step mechanism for the photoinduced intramolecular electron transfer in oligo(p-phenylene vinylene)-fullerene dyads

The intramolecular photoinduced charge separation within two oligo(p-phenylene vinylene) fulleropyrrolidine dyads with three and four phenyl rings (OPV3-C₆₀ and OPV4-C₆₀) is investigated with femtosecond pump-probe spectroscopy in solvents of different polarity. Photoexcitation of the OPV moiety of these dyads results in an ultrafast singlet-energy transfer reaction to create the fullerene singlet excited state with a time constant of 190 fs, irrespective of the polarity of the medium. In a polar solvent, intramolecular electron transfer occurs from the OPV moiety to the C₆₀ moiety with a time constant of 10--13 ps as a secondary reaction, subsequent to the ultrafast singlet-energy transfer. The charge-separated state has a lifetime of 50--90 ps and recombines to the ground state.