Photoinduced energy and electron transfer in oligo(p-phenylene vinylene)-fullerene dyads

The intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene)-fulleropyrrolidine dyad with four phenyl rings (OPV4-C-60) has been investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state. In solution, photoexcitation of the OPV4 moiety of OPV4-C-60 results in an ultrafast (<190 fs) singlet energy transfer reaction, creating the fullerene singlet excited state. In polar solvents, the ultrafast energy transfer is followed in the picosecond time domain by an intramolecular electron transfer. In accordance with Marcus theory, the rates for forward and backward intramolecular electron transfer in OPV4-C-60 are influenced by the polarity of the solvent. In the solid state the photophysics of OPV4-C-60 is dramatically different. In thin films, the forward electron transfer proceeds within 500 fs, irrespective of which chromophore is photoexcited. The increased rate for charge separation in the solid state is attributed to a more favorable orientation of the donor and acceptor that results in an intermolecular electron transfer. In the films, energy and electron transfer processes compete at the earliest moments after photoexcitation. In the solid state, the photogenerated electrons and holes have long lifetimes as a result of migration of these charges to thermodynamically more favorable sites in the film.