Synthesis, Photophysical Properties, and Photovoltaic Devices of Oligo(<i>p</i>-phenylene vinylene)-fullerene Dyads

Abstract

The synthesis of a homologous series of oligo( p -phenylene vinylene)-fulleropyrrolidines (OPV n -C 60, n = 1−4, where n is the number of phenyl rings) is described. The photophysical properties of these donor−acceptor dyads and the corresponding model compounds, α,ω-dimethyl-2,5-bis(2-( S )-methylbutoxy)-1,4-phenylene vinylene oligomers (OPV n, n = 2−4) and N -methylfulleropyrrolidine (MP-C 60 ), are studied as a function of the conjugation length in solvents of different polarity and as thin films. Fast singlet energy transfer occurs after photoexcitation of the OPV n moiety of the dyads toward the fullerene moiety in an apolar solvent. Photoexcitation of the dyads in a polar solvent results in electron transfer for OPV3-C 60 and OPV4-C 60, and to some extent for OPV2-C 60, but not for OPV1-C 60 . These results are compared to the results obtained for mixtures of OPV n and MP-C 60 in the same solvents. The solvent-dependent change in free energy for charge separation of the donor−acceptor systems is calculated from the Weller equation, and the rate constants for energy and electron transfer are derived from the fluorescence lifetime and quenching. The results show that in a polar solvent electron transfer in these dyads is likely to occur via a two-step process, that is, a very fast singlet energy transfer prior to charge separation. In thin solid films of OPV3-C 60 and OPV4-C 60, a long-lived charge-separated state is formed after photoexcitation. The long lifetime in the film is attributed to the migration of charges to different molecules. A flexible photovoltaic device is prepared from OPV4-C 60 .