Probing molecular wire: synthesis of donor-acceptor ensembles exhibiting long range electron transfer

Abstract

A series of donor–acceptor arrays (C60–oligo-PPV–exTTF; 16–20) incorporating p-conjugated oligo(phenylenevinylene) wires (oligo-PPV) of different length between p-extended tetrathiafulvalene (exTTF) as electron donor and C60 as electron acceptor has been prepared by multistep convergent synthetic approaches. The electronic interactions between the three electroactive species present in 16–20 were investigated by UV-visible spectroscopy and cyclic voltammetry (CV). Our studies clearly show that, although the C60 units are connected to the exTTF donors through a p-conjugated oligo- PPV framework, no significant electronic interactions are observed in the ground state. Interestingly, photoinduced electron-transfer processes over distances of up to 50 6 afford highly stabilized radical ion pairs. The measured lifetimes for the photogenerated charge-separated states are in the range of hundreds of nanoseconds (~500 ns) in benzonitrile, regardless of the oligomer length (i.e., from the monomer to the pentamer). A different lifetime (4.35 ms) is observed for the heptamer-containing array. This difference in lifetime has been accounted for by the loss of planarity of the oPPV moiety that increases with the wire length, as established by semi-empirical (PM3) theoretical calculations carried out with 19 and 20. The charge recombination dynamics reveal a very low attenuation factor (b=0.01_0.005 6_1). This b value, as well as the strong electron coupling (V~5.5 cm_1) between the donor and the acceptor units, clearly reveals a nanowire behavior for the p-conjugated oligomer, which paves the way for applications in nanotechnology.