Properties of excited radical cations of substituted oligothiophenes.

Excited-state properties of radical cations of substituted oligothiophenes ( nT (+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (+) in the excited state ( nT (+)). The D 1 state lifetime was estimated to be 34 +/- 4, 24 +/- 2, and 18 +/- 1 ps for 3T (+), 4T (+), and 5T (+), respectively. In the presence of hole acceptor (Q), bleaching of nT (+) and growth of Q (+) were observed upon selective excitation of nT (+) during the nanosecond-nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT (+)(D 1) to Q. Recovery of nT (+) was also observed together with decay of Q (+) because of regeneration of nT (+) by hole transfer from Q (+) to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate ( k HT) from nT (+)(D 1) to Q depended on the free-energy change for hole transfer (-Delta G = 1.41-0.46 eV). The estimated k HT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1-1.0 M).