Photoinduced electron transfer processes of C-60-doped poly(N-vinylcarbazole) films as revealed by picosecond laser photolysis

Photoinduced electron transfer dynamics of poly(N-vinylcarbazole) (PVCz) films doped with a small amount of C-60 was investigated by using picosecond and nanosecond laser flash photolysis techniques under various experimental conditions. From the rime evolution of transient absorption spectra in the 600-1180 nm wavelength region, it was revealed for 0.7 mol % C-60-doped PVCz films that the charge-separated state (PVCz(+)-C-60-) between PVCz and C-60 was produced immediately after excitation with a picosecond 532 nm laser pulse. The decay of the initial charge-separated state with a time constant of 1.2 ns comprised the following three channels: the charge-recombination process, the hole-migration reaction to the neighboring carbazolyl chromophores, and the formation of the local triplet stale of C-60 (C-3(60)*). The reaction yield of the hole migration was estimated to be 30%, The present results were quite different from those reported previously for densely C-60-doped PVCz films excited with a picosecond 355 nm laser pulse (J. Phys. Chem. 1995, 99, 1199). In order to elucidate factors causing the difference, the excitation intensity effect on the dynamics was examined, and it was found that an increase in the excitation laser intensity induced rapid formation of C-3(60)* by a nonlinear process immediately after excitation of a picosecond laser pulse. Under the condition where the excitation intensity was rather weak and the dense population of the excited state was avoided, the deactivation dynamics of the charge-separated state produced by the excitation al 355 nm was found to be almost the same as that by the 532 nm excitation. The nanosecond to microsecond dynamics of C-60-doped PVCz films showed that the effective charge separation did not take place between the triplet state of C-60 and PVCz. From these results, it was concluded that the hole-migration process inducing free carriers was effective in the initial charge-separated state of C-60-doped PVCz films, and thus, relatively high reaction yield of this process was responsible for the enhancement of the photoconductivity in the films.