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) (PVC z) films doped with a small amount of C-60 was investigated by using p icosecond and nanosecond laser flash photolysis techniques under vario us experimental conditions. From the rime evolution of transient absor ption spectra in the 600-1180 nm wavelength region, it was revealed fo r 0.7 mol % C-60-doped PVCz films that the charge-separated state (PVC z(+)-C-60-) between PVCz and C-60 was produced immediately after excit ation with a picosecond 532 nm laser pulse. The decay of the initial c harge-separated state with a time constant of 1.2 ns comprised the fol lowing three channels: the charge-recombination process, the hole-migr ation reaction to the neighboring carbazolyl chromophores, and the for mation of the local triplet stale of C-60 (C-3(60)). The reaction yie ld of the hole migration was estimated to be 30%, The present results were quite different from those reported previously for densely C-60-d oped PVCz films excited with a picosecond 355 nm laser pulse (J. Phys. Chem. 1995, 99, 1199). In order to elucidate factors causing the diff erence, the excitation intensity effect on the dynamics was examined, and it was found that an increase in the excitation laser intensity in duced rapid formation of C-3(60) by a nonlinear process immediately a fter 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 mi crosecond 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-migratio n process inducing free carriers was effective in the initial charge-s eparated state of C-60-doped PVCz films, and thus, relatively high rea ction yield of this process was responsible for the enhancement of the photoconductivity in the films.