RESEARCH ON THE PRIMARY PROCESSES OF PHOTOCONDUCTION IN AROMATIC VINYL POLYMERS BY MEANS OF PICOSECOND AND FEMTOSECOND VISIBLE AND NEAR-IR LASER SPECTROSCOPY

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The transport of electrons initiated by photoexcitation is ubiquitous in nature as well as in artificial systems. Photoconductivity in aromatic vinyl polymers is one of the typical phenomena closely related to these fundamental processes. Since the band structures in the electronic states are not plausible because of rather small interaction among the pendant aromatic groups, hopping model has been proposed for this carrier transport processes. Still unclear are the factors regulating the charge shift reaction between two neighboring aromatic groups in a molecular level. To obtain deeper insights into this efficient electron transfer reaction, photoinduced electron transfer dynamics (charge separation, charge recombination and hole transfer reactions) in poly (N-vinylcarbazole) (PVCz) and solvent effects on the reaction profiles were studied by means of picosecond and femtosecond transient absorption spectroscopy and transient dichroism measurements.Electron transfer dynamics in PVCz in rather polar solutions was well described by the simple scheme that the cation state of carbazolyl (Cz^+) moiety continuously migrates along pendant Cz moieties in the polymer chain with the charge recombination at the initial position of the charge separation. Rate constant of the hole transfer (HT) from the Cz^+ to neighboring ones was (230 ps)^<-1> in pyridine. With a decrease in the solvent polarity, the HT rate constants drastically decreased and it was 【less than or equal】 ca. 10^7s^<-1> in chloroform solution. This solvent polarity effect was accounted for not only by the increase in the attractive Coulombic interaction with decreasing solvent polarity but also by the delocalization of the cationic state over Cz units. By integrating the present results with those in other aromatic vinyl polymers, the factors regulating the hole transfer process in aromatic vinyl polymers were discussed.