RESEARCH ON THE FLUCTUATION MODES CONTROLLING THE ULTRAFAST CHARGE MIGRATION IN AROMATIC VINYL POLYMERS.

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The transport of electrons initiated by photoexcitation is ubiquitous in nature as well as in artificial systems. Photocon-ductivity 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, we have studied temperature dependence of the hole escape process in one of the typical photoconductive polymers, poly (N-vinylcarbazole) (PVCz), doped with electron acceptors by picosecond transient absorption spectroscopy and dichroism measurements and found that no temperature dependence was observed for the rate constant of the hole shift reaction in the initial stages after the photoexcitation in the temperature region between 77 and 295K.The present hole transfer rate constant and its temperature independence are not rationally accounted for by the standard theory of the ET reaction assuming very weak interaction between the reactants. By integrating the present results with the previous ones on the solution phase, it is indicated that large electronic interaction between the aromatics is an important factor regulating the rapid hole transfer also in the solid polymer films. In solid phase, the small fluctuation of the carbazolyl moiety in the hindered space of the polymer film (zero-point motion) may fluctuate the transfer integral and may lead to the rapid hole transfer reaction when the mutual orientation takes a favorable condition. This contribution of the fluctuation in the electronic interaction is strongly suggested to play an important role in the rapid hole migration in the aromatic vinyl polymer systems.