Raman scattering study on organic semiconductors having gigantic dielectric constant

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Mixed stack organic semiconductors, composed of oneーdimensional chains of alternately stacked donor and acceptor molecules have an instability to molecular valence change. These materials show anomalous response to the electric field. One is a gigantic dielectric response and the other is a current switching effect. Especially, the latter has been attracted much attention in this decade due to possible application to molecular electronics. We have investigated the transport, dielectric, and optical properties of these materials. Raman and infrared spectra were measured to investigate the microscopic properties of the switch-on state.From the experimental results, we have concluded that the stacking fault on the one-dimensional chain (in other words, solitons) play important roles in the dynamical properties of these compounds. The ionic charge transfer compounds undergo a structural phase transition. The switching effect was found to occur conspicuously in the low temperature phase, where neighboring molecules are dimerized. It was found that this dimerization order is partially broken by too much current mediated by solitonic charge carriers.