Substances which will give rise to a color change (absorption spectral change) or the like upon photoirradiation are being used for extensive development of a variety of functional materials for use in recording, display and switching.
For example, in high-density recording, the properties of the optical device, such as the transmittance used in the system, make it desirable to use a material which causes a change in the absorption spectrum or refractive index in the visible or near-infrared region with a short-wavelength light such as one as short as about 400 nm. In view of the increasing speed of information processing, materials are required that exhibit a very high speed response in the near-infrared region, i.e., the wavelength region for optical telecommunication. In addition, for application to electronic paper and the like, there are eagerly awaited materials which generate a color change in response not only to light but also to electric or electronic signals. However, very few materials are found which fully satisfy these requirements.
An ion-pair charge-transfer complex is a charge-transfer complex composed of a cationic component which serves as the electron acceptor and an anionic component which constitutes the counter ion and serves as the electron donor. The proximity of the donor and the acceptor attributable to an oxidation-reduction reaction in an ion-pair charge-transfer (sometimes abbreviated as IPCT hereinafter) complex causes a face-to-face charge-transfer interaction in addition to the coulombic interaction, and therefore the complex will develop new types of colors and properties which are not observed at all with the cation or anion alone. In the case of an ion-pair charge-transfer complex or an extended aromatic conjugated system such as X in the formula (I) set out later, photoirradiation corresponding to the absorption by the cation alone will give rise to a photoinduced electron transfer from the anion to cation thereby producing a one-electron reductant (radical cation).
Previously, the present inventors invented an IPCT complex of a “polymer” composed of bipyridinium cation as the backbone (Japanese Patent No. 3720277 “Polymeric photoresponsive materials and elements utilizing photoinduced electron transfer reaction”: Patent Reference No. 1). This IPCT complex polymer is suitable for use as a photo-functional material, as it causes, upon photoirradiation, an absorption spectral change (color change) at the visible to near-infrared region over a wide wavelength range for a wide range of response time. The material has the additional advantage of being easy to make into film because it is a polymer. However, the drawback is that it has a low glass-transition temperature making it difficult to apply the material to a device.    Patent Reference No. 1: Japanese Patent No. 3720277 “Polymeric photoresponsive materials and photoresponsive elements utilizing photoinduced electron transfer reaction”.