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Photochromic Dyes

 Chromism is a phenomenon that reversibly changes optical properties (eg. photo absorption and emission) by an external stimulus. A molecular structure changes in many cases while chromism occurs, and then refractive index, dielectricity, redox potentials, and melting point also change. Photochromism is a photo-induced chromism. Photochromic materials are further developed for a light-modulating materials, optical recording media, optical switches, and functional inks.
 A structural isomerization during photochromism involves photo-induced geometrical isomerizations (eg. cis-trans isomerization) and photo-induced ring-opening and -closure reactions. Fischer et al. first reported the photochromic behavior of spiropyrans.1) UV irradiation to a spiropyran undertakes a ring-opening of the spiro carbon. The structural change leads to a blue shift of photo absorption up to the visible region. Irie et al. first observed photochromism of diarylethenes.2) The diarylethene also shows photo-induced ring-opening and -closure reactions. UV irradiation to the diarylethene undertakes a ring-closure (with coloration) whereas a spiropyran does a ring-opening (with coloration) by UV irradiation. The ring-opened spiropyran thermally closes the ring (T-type). On the other hand, the ring-closed diarylethene is thermally stable but opens to form the original compound by visible light irradiation (P-type). It is well known that diarylethene is a photochromic compound with high photo sensitivity and repetition durability.3) A fulgide is also a P-type photochromic compound exhibiting a ring-closure by a photo irradiation.4)
 Hayashi and Maeda observed photochromism of an imidazole dimer, hexaarylbiimidazole (HABI).5) A light irradiation to HABI undertakes dissociation of the dimer. The photochromic mechanism is different from those of the others doing photo isomerizations. A triphenylimidazolyl radical formed by the dissociation thermally recombines to reform the original HABI, thus this is a T-type photochromism. The triphenylimidazolyl radical is also useful as a photopolymerization initiator.6)

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