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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)
- 1) E. Fischer, Y. Hirshberg, J. Chem. Soc. 1952, 4522.
- 2) Review: M. Irie, Chem. Rev. 2000, 100, 1685.
- 3) K. Matsuda, M. Irie, TCIMAIL 2003, number 120, 2. [PDF]
- 4) J. Harada, R. Nakajima, K. Ogawa, J. Am. Chem. Soc. 2008, 130, 7085.
- 5) K. Maeda, T. Hayashi, Bull. Chem. Soc. Jpn. 1969, 42, 3509.
- 6) Review: B. M. Monroe, G. C. Weed, Chem. Rev. 1993, 93, 435.