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Updated: Feb 17, 2026

Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
Published on: February 7, 2022
Photoisomerization and thermal isomerization of arylazoimidazoles
Joe Otsuki1, Kazuya Suwa, Kamal Krishna Sarker
1College of Science and Technology, Nihon University, 1-8-14 Kanda Surugadai, Tokyo 101-8308, Japan. otsuki@chem.cst.nihon-u.ac.jp
Abstract:
Photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles are investigated. Absorption spectra are characterized by a structured pipi* absorption band around 330-400 nm with a tail on the lower energy side extending to 500 nm corresponding to an npi* transition. The trans-to-cis photoisomerization occurs on excitation into these absorption bands. The quantum yields are dependent on the excitation wavelength, as observed for azobenzene derivatives, but are generally larger than those of azobenzene. The thermal cis-to-trans isomerization rates are also generally larger than that of azobenzene and are comparable to those of 4-N,N-dimethylaminoazobenzene and 4-nitroazobenzene. Arylazoimidazoles with no substituent on the imidazole nitrogen are unique in that the quantum yield for the trans-to-cis photoisomerization and the rate of thermal cis-to-trans isomerization are particularly large. It is proposed that the fast thermal isomerization is due to an involvement of self-catalyzed and protic molecule-assisted tautomerization to a hydrazone form.
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