The curious case of the photochemistry of 2-hydroxyphenylazo-3,5-dimethylisoxazole: unravelling the process among tautomerization, photoisomerization, and conformational changes.

Considering the growing interest in azoheteroarene photoswitches, the molecular level process underlying photochemistry is intriguing. In our earlier investigation on phenylazo-3,5-dimethylisoxazole, besides E - Z photoisomerization, we also observed light-induced phase transition that supports the possible conformational changes under neat conditions. Furthermore, hydrogen bond-forming groups such as -OH, at the ortho position to the azo chromophore, can potentially hamper the isomerization through tautomerism. All of them develop a curiosity in the photochemical outcome of 2-hydroxyphenylazo-3,5-dimethylisoxazole (HPAI, 1). Herein, we report the photochemistry of HPAI in an argon matrix at 4 K, followed by infrared spectroscopy. Through experiments and computations, we identified the E - Z photoisomerization in HPAI as the only observed channel among the above-mentioned possibilities.