Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its Derivatives: A Theoretical Study.

Hydroxyphenyl benzothiazole (HBT), is a well-known organic system based on its special characteristic of the excited state hydrogen transfer (ESHT) following photoexcitation. However, the capability of this system regarding photochromism and photoswitching has not been addressed yet. In this study, we have investigated this issue by the aim of the MP2, CC2, ADC(2), and CASSCF theoretical methods. Also, we have considered several electron withdrawing groups and investigated their effects on the photophysical characteristics and spectroscopic properties of the enol and keto tautomers of the titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characteristics required for photochromism. Also, substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems. Our theoretical results verify that different substitutions alter the UV absorption of HBT systems from 330 to 351 nm and also the corresponding absorption wavelength of the γ-forms of 526-545 nm.