Halogen-modulated tautomerism towards reversable tunable photoswitching in N-(3,5-di-tert-butylsalicylidene)-4-halobenzenes: Insights from DFT and TD-DFT

Despite substantial experimental work on halogenated salicylideneaniline systems, a deeper theoretical understanding of their structure–property relationships remains an active area of research. This study presents a first-principles investigation of the structural stability, nonlinear optical properties, and ultraviolet absorption characteristics of halogen-substituted salicylideneaniline derivatives, namely N-(3,5-di-tert-butylsalicylidene)-4-halobenzenes. Our results reveal that the enol and cis-keto forms exhibit stronger intramolecular hydrogen bonding than the trans-keto forms, contributing to enhanced stability and nonlinear optical response. The pronounced π-conjugation in these compounds facilitates efficient ultraviolet light absorption and promotes photoswitching between enol and cis-keto forms. Notably, the cis-keto forms of Br- and I-substituted analogs exhibit nonlinear optical responses 2–3 times greater than the parent compound, corroborated by red-shifted TD-DFT-predicted absorption spectra. Dihedral angle analysis offers further insight into the photo-induced nonlinear optical response in the UV–Vis region. While the enol forms exhibit inherent photochromic properties, the cis-keto forms of the bulkier halogen-substituted salicylideneaniline derivatives display intrinsic thermochromic behavior. Moreover, the compounds exhibit reversible photoswitching behavior depending on tautomeric configurations.