The design of azo-imine derivatives with tunable photochromic and nonlinear optical (NLO) properties is crucial for developing advanced optoelectronic materials. In this study, three new azo-imine derivatives ( C1–C3 ) incorporating methyl (−CH 3 ), methoxy (−OCH 3 ), and chloro ( Cl) substituents, along with an octyloxy chain, were synthesized and characterized through a combination of experimental and theoretical approaches. FT-IR and 1 H NMR spectra confirmed the successful formation of the azo-imine framework. DFT calculations at the B3LYP/6–311++G(d,p) level were used to examine optimized geometries and electronic structures. UV–Vis spectra showed weak absorption bands at 272–274 nm and intense π–π* transitions at 354–360 nm, with substituent-dependent spectral shifts reflecting modulation of conjugation and electron density. Photochromic studies demonstrated that C1 undergoes the fastest E → Z isomerization, whereas the chloro-substituted derivative C3 exhibits the highest photoisomerization efficiency and the most rapid Z → E thermal recovery. The electron-donating methoxy group in C2 enhances conjugation but reduces photoisomerization efficiency. Thermogravimetric analyses confirmed high thermal stability for all derivatives. Frontier molecular orbital and DOS analyses revealed a reduced HOMO–LUMO gap for C2 , indicating facilitated electronic transitions. NBO results showed dominant σ → σ* interactions with Lewis-type contributions of 97.545%, 97.564%, and 97.606% for C1 , C2 , and C3 , respectively. Transition density matrix and excitation binding energy analyses indicated strong charge-transfer character and effective electron delocalization in C2 . Nonlinear optical (NLO) responses increased markedly with solvent polarity, and in the aqueous phase, C2 exhibited the highest β total (878 × 10 −31 esu), surpassing urea by more than two orders of magnitude. These results underscore the significant role of substituent effects in governing photochromic reversibility and nonlinear optical (NLO) performance, with C3 exhibiting the highest photoswitching efficiency and C2 emerging as the most promising NLO-active candidate. Thus, this study offers valuable insights for the rational design of azo-imine systems with tunable photophysical and optical properties, paving the way for advanced optoelectronic applications. • Three novel azo-imine derivatives ( C1-C3 ) were synthesized and characterized. • DFT/TD-DFT was used to optimize structures and study electronic and NLO features. • All compounds exhibited reversible E ↔ Z isomerization; C3 showed the best photoresponse. • NBO, TDM, and E b revealed the strongest charge delocalization in C2 . • Substituents and solvent polarity enhance NLO; C2 showed superior performance.
Islam et al. (Wed,) studied this question.