Experimental and DFT/TD-DFT Insights into Spectroscopic, Solvatochromic, and Nonlinear Optical Properties of Methoxy-Substituted N-Benzylideneaniline Schiff Base Regioisomers

Schiff bases bearing donor–acceptor substituents represent versatile D–π–A systems with tunable electronic and optical properties. Herein, we report combined experimental spectroscopic and DFT/TD-DFT (B3LYP/6-311++G(d,p)) investigations of methoxy-substituted N-benzylideneaniline Schiff base regioisomers, benchmarked against unsubstituted analogues, revealing substituent position–controlled spectral and nonlinear optical properties. The FT-IR and NMR results demonstrate that substituent identity and position significantly influence the vibrational frequencies of characteristic IR bands and shielding/deshielding patterns of proton and carbon nuclei. UV–Vis and TD-DFT analyses reveal bathochromic shifts of the π → π* transitions in polar media with solvatochromic sensitivity decreasing in the order: ortho > para ≫ meta. Molecular electrostatic potential and Mulliken charge analyses indicate that methoxy substitution on the benzylidene ring enhances resonance donation and conjugation in the ortho- and para-isomers, leading to higher molecular electron polarization and consequently stronger bathochromic shifts and solvatochromic responses than the meta-isomer and unsubstituted analogues. Frontier molecular orbital (FMO) analysis further confirms that substituent position and donor strength critically govern the optical response and chemical reactivity of these systems. DFT-calculated polarizabilities reveal that donor–acceptor strength tuned by π-conjugation significantly enhances the NLO responses of these systems. The positional methoxy substitution provides an effective strategy for tuning the electronic and photophysical properties of Schiff bases, highlighting their potential for nonlinear optical and photonic applications.