Hydrogen-bond assisted charge transfer and enhanced nonlinear optical response in 4-methylbenzylammonium hydrogen maleate

Although the growth and experimental characterization of 4-methylbenzylammonium hydrogen maleate (4MLBAHM) single crystals have been previously reported, a fundamental understanding of the origin of its structural stability and nonlinear optical (NLO) response remains unexplored. In the present work, a detailed Density Functional Theory (DFT) investigation has been performed to establish the structure–property relationship of 4MLBAHM, with particular emphasis on the role of hydrogen bonding in driving charge-transfer mechanisms and its physicochemical behavior. The optimized geometry at the B3LYP/6–311G(d,p) level shows good agreement with experimental crystallographic data, validating the computational approach. Natural Bond Orbital (NBO) analysis reveals strong donor–acceptor interactions associated with proton-transfer-assisted hydrogen bonding, leading to significant intramolecular charge delocalization. Frontier molecular orbital analysis further confirms enhanced electronic polarization across the molecular framework. Topological and vibrational analyses demonstrate that O–H⋯O, H⋯H⋯N and C–H⋯O interactions play a dominant role in modulating structural distortion and vibrational characteristics, while Hirshfeld surface analysis highlights the contribution of weak intermolecular contacts in stabilizing the supramolecular architecture. The first-order hyperpolarizability is significantly enhanced compared to standard reference systems, which is attributed to the synergistic interplay between hydrogen bonding, aromatic conjugation and directional charge transfer. These results provide clear insight into the microscopic origin of NLO activity and establish 4MLBAHM as a potential candidate for optoelectronic and photonic applications.