Synthesis, characterization, and comprehensive analysis of 2-methylquinolinium picrate: crystal engineering, optical, spectroscopic, thermal stability, and nonlinear optical properties for opto-electronic applications

Transparent pale yellow crystals of 2-methylquinolinium picrate (2MQPA) were synthesized from picric acid and 2-methylquinoline using methanol as a solvent. The salt formation occurred through hydrogen bonding between the phenolate group of picric acid and the tertiary nitrogen of 2-methylquinoline. Optical characteristics, including transmittance and photoluminescence, were examined using UV–Vis spectroscopy. Functional groups were identified through Fourier Transform Infrared (FTIR) spectroscopy, while thermal stability and decomposition behavior were analyzed via thermogravimetric-differential thermal analysis (TG–DTA) and differential scanning calorimetry (DSC). The third-order nonlinear optical (NLO) response was investigated using the Z-scan technique. Hirshfeld surface analysis highlighted significant intermolecular interactions, including H–H, C–H, O–H, and N–O contacts. Computational studies optimized the crystal structure, and density functional theory (DFT) calculations confirmed intermolecular charge transfer interactions and N–H⋯O hydrogen bonding, offering detailed insights into the electronic properties of 2MQPA.