Synthesis, Crystal Structures, and Third‐Order Nonlinear Optical Properties of Quinolinium and Isoquinolinium Dinitrophenolate Proton‐Transfer Complexes

ABSTRACT This study explores the synthesis, structural properties, and third‐order nonlinear optical (NLO) behavior of two newly developed proton‐transfer complexes: 4‐methylquinolinium 2,4‐dinitrophenolate (4MQ‐DNP) and 1,2,3,4‐tetrahydroisoquinolin‐2‐ium 2,4‐dinitrophenolate (THIQ‐DNP). Both complexes were synthesized through equimolar reactions in ethanol, followed by slow evaporation to grow well‐defined crystals. Fourier‐transform infrared analysis confirmed key molecular interactions, while diffuse reflectance spectroscopy indicated significant transparency in the UV–visible range. Thermal analysis revealed single‐step decomposition processes, indicating good thermal stability. Complexes 4MQ‐DNP and THIQ‐DNP both exhibit monoclinic crystal symmetry, with 4MQ‐DNP belonging to the P 2 1 / n space group and THIQ‐DNP crystallizing in the P 2 1 / c space group. Scanning electron microscopy unveiled distinct surface morphologies, and Z‐scan analysis demonstrated self‐defocusing behavior, attributing a high third‐order susceptibility to both complexes. Additionally, single‐crystal x‐ray diffraction and Hirshfeld surface analysis provided insights into the packing and intermolecular interactions, highlighting π•••π stacking and hydrogen bonding as structural stabilizers. These findings suggest that the complexes hold strong potential for use in optoelectronic devices.