Synthesis, crystal structure, and nonlinear optical investigation of a 2-amino-4,6-dimethylpyrimidine–hippuric acid cocrystal

This work reports the design and comprehensive investigation of an organic cocrystal formed between 2-amino-4,6-dimethylpyrimidine (2A46DMP) and hippuric acid (HA), with a focus on supramolecular structure–property relationships governing nonlinear optical (NLO) behavior. Single-crystal X-ray diffraction reveals a centrosymmetric monoclinic structure stabilized by a robust acid–pyrimidine heterosynthon (R₂²(8)) motif, complemented by auxiliary N–H···O and C–H···π interactions that drive three-dimensional packing. Hirshfeld surface and fingerprint analyses quantify the dominance of H···H and O···H contacts and highlight stabilization of the cocrystal lattice by van der Waals forces and directional hydrogen bonding. Spectroscopic investigations (FT-IR, FT-Raman, and NMR) confirm the formation of a neutral cocrystal without proton transfer, consistent with ΔpK_a considerations. Thermal analysis demonstrates enhanced lattice stability arising from cooperative intermolecular interactions. Optical studies reveal a wide direct bandgap (approximately 3.6 eV) and good transparency, while Z-scan measurements indicate saturable absorption, a negative nonlinear absorption coefficient, and a negative nonlinear refractive index. The observed third-order susceptibility is attributed to hydrogen-bond-mediated electronic polarization and supramolecular packing, providing insight into the origin of the nonlinear response. This study highlights how functional group complementarity and synthon hierarchy can be exploited to tune optical and electronic properties in organic cocrystals, offering a rational design strategy for advanced photonic materials.