Directional growth, physicochemical and quantum chemical investigations on pyridinium 2-carboxylate: 4-nitrophenol (P2C4N) single crystal for nonlinear optical (NLO) applications

Abstract

An organic nonlinear optical (NLO) single crystal of pyridinium 2-carboxylate: 4-nitrophenol (P2C4N) was grown by the Sankaranarayanan-Ramasamy (SR) method using the (1 1 −1) plane. A transparent crystal of size 180 mm in length and 10 mm in diameter was grown over a period of 60 days. The lattice parameters and the molecular structure of the grown crystal were confirmed by single crystal XRD and nuclear magnetic resonance (NMR) spectrum analysis, respectively. The crystalline perfection of the SR method-grown crystal was evaluated by HRXRD analysis; from the observed results, it is evident that the crystal quality is quite good. UV-Vis-NIR analysis shows that the SR method-grown crystal has good transparency (90%) in the visible and NIR regions. The band gap of the title crystal was estimated by the Tauc's plot method and was found to be 3 eV. The title material is thermally stable up to 141 °C, and it demonstrates positive photoconductive behaviour. Vickers microhardness studies revealed that the grown crystal can be categorized as a soft material. The laser damage threshold of P2C4N was found to be 2.9 GW cm⁻², which shows that the grown crystal possesses excellent resistance to high power radiation. Z-Scan studies proved that the grown crystal possesses self-defocusing effects and negative nonlinearity. The theoretical spectra were calculated using the B3LYP/cc-pVTZ basis set and were compared with the experimental FT-IR and FT-Raman spectra of the title molecule. The full vibrational assignments of the observed spectra have been proposed by normal coordinate analysis (NCA) followed by force-field calculations. Quantum chemical studies, such as optimized geometry, HOMO–LUMO, natural bonding orbital (NBO) analysis, AIM analysis, hyperpolarizability and molecular electrostatic potential (MEP) were performed for the title molecule. The results are discussed in detail.