Investigation on the growth, structural, HOMO–LUMO and optical studies of 1-ethyl-2-[2-(4-hydroxy-phenyl)-vinyl]-pyridinium iodide (HSPI) – a new stilbazolium derivative for third-order NLO applications

Abstract

The organic third-order non-linear optical crystal 1-ethyl-2-[2-(4-hydroxy-phenyl)-vinyl]-pyridinium iodide (HSPI), a new derivative of the stilbazolium family, was successfully synthesized and the optical quality single crystal was grown by a slow evaporation technique for the first time. The crystal structure of HSPI was confirmed by single crystal X-ray diffraction studies and it was found that it belongs to the monoclinic system with centrosymmetric space group C2/c. The molecular structure of the grown crystal was further confirmed by ¹H NMR and FTIR analysis. Theoretical calculations were performed to derive HOMO–LUMO energies and dipole moment by using the Spartan'14 V1.0.1 program. The UV-Vis-NIR spectral studies exhibit a huge transparency (∼70%) in the visible and near-IR (from 468 to 1100 nm) spectral regions that make it a potential material for non-linear device applications in this spectral range. The luminescence spectrum of the grown crystal showed green emission at 528 nm. The thermal stability of HSPI was found to be 226.6 °C. Mechanical behaviour was estimated by Vickers microhardness test. Dielectric studies were carried out as a function of frequency, and the results are discussed. The growth mechanism of HSPI was assessed by chemical etching studies. The title crystal shows excellent resistance to laser radiation with a high threshold up to 6.93 GW cm⁻² and it is compared with other NLO materials. The third-order nonlinear optical susceptibility of HSPI was derived using the Z-scan technique, and it was 4.40 × 10⁻⁴ esu. The negative nonlinear refractive index, n₂ = −7.29 × 10⁻¹² m² W⁻¹, is an indication of self-defocusing optical nonlinearity of the sample. It is believed that HSPI is a promising new candidate for developing efficient photonic and optical power limiting devices.