A New Nonlinear Optical Stilbazolium Family Crystal of (E)‑1-Ethyl-2-(4-nitrostyryl) Pyridin-1-ium Iodide: Synthesis, Crystal Structure, and Its Third-Order Nonlinear Optical Properties

The derivative of an organic stilbazolium single crystal, (E)-1-ethyl-2-(4-nitrostyryl) pyridin-1-ium iodide (hereafter abbreviated as NSPI), was synthesized by the Knoevenagel condensation method. Single crystal of NSPI of size 13 × 12 × 3 mm³ were obtained from a 1:1 ratio of methanol and acetonitrile mixed solvent using the slow evaporation technique. X-ray structural analysis has indicated that NSPI crystallizes in the centric triclinic system with the P1̅ space group. In addition, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopic analyses were carried out to confirm the molecular structure of the grown crystal. The small energy difference of the frontier molecular orbitals indicates the possible nonlinear optical properties present in the title crystal. Thermal studies confirm that the material has a good thermal stability of about 151 °C. Etching studies provide the layered growth of the NSPI crystal with fewer dislocations. The optical transmission range and the band gap were assessed by UV–vis transmission study. Red emission taking place at 721 nm was identified from a photoluminescence study. The NSPI crystal has good resistance to laser radiation against the Nd:YAG laser at 1064 nm. The third-order NLO efficiency of the NSPI crystal was determined via the Z-scan technique with a He–Ne laser excitation source operating at 632.8 nm.