Synthesis, structure, third-order nonlinear optical properties and Hirshfeld surface analysis of 18-crown-6 with dimethylpyridin-1-ium iodide isomers

Supramolecular complexes of 18-crown-6 with isomers of dimethylpyridin-1-ium iodide have been successfully synthesized and characterized using various analytical techniques, including single-crystal X-ray diffraction (SCXRD). SCXRD study revealed the crystal structures of bis(1,2-dimethylpyridin-1-ium iodide)-18-crown-6 (I) with a monoclinic structure in the centric space group P2₁/n, bis(1,3-dimethylpyridin-1-ium iodide)-18-crown-6 (II), and bis(1,4-dimethylpyridin-1-ium iodide)-18-crown-6 (III), both exhibiting triclinic structures in the centric space group Pī. The band gap energies are determined using diffuse reflectance data through the Kubelka-Munk algorithm. The thermal stability of these cocrystals was assessed through differential thermal and thermogravimetric studies, while their surface morphology was analyzed using scanning electron microscopy. The third-order nonlinear optical susceptibilities of cocrystals (I), (II), and (III) were measured at 3.54 × 10⁻⁶ esu, 4.05 × 10⁻⁶ esu, and 4.08 × 10⁻⁶ esu, respectively, indicating their promising utility in nonlinear optical applications.