Electronic Structure, Nonlinear Optical Response, and Non-Covalent Interactions in Bi-(2-Amino-5-Methylpyridinium) Hexachlorostannate: Experimental and DFT Investigation

Abstract

This work presents an experimental and theoretical study of an aromatic nitrogen-containing ligand and its halotin complex, aiming to link molecular structure, interactions and nonlinear optical (NLO) properties. The geometries obtained from X-ray diffraction and DFT calculations are consistent, and the UV‑visible spectra reveal mainly intra-ligand transitions in the ultraviolet. Electronic analysis shows a lower HOMO–LUMO gap for the ligand than for the complex, as well as anisotropic polarisation associated with σ holes along the Sn–Cl bond. Reactivity descriptors indicate increased electronegativity, hardness and electrophilicity for the complex. The study of non-covalent interactions highlights the predominance of van der Waals forces, with some localized attractive interactions. In terms of NLO, the ligand exhibits higher first-order hyperpolarizability (β₀), while the complex is characterised by enhanced average polarizability (α) and second-order hyperpolarizability (γ), as well as increased dynamic response at 532 nm. Together, these findings provide avenues for the design of organic-inorganic hybrid materials suitable for second- and third-order optical applications.