Growth, Crystal Structure, Optical Limiting and Third-Order Nonlinear Optical Properties of Bis(Thiourea) Lead Dinitrate

Single crystals of bis(thiourea) lead dinitrate (BTD) were successfully grown by the slow evaporation solution growth technique. Single-crystal X-ray diffraction analysis revealed that BTD crystallizes in the centrosymmetric monoclinic crystal system with the space group P21/c. The phase purity and crystallinity of the grown crystals were confirmed by powder X-ray diffraction, while Fourier transform infrared (FTIR) spectroscopy verified the coordination of thiourea molecules with Pb2+ ions through characteristic vibrational bands. Surface morphology examined by scanning electron microscopy (SEM) showed well-developed crystal facets with only minor growth defects. Thermogravimetric and differential thermal analyses demonstrated that the crystal is thermally stable up to 188 °C. Diffuse reflectance spectroscopy (DRS) indicated excellent optical transparency in the visible region with an optical band gap of 3.27 eV, highlighting the material’s suitability for photonic applications. Photoluminescence studies exhibited an intense blue emission centered at 392 nm. Density functional theory (DFT) calculations, complemented by quantum theory of atoms in molecules (QTAIM), non-covalent interaction–reduced density gradient (NCI–RDG), electron localization function (ELF) and interaction region indicator (IRI) analyses, were performed to investigate the electronic band structure, density of states, molecular polarizability, and first-order hyperpolarizability, as well as the Pb(II) coordination environment and intermolecular interactions, providing insight into the electronic transitions governing the optical response. Hirshfeld surface and two-dimensional fingerprint analyses revealed that the crystal packing is predominantly stabilized by O···H/H···O hydrogen-bonding interactions. The third-order nonlinear optical properties were evaluated using the Z-scan technique, which demonstrated positive nonlinear refraction, self-focusing behavior, and significant nonlinear absorption, with a third-order nonlinear susceptibility of 0.9909 × 10−13 esu. Furthermore, the crystal exhibited efficient optical limiting with a threshold of 24.44 J cm−2, indicating that BTD is a promising candidate for nonlinear photonic and optical limiting applications. The potential application of these materials also highlights the importance of resource efficiency and sustainable manufacturing in future materials development.