Synthesis, crystal growth, structural characterization and third-order NLO properties of caesium trihydrodiphthalate

Single crystals of (benzene-1,2-dicarboxylic acid-κO)(2-carboxybenzoato-κO) caesium(I), also known as caesium trihydrodiphthalate (CsADP), were successfully synthesized through a controlled hydrothermal reaction between caesium sulfate and phthalic acid in a 1:2 molar ratio. The resulting crystals, grown over 26–28 days, were analyzed using various techniques. FT-IR spectroscopy revealed characteristic vibrational bands, while powder XRD and single-crystal XRD confirmed phase purity and structural properties. CsADP crystallizes in the orthorhombic system with a centrosymmetric space group (Pbcn). The caesium ion is coordinated by ten oxygen atoms, with Cs···O bond lengths ranging from 3.085 to 3.610 Å, forming a distorted bicapped square antiprism geometry. Thermal analysis indicated stability up to 240 °C, while UV–Vis spectroscopy demonstrated transparency, with a bandgap of 4.17 eV. Nonlinear optical (NLO) properties were investigated using Z-scan techniques, revealing reverse saturation absorption and a strong third-order susceptibility, highlighting CsADP as a promising candidate for NLO applications. Hirshfeld surface analysis identified dominant O···H/H···O interactions, contributing to its NLO behaviour. Additionally, molecular electrostatic potential and Mulliken population analysis provided insights into the charge distribution within the structure.