Exploration of Chromone-Based Thiosemicarbazone Derivatives: SC-XRD/DFT, Spectral (IR, UV–Vis) Characterization, and Quantum Chemical Analysis

[Image: see text] By the condensation of thiosemicarbazide with coumarin aldehyde, two novel substituted thiosemicarbazones with chemical formulae C(24)H(25)N(3)O(3)S (3a) and C(26)H(23)N(3)O(3)S (3b) have been synthesized. The synthesized compounds were resolved using SC-XRD, and structure elucidation was carried out using (1)H NMR, (13)C NMR, UV–visible, and FT-IR spectroscopic analyses. Computational calculations at the B3LYP/6-311+G(d,p) level of theory were performed to countercheck the experimental (UV–vis, FT-IR) findings and explore the electronic (FMO, NBO, MEP) properties of 3a–b. The nonlinear optical (NLO) properties of 3a–b were estimated using B3LYP, HF, LC-BLYP, CAM-B3LYP, M062X, and M06 functionals in combination with the 6-311+G(d,p) basis set. The crystallographic data revealed that compounds were crystallized as an orthorhombic crystal lattice with the Pbcn space group and the triclinic crystal lattice with the P̅1 space group. A good concurrence among experimental SC-XRD-generated bond lengths, bond angles, FT-IR, UV–vis, and corresponding DFT results was found, which confirms the purity of both compounds. The NBO analysis confirmed the presence of intramolecular hydrogen bonding and hyperconjugative interactions, which not only were the pivotal cause of stability of the investigated compounds but also led to an overwhelming NLO response. The energy differences calculated for HOMO/LUMO are 3.053 and 3.118 eV in 3a and 3b, respectively. The crystal 3b showed a higher value of first-order polarizability at all levels of theory than 3a. Overall results show that the crystals under investigation are polarized in nature with a good dipole moment. A comparative analysis with urea molecules clearly indicates that the studied compounds are acceptable NLO candidates and they can be used for future technological applications.