Investigation of the Structural, Geometric, Spectroscopic, and Nonlinear Optical Properties of (E)-2-(2,4-dihydroxybenzylidene) thiosemicarbazone and (E)-2-[(1H-indol-3-yl)methylene] thiosemicarbazone complexes: A DFT Study

The optimized molecular geometries, vibrational frequencies, and both ¹H and ¹³C NMR spectra of (E)-2-(2,4-dihydroxybenzylidene)thiosemicarbazone (1) and (E)-2-[(1H-indol-3-yl)methylene]thiosemicarbazone (2) complexes were investigated using density functional theory (DFT) methods at the HSEh1PBE/6–311++G(d,p) and B3LYP/6–311++G(d,p) levels. NMR chemical shifts play a crucial role in elucidating the relationship between molecular structure and electronic properties. In addition, frontier molecular orbitals (FMOs) are essential for understanding the reactivity, as well as the electrical and optical properties, of molecules. The FMOs and related global reactivity descriptors—chemical hardness (η), electronegativity (χ), chemical potential (μ), and softness (S)—were also calculated using the same computational methods. Density Functional Theory (DFT) calculations were performed to investigate the nonlinear optical (NLO) properties of complexes 1 and 2. The results indicate that both complexes exhibit promising NLO behavior, with high second harmonic generation (SHG) efficiency.