The Synthesis and Characterization of Transition Metal Complexes of Mono Thiourea as Ligand and its Nonlinear Optic Application

1,3-Diphenylthiourea (DPTU) is a promising candidate as a nonlinear optical (NLO) material due to its electronic structure, high dipole moment, and potential for strong nonlinear optical responses. It could be used in various photonic applications such as laser frequency conversion, optical switching, and signal processing. This study investigates the nonlinear optics (NLO) properties of metal complexes formed by using 1,3- diphenylthiourea as a ligand with different transition metals (copper, cadmium, and cobalt, labeled CA, CB, and CC respectively). Ligand was synthesized with a 70% yield. The characterization was performed using Fourier Transform Infrared (FT-IR), Nuclear Magnetic Resonance (NMR), and Ultraviolet-Visible (UV-Vis) spectroscopy. Density Functional Theory (DFT) calculations with the 6-31G (d,p) basis set at the B3LYP level were used to optimize the structure of thiourea and compute the IR, 1H NMR, and UV-Vis spectra, which were then validated against experimental data. FT-IR analysis showed the C=S stretching peak at 1242.98 cm⁻¹ (experimental) and 1200.34 cm⁻¹ (computational) for the ligand. Upon complexation, the C=S stretching frequencies shifted to 1028.01 cm⁻¹ (CA), 1183.81 cm⁻¹ (CB), and 1205.50 cm⁻¹ (CC) experimentally, and to 1035.74 cm⁻¹ (CA), 1196.70 cm⁻¹ (CB), and 1221.18 cm⁻¹ (CC) computationally, with deviations under 5%. NMR analysis revealed amine peak signals at ẟ 9.633 ppm (H, NHa) and ẟ 8.169 ppm (H, NHb) for the ligand, shifting to ẟ 8.621 ppm (H, NHa) and ẟ 7.402 ppm (H, NHb) for CA. UV-Vis spectroscopy showed a π–π* transition for the ligand at 238 nm, shifting to 253 nm (CA), 266 nm (CB), and 265 nm (CC), with additional Metal-to-Ligand Charge Transfer (MLCT) transitions at 701 nm (CA), 429 nm (CB), and 507 nm (CC). DFT calculations indicated that the cobalt complex (CC) exhibits the highest NLO properties, with a frequency-dependent first hyperpolarizability (βtot) value of 6030.985 x10⁻³⁰ esu at 1064 nm, supported by a high dipole moment and low energy gap, making it the best candidate for NLO applications.