Synthesis, Characterization, and DFT Insights into Non-linear Optical Properties of Ethoxysalicylaldehyde Thiosemicarbazone Derivatives

Abstract

In this study, a series of novel 3-ethoxysalicylaldehyde-based thiosemicarbazones (1–4) were synthesized via a condensation reaction between 3-ethoxysalicylaldehyde and substituted thiosemicarbazides in ethanol. The synthesized compounds (1–4) were characterized using various spectroscopic techniques including NMR (¹H and ¹³C), UV–Visible, and FT-IR spectroscopy. To explore their electronic properties, DFT and TD-DFT calculations were performed, focusing on nonlinear optical (NLO) properties, frontier molecular orbitals (FMOs), vibrational bands, UV–Vis absorption, and NBO analysis. Global reactivity descriptors showed that compounds 1–4 exhibit appropriate hardness and softness values, contributing to their stability. NBO analysis further revealed that strong hyperconjugations significantly contributed to their molecular stability. Among the compounds, 3 exhibited the narrowest band gap (4.721 eV), the highest softness (0.211 eV⁻¹), and the lowest hardness (2.360 eV), highlighting its notable electronic properties. A comparative study with urea indicated that all compounds possess higher values of hyperpolarizability than standard urea. The decreasing order observed for the β_total of studied species is found as 3 > 4 > 2 > 1. Among all synthesized compounds, compound 3 exhibited the highest value of β_total (857.582 a.u), indicating its potential as an NLO material. Furthermore, GRDs suggest that the electronic properties of the synthesized compounds particularly compound 3 may contribute in biological field.