Synthesis, crystal structure analysis, spectral characterization and nonlinear optical exploration of potent thiosemicarbazones based compounds: A DFT refine experimental study

The thiosemicarbazones have exciting biological and nonlinear optical (NLO) applications. The present study reports detail experimental and computational studies of three novel ferrocene-substituted thiosemicarbazones: (E/Z)-4-benzyl-1-(1-ferrocenylethyl)thiosemicarbazones (1), (E/Z)-4-(4-chlorobenzyl)-1-(1-ferrocenyl-ethyl) thiosemicarbazone (2) and (E/Z)-4-(2-bromo benzyl)-1-(1-ferrocenylethyl)thiosemicarbazone (3). These compounds were synthesized and resolved into their single crystal structures for the estimation of unit cells, space groups, bond angles and bond lengths. Chemical structures of 1-3 were further characterized spectroscopically employing nuclear magnetic resonance technique (1H NMR), infrared (FT-IR), mass and UV-Visible studies. Computational studies of 1-3 were performed using density functional theory (DFT) tools at M06 level of theory and 6-31 + G(d,p) basis set combination to gain the optimized geometry. A good correlation was found between experimental SC-XRD structures and DFT optimized geometries. Electronic properties including natural bond orbital (NBO) analysis, frontier molecular orbitals (FMOs) analysis, spectroscopic FT-IR data and NLO properties were calculated using same M06/6-31 + G(d,p) level of theory. NBO analysis confirmed the formation of charge separation state due to successful migration of electrons from donor to acceptor unit through pi-bridge. Global reactivity parameters were estimated using energies of FMOs which described that 1-3 are chemically hard and stable molecules. Vertical electronic transition states were calculated using time-dependent DFT (TDDFT) at same level of theory. NLO properties of 1-3 were computed 5.77, 3.48 and 8.93 times greater than the standard urea molecule respectively. Two-state model confirmed the potential of synthesized molecules as NLO candidates.