Materials with nonlinear optical properties have significant applications in nuclear science, biophysics, medicine, chemical dynamics, solid physics & materials science. We show how π bridges, donors & acceptors can be reconfigured to improve optical properties.

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The role of thioureas in crystal engineering as robust supramolecular synthons is now recognized, but their analogs, namely thiosemicarbazones/N-iminothioureas, have not received the attention they deserve.

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In this study, seven derivatives of salicylaldehyde thiosemicarbazones (1–7) were synthesized by refluxing substituted thiosemicarbazide and salicylaldehyde in an ethanol solvent. Different spectral techniques (UV–vis, IR, and NMR) were used to analyze the prepared compounds (1–7). Accompanied by the experimental study, quantum chemical studies were also carried out at the M06/6-311G­(d,p) level. A comparative analysis of the UV–visible spectra and vibrational frequencies between computational and experimental findings was also performed. These comparative data disclosed that both studies were observed to be in excellent agreement. Furthermore, natural bond orbital investigations revealed that nonbonding transitions were significant for the stability of prepared molecules. In addition, frontier molecular orbital (FMO) findings described that a promising charge transfer phenomenon was found in 1–7. The energies of FMOs were further used to determine global reactivity parameters (GRPs). These GRP factors revealed that all synthesized compounds (1–7) contain a greater hardness value (η = 2.1 eV) and a lower softness value (σ = 0.24 eV), which indicated that these compounds were less reactive and more stable. Nonlinear optical (NLO) evaluation displayed that compound 5 consisted of greater values of linear polarizability ⟨α⟩ and third-order polarizability ⟨γ⟩ of 324.93 and 1.69 × 105 a.u., respectively, while compound 3 exhibited a larger value of second-order polarizability (βtotal) of 508.41 a.u. The NLO behavior of these prepared compounds may be significant for the hi-tech NLO applications.