Quantum chemical calculations based on the density functional theory (DFT) method were performed on five polydentate Schiff base compounds (PSCs), act as corrosion inhibitors for iron in aerated 2.0 M HNO₃ and 2.0 M NaOH, aiming to analyze the characteristics of the inhibitor/surface mechanism as well as to analyze the experimental data and to describe the structural nature of the inhibitor on the corrosion process. The relationship between molecular structure of PSC and their inhibition efficiencies was discussed. The inhibition effect has been investigated using weight loss, thermometric, potentiodynamic polarization measurements and quantum chemical study. Among the compounds studied, 4-(2-hydroxynaphthylideneamino) antipyrine (PSC_1) exhibited the best inhibition efficiency IE (%) 78.4 % at 10⁻⁴ M at 303 K. The high IE of (PSCs) was attributed to the blocking of active sites by adsorption of inhibitor molecules on the iron surface. Adsorption of (PSCs) inhibitors is spontaneous, exothermic, obeyed Temkin isotherm and is regarded as physiochemical mechanism. Polarization measurements indicate that all the examined Schiff bases are of mixed-type inhibitor in HNO₃; causing only inhibition of the cathode process in NaOH. The results showed that the inhibition efficiency of PSCs increased with the increase in E_HOMO and decrease in E _LUMO − E _HOMO; and the areas containing N and O atoms are most possible sites for bonding the metal iron surface by donating electrons to the metal.

The electronic absorption spectra, ground-state geometries and electronic structures of symmetric and asymmetric squaraine dyes (SQD1–SQD4) were investigated using density functional theory (DFT) and time-dependent (TD-DFT) density functional theory at the B3LYP/6-311++G** level. The calculated ground-state geometries reveal pronounced conjugation in these dyes. Long-range corrected time dependent density functionals Perdew, Burke and Ernzerhof (PBE, PBE1PBE (PBE0)), and the exchange functional of Tao, Perdew, Staroverov, and Scuseria (TPSSh) with 6-311++G** basis set were employed to examine optical absorption properties. In an extensive comparison between the optical data and DFT benchmark calculations, the BEP functional with 6-311++G** basis set was found to be the most appropriate in describing the electronic absorption spectra. The calculated energy values of lowest unoccupied molecular orbitals (LUMO) were 3.41, 3.19, 3.38 and 3.23 eV for SQD1, SQD2, SQD3, and SQD4, respectively. These values lie above the LUMO energy (−4.26 eV) of the conduction band of TiO₂ nanoparticles indicating possible electron injection from the excited dyes to the conduction band of the TiO₂ in dye-sensitized solar cells (DSSCs). Also, aromaticity computation for these dyes are in good agreement with the data obtained optically and geometrically with SQD4 as the highest aromatic structure. Based on the optimized molecular geometries, relative positions of the frontier orbitals, and the absorption maxima, we propose that these dyes are suitable components of photovoltaic DSSC devices.