DFT investigation of 5,8-diaminoquinoxaline-based alkyl bis-Schiff bases: geometry, spectroscopic analysis, MEP, frontier orbitals, and nonlinear optical characteristics

This work presents a comprehensive density functional theory (DFT) study of bis-Schiff bases (III) formed by condensation of 5,8-diaminoquinoxaline with aliphatic aldehydes (II) (R = H, Me, Et, n-Pr, iso-Pr, n-Bu), with emphasis on their structural, electronic, spectroscopic, and nonlinear optical (NLO) properties. Initial molecular models were fully optimized at the B3LYP/6-311 + + G (d, p) level, without symmetry constraints, using Gaussian 09. Calculations included dipole moments (µ), mean polarizabilities (αave), first static hyperpolarizabilities (βtotal) via the finite-field approach, frontier molecular orbitals, molecular electrostatic potential (MEP) maps, and simulated IR and UV–Vis spectra. The bis-Schiff bases adopt stable ground-state geometries with electronic structures characterized by well-defined HOMO–LUMO separations that correlate with computed optical transitions. MEP analyses identify the principal sites for electrophilic and nucleophilic attack, providing insight into reactivity and potential biological interaction points. Computed polarizabilities and βtotal values indicate appreciable NLO responses that depend sensitively on alkyl substitution and intramolecular charge-transfer characteristics. Simulated IR and UV–Vis spectra agree qualitatively with the expected vibrational and electronic features of Schiff bases, supporting the assignments of key transitions and functional-group vibrations.