Theoretical Exploration of Nonlinear Optical Properties in Thiophene-Based Azo Dyes

A comprehensive computational study was conducted on the nonlinear optical (NLO) properties, intramolecular charge transfer (ICT), electronic structure, electrostatic potential, Mulliken charge distribution, and frontier molecular orbital energies of thiophene-based heterocyclic azo dyes using density functional theory (DFT). The dye D6 exhibited a red-shift in its vertical excitation energy, in agreement with experimental data, indicating effective ICT characteristics. The incorporation of a nitro group significantly decreased the HOMO-LUMO energy gap, which contributed to enhanced polarizability (alpha) and both first-order (beta) and second-order (gamma) hyperpolarizabilities. All computed NLO values were within the fundamental theoretical limits, ensuring structural stability and practical applicability. Furthermore, the NLO responses evaluated at laser frequencies and EFISH (electric field-induced second harmonic generation) frequencies showed a marked enhancement, suggesting strong frequency-dependent optical activity. Comparative analysis revealed that the thiophene-based azo dyes demonstrated more than a twofold increase in NLO response relative to previously reported analogues. Among them, nitro-substituted derivatives (D4-D6) displayed superior performance across all evaluated parameters, highlighting the influence of electron-withdrawing substituents in tuning NLO behavior. These findings position nitro-substituted thiophene-based azo dyes as promising materials for advanced nonlinear optical applications such as photonic switches, optical data storage, and frequency conversion technologies.