High-Order Harmonics and Photoluminescence in Azo-Phenylthiophene Derivatives

This paper presents the linear and nonlinear optical (NLO) properties of two donor–acceptor organic materials, namely, N,N-dimethyl-4-((E)-(4-((E)-((5-phenylthiophen-2-yl)methylene)amino)phenyl)diazenyl)aniline A and (E)-4-((E)-(4-nitrophenyl)diazenyl)-N-((5-phenylthiophen-2-yl)methylene)aniline B. The studied compounds differ from each other by the nature of the substituent (donor or acceptor) on the azobenzene moiety, giving rise to D−π–D and D−π–A systems, respectively, for A and B. The thin-film deposition process was carried out using two different techniques: physical vapor deposition (PVD) and spin-coating. The aim of this work is to elucidate the influence of both the molecular structure of the compounds and thin-film deposition technique on their linear and nonlinear optical responses. Absorbance, photoluminescence, and decay time were used to measure the linear optical properties, while second and third harmonic generation techniques were used as tools for the nonlinear optical responses. The comparison of the results obtained for both A and B indicates a much better NLO performance with a value of 8.18 ± 0.09 pm V^–1 for compound B with a D−π–A shape and a value of 1.32 ± 0.07 pm V^–1 for compound A with a D−π–D structure. A more detailed analysis of the NLO properties revealed a noteworthy finding: the compounds exhibited considerably elevated third-order nonlinear susceptibility values in comparison to the reference material, with discrepancies spanning 1–2 orders of magnitude. Of particular interest was compound B, which demonstrated the highest 𝜒(3)elec${{\chi }^{(3)}}_{\mathrm{elec}}$${{\chi }^{(3)}}_{\mathrm{elec}}$ value equal (225.91 ± 0.92) × 10^–22 m² V^–2. The research was completed by theoretical quantum chemical calculations, which included the determination of dipole moments and the evaluation of molecular orbital frontier highest occupied molecular orbital and lowest unoccupied molecular orbital energies. These comprehensive studies demonstrate the significant potential of azo-based phenylthiophene derivatives in optoelectronics and quantum optics applications and also show that it is a valuable candidate for the production of organic light-emitting diodes.