High-Order Harmonics and Photoluminescence in Azo-Phenylthiophene\nDerivatives

This\npaper presents the linear and nonlinear optical (NLO) properties\nof two donor–acceptor organic materials, namely, <i>N</i>,<i>N</i>-dimethyl-4-((<i>E</i>)-(4-((<i>E</i>)-((5-phenylthiophen-2-yl)methylene)amino)phenyl)diazenyl)aniline <b>A</b> and (<i>E</i>)-4-((<i>E</i>)-(4-nitrophenyl)diazenyl)-<i>N</i>-((5-phenylthiophen-2-yl)methylene)aniline <b>B</b>. The studied compounds differ from each other by the nature of the\nsubstituent (donor or acceptor) on the azobenzene moiety, giving rise\nto D−π–D and D−π–A systems,\nrespectively, for <b>A</b> and <b>B</b>. The thin-film\ndeposition process was carried out using two different techniques:\nphysical vapor deposition (PVD) and spin-coating. The aim of this\nwork is to elucidate the influence of both the molecular structure\nof the compounds and thin-film deposition technique on their linear\nand nonlinear optical responses. Absorbance, photoluminescence, and\ndecay time were used to measure the linear optical properties, while\nsecond and third harmonic generation techniques were used as tools\nfor the nonlinear optical responses. The comparison of the results\nobtained for both <b>A</b> and <b>B</b> indicates a much\nbetter NLO performance with a value of 8.18 ± 0.09 pm V<sup>–1</sup> for compound <b>B</b> with a D−π–A shape\nand a value of 1.32 ± 0.07 pm V<sup>–1</sup> for compound <b>A</b> with a D−π–D structure. A more detailed\nanalysis of the NLO properties revealed a noteworthy finding: the\ncompounds exhibited considerably elevated third-order nonlinear susceptibility\nvalues in comparison to the reference material, with discrepancies\nspanning 1–2 orders of magnitude. Of particular interest was\ncompound <b>B</b>, which demonstrated the highest χ(3)elec value equal (225.91 ± 0.92) ×\n10<sup>–22</sup> m<sup>2</sup> V<sup>–2</sup>. The research\nwas completed by theoretical quantum chemical calculations, which\nincluded the determination of dipole moments and the evaluation of\nmolecular orbital frontier highest occupied molecular orbital and\nlowest unoccupied molecular orbital energies. These comprehensive\nstudies demonstrate the significant potential of azo-based phenylthiophene\nderivatives in optoelectronics and quantum optics applications and\nalso show that it is a valuable candidate for the production of organic\nlight-emitting diodes.