Elucidating the Photophysics and Nonlinear Optical Properties of a Novel Azo Prototype for Possible Photonic Applications: A Quantum Chemical Analysis

The photophysics and nonlinear optical responses of a novel nitrothiazol-methoxyphenol molecule were investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods with the polarizable continuum model to take the solvent effect into account. Special attention is paid to the description of the lowest absorption band, characterized as a strong π → π* state in the visible region of the spectrum. The TD-DFT emission spectrum analysis reveals a significant Stokes shift of more than 120 nm for the π → π* state in gas phase condition. The results show a great influence of the solvent polarity on the nonlinear optical (NLO) response of the molecule. Specifically, the second harmonic generation hyperpolarizability β(−2ω; ω, ω) shows a large variation from gas to aqueous solvent (82 × 10^–30 to 162 × 10^–30 esu), exhibiting notably higher values than those reported for standard compounds such as urea (0.34 × 10^–30 esu) and p-nitroaniline (6.42 × 10^–30 esu). Furthermore, a two-photon absorption analysis indicates a large cross-section (δ^2PA = 77 GM) with superior performance compared to several dyes. These results make the molecule quite interesting for nonlinear optics.