First hyperpolarizability in proton-transfer benzoxazoles: computer-aided design, synthesis and study of a new model compound Original Research Article

With regard to second-order nonlinear optics (NLO) applications, a new class of 2-(2′-hydroxyphenyl)benzoxazoles (HBO) was designed for a combination of high first hyperpolarizability, β, with good photothermal stability, in association with a fast excited state intramolecular proton transfer (ESIPT) mechanism. Semi-empirical optimization of molecular structures and ab initio calculations of dipole moments were performed. Clear evidence was found that conditions such as conjugation efficiency and electron donor/acceptor strength cannot be evaluated separately, due to structural changes in molecular spatial distribution. Experimentally, a new fluorescent molecule of the HBO family, 2-(2′-hydroxy-4′-aminophenyl)-6-nitrobenzoxazole (BO6), was synthesized, purified and characterized, including solvent environments of distinct polarities. Hyper-Rayleigh scattering, UV–Vis absorption and emission spectroscopy, differential scanning calorimetry and thermogravimetric analysis of BO6 show a significant β (213.4±25.7×10−30 esu in acetone, at 1064 nm) and thermal stability up to 270 °C. Such results, in this first study of ESIPT dyes for second-order NLO to our best knowledge, indicate that the HBO family well deserves further attention towards promising application materials.