Synthesis and chemical–optical characterization of novel two-photon fluorescent borinates derived from Schiff bases

Herein is reported the synthesis and characterization of eight borinates (2a–h) prepared from the reaction of the corresponding bidentate imine ligands (1a–h) and diphenylborinic acid. All compounds were characterized by MS, IR and NMR spectroscopies whereas the molecular structures of derivatives 2c and 2e were also studied by X-ray diffraction analysis that corroborated the presence of a tetra-coordinated boron atom. In addition, the linear and nonlinear optical (NLO) characterization of these molecules having D–π–CN+–π–D and D–π–CN+–π–A electronic systems was performed, i.e., the third-order nonlinear optical responses were measured by using the two-photon excited fluorescence and the third-harmonic generation (χ3 determination) techniques. The borinates showed two-photon absorption (2PA) effect in solution under femtosecond excitation: 2g and 2h having D–π–CN+–π–A architecture showed the largest 2PA cross-sections (σ2PA) values at 760 nm: 124 and 174 GM (1 GM = 10−50 cm4 s), respectively, while the values for 2e and 2f that possess D–π–CN+–π–D structure were 12 and 37 GM. The χ3 values, with nanosecond excitation, were of the order of 10−12 esu at 1200 nm.