O,S-Chelated bis(pentafluorophenyl)boron and diphenylboron-β-thioketonates: synthesis, photophysical, electrochemical and NLO properties.

Boron-β-diketonates are classical emissive materials that have been utilized in various fields, however, boron monothio-β-thioketonates, where one oxygen atom is exchanged for a sulphur atom, have not been explored in detail. To gain a better understanding of this class of materials, we synthesised various aryl substituted monothio-β-diketonate boron complexes with two different aryl substitutions on the boron center and studied their structural, optical and electrochemical properties. Single crystal X-ray analysis revealed that there is considerable deviation in B-O and B-S bond lengths for bis(pentafluorophenyl)boron complexes against diphenyl boron complexes. The bis(pentafluorophenyl)boron complexes have a relatively high absorption coefficient over diphenyl boron complexes. More importantly, a striking difference was observed for the emission behaviour of these compounds. The bis(pentafluorophenyl)boron complexes exhibit weak emission in the solution as well as in the solid state, whereas diphenyl boron complexes do not show any emission in either solution or the solid state. Further, the electrochemical study reveals that diphenyl boron complexes show a reduction potential that is more negative compared to the bis(pentafluorophenyl)boron complexes. The high absorption coefficient of the compounds pointed towards the possibility of high first order hyperpolarizability upon optical excitation, which motivated us to ascertain the nonlinear optical coefficients in the near infrared range, towards applicability of such compounds in optical limiting and switching. The open aperture -scan measurements at ultrashort time scales elucidated a few critical features of such compounds towards optical limiting applications.