Optical and electronic properties of air-stable organoboron compounds with strongly electron-accepting bis(fluoromesityl)boryl groups

Abstract

Three compounds with phenyl (<b>1</b>), 4-<i>tert</i>-butylphenyl (<b>2</b>) and 4-<i>N</i>,<i>N</i>-diphenylaminophenyl (<b>3</b>) groups attached to bis(fluoromesityl)boryl ((FMes)₂B) through B-C bonds have been prepared. The restricted rotation about the B-C bonds of boron-bonded aryl rings in solution has been studied by variable-temperature ¹⁹F NMR spectroscopy, and through-space F-F coupling has been observed for <b>3</b> at low temperature. Steric congestion inhibits binding of <b>1</b> by Lewis bases DABCO and <i>t</i>Bu₃P and the activation of H₂ in their presence. Photophysical and electrochemical studies have been carried out on <b>2</b>, <b>3</b>, and an analogue of <b>3</b> containing a bis(mesityl)boryl ((Mes)₂B) group, namely <b>4</b>. Both <b>2</b> and <b>3</b> show bright emission in nonpolar solvents and in the solid-state, very strong electron-accepting ability as measured by cyclic voltammetry, and good air-stability. In addition, <b>2</b> displayed unusually long-lived emission (<i>τ</i> = 2.47 s) in 2-MeTHF at 77 K. The much stronger acceptor strength of (FMes)₂B than (Mes)₂B leads to significantly red-shifted emission in solution and the solid state, stronger emission solvatochromism, and significantly lower reduction potentials. Theoretical calculations confirm that <b>2</b> and <b>3</b> tend to form highly twisted excited states with good conjugation between one FMes group and the boron atom, which correlate well with their blue-shifted solid-state emissions and low <i>k</i>_r values in solution.