Three-coordinate organoboron with a BN bond: substituent effects, luminescence/electroluminescence and reactions with fluoride

Abstract

Five new three-coordinate boron compounds with a BN bond involving an indole or a substituted indole ligand including B(mesityl)₂(indolyl), (1), B(mesityl)₂(2-Me-indolyl), (2), B(mesityl)₂(3-Me-indolyl), (3), B(mesityl)₂ (7-Me-indolyl), (4), and B(mesityl)₂(3-Ph-indolyl), (5) have been synthesized. The structures of these new compounds were determined by X-ray diffraction analyses. All five compounds are luminescent involving a charge transfer transition between the indolyl π orbital and the boron p_π orbital. The substituent group on the indolyl ring was found to have a subtle impact on the electronic properties of compounds. NMR experiments established that the methyl group at the 7-position of indole is most effective in blocking the rotation of the mesityl group around the B–C bond. The addition of fluoride ions to this group of compounds causes luminescent quenching and an irreversible decomposition of the compounds due to the reaction of the F⁻ adduct with water molecules. The potential use of this group of compounds as blue emitters in electroluminescent (EL) devices was demonstrated by the successful fabrication of a four-layer EL device where 1 was used as the emitter. The EL device displays a blue emission with a maximum luminescence being 1037 cd m⁻² and a maximum current efficiency about 0.7 cd A⁻¹ at 5 V.