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

Five new three-coordinate boron compounds with a B=N 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 p orbital and the boron p p 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.