Organoboron Compounds with an 8-Hydroxyquinolato Chelate and Its Derivatives:  Substituent Effects on Structures and Luminescence

Four new luminescent organoboron complexes have been synthesized and fully characterized. These compounds are four-coordinate boron chelated by either 8-hydroxyquinolato (q) or functionalized 8-hydroxylquinolato ligands, including BPh₂(5-(1-naphthyl)-q) (1), BPh₂(5-(2-benzothienyl)-q) (2), B(2-benzothienyl)₂q (3), and B(2-benzothienyl)₂(2-Me-q) (4). All four compounds have a tetrahedral geometry as established by X-ray diffraction analyses. In solution, compounds 1−4 have an emission maximum at 534, 565, 501, and 496 nm, respectively, at room temperature. They emit similar colors in the solid states without red shifts of the emission band due to the lack of significant intermolecular interactions in the crystal lattices. The substituent group at C5 or C2 position of the 8-hydroxyquinolato ligand has been observed to have a significant impact on the emission energy and the emission quantum efficiency of the boron complexes. Molecular orbital calculations (Gaussian 98) showed that the electronic transition of 1 and 2 is a π−π* transition centered on the functionalized 8-hydroxyquinolato group and the electronic transition of 3 and 4 is an interligand charge transfer from the 2-benzothienyl ligand to the hydroxyquinolato ring. A double-layer electroluminescent device using 3 as the emitter has been fabricated, which produced a broad emission band with a significant contribution of exciplex emission.