Diboron and triboron compounds based on linear and star-shaped conjugated ligands with 8-hydroxyquinolate functionality: impact of intermolecular interaction and boron coordination on luminescence.

New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized using Suzuki-Miyaura coupling methods (R = MeO, L1-L5; R = CH3OCH2O, L1'-L5'). When treated with HCl, L1'-L5' are converted readily to the corresponding 8-hydroxyquinoline compounds L(OH)1-L(OH)5 which react readily with BPh3 in refluxing THF to produce the corresponding polyboron chelate compounds B1-B5 in good yields. L1-L5 and B1-B5 display similar thermal stability with Td at approximately 300 degrees C. Experimental and molecular orbital calculation results showed that the chelation by boron stabilizes the LUMO level of the ligand and narrows the HOMO-LUMO gap, resulting in the blue emission of the ligands and the green or orange emission of the boron compounds. Crystal structures of L1, L3, and L5 showed that these molecules have layered arrangements in the solid state with significant intermolecular pi-pi interactions. The linear diboron B5 displays concentration and temperature-dependent emission in solution, attributable to intermolecular interactions. The properties of a monoboron compound BPh2(5-Ph-8-MeO-q) (B0) and its corresponding free ligand L0 were investigated and compared to the closely related diboron compound B1 and the ligand L1, which revealed that the increase of the number of chromophores linked by an aromatic group has a significant impact on thermal stability and the HOMO and LUMO energy levels.