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 ∼300 °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 π−π 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.