Organoboron Quinolinolates with Extended Conjugated Chromophores:  Synthesis, Structure, and Electronic and Electroluminescent Properties

Luminescent organoboron complexes featuring substituted 8-quinolinolates as chelating ligands have been synthesized and characterized. Substitution of the quinolinolate has been achieved in the 5 and the 7 positions introducing phenyl, biphenyl, and 9,9‘-dihexylfluorenyl substituents by reacting 8-benzyloxy-5,7-dibromoquinoline with the corresponding boronic acid derivatives in a Suzuki-type cross-coupling reaction. Upon deprotection, the 8-quinolinol derivatives have been transformed into the target compounds upon reaction with BPh₃. (κ²-(N,O)-5,7-dibromo-8-quinolinolate)diphenylborane (9), (κ²-(N,O)-5,7-diphenyl-8-quinolinolate)diphenylborane (10), (κ²-(N,O)-5,7-bis(biphenyl-4-yl)-8-quinolinolate)diphenylborane (11), and (κ²-(N,O)-5,7-bis(9,9-dihexylfluoren-2-yl)-8-quinolinolate)diphenylborane (12) have been investigated by nuclear magnetic resonance and infrared spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, thermal analysis, cyclic voltammetry, UV−vis absorption spectroscopy, and luminescence measurements in solution and in the solid state to retrieve a concise knowledge of the introduced compounds. Special emphasis has been placed on studying the consequences of extension of the conjugated π system of the quinolinolate ligand on materials properties. Small molecule organic light emitting devices (SMOLED) based on compounds 10, 11, and 12 give bright yellow to orange electroluminescence with low onset voltages (3.5−3.7 V) and continuous wave luminance intensities exceeding 1000 cd/m², while devices based on 9 give poor results, proving 9 to be an unsuitable emissive material.