Phosphorescence Color Tuning of Ionic Iridium Complexes by Manipulating Excited State Properties

The new ionic iridium complexes, [Ir(bpy)(2)(CN)(2)](-) and [Ir(dtbbpy)(2)(CN)(2)](+), were prepared and their luminescence properties were investigated, where bpy, dtbbpy and CN represent 2,2'-dipyridyl, 4,4'-di-tert-butyl-2,2'-dipyridyl and cyanide, respectively. We expected that introduction of the electron-withdrawing nonchromophoric ligand, -CN, to an Ir complex might effectively lower the HOMO level and lead to the blue-shifted emission compared to [Ir(bpy)(3)](3+). The main ligand with a sterically bulky group was also considered to orient the binding between the main ligand and iridium center and to improve the solubility. Thus, the new ionic complex, [Ir(dtbbpy)(2)(CN)(2)](+), was synthesized and its properties was compared with the other complex, [Ir(bpy)(2)(CN)(2)](+). The maximum PL peak of [Ir(dtbbpy)(2)(CN)(2)](+) appeared at 491 nm, which was hypsochromically shifted, compared with 528 and 566 nm of [Ir(bpy)(3)](3+) and [Ir(bpy)(2)(CN)(2)](-), respectively. Investigation of their electrochemical properties led us to inferring that the cyanide ligand could effectively modulate the HOMO level of its iridium complex for the blue emission. Also, the introduction of the bulky alkyl group in the main ligand might effectively cause direct bindings between both Ns of biphenyl and the iridium center, which also might result in the relative blue-shifted emission in comparison with PL peak of [Ir(bpy)(2)(CN)(2)](-) in which biphenyl was C-and N-bound to the Ir center.