Blue-emitting cationic iridium complexes with 2-(1H-pyrazol-1-yl)pyridine as the ancillary ligand for efficient light-emitting electrochemical cells

Two blue-emitting cationic iridium complexes with 2-(IH-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands, namely, [lr(PPY)2(PZPY)1PF6 and [Ir(dfPPY)2(PZPY)JJPF6 (ppy is 2-ph enyl pyri dine, dfppy is 2-(2,4-difluorophenyl) pyridine, and PF6_ is hexafluorophosph ate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH3CN solutions, [Ir(PPY)2(PZPY)]PF6 emits blue-green light (475 nm), which is blue-shifted by more than 100 nm with respect to the typical cationic iridium complex [lr(ppy)2(dtb-bpy)]PF6 (dtb-bpy is 4,4'-di-tert-butyl-2,2'-bipyridine); [lr(dfppY)2(PZPY)IPF6 with fluorine-substituted cyclometalated ligands shows further blue-shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand-centered 3 7r_7r * states of the cyclometalated ligands (ppy or dfppy). Light-emitting electrochemical cells (LECs) based on [Ir(PPY)2(pzpy)]PF6 gave green-blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A-' when an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was added into the light-emitting layer. LECs based on [1.r(dfpPY)2(PZPYAPF6 gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs-based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron -donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.