Decreased Turn-On Times of Single-Component Light-Emitting Electrochemical Cells by Tethering an Ionic Iridium Complex with Imidazolium Moieties

We report a significant decrease in turn-on times of light-emitting electrochemical cells (LECs) by tethering imidazolium moieties onto a cationic Ir complex. The introduction of two imidazolium groups at the ends of the two alkyl side chains of [Ir(ppy)(2) (dC6-daf)](+)(PF6)(-) (ppy=2-phenylpyridine, dC6-daf=9,9'-dihexyl-4,5-diazafluorene) gave the complex [Ir(ppy)(2)(dC6MIM-daf)](3+)[(PF6)(-)](3) (dC6MIM-daf=9,9-bis[6-(3-methylimidazolium)hexyl]-1-yl-4,5-diazafluorene). Both complexes exhibited similar photoluminescent/electrochemical properties and comparable electroluminescent efficiencies. The turn-on times of the LECs based on the latter complex, however. were much lower than those of devices based on the former. The improvement is ascribed to increased concentrations of mobile counterions ((PF6)-) in the neat films and a consequent increase in neat-film ionic conductivity. These results demonstrate that the technique is useful for molecular modifications of ionic transition-metal complexes (ITMCs) to improve the turn-on times of LECs and to realize single-component ITMC LECs compatible with simple driving schemes.