Quinoxaline-Based Cationic Cyclometalated Iridium Complexes for Deep-Red Light-Emitting Electrochemical Cells

Three new red-emitting cationic cyclometalated iridium complexes with formula [Ir(C^N)2(N^N)][ClO4] (C^N = substituted quinoxaline[X-dpqx], N^N = 2-(1-benzyl-1H-benzo[d]imidazole-2-yl) quinoline, namely: IrQ1, 2, 3, where X = H, Cl, and OCH3, respectively) have been synthesized for application as an emitter in light-emitting electrochemical cells (LECs). The (quasi-) reversible redox behavior of the complexes confirmed their electrochemical stability as an important feature of emitters for achieving efficient LECs. Complexes IrQ1, 2, and 3 exhibit intense emissions in the red region (622-645 nm), with photoluminescence quantum yields (PLQYs) of 7, 18, and 3%, respectively, in a CH2Cl2 solution. Furthermore, the neat-film PL of the complexes showed a significant red shift in emission by about 55-80 nm. To investigate the stability of the complexes against water exchange reactions during LEC operation, for the first time, their electrocatalytic activity for the water splitting reaction was examined, and their high inertness, especially complex IrQ2, was confirmed. With the ligand substitution strategy, an efficient and stable LEC with IrQ2 as the emitter was fabricated, which illustrated a deep-red emission (661 nm) with a luminance of 19 cdm-2 and an external quantum efficiency (EQE) of 0.95%. The obtained EQE value is several times higher than that of similar diphenylquinoxaline/Ir-based LECs.