Exploring the effect of the cyclometallating ligand in 2-(pyridine-2-yl)benzo[d]thiazole-containing iridium(iii) complexes for stable light-emitting electrochemical cells

Abstract

The preparation and characterization of a series of iridium(III) ionic transition-metal complexes for application in light-emitting electrochemical cells (LECs) are reported. The complexes are of the type [Ir(C^N)₂(N^N)][PF₆] in which C^N is one of the cyclometallating ligands 2-(3-(tert-butyl)phenyl)pyridine (tppy), 2-phenylbenzo[d]thiazole (pbtz), 1-phenyl-1H-pyrazole (ppz) and 1-phenylisoquninoline (piq), and N^N is 2-(pyridine-2-yl)benzo[d]thiazole (btzpy). The variation in the C^N ligands allows the HOMO energy level to be tuned, leading to HOMO–LUMO gaps in the range 2.76–3.01 eV and values of E^ox_1/2 of 0.81–1.11 V. In solution, the complexes are orange to deep-red emitters (λ_max in the range 600–660 nm), with quantum yields between 2% for [Ir(tppy)₂(btzpy)][PF₆] and 41% for [Ir(pbtz)₂(btzpy)][PF₆]. Similar trends for the emission maxima and photoluminescence quantum yields are observed in the solid state. Density functional theory (DFT) calculations support the charge transfer nature of the emission. Very bright electroluminescence was observed for LECs containing [Ir(pbtz)₂(btzpy)][PF₆], although the device was not stable under continuous operation; this is attributed to an unbalanced charge distribution and/or to a fast ionic migration. Significantly, LECs fabricated with [Ir(tppy)₂(btzpy)][PF₆] in the active layer are very stable, produce pure red emission and show no signs of degradation over a period of 5 days of continuous operation.