Cationic Ir(III) emitters with near-infrared emission beyond 800 nm and their use in light-emitting electrochemical cells.

Near-infrared (NIR) solid-state light-emitting devices have recently received much attention as NIR light sources, which can penetrate deep into human tissue and are suitable for bioimaging and labeling. On the other hand, solid-state NIR light-emitting electrochemical cells (LECs) show several promising advantages over NIR organic light-emitting devices (OLEDs). However, in the reported ionic transition metal complex (iTMC)-based NIR LECs, there is currently no iridium-based LEC that can display NIR electroluminescence (EL) peaks near or above 800 nm. In this study, we demonstrate a simple method for adjusting the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) with iridium-based iTMCs to generate NIR emission. We show a series of novel ionic-iridium complexes with very small energy gaps, namely NIR1-NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels, and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and are applied to the LEC components, showing a maximum external quantum efficiency (EQE) of 0.05% in EL devices. By using the host (RED)-guest emissive system, the highest EQE of the LEC can be further enhanced to above 0.1%.