Cationic Ir ^III Emitters with Near‐Infrared Emission Beyond 800 nm and Their Use in Light‐Emitting Electrochemical Cells

Abstract

Solid‐state near‐infrared (NIR) light‐emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. In addition, solid‐state NIR light‐emitting electrochemical cells (LECs) have shown several promising advantages over NIR organic light‐emitting devices (OLEDs). However, among the reported NIR LECs based on ionic transition‐metal complexes (iTMCs), there is currently no iridium‐based LEC that displays NIR electroluminescence (EL) peaks near to or above 800 nm. In this report we demonstrate a simple method for adjusting the energy gap between the highest‐occupied molecular orbital (HOMO) and the lowest‐unoccupied molecular orbital (LUMO) of iridium‐based iTMCs to generate NIR emission. We describe 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 the complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and have been applied as components in LECs, showing a maximum external quantum efficiency (EQE) of 0.05 % in the EL devices. By using a host–guest emissive system, with the iridium complex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1 %.