Axially Chiral TADF Imidazolium Salts for Circularly Polarized Light-Emitting Electrochemical Cells

Abstract

A pair of axially chiral thermally activated delayed fluorescent (TADF) enantiomers, R-TCBN-ImEtPF6 and S-TCBN-ImEtPF6, with intrinsic ionic characteristics were efficiently synthesized by introducing imidazolium hexafluorophosphate to chiral TADF unit. The TADF imidazolium salts exhibited a high photoluminescence quantum yield (PLQY) of up to 92%, a small singlet–triplet energy gap (∆EST) of 0.04 eV, as well as reversible redox properties. Furthermore, the enantiomers showed distinct mirror-image CD and CPL activities with glum values of -3.7×10-3 and +3.4×10-3. Notably, by doping the axial TADF imidazolium salts into achiral TADF sensitizer, sandwich-structured light-emitting electrochemical cells (LECs) without the addition of ionic liquids (ILs) or ionic transition-metal compounds (iTMCs) were fabricated. When driven at 50 A m-2, the LECs displayed an EQE of up to 5.2% and strong circularly polarized electroluminescence (CPEL) with gEL values of +3.3×10-3 and -3.0×10-3. This represents the first CP-LEC based on TADF materials and offers a promising strategy for the development of high-performance CPEL devices.