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 (▵E_ST) of 0.04 eV, as well as reversible redox properties. Furthermore, the enantiomers showed distinct mirror-image CD and CPL activities with g_lum 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 g_EL 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.