Circularly Polarized Electroluminescence from Sterically Engineered Ruthenium(II) Complexes Enabled by a Chiral Anion Strategy in OLEDs

Abstract

Circularly polarized luminescence (CPL) has garnered significant interest for advanced optoelectronic applications, with circularly polarized electroluminescence (CPEL) offering particular promise for next-generation displays. Ruthenium(II) polypyridine complexes, despite their well-established photophysics, have seen limited success in CPEL due to the difficulty of accessing enantiomerically pure emitters and their modest device efficiencies. Here, we overcome these limitations through a combined molecular and device-level strategy. Introducing bulky substituents onto bipyridine ligands enhances the radiative decay rate while suppressing non-radiative pathways, yielding a high solid-state photoluminescence quantum yield (PLQY) of 19.7%. A chiral anion strategy using camphorsulfonate counterions circumvents the need for chiral resolution, providing a practical route to chiral emission. By incorporating 2 wt% of a chiral ionic liquid into the emissive layer of an organic light-emitting diode (OLED), we obtain CPEL signals with dissymmetry factors (|gEL|) of ~10⁻3. The optimized device achieves an external quantum efficiency (EQE) of 6.16%, representing the highest value reported for Ru(II)-based CPEL devices to date. This work demonstrates that combining molecular design with device engineering offers a practical approach to advancing Ru(II) complexes for CPEL applications.