A chiral spirofluorene-embedded multiple-resonance thermally activated delayed fluorescence emitter for efficient pure-green circularly polarized electroluminescence

Abstract

The simultaneous achievement of chiral multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrowband and circularly polarized electroluminescence (CPEL) poses a challenge. Herein, a MR-TADF emitter, Spiro-BNCz, embedding spirofluorene structure was developed and chiral separated, whose emission peaks at 528 nm in toluene with Commission Internationale de L'Eclairage coordinates of (0.26, 0.69). The conjugation extension caused by embedding sulfur substituted spirofluorene on B/N framework shortens the singlet-triplet energy gap and increases spin-orbital coupling matrix element. Therefore, a fast reverse intersystem crossing rate constant of 2.8 × 10⁶ s⁻¹ and a high photoluminescence efficiency of 92% in film were achieved. The organic light-emitting diode (OLED) exhibits a maximum external quantum efficiency of 32.3%. Particularly, the circularly polarized OLEDs based on Spiro-BNCz enantiomers show symmetric CPEL with dissymmetry factors (|g_EL|) ≈ 10⁻³.