Spiral-fluorene-integrated sterically shielded multi-resonance TADF emitter: simultaneously achieving narrowband emission and restraining Dexter energy transfer.

The fundamental challenge in achieving high doping concentrations for multiple resonance emitters, while simultaneously suppressing Dexter energy transfer (DET)-induced concentration quenching, stems from their intrinsically long-lived exciton states. Moving beyond conventional steric hindrance strategies for intermolecular separation, we utilize terminal spirofluorene interactions to promote lamellar molecular stacking. This configuration enhances host-guest separation, achieving a Förster resonance energy transfer (FRET) radius of 3.19 nm, which effectively suppresses DET (with a DET rate constant: κDET = 3.48 × 105 s-1) while maintaining efficient FRET (with a FRET rate constant: κFRET = 1.59 × 108 s-1). This collective molecular orchestration breaks the concentration ceiling inherent to B/N-based systems without inducing spectral broadening (full-width-at-half-maximum, FWHM = 19/20 nm). As a proof of concept, our devices set new efficiency records for binary organic light-emitting diodes (OLEDs), reaching a peak external quantum efficiency (EQE) of 33.2% at 3%-5% doping in non-sensitized configurations and 36.9% with interlayer sensitization. This work introduces a materials design paradigm that successfully resolves the critical doping-concentration paradox in multiple resonance thermally activated delayed fluorescence (MR-TADF) systems, thereby enhancing their potential for commercial application.