Multi-resonance emitters with phosphorus-bridged cyclization: spectral narrowing synergized with accelerated reverse...

While intramolecular cyclization effectively modulates photoelectronic properties of multi-resonance (MR)-thermally activated delayed fluorescence (TADF) emitters, simultaneous narrowing full width at half maxima (FWHM) of spectra and accelerating reverse intersystem crossing (RISC) remain a formidable challenge. Here, we introduce a phosphorus-carbon-bridged cyclization in MR skeletons to synergistically suppress high-frequency molecular vibrations via skeleton rigidification and enhance spin-orbital coupling through introducing heavy-atom effects. Implementing this approach, two blue emitters, phenylphosphine oxide-bridged (BCzBN-PO) and phenylphosphine sulfide-bridged (BCzBN-PS), are developed and exhibit emission peaks at 467 and 474 nm with FWHMs of 19 and 18 nm, respectively. Moreover, benefiting from the additional heavy atom effect of sulfur complementing that of phosphorus, BCzBN-PS achieved a kRISC of 8.5 × 10⁵ s^-1, nearly 8-fold higher than that of BCzBN-PO (1.1 × 10⁵ s^-1). In the non-sensitized device architecture, both emitters exhibited narrowband emission with a FWHM < 30 nm and a maximum external quantum efficiency (EQE) > 20%. Notably, BCzBN-PS, leveraging its higher upconversion rate, demonstrated a superior maximum EQE and lower efficiency roll-off. Furthermore, in the TADF-sensitized device configuration, the organic light-emitting diodes further validated the enhanced upconversion efficiency-evidenced by BCzBN-PS delivering a higher maximum EQE than BCzBN-PO (43.0% vs. 41.2%) and a reduced efficiency roll-off (30.1% vs. 25.9% at 1000 cd m^-2). This work establishes a molecular engineering paradigm that balances color purity and exciton utilization efficiency, paving new avenues for high-performance narrowband electroluminescence.