Simultaneously Improving Radiative Decay and Reverse Intersystem Crossing in Space-Confined Through-Space Charge-Transfer (TSCT) Emitter by Strong Intermolecular TSCT Enabled by a Planar Donor

Abstract

Simultaneously improving radiative decay and reverse intersystem crossing (RISC) of through-space charge-transfer (TSCT) thermally-activated delayed fluorescence (TADF) emitters has remained a challenge. Here, space-confined TSCT-TADF emitters are developed with a planar indolo[3,2,1-kl]phenoxazine donor, a spiro-carbon-fixed benzophenone acceptor, and a fluorene (for 1) or xanthene (for 2) bridge. Close, cofacial donor/acceptor alignments and efficient intramolecular TSCT have been observed for both emitters. Intriguingly, emitter 1 exhibits an intermolecular donor/acceptor interaction as strong as the intramolecular one, which opens efficient intermolecular TSCT, improving the radiative decay rate (k_r,s) to the same level as emitter 2 featuring a much stronger intramolecular donor/acceptor interaction. The intermolecular TSCT also largely enhances the RISC rate (k_RISC). In the 20 wt.% doped films, emitters 1 and 2 show green TADF with high k_r,s/k_RISC at 1.1 × 10⁷ s⁻¹/1.3 × 10⁶ s⁻¹ and 1.2 × 10⁷ s⁻¹/7.7 × 10⁵ s⁻¹, respectively. Organic light–emitting diodes (OLEDs) using the emitters show high external quantum efficiencies (EQEs) up to 27.5% and low-efficiency roll-offs. Hyperfluorescent OLEDs using the emitters as sensitizers afford narrowband blue-green emission with high EQEs up to 30.6% and largely suppressed efficiency roll-offs. The work reveals that strengthening intermolecular TSCT is a promising avenue to simultaneously improve k_r,s, and k_RISC of TSCT-TADF emitters for high-performance OLEDs.