Regulating the Distance Between Donor and Acceptor in Space‐Confined Through‐Space Charge Transfer Emitters for Fast Radiative Decays and High‐Efficiency Organic Light‐Emitting Diodes With Low Efficiency Roll‐Offs

Abstract Through‐space charge transfer (TSCT) emitters featuring thermally‐activated delayed fluorescence (TADF) are extensively researched but suffer from low radiative decay rates (k_r,s) due to insufficient through‐space donor/acceptor interactions. Here, space‐confined TSCT TADF emitters 1–3 with a chemically fixed benzophenone acceptor and a triphenylamine donor on different bridges, that is, 1‐methyl‐9,10‐dihydroanthracene for 1, 4‐methyl‐9H‐xanthene for 2 and 4‐methyl‐9H‐thioxanthene for 3, which exhibit reinforced donor/acceptor interactions with shortened donor–acceptor distances, are reported. It is unveiled that there exists an optimal distance between the fixing sites of donor and acceptor. The emitter 2 with such an optimal distance shows both strong donor/acceptor interactions and high molecular rigidity, whereas the emitter 3 with a too short distance exhibits a twisted molecular structure and increased non‐radiative deactivation. In solution, 1–3 shows high k_r,s up to 3.0 × 10⁷ s⁻¹. In doped films, 1–3 exhibits green emission with high k_r,s up to 8.3 × 10⁶ s⁻¹ and photoluminescent efficiency up to 0.96. Organic light‐emitting diodes based on 1–3 afford high external quantum efficiencies up to 23.1% and largely alleviated efficiency roll‐offs. The work demonstrates that using rigid bridges that render an optimal donor–acceptor distance is crucial to the development of highly efficient TSCT emitters with fast radiative decays for electroluminescence applications.