A Face-to-Face Orchestration of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence.

Intramolecular t hrough-space charge-transfer (TSCT) excited state has been exploited for developing thermally activated delayed fluorescence (TADF) emitters, whereas the tuning of excited state dynamics via conformational engineering remains sparse. Herein we designed a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π - π interactions, the radiative decay rate of the lowest singlet excited state (S 1 ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields up to 0.99 in doped thin films. High-efficiency electroluminescence device with maximum external quantum efficiency (EQE) of 23.96% has been achieved which maintains at >20% at the brightness of 1000 cd m -2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.