Optimal Dihedral Angle in Twisted Donor–Acceptor Organic Emitters for Maximized Thermally Activated Delayed Fluorescence
Yuhao Shi, Huili Ma, Zhiyu Sun, Weijun Zhao, Guang‐Yan Sun, Qian Peng
Abstract
Abstract The twisted donor–acceptor (D‐A) organic formwork with a large dihedral angle ( θ DA ) is usually adopted to narrow the singlet‐triplet energy gap for obtaining excellent thermally activated delayed fluorescence (TADF) emitters. However, the dependence of overall TADF properties on θ DA has not been systematically investigated to this day. Taking new designed CzBP , CzBP‐1M and CzBP‐2M via introducing methyl as investigated models, it is found that (i) with increasing θ DA , the charge transfer component in S 1 is larger than that in T 1 in varying degrees, leading to non‐monotonic spin‐orbit couplings; (ii) the electron‐vibration couplings between S 1 and T 1 states become the largest when θ DA approaching 80°, facilitating phonon‐driven up‐conversion; (iii) the overall TADF rate reaches a peak at θ DA ≈80°. By this, the TADF on/off switching is realized via methyl moiety for regulating θ DA from theoretical prediction to experimental confirmation. Importantly, the θ DA near 80° would be a good descriptor for screening excellent D‐A type TADF emitters.