Optimal Dihedral Angle in Twisted Donor-Acceptor Organic Emitters for Maximized Thermally Activated Delayed Fluorescence.

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.