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 (theta(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 theta(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 theta(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 theta(DA) approaching 80 degrees, facilitating phonon-driven up-conversion; (iii) the overall TADF rate reaches a peak at theta(DA)approximate to 80 degrees. By this, the TADF on/off switching is realized via methyl moiety for regulating theta(DA) from theoretical prediction to experimental confirmation. Importantly, the theta(DA) near 80 degrees would be a good descriptor for screening excellent D-A type TADF emitters.

Automated summary

The twisted donor-acceptor (D-A) organic formwork with a large dihedral angle (theta(DA)) is found to play a crucial role in achieving excellent thermally activated delayed fluorescence (TADF) emitters. The overall TADF properties depend on the value of theta(DA), with a peak at around 80 degrees. The introduction of a methyl moiety allows for the regulation of theta(DA) and enables the on/off switching of TADF emission.