Design of Intramolecular Dihedral Angle between Electronic Donor and Acceptor in Thermally Activated Delayed Fluorescence Molecules.

In order to improve the exciton utilization efficiency (η exc ) of organic light-emitting materials, we addressed the ideal donor-acceptor dihedral angle (θ D-A ) in the TADF molecule by striking a balance between two photophysical processes. One is the conversion of triplet excitons into singlet excitons, and the other is the radiative process from a low-lying excited state to the ground state. Using a combination of first-principles calculations and molecular dynamics simulations, we investigated the impact of θ D-A on the splitting energy and spin-orbit coupling between singlet and triplet excitons as well as the transition dipole moment for carbazole benzonitrile (CzBN) derivatives. By comparison with the reverse intersystem crossing rate ( k rISC ), fluorescence emission rate ( k r ), and η exc , we proposed a potential highest η exc (of 94.4%) with the ideal θ D-A of 77° for blue light CzBN derivatives; the calculated results have a good agreement with experimental measurement. The structure-efficiency physical connection between the molecular structure (θ D-A ) and efficiency provided an ideal parameter for a potential candidate for blue TADF-OLED materials.