Getting the Right Twist: Influence of Donor-Acceptor Dihedral Angle on Exciton Kinetics and Singlet-Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet–triplet gap. The angular dependence of the singlet–triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground-state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86°. Surprisingly, an experimentally obtained energy gap of 72 ± 5 meV can only be explained by torsion angles in the range of 70–75°. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediatel...