High-Lying Triplet Excitons of Thermally Activated Delayed Fluorescence Molecules

Abstract

We investigate high-lying triplet excitons involving a transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO) for 18 thermally activated delayed fluorescence (TADF) molecules, within the first-principles one-shot GW+Bethe–Salpeter method. On the basis of our exciton analysis using the exciton wave functions, detailed exciton features are discussed in terms of exciton size, exciton binding energy, electron–hole separation distance, exciton map, and the overlap strength between the electron and hole wave functions. Contrary to our expectation, no exciton that could be purely classified as a charge-transfer exciton is found in our exciton map; moreover, the energy difference between the lowest singlet exciton and the high-lying triplet exciton is nearly zero for some TADF molecules. Our simulation strongly suggests that the hot-exciton process involving a high-lying triplet exciton is more likely to occur in the TADF mechanism than the conventionally considered process between the lowest singlet and triplet excitons, and our results support those of recent experiments. We propose a new method for calculating the energy difference between singlet and triplet excitons from the expectation value of the exchange bare Coulomb interaction and demonstrate that the combined use with exciton map is efficient and accurate for screening TADF molecules.