Theoretical study on photophysical properties of a series of functional pyrimidine-based organic light-emitting diodes emitters presenting thermally activated delayed fluorescence.

Abstract

Issue concerning accurate prediction of the reverse intersystem crossing rate (kRISC ) is critical for developing novel efficient thermally activated delayed fluorescence (TADF) materials. In this contribution, the kRISC rates from the lowest excited triplet T1 state to the lowest excited singlet S1 state were evaluated for five donor-π-acceptor-type pyrimidine-based TADF emitters using the semiclassical Marcus theory. Both the singlet-triplet energy difference (ΔEST ) and spin-orbit coupling (V) between the S1 and T1 states were investigated by performing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. In addition, their fluorescence emission wavelengths (λem ) were also calculated at the TD-DFT level. The predicted kRISC and λem values are found to reproduce well the available experimental findings. The present results reveal that the kRISC rates of molecules possessing the unsymmetrical diphenyl pyrimidine acceptor core are calculated to be slightly larger than those of their analogues with the symmetrical diphenyl pyrimidine. In addition, introducing two tert-butyl groups into the 2,7-positions of the donor moiety of the latter is also an effective method for increasing kRISC when designing TADF emitters. Such a difference is related to the nature of the T1 excited state. A more remarkable charge-transfer (CT) contribution to the state can achieve a smaller ΔEST , leading to a more efficient RISC process, and consequently a shorter delayed fluorescence lifetime as observed experimentally. © 2019 Wiley Periodicals, Inc.