On Predicting the Excited-State Properties of Thermally Activated Delayed Fluorescence Emitters

Limitations imposed by spin statistics governing exciton formation has meant that most efficient organic light-emitting diodes (OLEDs) have relied upon complexes containing heavy metals. This can be overcome by exploiting thermally activated delayed fluorescence (TADF), which has opened the opportunity to design emitters composed only of lighter, more abundant elements. For these complexes, charge-transfer excitations play a central role, meaning that modeling their properties within the framework of time-dependent density functional theory (TD-DFT) is challenging. Herein, two computational approaches to rectify this are explored. First is an analysis based on the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This qualitative approach provides a satisfactory and very computationally efficient prediction of the energy gap between the lowest singlet and triplet excited states, crucial for TADF. In the second approach, the excited-state properties are explicitly calculated using TD-DFT by optimizing the range separation parameter within range-corrected functionals. This yields quantitative agreement with experimental results and can therefore be used to rationalize the photophysical properties of these complexes.