Determining the Energy Gap between the S₁ and T₁ States of Thermally Activated Delayed Fluorescence Molecular Systems Using Transient Fluorescence Spectroscopy

The energy gap (ΔE_S–T) between the lowest single and triple excited states is a crucial parameter for thermally activated delayed fluorescence (TADF) molecular systems with high quantum yield. However, a reliable experimental approach to precisely determine this value is challenging. Here, we introduce a new, simple, and efficient strategy to accurately obtain the ΔE_S–T in TADF systems from time-resolved fluorescence spectroscopy using a recently reported TADF molecule, DMACPDO, as a representative. By introducing an explicit model to describe the corresponding singlet–triplet coupling system, elusive intersystem crossing and reverse intersystem crossing rates can be extracted by fitting the kinetics of the observed fluorescence. The ΔE_S–T value can then be determined. Moreover, our modeling accurately explained the opposite trend in fluorescence intensity of DMACPDO with solvent polarity under air-saturated and deoxygenated conditions. Additionally, the validity of this approach has been demonstrated in another well-known TADF molecule, 4CzIPN. We demonstrate how this approach of determining ΔE_S–T sheds light on a deeper understanding of energy-loss mechanisms involved in related photoconversion processes.