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

The energy gap (ΔES-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 ΔES-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 ΔES-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 ΔES-T sheds light on a deeper understanding of energy-loss mechanisms involved in related photoconversion processes.