Determining the Energy Gap between the S1and T1States ofThermally Activated Delayed Fluorescence Molecular Systems UsingTransient Fluorescence Spectroscopy

The energy gap (Delta ES-T) between the lowest single andtriple excited states is a crucial parameter for thermally activated delayedfluorescence (TADF) molecular systems with high quantum yield.However, a reliable experimental approach to precisely determine thisvalue is challenging. Here, we introduce a new, simple, and efficientstrategy to accurately obtain the Delta ES-Tin TADF systems from time-resolvedfluorescence spectroscopy using a recently reported TADFmolecule, DMACPDO, as a representative. By introducing an explicitmodel to describe the corresponding singlet-triplet coupling system,elusive intersystem crossing and reverse intersystem crossing rates can beextracted byfitting the kinetics of the observedfluorescence. The Delta ES-Tvalue can then be determined. Moreover, our modeling accuratelyexplained the opposite trend influorescence intensity of DMACPDOwith solvent polarity under air-saturated and deoxygenated conditions. Additionally, the validity of this approach has beendemonstrated in another well-known TADF molecule, 4CzIPN. We demonstrate how this approach of determining Delta ES-Tshedslight on a deeper understanding of energy-loss mechanisms involved in related photoconversion processes.

Automated summary

This study introduces a new, simple, and efficient strategy to accurately obtain the energy gap (Delta ES-T) in thermally activated delayed fluorescence (TADF) systems. By using time-resolved fluorescence spectroscopy and a specific TADF molecule as a representative, the elusive singlet-triplet coupling dynamics can be described and the Delta ES-T value can be determined. The approach is validated in different TADF molecules and sheds light on the energy-loss mechanisms in related photoconversion processes.