Determining the Energy Gap between the S<sub>1</sub> and T<sub>1</sub> States of Thermally Activated Delayed Fluorescence Molecular Systems Using Transient Fluorescence Spectroscopy
Min Zheng, Yuanming Li, Yaxiong Wei, Lin Chen, Xiaoguo Zhou, Shilin Liu
Abstract
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.