Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor-Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy.

The photophysical processes involved in the electron donor-acceptor thermally activated delayed fluorescence (TADF) of emitters are complicated and controversial. The recent consensus is that at least three states are involved, i.e. the singlet charge transfer state (1CT), the triplet localized excited state (3LE) and the triplet CT state (3CT). It is clear the very often used steady state and time-resolved luminescence spectroscopic methods are unable to present direct evidence for the dark states, i.e. the 3LE and 3CT states, as well as the interconversion of these states. Concerning this aspect, the femtosecond-nanosecond transient absorption spectroscopic methods are in particular interests. Both the emissive state and the dark state can be detected in these spectra, and interconversion of the states involved in TADF process can be also revealed. This review article focuses on the recent development of using the transient absorption spectra to study the photophysics of the TADF emitters.