We hereby report the results of our direct investigation into the excited-state dynamics of thermally activated delayed fluorescence (TADF) molecules in solution using pump-probe transient absorption spectroscopy (TAS). We found that the charge-transfer (CT) state commonly stated for TADF molecules encompasses two forms: localized and delocalized CT states. A highly efficient TADF molecule, 4CzIPN [Uoyama et al., Nature, 492, 234-238 (2012)], showed both the localized and delocalized CT states, while an inefficient TADF molecule, 2CzPN, exhibited only a localized CT state. By analyzing the time profile of triplet species observed in TAS, we propose that the reverse intersystem crossing (RISC) of 4CzIPN occurs via a mutual interaction in multiple energy levels of localized neutral and CT states, and delocalized CT states.
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