Excited-state dynamics of highly efficient thermally activated delayed fluorescence molecules

Abstract

The excited-state dynamics of organic molecules exhibiting thermally activated delayed fluorescence (TADF) was investigated by transient absorption spectroscopy aiming to reveal a mechanism leading to a highly efficient TADF. We found experimentally that both decreasing the ΔEST and increasing the spin-orbit coupling (SOC) between the singlet and triplet excited states were crucial for the efficiency. In particular, unexpected from the general assumption that the SOC of the purely organic TADF molecules is negligibly weak, the increase of the SOC is indeed the key for the TADF emission and is realized by closing a local excitonic triplet state to a charge-transfer singlet state. This situation is achieved by tuning the degree of the charge-transfer state. Some history of the TADF and its related recent research are also introduced.