Nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence

Despite significant efforts, a complete mechanistic understanding of thermally activated delayed fluorescence (TADF) materials has not yet been fully uncovered. Part of the complexity arises from the apparent dichotomy between the need for close energy resonance and for a significant spin-orbit coupling between alike charge-transfer singlet and triplet excitations. Here we show, in the case of reference carbazole derivatives, that this dichotomy can be resolved in a fully atomistic model accounting for thermal fluctuations of the molecular conformations and microscopic electronic polarization effects in amorphous films. These effects yield electronic excitations with a dynamically mixed charge-transfer and localized character, resulting in thermally averaged singlet-triplet energy differences and interconversion rates in excellent agreement with careful spectroscopic studies. The work in Mons was supported by the Programme d’Excellence de la Région Wallonne (OPTI2MAT project), the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 646176 (EXTMOS project), and FNRS-FRFC. Computational resources were provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by theWalloon Region under Grant Agreement No. 1117545. B.Y. and T.Q.N. thank the Department of the Navy, Office of Naval Research (Award No. N00014-14-1-0580) for support. L.M. acknowledges funding by the French national grant ANR-10-LABX-0042-AMADEus managed by the National Research Agency under the initiative of excellence IdEx Bordeaux program (reference ANR-10-IDEX-0003-02). G.D. acknowledges support from EU through the FP7-PEOPLE-2013-IEF program (Project No. 625198).