Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (EST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show:
(i) How the nature of these excited can be characterized and how this nature can be tuned by varying the nature of the electron donating (D) or accepting (A) units in D-A(-D) compounds.
(ii) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films.
For both topics, we will demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes and that overall, the conversion of triplet-to-singlet is a dynamic process gated by conformational fluctuations.
Yoann Olivier, Brett Yurash, Luca Muccioli, Gabriele D'Avino, Monica Moral, Oleksandr Mikhnenko, Juan-Carlos Sancho-Garcia, Chihaya Adachi, Thuc-Quyen Nguyen, and David Beljonne, "On the nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence (Conference Presentation)," Proc. SPIE 10687, Organic Electronics and Photonics: Fundamentals and Devices, 1068705 (Presented at SPIE Photonics Europe: April 24, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307652.5788751033001.
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