Enhancing Thermally Activated Delayed Fluorescence by Fine-Tuning the Dendron Donor Strength

Eimantas Duda, David Hall, Sergey Bagnich, Cameron L. Carpenter-Warren, Rishabh Saxena, Michael Y. Wong, David B. Cordes, Alexandra M.Z. Slawin, David Beljonne, Yoann Olivier, Eli Zysman-Colman, Anna Köhler

Research output: Contribution to journalArticlepeer-review

Abstract

Thermally activated delayed fluorescence (TADF) relies on a small energy gap between the emissive singlet and the nonemissive triplet state, obtained by reducing the wave function overlap between donor and acceptor moieties. Efficient emission, however, requires maintaining a good oscillator strength, which is itself based on sufficient overlap of the wave functions between donor and acceptor moieties. We demonstrate an approach to subtly fine-tune the required wave function overlap by employing donor dendrons of changing functionality. We use a carbazolyl-phthalonitrile based donor–acceptor core (2CzPN) as a reference emitter and progressively localize the hole density through substitution at the 3,6-positions of the carbazole donors (Cz) with further carbazole, (4-tert-butylphenyl)amine (tBuDPA), and phenoxazine (PXZ). Using detailed photoluminescence studies, complemented with density functional theory (DFT) calculations, we show that this approach permits a gradual decrease of the singlet–triplet gap, ΔEST, from 300 to around 10 meV in toluene, yet we also demonstrate why a small ΔEST alone is not enough. While sufficient oscillator strength is maintained with the Cz- and tBuDPA-based donor dendrons, this is not the case for the PXZ-based donor dendron, where the wave function overlap is reduced too strongly. Overall, we find the donor dendron extension approach allows successful fine-tuning of the emitter photoluminescence properties.

Original languageEnglish
Pages (from-to)552-562
Number of pages11
JournalThe Journal of Physical Chemistry. B, Condensed matter, materials, surfaces, interfaces & biophysical
Volume126
Issue number2
DOIs
Publication statusPublished - 20 Jan 2022

Funding

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 812872 (TADFlife). S.B. acknowledges support from the German Science Foundation (392306670/HU2362). The St Andrews team would also like to thank the Leverhulme Trust (RPG-2016-047) and EPSRC (EP/P010482/1) for financial support. Computational resources have been 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. D.B. is a FNRS Research Director. Y.O. acknowledges funding from the FRS-FNRS under the grant F.4534.21 (MIS-IMAGINE). EZ-C is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). Y.O. is grateful for the fruitful discussions with Prof. Juan-Carlos Sancho-Garcia from the University of Alicante and Prof. Luca Muccioli from the University of Bologna.

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