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

• Published in: The journal of physical chemistry. B 2022-01, Vol.126 (2), p.552-562
• Main Authors: Duda, Eimantas, Hall, David, Bagnich, Sergey, Carpenter-Warren, Cameron L., Saxena, Rishabh, Wong, Michael Y., Cordes, David B., Slawin, Alexandra M. Z., Beljonne, David, Olivier, Yoann, Zysman-Colman, Eli, Köhler, Anna
• Format: Article
• Language: English
• Subjects: B: Soft Matter, Fluid Interfaces, Colloids, Polymers, and Glassy Materials
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.1c05749

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 ( t BuDPA), 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, ΔE ST, from 300 to around 10 meV in toluene, yet we also demonstrate why a small ΔE ST alone is not enough. While sufficient oscillator strength is maintained with the Cz- and t BuDPA-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.