Excited-State Modulation in Donor-Substituted Multiresonant Thermally Activated Delayed Fluorescence Emitters

Sen Wu; Wenbo Li; Kou Yoshida; David Hall; Subeesh Madayanad Suresh; Thomas Sayner; Junyi Gong; David Beljonne; Yoann Olivier; Ifor D W Samuel; Eli Zysman-Colman

doi:10.1021/acsami.2c02756

Excited-State Modulation in Donor-Substituted Multiresonant Thermally Activated Delayed Fluorescence Emitters

ACS Appl Mater Interfaces. 2022 May 18;14(19):22341-22352. doi: 10.1021/acsami.2c02756. Epub 2022 May 9.

Authors

Affiliations

¹ Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, United Kingdom.
² Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, United Kingdom.
³ Laboratory for Chemistry of Novel Materials, University of Mons, 7000 Mons, Belgium.
⁴ Laboratory for Computational Modeling of Functional Materials & Solid State Physics Laboratory, Namur Institute of Structured Matter, University of Namur, Rue de Bruxelles, 61, 5000 Namur, Belgium.

Abstract

Strategies to tune the emission of multiresonant thermally activated delayed fluorescence (MR-TADF) emitters remain rare. Here, we explore the effect of donor substitution about a MR-TADF core on the emission energy and the nature of the excited state. We decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short-range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (LRCT) that is typically observed in donor-acceptor TADF emitters. The impact is that in three examples that emit from a SRCT state, Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, the emission remains narrow, while in four examples that emit via a LRCT state, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, λ_EL, of 511, 492, and 547 nm with moderate full width at half-maxima (fwhm) of 62, 61, and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQE_max) of 24.4, 23.0, and 24.4%, which are among the highest reported with ketone-based MR-TADF emitters. OLEDs with D-A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQE_max of 23.8 and 20.2%, but accompanied by broad emission at λ_EL of 549 and 527 nm, respectively. Notably, the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m^-2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs.

Keywords: donor decoration; multiresonant thermally activated delayed fluorescence; narrowband emission; organic light-emitting diodes; short-range charge transfer; thermally activated delayed fluorescence.