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

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, <b>DiKTa</b>. 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, <b>Cz-DiKTa</b>, <b>Cz-Ph-DiKTa</b>, and <b>3Cz-DiKTa</b>, the emission remains narrow, while in four examples that emit via a LRCT state, <b>TMCz-DiKTa</b>, <b>DMAC-DiKTa</b>, <b>3TMCz-DiKTa</b>, and <b>3DMAC-DiKTa</b>, 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, <b>DMAC-DiKTa</b> and <b>TMCz-DiKTa</b>, 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 <b>DMAC-DiKTa</b>-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.