Thermally Activated Delayed Fluorescence in 1,3,4-Oxadiazoles with π-Extended Donors

Here, we describe the synthesis of five 1,3,4-oxadiazole-based donor-acceptor materials, using dendritic carbazole-based donors 9'H-9,3':6'9″-tercarbazole (terCBz) and N3,N3,N6,N6-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC). Due to the strongly donating and highly twisted nature of the TTAC donor as well as the spatially separated hole-particle wavefunctions, three of the five compounds exhibited thermally activated delayed fluorescence (TADF) in spite of a relatively large ΔE_ST measured through phosphorimetry (0.33-0.37 eV). These materials demonstrated photoluminescence quantum yields as high as 0.89 in toluene, with emission maxima ranging from 474 to 495 nm in the solid state. Additionally, two materials containing only terCBZ donor(s) exhibited deep blue fluorescence, with Commission Internationale de l'éclairage coordinates of (0.16, 0.05); the weaker nature of the terCBz donor results in a prohibitively large ΔE_ST (0.68-0.77 eV). A gap-tuned range-separated hybrid functional (ωB97XD*) was used to rigorously calculate triplet energies, while a systematic analysis of electronic structures and photophysical properties provided further insight into the properties of these materials. These findings ultimately contribute a synthetically facile approach toward highly emissive TADF emitters using a 1,3,4-oxadiazole motif.