Dimesitylarylborane-based luminescent emitters exhibiting highly-efficient thermally activated delayed fluorescence for organic light-emitting diodes

Abstract

Triarylboranes are widely used as luminescent molecules. However, there are few reports focused on thermally activated delayed fluorescence (TADF) characteristics. In this study, new donor-acceptor triarylboranes exhibiting TADF characteristics are designed, synthesized, and applied to organic light-emitting diodes (OLEDs) as an emitter. Electro-accepting dimesitylphenylborane connected with carbazole (7), 9,9-dimethylacridane (8), and phenoxazine (9) as electron-donating units are synthesized in only two steps with good yield. Compounds 8 and 9 exhibit light blue and green TADF with good PL quantum yields of 89 and 87% in toluene, respectively. On the other hand, compound 7 shows normal deep blue fluorescence without TADF characteristics. Density functional theory and time-dependent density functional theory studies reveal that high electron-donating ability of donor unit and large dihedral angles between cross-linking phenylene and donor units are attributed to spatially separate the HOMO and LUMO, which results in lowering the energy gap between lowest singlet (S1) and triplet (T1) excited states and accelerating reverse intersystem crossing of excitons from T1 to S1 states. OLEDs using compounds 8 and 9 as emitters exhibit light blue and green emission with very good external quantum efficiencies of 16.0 and 17.3%, respectively.