Impact of boryl acceptors in para-acridine-appended triarylboron emitters on blue thermally activated delayed fluorescence OLEDs

Abstract

Two pairs of para-donor-appended triarylboron compounds that are derived from 9,9-dialkylacridine (9,9-dimethylacridine (DMAC) or 9,9-diphenylacridine (DPAC)) donors and boryl (9-boraanthryl (BA) or dimesitylboryl (BMes(2))) acceptors are prepared and characterized. X-ray diffraction studies reveal that the BA ring is nearly orthogonal to the phenylene linker, whereas the BMes(2) moiety is tilted by ca. 60 degrees. The BA-containing compounds exhibit blue thermally activated delayed fluorescence (TADF) in both the solution and film states. In contrast, BMes(2)-containing compounds display normal blue fluorescence in solution, while TADF is induced in their films. The small Delta E-ST values (<0.1 eV) and high k(RISC) (ca. 10(6) s(-1)) of the doped films of the BA-containing compounds support the strong TADF character with the short-lived delayed fluorescence (tau(d) = 1.6-2.0 mu s). The theoretical studies along with the experimental results reveal that the electronic transitions in the BA-containing compounds proceed by the stronger charge transfer from the donor to the acceptor moieties than the BMes(2)-containing compounds. Pure to deep blue TADF-OLEDs are realized with all compounds as the dopants. The devices employing the DMAC donor-based emitters exhibit high device performance with a maximum external quantum efficiency (EQE) of 21.3%. However, lower EQEs (<12.8%) and severe efficiency roll-off are observed for the devices with the DPAC donor-based emitters.