Modified triphenylamine donors with shallower HOMO energy levels to construct long-wavelength TADF emitters of efficient organic light-emitting diodes

Abstract

Triphenylamine (TPA) is the most promising donor fragment for the construction of long-wavelength thermally activated delayed fluorescence (TADF) emitters owing to its suitable dihedral angle that could enhance radiative decay to compete with the serious non-radiative decay. However, the moderate electron-donating capacity of TPA seriously limits the selection of acceptor for constructing long-wavelength TADF emitters with narrow bandgaps. To address this issue, in this work, the peripheral benzene of TPA was replaced with 1,4-benzodioxane and anisole to obtain two new electron-donating units N-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-N-phenyl-2,3-dihydrobenzo[b][1,4]dioxin-6-amine (TPADBO, −5.02 eV) and 4‑methoxy-N-(4-methoxyphenyl)-N-phenylaniline (TPAMO, −5.00 eV), which possess much shallower highest occupied molecule orbital (HOMO) energy levels than the prototype TPA (−5.33 eV). Based on TPA and the modified TPA donor fragments, three TADF emitters were designed and synthesized, namely Py-TPA, Py-TPADBO and Py-TPAMO, with the same acceptor fragment 12-(2,6-diisopropylphenyl)pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (Py). Among them, Py-TPAMO exhibits the highest photoluminescence quantum yield of 78.4 % and the smallest singlet-triplet energy gap, which is because the introduction of anisole does not cause significant molecule deformation for the excited Py-TPAMO. And Py-TPAMO-based OLEDs successfully realize a maximum external quantum efficiency of 25.5 % with the emission peak at 605 nm. This work provides a series of candidate of donor fragments for the development of efficient long-wavelength TADF emitters.