Achieving highly efficient bipolar near-ultraviolet emitters via regulating the energy levels of the excited states by a co-acceptor system

With the 4-(10H-phenoxazin-10-yl)benzonitrile oxide 5,5-dioxide (PhCNTzO) unit as a co-acceptor and 9-phenyl-9H-carbazole (PhCz) and 9,9-dimethyl-10-phenyl-9,10-dihydroacridine (PhAd) groups as electron donors, three novel bipolar near-ultraviolet (NUV) emitters (PhCNTzO-mCz, PhCNTzO-Cz, and PhCNTzO-Ad) were designed and synthesized. In order to regulate the energy levels of these emitters to achieve NUV emission, 10H-phenothiazine 5,5-dioxide (TzO) was introduced between the p-cyanophenyl group and PhCz or PhAd units. It was shown that the sulfonyl unit together with the p-cyanophenyl group could serve as a co-acceptor system to elevate the energy levels of the excited states from these emitters to furnish NUV emission by fulfilling electron transition from the PhCz or PhAd group with a weak electron-donating ability to the p-cyanophenyl unit. The photoluminescence (PL) peak with higher energy in toluene for these PhCNTzO-based emitters was at 381 nm with a very small CIE_y only ca. 0.03. Hence, these PhCNTzO-based emitters showed NUV behavior. Importantly, the photoluminescent quantum yields (PLQYs) of these emitters in toluene were more than 82.7%. As an excellent consequence, organic light-emitting diodes (OLEDs) with NUV emission based on these PhCNTzO-based emitters could achieve a forward-viewing maximum external quantum efficiency (η_ext) of 5.67% and an EL peak with a higher energy at 416 nm with a CIE_y of 0.06. These encouraging EL data show the great potential of the PhCNTzO moiety as a co-acceptor in developing highly efficient NUV emitters.