Design of donor-acceptor-acceptor (D-A-A′)-type fluorescence emitters based on benzothiadiazole with the hybridized local and charge-transfer (HLCT) excited state feature for green to deep-red emitting OLEDs

Organic light-emitting diodes (OLEDs) have significant applications in solid-state lightings and flat-panel displays. The development of novel organic emitters to meet the demands of high-performance OLEDs is attracting much attention. The electron withdrawing group benzothiadiazole (BTZ) is widely used in organic emitters due to its rigid planar structure which will benefit the emission performance. At present, the chemical structures of organic emitters based on BTZ skeleton are mostly dominated by the symmetric D-A-D configuration. Herein, we designed and synthesized a serial of asymmetric donor-acceptor-acceptor (D-A-A′)-type fluorescence emitters with the hybridized local and charge-transfer (HLCT) excited state feature. Due to its extended π-conjugation, the D-A-A′ type molecular architecture is conducive to enhancing intramolecular charge transfer and achieving redshift of emitter. Notably, the emission of OLED devices can be finely tuned from green to deep red by varying the doping concentrations of these fluorescent emitters within the host material, thereby enabling a broad spectrum of light colors. Moreover, it is worth highlighting that the introduction of triphenylamine groups can enable devices at much higher doping levels to achieve higher EQEs. Compared with the maximum EQE of 3.1 % for the10 wt% doped OLED based on 7b, the maximum EQEs of devices based on 7b at the doping concentration of 50 wt% and 100 wt% increase to 4.8 % and 3.5 %, respectively. Especially, the non-doped OLED based on 7b exhibits excellent red color purity with the CIE coordinate of (0.63, 0.36), which is very close to the Rec. 709 standard red color CIE coordinate (0.64, 0.33).