Design, Synthesis And Properties Of Donor-Acceptor Type Organic Deep-Blue Fluorescent Materials

Organic light-emitting diodes(OLEDs) have made great progress in flat-panel displays and solid-state lighting resources in the past decades. In full-color displays, blue/deep-blue emission plays an extremely important role. On the one hand, it is one of three primary colors. On the other hand, blue/deep-blue emitters commonly act as host to generate emission of other colors by energy transfer. Because of the intrinsic wide band gap, blue/deep-blue OLEDs often exhibit poorer performance than that of green/red OLEDs. According to the European Broadcasting Union(EBU) standard, the Commission International de l’Eclairage coordinates(CIE(x, y)) of a pure blue emission should reach the value of(0.15, 0.06). However, high performance blue emitters which meet the EBU standard are rare.Phosphorescent emitters are commonly utilized to gain high efficiency, as they can harvest both singlet and triplet excitons. Due to the intrinsic wide band gap, a few satisfactory deep-blue phosphorescent OLEDs that exhibited high efficiency with CIEy values smaller than 0.10 have been reported. In contrast, fluorescent blue emitters are relatively easy to obtain high color saturation but difficult to reach high efficiency. For fluorescent emitters, donor-acceptor(D-A) type molecules have attracted much interest recently. On the one hand, D-A type molecules tend to facilitate the carries injection and balance in devices. On the other hand, they can induce intra-molecular charge transfer(ICT) state. Owing to the ICT state, D-A type emitters commonly suffer from the red-shift of emission, which is unfavorable for deep-blue emission. Thus, selecting different donor or acceptor to control the strength of ICT is a rational strategy to achieve deep-blue emission.Firstly, we choose 9,10-diphenyl-anthracene as a chromophore to generate deep-blue emission, which exhibits high fluorescent efficiency. We also notice that imidazole has an interesting property that it can behave respectively as a mild acceptor or donor depending on whether it is substituted with neutral moiety at its N1 and C2 positions. Thus, we designed and synthesized two asymmetrical D-A type deep-blue emitters, DPA-PPI and DPA-PIM. Both DPA-PPI and DPA-PIM exhibited high thermal stabilities, bipolar properties and deep-blue emission. Using DPA-PPI and DPA-PIM as emitters, the non-doped devices exhibited blue emission. Especially, the non-doped device based on DPA-PIM achieved maximum external quantum efficiency(EQE) of 6.5 % with a CIE coordinate of(0.15, 0.08). Moreover, the two emitters in doped devices achieved high EQEs over 5 % and pure deep-blue emission with CIE coordinates of(0.15, 0.06) for DPA-PPI and(0.15, 0.05) for DPA-PIM, which perfectly matched the EBU blue standard.We also find that 3,4,5-triphenyl-4H-triazole(TAZ) is similar to imidazole with good electron-transporting property. Due to its ultraviolent emission, TAZ has been commonly used as a build block for electron-transporting materials and host for phosphorescent emitters. Recently, several green/sky-green thermally activated delayed fluorescence(TADF) emitters based on TAZ have been reported, which indicate TAZ could exhibit deep-blue emission while attaching an appropriate electron-donor. Thus, we chose triphenylamine(TPA) as a donor and TAZ as acceptor, designed two D-A type emitters, 3TPA-TAZ and 4TPA-TAZ. Due to the ICT character, 3TPA-TAZ and 4TPA-TAZ exhibited expected deep-blue emission with high fluorescent quantum yields. In the non-doped devices, both of the two emitters achieved pure deep-blue emission with CIE coordinates of(0.15, 0.05) for 3TPA-TAZ and(0.16, 0.04) for 4TAP-TAZ. In addition, the maximum EQE of the non-doped device based on 3TPA-TAZ reached to 4.2 %, which is among the best reported deep-blue non-doped OLEDs with CIEy coordinate below 0.06.