Solution-processable, high current efficiency deep-blue organic light-emitting diodes based on novel biphenyl-imidazole derivatives

A series of bipolar biphenyl-imidazole derivatives with fully twisting structures were designed and synthesized by the introduction of an aryl moiety with electron-donating or electron-withdrawing substituents to the biphenyl-imidazole core. The thermal, photophysical, electrochemical and computational properties of compounds 2a-2e were investigated. The five biphenyl-imidazole derivatives show good thermal stability and possess appropriate energy levels for carrier injections. The nonplanar fully twisting structures could reduce the molecular aggregation and enable them to exhibit favorable fluorescence quantum yields. Meanwhile, their wide optical energy gaps result in efficient deep-blue emission. Moreover, we successfully fabricated deep-blue organic light-emitting diodes (OLEDs) using a solution processing method by blending PVK with compounds 2a-2e as emitting layers. By incorporating an effective n-doped electron injection layer, the optimal device VI based on compound 2c showed a greatly improved EL performance with CIE coordinates of (0.156, 0.085). Particularly, it presents a low turn-on voltage of 3.6 V, a maximum power efficiency of 5.26 lm W-1, and a very high maximum current efficiency of 6.12 cd A(-1) with small efficiency roll-off (19%). Our optimal device exhibits an advantage in current efficiency compared to the most reported imidazole-based deep-blue OLEDs. This work provides a feasible strategy for the design and synthesis of biphenyl-imidazole deep-blue materials, which may have great application prospects in the field of high-performance OLEDs.