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Synthesis, Properties and Applications of Biphenyl Functionalized 9,9-Bis(4-diphenylaminophenyl)fluorenes as Bifunctional Materials for Organic Electroluminescent Devices



Four new bifunctional materials, namely BPTF, CBPTF, CMBPTF and BPVTF, having 9,9-bis(4-diphenylaminophenyl)fluorene as a molecular platform and biphenyl derivatives as end-capping substituents were synthesized and characterized. Their optimized structures revealed that both triphenylamine moieties at the C-9 position of the fluorene ring generated a bulky molecular structure. These molecules showed strong blue emission in both solution and in the solid state, and were thermally stable amorphous materials with glass transition temperatures up to 207 °C. The abilities of these materials to act as blue-light-emitting materials for blue OLEDs and hole-transporting materials for green OLEDs in terms of device performance and thermal properties were superior to commonly used N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB). Efficient, and bright nondoped deep-blue and Alq3-based green OLEDs with maximum luminance efficiencies and CIE coordinates of 2.48 cd A–1 and x = 0.15 and y = 0.07, and 4.40 cd A–1 and x = 0.28 and y = 0.52 were achieved, respectively, with BPTF having two biphenyl substituents as active layers. Both devices showed low turn-on voltages of 3.1 and 2.8 V, respectively. Notably, the color purities of these deep-blue and green devices were close to the National Television Standards Committee blue (CIE coordinates of x = 0.14 and y = 0.08) and green (CIE coordinates of x = 0.26 and y = 0.65) standards.