Preparation of Cyano-Substituted Tetraphenylethylene Derivatives and Their Applications in Solution-Processable OLEDs

Creation of organic luminescent materials with high solid-state efficiency is of vital importance for their applications in optoelectronic fields. Here, a series of AIE luminogens (AIE gens), (Z)-2,3-bis(4-(9,9-bis(6-(9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)phenyl)-3-phenylacrylonitrile (SFC), and 2,3-bis(4-(9,9-bis(6-(9H-carbazol-9-yl)hexyl)-9H-fluoren-2-yl)phenyl)fumaronitrile (DFC), utilizing 2,3,3-triphenylacrylonitrile and 2,3-diphenylfumaronitrile as respective centers, are designed and synthesized by Suzuki coupling reactions with high yields. The cis- and trans-isomers of DFC are also successfully obtained. All of them are thermally stable and show good solubility in common organic solvents. They all emit weakly in solution, but become strong emitters when fabricated into solid films. It is found introduction of one additional cyano group in DFC induced a big red-shift in solid-state emission, owing to its high electron-withdrawing ability. The cis- and trans-DFC show similar photophysical and Cyclic voltammogram (CV) behaviors. Non-doped solution-processed organic light-emitting diodes (OLEDs) using the three compounds as light-emitting layers are fabricated. SFC gives the best device performance with a maximum luminance of 5201 cd m^-2, a maximum current efficiency of 3.67 cd A^-1 and a maximum external quantum efficiencies (EQE) of 1.37%. Red-shifted EL spectra are observed for cis- and trans-DFC-based device, and the OLED using trans-DFC as active layer exhibits better performance, which might derive from their different conformation in film state.