Binaphthyl-Containing Green- and Red-Emitting Molecules for Solution-Processable Organic Light-Emitting Diodes

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution-processable, electroluminescent organic molecules. In this paper, a non-planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthyl-containing molecules (BNCMs): green-emitting BBB and TBT as well as red-emitting BTBTB and TBBBT, are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT, TBBBT, and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film-forming ability of the synthesized BNCMs, double-layer structured organic light-emitting diodes (OLEDs) with BNCMs as emitting layer and poly(N-vinylcarbazole) (PVK) or a blend of poly [N,N'-bis(4-butyl phenyl)-N,N'-bis(phenyl) benzidine] and PVK as hole-transporting layer are fabricated by a simple solution spin-coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 831.5 cd . m(-2) and a maximum luminous efficiency of 1.95 cd.A(-1) at a low turn-on voltage of 2.2V. This is one of the best performances of a spin-coated OLED reported so far. In addition, by doping the green and red BNCMs into a blue-emitting host material poly(9,9-dioctylfluorene-2,7-diyl) high performance white light-emitting diodes with pure white light emission and a maximum luminance of 4000 cd . m(-2) are realized.