Synthesis and electroluminescence properties of a series of hyper-branched light-emitting polymers

A series of hyper-branched polyalkylfluorenes (HBPFs) with an electron-conducting group, triazine and a hole-conducting group, triphenylamine as molecular cores was synthesized by the Suzuki coupling reaction. This series of HBPFs with different conjugated lengths was end-capped with a hole-conducting group, triphenylamine, and a rigid moiety, tetraphenylsilane. Higher brightness and efficiency were observed for the triazine-cored HBPF-based devices as compared to the triphenylamine-cored HBPF-based devices. For the HBPF containing the triazine core and tetraphenylsilane end-cap group, brightness and efficiency can be achieved as high as 1702 cd/m2 and 0.72 cd/A, respectively. Moreover, for polymers having triphenylamine as the molecular core, a narrower electroluminescence (EL) emission band was observed as compared to those of the triazine-cored HBPF-based devices. As a result, deep blue emission (Commission Internationale de L'Eclairage (CIEx,y) coordinates (0.16, 0.09)) were observed. In addition, excimer formation of the alkyfluorene chains was induced during the electrical driving process for devices based on HBPF consisting of the triphenylamine core and tetraphenylsilane end-cap group. EL quenching of the electrically induced excimer was therefore observed. Correlations between the molecular structure, thermal stability, and optical and EL properties of the HBPFs with different electron- or hole-conducting groups as molecular cores and end-cap groups are discussed in detail.