Photoluminescent poly(p-phenylenevinylene)s with an aromatic oxadiazole moiety as the side chain: synthesis, electrochemistry, and spectroscopy study

Two poly(p-phenylenevinylene) (PPV) based polymers functionalized with an electron-deficient oxadiazole segment as the side chain by mimicking the chemical structure of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) have been successfully synthesized through the Gilch route and Wittig method. The obtained polymer II, which is a copolymer, is completely soluble in conventional organic solvents. However, polymer I, which is a homopolymer, is not soluble in any common organic solvents tried. The structure and purity of II have been characterized by FT-IR, ¹H NMR, ¹³C NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-vis and photoluminescence (PL) spectroscopy, and electrochemical analysis. The TGA results indicate that II has very high thermal stability, while DSC investigation demonstrates that the glass transition temperature (T_g) of II is higher than 200°C, which might be a merit for the long-life operation of light-emitting devices. The absorption spectrum of film sample of II reveals two peaks, and the edge absorption corresponds to a band gap of 2.36 eV. The photoluminescence spectra indicate that this polymer is an orange-yellow emitting material. Electrochemical analysis through cyclic voltammetry demonstrates that this polymer is electroactive, showing reversible n-doping and p-dopable processes. The onset potential of reduction is comparable to that of PBD, which means the electron affinity of this polymer is readily enhanced by introducing an electron-withdrawing group as the side chain, and the situation of imbalance of charge injection ability could be improved. The HOMO and LUMO energy levels have also been estimated.