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

Abstract

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 (Tg) 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.