Photophysical, electrochemical and thermal properties of new (co)polyimides incorporating oxadiazole moieties

New polyimides and copolyimides were obtained by polycondensation of 2,5-bis(p-aminophenyl)-1,3,4-oxadiazole with one of the three aromatic dianhydrides: 1,4,5,8-naphthalenetetracarboxylic dianhydride, 4,4′-(4,4′-isopropylidene-diphenoxy)bis(phthalic anhydride) and 4,4′-tetraphthaloyl-bis(1,8-naphthalene dicarboxylic)dianhydride. The photophysical properties such as UV absorption, photoluminescence (PL), electrochemical and spectroelectrochemical properties of the copolymers were evaluated. Additionally, their thermal behavior was investigated. The polymers dissolved in NMP and in the solid state as blend with nonemissive poly(methyl methacrylate) emitted blue light with the highest intensity under excitation wavelength (λex) 340 nm, whereas, under λex = 405 nm green fluorescence was observed with double-exponential lifetime. Longer decay constants were estimated for polymers in blend as compared with the ones obtained for solutions. The electronic properties, such as, HOMO–LUMO levels and band gaps were calculated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) and for models of polyimides were calculated theoretically by density functional theory (DFT). All of the polymers exhibited reversible reduction and non-reversible oxidation process. The electrochemical band gaps based on DPV measurements were in the range of 1.79–2.50 eV. Spectral changes in UV–vis region under applied potential were observed. Decreasing of applied potential to negative values resulted in appearance of new band around 600 nm in the case of film prepared from polyimide with naphthalene imide rings and with copolyimide containing naphthalene diimide and phthalic imide structures. The obtained polymers exhibited glass transition temperatures (Tg) in the range of 227–336 °C and decomposition temperature (Td) in the range of 382–505 °C.