Synthesis, characterization, thermal stability, conductivity and band gap of oligo-4-[(2-hydroxybenzylidene)amino]benzoic acid

In this study, the oxidative polycondensation reaction conditions of 4-[(2-hydroxybenzylidene)amino]benzoic acid (4-HBAB) by using oxidants such as air O2, H2O2 and NaOCl were studied in an aqueous alkaline medium between 40 and 90 °C. The structures of the synthesized monomer and oligomer were confirmed by FT-IR, UV–vis, NMR and elemental analysis. The characterization was made by TG-DTA, size exclusion chromatography (SEC) and solubility tests. At the optimum reaction conditions, the yield of oligo-4-[(2-hydroxybenzylidene)amino]benzoic acid (O-4-HBAB) was found to be 68% (for air O2 oxidant), 70% (for air H2O2 oxidant) 53% (for NaOCl oxidant). According to the SEC analysis, the number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (PDI) values of O-4-HBAB were found to be 932, 1469 g mol−1 and 1.576, using H2O2, and 1895, 2560 g mol−1 and 1.354, using air O2 and 2320, 3015 g mol−1 and 1.300, using NaOCl, respectively. According to TG analysis, the weight losses of 4-HBAB and O-4-HBAB were found to be 96.86% and 73.10% at 1000 °C, respectively. O-4-HBAB was shown higher stability against thermal decomposition. Also, electrical conductivity of the O-4-HBAB was measured, showing that the polymer is a typical semiconductor. Electrochemical HOMO, LUMO and energy gaps (Eg) of 4-HBAB and O-4-HBAB were found to be −6.28, −6.36; −2.39, −2.64; 3.89 and 3.72 eV, respectively. According to UV–vis measurements, optical band gap (Eg) of 4-HBAB and O-4-HBAB were found to be 3.23 and 3.09 eV, respectively.