2,1,3-Benzothiadiazole-based fluorophores. Synthesis, electrochemical, thermal and photophysical characterization

Abstract

Three photoactive compounds with π-extended conjugation based on the 2,1,3-benzothiadiazole unit were synthesized and characterized. The compounds exhibited absorption in the violet-blue region with molar absorptivity coefficients and radiative rate constants arising from spin and symmetry allowed 1ππ* electronic transitions. An emission located in the green region with a large Stokes shift was observed, which was most likely due to a charge-transfer mechanism in the excited state. In spin-coated films a dependence on the fluorescence emission intensity with the size of the alkoxy chain could be observed, where an effective non-radiactive channel seems to be present to deactivate the excited state. The thermal properties were analyzed by differential scanning calorimetry (DSC), and all final compounds exhibited a similar behavior with a crystal-isotropic liquid transition during the heating scan and isotropic liquid-crystal transition during the cooling process. Thermogravimetric analysis indicated a main thermal event with an initial decomposition temperature that was higher than 340 °C. The electrochemical characterization indicate that the compounds exhibited a reversible peak at −1.48 V and an irreversible oxidation process at 0.94 V versus Ag/Ag+. The electrochemical band gap was calculated to be approximately 2.30 eV versus NHE. The spectroelectrochemical measurements demonstrated changes in the absorption spectra due to changes in the electronic structure of the conjugated molecules under oxidative and reductive potentials.