New heterocyclic conjugated azomethines containing triphenylamine units with optical and electrochemical responses towards the acid environment

[Display omitted] •Heterocyclic azomethines containing ortho-linked triphenylamine units are developed.•They exhibit high organo-solubility/film forming ability and high thermostability.•Their opto-electronic behavior is modulated through the structural pattern.•Different spectral changes occurred by doping with hydrochloric and trifluoroacetic acids. A series of heteroaromatic azomethines containing phenyl, pyridine, thiophene or furan ring and ortho-catenated triphenylamine (TPA) core was synthesized and used to investigate the effect of the structural variation on opto-electronic and acid-sensing properties. The twisted structure induced by the TPA core enabled good solubility in organic solvents, including chloroform, acetone or ethyl acetate. Meanwhile, they preserved good thermal stability and showed high glass transition temperatures. The modulation of optical and electronic properties of azomethines was most likely due to two effects: the degree of coplanarity induced by the heterocyclic moiety and the electronic effect of the π-rich heterocycle that is electron-withdrawing (pyridine) or electron-acceptor (thiophene and furan). A detailed study was accomplished with respect to the acid recognition capability promoted by the electronic and basicity character of the azomethine center. The spectroscopic and electrochemical responses to acid environment were followed by cyclic voltammetry, FTIR, fluorescence and UV–vis spectroscopy. Different spectral changes occurred when doping was performed with hydrochloric and trifluoroacetic acids. The sensing properties toward the environmental pH modification were mostly accompanied by fluorescence quenching and driven by different recognition principles with respect to the acid dopant type.