Synthesis, LC self-assembly and photophysical behavior of stimuli responsive cyanostilbene-based hexacatenar mesogens with multicolour switching

Abstract

A novel series of cyanostilbene-based hexacatenar mesogens consisting of two cyanostilbene units and one central pyridine in the π-conjugated rigid core were obtained by Suzuki coupling and Knoevenagel reactions. POM, DSC, SAXS, UV–vis spectrophotometer and fluorescence spectrophotometer were employed to explore the liquid crystalline, photophysical and stimuli responsive properties. All the biscyanovinyl diphenylpyridine-based hexacatenars can self-assemble into rectangular columnar phase with P2mm symmetry in bulk state. All the mesogens display weak solvatochromism due to the weak intramolecular charge transfer (ICT) and aggregation-induced enhanced emission (AIEE) due to non-planar structure and the long π-conjugated core. Temperature inducing hypochromatic shifts of fluorescence spectra, fluorescence quenching and the change in fluorescence color due to the phase transition and molecular planarization was well demonstrated. In addition, volatile acids could induce reversible changes of dual solid color, dual fluorescence color, fluorescence quantum yield and fluorescence lifetime owing to the protonation and deprotonation of pyridine. DFT analysis demonstrated that the protonation could induce more distinct molecular distortion and ICT, causing bathochromic-shifted absorption and emission spectra. Therefore, the introduction of different functional building blocks into liquid crystalline molecules is beneficial for the development of multifunctional materials.