Insight into the effect of terminal aromatic group on the mesomorphic, emissive and stimuli-responsive properties of cyanostyrene-based derivatives with multiple applications

Three novel series of cyanostyrene-based derivatives containing pyridine, cyanostyrene and terminal phenyl, naphthyl and anthryl as π-conjugated aromatic unit were synthesized by Suzuki coupling and Knoevenagel reactions. A slight modification in chemical structures induced significant differences in self-assembly property in bulk state, emissive properties in both solution and aggregated states, mechanochromic properties and acidochromism properties. The terminal phenyl cyanostyrene-based derivatives exhibited a mesophase transition from nematic phase to smectic A phase upon elongation of the terminal chain and decreasing temperature, whereas the other terminal naphthyl and anthryl cyanostyrene-based derivatives were non-mesogen, which might be attributed to the increased twisted molecular configuration and geometric anisotropy. All the compounds displayed positive solvatochromic behaviors and the redshift in emission spectra gradually increased from terminal naphthyl compound via phenyl compound to anthryl compound attributing to the enhancement in intramolecular charge transfer. All the compounds displayed emission in both solution and aggregated states due to the twisted molecular configurations or/and distinct intramolecular charge transfer. The terminal phenyl and naphthyl compounds displayed extremely weak mechanochromism, whereas the terminal anthryl compound displayed distinct mechanochromism due to the highly twisted molecular configuration of the anthryl compound. Reversible high-contrast acidochromism was realized for all the compounds due to the reversible protonation and deprotonation process of pyridine. In addition, the good applications in security paper, encrypted ink and bioimaging were also demonstrated. This investigation elucidates that a slight change in chemical structures could induce big differences in characteristics in different states, which afforded effective ways for the construction of multifunctional materials.