Positional isomerism mediated the self-assembly and optical properties of amphiphilic cyanostyrene-based mesogens

Two novel amphiphiles containing phenylthiophene and cyanostyrene group in the rigid core and a methyl and an oligooxyethylene unit at the terminal were synthesized by Suzuki coupling and Knoevenagel condensation reactions. The mesomorphic and optical behaviors were well demonstrated by POM, DSC, X-ray diffraction, dynamic light scattering (DLS), UV–vis spectra, fluorescence emission spectra, fluorescence lifetimes, density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The positional isomer with terminal thiophene ring exhibits an enantiotropic nematic phase whereas the positional isomer with terminal benzene ring is non-mesogen, which might be attributed to the increasing intermolecular interaction induced by the position of thiophene ring. The UV–vis spectra of both compounds show little difference due to the similar rigid core. The emission spectra of the positional isomer with terminal benzene ring showed a distinct bathochromic shift in both solution and aggregated states than those of the positional isomer with terminal thiophene ring, which attributed to the distance between the cyano group and thiophene ring. The shorter distance could induce more obvious intramolecular charge transfer (ICT). Both positional isomers show AIE characteristics due to the restricted intermolecular rotation. Positional isomerism also induces a difference in reversible mechanochromic properties due to the changes in the molecular crystalline arrangement before and after grinding. The reversible fluorescence switching properties in the rewritable luminescent paper was also demonstrated. Therefore, positional isomerism could lead to distinct transition of self-assembly and optical properties, which paved a method to construct multifunctional molecules with different properties and applications.