Dual-state emission luminescent liquid crystals: synthesis, self-assembly and emission behavior

A series of luminescent liquid crystals incorporating a central thieno[3,2-b]thiophene core and two cyanostilbene peripheral units was designed and fabricated through Suzuki coupling and Knoevenagel reactions. Their mesomorphic properties were characterised via polarised optical microscopy, differential scanning calorimetry and small-angle X-ray scattering, confirming the transition from a hexagonal columnar mesophase with p6mm lattice to a rectangular columnar mesophase with p2mm lattice. Extremely weak solvatochromism in organic solvents and dual-state emission were observed, arising from weak intramolecular charge transfer and the long rigid aromatic core. Density functional theory calculations further supported these findings, revealing optimised geometry and spatial distributions of the highest occupied molecular orbitals and lowest unoccupied molecular orbitals consistent with weak intramolecular charge-transfer and molecular twisting. Fluorescence enhancement triggered by temperature was realised in a hexacatenar liquid crystal material. Leveraging their dual-state emission, these luminescent liquid crystals were proved to be highly suitable for advanced applications, including the fabrication of anti-counterfeiting labels and the development of latent fingerprints. Therefore, this study establishes a viable molecular design strategy towards dual-state emission active luminescent liquid crystalline materials with favourable performance and broad application potential.