Design of Near-UV Photoluminescent Liquid-Crystalline Dimers: Roles of Fluorinated Aromatic Ring Position and Flexible Linker

Near-ultraviolet photoluminescence liquid-crystalline molecules (PLLCs) have attracted attention for temperature-responsive photoluminescence (PL) modulation and ON/OFF sensing under external stimuli. We recently developed mesogenic dimers composed of two hexyloxy-substituted, fluorinated tolane-type cores linked by alkylene-1,n-dioxy chains that exhibited near-UV PL in the solid state. However, the formation of LC phases and the temperature range of the LC state were limited. To improve LC phase stability, in this study, we extended the flexible terminal chains and repositioned the fluorinated aromatic rings from the outer to the inner core positions. Accordingly, we synthesized mesogenic dimers with even-numbered alkylene-1,n-dioxy linkers (hexylene, octylene, and decylene) and outer- or inner-ring fluorination. Outer-ring fluorination led to high melting temperatures and stable crystalline phases with limited mesophase formation. In contrast, inner-ring fluorination induced nematic phases upon heating and cooling owing to zig-zag molecular structures that disrupted crystallinity. Photophysical studies confirmed near-UV PL in solution and solid states; however, the quantum yield of the solution PL was low (<0.01). In the solid state, the PL efficiencies and wavelengths were influenced by the fluorinated aromatic ring position and linker length. This study provides important molecular design criteria for developing stable LC materials with tunable near-UV luminescence for temperature-responsive optical devices.