Multi-State Photoluminescence of Donor–π–Acceptor Tetrafluorinated Tolane Mesogenic Dimers in Solution, Crystal, and Liquid-Crystalline Phases

Photoluminescent liquid crystals with photoluminescence (PL) and liquid-crystalline (LC) properties have attracted attention as PL-switching materials owing to their thermally induced phase transitions, such as crystal → smectic A/nematic → isotropic phase transitions. Our group previously developed tetrafluorinated tolane mesogenic dimers linked by flexible alkylene-1,n-dioxy spacers, demonstrating that the position of the tetrafluorinated aromatic ring critically influences the LC behavior. However, these compounds exhibited very weak fluorescence owing to an insufficient D–π–A character of the π-conjugated mesogens, which facilitated internal conversion from emissive ππ* to non-emissive πσ* states. We designed and synthesized derivatives in which the mesogen–spacer linkage was modified from ether to ester, thereby enhancing the D–π–A character. Thermal and structural analyses revealed spacer-length parity effects: even-numbered spacers induced nematic phases, whereas odd-numbered spacers stabilized smectic A phases. Photophysical studies revealed multi-state PL across solution, crystal, and LC phases. Strong blue PL (ΦPL = 0.39–0.48) was observed in solution, while crystals exhibited aggregation-induced emission enhancement (ΦPL = 0.48–0.77) with spectral diversity. In LC states, ΦPL values up to 0.36 were maintained, showing reversible intensity and spectral shifts with phase transitions. These findings establish design principles that correlate spacer parity, phase behavior, and PL properties, enabling potential applications in PL thermosensors and responsive optoelectronic devices.