Nematic liquid crystals (NLCs), that is, fluids with optical anisotropy as well as electric- and magnetic-field responsiveness, have been widely used in commercial liquid crystal displays. Recent advancements have extended the scope of NLC molecules to large calamitic π-conjugated systems, which heralds prospects for novel applications that exploit their superior electronic or optical functionalities in, for example, electric field controlled fluorescence switch devices. However, NLC phases of such extended π-systems usually flow only at high temperatures, which hampers device applications that operate around room temperature. Here, we show near-room-temperature NLCs of a π-conjugated fluorophore by introducing a flexible cyclic structure into the mesogenic core. 3,8-Bis(4-propylphenyl)-6,7-dihydro-5H-benzo[7]annulene (DPB[7]-C3) has a nematic phase in a significantly lower temperature range (52.6–160.4°C) than the DPB[7]-C3 analog without flexible alkylene bridges, (E)-4-propyl-4′-(4-propylstyryl)-1,1′-biphenyl (248–262°C). We attribute this large decrease in the phase transition temperature to large intramolecular conformational entropies that arise from the geometric change of the cyclic structure, which involves rotational motion of single biaryl-bonds and bending motions along the long molecular axis in the thermal equilibrium state. The practical utility of these NLC molecules is demonstrated by preparing an electric-field-responsive large-area fluorescent switch device with a sub-millisecond response time from a mixture of 3,8-bis(4-alkylphenyl)-6,7-dihydro-5H-benzo[7]annulenes (DPB[7]-Cns).
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