Ferroelectric Properties and Ambipolar Carrier Transport of 9-Fluorenone–Based Liquid Crystals

The functional integration of chiral liquid crystals and π-conjugated compounds has great potential for creating novel exotic materials. We synthesized a series of chiral donor–acceptor (D–A)-type fluorenone derivatives to investigate the influence of molecular structure upon their phase-transition behavior, ferroelectricity, and photophysical and photoconductive properties. Polarizing optical microscopy and differential scanning calorimetry analyses revealed that several D–A-type fluorenone derivatives exhibited liquid crystal (LC) phases. These chiral LC fluorenone derivatives exhibited polarization hysteresis behavior in the chiral smectic C (SmC*) phase. Among the four ferroelectric liquid-crystalline fluorenone derivatives, (R,R)-2a exhibited the largest spontaneous polarization (over 3.0 × 102 nC cm–2). The formation of intramolecular charge-transfer states in each compound was evidenced in the UV–vis absorption spectra. Using the time-of-flight method, the ambipolar carrier transport in the SmC* phases of the fluorenone-based LCs was clarified. The hole and electron mobilities in the SmC* phases were on the order of 10–5 cm2 V–1 s–1, which is on par with the carrier mobilities of low-ordered smectic phases in conventional LC semiconductors.