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Article

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

by
Sou-un Doi
1,
Syota Yamada
1,
Ken’ichi Aoki
1,2 and
Atsushi Seki
1,2,3,*
1
Department of Chemistry, Graduate School of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
2
Department of Chemistry, Faculty of Science Division II, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
3
Material and Biological Engineering Course, Department of Industrial Systems Engineering, National Institute of Technology (KOSEN), Hachinohe College, 16-1 Uwanotai, Tamonoki, Hachinohe, Aomori 039-1192, Japan
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(12), 1021; https://doi.org/10.3390/cryst15121021 (registering DOI)
Submission received: 4 November 2025 / Revised: 20 November 2025 / Accepted: 27 November 2025 / Published: 28 November 2025
(This article belongs to the Special Issue State-of-the-Art Liquid Crystals Research in Japan (2nd Edition))
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Abstract

The functional integration of chiral liquid crystals and π-conjugated compounds has great potential for creating novel exotic materials. A series of chiral donor–acceptor (D–A)-type fluorenone derivatives was synthesized to investigate the influence of molecular structure upon their liquid-crystalline phase-transition behavior, ferroelectricity, photophysical properties, and photoconductive properties. Polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) analyses revealed that several D–A-type fluorenone derivatives exhibited liquid crystal (LC) phases. These chiral LC fluorenone derivatives exhibited polarization hysteresis in the chiral smectic C (SmC*) phase. Among the four fluorenone-based ferroelectric liquid crystals (FLCs), (R,R)-2a exhibited the largest spontaneous polarization (over 3.0 × 102 nC cm−2). The formation of intramolecular charge-transfer (ICT) states in each compound was evidenced by the UV–vis absorption spectroscopy. Ambipolar carrier transport in the SmC* phases of the fluorenone-based FLCs was elucidated by the time-of-flight (TOF) method. The mobilities of holes and electrons 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.

1. Introduction

Chiral molecular assemblies exhibit characteristic properties absent in the achiral counterparts [1,2,3,4,5,6,7,8,9]. The hierarchical suprastructures reflecting molecular chirality offer platforms for biopharmacological, chemical, and physical functions [1,2,3,4,5,6,7,8,9]. While helical self-assembly is the most common chiral supramolecular system, molecular chirality often triggers the formation of broken-symmetry aggregated structures. Because the symmetry reduction of self-organized structures results in the stabilization of polar structures, ferroelectric behavior is observed in some chiral suprastructures [10,11,12]. The incorporation of molecular chirality contributes to the manifestation of various unique integrated functionalities [13,14]. Thus, various chiral soft materials such as chiral polymers [5,15,16], chiral supramolecular polymers [16,17], and chiral liquid crystals (CLCs) [1,2,3] have been widely developed and examined. Remarkably, CLCs exhibit high responsiveness to temperature and electric fields, owing to their dynamic features [1,2]. In the past decade, we have paid attention to chiral smectic liquid crystals forming polar structures [18,19,20,21,22,23]. The most classical ferroelectric liquid crystal (FLC) is a CLC that exhibits a chiral smectic C (SmC*) phase [10,11]. Classical CLCs have been developed and tried to be applied to optical sensors [24] and high-speed LC displays [25].
Aromatic compounds, including oligoacenes [26] and oligothiophenes [27], have been widely studied as organic semiconductors owing to their structural diversity, favorable processability, and good carrier-transport properties in condensed states [28,29,30,31]. LC compounds integrating a planar aromatic mesogenic core can be regarded as semiconductors. In the past few decades, the anisotropic electronic conduction in LC structures has been deeply discussed [32,33,34,35,36,37]. The transport of electronic charge carriers in a variety of LC phases has also been extensively studied [38,39,40,41,42,43,44,45]. Furthermore, LC semiconductors have been applied in lightweight and flexible organic electronic devices such as thin-film field-effect transistors [46,47,48,49], light-emitting diodes [50,51,52], and solar cells [53,54].
Whereas typical CLCs are insulators, CLCs with extended π-conjugated cores exhibit electronic carrier transport [18,19,20,21,22,23,55,56,57,58]. These chiral π-conjugated LCs have the potential to create novel function-integrated materials. For example, ferroelectric π-conjugated LCs exhibiting a bulk photovoltaic effect (BPVE) have been developed [18,21,23]. The BPVE in LC phases results from the coupling effect of spontaneous polarization and carrier transport [19]. The BPVE, one of the bandgap-independent photovoltaic effects, clearly differs from classical photovoltaic effects based on p-n junctions. Therefore, the development of materials incorporating CLCs is important.
Thermally stable fluorenone is an easily modified rigid electron acceptor unit with a planar π-conjugated system [59,60,61]. The central carbonyl moiety is a polar group, which can contribute to the enhancement of spontaneous polarization. The incorporation of donor units onto fluorenone can cause ICT, which results in the formation of local electrical polarization as well as the appearance of a broad light-absorption band in the long-wavelength range [62,63]. In addition, some LC D–A-type fluorenone derivatives exhibit good carrier-transport properties [60].
In this study, a series of fluorenone derivatives incorporating chiral moieties was synthesized (Figure 1). After basic characterization of the LC properties, the electronic properties of the chiral fluorenone derivatives were studied.

2. Materials and Methods

2.1. General Procedures and Materials

All chemical reagents and solvents were purchased from FUJIFILM Wako Pure Chemical (Osaka, Japan), Kanto Chemicals (Tokyo, Japan), and Tokyo Chemical Industry (Tokyo, Japan). All reagents, including solvents, were used as received without further purification. All reactions were conducted in a thoroughly dried flask under an argon atmosphere with continuous stirring. Dehydrated solvents were used for the organic synthesis experiments. All 1H and 13C NMR spectra were recorded on a Bruker (Osaka, Japan) Biospin AVANCE NEO 400 spectrometer (400 MHz for 1H NMR spectra, 100 MHz for 13C NMR spectra). All chemical shifts (δ) in the 1H and 13C NMR spectra are quoted in ppm. Tetramethylsilane of 0.03 vol% was used as the internal standard to determine δ = 0.00 ppm. A SCIEX (Tokyo, Japan) X500R QTOF spectrometer was used for high-resolution mass spectrometry (HRMS) measurements by electrospray ionization (ESI).

2.2. Synthesis and Characterization of Target Compounds

A scheme and details of the synthesis of the target compounds are given in the attached Supplementary File (Section S1. Synthesis).

2.2.1. Characterization of (R)-1a

1H NMR (400 MHz, CDCl3): δ [ppm] = 7.85 (dd, J = 10.0, 1.3 Hz, 2H), 7.69–7.62 (m, 2H), 7.53 (d, J = 7.7 Hz, 1H), 7.49 (d, J = 7.8 Hz, 1H), 7.35 (dd, J = 12.3, 2.2 Hz, 1H), 7.32–7.29 (m, 1H), 7.20 (d, J = 3.6 Hz, 1H), 7.03 (t, J = 8.5 Hz, 1H), 6.77 (d, J = 3.6 Hz, 1H), 4.41 (sextet, J = 6.1 Hz, 1H), 2.83 (t, J = 7.5 Hz, 2H), 1.88–1.57 (m, 4H), 1.52–1.25 (m, 17H), 0.95–0.84 (m, 6H); 13C-NMR (100 MHz, CDCl3): δ [ppm] = 193.6, 153.8 (d, J = 244.7 Hz), 146.7, 146.1 (d, J = 10.7 Hz), 143.0, 142.3, 140.6 (d, J = 1.6 Hz), 140.3, 135.8, 135.2, 133.1 (d, J = 6.5 Hz), 132.7, 131.2, 125.3, 123.4, 122.5, 122.4 (d, J = 3.1 Hz), 121.1, 120.8, 120.7, 117.6 (d, J = 2.2 Hz), 114.7 (d, J = 20.2 Hz), 76.4, 36.5, 31.8, 31.6, 30.3, 29.3, 28.8, 25.5, 22.6, 22.6, 19.9, 14.1; HRMS (ESI): molecular weight (C37H41FO2S) = 568.2811; m/z calcd. for [C37H42FO2S]+ ([M + H]+) = 569.2884; m/z found = 569.2878.

2.2.2. Characterization of (R,R)-2a

1H NMR (400 MHz, CDCl3): δ [ppm] = 7.85 (d, J = 1.3 Hz, 2H), 7.66 (dd, J = 7.7, 1.7 Hz, 2H), 7.57 (d, J = 7.8 Hz, 2H), 7.36 (dd, J = 12.2, 2.2 Hz, 2H), 7.33–7.29 (m, 2H), 7.04 (t, J = 8.5 Hz, 2H), 4.41 (sextet, J = 6.1 Hz, 2H), 1.89–1.73 (m, 2H), 1.69–1.57 (m, 2H), 1.52–1.24 (m, 22H), 0.89 (t, J = 6.9 Hz, 6H); 13C-NMR (100 MHz, CDCl3): δ [ppm] = 193.6, 153.8 (d, J = 244.4 Hz), 146.1 (d, J = 10.8 Hz), 142.8, 140.7 (d, J = 1.8 Hz), 135.2, 133.0 (d, J = 6.7 Hz), 132.7, 122.5, 122.4 (d, J = 3.4 Hz), 120.8, 117.6 (d, J = 2.2 Hz), 114.7 (d, J = 19.7 Hz), 76.4, 36.5, 31.8, 29.3, 25.5, 22.6, 19.8, 14.1; HRMS (ESI): molecular weight (C41H46F2O3) = 624.3415; m/z calcd. for [C41H47F2O3]+ ([M + H]+) = 625.3488; m/z found = 625.3486.

2.2.3. Characterization of (R,R)-2b

1H NMR (400 MHz, CDCl3): δ [ppm] = 7.85 (d, J = 1.4 Hz, 2H), 7.66 (dd, J = 7.8, 1.8 Hz, 2H), 7.56 (d, J = 7.7 Hz, 2H), 7.35 (dd, J = 12.3, 2.3 Hz, 2H), 7.33–7.28 (m, 2H), 7.04 (t, J = 8.6 Hz, 2H), 4.24 (quintet, J = 5.9 Hz, 2H), 1.80–1.61 (m, 8H), 1.53–1.24 (m, 12H), 1.00 (t, J = 7.4 Hz, 6H), 0.89 (t, J = 7.0 Hz, 6H); 13C-NMR (100 MHz, CDCl3): δ [ppm] = 193.6, 153.7 (d, J = 244.7 Hz), 146.6 (d, J = 10.8 Hz), 142.8, 140.8 (d, J = 2.0 Hz), 135.2, 132.9 (d, J = 6.5 Hz), 132.7, 122.5, 122.4 (d, J = 3.4 Hz), 120.8, 117.5 (d, J = 2.5 Hz), 114.8 (d, J = 19.7 Hz), 81.5, 33.4, 31.9, 26.6, 25.0, 22.6, 14.0, 9.5; HRMS (ESI): molecular weight (C41H46F2O3) = 624.3415; m/z calcd. for [C41H47F2O3]+ ([M + H]+) = 625.3488; m/z found = 625.3488.

2.2.4. Characterization of (R,R)-3a

1H NMR (400 MHz, CDCl3): δ [ppm] = 7.81 (d, J = 1.4 Hz, 2H), 7.65 (dd, J = 7.8, 1.8 Hz, 2H), 7.58 (d, J = 7.8 Hz, 2H), 7.15 (d, J = 9.1 Hz, 4H), 4.34 (sextet, J = 6.1 Hz, 2H), 1.87–1.72 (m, 2H), 1.67–1.57 (m, 2H), 1.56–1.23 (m, 22 H), 0.89 (t, J = 6.8 Hz, 6H); 13C-NMR (100 MHz, CDCl3): δ [ppm] = 193.0, 156.8 (dd, J = 246.9, 6.5 Hz), 143.3, 139.9, 135.3, 134.6 (d, J = 17.7 Hz), 134.6 (d, J = 14.4 Hz), 134.4, 132.9, 122.6, 121.0, 110.3 (dd, J = 16.8, 7.0 Hz), 81.2, 36.9, 31.8, 29.3, 25.3, 22.6, 20.1, 14.1; HRMS (ESI): molecular weight (C41H44F4O3) = 660.3227; m/z calcd. for [C41H45F4O3]+ ([M + H]+) = 661.3299; m/z found = 661.3298.

2.2.5. Characterization of (R,R)-4a

1H NMR (400 MHz, CDCl3): δ [ppm] = 7.76 (d, J = 1.5 Hz, 2H), 7.61 (dd, J = 7.7, 1.5 Hz, 2H), 7.49 (d, J = 7.7 Hz, 2H), 7.28–7.19 (m, 4H), 6.92 (t, J = 8.7 Hz, 2H), 4.40 (sextet, J = 6.1 Hz, 2H), 1.87–1.71 (m, 2H), 1.66–1.55 (m, 2H), 1.52–1.22 (m, 22H), 0.89 (t, J = 6.9 Hz, 6H); 13C NMR (100 MHz, CDCl3): δ [ppm] = 192.2, 152.9 (d, J = 244.7 Hz), 147.1 (d, J = 10.3 Hz), 143.1, 137.7, 134.4, 128.2 (d, J = 3.4 Hz), 127.3, 124.4, 120.6, 119.5 (d, J = 20.2 Hz), 116.7 (d, J = 2.5 Hz), 115.2 (d, J = 8.5 Hz), 90.5 (d, J = 2.7 Hz), 87.9, 76.3, 36.4, 31.8, 29.2, 25.4, 22.6, 19.8, 14.1; HRMS (ESI): molecular weight (C45H46F2O3) = 672.3415; m/z calcd. for [C45H47F2O3]+ ([M + H]+) = 673.3488; m/z found = 673.3484.

2.3. Characterization of Light-Absorption Properties

UV–vis absorption spectra were obtained using a JEOL V-650 spectrometer (JEOL Corporation, Tokyo, Japan). The absorption spectra of dilute solutions (10 μM) were measured using a pair of quartz cells (cell path length: 1 cm). The absorption spectra in the bulk state were measured using the sample-filled ITO sandwich cells (cell gap: 2 μm).

2.4. Characterization of LC Properties

The LC properties were investigated using POM, DSC, and XRD. A polarizing optical microscope (Olympus BH2, Olympus Corporation, Tokyo, Japan) with a temperature-controlled hot stage (METTLER TOLEDO FP90 and FP82HT) was used to observe the optical texture. The POM images were recorded by a digital camera (AS ONE HDCE-X1, AS ONE Corporation, Osaka, Japan). The chiral fluorenone derivatives were filled into indium tin oxide (ITO) sandwich cells (KSSO-02/A311P1NSS05, cell gap: 2 μm, EHC Corporation, Tokyo, Japan) for POM observations. The polyimide-free ITO surface was rubbed to facilitate planar orientation of the LC phases. DSC measurements were performed using a DSC-60 equipped with a TAC-60L cooling system (SHIMADZU Corporation, Kyoto, Japan. Approximately 2–3 mg of each sample was taken and sealed in an aluminum pan. The scan rate for the POM and DSC measurements was 10 °C min−1. A Rigaku Miniflex (Ni-filtered Cu Kα radiation, Rigaku Corporation, Tokyo, Japan) equipped with a thermal control system was used for XRD analysis.

2.5. Evaluation of Ferroelectric Properties

The ferroelectric properties of each LC compound were evaluated by the Sawyer-Tower (ST) method. A triangular-wave bias (±25 kV cm−1, 100 Hz) was generated by a function generator (WF1973, NF Corporation, Yokohama, Japan). The generated bias was applied to the 2-μm-thick LC sample cells. A digital oscilloscope (TDS 3044B, Tektronix & Fluke Corporation, Tokyo, Japan) was used for recording the polarization of the sample. The polarization was detected as a voltage drop through the standard capacitor (33 nF) connected in series to the sample. The preparation of sample-filled LC cells and temperature control were followed by the previously reported method [18,19,20,21,22,23].

2.6. Evaluation of Carrier Transport Characteristics

The TOF technique was used to assess carrier transport properties in the LC phases. 25-μm-thick LC cells (KSSO-25/A307P1NSS, EHC Corporation, Tokyo, Japan) were used for the measurement. The sample-filled cells were prepared using the same methods as for the ST polarization hysteresis measurements. DC electric fields were applied to the LC sample using an electrometer (R8252, ADC Corporation, Miyako Namegawa, Japan). The third harmonic generation of an Nd:YAG pulsed laser (wavelength = 356 nm, pulse duration = 2 ns; MiniLite II, Continuum Corporation, San Jose, California, United States of America) was used for excitation. The photo-induced displacement currents were converted to voltage drops through a serial resistor. The voltage drops derived from photo-induced displacement currents were recorded on a digital oscilloscope (TDS 3044B, Tektronix & Fluke Corporation, Tokyo, Japan). The transit time (tT) of the photo-generated charge carriers was determined from the kink point on the transient photocurrent curve. Positive and negative charge carrier mobilities (μ) were estimated from the equation μ = d2/(V·tT), where d signifies the sample thickness (25 μm), and V represents the applied voltage. The error of carrier mobilities could be approximately ±20%, which is mainly caused by the error of sample thickness (±5 μm) and uncertainty in tT.

2.7. Evaluation of Current–Voltage Characteristics

The evaluation of current–voltage characteristics was carried out in the polarized ordered smectic phase under dark conditions and illumination with LED white light (20 mW cm−2). The sample-filled 2-μm-thick LC cells were used for the measurement. The poled sample was prepared by following the process. The sample LC cell was placed on a hot stage. The temperature of the LC cell was controlled by a PID thermo-controller. In the SmC* phase, the DC bias voltage (±20 V, ±100 kV cm−1) was applied to the sample by using an electrometer (R8252, ADC Corporation, Miyako Namegawa, Japan). The poled SmC* sample was cooled to the ordered smectic phase under the DC bias (±20 V, ±100 kV cm−1). After removing the external electric field, the polar structure was partially retained to generate an internal electric field. The positively and negatively polarized ordered smectic states were defined by the direction of the internal electric field during the poling treatment.

3. Results and Discussion

3.1. Light-Absorption Properties

The UV–vis absorption spectra of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a in tetrahydrofuran (10 μM) are shown in Figure 2a. In the absorption spectra of each compound, several strong absorption peaks appeared in the range of 290–360 nm, with a weak, broad absorption band spanning 400 to 550 nm. While the intense absorption bands in the short wavelength range (high energy region) contribute to local excitation of the central core, the broad absorption band from 400 to 550 nm is probably attributed to an ICT transition. Among the four compounds, (R,R)-4a showed the largest molar coefficient of the ICT absorption band. A similar enhancement of the molar coefficient upon insertion of an ethynyl linker between electron-deficient fluorenone and phenyl rings was reported for the analogous D–A–D-type fluorenone derivatives [64]. Among the solutions of the four fluorenone derivatives, the THF solution of (R)-1a exhibited the lowest absorption edge energy in the spectrum. The substitution of the electron-rich thiophene moiety probably stabilized the ICT excited state and narrowed the gap between the ICT ground and excited states. Among the film samples of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a, the LC film of (R)-1a covered the widest range in the visible light region of the absorption spectrum, due to the molecular alignment in the LC cell, and planarity of π-conjugated unit in the aggregated state (Figure 2b).

3.2. Liquid-Crystalline Properties

3.2.1. Polarizing Optical Microscopy

In the POM analysis on cooling from the isotropic liquid (IL) state of chiral fluorenone derivative (R)-1a, a fan-shaped texture was seen at ~185 °C (Figure 3a). The characteristic broken-fan-shaped domains indicate the formation of a tilted-layer structure in the LC phase at that temperature. By contrast, a complex optical texture with many disclination lines was observed at 90 °C (Figure 3b). The apparent change in optical texture indicated a phase transition from SmC* to ordered smectic phases. The complicated texture in the ordered phase (Figure 3b) was retained without conspicuous domain shape changes upon further cooling to ambient temperature. The POM analysis of (R,R)-2a implied that the chiral 2,7-disubstituted fluorenone derivative exhibited several LC phases during the cooling process from the IL phase (Figure 4a–c). The distinct fan-shaped texture of the smectic phase appeared at 153 °C on cooling from the IL phase (Figure 4a). The fan-shaped domain with smooth interfaces indicated the appearance of the chiral smectic A (SmA*) phase. After the sample cooled to 135 °C, the distinctive fan-shaped domains were observed (Figure 4b). Furthermore, a textural change owing to the phase transition was evident at ~50 °C (Figure 4c). By contrast, (R,R)-2b was a clear viscous liquid at room temperature. In the POM study of (R,R)-2b, the image appeared dark and homogeneous owing to the lack of birefringence above room temperature. Upon cooling from the IL phase of (R,R)-3a, the fan-shaped texture was seen at 170 °C (Figure 5a). The stripes in the fan-like domains became prominent when the sample cooled to ~130 °C (Figure 5b). Considering the phase transition sequence, the high- and low-temperature LC phases should be SmA* and SmC* phases, respectively. A more explicit optical textural change was observed below 95 °C, during the cooling process (Figure 5c). (R,R)-4a formed broken-fan-shaped domains at 95 °C (Figure 6a). The domain shapes were mostly retained after cooling to 65 °C (Figure 6b).

3.2.2. Differential Scanning Calorimetry

Two endothermic peaks of phase transitions appear during the second heating scan in the DSC thermogram of (R)-1a (Figure 7a). The first peak at 109 °C corresponds to the transition from an ordered phase to the SmC* phase. The transition enthalpy of 11 kJ mol−1 is almost equal to that of the ordered smectic-SmC* phase transition in other systems [18,21]. The phase transition from SmC* to IL gives rise to a second peak at 192 °C, which is the clearing temperature. On cooling scan, the metastable LC (SmXm) phase appeared transiently between the SmC* and SmX* phases. The DSC thermogram of (R,R)-2a reveals three endothermic peaks during the second heating scan (Figure 7b). The peak at 77 °C indicates an ordered smectic-SmC* phase transition, while the second small peak at 140 °C represents the SmC*–SmA* phase transition. (R,R)-2a exhibits the SmA* phase between 140 °C and 156 °C. Notably, no endothermic peaks corresponding to first-order phase transitions are apparent in the DSC thermogram of (R,R)-2b. The DSC study also indicates that (R,R)-2b is not an LC. Among the five compounds, (R,R)-3a shows the most complex phase-transition behavior. A few peaks appeared below 90 °C in the DSC thermogram of (R,R)-3a (Figure 7c). These peaks imply gradual disruption of the intralayer bond order in the ordered smectic phase. Such a multi-step transformation of the intralayer order would also be related to the emergence of metastable phases during the first cooling scan. (R,R)-3a exhibits the ordered smectic-SmC* phase transition at ~100 °C on the second heating process. Subsequently, the SmC*–SmA* phase transition occurs at 137 °C. The relatively stable SmA* phase was retained at ~180 °C. (R,R)-4a bearing a rigid ethynyl-bridged D–A core exhibits a crystalline (Cr) phase at ambient temperature. On the heating scan, the Cr phase was transformed into the SmC* phase at 92 °C, which was retained below the clearing temperature of 127 °C (Figure 7d).
The thermal phase-transition behaviors of fluorenone derivatives on the second heating scan are summarized in Table 1. While the fluorenone derivative (R,R)-2b is not an LC, the other four compounds exhibit an enantiotropic SmC* phase. Notably, direct modification with donor units on 2- and 7-positions of the fluorenone core stabilizes the SmA* phase. The asymmetrically modified fluorenone derivative (R)-1a exhibited the highest clearing temperature among the four LC fluorenone derivatives. Analysis of the UV–vis absorption spectra (Section 3.1, Figure 2) indicated that the π-conjugated system was well-developed owing to the higher planarity of the rigid mesogenic core in compound (R)-1a among the five chiral fluorenone derivatives. In addition, the free volume of motile side chains in (R)-1a would be smaller than the other compounds when the molecular structure is considered. Thus, the weak dynamic effect that disturbs the molecular packing and enhances intermolecular interactions between the central cores resulted in the high clearing temperature of (R)-1a. In the case of (R,R)-2b, the steric and dynamic effects of the bulky, chiral branched aliphatic chain interfered with aggregation and thus reduced the liquid crystallinity.

3.2.3. X-Ray Diffraction

To corroborate the identification of the LC phases, the four LC fluorenone derivatives were analyzed using XRD. The XRD profiles are shown in Figure 8. The diffraction profile at 109 °C (Figure 8a, upper) shows three diffraction peaks, which support the smectic layer structure. The layer spacing (L) in the SmC* phase of (R)-1a was experimentally estimated to be 29.8 Å. Because the shorter layer spacing than the calculated molecular length (34.5 Å) of (R)-1a by molecular mechanics (MM) methods indicates a tilt of the LC molecules relative to the layer normal, the XRD profile is consistent with the SmC* phase. The presumed tilt angle was ~30°. The XRD profiles of (R)-1a at 80 °C (Figure 8a, bottom) indicated the emergence of an ordered smectic (SmX*) phase below 109 °C. The value of L in the SmX* phase was 29.1 Å. The wide-angle diffractions (2θ > 15°) correspond to intralayer order. Therefore, the SmX* phase is likely a chiral smectic F (SmF*) or chiral smectic I (SmI*) phase [65,66]. In the diffraction profile of (R,R)-2a at 142 °C (Figure 8b, upper), periodic diffraction peaks in the small-angle region (2θ < 10°) indicate the LC layer structure. The value of L at 142 °C was estimated to be 31.6 Å, which is shorter than the molecular length of (R,R)-2a (36.4 Å) obtained by MM calculation. Based on the phase transition sequence and results of the POM analysis, the interdigitated SmA* structure was probably formed in the high-temperature LC phase. In the second smectic phase at 125 °C, the experimental value of L is 30.7 Å, which is smaller than the value of L at 142 °C (Figure 8b, middle). Because the shrinkage of L suggests that the molecules of (R,R)-2a are tilted to the layer normal in the LC phase at 125 °C, we consider that (R,R)-2a exhibits the SmC* phase in the temperature range of 77–140 °C on heating scan. In the XRD profile of (R,R)-2a at 30 °C (Figure 8b, bottom), several weak diffraction peaks are seen in the wide-angle region in addition to several small-angle diffractions. These wide-angle peaks correspond to intralayer bond order in the ordered smectic (SmX1*) phase. The XRD profile of (R,R)-3a at 180 °C (Figure 8c, upper) was consistent with the interdigitated-type SmA* structure, which is similar to the LC structure in the high-temperature smectic phase of (R,R)-2a. At 180 °C, the layer spacing L was 32.1 Å, which is shorter than the calculated molecular length of (R,R)-3a (37.2 Å) by MM methods. After the cooling-induced phase transition, L was shortened to 30.5 Å (Figure 8c, middle), implying the formation of a SmC* tilted layer structure at 130 °C. The XRD profile at 50 °C (Figure 8c, bottom) shows several peaks originating from the layered structure, including wide-angle peaks, which indicate intramolecular order. The complex XRD profile reflects the ordered smectic (SmX1*) phase that appears below 98 °C. The XRD profile of compound (R,R)-4a featured fewer peaks (Figure 8d, upper). Since the layer spacing L (31.1 Å) at 110 °C is shorter than the calculated molecular length of (R,R)-4a (42.2 Å), the XRD pattern at 110 °C is consistent with the SmC* phase. By contrast, the XRD profile at 35 °C indicated crystalline order below 92 °C (Figure 8d, bottom).

3.3. Ferroelectric Properties

We examined the ferroelectric properties of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a. All of these compounds exhibited ferroelectric hysteresis behavior in the SmC* phase (Figure 9). Although the spontaneous polarization (Ps) in the SmC* phase of (R,R)-2a reached 3.0 × 102 nC cm−2 at 110 °C, bilaterally asymmetric compound (R)-1a exhibited the lowest Ps among the four compounds (1.0 × 102 nC cm−2 at 150 °C). The Ps values for (R,R)-2a and (R,R)-3a were almost equal at 110 °C (~2.0 × 102 nC cm−2). Comparing the Ps values for the ferroelectric π-conjugated LCs in our previous work [18,21,23], each compound showed a sufficiently large Ps in the SmC* phase. Furthermore, the Ps value for each chiral fluorenone derivative outperformed those of classical FLCs, such as chiral alkyl 4-n-alkyloxybiphenyl-4′-carboxylates (12–36 nC cm−2) and chiral alkyl N-(4′-n-dodecyloxybenzylidene)-4-aminocinnamates (3–10 nC cm−2) [67,68]. The electrical dipole of the central carbonyl unit in the fluorenone core probably contributed to the large Ps value. The fluorine groups on the wings should have also affected ferroelectric polarization. For those ferroelectric fluorenone derivatives, the Ps tended to increase with decreasing temperature in the SmC* phase until reaching saturation (Figures S16–S19, Supplementary File). As oblique evidence of the change in molecular orientation in response to an external electric field, the optical texture change upon application of a voltage is shown in Figures S20–S23 (Supplementary File).

3.4. Carrier Transport Characteristics

The carrier transport characteristics of the LC phases of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a were evaluated using the cells with 25 μm gaps by the TOF method. For each LC sample, transient photocurrent curves of the positive and negative carriers were measured in the SmC* phases (Figure 10 and Figure 11). The typical double logarithmic plots of the transient curves for (R)-1a are displayed in Figure 10. In the photocurrent decays of (R)-1a, we found kink points that signify the transit times of both carriers (Figure 8a,b). The systematic shortening of these transit times with increasing applied voltage was found. The transient curves implied the ambipolar carrier transport in the SmC* phase of (R)-1a. While the mobility of negative carriers in the SmC* phases for (R)-1a at 160 °C was determined to be 4 × 10−5 cm2 V−1 s−1 (−200 V) from the transit times, the mobility of positive carriers at the same temperature reached ~6 × 10−5 cm2 V−1 s−1 (+200 V). The kink points also appeared in the photocurrent decays of each carrier in (R,R)-2a (Figure 11). Even in these two compounds, the transit time shortened with increasing applied voltage. The transient photocurrent profiles of (R,R)-2a, (R,R)-3a, and (R,R)-4a revealed ambipolar carrier transport in the SmC* phases of each compound. The mobilities of the positive and negative carriers in the FLC phase reached 10−5 cm2 V−1 s−1, which was on par with the hole and electron mobilities in the LC phases with loose π-stacks. Therefore, we considered that the transient photocurrent curves for the positive and negative carriers were attributed to hole and electron transport, respectively. In the SmC* phase of (R,R)-2a, the electron mobility was slightly higher than that of the holes (Table 2). Among the four fluorenone derivatives, the asymmetrically substituted fluorenone derivative (R)-1a showed the highest hole mobility in the SmC* phase. In addition to the thermal promotion of carrier hopping, the electron-donating thiophene unit containing a sulfur atom, which has a large atomic radius, probably contributed to improving the overlap of the molecular orbitals for the hole-conduction pathway. Comparing the electron mobilities in the SmC* phase, (R,R)-2a exhibited the highest mobility. As can be deduced from the XRD results, the (R,R)-2a molecules formed a relatively well-ordered layer structure in the SmC* phase among the four compounds. Therefore, the realization of significant orbital overlap between neighboring central fluorenone cores contributed to the high electron mobility. Similarly, the determination of carrier mobility in the ordered smectic phase was attempted. However, the obtained transient curves for the electronic charge carriers were dispersive decays. Therefore, the determination of the transient time for the hole and electron unfortunately proved difficult.

3.5. Current–Voltage Characteristics

The LC fluorenone derivatives subject to evaluation of current–voltage characteristics were narrowed down to (R)-1a and (R,R)-2a based on the spontaneous polarization and carrier mobility. In order to reduce the impact of the molecular reorientation induced by the external bias voltage, the fixation of the polar structure in the electrically poled SmC* phase was required. While insufficient polarization retention in (R)-1a, we succeeded in the partial fixation of polar structure in the SmC* phase of (R,R)-2a under the application of DC voltage. Thus, current–voltage characteristics were evaluated for only (R,R)-2a. Figure 12 depicts the current density–voltage (J–V) curves in polarized ordered smectic states for (R,R)-2a under the white LED light illumination (20 mW cm−2). The direction of the internal electric field induced by the external bias on the poling treatment is clearly reflected in the J–V curves. The polarity inversion in the current–voltage characteristics was found. The unique features are probably based on the BPVE. In the positively polarized state, the open-circuit voltage (Voc) and short-circuit current density (Jsc) were approximately +0.8 V and −0.05 μA cm−2, respectively. On the other hand, the Voc in the reversed negatively polarized state was −0.2 V. The Jsc in the negatively polarized state was +0.01 μA cm−2. The low performance probably resulted from imperfect polarization fixation and low carrier transport efficiency. Although these values were inadequate for the application for solar cells, the large Voc shift of ~1.0 V should be profitable for multifunctional devices such as integrated switch, integrated memory, and logic gate that are controlled by temperature, external electric field, and light.

4. Conclusions

In this study, five chiral fluorenone derivatives were synthesized. Among them, only (R,R)-2b was a non-LC compound. The other fluorenone derivatives exhibited chiral smectic LC phases, including the SmC* phase. This indicates that the branched structure of the chiral chain strongly affected the emergence of LC phases in this system. The introduction of chiral (2-octyloxy)phenyl units on the 2- and 7-positions of the 9-fluorenone core promoted the formation of the SmA* structure. The dielectric polarization measurements revealed that (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a exhibited ferroelectric behavior in their SmC* phases, with planar orientations in 2 µm thick LC cells. In particular, the fluorenone derivative (R,R)-2a exhibited the largest spontaneous polarization of the four compounds. The FLC fluorenone derivatives synthesized in this study exhibited ambipolar carrier transport in the SmC* phases. In their SmC* phases, the hole and electron mobilities were on the order of 10−5 cm2 V−1 s−1, which is on par with the carrier mobilities in the SmC and SmC* phases of conventional LC semiconductors. Although the hole mobility was slightly higher than the electron mobility in the SmC* phase of (R)-1a, the electron mobility was higher than the hole mobility in the SmC* phases of (R,R)-2a, (R,R)-3a, and (R,R)-4a. The UV–vis absorption spectra in dilute THF solution revealed that (R,R)-4a possessed the most favorable visible-light-absorption properties owing to the expansion of the π-conjugation system by the insertion of the ethynyl linker. We believe that these findings can inform the molecular design of novel ferroelectric π-conjugated LCs for the application of next-generation organic optoelectronic and electro-optic devices.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cryst15121021/s1, Scheme S1: Synthesis of (R)-1a, (R,R)-2a, (R,R)-2b, (R,R)-3a, and (R,R)-4a; Figure S1: 1H NMR spectrum of (R)-1a; Figure S2: 13C NMR spectrum of (R)-1a; Figure S3: 1H NMR spectrum of (R,R)-2a; Figure S4: 13C NMR spectrum of (R,R)-2a; Figure S5: 1H NMR spectrum of (R,R)-2b; Figure S6: 13C NMR spectrum of (R,R)-2b; Figure S7: 1H NMR spectrum of (R,R)-3a; Figure S8: 13C NMR spectrum of (R,R)-3a; Figure S9: 1H NMR spectrum of (R,R)-4a; Figure S10: 13C NMR spectrum of (R,R)-4a; Figure S11: High-resolution ESI mass spectrum of (R)-1a; Figure S12: High-resolution ESI mass spectrum of (R,R)-2a; Figure S13: High-resolution ESI mass spectrum of (R,R)-2b; Figure S14: High-resolution ESI mass spectrum of (R,R)-3a; Figure S15: High-resolution ESI mass spectrum of (R,R)-4a; Figure S16: (a) Polarization hysteresis loops and (b) the Ps as a function of temperature in the SmC* phase of (R)-1a; Figure S17: (a) Polarization hysteresis loops and (b) the Ps as a function of temperature in the SmC* phase of (R,R)-2a; Figure S18: (a) Polarization hysteresis loops and (b) the Ps as a function of temperature in the SmC* phase of (R,R)-3a; Figure S19: (a) Polarization hysteresis loops and (b) the Ps as a function of temperature in the SmC* phase of (R,R)-4a. Figure S20: Changes in POM textures by the application of DC bias voltage to the LC cell of (R)-1a at 110 °C; Figure S21: Changes in POM textures by the application of DC bias voltage to the LC cell of (R,R)-2a at 110 °C; Figure S22: Changes in POM textures by the application of DC bias voltage to the LC cell of (R,R)-3a at 110 °C; Figure S23: Changes in POM textures by the application of DC bias voltage to the LC cell of (R,R)-4a at 110 °C.

Author Contributions

Conceptualization, A.S.; methodology, A.S.; validation, S.-u.D., S.Y. and A.S.; formal analysis, S.-u.D., S.Y. and A.S.; investigation, S.-u.D., S.Y. and A.S.; resources, K.A. and A.S.; data curation, S.-u.D., S.Y. and A.S.; writing—original draft preparation, S.-u.D. and A.S.; writing—review and editing, A.S., S.-u.D., S.Y. and K.A.; visualization, A.S. and S.-u.D.; supervision, K.A. and A.S.; project administration, A.S.; funding acquisition, K.A. and A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by a Grant-in-Aid for Early-Career Scientists (No. 24K17746) from the Japan Society for the Promotion of Science (JSPS), and a Research Grant (2023, No. 337) from the JKA, Japan, for A.S. This study was also financially supported by a research fund from the Tokyo University of Science for K.A. and A.S.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank M. Funahashi (Kobe University) for helping with electronic measurements, and Y. Yoshimura (Tokyo University of Science) for helping with HRMS measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Molecular structures of the chiral fluorenone derivatives.
Figure 1. Molecular structures of the chiral fluorenone derivatives.
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Figure 2. UV–vis absorption spectra of chiral fluorenone derivatives (a) in THF solution (10 μM) and of (b) bulk sample at room temperature in a 2 μm thick cell.
Figure 2. UV–vis absorption spectra of chiral fluorenone derivatives (a) in THF solution (10 μM) and of (b) bulk sample at room temperature in a 2 μm thick cell.
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Figure 3. POM images of (R)-1a at (a) 185 °C and (b) 90 °C upon cooling.
Figure 3. POM images of (R)-1a at (a) 185 °C and (b) 90 °C upon cooling.
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Figure 4. POM images of (R,R)-2a at (a) 153 °C, (b) 135 °C, and (c) 50 °C upon cooling.
Figure 4. POM images of (R,R)-2a at (a) 153 °C, (b) 135 °C, and (c) 50 °C upon cooling.
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Crystals 15 01021 g005
Figure 5. POM images of (R,R)-3a at (a) 170 °C, (b) 110 °C, (c) 80 °C, and (d) 44 °C upon cooling.
Figure 5. POM images of (R,R)-3a at (a) 170 °C, (b) 110 °C, (c) 80 °C, and (d) 44 °C upon cooling.
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Crystals 15 01021 g006
Figure 6. POM images of (R,R)-4a at (a) 95 °C, and (b) 65 °C upon cooling.
Figure 6. POM images of (R,R)-4a at (a) 95 °C, and (b) 65 °C upon cooling.
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Crystals 15 01021 g007
Figure 7. DSC thermograms of (a) (R)-1a, (b) (R,R)-2a, (c) (R,R)-3a, and (d) (R,R)-4a at a scan rate of 10 °C min−1. Cr, IL, SmA*, and SmC* denote the crystal, isotropic liquid, chiral smectic A, and chiral smectic C phases, respectively. The abbreviations SmX, SmX1*, SmX2*, and SmX3* denote unidentified ordered chiral smectic phases. The abbreviations Crm, and SmXm denote metastable crystal and metastable chiral smectic phases, respectively.
Figure 7. DSC thermograms of (a) (R)-1a, (b) (R,R)-2a, (c) (R,R)-3a, and (d) (R,R)-4a at a scan rate of 10 °C min−1. Cr, IL, SmA*, and SmC* denote the crystal, isotropic liquid, chiral smectic A, and chiral smectic C phases, respectively. The abbreviations SmX, SmX1*, SmX2*, and SmX3* denote unidentified ordered chiral smectic phases. The abbreviations Crm, and SmXm denote metastable crystal and metastable chiral smectic phases, respectively.
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Figure 8. XRD profiles of (a) (R)-1a, (b) (R,R)-2a, (c) (R,R)-3a, and (d) (R,R)-4a at varying temperatures.
Figure 8. XRD profiles of (a) (R)-1a, (b) (R,R)-2a, (c) (R,R)-3a, and (d) (R,R)-4a at varying temperatures.
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Figure 9. Polarization hysteresis loops of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a in the SmC* phases. The samples were planar oriented in 2-μm-thick LC cells. Triangular-wave bias (±5 V, ±25 kV cm−1, 100 Hz) was applied to the cells.
Figure 9. Polarization hysteresis loops of (R)-1a, (R,R)-2a, (R,R)-3a, and (R,R)-4a in the SmC* phases. The samples were planar oriented in 2-μm-thick LC cells. Triangular-wave bias (±5 V, ±25 kV cm−1, 100 Hz) was applied to the cells.
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Figure 10. Double logarithmic plots of transient photocurrent curves for (a) positive and (b) negative carriers in the SmC* phase (160 °C) of (R)-1a. The black arrows in the plots indicate kink points corresponding to the transit times, that is, the time required for the charge carriers to reach the counter electrode.
Figure 10. Double logarithmic plots of transient photocurrent curves for (a) positive and (b) negative carriers in the SmC* phase (160 °C) of (R)-1a. The black arrows in the plots indicate kink points corresponding to the transit times, that is, the time required for the charge carriers to reach the counter electrode.
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Figure 11. Double logarithmic plots of transient photocurrent curves for (a) positive and (b) negative carriers in the SmC* phase (120 °C) of (R,R)-2a. The black arrows in the plots indicate kink points corresponding to the transit times, that is, the time required for the charge carriers to reach the counter electrode.
Figure 11. Double logarithmic plots of transient photocurrent curves for (a) positive and (b) negative carriers in the SmC* phase (120 °C) of (R,R)-2a. The black arrows in the plots indicate kink points corresponding to the transit times, that is, the time required for the charge carriers to reach the counter electrode.
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Crystals 15 01021 g012
Figure 12. Current density–voltage curves for (R,R)-2a in polarized ordered smectic states at 60 °C under illumination of white LED light (20 mW cm−2).
Figure 12. Current density–voltage curves for (R,R)-2a in polarized ordered smectic states at 60 °C under illumination of white LED light (20 mW cm−2).
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Table 1. Phase-transition behavior of chiral fluorenone derivatives.
Table 1. Phase-transition behavior of chiral fluorenone derivatives.
CompoundPhase Transition Temperature/°C (Enthalpy/kJ mol−1) 1
(R)-1aSmX* 109 (11) SmC* 192 (8) IL
(R,R)-2aSmX1* 77 (16) SmC* 140 (1) SmA* 156 (3) IL
(R,R)-2bG 44 IL
(R,R)-3aSmX3* −33 (2) SmX2* 79 (4) SmX1* 98 (11) SmC* 137 (1) SmA* 183 (2) IL
(R,R)-4aCr 92 (23) SmC* 127 (2) IL
1 The phase transition temperature and enthalpy were estimated from the second heating scans of DSC thermograms.
Table 2. Carrier mobilities of chiral fluorenone derivatives.
Table 2. Carrier mobilities of chiral fluorenone derivatives.
CompoundT/°CV/Vμ/cm2 V−1 s−1
(Positive Carrier)		V/Vμ/cm2 V−1 s−1
(Negative Carrier)
(R)-1a160+2006 × 10−5−2004 × 10−5
(R,R)-2a120+2003 × 10−5−2006 × 10−5
(R,R)-3a130+2001 × 10−5−2001 × 10−5
(R,R)-4a120+2001 × 10−5−2001 × 10−5
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Doi, S.-u.; Yamada, S.; Aoki, K.; Seki, A. Ferroelectric Properties and Ambipolar Carrier Transport of 9-Fluorenone-Based Liquid Crystals. Crystals 2025, 15, 1021. https://doi.org/10.3390/cryst15121021

AMA Style

Doi S-u, Yamada S, Aoki K, Seki A. Ferroelectric Properties and Ambipolar Carrier Transport of 9-Fluorenone-Based Liquid Crystals. Crystals. 2025; 15(12):1021. https://doi.org/10.3390/cryst15121021

Chicago/Turabian Style

Doi, Sou-un, Syota Yamada, Ken’ichi Aoki, and Atsushi Seki. 2025. "Ferroelectric Properties and Ambipolar Carrier Transport of 9-Fluorenone-Based Liquid Crystals" Crystals 15, no. 12: 1021. https://doi.org/10.3390/cryst15121021

APA Style

Doi, S.-u., Yamada, S., Aoki, K., & Seki, A. (2025). Ferroelectric Properties and Ambipolar Carrier Transport of 9-Fluorenone-Based Liquid Crystals. Crystals, 15(12), 1021. https://doi.org/10.3390/cryst15121021

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