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Article

Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase

by
Yuki Arakawa
* and
Takuma Shiba
Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441-8580, Aichi, Japan
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(1), 69; https://doi.org/10.3390/cryst16010069
Submission received: 29 December 2025 / Revised: 15 January 2026 / Accepted: 16 January 2026 / Published: 21 January 2026
(This article belongs to the Special Issue State-of-the-Art Liquid Crystals Research in Japan (2nd Edition))
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Abstract

Bent-shaped liquid crystal (LC) dimers, trimers, and oligomers are intriguing because of their unique liquid crystallinities, which have gained further impetus after the identification of the twist-bend nematic (NTB) phase in these molecules. LC trimers exhibiting the NTB phase still remain relatively rare compared to the predominant LC dimers. We report the first homologs of selenium-linked LC trimers, 4,4′-bis[ω-(4-cyanobiphenyl-4′-ylseleno)alkoxy]biphenyls (CBSenOBOnSeCB) with carbon numbers in the alkyl-chain spacers, n = 7 or 9). Polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD) measurements were performed to investigate the phase transition behavior and mesophase structures of the trimers. Both CBSenOBOnSeCB trimers exhibited nematic (N) and NTB phases. The XRD measurements revealed the presence of smectic A-like cybotactic clusters with a triply intercalated structure in the N and NTB phases. The LC phase transition temperatures of CBSenOBOnSeCB were lower than those of the already-known ether-linked CBOnOBOnOCB and thioether-linked CBSnOBOnSCB counterparts. This trend is ascribed to the enhanced molecular bending and molecular flexibility of CBSenOBOnSeCB, which are caused by the smaller bond angle and greater bond flexibility of C–Se–C compared to C–O–C and C–S–C. This study offers a new molecular design for multiply linked LC oligomers with heavier chalcogen atoms.

1. Introduction

Generally, liquid crystal (LC) dimers, trimers, tetramers, and higher oligomers alternately and linearly comprise rigid mesogenic groups and flexible alkyl spacers. An intriguing feature of them is the odd-even effects of the number of alkyl chain atoms on various LC properties, which are associated with different molecular shapes caused by the relative orientations of the rigid mesogenic groups. Even- and odd-numbered spacers render molecular shapes relatively linear and bent, respectively. Odd ones, particularly bent-shaped dimers, have drawn significant attention due to their rich liquid crystallinities associated with ferroelectricity, frustration, and chirality [1,2,3,4,5,6,7,8].
Over the last decade, the identification of the twist-bend nematic (NTB) phase has provided further impetus for research on bent-shaped LC dimers, trimers, and higher oligomers [9,10,11,12]. The NTB phase features heliconical structures with very short pitches of approximately 10 nm [13,14,15,16]. Despite its helicity (i.e., chirality), this phase can be induced by achiral bent-shaped molecules. In fact, two domains of right- and left-handed helices equally coexist so that the NTB phase is macroscopically achiral [17]. Optical-electrical and viscoelastic applications using twist-bend nematogenic dimers have been examined [18,19,20,21,22,23,24,25,26,27,28,29,30]. Theoretical and computational studies investigated the NTB phase and potential phase sequences for bent-shaped molecules in terms of different approaches [31,32,33,34,35,36,37,38,39,40,41]. However, the nanoscopic structure of the NTB phase still remains an open question and requires further investigation [42,43,44].
The overwhelming majority of twist-bend nematogens are LC dimers [45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71]. In contrast, relatively fewer studies have explored higher oligomeric LC analogs such as trimers [72,73,74,75,76], tetramers [73,77,78,79,80], hexamers [78], non-linear oligomers [81,82,83], and polymers [84], as well as bent-core molecules [85,86]. The key to the NTB phase is molecular bending, one of whose crucial factors is the type of linkage between the rigid mesogenic groups and flexible alkyl spacers [87], such as methylene, ether, and ester. For example, twist-bend nematogens typically consist of methylene linkages (C–CH2–C angle: ~110°), whereas ether-linked dimers (C–O–C angle: ~118°) are relatively unfavorable for the NTB phase in terms of molecular bending. The smaller the linkage angle, the more preferable it is for the twist-bend nematogen, at least to some extent. Heavier chalcogen linkages, such as thioether (C–S–C) and selenoether (C–Se–C), have smaller inner angles (approximately 100°) than methylene (C–CH2–C) and ether (C–O–C) linkages, which is due to the deviation from sp3 orbital hybridization. When placed between a mesogenic group and an alkyl spacer, they can make the dimers and other oligomers curved more than methylene and ether [88,89,90]. Accordingly, many thioether- [88,90,91,92,93,94,95,96,97,98,99,100,101,102,103] and selenoether-linked LC dimers [89,102] form the NTB phase. Thioether-linked LC trimers also exhibit the NTB phase, in which outer linker positions are favorable [75,76,93], as shown in Figure 1 for cyanobiphenyl-based LC trimers (CBSnOBOnSCB). Actually, odd homologs of ether-linked ones (CBOnOBOnOCB) also exhibit the NTB phase [74]. However, selenium-linked LC trimers have not yet been reported. LC trimers can bridge relatively smaller dimers and larger oligomers toward polymers; therefore, understanding them is important. Furthermore, the synthesis of selenium-containing LC molecules is useful for analyzing detailed LC phase structures using resonant X-ray scattering focused on the selenium atom [104,105,106,107,108,109,110,111,112,113].
In this context, we synthesized two homologs of bent-shaped selenoether-linked CBSenOBOnSeCB trimers possessing an odd number of carbon atoms in flexible spacers (n = 7 and 9). The phase transitions and characterization of these trimers were investigated using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) measurement, and compared with those of the corresponding ether-linked CBOnOBOnOCB [74] and thioether-inked CBSnOBOnSCB [75] trimers.

2. Materials and Methods

All reagents were purchased from commercial suppliers, Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), FUJIFILM Wako Pure Chemical Co. (Osaka, Japan), Kanto Chemical Co., Inc. (Tokyo, Japan), and Nacalai Tesque Inc. (Kyoto, Japan), and used as received. The CBSenOBOnSeCB trimers (n = 7 and 9) were synthesized according to Scheme 1, for which the procedures and molecular characterization data are provided in the Supplementary Materials. The synthesis of the BrnSePhBr intermediates has been previously reported in our earlier work [102]. The molecular structures were characterized using 1H and 13C NMR spectroscopy with a JEOL JNM-ECS400 or JNM ECS 500 spectrometer (JEOL Ltd., Tokyo, Japan) and using high-resolution mass spectrometry (HRMS) with a Bruker compact spectrometer (Bruker Co., Billerica, MA, USA). Phase transitions and identification were determined by POM using an Olympus BX50 microscope (Olympus, Tokyo, Japan) with a Linkam LK-600 PM hot stage (Linkam Scientific Instruments Ltd., Salfords, UK) and an Olympus BX51 microscope (Olympus Co., Japan) with a Mettler Toledo HS82 hot stage (Mettler Toledo International Inc., Zurich, Switzerland). A glass cell with a thickness of 3 μm, uniaxially rubbed polyimide surfaces, and planar anchoring was purchased from EHC Co. (Tokyo, Japan). The phase transition temperatures and associated enthalpy changes (ΔH) were measured using a Shimadzu DSC-60 plus differential scanning calorimeter at a rate of 10 °C min−1 under a nitrogen gas flow of 50 mL min−1 (Shimadzu Co., Kyoto, Japan). XRD measurements were performed using a Bruker D8 DISCOVER diffractometer equipped with a VÅNTEC-500 detector and a Cu Kα X-ray source (Bruker Co., USA). The CBSe9OBO9SeCB sample was placed in a 1.5 mm diameter glass capillary tube (WJM-Glas Müller GmbH, Berlin, Germany). The capillary tube was sandwiched between permanent magnets in a holder and set in a temperature controller. X-ray diffraction was detected upon cooling from the isotropic (Iso) phase temperature of the sample. X-rays were irradiated for 300 and 150 s in the N and NTB phases, respectively.

3. Results and Discussion

The phase transition results for CBSenOBOnSeCB (n = 7 and 9) are summarized in Table 1. Upon heating, CBSe7OBO7SeCB did not display any mesophases (Figure S1), whereas CBSe9OBO9SeCB exhibited a conventional nematic (N) phase at ~136 °C (Figure S2). Upon cooling, both trimers exhibited N and NTB phases; thus, their NTB phases were monotropic. The optical textures of the trimers in non-surface-treated glass cells are shown in Figure 2. Upon cooling from the Iso phase, the marble and schlieren textures of the conventional N phase appeared and transformed into blocky textures typical of the NTB phase. Specifically, the CBSe7OBO7SeCB sample mainly crystallized during POM and DSC, and its NTB formation (~95 °C) was determined in small supercooled N-phase droplets using POM, as shown in Figure 2a–c. In contrast, the N–NTB phase transition temperature (TNNTB) of CBSe9OBO9SeCB (98.6 °C) was higher than its crystallization temperature (92.5 °C). Thus, its NTB texture is easy to observe (Figure 2d,e), and the N–NTB phase transition was detected as a second-order-like baseline shift in the DSC curves (Figure 3). POM for CBSe9OBO9SeCB was also performed using a uniaxially and antiparallelly rubbed polyimide surface cell with planar anchoring, as shown in Figure 4. In the N phase, uniform birefringent color and dark textures were observed when the rubbing direction of the cell was oriented at 45° and 0°, respectively, to the polarization directions of the two polarizers with crossed Nicols. This ensured the uniaxial orientation of the N director of the trimer sample along the rubbing direction. When entering into the NTB phase temperature, pale stripes appeared along the rubbing direction, which became apparent as the temperature decreased. This stripe texture supports the NTB phase formation, which could be ascribed to the undulation of the pseudo-layers or Helfrich–Hurault buckling instability [114,115].
In addition, we performed XRD measurements in the N and NTB phases of the CBSe9OBO9SeCB sample under a magnetic field. The obtained 2D- and 1D-XRD patterns are shown in Figure 5. In the N phase at 130 °C, two wide-angle diffractions centered at 2θ = 19.2° were observed on the equator. These diffractions originated from the lateral intermolecular correlation and ensured that the N director oriented the magnetic field direction, as indicated by the double-headed arrow in Figure 5a. The real d-spacing value of the wide-angle diffraction (dWAX) was 4.6 Å, which is typical of the N phase. The dWAX value decreased with decreasing temperature, indicating that the intermolecular distance decreased. In addition, two small-angle diffractions centered at 2θ = 5.5° were observed on the meridian. The real d-spacing value of the small-angle diffraction (dSAX) was ~16.1 Å, which is approximately one-third of the molecular length of the trimer as a triply intercalated intermolecular arrangement [74,75,116]. These small-angle diffractions are caused by the presence of local smectic-like molecular assemblies in the N phase, known as the cybotactic N (NCyb) phase. Because the diffraction angles do not split on the meridian, the NCyb phase should contain non-tilted smectic A (SmA) clusters, as observed for other cyanobiphenyl-based LC trimers [74,75]. The small-angle diffraction intensity increased relative to the wide-angle diffraction intensity as the temperature decreased in the N phase, suggesting the growth of cybotactic clusters. The small-angle diffraction was also detected at 2θ = 5.3° in the NTB phase. This indicates that the cybotactic correlation persists in the NTB phase. The X-ray diffraction intensity in the NTB phase was lower than that in the N phase because of the shorter X-ray irradiation time for the NTB phase than that for the N phase to avoid sample crystallization.
Finally, we compared the phase transition behavior and XRD results of CBSenOBOnSeCB (n = 7 and 9) with those of previously reported ether-linked CBOnOBOnOCB and thioether-linked CBSnOBOnSCB trimers (n = 7 and 9), all of which have the same alkyl chain length. The CBOnOBOnOCB and CBSnOBOnSCB data were obtained from our previous reports [74] and [75], respectively. The Iso–N phase transition temperature (TIN) and TNNTB values of the trimers are plotted in Figure 6 as a function of n. It is apparent that both TIN and TNNTB values follow the order CBOnOBOnOCB (ether) > CBSnOBOnSCB (thioether) > CBSenOBOnSeCB (selenoether). In addition, Figure 7 shows the entropy changes at the I–N phase transitions (ΔSIN) of the trimers, which are in the same order as TIN and TNNTB. This trend could be a manifestation of the orders for molecular bending (biaxiality) and flexibility of the LC trimers with different linkages [89,93,117]. In general, the higher the chalcogen atom (X), the smaller the bond angles with carbon atoms (i.e., C–X–C) and the higher their bond flexibility because of the deviation from sp3 hybridization and longer bond lengths. The most flexible and smallest-angle C–Se–C bond makes the LC trimers more bent and flexible, whereas C–O–C renders the dimers more linear and rigid; therefore, C–S–C should lead to the intermediates. This trend is consistent with those of ether-, thioether-, and selenoether-linked LC dimers [89,102].
The dWAX and dSAX values of the N phases of the CBO9OBO9OCB, CBS9OBO9SCB, and CBSe9OBO9SeCB trimers are plotted as a function of the shifted temperature (ΔT = TIN–T) in Figure 8. CBO9OBO9OCB and CBS9OBO9SCB also exhibit N and NTB phases with triple intercalation in the cybotactic A clusters. Although the dWAX values of the three trimers were nearly comparable, they decreased in the order CBO9OBO9OCB > CBS9OBO9SCB > CBSe9OBO9SeCB in the N phase. This suggests that the heavier the chalcogen atom, the closer the intermolecular distances at similar ΔT values. The dSAX values of selenium-linked CBSe9OBO9SeCB were smaller than those of CBO9OBO9OCB and CBS9OBO9SCB, whose values were relatively comparable. The cause of this is not yet clear but could be associated with molecular bending.

4. Conclusions

We synthesized bent-shaped selenium-linked LC trimers, CBSenOBOnSeCB (n = 7 and 9), and investigated their phase transitions and mesophase structures. Both trimers exhibited N and NTB phases. XRD measurements revealed non-split small-angle diffractions in the fluid N and NTB phases, indicating the presence of SmA-like cybotactic clusters with triple intercalation. The TIN and TNNTB values of CBSenOBOnSeCB were lower than those of the ether-linked CBOnOBOnOCB and thioether-linked CBSnOBOnSCB counterparts, which was attributed to the molecular bending and flexibility increased by the selenoether linkage. This study introduces a new molecular design featuring selenium-linked bent-shaped LC trimers, which can incorporate other mesogenic structures and holds the potential for the development of multiply linked higher LC oligomers with heavy chalcogen atoms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst16010069/s1, Figure S1: DSC curves of CBSe7OBO7SeCB at a rate of 10 °C/min; Figure S2: A POM image of the N phase at 136 °C upon heating of CBSe9OBO9SeCB.

Author Contributions

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

Funding

This research was funded by the Japan Society for the Promotion of Science (KAKENHI grant numbers 17K14493 and 20K15351).

Data Availability Statement

The data for this study are presented in the article and the Supplementary Materials.

Acknowledgments

We would like to thank Masatoshi Tokita for providing us with the opportunity to conduct XRD measurements at Tokyo Institute of Technology (presently Institute of Science Tokyo). We are also grateful to T. Shiokawa at Division of Instrumental Analysis, Okayama University, for the HRMS measurements. The NMR measurements were performed using shared equipment of the Cooperative Research Facility Center (CRFC), Toyohashi University of Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 16 00069 g001
Figure 1. Molecular structures of ether-, thioether-, and selenoether-linked cyanobiphenyl-based liquid crystal trimers (CBOnOBOnOCB, CBSnOBOnSCB, and CBSenOBOnSeCB, respectively).
Figure 1. Molecular structures of ether-, thioether-, and selenoether-linked cyanobiphenyl-based liquid crystal trimers (CBOnOBOnOCB, CBSnOBOnSCB, and CBSenOBOnSeCB, respectively).
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Crystals 16 00069 sch001
Scheme 1. Synthesis route for CBSenOBOnSeCB.
Scheme 1. Synthesis route for CBSenOBOnSeCB.
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Figure 2. Optical textures of (a) the N phase at 110 °C, (b) the NTB phase at 95 °C, and (c) the Cr phase at 92 °C upon cooling of CBSe7OBO7SeCB, and (d) the N phase at 120 °C, (e) the NTB phase at 96 °C, and (f) the Cr phase at 64 °C upon cooling of CBSe9OBO9SeCB. P and A in the panel (a) refer to the polarization directions of the polarizer and analyzer, respectively. The scale bar, P, and A in the panel (a) apply to the other panels.
Figure 2. Optical textures of (a) the N phase at 110 °C, (b) the NTB phase at 95 °C, and (c) the Cr phase at 92 °C upon cooling of CBSe7OBO7SeCB, and (d) the N phase at 120 °C, (e) the NTB phase at 96 °C, and (f) the Cr phase at 64 °C upon cooling of CBSe9OBO9SeCB. P and A in the panel (a) refer to the polarization directions of the polarizer and analyzer, respectively. The scale bar, P, and A in the panel (a) apply to the other panels.
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Crystals 16 00069 g003
Figure 3. DSC curves of CBSe9OBO9SeCB at a rate of 10 °C min−1.
Figure 3. DSC curves of CBSe9OBO9SeCB at a rate of 10 °C min−1.
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Figure 4. Optical textures of CBSe9OBO9SeCB in a 3 μm thickness cell with uniaxially rubbed polyimide surfaces and planar anchoring: (a) N phase at 100 °C and NTB phase at (b) 95 and (c) 90 °C. The scale bar, rubbing direction (R), P and A in the panel (a) apply to the other panels.
Figure 4. Optical textures of CBSe9OBO9SeCB in a 3 μm thickness cell with uniaxially rubbed polyimide surfaces and planar anchoring: (a) N phase at 100 °C and NTB phase at (b) 95 and (c) 90 °C. The scale bar, rubbing direction (R), P and A in the panel (a) apply to the other panels.
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Figure 5. 2D-XRD patterns in the (a) N and (b) NTB phases and (c) 1D-XRD patterns in the N and NTB phases of CBSe9OBO9SeCB. The magnetic field direction indicated by the double headed arrow in the panel (a) applies to the panel (b).
Figure 5. 2D-XRD patterns in the (a) N and (b) NTB phases and (c) 1D-XRD patterns in the N and NTB phases of CBSe9OBO9SeCB. The magnetic field direction indicated by the double headed arrow in the panel (a) applies to the panel (b).
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Figure 6. (a) Iso–N phase transition temperature (TIN) and (b) N–NTB phase transition temperature (TNNTB) of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (n = 7 and 9) (circles), represented by O, S, and Se, respectively.
Figure 6. (a) Iso–N phase transition temperature (TIN) and (b) N–NTB phase transition temperature (TNNTB) of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (n = 7 and 9) (circles), represented by O, S, and Se, respectively.
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Figure 7. Entropy changes at the Iso–N phase transition (ΔSIN) of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (n = 7 and 9) (circles), represented by O, S, and Se, respectively.
Figure 7. Entropy changes at the Iso–N phase transition (ΔSIN) of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (n = 7 and 9) (circles), represented by O, S, and Se, respectively.
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Crystals 16 00069 g008
Figure 8. The d-spacings of (a) the wide-angle diffraction (dWAX) and (b) the small-angle diffraction (dSAX) in the N phases of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (circles) (n = 7 and 9), indicated by O, S, and Se, respectively.
Figure 8. The d-spacings of (a) the wide-angle diffraction (dWAX) and (b) the small-angle diffraction (dSAX) in the N phases of CBOnOBOnOCB (squares) [74], CBSnOBOnSCB (triangles) [75], and CBSenOBOnSeCB (circles) (n = 7 and 9), indicated by O, S, and Se, respectively.
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Table 1. Melting point (Tm, °C) upon 2nd heating, phase sequence, phase transition temperature (°C), and their associated enthalpy change (ΔH, in brackets, kJ mol−1) upon cooling of CBSenOBOnSeCB.
Table 1. Melting point (Tm, °C) upon 2nd heating, phase sequence, phase transition temperature (°C), and their associated enthalpy change (ΔH, in brackets, kJ mol−1) upon cooling of CBSenOBOnSeCB.
Trimer CodeTm [ΔH]Phase Transition Temperature [ΔH] Upon Cooling
CBSe7OBO7SeCB156.7 [65.4]Cr111.6 [59.0]NTB95 aN134.9 [1.9]Iso
CBSe9OBO9SeCB135.2 [53.4]Cr92.5 [29.1]NTB98.6 [-]N135.8 [3.8]Iso
a Determined by POM.
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Arakawa, Y.; Shiba, T. Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase. Crystals 2026, 16, 69. https://doi.org/10.3390/cryst16010069

AMA Style

Arakawa Y, Shiba T. Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase. Crystals. 2026; 16(1):69. https://doi.org/10.3390/cryst16010069

Chicago/Turabian Style

Arakawa, Yuki, and Takuma Shiba. 2026. "Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase" Crystals 16, no. 1: 69. https://doi.org/10.3390/cryst16010069

APA Style

Arakawa, Y., & Shiba, T. (2026). Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase. Crystals, 16(1), 69. https://doi.org/10.3390/cryst16010069

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