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Article

Cyanobiphenyl- and Cyanoterphenyl-Based Liquid Crystal Dimers (CBnCT): The Enantiotropic Twist-Bend Nematic Phase

by
Yamato Shimoura
and
Yuki Arakawa
*
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 2025, 15(2), 120; https://doi.org/10.3390/cryst15020120
Submission received: 20 December 2024 / Revised: 20 January 2025 / Accepted: 21 January 2025 / Published: 23 January 2025
(This article belongs to the Special Issue Advances in Liquid Crystal Dimers and Oligomers)
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Abstract

:
We report the first homologous series of methylene-linked cyanobiphenyl- and cyanoterphenyl-based liquid crystal (LC) dimers (CBnCT). To induce the heliconical twist-bend nematic (NTB) phase through bent molecular shapes, the CBnCT homologs have an odd-numbered flexible alkylene spacer (n) ranging from 1 to 17. Polarized optical microscopy and differential scanning calorimetry are used to identify phases and analyze the phase-transition behavior. Except for n = 1, all the CBnCT homologs exhibit the conventional nematic (N) and NTB phases. The CBnCT dimers with n = 3 and 5 show a monotropic NTB phase, while those with n = 7, 9, 11, 13, 15, and 17 demonstrate an enantiotropic NTB phase below the conventional N phase temperature. The NTB phases of the CBnCT dimers (n = 7, 9, and 11) remain stable down to room temperature and vitrify without crystallization. Compared with cyanobiphenyl-based LC dimer homologs (CBnCB), the CBnCT dimers show significantly broader N and NTB phase temperature ranges with higher isotropic and NTB–N phase-transition temperatures. The NTB phase temperature ranges of CBnCT (n = 7, 9, 11, and 13) are over 100 °C. Additionally, more CBnCT homologs exhibit the enantiotropic NTB phase than the CBnCB ones. These enhancements result from increased π-conjugation and asymmetric molecular structures. Furthermore, CB9CT exhibits higher birefringence than CB9CB owing to its longer π-conjugated terphenyl moiety.

1. Introduction

Liquid crystal (LC) dimers comprise two rigid mesogenic units linked by a central aliphatic spacer chain. Depending on whether the mesogenic structures are identical or different and whether the spacer is symmetric or asymmetric, LC dimers are categorized as symmetric or asymmetric. The first report on LC dimers dates back to the early 1900s [1]. Embryonic studies primarily focused on LC dimers as simplified models for main-chain LC polymers, in which rigid mesogenic structures alternate with aliphatic chains [2,3,4]. One notable feature of LC dimers is the odd–even effect, which is influenced by the number of carbon atoms in the central spacer. Specifically, the relative orientations of the two mesogenic arms are roughly parallel for even-numbered spacers and oblique for odd-numbered spacers. Consequently, even-spacer dimers exhibit properties similar to rod-like LCs, while odd-spacer dimers display those like bent-shaped LCs. This difference is similarly observed for trimers, even higher oligomers, and polymers. Many studies have focused more on odd-numbered bent-shaped LC dimers due to their unique LC phases, such as frustrated, spontaneously polar, and chiral smectic phases as observed for achiral bent-core molecules [5,6,7,8,9,10].
In the past decade, the identification of the twist-bend nematic (NTB) phase has further increased interest in bent LC dimers. This heliconical mesophase was first theoretically predicted [11,12] and later experimentally identified through the reinvestigation of the previously undefined nematic (NX) phase in cyanobiphenyl-based LC dimers (CBnCB; n = 7, 9, and 11), as shown in Figure 1 [13,14,15]. Similar NX phases have also been reported for bent main-chain polymers [16,17] and bent-core molecules [18,19] in addition to bent-shaped dimers [20], all of which contribute to the NTB phase. This phase can be formed by achiral bent molecules and is characterized by nanoscopic heliconical structures with the degeneracy of both handed chiral domains. These heliconical structures provide a pseudo-layered nature to the NTB phase, which resembles the layered smectic phase rather than the conventional fluid nematic (N) phase [21,22,23,24,25,26]. Electron microscopy [27,28] and resonant X-ray scattering [29,30,31,32,33] have confirmed nanoscopic periodicities of approximately 10 nm with double helices [33,34]. Some twist-bend nematogenic dimers have been utilized in various applications [35,36,37,38,39,40,41,42,43,44,45,46,47]. Structure–property relationships have been examined from theoretical perspectives [48,49,50,51,52,53,54]. However, an alternative heliconical model, termed the polar-twisted nematic phase, has also been proposed [55,56,57]. This model describes the significantly short helical pitches of the NX phase, and the detailed structure and term of the NX phase remain debatable.
To date, numerous bent LC dimers [20,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93], LC trimers [19,77,81,94,95,96,97,98,99,100,101], higher oligomers [102,103,104], polymers [16,17,105], and bent-core molecules [18,106,107] have been reported to form the NTB phase. The formation of the NTB phase by a duplexed oligomer suggests the potential presence of a double helical structure [108]. Typically, the NTB phase is monotropic, forming from the conventional N phase upon cooling in the normal phase sequence isotropic (Iso)–N–NTB. However, several dimers can exhibit the NTB phase directly from the Iso phase, without the conventional N phase [65,70,82,109,110,111,112]. Detailed reviews of twist-bend nematogenic molecules were provided by Mandle [113,114].
The bent molecular shape is a primary design factor for inducing the NTB phase. In dimeric structures, the bond angle at the two linkage positions connecting the mesogenic parts and flexible spacers plays a crucial role in determining molecular curvature [2,32,64,77,115]. Traditional twist-bend nematogenic dimers are based on methylene linkages (C–CH2–C; ca. 110°) rather than ether linkages (C–O–C; ca. 118°), as seen in cyanobiphenyl-based CBnCB and CBOnOCB dimers (Figure 1). The smaller bond angle of methylene makes molecular shapes more bent compared to ether, offering an advantage for forming the NTB phase in the methylene-linked CBnCB dimers rather than in the ether-linked CBOnOCB counterparts [58,69,92]. Actually, the ether-linked CBOnOCB dimers (n = 3, 5, 7, 9, and 11) also form the NTB phase from supercooled N domains [69,81,89].
Among twist-bend nematogens, the methylene-linked CBnCB dimers (n = 7 and 9) are the most well known, as they are the first examples for the NTB phase and display rare enantiotropic NTB phases. The homologous series of CBnCB, with n = 1 to 20, has been developed [58,92,116]. These homologs are structurally symmetric, with two identical mesogenic arms and a symmetric methylene spacer. Mixtures of the CBnCB homologs (n = 7, 9, and 11) strongly inhibit crystallization, maintaining the NTB phase for over a year [117]. Additionally, we previously reported that asymmetric structures in bent LC dimers are particularly effective in realizing the NTB phase [76,78,85]. Asymmetric arm combinations help prevent crystallization from the conventional N phase, often resulting in NTB phases or their glasses at room temperature. Furthermore, shortening the spacers of LC dimers often leads to the rare direct Iso–NTB phase transition [65,70,82,111]. In this context, methylene-linked cyanobiphenyl- and cyanoterphenyl-based asymmetric dimer homologs (CBnCT), as shown in Figure 2, are worthy of study. The cyanoterphenyl arm’s extended π-conjugation may not only enhance thermal LC phase stability, but also increase birefringence (Δn) [118,119,120,121].
Therefore, we report the synthesis and phase-transition behavior of a new homologous series of CBnCT dimers (Figure 2). To induce the NTB phase, odd-spacer CBnCT dimers (n = 1, 3, 5, 7, 9, 11, 13, 15, and 17) with bent shapes were developed. Their phase transitions were investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). Additionally, the temperature dependence of Δn was evaluated for CB9CT and CB9CB. The properties of the CBnCT homologs were compared with those of their CBnCB counterparts to assess the effects of the cyanoterphenyl arm’s extended π-conjugation and the impact of asymmetric dimer structures.

2. Materials and Methods

The CBnCT homologs were synthesized as outlined in Scheme 1. Dicarboxylic chlorides (COCln–2COCl) were synthesized from the corresponding dicarboxylic acids using oxalyl chloride, catalyzed by N,N-dimethylformamide (DMF). The resulting dicarboxylic halides underwent the Friedel–Crafts reaction with bromobenzene, yielding n-bis(4-bromophenyl)alkane-1,n-dione (BrPhCOn–2COPhBr). Subsequently, each BrPhCOn–2COPhBr homolog was subjected to the Wolff–Kishner reduction to produce 1,n-bis(4-bromophenyl)alkane homologs (BrPhnPhBr). The two shortest BrPhnPhBr homologs (n = 1 and 3) were synthesized by the reduction of the corresponding ketones with triethylsilane (TES) with reference to our previous work [111]. 4′-(n-(4-Bromophenyl)alkyl)-[1,1′-biphenyl]-4-carbonitrile homologs (CBnPhBr) were obtained via Suzuki–Miyaura coupling between the corresponding BrPhnPhBr and 4-cyanobenzene boronic acid pinacol ester. Finally, the CBnCT homologs were synthesized through the same cross-coupling reaction of the corresponding CBnPhBr and 4-cyanobiphenyl boronic acid pinacol ester. The molecular structures were confirmed using 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, recorded on a JEOL (Akishima, Japan) JNM-ECS400 (400 MHz for 1H, 100 MHz for 13C NMR) or a JEOL (Akishima, Japan) JNM-ECX500 (500 MHz for 1H, 126 MHz for 13C NMR) instrument. In addition, high-resolution mass spectrometry (HRMS) was conducted using high-performance liquid chromatography (HPLC; LC/MS/MS system with Agilent (Santa Clara, CA, USA) 1200 HPLC-Chip and 6520 Accurate-Mass Q-TOF (Agilent, Santa Clara, CA, USA)); the characterization data are provided in the Supplementary Materials.
The phase identification and phase-transition behaviors were investigated using POM and DSC. POM was carried out using an Olympus BX50 (Olympus Corporation, Tokyo, Japan) polarized optical microscope equipped with a Linkam LK-600PM (Linkam Scientific Instruments, Surrey, UK) temperature controller and an Olympus BX53M (Olympus Corporation, Tokyo, Japan) optical microscope equipped with a Mettler Toledo HS82 hot-stage system (Mettler Toledo, Zurich, Switzerland). An EHC polyimide surface cell with thicknesses of 7 µm was used for planar and uniaxial alignment observation. DSC was performed with a Shimadzu DSC-60 (Shimadzu, Kyoto, Japan) Plus instrument at a rate of 10 °C min−1 under a flow of nitrogen gas (50 mL min−1). Liquid nitrogen (Liq. N2) was added for cooling.
The molecular structures of CB9CT and CB9CB, with their all-trans conformation in the spacers, were optimized by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level [122,123] using Gaussian 16 [124]. Gauss View 6 was used for visualization [125].
The temperature dependence of Δn for CB9CT and CB9CB was evaluated by microscopic spectroscopy with reference to our previous study [126]. Each CB9CT or CB9CB sample was inserted into a uniaxially rubbed polyimide-surface 3-μm EHC cell for planar alignment over its isotropic (Iso) point on a Mettler FP82HT (Mettler Toledo, Zurich, Switzerland) hot stage. The uniaxial alignment state of the N phase was confirmed by POM using a Nikon LV100 Pol optical microscope (Nikon, Tokyo, Japan). Transmittance spectra through the nematic phases of the samples with the N director at 45° under the cross-Nicol condition were collected during cooling processes using an Ocean Optics USB4000 spectrometer (Ocean Optics, Florida, USA) through the Nikon LV100 Pol optical microscope. The detected transmittance (T) was fitted to Equation (1) with Cauchy’s equation, Equation (2), using coefficients a, b, c, and d:
T = A s i n 2 π d Δ n λ
Δ n = a + b λ 2 + c λ 4 + d λ 6
where d and λ in Equation (1) refer to the cell gap and wavelength of light, respectively. After going into the NTB phase temperature, the transmitted light spectra could not be fitted to the above equations, meaning the sample alignment became non-uniaxial.

3. Results and Discussion

Phase-transition results of CBnCT are summarized in Table 1, and Figure 3 shows phase-transition temperatures as a function of n. The shortest spacer CB1CT homolog did not show LC phases. In contrast, all other CBnCT homologs exhibited both the NTB phase and the conventional N phase. Specifically, CBnCT (n = 3 and 5) homologs show a monotropic NTB phase, whereas CBnCT (n = 7, 9, 11, 13, 15, and 17) exhibit an enantiotropic NTB phase, below the temperature of the conventional N phase. The phase-transition behavior of CB7CT is superficially in agreement with Ref. [74]. The N–NTB or NTB–N phase-transition temperatures (TNNTB upon cooling and TNTBN upon heating, respectively) range from 130 °C to 200 °C, which are significantly higher than those (approximately 80–100 °C) of typical LC twist-bend nematogenic dimers. The TNNTB value of CB5CT is the highest (approximately 184 °C). Notably, the NTB phase temperature ranges upon cooling of CBnCT (n = 7, 9, 11, and 13) are over 100 °C. The phase behavior of the CBnCT homologs is compared with those of the cyanobiphenyl-based CBnCB homologs later in this section.
The NTB phases of CBnCT were characterized by optical birefringent textures through POM, as shown in Figure 4 for n = 9. The optical texture of the NTB phase from the crystal phase upon heating contain focal-conic-like and blocky textures (Figure 4a). The optical textures of the NTB phase were finer upon cooling compared to those during heating processes. The samples often resulted in stripe- or rope-like textures in the NTB phase (Figure 4c). These textures indicate the pseudo-layered nature of the NTB phase due to the undulation of the heliconical structures [22,25,28]. POM images of the other homologs are shown in the Supplementary Materials. Differently from the other homologs, CB3CT showed some ambiguous textures and a fan-like texture for the NTB phase resulting from the conventional N phase in a nontreated glass cell, as shown in Figure 5a,b. This NTB texture is similar to those of the NTB phases formed directly from the Iso phases [82,111,112]. This study also examined the possibility of achieving the direct Iso–NTB phase transition for short spacer homologs, and the N phase temperature range of CB3CT was only approximately 6 °C. Regardless, the presence of the N phase is useful for achieving uniaxial alignments in its LC phases. In a uniaxially rubbed planar-alignment cell, the monochroic birefringent N texture shown in Figure 5e transitioned to blocky and striped textures with focal-conic textures as the temperature decreased (Figure 5d and Figure 5c, respectively). In addition, double helix textures were observed, as shown in Figure 5c [71]. These textures support the realization of the NTB phase for CB3CT.
The DSC curves obtained upon cooling the CBnCT homologs are shown in Figure 6. It is evident that the NTB–N (or N–NTB) phase-transition peaks become more akin to first-order transitions with discernible enthalpy changes for shorter spacer CBnCT dimers (n < 7). In particular, the N–NTB phase-transition peak of CB3CT is strongly first order, whose enthalpy change is higher than that of the Iso–N phase transition (Table 1), which is similar to those of the direct N–NTB phase transitions [49,65,70,82,109,112]. As n increases, the NTB–N (or N–NTB) phase-transition peaks resemble second-order ones with a baseline shift. Therefore, the enthalpy changes at NTB–N (or N–NTB) phase transitions for n ≥ 7 are not shown in Table 1. This trend aligns with typical twist-bend nematogenic dimer homologs [69,82,85]. Upon cooling, the NTB phase of CBnCT dimers (n = 7, 9, and 11) remained stable down to below room temperature and eventually vitrified (Figure 6). Their glass-transition temperatures (Tg) are comparable in approximately 15–20 °C, which are similar to those of typical LC dimers.
The phase-transition behavior of the CBnCT homologs was compared with that of the cyanobiphenyl-based CBnCB homologs. The melting temperature (Tm), temperature range of the N phase (ΔTN), Iso–N phase-transition temperature (TIN), and TNNTB of CBnCT and CBnCB are summarized in Figure 7 as functions of n. The data for the CBnCB dimers were obtained from Ref. [111] for n = 3; Ref. [69] for n = 5, 7, 9, 11, and 13; Ref. [92] for n = 1 and 15; and Ref. [87] for n = 17.
Specifically, CB1CT and CB1CB do not form LC phases, with Tm values of 224.8 and 206 °C, respectively. CB3CB exhibits a monotropic direct Iso–NTB phase transition upon cooling, whereas CB3CT shows the monotropic both conventional N and NTB phases. The CBnCB dimers (n = 5, 11, 13, and 17) show the monotropic NTB phase, while the CBnCB dimers (n = 7, 9, and 15) form the enantiotropic NTB phase. As mentioned earlier, the cyanoterphenyl-based CBnCT (n = 7, 9, 11, 13, 15, and 17) exhibits the enantiotropic NTB phase. Thus, six CBnCT homologs exhibit the enantiotropic NTB phase, which is a broader homologous range compared to the three homologs for CBnCB (n = 7, 9, and 15). Therefore, replacing one cyanobiphenyl of CBnCB with the cyanoterphenyl group expands the number of homologs that can form the enantiotropic NTB phase.
The Tm values of CBnCT are clearly higher than those of CBnCB for shorter n, whereas those of both homologs are roughly comparable for an n larger than 9, as shown in Figure 7a. The presence of the longer π-conjugated cyanoterphenyl moieties likely increases Tm compared to that of cyanobiphenyl for a shorter n. However, the asymmetric structure strongly compensates for the Tm values [127], resulting in a moderate increase in the Tm values of the shorter-n CBnCT homologs compared with those of the corresponding CBnCB homologs. In addition to the asymmetric structure, lengthening the spacer should contribute to lowering Tm, leading to comparable Tm values for both longer-n homologous series. Figure 7b shows that the temperature ranges of the N phases of the CBnCT homologs are significantly broader than those of CBnCB for the same n. The TIN values of CBnCT are particularly higher than those of CBnCB (Figure 7c). Meanwhile, the TNNTB values of CBnCT are modestly higher than those of CBnCB, as shown in Figure 7d. Thus, an increase in both TIN and TNNTB in the cyanoterphenyl arm was observed compared with the cyanobiphenyl arm, owing to the more anisotropic π-conjugated arm structure of the former system [76]. The difference in TNNTB between both homologs is clearly smaller than that in TIN. This can be attributed to the difference in the nature of the N and NTB phase formation, which is significantly influenced by molecular anisotropy and bent shape, respectively. Specifically, anisotropic structures stabilize the conventional N phase, and thus, the higher anisotropy of the terphenyl arm simply increases TIN (or TNI). However, NTB phase formation is driven by bent molecular structures. Therefore, TNNTB is less affected by molecular anisotropic factors (shapes and polarizabilities) compared with TIN. Consequently, the increase in TNNTB caused by the terphenyl arm of CBnCT was moderate compared to that in TIN.
The molecular bend in LC dimers is strongly influenced by linkages between the arm and flexible spacer [2,32,64,77,115]. In this study, the CBnCT and CBnCB homologs have the same methylene linkages. Figure 8 shows the molecular structures of CB9CT and CB9CB optimized by DFT calculations using Gaussian 16 with the B3LYP/6-31G(d) basis set. The inter-arm angles between the long axes of the two mesogenic arms of both dimers are approximately 111°, which is comparable with reported values for CB9CB [32,72,128]. Thus, their molecular bent shapes should be comparable. This fact also supports that the more anisotropic and longer π-conjugated cyanobiphenyl moiety modestly increased the TNNTB values of CBnCT compared with those of CBnCB.
Overall, the presence of the cyanoterphenyl in CBnCT increases both TIN (or TNI) and TNNTB (or TNTBN), as well as the number of enantiotropic NTB homologs compared with those of CBnCB. In other words, increasing π-conjugation in the mesogenic arm and combining asymmetric arms expands both the N and NTB phases [76]. This asymmetric structure of CBnCT should contribute to faster melting and prevent crystallization, enabling the realization of the NTB phases compared with symmetric dimers.

Birefringence

The temperature dependence of Δn for CB9CT and CB9CB was evaluated in their N phases to compare the effects of the terphenyl and biphenyl moieties. The Δn values at 550 nm are plotted as a function of measurement temperature (T) and the shifted temperature (ΔT = TIN − T) in Figure 9a and Figure 9b, respectively. As shown in Figure 9, the Δn values of both dimers increase with decreasing T or increasing ΔT. This behavior is attributed to the increased orientational order parameters (S) at lower temperatures, as described by Haller’s approximation [129]:
Δn = Δn0 S and S = (1 − T/Ti)β,
where Δn0 represents the Δn value at S = 1, and T, Ti, and β denote the measurement temperature, approximated isotropic temperature, and fitting constant, respectively. Figure 9 shows that the Δn values of CB9CT are significantly higher than those of CB9CB. This difference is attributed to the higher molecular polarizability and anisotropy provided by the cyanoterphenyl moiety compared with the biphenyl one, as indicated by the polarizabilities of the optimized dimer structures of CB9CT and CB9CB listed in Table 2. For the polarizability components (αxx, αyy, and αzz), the x-axis corresponds to the molecular long axis and the y- and z-axes are the two shorter axes perpendicular to the x-axis. The mean polarizability (α) and polarizability anisotropy (Δα) are given by α = (αxx + αyy + αzz)/3 and Δα = αxx − (αyy + αzz)/2. The α and Δα values are higher for CB9CT than for CB9CB, due to the longer π-conjugated cyanoterphenyl moiety compared with cyanobiphenyl. The Δn values of both dimers were well fitted to Haller’s equation, allowing the extrapolation of Δn0, Ti, and β values, which are summarized in Table 2. The obtained β values fall within the typical range for nematic phases. The Δn0 value (0.39) of CB9CT is also higher than that of CB9CB (0.27).

4. Conclusions

In this study, we synthesized the first homologous series of methylene-linked CBnCT (n = 1, 3, 5, 7, 9, 11, 13, 15, and 17). Except for n = 1, all CBnCT homologs exhibited the NTB phase. Specifically, CBnCT (n = 3 and 5) showed a monotropic NTB phase, whereas CBnCT with n = 7, 9, 11, 13, 15, and 17 exhibited an enantiotropic NTB phase. The replacement of cyanobiphenyl with cyanoterphenyl significantly broadened the temperature ranges of both N and NTB phases with higher TIN (or TNI) and TNNTB (or TNTBN) than those of CBnCB ones. In particular, the NTB phase temperature ranges of CBnCT (n = 7, 9, 11, and 13) were over 100 °C. More CBnCT homologs formed the enantiotropic NTB phase compared to the CBnCB ones. The incorporation of a longer π-conjugated structure in the mesogenic arm and an asymmetric arm combination served to stabilize both the N and NTB phases in these LC dimers. Additionally, the Δn values were higher for CB9CT than for CB9CB. This study provides useful insights into molecular design, demonstrating that the highly anisotropic π-conjugated terphenyl structure can effectively enhance the NTB phase and increase birefringence in LC dimers.

Supplementary Materials

The Supplementary Materials can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15020120/s1.

Author Contributions

Conceptualization, Y.A.; Methodology, Y.A.; Validation, Y.A.; Formal analysis, Y.S. and Y.A.; Investigation, Y.S. and Y.A.; Resources, Y.A.; Data curation, Y.A.; Writing—original draft preparation, Y.S. and Y.A.; Writing—review and editing, Y.S. and Y.A.; 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 20K15351 and 23K04874).

Data Availability Statement

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

Acknowledgments

We would like to thank Masatoshi Tokita (Institute of Science Tokyo) for the birefringence measurements and Tsugumi Shiokawa and Hiroko Tada at the Division of Instrumental Analysis (Okayama University) for HRMS measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00120 g001
Figure 1. Molecular structures of the methylene-linked CBnCB dimers and ether-linked CBOnOCB dimers.
Figure 1. Molecular structures of the methylene-linked CBnCB dimers and ether-linked CBOnOCB dimers.
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Crystals 15 00120 g002
Figure 2. Molecular structures of the methylene-linked CBnCT homologs synthesized in this study.
Figure 2. Molecular structures of the methylene-linked CBnCT homologs synthesized in this study.
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Crystals 15 00120 sch001
Scheme 1. Synthesis routes to CBnCT.
Scheme 1. Synthesis routes to CBnCT.
Crystals 15 00120 sch001
Crystals 15 00120 g003
Figure 3. Phase-transition temperatures of CBnCT as a function of n upon (a) heating and (b) cooling.
Figure 3. Phase-transition temperatures of CBnCT as a function of n upon (a) heating and (b) cooling.
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Crystals 15 00120 g004
Figure 4. POM images of the NTB and N phases of CB9CT upon (a,b) heating and (c,d) cooling.
Figure 4. POM images of the NTB and N phases of CB9CT upon (a,b) heating and (c,d) cooling.
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Crystals 15 00120 g005
Figure 5. POM images of the NTB and N phases of CB3CT upon cooling in (a,b) a non-treated glass cell and (c–e) a uniaxially rubbed planar alignment cell with a 7 µm thickness. The scale bars in (a,c) show 50 μm. The arrows with A, P, and R in (e) represent the directions of the analyzer, polarizer, and rubbing, respectively, for (c–e). The inset in (c) is a zoomed-in image of the double helical textures.
Figure 5. POM images of the NTB and N phases of CB3CT upon cooling in (a,b) a non-treated glass cell and (c–e) a uniaxially rubbed planar alignment cell with a 7 µm thickness. The scale bars in (a,c) show 50 μm. The arrows with A, P, and R in (e) represent the directions of the analyzer, polarizer, and rubbing, respectively, for (c–e). The inset in (c) is a zoomed-in image of the double helical textures.
Crystals 15 00120 g005
Crystals 15 00120 g006
Figure 6. DSC curves of CBnCT (n = 3, 5, 7, 9, 11, and 13) upon cooling. Liq. N2 indicates peaks resulting from the liquid nitrogen added for cooling.
Figure 6. DSC curves of CBnCT (n = 3, 5, 7, 9, 11, and 13) upon cooling. Liq. N2 indicates peaks resulting from the liquid nitrogen added for cooling.
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Crystals 15 00120 g007
Figure 7. (a) Tm, (b) ΔTN, (c) TIN, and (d) TNNTB of CBnCT and CBnCB. The data for CB3CB (green triangles) in panels (c,d) represent the Iso–NTB phase transition temperature (i.e., TINTB).
Figure 7. (a) Tm, (b) ΔTN, (c) TIN, and (d) TNNTB of CBnCT and CBnCB. The data for CB3CB (green triangles) in panels (c,d) represent the Iso–NTB phase transition temperature (i.e., TINTB).
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Crystals 15 00120 g008
Figure 8. Molecular structures of CB9CT and CB9CB, optimized using Gaussian 16 with the B3LYP/6-31G(d) basis set.
Figure 8. Molecular structures of CB9CT and CB9CB, optimized using Gaussian 16 with the B3LYP/6-31G(d) basis set.
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Crystals 15 00120 g009
Figure 9. Temperature dependence of Δn of CB9CT and CB9CB as functions of (a) T and (b) ΔT.
Figure 9. Temperature dependence of Δn of CB9CT and CB9CB as functions of (a) T and (b) ΔT.
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Table 1. Phase sequences, phase-transition temperatures (°C), and associated enthalpy changes (values in parentheses, kJ mol−1) for CBnCT homologs. The data in the top and bottom rows were obtained upon heating and cooling, respectively.
Table 1. Phase sequences, phase-transition temperatures (°C), and associated enthalpy changes (values in parentheses, kJ mol−1) for CBnCT homologs. The data in the top and bottom rows were obtained upon heating and cooling, respectively.
nPhase-Transition Temperatures/°C and Enthalpy Changes/kJ mol–1
1Cr224.6 (39.7)Iso
Iso159.3 (−32.9)Cr
3Cr189.4 (33.5)Iso
Iso180.2 (−0.23)N174.5 (−1.0)NTB121.2 (−21.2)Cr
5Cr196.1 (37.4)N249.7 (1.06)Iso
Iso246.7 (−1.09)N184.2 (−0.14)NTB107.4 (−20.3)Cr
7Cr150.8 (32.4)NTB179.7N248.8 (1.52)Iso
Iso246.6 (−1.49)N177.6NTB16.9G
9Cr120.2 (19.9)NTB152.6N264.8 (2.56)Iso
Iso262.5 (−2.76)N150.6NTB17.5G
11Cr95.0 (28.9)NTB151.3N246.9 (2.79)Iso
Iso243.4 (−3.04)N147.6NTB18.7G
13Cr106.2 (41.4)NTB144.7N227.5 (3.19)Iso
Iso224.6 (−3.03)N140.1NTB38.4 (−11.5)Cr
15Cr106.6 (24.6)NTB136.2N219.4 (2.95)Iso
Iso216.5 (−2.96)N134.4NTB68.5 (−15.6)Cr
17Cr106.5 (33.8)NTB132.4N216.1 (3.86)Iso
Iso214.2 (−4.16)N132.1NTB81.4 (−28.1)Cr
Table 2. Polarizability factors from the DFT-optimized structures, along with Δn0, Ti, and β values for CB9CT and CB9CB.
Table 2. Polarizability factors from the DFT-optimized structures, along with Δn0, Ti, and β values for CB9CT and CB9CB.
CodeαxxαyyαzzαΔαΔn0Tiβ
CB9CT745.5420.6319.9495.3375.20.39251.90.14
CB9CB596.3335.4299.3410.3278.90.27124.10.16
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Shimoura, Y.; Arakawa, Y. Cyanobiphenyl- and Cyanoterphenyl-Based Liquid Crystal Dimers (CBnCT): The Enantiotropic Twist-Bend Nematic Phase. Crystals 2025, 15, 120. https://doi.org/10.3390/cryst15020120

AMA Style

Shimoura Y, Arakawa Y. Cyanobiphenyl- and Cyanoterphenyl-Based Liquid Crystal Dimers (CBnCT): The Enantiotropic Twist-Bend Nematic Phase. Crystals. 2025; 15(2):120. https://doi.org/10.3390/cryst15020120

Chicago/Turabian Style

Shimoura, Yamato, and Yuki Arakawa. 2025. "Cyanobiphenyl- and Cyanoterphenyl-Based Liquid Crystal Dimers (CBnCT): The Enantiotropic Twist-Bend Nematic Phase" Crystals 15, no. 2: 120. https://doi.org/10.3390/cryst15020120

APA Style

Shimoura, Y., & Arakawa, Y. (2025). Cyanobiphenyl- and Cyanoterphenyl-Based Liquid Crystal Dimers (CBnCT): The Enantiotropic Twist-Bend Nematic Phase. Crystals, 15(2), 120. https://doi.org/10.3390/cryst15020120

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