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Article

Twist–Bend Nematic Phase Behavior of Cyanobiphenyl-Based Dimers with Propane, Ethoxy, and Ethylthio Spacers

by
Yuki Arakawa
*,
Yuto Arai
,
Kyohei Horita
,
Kenta Komatsu
and
Hideto Tsuji
Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441-8580, Japan
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(12), 1734; https://doi.org/10.3390/cryst12121734
Submission received: 26 October 2022 / Revised: 18 November 2022 / Accepted: 20 November 2022 / Published: 1 December 2022
(This article belongs to the Special Issue State-of-the-Art Liquid Crystals Research in Japan)
Browse Figures
Versions Notes

Abstract

:
The twist–bend nematic (NTB) phase is a liquid crystal (LC) phase with a heliconical structure that typically forms below the temperature of the conventional nematic (N) phase. By contrast, the direct transition between the NTB and isotropic (Iso) phases without the intermediation of the N phase rarely occurs. Herein, we demonstrate the effects of linkage type (i.e., methylene, ether, and thioether) on the typical Iso–N–NTB and rare direct Iso–NTB phase-transition behaviors of cyanobiphenyl (CB) dimers CB3CB, CB2OCB, and CB2SCB bearing three-atom-based propane, ethoxy, and ethylthio spacers, respectively. In our previous study, CB2SCB exhibited the monotropic direct Iso–NTB phase transition. In this study, we report that CB3CB also shows the direct Iso–NTB phase transition, whereas CB2OCB exhibits the typical Iso–N–NTB phase sequence with decreasing temperature. The Iso–LC (Iso–NTB or Iso–N) phase-transition temperatures upon cooling show the order CB2OCB (108 °C) > CB3CB (49 °C) > CB2SCB (43 °C). The thioether-linked CB2SCB is vitrifiable, whereas CB3CB and CB2OCB exhibit strong crystallization tendencies. The phase-transition behaviors are also discussed in terms of the three bent homologous series with different oligomethylene spacers n: CBnCB, CBnOCB, and CBnSCB.

1. Introduction

Liquid crystal (LC) phases are mesophases between anisotropic crystals and isotropic liquids. A nematic (N) phase generally does not have an apparent layered structure and is recognized as the most fluid LC phase. After the heliconical twist–bend nematic (NTB) phase was predicted [1,2,3], it was experimentally assigned to unknown N (NX) phases below the conventional N phase of bent molecules (dimers) in the last decade [4,5]. The NTB phase possesses a heliconical director precession with a pitch of approximately 10 nm [6,7,8]. The heliconical structures of the NTB phase result in optical textures and physical properties like those of layered smectic (Sm) phases rather than the conventional N phase [9,10,11,12,13]. Therefore, the NTB phase is often considered a pseudo-layered phase. However, the phase identification of the NTB phase for the NX phase is still under discussion and further study is required to elucidate the detailed structure owing to its elusive nature. Alternatively, a polar twisted nematic (NPT) phase model for the NX phase instead of the NTB phase has been proposed [14,15]. In this paper, the widely recognized term, NTB phase, is used.
The NTB phase can be formed only by bent molecules, such as bent dimers [4,5,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39], linear oligomers (e.g., trimers, tetramers, and hexamers) [40,41,42,43,44,45,46,47,48,49], duplexed hexamers [50], polymers [51], hydrogen-bonded dimers and trimers [52,53] with odd-number atom spacers, and bent-core molecules [54,55,56]. Mandle reviewed the structure–property relationship of the NTB phase and summarized the recent progress in this topic [57]. Theoretical simulation studies have examined the relationship between the curvature of various bent dimers and the incidence of the NTB phase [58,59,60,61]. In nearly all cases of the reported bent molecules, the NTB phase continuously formed at a temperature below the temperature of the conventional N phase, resulting in a typical isotropic (Iso)–N–NTB phase sequence with decreasing temperature. For example, the homologous series of bent symmetric methylene- and asymmetric methylene-/ether- and methylene-/thioether-linked cyanobiphenyl (CB) dimers with n number of carbon atoms in the oligomethylene spacers, CBnCB (n = 5, 7, 9, 11, and 13) [17,25], CBnOCB (n = 4, 6, 8, and 10) [24,25], and CBnSCB (n = 4, 6, 8, and 10) [62], respectively, are known to exhibit the typical Iso–N–NTB phase sequence (Figure 1a).
However, in a few cases, an NTB phase can be directly formed from the Iso phase without the intermediate N phase. Archbold et al. reported that binary mixtures of a dimer with the Iso–N–NTB phase sequence and a chiral dopant (6–10 wt%) exhibit the direct Iso–NTB phase transition (Figure 1b, right) [63]. Dawood et al. reported that two imine-linked dimers with a central propane spacer and terminal methoxy or ethoxy groups (m = 1 and 2, respectively) (Figure 1b, top left) show the direct Iso–NTB phase transition [64,65]. We reported that bent CBnSCB (n = 4, 6, 8, and 10) demonstrates the Iso–N–NTB phase sequence, as described earlier, whereas only the shortest ethylthio-linked CB2SCB exhibits the direct Iso–NTB phase transition upon cooling (Figure 1b, center left) [62]. Shortening the flexible spacer n of the CBnSCB dimers lowers the Iso–N phase-transition temperature (TIN) upon cooling, thereby narrowing the intermediate N-phase temperature range (ΔTN), which leads to the direct Iso–NTB phase transition of CB2SCB. Moreover, Wang et al. recently disclosed that a phosphorus-bridged LC dimer exhibits the direct Iso–NTB phase transition (Figure 1b, bottom left) [66].
Considering that, in our previous study, CB2SCB demonstrated the rare direct Iso–NTB phase transition [62], propane-linked CB3CB and ethoxy-linked CB2OCB bearing the same three-atom-based spacers (as shown in Figure 1c) are worthy of further investigation whether they exhibit the rare Iso–NTB phase or not. The influence of the different linkage types on the phase transition of such short-spacer dimers has yet to be reported. In this study, we investigated the phase-transition behaviors of three CB-based dimers with different three-atom-based spacers, namely CB3CB, CB2OCB, and CB2SCB. The phase-transition behavior of the previously reported CB3CB that did not show the LC phase [67] was re-investigated, while that of CB2OCB was explored for the first time. The phase-transition data of CB2SCB were obtained from our previous study [62]. We then compared the phase-transition behaviors of three series of CBnCB, CBnOCB, and CBnSCB homologs. Finally, the effects of linkage-type on the NTB phase-transition behavior of dimers with short spacers, particularly on the occurrence of the direct Iso–NTB phase transition, were explored.

2. Materials and Methods

2.1. General

The synthetic routes of CB3CB and CB2OCB are shown in Scheme 1. The molecular structures were analyzed using 1H and 13C nuclear magnetic resonance (NMR) spectroscopy on a JNM ECX 500 spectrometer (JEOL Ltd., Tokyo, Japan). Phase identification was conducted via polarized optical microscopy (POM) using a BX50 microscope (Olympus Corp., Tokyo, Japan) on an LK-600 PM hot stage (Linkam, Surrey, UK). The phase-transition temperatures and associated enthalpy changes were determined using differential scanning calorimetry (DSC) on a DSC-60 Plus (Shimadzu Corp., Kyoto, Japan). Calibration was performed using indium, and the measurements were performed over heating/cooling/heating cycles at a rate of 10 °C min−1 under an N2 gas flow (50 mL min−1).

2.2. Synthesis

2.2.1. 1,3-Bis(4-cyanobiphenyl-4′-yl)propane (CB3CB)

(E)-1,3-Bis(4-bromophenyl)prop-2-en-1-one

This compound was synthesized referring to a previously reported method [68]. 4′-Bromoacetophenone (2.19 g, 11.0 mmol), p-bromobenzaldehyde (2.04 g, 11.0 mmol), sodium ethoxide (NaOEt) (1.12 g, 16.5 mmol), ethanol (EtOH) (30 mL), and distilled water (10 mL) were added to a round-bottom flask. The resultant mixture was stirred at ambient temperature for 1 h. The reaction mixture was filtered, and the residue was rinsed with copious amounts of methanol to afford the target compound as a pale-yellow solid (86%). 1H NMR (500 MHz, CDCl3) δ 7.89 (d, J = 9.0 Hz, Ar–H, 2H), 7.75 (d, J = 16.0 Hz, CO–CH, 1H), 7.65 (d, J = 9.0 Hz, Ar–H, 2H), 7.56 (d, J = 9.0 Hz, Ar–H, 2H), 7.51 (d, J = 9.0 Hz, Ar–H, 2H), 7.47 (d, J = 16.0 Hz, CO–CH=CH, 1H) ppm. 13C NMR (126 MHz, CDCl3) δ 189.1, 143.9, 136.7, 133.7, 132.3, 132.0, 130.0, 129.8, 128.1, 125.1, 121.9 ppm.

1,3-Bis(4-bromophenyl)propane

This compound was also synthesized referring to the literature [68]. (E)-1,3-Bis(4-bromophenyl)prop-2-en-1-one (1.10 g, 2.99 mmol) was added to a two-necked round-bottom flask, which was then purged with argon gas. Trifluoroacetic acid (TFA) (11 mL) was then added into the flask, followed by adding triethylsilane (TES) (4.8 mL, 30 mmol) dropwise. The resultant mixture was stirred at ambient temperature for 1 h. TES (0.95 mL, 5.94 mmol) was added to the reaction mixture, which was further stirred for 18 h. The reaction mixture was poured into distilled water, extracted with dichloromethane, and washed with brine. The obtained solution was then dried over magnesium sulfate (MgSO4), and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using hexane as an eluent to afford 1,3-bis(4-bromophenyl)propane as a colorless solid (78%). 1H NMR (500 MHz, CDCl3) δ 7.39 (d, J = 8.5 Hz, Ar–H, 4H), 7.04 (d, J = 8.5 Hz, Ar–H, 4H), 2.58 (t, J = 7.5 Hz, Ar–CH2, 4H), 1.90 (tt, J = 7.5 and 7.5 Hz, Ar–CH2–CH2, 2H) ppm. 13C NMR (126 MHz, CDCl3) δ 140.9, 131.4, 130.2, 119.5, 34.6, 32.6 ppm.

CB3CB

1,3-Bis(4-bromophenyl)propane (200 mg, 0.565 mmol), 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (267 mg, 1.17 mmol), cesium carbonate (Cs2CO3) (748 mg, 2.29 mmol), and tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] (41.5 mg, 35.9 µmol) were added to a two-necked round-bottom flask, which was then purged with argon gas. Tetrahydrofuran (THF) (5 mL) was degassed by bubbling with argon gas and added to the flask. The resultant mixture was stirred at reflux temperature for 3 h. Subsequently, Pd(PPh3)4 (39.5 mg, 34.2 µmol) was added to the reaction mixture, which was further stirred for 4 h. The reaction mixture was extracted with dichloromethane and washed with brine. The solution was then dried over MgSO4, and the volatile solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using dichloromethane/hexane (1:1, v/v) and recrystallized in a dichloromethane/hexane mixture to afford CB3CB as a colorless solid (51%). 1H NMR (500 MHz, CDCl3) δ 7.71 (d, J = 8.5 Hz, Ar–H, 4H), 7.67 (d, J = 8.5 Hz, Ar–H, 4H), 7.53 (d, J = 8.5 Hz, Ar–H, 4H), 7.31 (d, J = 8.5 Hz, Ar–H, 4H), 2.74 (t, J = 7.5 Hz, Ar–CH2, 4H), 2.04 (tt, J = 7.5 and 7.5 Hz, Ar–CH2–CH2, 2H) ppm. 13C NMR (126 MHz, CDCl3) δ 145.5, 142.9, 136.7, 132.6, 129.2, 127.5, 127.2, 119.0, 110.6, 35.0, 32.7 ppm.

2.2.2. (4-Cyanobiphenyl-4′-yloxy)-2-(4-cyanobiphenyl-4′-yl)ethane (CB2OCB)

1-Bromo-4-[2-(4-bromophenoxy)ethyl]benzene

4-Bromophenethyl bromide (690 mg, 2.61 mmol), 4-bromophenol (302 mg, 1.75 mmol), and potassium carbonate (K2CO3) (617 mg, 4.46 mmol) were added to a two-necked round-bottom flask, which was then purged with argon gas. N,N-Dimethylformamide (DMF) (5 mL) was degassed by bubbling argon gas and then added to the flask. The resultant mixture was stirred at 90 °C for 18 h. More 4-bromophenethyl bromide (690 mg, 2.61 mmol) was added to the reaction mixture, which was further stirred for 5 h. The reaction mixture was extracted with ethyl acetate and washed with brine. The obtained solution was dried over MgSO4, and then the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using hexane/ethyl acetate (20:1, v/v) to afford 1-Bromo-4-[2-(4-bromophenoxy)ethyl]benzene (28%). 1H NMR (500 MHz, CDCl3) δ 7.43 (d, J = 8.5 Hz, Ar–H, 2H), 7.35 (d, J = 8.5 Hz, Ar–H, 2H), 7.14 (d, J = 8.5 Hz, Ar–H, 2H), 6.75 (d, J = 8.5 Hz, Ar–H, 2H), 4.11 (t, J = 6.5 Hz, Ar–O–CH2, 2H), 3.03 (t, J = 6.8 Hz, Ar–O–CH2–CH2, 2H) ppm. 13C NMR (126 MHz, CDCl3) δ 157.8, 137.1, 132.3, 131.6, 130.7, 120.4, 116.3, 113.0, 68.5, 35.1 ppm.

CB2OCB

1-Bromo-4-[2-(4-bromophenoxy)ethyl]benzene (96.7 mg, 0.272 mmol), 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (140 mg, 0.611 mmol), Cs2CO3 (182 mg, 0.559 mmol), and Pd(PPh3)4 (64.1 mg, 55.5 µmol) were added to a two-necked round-bottom flask, which was then purged with argon gas. THF (3 mL) was degassed by bubbling argon gas and then added to the flask. The resultant mixture was stirred at reflux temperature for 16 h. An arbitrary amount of Pd(PPh3)4 was added to the reaction mixture. The mixture was then stirred for 16 h, extracted with dichloromethane, and washed with brine. The solution was dried over MgSO4, and the volatile solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using dichloromethane/hexane (5:1, v/v) and recrystallized in a dichloromethane/hexane mixture to afford CB2OCB (20%). 1H NMR (500 MHz, CDCl3) δ 7.72 (d, J = 8.5 Hz, Ar–H, 2H), 7.69 (d, J = 8.5 Hz, Ar–H, 2H), 7.67(d, J = 8.0 Hz, Ar–H, 2H), 7.63 (d, J = 8.5 Hz, Ar–H, 2H), 7.55 (d, J = 8.5 Hz, Ar–H, 2H), 7.52 (d, J = 9.0 Hz, Ar–H, 2H), 7.42 (d, J = 8.0 Hz, Ar–H, 2H), 7.00 (d, J = 9.0 Hz, Ar–H, 2H), 4.27 (t, J = 6.5 Hz, Ar–O–CH2, 2H), 3.18 (t, J = 6.5 Hz, Ar–O–CH2–CH2, 2H) ppm. 13C NMR (126 MHz, CDCl3) δ 159.3, 145.3, 145.1, 138.9, 137.5, 132.59, 132.56, 131.7, 129.8, 128.4, 127.5, 127.3, 127.1, 119.1, 118.9, 115.1, 110.8, 110.1, 68.5, 35.3 ppm.

3. Results

3.1. Phase-Transition Behaviors of CB3CB, CB2OCB, and CB2SCB

As shown in the DSC curves (Figure 2a), the methylene-linked CB3CB sample exhibited a melting temperature (Tm) of ~149 °C, where it transitioned to the Iso phase without forming the LC phase upon heating. Upon cooling, the CB3CB sample crystallized at ~80 °C from the Iso phase. These phase-transition temperatures were higher than those (142.1 and 69.1 °C, respectively) reported in the literature [67], where CB3CB did not exhibit the LC phase. However, POM observations in this study revealed that in the supercooled Iso phase of CB3CB (Figure 3a), which does not undergo crystallization at ~80 °C, birefringent textures appear (Figure 3b), which then grow mixed textures including fan-, focal-conic-, and rope-like domains, as shown in Figure 3c,d. Besides, we did not observe typical N-phase textures such as marble and schlieren textures during this phase transition, as seen in Figure 3. This texture behavior is similar to the direct Iso–NTB phase transition of CB2SCB [62]. Therefore, it was revealed that CB3CB also shows the monotropic direct Iso–NTB phase transition at ~49 °C. During the heating after the cooling, the observed NTB phase of CB3CB transitioned to the Iso phase at ~55 °C. The CB3CB sample displayed a strong crystallization tendency and did not vitrify upon cooling, even at a higher rate of 30 °C min−1. Additionally, the ether-linked CB2OCB did not show LC phases over the first and second heating cycles, where it exhibited different Tm values of 164.4 and 139.7 °C, respectively; Figure 2b represents the latter. This result indicates the existence of crystal polymorphs that depend on the crystallization conditions. Upon cooling, most of the Iso-phase domains and droplets of CB2OCB crystallize at ~104 °C, as shown by the exothermic peak in Figure 2b. However, the POM images reveal the formation of the N and NTB phases at ~108 and 78 °C, respectively, in the supercooled Iso phase, as confirmed by the marble/schlieren textures (Figure 4a) and blocky texture (Figure 4b), respectively. Because of the strong crystallization tendencies of CB3CB and CB2OCB, as shown by the DSC curves (Figure 2a,b, respectively), their LC phases were not investigated by X-ray diffractometry. The strong crystallization tendency of these molecules differs from the vitrifiable CB2SCB, with a glass transition temperature of ~20 °C, as shown in Figure 2c [62]. The Tm values upon the second heating, the associated enthalpy changes (ΔH), and the Iso–NTB, Iso–N, and N–NTB phase-transition temperatures upon the cooling (TINTB, TIN, and TNNTB, respectively) of CB3CB, CB2OCB, and CB2SCB are summarized in Table 1. For simplicity, the crystallization and glass transition temperatures upon cooling are not listed in Table 1.
Thus, CB3CB and CB2SCB exhibited the direct Iso–NTB phase transition, whereas CB2OCB showed the typical Iso–N–NTB phase transition. Moreover, the CB3CB and CB2OCB samples displayed a stronger crystallization tendency compared with the thioether-linked CB2SCB, which had a vitrifiable NTB phase [62]. The Iso–LC phase-transition temperatures (i.e., TINTB or TIN) showed the order CB2OCB (108 °C) >> CB3CB (49 °C) > CB2SCB (43 °C), which translated to the order ether >> methylene > thioether in terms of linkage type. Naturally, the NTB phase-transition temperatures (i.e., TINTB or TNNTB) also showed the same order: ether >> methylene > thioether. This particularly high phase-transition temperature (especially for the LC–Iso or Iso–LC phase transition) with the ether linkage is typical for usual calamitic LCs [69,70,71], including bent LC dimers [28,30,31,33,37,39]. Additionally, CB2SCB is vitrifiable, whereas CB3CB and CB2OCB strongly crystallize. These trends in the LC phase-transition temperatures and crystallization or vitrification abilities of the three dimers could be attributed to their different linkages, that is, ether (C–O–C), methylene (C–CH2–C), and thioether (C–S–C). The C–O–C bond angle (118°) is larger than those of the other linkage types, which renders CB2OCB more anisotropic [72]. In addition, The higher rotational barrier [73,74] and stronger electron-donating property of the Ph–O bond may contribute to the molecular rigidity and intermolecular interactions of CB2OCB, respectively. These could result in higher TIN and TNNTB, a larger ΔTN, and, possibly, at least in part, a stronger crystallization tendency. The higher TINTB of CB3CB compared with that of CB2SCB is attributed to its higher molecular anisotropy owing to the larger C–CH2–C bond angle (~110°) compared with the C–S–C angle (~100°), as well as higher rigidity due to the higher rotational barrier of the C–CH2 bond compared with that of the C–S bond [75]. The higher Tm and TIN and stronger crystallization tendency of CB3CB compared with those of CB2SCB could also be attributed to the symmetric molecular structure of the former. A more bent, flexible C–S–C linkage and molecular asymmetry endow CB2SCB with lower phase-transition temperatures and a vitrification ability compared with CB3CB and CB2OCB [33,39,62].

3.2. Phase-Transition Behaviors of CBnCB, CBnOCB, and CBnSCB

As described in the Introduction, each bent CB-based dimer homolog series with the longer spacers CBnCB (n = 5, 7, 9, 11, and 13) [17,25], CBnOCB (n = 4, 6, 8, and 10) [24,25], and CBnSCB (n = 4, 6, 8, and 10) [62] exhibit the typical Iso–N–NTB phase sequence. The TIN, TNNTB, and ΔTN of these homologous series are plotted in Figure 5a–c, respectively, as a function of the total number of atoms in the spacer chain lengths, i.e., n for CBnCB and n + 1 for CBnOCB and CBnSCB, including the O and S atoms. The TINTB values of CB3CB and CB2SCB are included in the plots shown in Figure 5a,b.
Overall, the TIN and TNNTB values were approximately in the order of CBnOCB > CBnCB > CBnSCB, which could be similarly ascribed to the characteristics of the linkers described in Section 3.1 for the shortest homologs. Nevertheless, the TNNTB values of the ether-linked CBnOCB were relatively close to or partly lower than those of the methylene-linked CBnCB. This observation may partly be attributed to the characteristics of the more anisotropic structure of CBnOCB because the NTB phase formation for a more anisotropic molecular structure likely requires greater supercooling of the N phase compared to a more bent one [33]. Consequently, the ΔTN values are in the order of CBnOCB > CBnSCB > CBnCB for all n, as shown in Figure 5c. The ΔTN values of the ether-linked CBnOCB homologs are significantly larger than those of the others for all n owing to their high TIN values.
Next, we investigated the n dependence of the phase-transition temperatures of the three homologs. The TIN (or TINTB) and TNNTB values for all the CBnCB, CBnOCB, and CBnSCB homologs increase with increasing n and then level off or gradually decline with a further increase in n, as shown in Figure 5a,b. Consequently, with increasing n, ΔTN increases for all the homologs, reaches a maximum, and then gradually declines for CBnCB and CBnOCB, as shown in Figure 5c. These trends of TIN (or TINTB), TNNTB, and ΔTN for all the dimer homologs could be attributed to the average molecular shape and flexibility with increasing/decreasing n, as shown in Figure 5d [39]. The shortest dimers could be more bent (strong biaxiality); hence, their TIN (or TINTB) and TNNTB values were lower than those of the longer dimers. With increasing n, the average molecular shape of the bent dimer homologs becomes more linear (or anisotropic), thereby increasing TIN and TNNTB to some extent. However, further lengthening of the central spacer could enhance the molecular flexibility and dilute the polarizable mesogenic arms that increase the phase-transition temperatures; hence, these phase-transition temperatures nearly remain constant or gradually decline. Thus, the molecular biaxiality (molecular curvature) of the dimer homologs increases with decreasing n, which principally decreases TIN, and consequently, ΔTN. This results in ΔTN = 0, i.e., the direct Iso–NTB phase transition for the shortest CB3CB and CB2SCB [62].

4. Conclusions

In this study, we evaluated the phase-transition behaviors of three CB-based dimers with propane, ethoxy, and ethylthio spacers. Analogous to the previously reported CB2SCB, the short CB3CB exhibited the rare direct Iso–NTB phase transition, whereas CB2OCB showed the typical Iso–N–NTB phase transition. CB3CB and CB2OCB have strong crystallization tendencies, whereas the thioether-linked CB2SCB exhibited a vitrifiable NTB phase. The NTB phase-transition temperature (TINTB or TNNTB) decreased in the order CB2OCB (76 °C) > CB3CB (49 °C) > CB2SCB (43 °C). The phase-transition behaviors of all the CBnCB, CBnOCB, and CBnSCB homologs, including those with longer chains, were comprehensively examined. The more anisotropic ether-linked CBnOCB series showed significantly higher TIN and wider ΔTN for all n. Regarding shorter spacers, the phase-transition temperatures decreased, especially TIN. Hence, the ΔTN for all three homologous series decreased, resulting in the direct Iso–NTB phase transition for the short-spacer-bearing CB3CB and CB2SCB. This phenomenon could partly be ascribed to their bent molecular geometry or enhanced molecular biaxiality owing to their short lengths. Our findings provide new insights into the effects of linkage types on the molecular design of LC dimers that exhibit the direct Iso–NTB phase transition.

Author Contributions

Conceptualization, Y.A. (Yuki Arakawa); methodology, Y.A. (Yuki Arakawa); validation, Y.A. (Yuki Arakawa); formal analysis, Y.A. (Yuki Arakawa), Y.A. (Yuto Arai), K.H., and K.K.; investigation, Y.A. (Yuki Arakawa), Y.A. (Yuto Arai), K.H., and K.K.; resources, Y.A. (Yuki Arakawa) and H.T.; data curation, Y.A. (Yuki Arakawa); writing—original draft preparation, Y.A. (Yuki Arakawa); writing—review and editing, Y.A. (Yuki Arakawa), Y.A. (Yuto Arai), K.H., and H.T.; supervision, Y.A. (Yuki Arakawa) and H.T.; project administration, Y.A. (Yuki Arakawa); funding acquisition, Y.A. (Yuki Arakawa). 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) and Toyohashi University of Technology.

Data Availability Statement

Data are presented in the article.

Conflicts of Interest

The authors declare no financial conflict of interest.

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Crystals 12 01734 g001 550
Figure 1. Bent dimer systems exhibiting the typical Iso–N–NTB and rare direct Iso–NTB phase transitions investigated in previous studies and the present study. (a) Previous work on bent-shaped CB dimer homologs with longer spacers, including methylene-linked CBnCB (odd n = 5, 7, 9, 11, and 13) [17,25], methylene-/ether-linked CBnOCB (even n = 4, 6, 8, and 10) [24,25], and methylene-/thioether-linked CBnSCB (n = 4, 6, 8, and 10) [62] that exhibit the typical Iso–N–NTB phase sequence. (b) Previous work on dimers showing the rare Iso–NTB phase sequence from binary mixtures reported by Archbold et al. [63] (right) and single-component dimers reported by Dawood et al. [64,65] (imine-linked dimers, top left), our group [62] (ethylthio-linked CB2SCB, center left), and Wang et al. [66] (a phosphine-bridged dimer, bottom left). (c) Single-component dimers CB3CB and CB2OCB in the present study.
Figure 1. Bent dimer systems exhibiting the typical Iso–N–NTB and rare direct Iso–NTB phase transitions investigated in previous studies and the present study. (a) Previous work on bent-shaped CB dimer homologs with longer spacers, including methylene-linked CBnCB (odd n = 5, 7, 9, 11, and 13) [17,25], methylene-/ether-linked CBnOCB (even n = 4, 6, 8, and 10) [24,25], and methylene-/thioether-linked CBnSCB (n = 4, 6, 8, and 10) [62] that exhibit the typical Iso–N–NTB phase sequence. (b) Previous work on dimers showing the rare Iso–NTB phase sequence from binary mixtures reported by Archbold et al. [63] (right) and single-component dimers reported by Dawood et al. [64,65] (imine-linked dimers, top left), our group [62] (ethylthio-linked CB2SCB, center left), and Wang et al. [66] (a phosphine-bridged dimer, bottom left). (c) Single-component dimers CB3CB and CB2OCB in the present study.
Crystals 12 01734 g001
Crystals 12 01734 sch001 550
Scheme 1. Synthesis of (a) CB3CB and (b) CB2OCB.
Scheme 1. Synthesis of (a) CB3CB and (b) CB2OCB.
Crystals 12 01734 sch001
Crystals 12 01734 g002 550
Figure 2. DSC curves of (a) CB3CB, (b) CB2OCB, and (c) CB2SCB upon the second heating (red lines) and cooling (blue lines) cycles at a rate of 10 °C min−1. Cr and G denote the crystal phase and glassy state, respectively. Panel (c) is reproduced from Ref. [62].
Figure 2. DSC curves of (a) CB3CB, (b) CB2OCB, and (c) CB2SCB upon the second heating (red lines) and cooling (blue lines) cycles at a rate of 10 °C min−1. Cr and G denote the crystal phase and glassy state, respectively. Panel (c) is reproduced from Ref. [62].
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Crystals 12 01734 g003 550
Figure 3. POM images during the Iso–NTB phase transition of CB3CB: (a) the Iso phase (52 °C), (b) the NTB texture appearance in the Iso phase (50 °C), (c,d) growth in the NTB texture (48.5 and 47.5 °C, respectively).
Figure 3. POM images during the Iso–NTB phase transition of CB3CB: (a) the Iso phase (52 °C), (b) the NTB texture appearance in the Iso phase (50 °C), (c,d) growth in the NTB texture (48.5 and 47.5 °C, respectively).
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Crystals 12 01734 g004 550
Figure 4. POM images of (a) the N phase (104 °C) and (b) the NTB phase (69 °C) of CB2OCB.
Figure 4. POM images of (a) the N phase (104 °C) and (b) the NTB phase (69 °C) of CB2OCB.
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Crystals 12 01734 g005 550
Figure 5. (a) TIN, (b) TNNTB, (c) ΔTN, and (d) schematic models of dimers with shorter and longer spacers showing anisotropy and flexibility. In (a,b), the TINTB values are plotted for CB3CB and CB2SCB because they exhibit the Iso–NTB phase transition. Panel (d) was reproduced from Ref. [39].
Figure 5. (a) TIN, (b) TNNTB, (c) ΔTN, and (d) schematic models of dimers with shorter and longer spacers showing anisotropy and flexibility. In (a,b), the TINTB values are plotted for CB3CB and CB2SCB because they exhibit the Iso–NTB phase transition. Panel (d) was reproduced from Ref. [39].
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Table
Table 1. Tm and the associated ΔH upon second heating and TINTB, TIN, and TNNTB upon cooling for CB3CB, CB2OCB, and CB2SCB.
Table 1. Tm and the associated ΔH upon second heating and TINTB, TIN, and TNNTB upon cooling for CB3CB, CB2OCB, and CB2SCB.
SampleHeatingCooling
CB3CBTm = 149.0 °C (ΔH = 30.9 kJ mol−1) TINTB = 49 °C a
CB2OCBTm = 139.7 °C (ΔH = 25.5 kJ mol−1) TIN = 108 °C a, TNNTB = 78 °C a
CB2SCBTm = 131.0 °C (ΔH = 24.4 kJ mol−1) b,c TINTB = 42.6 °C b
a Determined by POM. b Obtained from Ref. [62]. c Obtained upon first heating.
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Arakawa, Y.; Arai, Y.; Horita, K.; Komatsu, K.; Tsuji, H. Twist–Bend Nematic Phase Behavior of Cyanobiphenyl-Based Dimers with Propane, Ethoxy, and Ethylthio Spacers. Crystals 2022, 12, 1734. https://doi.org/10.3390/cryst12121734

AMA Style

Arakawa Y, Arai Y, Horita K, Komatsu K, Tsuji H. Twist–Bend Nematic Phase Behavior of Cyanobiphenyl-Based Dimers with Propane, Ethoxy, and Ethylthio Spacers. Crystals. 2022; 12(12):1734. https://doi.org/10.3390/cryst12121734

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Arakawa, Yuki, Yuto Arai, Kyohei Horita, Kenta Komatsu, and Hideto Tsuji. 2022. "Twist–Bend Nematic Phase Behavior of Cyanobiphenyl-Based Dimers with Propane, Ethoxy, and Ethylthio Spacers" Crystals 12, no. 12: 1734. https://doi.org/10.3390/cryst12121734

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