Carbonyl- and thioether-linked cyanobiphenyl-based liquid crystal dimers exhibiting twist-bend nematic phases
Introduction
Liquid crystals (LCs) are fascinating mesophases between solid crystals and fluidic isotropic (I) liquids. The nematic (N) phase is the most fluidic non-layered LC phase, which is important for current LC displays. The cholesteric (Ch) phase, one of the chiral variants of the N phase, has a helicoidal director helix with a pitch length usually greater than several hundred nanometers (Fig. 1). The twist-bend nematic (NTB) phase, another helical N phase with a heliconical director helix, is a newly identified N phase that is found in an achiral bent molecule (Fig. 1) [1]. The NTB phase forms when bent molecules undergo twist and bend deformations, spontaneously giving rise to right- and left-handed heliconical-nanostructured chiral degenerate domains of directors, even in achiral molecules. This species was first proposed independently by Meyer [2], and later by Dozov [3], and was simulated by Memmer [4]. Then, in-depth reinvestigation [1,[5], [6], [7]] and identification of the unknown nematic (Nx) phase below the conventional N phase of achiral bent LC dimers were conducted [1]. The heliconical pitches are considered to have nanoscale lengths from several to tens of nanometers based on electro-optical analysis [8], transmission electron microscopy [9,10], and resonant X-ray scattering [[11], [12], [13], [14], [15]], which is significantly shorter than those of the helicoidal Ch phase.
Owing to the parity of central flexible alkylene spacers, dimeric molecules are either approximately linear or have bent geometries. However, the linear geometry is not favorable in the formation of a heliconical structure in the NTB phase. Indeed, the majority of twist-bend nematogens are bent dimeric molecules containing odd carbon number alkylene spacers [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]]. The NTB phase has also been observed in higher oligomers [[46], [47], [48], [49], [50], [51], [52], [53]], polymers [54], classic bent-core molecules [55,56], and hydrogen-bonded molecules [57,58]. Furthermore, various studies to identify the potential applications of NTB materials have been carried out in the context of photonic devices [[59], [60], [61], [62]], memory devices [63], LC gels [64], elastic bodies [65], and photo alignment with polarized light [66].
In terms of molecular structure, the molecular bend and appearance of the NTB phase are strongly influenced by the linkage bond bridging the mesogenic arms and the central flexible spacer. In contrast to methylene-linked cyanobiphenyl (CB) dimers with odd numbers of alkylene spacers (n) (i.e., CBnCB), which are known to be twist-bend nematogens [1], their ether-linked counterparts (i.e., CBOnOCB) are conventional nematogens [67]. So far, various linkage combinations, the resulting molecular bend angles [26,36,37,39,40,45,68], and intramolecular torsions [32,33,68] were verified to understand molecular structures of LC dimers to induce the NTB phase.
Compared with LC dimers containing different linkages (e.g., methylene, ether, ester, and imine), as far as we are aware, LC dimers containing a carbonyl linkage (i.e., ketone-based LC dimers) are relatively rare. For example, Mandle et al. reported a few examples of carbonyl-linked (ketone- and diketone-type) CB-based LC dimers exhibiting the NTB phase [26]. In addition, Lesac et al. reported several bis(carbonyl)-linked (or diketone-based) LC dimers that exhibit LC phases [32,33,69], including the NTB phase [32,33]. We also reported an asymmetric carbonyl- and thioether-linked CB-based LC dimer containing a pentamethylene spacer (CBCO5SCB, Fig. 2), which exhibits the NTB phase [39]. However, its homologous series has not been fully developed, and hence, the influence of the spacer length on mesogenic behavior has not yet been revealed.
Herein, we report the synthesis of a homologous series of carbonyl- and thioether-linked CB-based dimers, CBCOnSCB, where n = 3, 5, 7, and 9 (Fig. 2). The phase transitions of CBCOnSCB are investigated and compared with those of previously reported symmetric bis(thioether)-linked CBSnSCB and asymmetric ether- and thioether-linked CBOnSCB analogs (Fig. 2) [36,[38], [39]]. Furthermore, single-crystal X-ray structural analysis of CBCO5SCB is also reported.
Section snippets
Experimental
CBCOnSCB homologs (n = 3, 7, and 9) were newly synthesized according to three steps; Friedel-Crafts acylation, thioetherification, and Suzuki-Miyaura coupling (Fig. 2), which were performed in a manner similar to the synthesis of CBCO5SCB [39]. Detailed synthetic procedures are described in the Supporting Information. The molecular structures were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy using a JNM-ECX500 spectrometer (JEOL Ltd., Tokyo, Japan) and by high-performance
Phase-transition behavior of CBCOnSCB
The phase-transition temperatures observed upon second heating and cooling are summarized in Table 1, and phase diagrams upon cooling as a function of n are shown in Fig. 3. All CBCOnSCB homologs were found to be mesogenic. Although the longest CBCO9SCB displayed only a monotropic N phase, CBCOnSCB (n = 3, 5, and 7) exhibited both an N phase and a monotropic NTB phase.
As previously reported, CBCO5SCB showed different melting points (Tm) of 145.0 °C and 120.9 °C upon its first and second
Conclusions
We developed a homologous series of ketone-type LC dimers, namely asymmetrically carbonyl- and thioether-linked cyanobiphenyl dimers (CBCOnSCB, n = 3, 5, 7, and 9), and evaluated their phase-transition behaviors. CBCOnSCB (n = 3, 5, and 7) exhibited both the NTB and N phases, whereas CBCO9SCB displayed only the N phase. The NTB phase temperature range expanded up to n = 5 with increasing n, prior to decreasing until the NTB phase disappeared at n = 9. Overall, the phase-transition temperatures
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the Japan Society for the Promotion of Science (KAKENHI grant no. 17K14493 and 20K15351), the Naito Research Grant, and research grants from the Toukai Foundation for Technology and Toyohashi University of Technology. We are grateful to Ms. Tsugumi Shiokawa and Dr. Hiroko Tada from the Division of Instrumental Analysis (Okayama University) for carrying out the mass spectrometry measurements.
References (81)
- et al.
Giant
(2020) - et al.
J. Mol. Liq.
(2019) - et al.
J. Mol. Struct.
(2020) - et al.
J. Mol. Liq.
(2020) - et al.
J. Mol. Liq
(2021) - et al.
Phys. Rev. E.
(2011) Structural Problems in Liquid Crystal Physics, Molecular Fluids: Summer School in Theoretical Physics, les Houches lectures 1973
Europhys. Lett.
(2011)Liq. Cryst.
(2002)- et al.
Phys. Rev. Lett.
(2010)
Appl. Phys. Lett.
Phys. Rev. E
Phys. Rev. Lett.
Nat. Commun.
Proc. Natl. Acad. Sci. U.S.A.
Phys. Rev. Lett.
Phys. Chem. Chem. Phys.
Soft Matter
Adv. Mater.
Liq. Cryst.
J. Phys. Chem. B
Phys. Chem. Chem. Phys.
Chem. Eur J.
Chem. Eur J.
Phys. Chem. Chem. Phys.
Angew. Chem. Int. Ed.
RSC Adv.
RSC Adv.
Liq. Cryst.
Sci. Rep.
Liq. Cryst.
Liq. Cryst.
J. Am. Chem. Soc.
Liq. Cryst.
Liq. Cryst.
Soft Matter
Liq. Cryst.
Liq. Cryst.
CrystEngComm
New J. Chem.
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2022, Journal of Molecular LiquidsCitation Excerpt :It is important to note that in all these studies, the thioether (C-S-C) bond angle was estimated at around 100° which is in consistent with the one reported in the present study (100.4°). This value is also in excellent agreement with the one (100.6°) obtained by single-crystal X-ray structural analysis [34]. Ascribed to the larger C-O-C angle 119.2°, CBO7OCB dimer has an almost linear molecular geometry with a molecular bend of 145.3° and therefore tends not to form the Ntb phase.