Counterion-Induced Nanosheet-to-Nanofilament Transition of

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Interface-Rich Materials and Assemblies

Counter Ion Induced Nanosheet to Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals Daoliang Wang, Qi Yan, Fei Zhong, Yahui Li, Ming Fu, Lingpu Meng, Youju Huang, and Liangbin Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02168 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Counter Ion Induced Nanosheet to Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals Daoliang Wangab, Qi Yana, Fei Zhonga, Yahui Lia, Ming Fub, Lingpu Menga, Youju Huangc* and Liangbin Lia* a

National Synchrotron Radiation Lab, CAS Key Laboratory of Soft Matter Chemistry, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, University of Science and Technology of China, Hefei 230026, China

b

Hefei Institute for Public Safety Research, Tsinghua University.

c

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,

Ningbo, 315201, China

*E-mail: [email protected].*E-mail: [email protected]

ABSTRACT: The smart flexibility of phase transition in liquid crystals (LCs) makes them suitable in various applications, and is an important research field of contemporary science, engineering, and technology. Unlike most reports focused on the bent-core LCs in the thermotropic situation, in our present study, we designed and synthesized a fully rigid bent-core molecule with the sulfonic acid group replacing conventional flexible chains. A rich variety of counter ions-induced supramolecular LC phase behaviors has been systematically investigated. It was found that the smectic phase with nanosheets tends to transform into the hexagonal phase with nanofilaments when the protons of the sulfonic acid group are partially replaced by alkali

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metal ions. The experimental results show that the nanoaggregate and phase transition are controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. Another interesting feature is that the achiral bent-core molecules self-assemble into columns by helical stacking and present macroscopic chirality, indicating that spontaneous chiral symmetry breaking occurs in the columnar phase. The fully rigid bent-core molecules reveal surprisingly hierarchical molecular self-assemblies with the smectic-to-hexagonal phase transition, which was not previously observed in supramolecular complexes. The findings will provide new possibilities for applications in LCs based photonic devices, bio-system switches, and supramolecular actuators.

KEYWORDS: liquid crystals, bent-core molecule, phase transition, counter ions

INTRODUCTION

Liquid crystals are one kind of matters with unique ordered structure lying between the completely isotropic phase, like a conventional liquid, and the completely ordered phase, like a crystal. Varying degrees of positional order, bond orientation order and molecular orientational order usually result in different types of liquid crystal (LC), such as nematic, smectic, cholesteric and hexagonal LCs.1-3 In the formation process of LC, the phase transition is a critical phenomenon with distinctive changes of structures and properties in the system. These smart phase transitions of LCs allow them suitable in various applications such as electro-optical displays, mass transport, functional surfaces, and energy conversion.4-8 David J. Thouless, who


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won the Nobel Prize in Physics 2016, also especially highlighted the importance of theoretical discoveries of topological phases and topological phase transitions of matter.9, 10 Thus, tuning LC phase transition, via externally controlling the order of simple molecular building blocks, would be an important area of contemporary science, engineering and technology.

Much effort has therefore been devoted to exploring the colorful LC phase behaviors in the past decades. The external force fields such as light, electricity, and temperature are widely used to tune the molecular position, orientation and rotation for the formation of LCs in thermotropic and lyotropic systems.3, 7, 11-19 It is widely common and effective to tune the temperature to investigate the phase behaviors in thermotropic LCs. Phase transitions are usually achieved either by heating a crystalline solid or by cooling an isotropic liquid, where significant changes for both positional and orientational order of mesogens take place. Unlike thermotropic LCs, which are typically composed of a single compound and present primarily temperaturedependent phase behaviors,20 lyotropic LCs usually have wide regions of stability with respect to temperature, pressure, and system composition.21 Conventional mesogens of lyotropic LCs are amphiphilic molecules containing one or more hydrophobic organic tails and a hydrophilic headgroup, such as detergents, phospholipids.21, 22 The amphiphilic molecules can spontaneously assemble in water to form phase-segregated assemblies with specific geometries and regular size, and their LC phase behaviors are mainly controlled by the molecular geometry and concentration.23-25 In the lyotropic systems, chromonic phases are another class of LCs and


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possess distinct structures and properties in water. Chromonic LCs are formed by a range of multi-ring aromatic compounds, which are generally disk-shaped or plank-shaped and possess ionic groups at their periphery for solubility in water.26,

27

Upon increasing the molecular

concentration, the nematic-hexagonal phase transition is often observed in chromonic systems.26, 28, 29

Apart from the force fields, the molecular structure is an important intrinsic factor to affect the LC phase behaviors. Up to now, LC mesogens with different shapes such as rod, fan, disk, bent-core, and other heterotypic shapes, have been synthesized and studied.30-36 In particular, bent-core mesogens have gained increasing concern due to their unique organized structures and physical properties from their non-complex structures. In contrast to rod-like and disk-like molecules, which can rotate freely along their molecular symmetry axes without affecting the order parameter of their LCs, the excluded volume of bent-core molecules becomes large and violates the liquid crystallinity when molecules rotate freely along their long molecular axes. These result in a variety of special LC properties, i.e., polarity and chirality, and plentiful phases, i.e. B1-B8 phases.32, 37, 38 However, most reports focused on bent-core LCs in the thermotropic situation, where flexible aliphatic chains were necessary to program LC. To the best of our knowledge, there is no report about bent-core LC that formed by fully rigid bent-core molecules without flexible chains, possibly due to numerous non-covalent interactions in the lyotropic system.


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Herein, we designed a fully rigid, bent-core molecule (designated as P52C) with sulfonic acid group replacing the conventional flexible chains (Fig.1A) and systematically investigated its lyotropic LC phase behavior. Thanks to the sulfonic acid group, P52C can be easily dissolved in the water, and at a critical concentration, a lyotropic LC phase, smectic phase with nanosheet structure appeared. It was very interesting to find that the smectic phase tended to transform into the hexagonal LC phase with nanofilament structure after the protons of the sulfonic acid group were partially replaced by alkali metal ions, which was not previously observed in supramolecular complexes of the lyotropic system. The smectic-to-hexagonal phase and nanosheet to nanofilament transitions were characterized by means of polarized optical microscopy (POM), small and wide angle X-ray scattering (SAXS and WAXS), and scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Our experimental results reveal surprisingly hierarchical molecular assemblies related across the smectic-tohexagonal phase transition and present new possibilities for applications in liquid crystals based photonic devices, bio-system switches, and supramolecular actuators.

EXPERIMENTAL SECTION 1.1. Materials Pyridine (Py) was distilled under reduced pressure over calcium hydride and then stored over 4Å molecular sieves. N, N-dimethylacetamide (DMAc) was purified by vacuum distillation over anhydrous magnesium sulfate and stored over molecular sieves (4 Å). 5-methylisophthalic acid


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was recrystallized from a water/ethanol mixture (1:2 volume ratio). All other regents and solvents were commercially available (Sinopharm Chemical Reagent Co. Ltd.) and were used without further purification. 1.2. Experimental Methods 1

H and

13

C NMR spectra were recorded on a Bruker AV300 NMR spectrometer at 300 and 75

MHz, respectively, in DMSO-d6 solutions. Mass spectral data were obtained on a Thermo LTQ instrument operated in electrospray ionization (ESI) mode to generate negative ion spectra using methanol/water mixture as a solvent, and formic acid as an additive agent. Polarizing optical micrographs were obtained using an Olympus (Japan) BX-51-P microscope system with a polarizing intermediate attachment and a full-wave retardation plate (λ= 530 nm). Transmission electron microscopy (TEM) observations were carried out on a JEM-2100F electron microscopy operated at 200 kV. The samples were stained with neutral phosphotungstic acid for 5 minutes and freely dried after being dropped on carbon-coated copper grids. Scanning electron microscopy (SEM) measurements were carried out on Sirion200 (FEI, America) with an acceleration voltage of 5 kV. SAXS/WAXS measurements were carried out using an in-house setup with a 30 W micro X-ray source (Incoatec, GmbH) and a multi-wire proportional chamber detector (Bruker, Hi-star). The radiation wavelength was 0.154 nm. Parts of the WAXS measurements were performed on synchrotron radiation X-ray scattering station with a radiation wavelength of 0.154 nm and Mar 345 image plate as a detector at NSRL (China). The Fit2D


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software package was used to analyze the 2-dimensional (2D) SAXS/WAXS patterns. All measurements were conducted at room temperature. 1.3. Synthesis of P52C molecules 1-methyl

N3 ， N5-bis[4'-(azobenzene-4-sulphonic

acid)phenyl]

benzene

-3,

5-

biscarboxamide (P52C). 4'-Aminoazobenzene-4-sulphonic acid (11.5 g, 41.5 mmol), 5methylisophthalic acid (3.73 g, 20.7 mmol) triphenyl phosphite (TPP, 25 mL) and Pyridine (Py, 20 mL) were dissolved in 130 mL of N, N-dimethylacetamide (DMAc) in a three-neck roundbottomed flask equipped with a magnetic stirrer under nitrogen. The mixture was then warmed up to 75 °C for 30 minutes under a nitrogen atmosphere. After that, the temperature was increased to 115 °C, and the solution was stirred for 5 hours. Then the reaction mixture was precipitated with acetone. The obtained solid was filtered and washed with acetone several times, and then dried in vacuo at 70 °C for 12 hours to give the crude product. The crude product was dissolved in distilled water and reprecipitated in HCl water at 0 °C. The precipitate was collected and freeze-dried in vacuo for 24 hours, and finally dried at 60 °C in a vacuum oven until it reached a constant weight. The yield was 83% (12.0 g). 1H NMR (300 MHz, DMSO-d6) δ (ppm) = 10.78 (s, 2H), 8.42 (s, 1H), 8.09 (d, J=8.6 Hz, 4H), 8.04 (s, 2H), 7.98 (d, J=8.6 Hz, 4H), 7.86 (d, J=8.1 Hz, 4H), 7.79 (d, J=8.2 Hz, 4H), 2.54 (s, 3H). 13C NMR (75 MHz, DMSO-d6) δ (ppm) = 166.0, 152.3, 150.6, 148.4, 142.8, 138.9, 135.4, 132.0, 127.2, 125.0, 124.1, 122.5, 121.0, 21.4. EI-TOF MS: calcd for (C33H26N6O8S2) 698.13; Found: m/z 697.24 (M-H+), 348.20 (M-2H+).


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Elemental analysis calcd (%) for C45H33N9O12S3: C, 56.73; H, 3.75; N, 12.03; found: C, 56.31; H, 3.81; N, 11.87. 1.4. Preparation of samples Hydrogen ions in sulfo group of P52C molecules were replaced with alkaline ions by using MOH (M: Li, Na, K, Cs) solutions. According to the displacing molar ratio of M: H and molecular concentration, calculated volumes of the alkaline solution with standard concentration were added to the P52C aqueous solutions to obtain the experimental samples.


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RESULTS AND DISCUSSION

Figure 1. (a) Chemical structure of P52C molecule and illustration of counter ions substitution for alkaline ions, photos and polarizing optical micrographs of P52C (b, d) and P52CK2 (c, e) molecules in water at a concentration of 0.143 mmol/mL.


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The chemical structure of the designed bent-core molecule is shown in Fig.1a, consisting of 5 phenyl rings and two sulfo groups at its periphery (denoted as P52C). Due to the presence of sulfo groups, P52C can be easily dissolved in water. Interestingly, when hydrogen ions of P52C were replaced with alkaline ions (Li, Na, K, Cs) at the same concentration, the macroscopic physical property significantly changed from initial solution state to gel-like state with high viscosity (Fig.1b and c). A dramatic change was also observed in POM measurements. Taking the aqueous sample at a concentration of 0.143 mmol/mL for example, POM images shows that LC phase was formed in the P52C solution and displayed a grain-like texture (Fig.1d), which, however, converted to a stringlike texture as the H+ were replaced with K+ in P52CK2 sample (Fig.1e).

Figure 2. Polarizing optical micrographs of P52C (a-c) and P52CK2 (d-f) molecules in water at concentrations of 0.086 mmol/mL and 0.158 mmol/mL, respectively, (a) normal POM of P52C sample; (b) same as (a), but using the full wave (λ = 530 nm) retardation plate whose slow axis is


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marked by a blue arrow; (c) same as (b), but rotating 90°counter clockwise, (d) P52CK2 sample before shearing, (e and f) shear directions perpendicular and parallel to the slow axis, respectively（the white arrow indicates the shear direction）. In order to clearly investigate the LC texture, the full wave (λ= 530 nm) retardation plate was added to the POM system. Interestingly, the grains without color difference observed in normal POM (Fig. 2a) can be discriminated between yellow and green by using the full wave (λ= 530 nm) retardation plate (Fig. 2b), and the colour inversion occurred after rotating the sample by 90°counter clockwise (Fig. 2c). In contrast to the grains, the shear flowed P52CK2 sample possess positive birefringence as the sample appeared green and yellow with the flowed direction parallel and perpendicular to the slow axis of the retardation plate, respectively (Fig. 2e and 2f). That is, the index of refraction along the flowed direction is higher than that in the perpendicular direction. As the bent-core molecules resemble the polyaramid, which is always optical negative with the molecular chains parallel to the flow direction.39, 40 The positive birefringence observed in P52CK2 sample suggests that the bent-core molecules are likely stacking perpendicular to the flow direction. Another important phenomenon is the grains in the same direction observed in Fig. 2b and 2c can be either yellow or green, indicating that the index of refraction in the grain plane is possible to be higher or lower than that along the normal, which may be attributed to the bent-core feature of P52C molecules.


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Figure 3. X-ray scattering patterns of P52C (a, c) and P52CK2 (b, d) aqueous samples at the same concentration of 0.143 mmol/mL: (a-b) small-angle region, (c-d) wide-angle region. The corresponding 1D SAXS (e) and WAXS (f) curves for the P52C and P52CK2 samples. To determine the structural evolution of the phase transition, small and wide-angle X-ray scattering (SAXS and WAXS) studies were conducted at room temperature. Fig. 3a, b and. Fig. 3c, d show the two-dimensional (2D) SAXS and WAXS patterns of P52C and P52CK2 aqueous samples. The corresponding one-dimensional (1D) SAXS and WAXS curves at the same concentration (0.143 mmol/mL) are shown in Fig. 3e and 3f, respectively. The obvious scattering difference indicates that the transition of LC phase and molecular organization take place after displacing the counter ions in P52C molecules in aqueous solution. According to the SAXS


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curves (Fig. 3e), P52C sample possesses three distinct diffraction peaks with a q value ratio of 1:2:3, which are indexed as (10), (20) and (30) and their d-spacings are calculated to be 9.01 nm, 4.51 nm and 3.01 nm, respectively, suggesting that lamellar structure is formed in the P52C aqueous solution. However, P52CK2 present three clear diffraction peaks and an inconspicuous peak with a q value ratio of 1:31/2:2:71/2, indicating that the lamellar phase has transformed into hexagonal LC phase, where the three clear peaks can be indexed as (10), (11) and (20) and their d-spacings are calculated to be 9.23 nm, 5.32 nm and 4.62 nm, respectively. In the wide-angle region (Fig. 3f), three diffraction peaks in the P52C sample are observed at 15.4 nm-1, 16.6 nm-1 and 18.7 nm-1, denoting period distances of 0.408 nm, 0.378 nm, and 0.336 nm, respectively. However, two diffraction peaks in P52CK2 sample are observed at17.7 nm-1 and 18.3 nm-1, denoting period distances of 0.355 nm and 0.343 nm, respectively. These different peak configurations in SAXS and WAXS between P52C and P52CK2 indicated that smectichexagonal phase transition and molecular rearrangement appeared upon replacing the counter ions of the bent-core molecules. Additionally, in contrast to thermotropic liquid crystals, which always have diffused diffraction peaks in the wide-angle region, denoting the short-range order of arrangement at the molecular level, the lyotropic liquid crystals formed by fully rigid bentcore molecules possess long-range order for both in-plane and interlayer arrangement.


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Figure 4. (a-b) Scanning electron microscopy images of the P52C aqueous sample after drying. (c-d) Transmission electron microscopy images of the P52CK2 aqueous sample (stained by neutral phosphotungstic acid). Fig. 4a, b, and 4c, d present the scanning electron microscopy (SEM) images and the transmission electron microscopy (TEM) images for P52C and P52CK2 samples, respectively. Lamella stacking morphology is observed for the P52C sample in Fig. 4a and b, and gives a lamellar thickness of about several nanometers. However, fibrous morphology is observed for P52CK2 in Fig. 4c and d, and gives a diameter of about 8 nm for single nanofilament. Interestingly, the nanofilaments tend to pack into microbundles with diameters from dozens to hundreds of nanometers, which may be packed hexagonally according to the SAXS results.


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Moreover, the diameter of single nanofilament is close to the inter-column spacing a of ~10 nm in the hexagonal phase, Thus, the lyotropic columnar phase is possibly formed via a hexagonal arrangement of the nanofilaments in the P52CK2 sample. The POM, X-ray scattering and electron microscopy results all suggest that dramatically structural transformation happened after the H+ of the sulfonic acid group were replaced by K+. Fig. 5 shows that different alkali metal ions such as Li、Na、K、Cs were used to replace the protons of the sulfonic acid group for investigation the effect on LC phase transition. All the SAXS curves (Fig. 5A) show three diffraction peaks (10), (11) and (20), indicating the formation of hexagonal LC phase. The distinctive feather is that at the same concentration, counter ions replaced P52C show decreased q value of (10) peak in sequence from Li, Na, K to Cs, indicating increased spacing a between adjacent columns (Fig. 5B). This may ascribe to the increased electrostatic repulsive force, as the counter ions possess increased ionization potential and size in the sequence of Li、Na、K、Cs.


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Figure 5. (A) 1D SAXS curves for P52C samples (0.158 mmol/mL) displacing sulfonic acid with different alkali metal ions Li, Na, K, Cs. (B) The distance a (center distance between adjacent columns) versus the ionic radius of alkali metal ions. To investigate the effect of the concentration of P52C and P52CK2 molecules in water on the phase transition, SAXS measurements were studied systematically (S-Fig. 1). Decreasing the concentration of the bent-core molecules leads to an expanding of the periodic distance for both the lamellar and hexagonal phases, which is manifested by the reduction of q value of (10) peak. It is worth noting that both the lamellar and hexagonal symmetries in two sample systems persist well with decreasing the concentrations from 0.201 mmol/mL to 0.086 mmol/mL. However, the ordered lamellar structure disappears rapidly when the concentration lowers to 0.057 mmol/mL, as the diffraction peaks of (10), (20) and (30) have all vanished from the SAXS curves. Therefore, the concentration of bent-core molecules in water is not the key parameter to affect the lamellar to hexagonal phase transition; instead, the LC structures are kept well within a certain range of concentration, but with expanding of the periodic distances.


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Figure 6. (A) 2D SAXS patterns of aqueous samples with different molar ratios of K+/P52C at a fixed molecular concentration of P52C of 0.158 mmol/mL, and the number in the lower left corner represents the K+/P52C molar ratio. (B) The corresponding 1D SAXS curves of samples at different K+/P52C molar ratios. (C) The periodic distance of (10) peaks in lamellar (L) and hexagonal (H) phases versus the molar ratio of K+/P52C. Fig 6A and B show 2D and 1D SAXS curves of samples with different molar ratios of K+ to P52C, which indicated that the phase transition was controlled by the molar ratio (R). When R is lower than 0.55, three diffraction peaks with a q value ratio of 1:2:3 and indexed as (10), (20) and (30) are found in the SAXS curves, indicating an ordered lamellar structure. As R is


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increased to 0.55, the SAXS curves undergo a significant change. Three new peaks indexed as (10), (11) and (20) emerge with a q value ratio of 1:31/2:2, suggesting that the lamellar-hexagonal phase transition has taken place. With the increase of R above 0.55, the hexagonal symmetry and long-range order keep well until R comes to 2.0, where three diffraction peaks (10), (11) and (20) still present well-defined position and sharp peak shape. However, if R increases to 2.2, the hexagonal symmetry and long-range order start to be destroyed, which is inferred by the substantial broadening of (10) peak and almost elimination of (11) and (20) peaks. After further increasing R to 4.0, only one diffused diffraction ring appears near beam-stop zone (low q value), indicating that the hexagonal symmetry with the long-range order has been destroyed thoroughly. However, WAXS and POM results show that the supramolecular aggregates are still preserved and “disordered” LC phase is formed (S-Fig. 2 and 3). Fig. 5C plots the periodic distance calculated from (10) peaks for both the lamellar and hexagonal phases versus the molar ratio of K+/P52C, which clearly presents the two-step phase transition with the increase of R. In Fig. 5C, two critical points denoting the two-step phase transition are resolved as 0.55 and 2.20 for R, respectively. Another fact which should be noted is that the periodic distance for (10) peak expands with the increase of R both in the lamellar and hexagonal phases, which probably stems from the increasing repulsive force as K+ possesses higher ionization potential as compared with H+ in the system.


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Fig. 7. Circular dichroism spectra of P52C (C) and P52CK2 (K-1, K-2) aqueous samples at concentrations of 0.029 mmol/mL, the K-1 and K-2 represent different testing batches. Based on SAXS data, the electron-density profiles were reconstructed for both the lamellar and hexagonal LC phases (Part 2 of support information). Noted that the lamellar LC phase gives two-peak profile in a long period of about 9 nm (S-Fig. 5), whereas the length of a P52C molecule is only about 3 nm, indicating that the smetic lamellae are nanoaggregates with the hierarchical superstructure. The electron-density map of the hexagonal LC phase is shown in SFig. 7, where columnar morphology is observed to form hexagonal symmetry. The electrondensity map also indicates that a column diameter is about 8 nm, which is in agreement with the diameter of single nanofilament observed in Fig 4d. As P52CK2 molecule is a bent-core, bananashaped molecule, it is possible to stack into the columnar structure.32,

41

In addition, X-ray

scattering patterns of the oriented sample (S-Fig. 8) and the POM images (Fig. 2e and f) show that the bent-core molecules are likely stacking perpendicular to the long axis of the columns (flow direction). Moreover, the WAXS results in Fig. 3f give a molecular layer spacing of about


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0.35 nm, which is a typical value for π-π stacking. Based on these results, the most possible selfassembly way is that P52CK2 molecule firstly assembled into an aggregate, and then followed a helical stacking model to finally form the column of hexagonal LC. Circular dichroism (CD) spectra of P52CK2 sample (Fig. 7) confirmed the helical stacking model in the columnar phases. As shown in Fig. 7, the CD spectrum of P52C sample gives no Cotton effect, whereas P52CK2 sample gives a bisignate signal with two Cotton effects around 320 nm and 355 nm and a crossover around 335 nm. However, positive or negative (designated by the sign of the first longwavelength band) CD bisignate signals may show up in different testing batches. Since the P52CK2 molecule is achiral, the detected macroscopic chirality indicating that chiral symmetry breaking exists in the columnar phases with a spontaneous and unpredictable way, similar to the other symmetry breaking systems. 42-45 To illustrate the two-step phase transition more directly, Fig. 8 shows a schematic model, in which R is the determining factor for the transition process. With the increase of R, the LC phase in P52C aqueous system takes an interesting evolution from smectic lamellar to hexagonal columnar and then to disordered phase. Two critical points of R at 0.55 and 2.2 indicate the two procedures of the phase transition, respectively. The origin of phase transition from sematic LC to hexagonal LC induced by the introduction of alkali metal ions should be attributed to the different ionization potential between alkali metal ions and H+. Alkali metal ions possess higher ionization potential as compared with H+ in the sulfo group, and the introduction of alkali metal


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ions will give rise to increasing of electrostatic repulsive force in the lamellar aggregates, which further lead to the destruction of lamellae and molecular rearrangement to form columnar aggregates with the hexagonal arrangement. Another fact which should be noted is that, around the critical point for phase transition (0.4 molar ratio), the long period in the lamellar phase is 9.2 nm, corresponding to the inter-column spacing (2 d10/√3=9.2 nm) in the hexagonal phase (Fig. 6C). Thus, the cylinders formed in the hexagonal phase may stem from the intra-lamella rearrangement of molecules in the lamellar phase, as the intra-lamellar rearrangement of molecules will perhaps lead to the easiest way to form hexagonal cylinders and give the comparable long period and inter-column spacing in lamellar phase and hexagonal phase, respectively.

Figure 8. Schematic representation of the two-step phase transition induced by alkali metal ions in the lyotropic bent-core liquid crystals.

Our results demonstrate that fully rigid bent-core molecules can adopt different selfassembled mode to construct nano aggregates and ordered phases in aqueous solution. The amphiphilic molecules with hydrophilic headgroup and hydrophobic flexible tails can form


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phase-segregated structures in water, which is mainly controlled by the molecular geometry and concentration.21, 25 However, the present fully rigid bent-core molecule is a new-type lyotropic mesogen. Our results show that the bent-core molecules can assemble to not only lamellar structure like rod mosegens but also columnar structure like disk mesogens. In other words, the bent-core molecules have great flexibility in the self-assembled behaviors, and the counter ions play a key role in the formations of their nanoaggregates and LC phases. Analogous phase transition behavior has also been reported by T. Kato et al in the thermotropic LCs of the folic acid derivatives by the addition of alkaline metal salts.46, 47 In our studied system, the displacing ratio of alkali metal ions rather than the molecular concentration determines the nano aggregates and phase transition process of the P52C molecules. Another important fact, which should be noted, is that the achiral P52CK2 molecules form columns by helical stacking and present macroscopic chirality. Unlike the flexible amphiphilic molecules, which can form columns with a wall of liquid layers by flexible hydrophobic tails, the rigid P52CK2 molecules cannot pack freely to shield their hydrophobic rods from the water. In order to shield their hydrophobic rod and reduce the electrostatic force among end ionic groups simultaneously, the helical stacking mode seems to be the best choice for the rigid bent-core molecules. In addition, because the bentcore molecules are achiral, right- and left-handed domains can be obtained by chance, as is evidenced by the mirror image CD curves in Fig. 7. This phenomenon is analogous to that observed for the thermotropic LCs of bent-core mesogens reported in previous literature.48 The macroscopy chirality observed in our system indicates that chiral symmetry breaking occur in the


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formation process of the nanoaggregates and LC phases, which may attribute to the cooperation interactions and directionality of hydrogen bonds according to previous literatures.49-52 There is no doubt that the nano aggregates and LC phases formed in water are regulated through the coordination of various interactions, such as dispersion forces, hydrophobic interactions, electrostatic force. CONCLUSION We have demonstrated that counter ions would successfully induce supramolecular LC phase transition in a bent-core LC system. A series of technical tools such as POM, WAXS, SAXS, TEM, SEM, CD were employed to systematically study this rich variety of counter ionsinduced supramolecular LC phase processes, including lamellar-to-hexagonal LC phase and ordered-to-disordered phase. It was found that the bent-core molecules of P52C tend to form the smectic lamellar phase, in which the lamella were nanoaggregates with hierarchical superstructure, and the interlamellar spacing was controlled by the concentration of P52C molecules. Another distinctive feature is that the system would transform into hexagonal LC phase if the protons of the sulfonic acid group were replaced by alkali metal ions. Moreover, the columns in the hexagonal LC phase were formed by the helical stacking of P52CK2 aggregates. The nanoaggregates and phase transition were verified to be controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. The bent-core molecule reveals a surprisingly complex molecular assembly with the smectic-to-hexagonal phase transition, which has not been previously observed in supramolecular complexes in the lyotropic system. The


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work provides new possibilities for applications in liquid crystals based photonic devices, biosystem switches, and supramolecular actuators.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Evolution of 1D SAXS curves with a decrease of concentration, polarizing optical micrographs of selected samples with different molar ratios, 1D WAXS curves for samples with different K+/P52C molar ratios, the electron-density profiles reconstructed by the SAXS data of P52C and P52CK2 aqueous samples, X-ray scattering patterns of the oriented sample.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]

*E-mail:: [email protected]

ACKNOWLEDGMENTS

We gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 51703117, 51473179, 51633009, 51873222), Fujian Province-Chinese Academy of Sciences


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STS project (2017T31010024), and Youth Innovation Promotion Association of Chinese Academy of Science (2016268).


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Counterion-Induced Nanosheet-to-Nanofilament Transition of

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