Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an

Subscriber access provided by MT ROYAL COLLEGE

Article

Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound Jitendra Kumar, and Veena Prasad J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11733 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound

Jitendra Kumar and Veena Prasad*

Centre for Nano and Soft Matter Sciences, P.B. No. 1329, Jalahalli, Bengaluru - 560 013, India

*

Corresponding author: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Here, we report the observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of non-symmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarising optical microscope, differential scanning calorimeter, X-ray diffraction and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency (f < 1 kHz) measured in the range 10 Hz - 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Liquid crystals (LCs) composed of achiral bent-core mesogens are of significant interest due to their intriguing properties, viz., some of their mesophases exhibit polarity and chirality due to close packing of molecules and spontaneous breaking of the mirror symmetry. As a result, they may respond to the external stimuli leading to ferroelectric and anti-ferroelectric states.1,2 The development of polar order in the mesophases of bent-core compounds provides significant potential for practical applications, e.g., electro-optical and nonlinear optical devices.3 Since the pioneering work by Niori et al.4 immense research have been carried out on bent-core compounds, not only for the sake of the interesting properties of their smectic mesophases (B2 phase) but also for their potential to form biaxial, ferroelectric, twist bend type of nematics.5-12 The nematic mesophases of these bent-core compounds may also exhibit unusual properties such as chiral domains13, sign inversion in dielectric anisotropy,14 fast switching time and large flexoelectricy,15 which are not observed in the case of classical nematics formed by calamitic mesogens. Thus, the bent-core nematics are quite distinct from that of calamitic nematics and hence, could be used for the design of new electro-optical devices. Nematic phases in bent-core mesogens are rarely observed as these molecules have a high tendency to form mesophases with positional long range order. The local polar order in bent-core nematics exists due to the short range molecular correlation forming smectic nanoclusters. The correlation length of polar order can be increased on applying sufficiently high electric field which results in a ferroelectric-like switching. It appears that the ferroelectricity and biaxiality are strongly correlated, viz., the observation of both of these properties have been reported in a bent-core compound based on 1,2,4-oxadiazoles.7 Photinos et al.16 argued that the formation of a biaxial phase proceeds via the formation of locally biaxial clusters of bent-core molecules that would be inevitably polar.



In our previous study,13 we synthesized and investigated the liquid crystalline properties of several homologous series of compounds including A-10 and D-10, shown in Figure 1. Although, all these bent-core compounds exhibit enantiotropic nematic mesophases and in some cases underlying smectic phases as well, none of them are found to be electrically switchable. In an effort to obtain electrically switchable nematic mesophase in such systems, we designed and synthesized a new bent-core azo compound BC-F with a lateral fluoro substitution, as shown in Figure 1. This compound exhibited switchable nematic and tilted smectic mesophases over a wide temperature range with a fairly low transition temperatures. To the best of our knowledge, this is the first example of a bent-core mesogen exhibiting both electrically switchable nematic and smectic mesophases.

Figure 1. Molecular structure of the newly synthesized bent-core azo compound BC-F and previously reported similar compounds.

MATERIAL The new bent-core compound BC-F in the present study was synthesized using a procedure described by us earlier,13 for similar compounds. The structure and the purity of the compound are confirmed by using organic spectroscopy and elemental analysis. The analytical data obtained are as follows:


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yield 22.3%. IR (KBr) νmax: 2917, 2851, 1723, 1609, 1509, 1469, 1259, 1174, 1076, 1017, 837, 758, 650 cm-1. 1H NMR (CDCl3, 400 MHz), δH: 8.14 – 8.26 (m, 4H, ArH), 7.84 – 7.92 (m, 2H, ArH) 7.27 – 7.45 (m, 7H, ArH), 6.97– 7.15 (m, 5H, ArH), 4.04 – 4.08 (m, 4H, – OCH2–), 3.06 (s, 3H, Ar–CH3), 1.26 – 1.85 (m, 36H, –CH2–), 0.87– 0.90 (m, 6H, –CH3). Elemental analysis: C56H67FN2O8 requires, C, 73.50; H, 7.38; N, 3.06 %; found, C, 73.66; H, 7.28; N, 3.08%.

EXPERIMENTAL Optical textural observations were made using polarising optical microscope (POM), Olympus BX51 POM system equipped with a Mettler Toledo FP 82HT hot stage. The phase transition temperatures and the associated enthalpies were determined by using the differential scanning calorimeter (DSC), Perkin Elmer DSC-Diamond series. The heating and cooling rates were 5 °C/min. X-ray diffraction (XRD) studies were performed as a function of temperature, using X-ray diffractometer with a microfocus beam generator, model Genix Cu/MAR 345 (Xenocs, France). The samples were filled in a Lindemann capillary tube (0.7 mm diameter) and the sample temperatures were controlled with a precision of 0.1 °C using a homemade heater and a temperature controller. The 2θ scans were generated from the 2D diffraction patterns using the software package FIT2D developed by A.P. Hammersley of the European Synchrotron Radiation Facility. The dielectric and electro-optical (E-O) measurements were carried out in commercially available indium tin oxide (ITO) coated planar and homeotropic cells (Instec Inc., USA) of 9 µm thickness. The effective area of planar and homeotropic cells is 5x5 mm2 and 10x10 mm2, respectively. A function generator (Agilent, 33250A) in conjunction with a high voltage amplifier (Trek, PZD700) was used for E-O study. The polarization reversal current was measured across a 2.15 kΩ resistance



connected in series with the sample cell by applying standard triangular wave technique,17 and current response was recorded using an oscilloscope (Agilent, DSO5014A). The real (ε′⊥) and imaginary part (ε′′⊥) of the complex permittivity of the sample were recorded in the frequency range from 10 Hz to 5 MHz using LCR meter (HIOKI IM3536). The measuring voltage was 0.4 Vrms for dielectric study. The dielectric strength and relaxation frequency were obtained by fitting the dielectric spectra to the Havriliak-Negami (H-N) equation.18

RESULTS AND DISCUSSION Thermal Behavior. In the present study, we designed and synthesized a new bentcore compound BC-F, composed of non-symmetric azo molecules with a lateral fluoro substitution. The DSC thermograms for this compound are shown in Figure 2 and the thermal analyses obtained are as follows:

14 12 Heat flow (mW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

SmC

10

NcybC

Cr

8 6

Cr

Iso

NcybC

SmC

Iso

4 2 0 60

80

100

120

140

160

180

Temperature (°C)

Figure 2. DSC thermograms for heating and cooling cycles of the compound BC-F. Cr 111.1 °C (∆H: 23.4 kJ/mol) SmC 118.4 °C (∆H: 10.1 kJ/mol) NcybC 170.7 °C (∆H: 0.4 kJ/mol) Iso. The compound A-10, with X = H in Figure 1, exhibits enantiotropic SmA and N mesophases in addition to a monotropic unidentified SmX1 mesophase.13 When X = Cl, the compound D-10 exhibits only an enatiotropic N mesophase.13 However, when Cl is replaced 6 ACS Paragon Plus Environment

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with highly electronegative fluorine atom, the compound BC-F of present study, exhibits enantiotropic nematic and smectic mesophases. Although it is stated that the effect of Cl is often similar to F on the mesomorphic properties,19,20 in the present case we found that the effect of F is much different from Cl. As can be seen, the compound BC-F exhibits a smectic mesophase in addition to a nematic mesophase. This may be because, the F being more electronegative than Cl, the compound BC-F is experiencing higher intermolecular attractions compared to interlayer attractions. May be due to this reason, the melting point of the compound BC-F is found to be lower than the D-10 and the isotropic point of BC-F is higher than D-10. The POM textures observed in the mesophases of the compound BC-F are shown in Figure 3. Figure 3a shows the nematic droplets formed just below the isotropic temperature, which is generally an indication of cybotactic nature of the nematic mesophase.21 Figure 3b shows the nematic texture in a homeotropic cell at 160 °C. As can be seen, two brush disclinations dominate in the schlieren texture, which is an indication of the possibility of a biaxial nematic phase.11 Nevertheless, the observation of two brush disclinations is not the confirmation of biaxiality. Additional experimental investigations, viz., conoscopy, Deuterium-NMR spectroscopy, electron spin resonance (ESR) spectroscopy etc. are required to confirm the nematic phase biaxiality.10

Figure 3. POM textures obtained on cooling from the isotropic liquid of the compound BCF: (a) nematic droplets between untreated glass slides (168.8 °C); (b) schlieren texture of 7 ACS Paragon Plus Environment


nematic phase in a homeotropic cell (160 °C); (c) smectic mesophase (115 °C) obtained on cooling (a). In a planar cell, uniform homogeneous texture was obtained for the nematic phase as shown in Figure 4. The bright and dark textures, when the rubbing direction of planar cell was set at 45° and 0° with respect to polarizer, are shown in Figure 4a and b, respectively, indicating that the molecules are uniformly aligned along the rubbing direction.

Figure 4. (a) and (b) textures as observed between crossed polarizers in planar cell with rubbing direction 45º and 0º with the polarizer or analyzer, respectively.

XRD Study. The mesomorphic behavior of the compound BC-F is also investigated by means of X-ray diffraction studies. A plot of intensity versus 2θ obtained from the XRD patterns in the nematic and smectic mesophases is shown in Figure 5a and b, respectively. The intensity of the small-angle peak in the nematic phase is higher than that of the wideangle scattering, indicating a cybotactic nature of this mesophase.22,23 In addition, the intensity of the small-angle scattering rises rapidly with decreasing temperature most probably due to increase in the size of the cybotactic clusters.24 The d value calculated from the small angle peak in this mesophase is temperature dependent, as shown in Figure 5c. It increases from 33.25 Å at 165 °C to 36.46 Å at 120 °C and then decreases on approaching the nematic to smectic transition temperature. This increase in d value on decreasing the temperature in the nematic phase is attributed to an increase in the lateral packing density of the bent-core 8 ACS Paragon Plus Environment

Page 8 of 28

Page 9 of 28

molecules leading to an additional chain stretching. Moreover, the maxima of diffuse scattering in the wide-angle region vary from 4.51 to 4.35 Å between 165 ºC and 120 ºC indicating an average intermolecular distance decreases with decreasing temperature. This type of temperature dependence of d value in bent-core compound is known.12,25 Also, the d value in the nematic phase is much smaller than the estimated molecular length. This indicates that the nematic mesophase is composed of skewed/SmC-like clusters. In the smectic mesophase, there appears two sharp reflections in the small angle region with d spacings of 34.43 Å and 17.14 Å, which are in the ratio of 2:1 indicating a lamellar structure. Here, the d value (34.43 Å) is smaller than the estimated molecular length (L=53.09 Å), obtained from Chem3D (MM2, energy minimization) assuming all-trans conformation for the alkoxy chains, indicating a tilted or SmC-like molecular arrangement within the layers. The tilt angle estimated from the molecular formula cosθ = d/L is found to be ~ 49º. 104

135 °C 155 °C 165 °C

(a)

(b)

Intensity (arb. unit)

500

400

300

200

105 °C

103

100 0

5

10

15

20

102

25

0

5

10

15 2θ θ (degree)

2θ θ (degree)

37.0 (c)

N

SmC

36.5 36.0 35.5 d/Å

Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35.0 34.5 34.0 33.5 33.0 90

100

110

120

130

140

150

Temperature (°C)


160

170

20

25


Figure 5. The intensity versus 2θ graph obtained for the compound BC-F (a) at different temperatures in the nematic mesophase (b) in the SmC mesophase and (c) temperature dependence of d-spacing of small angle reflection.

E-O Switching Study. The E-O switching measurements were carried out in a planar cell. Figure 6a shows the current response in the nematic and smectic mesophases at different temperatures. Under a triangular wave field of 7.7 Vpp µm-1 with 20 Hz frequency, a single current peak per half cycle was observed in the nematic phase, the intensity of which increases with increasing voltage. However, the current peak disappears at temperature few degrees above the isotropic transition temperature. In bent-core systems, smectic-like cybotactic clusters persists even in the isotropic phase, just few degrees above the isotropic point and thereby, switching current peak can be observed in this region also.26 Hence, the observed peak in the nematic phase can be considered due to the formation of polar order within the clusters under the application of an external field. In addition, a large dielectric permittivity value observed in the nematic phase, as described in the later section, is a characteristic feature of ferroelectric-like polar domains. The local polar domains in nematic phase can exist due to the bent shape of the molecules and dispersion interaction. The coherence length of these domains is small at high temperature and it increases with decreasing temperature as indicated by the increase in d values (Figure 5c). In the lower temperature region of nematic phase, we observed a less intense current response, which shifted to higher voltage resulting in lowering the polarization value. The shifting of switching current peak to high voltage in the lower temperature regions of nematic phase seems to be due to increase in size of the clusters. Hence, larger voltage is required to reverse the polarization direction. The temperature dependence of the polarization is shown in Figure 6b. The value of polarization goes on increasing on cooling the sample from the isotropic liquid and in the midst of nematic temperature range, it reaches a maximum and on further cooling, it 10 ACS Paragon Plus Environment

Page 10 of 28

Page 11 of 28

decreases. The maximum value of polarization is found to be ~105 nC cm-2 at 135 °C. This type of unusual behavior of polarization value in the nematic phase of a bent-core mesogen is reported earlier also.27,28 The observed value of polarization in nematic phase is relatively small compared to that of SmCP phase due to the smaller size of the clusters. Usually, the polarization in SmCP phase is found to be around 300-500 nC cm-2.9 Below the transition point to SmC phase, the polarization peak reappears and its value reaches to ~210 nC cm-2. The polarization value depends on the frequency and it decreases with increasing frequency as shown in Figure 6c. The polarization in the nematic phase can be measured upto 100 Hz. This observation also indicates that the switching current peak in the nematic mesophase is not due to the ionic impurity, as ionic impurity in mesophases cannot respond to frequencies more than 10 Hz.29 0.10

0.05

0.00

0 0.2 40

Applied voltage

Applied voltage (V)

178°C (Iso)

-20

(b)

(a)

0.1

20

-0.05

0.0

0 -20

200 Polarization (nC/cm2)

20

Applied voltage 160 °C (N) 140 °C (N) 125 °C (N) 107 °C (SmC)

Switching current (arb. unit)

40

100

-0.1

-40

-40

-0.2 -0.02

-0.02

0.00 0.02 Time (S)

0.00

0.04

0

-0.10 0.02

100

0.04

110

120

130

140

Temperature (°C)

Time (S)

180

(c)

160 140 Polarization (nC/cm2)

Applied voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80 60 40 20 0 0

10

20

30

40

50

60

70

80

90 100 110

Frequency(Hz)


150

160

170


Figure 6. (a) Switching current response of the compound BC-F at different temperatures in the nematic and SmC mesophases on applying triangular wave field (7.7 VPP µm-1, 20 Hz); Inset shows the current response in the isotropic phase (b) polarization as a function of temperature and (c) frequency dependence of polarization in the nematic phase under an electric field of 20 Vpp µm-1 at 150 ºC. Rather, unusual for bent-core compounds as reported earlier,30 the SmC phase of the compound BC-F also shows an interesting current response as a function of applied voltage, shown in Figure 7. The switching behavior in the smectic phase of this compound is different from those observed for ferro- or antiferroelectric behaviour of bent-core mesophases. Under the triangular wave field, the compound BC-F in the smectic phase develops a single current peak at low voltage and as the voltage is increased to 30 VPP µm-1, two more peaks appear during each half cycle as shown in Figure 7b. At this stage, circular domains with dark and bright fringes parallel to the smectic layer are observed with extinction cross being inclined with respect to the polarizer and analyzer, indicating a reorganization of molecules in the mesophase. On further increasing the applied field, the number of current peaks increases as shown in Figure 7c. Finally, at 35 VPP µm-1, all these peaks merged into three peaks accompanied by textural change, indicating the saturation. The emergence of three polarization current peaks for each half cycle as well as the textural aspects observed under the POM indicate a field induced ferrielectric state.31-34 At this stage, the fringe lines disappears indicating that the molecules have a synclinic arrangement in the layers leading to a ferroelectric state. The POM textures with and without applied field are shown in Figure 8. On reducing the voltage gradually from the state 7d, the number of peaks reduces to one and the texture relax back to its initial state after removing the voltage, which implies the randomization of polar order in the smectic layers of this mesophase. Thus, due to an intermediate ferrielectric state between two ferroelectric states, a tetrastable switching can be 12 ACS Paragon Plus Environment

Page 12 of 28

Page 13 of 28

obtained in this type of materials. Such materials are of significant interest because of their fundamental importance and potential applications. In order to understand this ferrielectric switching behavior, a structure model was proposed based on the assumption of competing interlayer interactions between neighboring and next neighbor smectic layers.35 Based on our results, we propose a model for the occurrence of the ferrielectric state and possible switching mechanism in this state, as shown in Figure 9. In the ferrielectric state, the ferroelectric region is separated by antiferroelectric layer, and the competing interlayer interactions between neighboring and next neighbor smectic layers can cause the ferrielectric-like state in LC compound. 60

0.04

0

0.00

-20

-0.04

-40

-0.08

-60 -0.04

-0.03

-0.02

-0.01

0.00

0.01

Applied voltage (V)

20

Current response (arb. unit)

0.08

100

0.10

50

0.05

0

0.00

-50

-0.05

-100

-0.10

-150 -0.04

0.02

-0.03

-0.02

Time (s)

-0.01

0.00

0.01


(b)

40 Applied voltage (V)

0.15

150

(a)

-0.15 0.02

Time (s) 200

0.05 50 0.00

0 -50

-0.05 -100

100

-0.03

-0.02

-0.01

0.00

0.01

0.1

50 0.0

0 -50

-0.1

-100 -150

-0.10

-150

0.2

150

Applied voltage (V)

100

-0.04

(d)

0.10

-200 -0.04

0.02


(c)


150

Applied voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.2 -0.03

-0.02

-0.01

0.00

0.01

0.02

Time (s)

Time(s)

Figure 7. Switching current response of the compound BC-F in the smectic mesophase at 110 °C under the electric field of (a) 12 VPP µm-1 (b) 30 VPP µm-1 (c) 32 VPP µm-1 and (d) 35 VPP µm-1, 20 Hz.



Figure 8. Evolution of texture of BC-F under triangular wave fields in the smectic mesophase at 110 °C (a) 0 V (b) 30 VPP µm-1 and (c) 35 VPP µm-1.

Figure 9. Schematic representation of molecular organization of the layers and possible switching mechanism in ferrielectric state with increasing voltage as follows: Ferroelectric (-) → Ferrielectric (-) → Ferrielectric (+) → Ferroelectric (+). The response time of a LC is an important parameter from the application point of view, i.e, lower the response time faster will be the electro-optical response of the device. The switching time is measured using a square wave voltage applied across a planar cell. The current response, as a function of applied field with a frequency of 20 Hz at 159 ºC in the nematic phase, has been depicted in Figure 10a. As can be seen, a polarization hump appears on the time scale away from the edge of the applied voltage. The time interval between the edge of the applied voltage and the maxima of the polarization hump gives the switching time


Page 14 of 28

Page 15 of 28

of the LC material.36 Under the low voltage, the polarization reversal current peak was relatively broad and it becomes sharper with increasing voltage. As shown in Figure 10b, the switching time decreases with increasing voltage because higher voltage forces molecules to switch faster. The fastest response time obtained in the nematic phase is 0.5 ms at an electric field of 11.1 V µm-1. (a)

11.1 V µm-1 10 V µm-1 9.4 V µm-1 7.7 V µm-1 6.6 V µm-1 5.5 V µm-1 4.4 V µm-1 3.3 V µm-1

0.2

0.1

4.0 (b)

20Hz

3.5 Switching time (ms)

0.3 Current response (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0 2.5 2.0 1.5 1.0

0.0

0.5 0.000

0.002

0.004

0.006

0.008

2

4

Time (S)

6 8 Electric field (V/µ µm)

10

12

Figure 10. Measurement of the switching times in the nematic phase of the compound BC-F at 159 ºC under the square wave voltage with 20 Hz; (a) polarization current-time behavior at different voltage and (b) switching time as a function of applied electric field. The flexoelectric behavior of a LC material contributes to the electro-optical properties of the material which is an important factor for display devices.37 Figure 11a and b show the electro-convection pattern obtained for the sample in planar cell on applying a dc and ac electric field, respectively. The sample showed the formation of static periodic pattern between the crossed polarizers at dc field of 1.5 V µm-1, wherein, the stripes are oriented along the rubbing direction. However, these stripes disappeared on increasing the applied dc voltage. Such a type of periodic stripe pattern has also been observed by others in bent-core nematics. It is stated that the formation of parallel stripe pattern in such nematic LCs may be due to the flexoelectric effect.38,39 However, under an ac field of 2.4 Vpp µm-1, the electrohydrodynamic (EHC) instability pattern was observed. 15 ACS Paragon Plus Environment


Page 16 of 28

Figure 11. Electro-convection pattern in the nematic phase at 150 ºC on applying a field of (a) 1.5 V µm-1 dc and (b) 2.4 Vpp µm-1, 20 Hz.

Dielectric Investigations. Dielectric relaxation spectroscopy is a useful tool to understand the collective response of LC compounds and molecular properties. The dielectric measurements were carried out using both planar and homeotropic cells in the frequency range between 10 Hz and 5 MHz. Temperature dependence of the dielectric strength (∆ε) and relaxation frequency (fR) are extracted in planar-aligned cell by fitting the real (ε′⊥) and imaginary part (ε′′⊥) of the dielectric permittivity to the following Havriliak-Negami (H-N) equation,18 ε∗⊥ ε⊥ ε ⊥ ε

∆

(1)

where ω is the angular frequency, ε∞ is the high-frequency dielectric constant, the relaxation time τ = 1/2πfR (where fR is the relaxation frequency), ∆ε is the relaxation strength, α and β are shape parameters, which describe the symmetric and asymmetric broadening of dielectric function, respectively. The distribution parameter β is found to be 1, while α varies from 0.93 to 0.98 over the entire nematic mesophase range indicating a slight distribution of relaxation times.


Page 17 of 28

1000 (a)

(b)

105 °C 111 °C 117 °C 128 °C 144 °C 156 °C 164 °C 170 °C

105 °C 111 °C 117 °C 128 °C 144 °C 156 °C 164 °C 170 °C

100 M1

ε⊥//

ε⊥/

100

10

10

M2

1 1 102

103

104 105 Frequency (Hz)

106

107

102

103


106

107

Figure 12. Behavior of (a) real (ε′⊥) and (b) imaginary (ε′′⊥) parts of dielectric permittivity as a function of frequency for the compound BC-F at different temperatures. 300

500 (a)

∆ε fR

0V 2V 3V 4V 5V

(b)

600

250 450

ε ′′ ⊥

400

400

f R (Hz)

200

∆ε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 100

350 200

50

300 100

110

120

130

140

150

160

170

180

102

Temperature (°C)

103

104 Frequency (Hz)

105

Figure 13. (a) Temperature dependence of the dielectric strength and the relaxation frequency in the nematic and SmC mesophases (b) effect of bias voltage on the M1 relaxation mode at 125 ºC in the nematic phase. The frequency dependence of the ε′⊥ and ε′′⊥ of dielectric permittivity for the selected temperatures is shown in Figure 12a and b, respectively. The dielectric spectra (ε′′⊥) clearly show a single relaxation process defined as M1 mode, in the measured frequency range, over the entire nematic temperature range. The observed large value of dielectric strength (~ 400) of M1 mode at low frequency for the compound BC-F is due to the formation of ferroelectric clusters, which is supported by the observation of ferroelectric-like switching in the nematic phase and the frequency dependent polarization behaviour. Thus, the M1 mode in nematic 17 ACS Paragon Plus Environment


phase can be ascribed to the collective motion of clusters. However, here, the observed value of dielectric strength is slightly larger compared to that of reported for a four-ring and 1, 2, 4oxadiazole derived bent-core compounds exhibiting nematic phase.27,40 The dielectric strength (∆ε) and relaxation frequency (fR) for the M1 mode present in nematic phase is shown in Figure 13a. Over the entire nematic range, ∆ε slightly increases with increasing temperature. Yet, relaxation frequency decreases with decreasing temperature due to an increase in the rotational viscosity of the sample. On further decreasing the temperature to SmC transition point, a discontinuity in the dielectric strength and the relaxation frequency is observed. The decrease in the dielectric strength and the increase of the relaxation frequency of M1 mode in the vicinity of nematic to SmC transition indicates the development of polar smectic structure with antipolar correlation. Since, fR is proportional to the strength of interlayer polar interactions in the system, i.e, stronger the interaction higher the relaxation frequency.41 Such an increase in the relaxation frequency is a typical behavior of a SmCPA phase of bent-core compounds.42 On further decreasing the temperature the relaxation frequency decreases again due to the increasing viscosity of the material. In SmC phase, an additional relaxation mode defined as M2 mode is appeared at around 1MHz. M2 mode is observed both in planar and homeotropic cells. The relaxation frequency of M2 mode decreases and dielectric strength slightly increases with decreasing temperature. This high frequency M2 mode is attributed to the molecular rotation around the short axis. These results, together with electro-optic switching, clearly indicate that the observed switching current peak in the smectic mesophase is due to reorientation of polar domains. In order to understand the collective behavior of M1 relaxation mode, the effect of dc bias on this mode was investigated. The relaxation frequency shifted to the lower frequency and dielectric strength decreased with increasing bias voltage as shown in Figure 13b. In our experiment, we could apply only up to 5 volt due to the limitation of the equipment used. We expect that this


Page 18 of 28

Page 19 of 28

relaxation mode would be suppressed on further increasing the bias voltage. This observation indicates that the observed M1 mode at low frequency can be assigned to the collective motion of the molecular dipoles. We performed the dielectric measurements in the mesophases of the compound BC-F filled in a homeotropic cell, as well. Relaxation processes similar to the above described for planar cell were observed. The dielectric loss factor (tan δ), ratio of ε″ and ε′ curves, obtained from the homeotropic cell plotted as a function of frequency ranging from 10 Hz to 1 MHz at different temperatures is shown in Figure 14. The tan δ curves show two overlapping peaks 5 165 °C 157 °C 151 °C 137 °C 129 °C 119 °C 117 °C 111 °C 106 °C

4 Ionic impurities 3 tan (δ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

10

1

10

2

3

4


5

10

6

10

Figure 14. Dielectric loss factor (tan δ) versus frequency in homeotropically aligned cell at different temperatures. existing in the low frequency range, the first one between 10 -150 Hz and the second one between 1- 5 kHz. The first one can be ascribed to the ionic impurities present in the LC material as it can be suppressed under a low bias voltage of 1 V. In addition, as can be seen from Figure 14, this peak shifts further towards the lower frequency with decreasing temperature irrespective of the nature of mesophase. A similar type of low frequency relaxation peak below the Goldstone mode was reported for ferroelectric LC materials, and it was suggested that this low frequency relaxation mode is due to the effective ionization– recombination assisted diffusion of slow ions present in LC system.43 However, the 19 ACS Paragon Plus Environment


relaxation frequency, corresponding to the M1 mode observed in dielectric spectra (ε′′), appeared at higher frequency region for the tan δ dielectric spectrum. The maximum of tan δ appears at frequencies, higher by a factor of (ε0 /ε∞)1/2 than the respective maximum for the dielectric loss (ε″).44 In the nematic phase, the relaxation frequency, fR, decreases linearly with decreasing temperature (except in the vicinity of the nematic to isotropic transition) following the Arrhenius expression,

exp

(2)

where Ea is the activation energy of relaxation, f0 is the temperature independent constant and kB is the Boltzmann constant. Figure 15 shows the behavior of relaxation frequency (fR), extracted from fitting the dielectric spectra (ε′′⊥) to equation 1, versus inverse temperature (1/T). The activation energy, Ea, for the M1 mode obtained from the slop of Arrhenius plot is found to be equal to 0.36 eV. This value is comparable with those obtained for other bentcore compounds.40,45 Higher activation energy implies a greater hindrance to the molecular process. In the case of compound BC-F, the lower value of Ea in the nematic mesophase suggests the lower hindrance of the collective movement of the molecules responsible for this process. 6.4 6.2 6.0 ln (fR)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Ea=0.36 eV

5.8 5.6 5.4 5.2 2.25

2.30

2.35

2.40 3

2.45

2.50

2.55

-1

10 /T (K )


2.60

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 15. Temperature dependence of the relaxation frequency in the nematic phase of the compound BC-F; The fR values are extracted from fitting the dielectric spectra (ε′′⊥) to equation 1.

CONCLUSIONS A thermally stable achiral bent-core azo compound exhibiting a ferroelectric-like switching nematic and ferrielectric smectic mesophases is reported. This is the first observation of ferroelectric nematic and ferrielectric smectic mesophases existing in the same compound. We performed X-ray, electro-optic switching and dielectric studies of this compound. The Xray studies revealed the presence of skewed cybotactic clusters in the nematic phase. The polar order in the cybotactic nematic phase is confirmed by polarization reversal current measurement and dielectric study. A ferroelectric-like switching response is observed in the nematic phase. The dielectric study in this phase shows a single relaxation process in the frequency range of 200 - 600 Hz due to the collective motion of the molecules. The tetrastable switching observed in the SmC phase at high voltage indicates a field induced ferrielectric state of this mesophase. Such ferroelectric nematic and ferrielectric smectic mesophases could open new avenues in electro-optic device technology.

ACKNOWLEDGEMENTS This research is supported, partially, by the Department of Science and Technology, Government of India - Nano Mission, through "Thematic Projects in Frontiers of Nano S&T (TPF-Nano)", Grant No. SR/NM/TP-25/2016. We sincerely acknowledge the help received from Ms. Rekha S. Hegde and Ms. Monika M, during the synthesis of the compound BC-F.

Conflicts of interest 21 ACS Paragon Plus Environment


Authors declare no conflict of interest.

REFERENCES (1)

Pelzl, G.; Diele, S.; Weissflog, W. Banana-shaped compounds-A new field of liquid

crystal. Adv. Mater. 1999, 11, 707-724. . (2)

Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova E.; Walba,

D. M. Spontaneous formation of macroscopic chiral domains in a fluid smectic phase of achiral molecules. Science 1997, 278, 1924-1927. (3)

Etxebarria, J.; Ros, M. B. Bent-core liquid crystal in the route to functional materials.

J. Mater. Chem. 2008, 18, 2919-2926. (4)

Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. Distinct ferroelectric

smectic liquid crystals consisting of banana shaped achiral molecules. J. Mater. Chem. 1996, 6, 1231-1233. (5)

Sreenilayam, S. P.; Panov, V. P.; Vij, J. K.; Shanker, G. The NTB phase in a achiral

asymmetrical bent-core liquid crystal terminated with symmetric alkyl chains. Liq. Cryst. 2017, 44, 244-253. (6)

Madsen, L. A.; Dingemans, T. J.; Kakata, M.; Samulski, E. T. Thermotropic biaxial

nematic liquid crystals. Phys. Rev. Lett. 2004, 92, 145505. (7)

Francescangeli, O.; Stanic, V.; Torgova, S. I.; Strigazzi, A.; Scaramuzza, N.; Ferrero,

C.; Dolbnya, I. P.; Weiss, T. M.; Berardi, R.; Muccioli, L.; et al. Ferroelectric response and induced biaxiality in the nematic phase of bent-core mesogens. Adv. Funct. Mater. 2009, 19, 2592-2600. (8)

Prasad, V.; Kang, S.-W.; Suresh, K. A.; Joshi, L.; Wang, Q.; Kumar, S. Thermotropic

uniaxial and biaxial nematic and smectic phases in bent-core mesogens. J. Am. Chem. Soc. 2005, 127, 17224-17227. 22 ACS Paragon Plus Environment

Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9)

Jakli, A. Liquid crystals of the twenty-first century-nematic phase of bent-core

molecules. Liq. Cryst. Rev. 2013, 1, 65-82. (10)

Tschierske, C.; Photinos, D. J. Biaxial nematic phases. J. Mater. Chem. 2010,

20, 4263-4294. (11)

Acharya, B. R.; Kang, S.-W.; Prasad, V.; Kumar, S. Role of molecular structure on x-

ray diffraction in uniaxial and biaxial phases of thermotropic liquid crystals. J. Phys. Chem. B 2009, 113, 3845-3852. (12)

Keith, C.; Lehmann, A.; Baumeister, U.; Prehm, M.; Tschierske, C. Nematic phases

of bent-core mesogens. Soft Matter 2010, 6, 1704-1721. (13)

Prasad, V.; Nagaveni, N. G.; Monika, M. Thermally stable azo-substituted bent-core

nematogens: Observation of chiral domains in the nematic mesophases composed of smectic nanoclusters. Liq. Cryst. 2017, DOI. 10.1080/02678292.2017.1371344. (14)

Jang, Y.; Panov, V. P.; Keith, C.; Tschierske, C.; Vij, J. K. Sign reversal in the

dielectric anisotropy as functions of temperature and frequency in the nematic phase of a bent-core mesogens. Appl. Phys. Lett. 2010, 97, 152903. (15)

Harden, J.; Mbanga, B.; Eber, N.; Fodor-Csorba, K.; Sprunt, S.; Gleeson, J. T.; Jakli,

A. Giant flexoelectricity of bent-core nematic liquid crystals. Phys. Rev. Lett. 2006, 97, 157802. (16)

Vanakaras, A. G.; Photinos, D. J. Thermotropic biaxial nematic liquid crystals:

Spontaneous or field stabilized? J. Chem. Phys. 2008, 128, 154512. (17)

Achard, M. F.; Bedel, J. P.; Marcerou, J. P.; Nguyen, H. T.; Rouillon, J. C. Switching

of banana liquid crystal mesophases under field. Eur. Phys. J. E 2003, 10, 129-134. (18)

Havriliak, S.; Negami, S. A complex plane representation of dielectric and

mechanical relaxation processes in some polymers. Polymer 1967, 8, 161-210.



(19)

Amaranatha, R.; Tschierske, C. Bent-core liquid crystals: Polar order, superstructural

chirality and spontaneous desymmetrisation in soft matter systems. J. Mater. Chem. 2006, 16, 907-961. (20)

Hegde, R. S.; Kumar, J.; Prasad, V.; Monika, M. The first examples of V-shaped

compounds exhibiting a B5 mesophase and direct transition from the isotropic to a polar biaxial smectic A mesophase. J. Mol. Liq. 2018, 249, 97-105. (21)

Monika, M.; Roy, A.; Prasad, V. Smectic nanoclusters in the nematic mesophases of

dimeric compounds composed of rod-like azo moieties with lateral substituents. New J. Chem. 2017, 41, 11576-11583. (22)

Vaupotic, N.; Szydlowska, J.; Salamonczyk, M.; Kovarova, A.; Svoboda, J.; Osipov,

M.; Pociecha, D.; Gorecka, E. Structure studies of the nematic phase formed by bent-core molecules. Phys. Rev. E 2009, 80, 030701. (23)

De Vries, A. X-ray diffraction studies of the structure of the skewed cybotactic

nematic phase: a review of the literature. J. Mol. Liq. 1986, 31, 193-202. (24)

Droulias, S.; Vanakaras, A. G.; Photinos, D. J. Extending the Maier-Saupe theory to

cybotactic nematics. Liq. Cryst. 2010, 37, 969-976. (25)

Alaasar, M.; Prehm, M.; May, K.; Eremin, A.; Tschierske, C. 4-cyanoresorcinol-based

bent-core mesogens with azobenzene wings: Emergence of sterically stabilized polar order in liquid crystalline phases. Adv. Funct. Mater. 2014, 24, 1703–1717. (26)

Hong, S. H.; Verduzco, R.; Williams, J. C.; Twieg, R. J.; DiMasi, E.; Pindak, R.;

Jakli, A.; Gleeson, J. T.; Sprunt, S. Short-range smectic order in bent-core nematic liquid crystals. Soft Matter 2010, 6, 4819-4827. (27)

Shanker, G.; Prehm, M.; Nagaraj, M.; Vij, J. K.; Weyland, M.; Eremin, A. Tschierske,

C. 1,2,4-oxadiazole-based bent-core liquid crystals with cybotactic nematic phases. ChemPhysChem. 2014, 15, 1323-1335.


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28)

Shanker, G.; Nagaraj, M.; Kocot, A.; Vij, J. K.; Prehm, M.; Tschierske, C. Nematic

phases in 1,2,4-oxadiazole-based bent-core liquid crystals: is there a ferroelectric switching? Adv. Funct. Mater. 2012, 22, 1671-1683. (29)

Kishikawa, K.; Inoue, T.; Hasegawa, N.; Takahashi, M.; Kohri, M.; Taniguchi, T.;

Kohmoto, S. Achiral straight-rod liquid crystals indicating local biaxiality and ferroelectric switching behaviour in the smectic A and nematic phases. J. Mater. Chem. 2015, 3, 35743581. (30)

Findeisen-Tandel, S.; Schroder, M. W.; Pelzl, G.; Baumeister, U.; Weissflog, W.;

Stern, S.; Nemes, A.; Stannarius, R.; Eremin, A. Multistage polar switching in bent-core mesogens. Eur. Phys. J. E 2008, 25, 395-402. (31)

Lee, J.; Chandani, A. D. L.; Itoh, K.; Ouchi, Y.; Takezoe, H.; Fukuda, A. Frequency

dependent switching behaviour under triangular waves in antiferroelectric and ferrielectric chiral smectic phases. Jpn. J. Appl. Phys. 1990, 29, 1122-1127. (32)

Itoh, K.; Kabe, M.; Miyachi, K.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.; Fukuda,

A. Devil’s staircase between antiferroelectric SCA* and ferroelectric SC* phases in liquid crystals observed in free-standing films under temperature gradients. J. Mater. Chem. 1997, 7, 407-416. (33)

Jaradat, S.; Brimicombe, P. D.; Osipov. M. A.; Pindak, R.; Gleeson, H. F. A field-

induced ferrielectric liquid crystal phase. Appl. Phys. Lett. 2011, 98, 043501. (34)

Soltani, T.; Gharbi, A.; Marcerou, J. P.; Gineste, S. Electric field induced phase

transition in a ferroelectric smectic-C* liquid crystal. Eur. Phys. J. Appl. Phys. 2011, 55, 10201. (35)

Stern, S.; Stannarius, R.; Eremin, A.; Weissflog, W. A model for a field-induced

ferrielectric state in a bent-core mesogen. Soft Matter 2009, 5, 4136-4140.



(36)

Page 26 of 28

Dey, K. C.; Mandal, P. K.; Dąbrowski, R. Effect of lateral fluorination in

antiferroelectric and ferroelectric mesophases: synchrotron x-ray diffraction, dielectric spectroscopy, and electro-optic study. J. Phys. Chem. Solids 2016, 88, 14-23. (37)

Buka, A.; Eber, N. Flexoelectricity in Liquid Crystals: Theory, Experiments and

Applications, Imperial College Press, London, 2013. (38)

Elamain, O.; Hegde, G.; Katalin, F.-C.; Komitov, L. Field-induced optically isotropic

state in bent-core nematic liquid crystals: unambiguous proof of filed-induced optical biaxiality. J. Phys. D: Appl. Phys. 2013, 46, 455101. (39)

Wiant D.; Gleeson, J.; Ebert, N.; Fodor-Csorba, K.; Jakli, A.; Toth-Katona, T.

Nonstandard electroconvection in a bent-core nematic liquid crystal. Phys. Rev. E 2005, 72, 041712. (40)

Turlapati, S.; Khan, R. K.; Ghosh, S.; Tadapatri, P.; Pratibha, R.; Nandiraju, V. S.

Rao. Existence of polar switching in the nematic and orthogonal smectic phases in novel four-ring bent-core compounds. J. Appl. Phys. 2016, 120, 174101. (41)

Pociecha, D.; Gorecka, E.; Cepic, M.; Vaupotic, N.; Gomola, K.; Mieczkowski, J.

Paraelectric-antiferroelectric phase transition in achiral liquid crystals. Phys. Rev. E 2005, 72, 060701. (42)

Pociecha, D.; Gorecka, E.; Cepic, M.; Vaupotic, N; Weissflog, W. Polar order and tilt

in achiral smectic phases. Phys. Rev. E 2006, 74, 021702. (43)

Chandran,

A.;

Prakash,

J.;

Ganguly,

P.;

Biradar,

A.

M.

Zirconia

nanoparticles/ferroelectric liquid crystal composites for ionic impurity-free memory applications. RSC Adv. 2013, 3, 17166-17173. (44)

Biradar, A. M.; Kilian, D.; Wrobel, S.; Haase, W. A sub-hertz frequency dielectric

relaxation process in a ferroelectric liquid crystal material. Liq. Cryst. 2000, 27, 225-231.


Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45)

Salamon, P.; Eber, N.; Buka, A.; Gleeson, J. T.; Sprunt, S.; Jakli, A. Dielectric

properties of mixtures of bent-core and a calamitic liquid crystal. Phys. Rev. E 2010, 81, 031711.

TOC GRAPHIC




Page 28 of 28

Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an

Recommend Documents