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Polar Switching in a Lyotropic Columnar Nematic Liquid Crystal Made of Bowl-Shaped Molecules Julia Guilleme, Emma Cavero, Teresa Sierra,* Josu Ortega, César L. Folcia, Jesus Etxebarria,* Tomás Torres,* and David González-Rodríguez* Columnar liquid crystals (LCs) formed by more or less planar aromatic cores[1] are promising materials for certain applications in organic electronics such as the construction of light-emitting diodes, photovoltaic cells, or field-effect transistors.[2] Many of these materials show interesting properties such as a virtually 1D electrical conductivity, which can be exploited in technologies that rely on directional charge transport.[3] On the other hand, these compounds usually show ease of processing, and can be readily integrated on semiconductor films for device construction.[4] It is widely recognized that the possibility of axial polar order in columnar phases would highly extend their range of applicability.[5] However, ferroelectric switching in such type of columnar LCs is rather rare. In fact, the first genuine ferroelectric columnar LC with spontaneous polarization along the columnar axis was only recently reported.[6] This material consists of fan-shaped molecules with a polar phthalonitrile group. Four fan-shaped molecules assemble into a conical object, which, in its turn, stacks up in columns to form a hexagonal columnar mesophase. Even more extraordinary is the polar nematic phase. The mere question of the existence of a ferroelectric nematic is considered an outstanding topic with both fundamental and practical interest.[7] Up to now, no unambiguous confirmation of this structure has been established in the literature, though it was claimed to occur for some polymeric glutamate molecules several years ago.[8] Some other reports in the same J. Guilleme, Prof. T. Torres, Dr. D. González-Rodríguez Departamento de Química Orgánica Universidad Autónoma de Madrid 28049 Madrid, Spain E-mail: [email protected]; [email protected] Dr. E. Cavero, Dr. T. Sierra Departamento de Química Orgánica Facultad de Ciencias –Instituto de Ciencia de Materiales de Aragón (ICMA) CSIC–Universidad de Zaragoza 50009 Zaragoza, Spain E-mail: [email protected] Dr. J. Ortega Departamento de Física Aplicada II Facultad de Ciencia y Tecnología UPV/EHU 48080 Bilbao, Spain Prof. C. L. Folcia, Prof. J. Etxebarria Departamento de Física de la Materia Condensada Facultad de Ciencia y Tecnología UPV/EHU 48080 Bilbao, Spain E-mail: [email protected]
DOI: 10.1002/adma.201500238
4280
wileyonlinelibrary.com
direction have also been recently published in materials with bent-core molecular units, albeit the results should be definitely confirmed.[9] On the other hand, it seems that such a phase is not forbidden from the theoretical point of view, and computer simulations have demonstrated the possibility of nematic ferroelectric order in structures constituted by elongated asymmetric particles.[10] Here, we wish to report on a significant step in that direction that, at difference to previous works, involves a columnar nematic mesophase.[11] Recently, we described a columnar LC that can be efficiently aligned parallel to an electric field and maintains its polar order upon electric field termination.[12] The strategy for the material design is based on the use of a rigid cone-shaped molecule [a boron subphthalocyanine (SubPc) derivative] with a strong axial dipole moment. In the LC phase, which is stable at room temperature, the molecules stack in columns without head-to-tail invariance (Figure 1a) and show permanent polarity along the columnar axis. However, though the material is pyroelectric, it cannot be switched. As a step further toward the design of a ferroelectrically switchable columnar mesophase, we have selected the SubPc molecule shown in Figure 1b (see ref. [13] and the Supporting Information). An amide group incorporated between the central core and each of the three peripheral molecular segments certainly enhances the cohesive forces between neighboring molecules, due to strong intermolecular hydrogen bonding interactions along the column.[14] As a result, the material shows a hexagonal columnar mesophase Colh in a very broad temperature range (see Figure S1–S3 in the Supporting Information). The phase sequence is Colh–(250 °C)–isotropic. However, the high viscosity of the mesophase and the high temperatures of its range of stability are not ideal for processing and possible applications. Therefore, we resorted to control these features by the addition of a suitable solvent. In general, lyotropic mesomorphism is not favored for typical disc-shaped molecules likely because columns are not stable enough in the presence of solvents.[1b,15] However, reinforcing interdisk interactions along the column allows the incorporation of solvent molecules without disrupting the columnar order of the mesophase. Several amphitropic columnar LCs have been described, which mainly consist of mesogenic molecules designed to implement several types of intermolecular interactions that stabilize the column, i.e., π-stacking, hydrogen bonding, dipole–dipole interactions.[16] In the present case, π-stacking and threefold H-bonding interactions between amide groups are further reinforced by dipole–dipole interactions due to the B-F axial dipole moment. As a matter of fact, we recently demonstrated that our compound self-assembles in
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4280–4284
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a) Top and side views of the axial dipolar SubPc BF macrocycle and its packing in columns without the head to tail invariance. The yellow spheres represent the arylamide substituents. b) Molecular structure of the tri(arylamide)SubPcBF. c) POM image of the lyotropic phase in dodecane (10 wt%). d) X-ray diffraction pattern recorded at r.t. Four different reflections are indicated. e) Textures observed by POM for the lyotropic mesophase in dodecane (10 wt%) under a low-frequency (0.1 Hz) square-wave field: left, +40 V; center, transient texture observed during voltage inversion; right, –40 V. Sample thickness, 5 µm.
very stable non-centrosymmetric convex-to-concave stacks in apolar solvents such as methylcyclohexane or dodecane at concentrations as low as 10−6 M.[13] We herein report that a solution of our compound at a concentration of 10 wt% (3.4 × 10−2 M) in dodecane displays mesomorphic behavior (Figure 1c,d), which is stable at room temperature. Indeed, the connections established between SubPc cores force strong segregation between cores and aliphatic chains, which interact with dodecane molecules, and this gives high stability to the lyomesophase. In this work, we will restrict ourselves to study the solution just at this small concentration, for which the viscosity is greatly reduced. The low viscosity together with the kind of texture observed by POM (Figure 1c) suggests the possibility of having a nematic phase at 10 wt%. To investigate more in detail the mesophase structure, the material was studied by X-ray diffraction. The results (Figure 1d and Figure S4, Supporting Information) ratify our hypothesis. At wide angles, a diffuse reflection at 4.5 Å (3) and a sharper peak at 4.2 Å (4) were detected. The latter is due to the interdisk stacking distance, whereas the diffuse reflection is typical of LCs and results from the disorder of the aliphatic chains and the solvent. In the small angle region, we observed two diffuse reflections at about 40 Å (1) and 15 Å (2). The first one accounts for the lateral intercolumnar distance
Adv. Mater. 2015, 27, 4280–4284
and the second one (very weak and broad) is connected with the average length of the dodecane molecules. The phase is thus characterized as a columnar nematic NCol.[17] The “nematic particles” would be constituted by column segments of several molecules (Figure 1a), whose stacking is responsible for the Bragg-like peak 4. Such a structure can be cataloged within the so-called chromonic liquid crystals.[18] A remarkable property of our NCol phase is that it shows electro-optic response. We carried out electro-optic studies in glass cells with transparent ITO electrodes. The sample thickness was 5 µm. When an electric field of about 10 V µm−1 was applied, extinction could be observed between crossed polarizers irrespective of the field polarity. When the electric field was turned off the black texture was retained, at least for several minutes. These observations imply that while the electric field is maintained, the solution has an optic axis parallel to it. Under a low-frequency (
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Polar Switching in a Lyotropic Columnar Nematic Liquid Crystal Made of Bowl-Shaped Molecules Julia Guilleme, Emma Cavero, Teresa Sierra,* Josu Ortega, César L. Folcia, Jesus Etxebarria,* Tomás Torres,* and David González-Rodríguez* Columnar liquid crystals (LCs) formed by more or less planar aromatic cores[1] are promising materials for certain applications in organic electronics such as the construction of light-emitting diodes, photovoltaic cells, or field-effect transistors.[2] Many of these materials show interesting properties such as a virtually 1D electrical conductivity, which can be exploited in technologies that rely on directional charge transport.[3] On the other hand, these compounds usually show ease of processing, and can be readily integrated on semiconductor films for device construction.[4] It is widely recognized that the possibility of axial polar order in columnar phases would highly extend their range of applicability.[5] However, ferroelectric switching in such type of columnar LCs is rather rare. In fact, the first genuine ferroelectric columnar LC with spontaneous polarization along the columnar axis was only recently reported.[6] This material consists of fan-shaped molecules with a polar phthalonitrile group. Four fan-shaped molecules assemble into a conical object, which, in its turn, stacks up in columns to form a hexagonal columnar mesophase. Even more extraordinary is the polar nematic phase. The mere question of the existence of a ferroelectric nematic is considered an outstanding topic with both fundamental and practical interest.[7] Up to now, no unambiguous confirmation of this structure has been established in the literature, though it was claimed to occur for some polymeric glutamate molecules several years ago.[8] Some other reports in the same J. Guilleme, Prof. T. Torres, Dr. D. González-Rodríguez Departamento de Química Orgánica Universidad Autónoma de Madrid 28049 Madrid, Spain E-mail: [email protected]; [email protected] Dr. E. Cavero, Dr. T. Sierra Departamento de Química Orgánica Facultad de Ciencias –Instituto de Ciencia de Materiales de Aragón (ICMA) CSIC–Universidad de Zaragoza 50009 Zaragoza, Spain E-mail: [email protected] Dr. J. Ortega Departamento de Física Aplicada II Facultad de Ciencia y Tecnología UPV/EHU 48080 Bilbao, Spain Prof. C. L. Folcia, Prof. J. Etxebarria Departamento de Física de la Materia Condensada Facultad de Ciencia y Tecnología UPV/EHU 48080 Bilbao, Spain E-mail: [email protected]
DOI: 10.1002/adma.201500238
4280
wileyonlinelibrary.com
direction have also been recently published in materials with bent-core molecular units, albeit the results should be definitely confirmed.[9] On the other hand, it seems that such a phase is not forbidden from the theoretical point of view, and computer simulations have demonstrated the possibility of nematic ferroelectric order in structures constituted by elongated asymmetric particles.[10] Here, we wish to report on a significant step in that direction that, at difference to previous works, involves a columnar nematic mesophase.[11] Recently, we described a columnar LC that can be efficiently aligned parallel to an electric field and maintains its polar order upon electric field termination.[12] The strategy for the material design is based on the use of a rigid cone-shaped molecule [a boron subphthalocyanine (SubPc) derivative] with a strong axial dipole moment. In the LC phase, which is stable at room temperature, the molecules stack in columns without head-to-tail invariance (Figure 1a) and show permanent polarity along the columnar axis. However, though the material is pyroelectric, it cannot be switched. As a step further toward the design of a ferroelectrically switchable columnar mesophase, we have selected the SubPc molecule shown in Figure 1b (see ref. [13] and the Supporting Information). An amide group incorporated between the central core and each of the three peripheral molecular segments certainly enhances the cohesive forces between neighboring molecules, due to strong intermolecular hydrogen bonding interactions along the column.[14] As a result, the material shows a hexagonal columnar mesophase Colh in a very broad temperature range (see Figure S1–S3 in the Supporting Information). The phase sequence is Colh–(250 °C)–isotropic. However, the high viscosity of the mesophase and the high temperatures of its range of stability are not ideal for processing and possible applications. Therefore, we resorted to control these features by the addition of a suitable solvent. In general, lyotropic mesomorphism is not favored for typical disc-shaped molecules likely because columns are not stable enough in the presence of solvents.[1b,15] However, reinforcing interdisk interactions along the column allows the incorporation of solvent molecules without disrupting the columnar order of the mesophase. Several amphitropic columnar LCs have been described, which mainly consist of mesogenic molecules designed to implement several types of intermolecular interactions that stabilize the column, i.e., π-stacking, hydrogen bonding, dipole–dipole interactions.[16] In the present case, π-stacking and threefold H-bonding interactions between amide groups are further reinforced by dipole–dipole interactions due to the B-F axial dipole moment. As a matter of fact, we recently demonstrated that our compound self-assembles in
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4280–4284
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a) Top and side views of the axial dipolar SubPc BF macrocycle and its packing in columns without the head to tail invariance. The yellow spheres represent the arylamide substituents. b) Molecular structure of the tri(arylamide)SubPcBF. c) POM image of the lyotropic phase in dodecane (10 wt%). d) X-ray diffraction pattern recorded at r.t. Four different reflections are indicated. e) Textures observed by POM for the lyotropic mesophase in dodecane (10 wt%) under a low-frequency (0.1 Hz) square-wave field: left, +40 V; center, transient texture observed during voltage inversion; right, –40 V. Sample thickness, 5 µm.
very stable non-centrosymmetric convex-to-concave stacks in apolar solvents such as methylcyclohexane or dodecane at concentrations as low as 10−6 M.[13] We herein report that a solution of our compound at a concentration of 10 wt% (3.4 × 10−2 M) in dodecane displays mesomorphic behavior (Figure 1c,d), which is stable at room temperature. Indeed, the connections established between SubPc cores force strong segregation between cores and aliphatic chains, which interact with dodecane molecules, and this gives high stability to the lyomesophase. In this work, we will restrict ourselves to study the solution just at this small concentration, for which the viscosity is greatly reduced. The low viscosity together with the kind of texture observed by POM (Figure 1c) suggests the possibility of having a nematic phase at 10 wt%. To investigate more in detail the mesophase structure, the material was studied by X-ray diffraction. The results (Figure 1d and Figure S4, Supporting Information) ratify our hypothesis. At wide angles, a diffuse reflection at 4.5 Å (3) and a sharper peak at 4.2 Å (4) were detected. The latter is due to the interdisk stacking distance, whereas the diffuse reflection is typical of LCs and results from the disorder of the aliphatic chains and the solvent. In the small angle region, we observed two diffuse reflections at about 40 Å (1) and 15 Å (2). The first one accounts for the lateral intercolumnar distance
Adv. Mater. 2015, 27, 4280–4284
and the second one (very weak and broad) is connected with the average length of the dodecane molecules. The phase is thus characterized as a columnar nematic NCol.[17] The “nematic particles” would be constituted by column segments of several molecules (Figure 1a), whose stacking is responsible for the Bragg-like peak 4. Such a structure can be cataloged within the so-called chromonic liquid crystals.[18] A remarkable property of our NCol phase is that it shows electro-optic response. We carried out electro-optic studies in glass cells with transparent ITO electrodes. The sample thickness was 5 µm. When an electric field of about 10 V µm−1 was applied, extinction could be observed between crossed polarizers irrespective of the field polarity. When the electric field was turned off the black texture was retained, at least for several minutes. These observations imply that while the electric field is maintained, the solution has an optic axis parallel to it. Under a low-frequency (
