A columnar liquid crystal with permanent polar order

Article abstract of DOI:10.1039/c4tc02662d

The self-assembly of axial dipolar subphthalocyanine molecules in the presence of electric fields leads to uniaxially oriented columnar liquid crystalline materials that exhibit permanent polarization.

Full text of DOI:10.1039/c4tc02662d

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Materials Chemistry C
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A columnar liquid crystal with permanent polar
order†
Cite this: J. Mater. Chem. C, 2015, 3,
J. Guilleme, J. Arago, E. Ortı, E. Cavero, T. Sierra, J. Ortega,d C. L. Folcia,e
985
a
b
b
c
c
*
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e
a
a
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J. Etxebarria, D. Gonzalez-Rodrıguez and T. Torres
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Received 21st November 2014
Accepted 19th December 2014
DOI: 10.1039/c4tc02662d
www.rsc.org/MaterialsC
The self-assembly of axial dipolar subphthalocyanine molecules in the or urea groups.4b,e,5,10 Spontaneous polarization in LC materials
presence of electric fields leads to uniaxially oriented columnar liquid typically relies on temporal conformational or orientational
crystalline materials that exhibit permanent polarization.
arrangements of dipoles and is quite sensitive in practice to
external conditions such as cell surfaces or applied elds, which
is valuable for ferroelectric switching applications.3,9d However,
for the same reason, the realization of columnar LC materials
with permanent polarization, which persists even in the pres-
ence of electric elds of opposite sense, remains a challenging
task. Polar order in the columnar materials developed so far is
eventually lost with time due to structural relaxation processes
that lead to dipole readjustments.
Here we study columnar LC lms that can be efficiently
aligned parallel to an electric eld and that maintain their polar
order upon electric eld suspension or even inversion. Our
strategy relies on the use of conformationally rigid (i.e. non-
invertible)11 bowl-shaped mesogens with a strong axial dipole
moment, like the C3-symmetric boron subphthalocyanine
Columnar liquid crystals (LCs) made of discotic p-conjugated
semiconductors offer a combination of attributes that are very
appealing in several optoelectronic technologies.1 Their
propensity to form stable columns by intermolecular p–p
interactions and their uid nature allows for their organization
into oriented materials by means of diverse techniques.2 Among
them, the use of electric elds arises as particularly useful3 since
(1) it can control homeotropic column alignment in dened
areas between patterned electrodes,4 and (2) it may lead to the
generation of thin lms that exhibit net polarization.5 The latter
is viewed as a property of great potential for those applications
that require an efficient directional transport of charge
carriers,6 such as eld-effect transistors and photovoltaic
devices, where the design of high dielectric layers and polar
electronic interfaces is gaining importance.7
In order to produce columnar LC materials that exhibit
macroscopic polarization a non-centrosymmetric organization
is required,8 and for such a goal several strategies have been
practiced.3 One of them is the use of molecules with dipoles that
are parallel to the column axis, either by the stacking of bowl-
shaped structures,5a,9 or by the incorporation of dipolar amide
a
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´
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Departamento de Quımica Organica, Universidad Autonoma de Madrid, 28049
Madrid, Spain. E-mail: tomas.torres@uam.es
bInstituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia,
Spain
c
´
´
Departamento de Quımica Organica, Facultad de Ciencias – Instituto de Ciencia de
´
Materiales de Aragon (ICMA), CSIC – Universidad de Zaragoza, 50009 Zaragoza,
Spain
d
´
´
Departamento de Fısica Aplicada II, Facultad de Ciencia y Tecnologıa, UPV/EHU,
48080, Bilbao, Spain
e
´
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Departamento de Fısica de la Materia Condensada, Facultad de Ciencia y Tecnologıa,
Fig. 1 (a) Top and side views of the axial dipolar SubPcBF macrocycle.
The yellow spheres represent the arylethynyl substituents. Head-to-
tail stacking leads to polarized columnar structures. (b) Structure of the
C3-symmetric SubPc 1.16,17
UPV/EHU, 48080, Bilbao, Spain
† Electronic supplementary information (ESI) available: Experimental details and
additional characterization data. Fig. S1–S9. See DOI: 10.1039/c4tc02662d
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(SubPc)12,13 1 (Fig. 1). The key to generate a head-to-tail non- attributed to the hexagonal cell parameter. Furthermore, an
centrosymmetric columnar stacking14,15 with a strong axial XRD diagram recorded in the crystalline state (8 weeks aer
dipole was the introduction of a small, highly electronegative thermal treatment) is consistent with the hexagonal columnar
atom linked to the central boron, such as uorine.
arrangement (see Fig. S4 and Table S1†). Finally, the diffuse
The thermal properties of 1 were studied by polarized optical maximum at wide angles, typically observed in all kind of
microscopy (POM) and differential scanning calorimetry (DSC). mesophases, is unusually broad in this case (Fig. 2b and S5†).
SubPc 1 displays pseudo focal-conic textures between crossed Part of the contribution to that reection can be attributed to
polarizers, which are characteristic of a hexagonal columnar the liquid-like order of the hydrocarbon chains (distances
mesophase (Fig. 2a).4b,5a,9b,c,10a According to DSC thermograms around 4.5 A), but the reection further extends at higher
˚
˚
(Fig. S2†), 1 is crystalline as synthesized but, once melted to the angles, covering distances down to 2.5 A. These small spacings
isotropic liquid (I), the mesophase appears on cooling at 89.2 should correspond to intracolumnar distances.
ꢀC, and maintains stable at room temperature for several weeks.
To gain more insight into the structure of the 1 assemblies in
During the second heating process the Colh mesophase the mesophase, semiempirical quantum-chemical calculations
undergoes a cold crystallization, with no texture change, and were carried out.17 A hexagonal columnar model, formed by 7
this crystal melts into the I phase at 101.2 ꢀC. Subsequent columns and 4 molecules per column, was built up and,
cooling–heating cycles show the same type of thermogram.
subsequently, its structure was fully relaxed at the PM7 level (see
The mesophase arrangement of 1 was studied by X-ray Fig. S7 and S8†). The average distance between the stacking axes
˚
diffraction (XRD) at room temperature. Experiments performed of the 1 columns is computed at 42.2 A, in good agreement with
˚
on samples previously heated up to the isotropic liquid, and the lattice parameter (39.0 A) derived from XRD data and the
cooled down at a rate of ca. 10 ꢀC minꢁ1, yielded an intense mean periodicity (40 A) measured in AFM images. Calculations
˚
maximum in the low-angle region and a broad diffuse also indicate that aggregation within a column is not only
maximum in the wide-angle region (Fig. 2b). The low-angle peak dominated by dispersion or van der Waals forces, but also by
can be related to the (10) reection of a hexagonal columnar strong Fdꢁ/Bd+ dipolar interactions between adjacent
phase. The absence of higher order reections18 makes it diffi- SubPcs.17,19
cult to unambiguously assign the mesophase type exclusively by
We were interested in studying the evolution of the axial
XRD. Nevertheless, according to POM observations, we propose dipole moment with the size of the system. Stacks of increasing
that the small-angle maximum corresponds to the (10) reec- length, comprising 1, 4, 8, 12, 16 and 20 molecules, were
tion of a hexagonal arrangement with a cell parameter (a) of computed using a column extracted from the PM7-optimized
˚
39.0 A. Additionally, AFM tapping mode images, taken at r.t. to a hexagonal lattice as a building block. Our results indicate that
thermally treated sample (vide supra) of 1 on mica, show line the dipole moment follows the stacking axis and systematically
patterns that are consistent with different domains of columns increases from 13.7 D for the 1 monomer to 281.6 D in a
arranged onto the surface (Fig. 2c–d and S3†). Surface analysis columnar icosamer (see Fig. S9†). Additionally, the inuence of
showed a mean periodicity between lines of 40 A, which is an electric eld (12 V mmꢁ1) on the energy of the dipolar stacks
˚
within the range of the distance measured by XRD and is was evaluated at the PM7 level (Fig. S9†). A slight stabilization/
destabilization of the columnar aggregate is seen when such a
eld is applied parallel/antiparallel to the polarization vector.
The energy difference between parallel and antiparallel
arrangements increases linearly with the stack size reaching 3.2
kcal molꢁ1 in the icosamer.
The Colh mesophase of 1 was next studied in commercial
Linkam cells, comprised of two ITO plates separated by 5 mm, in
order to investigate the electric eld columnar alignment. The
material in the cell was cooled from the isotropic liquid into the
Colh mesophase while applying a triangular-wave AC voltage (90
V
pp, 0.08 Hz). In this cooling process, the ITO area appeared
non-birefringent between crossed-polarizers, whereas a bire-
fringent texture became visible in the area without electrodes
(Fig. 3a). The absence of birefringence under a eld indicates a
uniaxial character of the mesophase thus reinforcing the
hexagonal symmetry assigned to the mesophase structure.4,9 On
heating the sample, this black texture persists up to the tran-
sition to the isotropic liquid at 101.2 ꢀC, which is clearly
observed in the birefringent areas without electrodes. Repeating
the cooling process without the electric eld led on the other
hand to the development of a birefringent texture on the full cell
area. Once the material reaches the mesophase on cooling, the
dark area between electrodes does not show up with the applied
Fig. 2 (a) POM photograph (ꢂ50) taken at r.t. on cooling from the I
phase. Transition temperatures (ꢀC) and enthalpy values (kJ molꢁ1
correspond to the second heating (h)–cooling (c) cycle recorded at 10
ꢀC minꢁ1. (b) X-ray diffractogram taken at r.t. on a thermally treated
sample (c) AFM topography and (d) 2D phase images of a thermally
treated sample of 1 deposited onto mica.
)
986 | J. Mater. Chem. C, 2015, 3, 985–989
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Fig. 4 (a) Scheme of the optical setup used for SHG interferometry.
The interferograms are obtained by rotating the glass plate. (b) SHG
interference patterns at 40 ꢀC obtained by rotating the glass plate. Blue
and red points correspond to a LC sample that was polarized with (+)
and (ꢁ) DC electric field, respectively, on cooling from the I phase.
Black points were obtained by reversing the (ꢁ) DC field without
heating the sample to the I phase.
Fig. 3 (a) POM photograph (ꢂ20; r.t.) of a 5 mm sandwich cell with ITO
electrodes. An electric field was applied during the I–Colh transition
that provoked the orientation of the columns. (b) Schematic illustration
showing the SHG light polarization for different polarizations of the
fundamental wave. (c) SHG signal at 40 ꢀC as a function of the angle of
incidence for the s / p conversion. The solid line is a theoretical fit
giving d31 ¼ 0.5 pm Vꢁ1. (d) Temperature dependence of the SHG signal
at an angle of incidence of 30ꢀ. The origin is the clearing point (Tc).
between the fundamental and SHG waves in the glass plate. The
phase shi is controlled by rotating the glass plate, and the
resulting SHG signal shows interference fringes as the rotation
angle is varied. When the LC sample is obtained from the
isotropic liquid under opposite DC elds interference patterns
in antiphase are attained (blue and red points in Fig. 4b). In
addition, the pattern remains unaltered if the eld is turned off
or even reversed within the mesophase range (black dots in
Fig. 4b). This indicates that the liquid crystal sample is
AC voltage. These observations suggest that the marked dipole
moment of the mesogens, perpendicular to the SubPc core,
prompt them to interact with the electric eld during the I–Colh
transition. Accordingly, unidirectional orientation of the
columns along the electric eld is achieved, and remains stable
in the mesophase once the electric eld is turned off.
We then investigated the LC material in the Linkam cells
described above using second harmonic generation (SHG),
which is a convenient and reliable method to determine the
existence of polarity in the presence and absence of an electric
eld. A setup20 including a Nd:YAG laser with a fundamental
wavelength l ¼ 1064 nm was used. The SHG signal is observed
in the mesophase when the material is cooled down from the
isotropic liquid under a DC eld. The SHG response is
preserved aer turning off the eld. The SHG light is p polar-
ized, regardless of whether the fundamental light is p or s
(Fig. 3b). In addition, the SHG signal is null at normal incidence
(Fig. 3c). These characteristics indicate that the material has a
macroscopic polar axis along the eld direction, with a
susceptibility tensor of the same symmetry as poled polymers.17
The induced polar axis is thermally stable and is kept up to the
clearing point (Fig. 3d). Remarkably, the SHG signal is main-
tained at room temperature for several weeks without appre-
ciable degradation.
We used SHG interferometry to analyze the switching
behavior of the 1 material. The setup (Fig. 4a) is designed to
permit the interference of the SHG signals from the sample and
a quartz plate. A relative phase shi between the two SHG waves
is produced as a consequence of the optical path difference
Fig. 5 Schematic representation of the polarization behavior of a LC
film of 1. Arrows represent groups of molecular dipoles which
assemble in columns in the mesophase. The polarization is developed
on cooling the material from the isotropic liquid under a DC electric
field. Within the mesophase, the polar order remains unaltered after
the electric field is switched off and cannot be reversed.
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permanently polarized once formed, but it is not electrically
invertible. In other words, the mesophase is not ferroelectric
but pyroelectric. The directional sense of the macroscopic
polarization can only be reversed by heating the material up to
the isotropic phase and cooling down again under an opposite
eld. A schematic illustration of the polarization behavior
under electric eld and temperature treatment is shown in
Fig. 5.
It is interesting to have an idea of the degree of polar order in
the structure. To evaluate this parameter, the magnitude of the
SHG signal was analyzed. By comparing an estimate of the
microscopic molecular hyperpolarizabilities21 with the macro-
scopic SHG signal, a minimum threshold for the polar order in
the mesophase of 35% was deduced17 for an applied electric
eld of 12 V mmꢁ1 in the I phase. This degree of order corre-
sponds to a remnant polarization of 400 nC cmꢁ2 (taking a
dipole moment of 13.7 D derived from our PM7 calculations).
Larger electric elds induce higher order, but elds above 15 V
mmꢁ1 are likely to produce dielectric breakdown.
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Conclusions
´
In conclusion, we have studied a columnar liquid crystal that
exhibits permanent polar order along the columnar axis. The
polarization orientation is developed under electric eld
application only when cooling the sample from the isotropic
phase. The degree of polar order is large-enough for device
applications,13 not sensitive to electric elds of opposite sense,
and can be conserved for weeks in the mesophase aer the
electric eld is switched off. This unique effect is ascribed to the
rigidity of our axial dipolar SubPc stacks and differs from
related LCs where polar order can eventually vanish or switch
due to conformational or orientational changes.
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Acknowledgements
Funding from MINECO (MAT2012-38538-CO3-01, 02 and
CTQ2012-31914), MICINN and MEC (F.P.U. fellowship (JG),
CTQ-2011-24187/BQU, CTQ2011-23659 and Consolider-Ingenio
Nanociencia Molecular, CDS2007-00010), Comunidad de
Madrid (S2013/MIT-2841 FOTOCARBON), European Research
Council (StG-279548), Aragon government, E04, Basque Country
Government GI/IT-449-10, Generalitat Valenciana (PROMETEO/
2012/053), and FEDER is acknowledged. We thank the
Advanced Microscopy Laboratory (LMA) for the AFM studies.
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Soc., 2003, 125, 8264; (b) K. Kishikawa, S. Nakahara,
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¨
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Journal of Materials Chemistry C
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´
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˚
˚
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´
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