Janus-type dendromesogens: A tool to control the nanosegregation and polar organization of bent-core structures

Article abstract of DOI:10.1021/cm302144f

Several first- or second-generation Janus-type codendrimers that combine promesogenic bent-core and rodlike molecular segments have been synthesized by the versatile CuAAC reaction and the materials have been characterized by POM, DSC, XRD, and SHG studies. Depending on the ratio between the number of rod- and bent-core units, these compounds form mesophases ranging from nematic to a complex polar smectic mesophase in which the two types of mesogenic moieties are segregated on a nanometric scale. In the case of a 1:1 bent/rod ratio, the materials form ferroelectric mesophases and the macroscopic polarization is stable in the absence of an applied electric field. The appearance of the different mesophases is explained in terms of the relative values of the transverse areas of the mesogenic cores.

Full text of DOI:10.1021/cm302144f

Article
pubs.acs.org/cm
Janus-Type Dendromesogens: A Tool to Control the
Nanosegregation and Polar Organization of Bent-Core Structures
Nelida Gimeno,† Jorge Vergara,† Miguel Cano,† Jose Luis Serrano,‡ M. Blanca Ros,*,† Josu Ortega,§
́
́
Cesar L. Folcia,⊥ Sofía Rodríguez-Conde,⊥ Gerardo Sanz-Enguita,⊥ and Jesus Etxebarria⊥
́
́
†Departamento de Química Organ
Zaragoza-CSIC, 50009-Zaragoza, Spain
‡Departamento de Química Organ
ica, Facultad de Ciencias, Instituto de Nanociencia de Aragon
50009-Zaragoza, Spain
́
ica, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragon
́
, Universidad de
́
́
, Universidad de Zaragoza,
⊥
§Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, and Departamento de Física de la Materia Condensada,
Facultad de Ciencia y Tecnología, UPV/EHU, 48080 Bilbao, Spain
S
* Supporting Information
ABSTRACT: Several first- or second-generation Janus-type
codendrimers that combine promesogenic bent-core and rodlike
molecular segments have been synthesized by the versatile
CuAAC reaction and the materials have been characterized by
POM, DSC, XRD, and SHG studies. Depending on the ratio
between the number of rod- and bent-core units, these
compounds form mesophases ranging from nematic to a
complex polar smectic mesophase in which the two types of mesogenic moieties are segregated on a nanometric scale. In
the case of a 1:1 bent/rod ratio, the materials form ferroelectric mesophases and the macroscopic polarization is stable in the
absence of an applied electric field. The appearance of the different mesophases is explained in terms of the relative values of the
transverse areas of the mesogenic cores.
KEYWORDS: Janus codendrimers, bent-shaped molecules, liquid crystals, ferroelectric material, bent-core liquid crystals
in the same material of bent molecules and other structures, a
design that allows the unique properties derived from both
systems to be combined. For this purpose, two different
approaches have been employed. In most cases, new materials
are prepared by physical blending of bent-shaped molecules
with the other components. This approach has provided new
mesophases and/or phase sequences that depend on the
concentrations of the components.11−15 For example, mixtures
with a small amount of achiral bent-core molecules have been
successfully used to enhance chiral effects12 or to induce
antiferroelectric behavior.13 Moreover, it has recently been
reported that the doping of nematic bent-core liquid crystals
with a small percentage of chiral additive yields blue phases14,15
(namely BPI and BPIII). Unfortunately, in many cases, the
limited molecular miscibility seriously limits the possibilities for
this kind of approach.
INTRODUCTION
■
In the search for functional or smart materials with tunable
properties, organized molecular assemblies represent a key
area.1,2 It has been demonstrated in numerous studies that
liquid crystals are excellent candidates for organizing active
units and a variety of mesophases have been successfully used
with this aim in recent decades.3 In the work described here we
focused on unconventional liquid crystals, the so-called bent-
core liquid crystals.4 These materials are a type of mesogen that
has compact molecular packing in the mesophase, thus
restricting the molecular rotational freedom. This packing
leads to strong polar order within layers or columns and many
interesting phenomena have been observed in these systems.
Among other properties, we can highlight the manifestation of
supramolecular chirality in the absence of chiral molecules4 and
the appearance of high macroscopic polarization values and
piezoelectric, flexoelectric, or nonlinear optical responses.4,5
Current research interest in this area is focused on the
structural characterization and understanding of the new types
of mesophases4−6 and the modulation of their physical
properties. In this sense, it is especially important to understand
how to control this kind of supramolecular packing through
appropriate molecular design for both low4−6 and high7−10
molecular weight compounds.
A different strategy has been proposed that involves the use
of dimeric molecules, where the bent-core structures are
covalently linked to another moiety to form nonsymmetric
molecules. Despite the fact that very few examples have been
reported to date, this approach offers the possibility of
obtaining interesting physical properties.16−19 For example,
Received: July 9, 2012
An efficient strategy to control or modify supramolecular
arrangements of bent-core molecules involves the incorporation
Revised: December 14, 2012
Published: December 24, 2012
© 2012 American Chemical Society
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Scheme 1. Schematic Representation of the Different Units Selected to Build up the Janus-Type Dendrimers (top) and the Six
Codendrimers Prepared from These Units (bottom)
Yelamaggad et al.17,18 reported the induction of a biaxial
nematic phase or broad blue mesophases in dimers based on
bent core-calamitic hybrids, and Weissflog et al.16a,19 studied
the structure−property relationships for these kinds of bent-
core based dimers.
Numerous attractive possibilities still remain unexplored in
the combination of bent-core and rod-like structures. One such
example is the case of block codendrimers, sometimes known
as hybrid or “Janus” dendrimers.20 Janus-like structures
represent a useful tool to exploit the cooperative nature of
the mesomorphism and, whereas dimers offer a fixed 1:1 bent/
rod ratio, Janus-like structures allow the manipulation of the
bent/rod ratio through the appropriate dendron combinations
among other possibilities.
Janus dendrimers have recently attracted a great deal of
attention as a new class of compounds since they combine two
differently functionalized segments on opposite sides of a single
molecule, thus yielding bifunctionalized products with a
perfectly controlled disposition of the peripheral groups. This
approach has been used to produce outstanding examples of
amphiphilic self-assembling materials or materials for medical
applications.21,22 Janus-type dendrimers can also present
mesomorphism20a,b,g,23 and this has proven to be a very
powerful and promising feature to control the supramolecular
organization of dissimilar chemical structures in which either
calamitic or discotic moieties have been incorporated.23f−h,24−29
We describe here the synthesis and characterization of the
first series of low generations Janus-type dendrimers that bear
rodlike and bent-shaped cores (Scheme 1). The work mainly
focuses on the different mesomorphism and physical properties,
which depend on the bent/rod ratio of the constituent cores of
the dendrimers. We concentrated on what we consider to be a
representative selection of structural motifs from the many
possibilities that exist. Our aim was to gain a better
understanding of the general rules that govern the meso-
morphic behavior of this kind of block dendrimer depending on
the bent/rod core ratio. Moreover, we also report the first series
of dendrons bearing bent-core units in their periphery.
As far as the bent-core units are concerned, two structures
based on the 3,4′-dihydroxybiphenyl core, a well-known
promoter of bent-core mesophases, were selected.4 The first
part of our research was focused on the use of a long
promesogenic unit (B2) that contained six aromatic rings, an n-
tetradecyloxy chain as the outer terminal tail and a long
[−(CH2)10−] flexible spacer − a system that was previously
used by us to produce bent-core dendrimers.30 Based on the
results obtained we decided to extend our study to a shorter
system (B1)30 that contained five aromatic rings, an n-octyloxy
terminal chain and a short [−(CH2)4−] spacer. These bent-
core moieties were attached to dendritic structures based on
bis-MPA31 dendrons of the first (G1) and second (G2)
generations functionalized at the focal point with an alkyne
group. A well-known rod-shaped promesogenic core, namely
the photoactive cyanoazobenzene (R), was chosen as a rod-like
unit and this was attached to the same type of dendron (G1
and G2) functionalized at the focal point as an azide. Calamitic
dendrons were attached to bent-core-based dendrons through a
triazole ring (TA) by applying a “click chemistry” approach
(CuAAC reaction).32 This methodology was used to prepare
six first- or second-generations Janus-dendrimers with different
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Scheme 2. General Synthesis of the Alkyne-Functionalized Molecules Based on B2 Structure Dendrons (Routes 1 and 2)
Scheme 3. General Synthesis of the Dendrons Based on the B1 Structure (Route 3), the R Moiety (Route 4) and Janus-Type
Codendrimers (Route 5)
bent-core/rod-core ratios and the properties of these
compounds were studied. Four molecules based on the longer
bent-core structure (B2) [B2-TA-RG1 (1:2 bent/rod units),
B2G1-TA-RG1 (2:2 bent/rod units), B2G1-TA-RG2 (2:4
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Table 1. Thermal and XRD Data for the Intermediate Compounds and the First- and Second-Generations Dendrons Derived
from the Bent-Core Units B1 and B2
a,b
compd
phase transitions (°C) (kJ mol−1
)
measured spacings (Å) Miller indices hkl layer thickness c (Å) theoretical molecular length (Å)
B2-CH2C≡CH
B2G1-Bn
Cr 84 [17.8] SmCP 118 [14.1] I
Cr 79 [2.6] SmCP 140 [34.7] I
40.0
92.9
46.0
30.4
42.3
27.8
43.0
89.6
44.8
29.9
42.8
29.9
89.6
44.8
29.4
53.3
26.1
20.4
17.3
53.3
26.5
20.7
17.4
002
001
002
003
002
003
002
001
002
003
002
003
001
002
003
100
200
110
120
100
200
110
120
80.0
92.1
84.0
72.4
75.6
70.8
73.2
79.2
74.3
B2G1-H
Cr 83 [18.1] SmCP 152 [35.6] I
B2G1-CH2C≡CH
B2G2-Bn
Cr 82 [8.3] SmCP 143 [37.7] I
Cr 62 [26.0] SmCP 141 [73.0] I
86.0
89.7
B2G2-H
Cr 81 [10.4] SmCP 166 [71.6] I
Cr 87 [12.2] SmCP 158 [77.1] I
87.7
89.6
B2G2-CH2C≡CH
76.9
59.7
B1G1-CH2C≡CH
B1G2-CH2C≡CH
Cr 76 [3.7] Colr 104 [30.2] I
a:22.1
b:52.2
c
Cr 70 [23.0] Colr 124 [61.7] I
a:22.4
b:53.1
59.7
a
b
Data determined by DSC, from second scans at a scanning rate of 10 °C/min. Cr, crystal; SmCP, smectic C polar mesophase; Colr, rectangular
c
columnar phase; I, isotropic liquid. Data from the first heating scan.
bent/rod units), and B2G2-TA-RG2 (4:4 bent/rod units)] and
two codendrimers containing the shorter bent-shaped structure
(B1) [B1G1-TA-RG2 (2:4 bent/rod units) and B1G2-TA-
RG2 (4:4 bent/rod units)] were prepared.
LIQUID CRYSTALLINE BEHAVIOR
■
The mesomorphic properties of the dendrons and the Janus-
type dendrimers were investigated by polarized light optical
microscopy (POM), differential scanning calorimetry (DSC)
and X-ray diffraction (XRD).
RESULTS
Bent-Core-Based Dendrons. Bent-core-based dendrons
have not been described previously and we present the
dendritic molecules under investigation here due to their
novelty. All of the bent-core first- and second-generations
dendrons described here form liquid crystalline phases and
their thermal and XRD data are gathered in Table 1. More
detailed information on the mesomorphic properties of these
dendrons, as well as data for the dendrons based on calamitic
molecules, is gathered in the Supporting Information. A
schematic representation of some of the mesophases that are
typical of bent-core molecules is also provided in Figure S1 in
the Supporting Information.
It can be seen that all of the bent-core dendrons present
mesophases with broad temperature ranges (more than 60 °C).
This fact confirms that dendritic molecules tend to stabilize
bent-core-type mesomorphism, not only for dendrimeric
structures30,35 but also for these bis-MPA-derived dendrons.
Once again, the supramolecular arrangement depends strongly
on the nature of the bent-core promesogenic unit. On the basis
of POM and XRD studies, a lamellar arrangement has been
assigned to the dendrons with the longest bent-core moiety
(B2) regardless of the generation of the dendron (G1 or G2)
or the focal point structure (-Bn, -H, or -CH2C≡CH), which
only modulates the transition temperatures slightly. On the
other hand, a columnar mesophase is proposed for the two
dendrons derived from the shortest bent-core unit B1.
Furthermore, concerning the clearing enthalpies, there is an
additive contribution of each mesogenic structure to the
■
Synthesis of the Compounds. Details of the synthetic
routes followed to prepare the dendrons and Janus-type
dendrimers are provided in Schemes 2 and 3. The nondendritic
bent-shaped alkyne B2-CH2C≡CH was prepared by function-
alization of the previously reported acid B2-OH33 with
propargylic alcohol, as shown in Route 1. Two alternative
synthetic routes were assessed in order to prepare the alkyne-
functionalized dendrons. In the case of bent-core dendrons
based on B2, the synthetic scheme shown in Route 2 was
employed and this provided several intermediate dendrons
(B2G1-Bn, B2G1-H, B2G1-CH2C≡CH, B2G2-Bn, B2G2-H,
and B2G2-CH2C≡CH). However, a simplified synthetic
pathway (Route 3) allowed us to obtain bent-core unit B1-
derived dendrons (B1G1-CH2C≡CH and B1G2-CH2C≡CH)
in a straightforward manner. On the other hand, dendrons
containing the calamitic units were prepared by Route 4, as
reported previously.34 All of the codendrimers were synthesized
using an alkyne and azide Huisgen cycloaddition with CuBr as
catalyst, N,N,N′,N′,N″-pentamethyldiethylenetriamine
(PMDETA) as ligand and DMF as solvent (Route 5, Scheme
3). The combination of bent-core and rod-like functionalized
dendrons gave access to codendrimers with a 1:1 or 1:2 bent/
rod ratio and allowed perfect control of the number of bent-
core and rod-like units on each side of the block codendrimer.
Further synthetic details and full characterization data for all
these compounds are gathered in the Supporting Information.
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Figure 1. (a) X-ray pattern of B2G2-Bn at small angles, (c, d) structural models deduced from (b) one of the electron density distributions
compatible with the experimental diagram. In c and d, the molecular tilt of the bent-core molecular segments has been omitted for clarity and an
antiferroelectric structure is represented by the antiparallel orientation of the dipole moments of the bent-core units.
transition enthalpy. Thus, the enthalpy for the first generation
(∼35 kJ/mol) is approximately double that of the bent-shaped
acid (18.4 kJ/mol) and half the enthalpy measured for the
second-generation dendron (∼70 kJ/mol). This phenomenon
has also been reported for other liquid crystalline dendritic
systems.30,34,36
observation indicates that the material has a homochiral
character (dark conglomerate without field and SmCP under
field).39
Janus-Type Dendromesogens. All of the first- and
second-generation Janus-type codendrimers described here
showed mesomorphism. Remarkably, the liquid crystal phases
remained on cooling below 100 °C. The phase sequences and
corresponding enthalpy values for the compounds are
summarized in Table 2. The XRD data are gathered in Table
3. It can be seen that, in general, the higher the generation the
broader the stabilization of the mesophase range (B2G1-TA-
RG1 vs B2G2-TA-RG2), which is not the trend reported for
the dendrons or homodendrimers.30
One can clearly distinguish between the mesogenic behavior
displayed by the codendrimers with 1:2 and 1:1 bent/rod
ratios. Two of the three codendrimers with a 1:2 bent/rod ratio
(B2-TA-RG1 and B1G1-TA-RG2) show diffuse reflections at
small angles (see Figure 3a), whereas the third (B2G1-TA-
RG2) presents a sharp peak in that region. The first kind of
pattern can be attributed to a nematic phase and that for the
third compound is consistent with a smectic mesophase. The
important fact, however, is that in all three cases the
corresponding distances (about 40 Å) are much smaller than
the molecular sizes. This indicates that the bent-cores and rod-
cores are homogeneously mixed in both the nematic and
smectic mesophases; i.e. there is no nanophase segregation − as
described in b and d in Figure 3.
The results obtained for dendron B2G2-Bn will be discussed
in detail as this material showed the most attractive properties,
reasonable transition temperatures and better chemical stability.
On cooling from the isotropic phase B2G2-Bn showed
schlieren textures of very low birefringence, which are typical
of the SmCP phase. The existence of a lamellar arrangement
was confirmed by the X-ray diffractograms, which showed up to
three reflections in the small-angle region in addition to the
diffuse wide-angle scattering. For this compound the theoretical
length (79.2 Å) calculated by Chemsketch37 is significantly
smaller than the layer spacing (89.7 Å). This finding indicates
that there are at least two dendrons per smectic layer. Similar
behavior was found for the other SmCP structures (Table 1).
The X-ray pattern (see Figure 1) contains a large (002) peak
and two small (001) and (003) reflections. Different models
consistent with the observed intensities could be proposed −
either dendrons in an elongated conformation with two
mesogenic cores on both sides of the focal point (presumably
the most stable conformation), or alternatively, completely
folded dendrons. In both cases, the molecules are tilted within
the layers.
In a first approximation, the smectic phase has a periodicity
equal to c/2 since the (002) peak is by far the most dominant.
However, the existence of the (001) and (003) peaks implies a
real periodicity of c. These reflections account for the small
difference in electronic density between the terminal chains
with and without the linking unit. A schematic representation of
the structures is shown in Figure 1 along with a possible profile
for the electronic density obtained by an inverse Fourier
transform that is compatible with the observed X-ray
intensities.4h,38
The SmCP mesophase of dendron B2G2-Bn was studied
under electric fields and it showed electrooptic switching. The
polarization curves indicate antiferroelectric behavior (see the
Supporting Information). In this case, the birefringence was
observed to increase on applying the electric field and, on
removal of the field, a dark texture with chiral domains
appeared on uncrossing the polarizers (see Figure 2). This
The structure shown in Figure 3e is another possibility that is
compatible with the XRD pattern of the smectic material. In
this case nanosegregation of both types of mesogenic segments
takes place. The single reflection observed would be the (003),
and the smaller angle reflections are assumed to be very weak.
In this case, the material would have a pseudoperiodicity of c/3
because of the similar electronic density of two calamitic cores
and one bent-core. Further analysis of this structure will be
presented in the Discussion section.
The electric field responses of B2-TA-RG1 and B1G1-TA-
RG2, both of which show the N mesophase, were investigated
in 5 μm glass cells. As expected, ferroelectric switching was not
detected. However, under a strong low-frequency electric field
(20 V/μm) an electrooptic response was observed due to the
dielectric anisotropy. The smectic mesophase of B2G1-TA-
RG2 also responded to the field. In this case the material
showed antiferroelectric switching for fields above 20 V/μm
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Table 2. Transition Temperatures and Enthalpies
Determined by DSC for Janus-Like Compounds B2-TA-
RG1, B2G1-TA-RG1, B2G1-TA-RG2, B2G2-TA-RG2,
B1G1-TA-RG2 and B1G2-TA-RG2
a,b
codendrimer
phase transition (°C) (kJ mol−1
)
B2-TA-RG1
Cr 71.6 [28.7] N 77.2 [12.8] I
B2G1-TA-RG1
B2G1-TA-RG2
B2G2-TA-RG2
B1G1-TA-RG2
B1G2-TA-RG2
Cr 89.9 [4.9] SmCSPF 104.2 [42.4] I
Cr 85.1 [58.6] SmCAPA 99.5 [46.3] I
Cr 89.5 [12.4] SmCSPF 130.9 [83.5] I
Cr 46.5 [11.7] N 78.9 [10.3] I
Cr 66.3[23.0] SmCSPF 92.5 [65.5] I
a
Data determined in the second scans at a scanning rate of 10 °C/min.
Cr, crystal; SmCSPF, polar smectic C synclinic ferroelectric
b
mesophase; SmCAPA, polar smectic C anticlinic antiferroelectric
mesophase; N, nematic phase; I, isotropic liquid.
Table 3. XRD Data for the Mesophases of Janus-Like
Compounds B2-TA-RG1, B2G1-TA-RG1, B2G1-TA-RG2,
B2G2-TA-RG2, B1G1-TA-RG2 and B1G2-TA-RG2
measured
spacings
(Å)
Miller
layer
theoretical
molecular
length (Å)
indices thickness
a
codendrimer mesophase
hkl
c (Å)
B2-TA-RG1
N
30−40
137.0
69.0
116
124
B2G1-TA-
RG1
SmCSPF
001
002
003
004
005
001
136.7
45.4
34.2
27.1
B2G1-TA-
RG2
SmCAPA
SmCSPF
44.6
44.7
126
128
B2G2-TA-
RG2
137.0
68.0
001
002
003
004
005
136.5
45.9
34.0
27.3
B1G1-TA-
RG2
N
30−40
102
B1G2-TA-
RG2
SmCSPF
108.7
54.5
36.2
27.7
21.7
001
002
003
004
005
108.7
107.2
a
N, nematic phase; SmCSPF, polar smectic C synclinic ferreoelectric
mesophase; SmCAPA, polar smectic C anticlinic antiferreoelectric
mesophase.
Figure 2. Microphotographs of the SmCP-DC mesophase of B2G2-
Bn: (a) in the absence of an electric field, (b) under an electric field
between crossed polarizers; (c, d) after removing the electric field and
uncrossing the polarizers, showing the chiral domains. Arrows indicate
transmission directions of the polarizers. (e, f) Textures of the SmCSPF
phase of compound B2G2-TA-RG2 at 113 °C under opposite electric
field polarity (10 V μm−1). After field removal the textures remain
unaltered in both cases, indicating a bistable ferroelectric ground state.
observed at low angles in addition to the wide-angle diffuse
halo. The mesophases are smectic because all the sharp
reflections can be indexed as (00l). The structure periodicities
are larger than the corresponding molecular lengths. In
addition, the intensity of the peaks follows a characteristic
sequence, with a dominant (003) reflection, a significant (002)
intensity and a small (001) peak (see Figure 4a). All of these
findings can be explained by the structural model proposed and
depicted in Figure 4b. As can be seen, the rod-core segments
are completely interdigitated, whereas the bent-core moieties
segregate in pairs between two consecutive layers of calamitic
structures. The peculiar pattern for the reflection intensities is
due to the complex structure of the smectic layers. A possible
electronic density consistent with the experimental intensities is
shown in Figure 4c and this was obtained by following the
Fourier transform procedure mentioned previously.4h,38 The
narrow density peaks account for the rod-core part and the
when the field was applied during the cooling process from the
isotropic phase. The smectic mesophase can thus be identified
as SmCAPA (see Figure S3 in the Supporting Information). The
lamellar mesomorphism of this codendrimer, which contains a
similar number of mesogenic units as B1G1-TA-RG2, should
be ascribed to a layer-ordering preference favored by the
presence of the more extended rigid core (B2) increasing the
intermolecular interactions.
The X-ray patterns of the codendrimers with a 1:1 bent/rod
ratio were completely different. Up to five Bragg peaks were
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Figure 3. (a) XRD diagram and (b) proposed structure for the nematic phase of B2-TA-RG1. (c) XRD diagram and (d, e) proposed structures for
the SmCAPA phase of B2G1-TA-RG2.
In order to ascertain whether the mesophase is intrinsically
ferroelectric, second-harmonic generation (SHG) experiments
were carried out (see the Supporting Information for
experimental details). In the three 1:1 codendrimers, an SHG
signal was detected in the absence of a field just below the
clearing point. A plot of SHG intensity versus temperature for
B2G1-TA-RG1 is shown in Figure 5 as an example.
Figure 4. (a) X-ray pattern of B2G1-TA-RG1 at small angles, and (b)
structural model deduced from (c) the electron density distributions
compatible with the experimental X-ray diagram. In b, the molecular
tilt of the bent-core molecular segments has been omitted for clarity
and the ferroelectric structure is represented by the parallel orientation
of the dipole moments of the bent-core units.
broader peaks are due to the bent-core moieties. The bent-core
part should be tilted by an angle of about 50° to fit the
observed spacing, and this is consistent with tilt angles within
Figure 5. SHG intensity of compound B2G1-TA-RG1 vs temperature.
The experiment was carried out in the absence of a field.
SmCP phases.
Rather remarkably, the three 1:1 compounds exhibit
ferroelectric smectic phases (SmCSPF). This point was proved
by the textures observed under an electric field and by SHG
measurements. Representative textures for this kind of mesogen
(B2G2-TA-RG2) are shown in panels e and f in Figure 2.
Under an electric field of 10 V μm−1, molecular switching
around the tilt cone (see Figure S1 in the Supporting
Information) was observed over the whole mesophase range.
The POM images indicate a synclinic arrangement under an
electric field. On removing the field the texture remained
unaltered regardless of the field polarity, which indicates that
the ferroelectric state is truly bistable.
SHG interferometry experiments were carried out in an effort
to gain a further insight into the ferroelectric behavior of the
ground state of these materials. A schematic representation of
the experiment is shown in Figure 6. Further experimental
details can be found in the Supporting Information. The
resulting SHG interference fringes are shown in Figure 6b. The
different curves were obtained in compound B2G1-TA-RG1
under electric fields with different polarity (+ open, − solid
circles) and after removal of the field (+ open, − solid squares).
The electric field was applied perpendicular to the sample
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Figure 6. (a) Schematic representation of the SHG interferometry
experiment. (b) SHG intensity of the interferometric experiment in
the same compound under an electric field with different polarity (+
open, − solid circles) and after field removal (+ open, − solid squares).
Fringes are in phase after field removal and a phase shift of π is
observed when the polarity is inverted. Lines connecting points are to
guide the eye.
substrate. The results shown in Figure 6b can be interpreted as
follows: If full circles (squares) represent |E1 + E22ω|2 then
2ω
open circles (squares) represent |E12ω − E22ω|2. In other words,
a phase shift of π in the SHG field generated by the sample is
observed after inverting the polarity, which indicates that the
ferroelectric state is fully switched by the field. Furthermore,
because a shift in the fringes was not observed on removing the
field, the bistability of the material is again confirmed.
Therefore, the ground state can be classified as SmCSPF.
Similar experiments were carried out on previous chiral bent-
core liquid crystals that exhibit ferroelectric smectic phases.40,41
Figure 7. (a) Schematic comparison between the effective cross-
section of a rod-like core and the lateral part (arm) of a bent-shaped
molecule. (b) Smectic layer of a compound with a 1:2 bent/rod ratio
in which the matching of the in-layer areas is represented
schematically. (c) The same scheme for a codendrimer with 2 rod-
like molecules and 2 bent-core molecules (1:1 bent/rod ratio). In this
case, segregation of rod and bent cores takes place and the rods
become interdigitated.
proposed model implies that rod-like structures of different
codendrimers interdigitate and form a sublayer, whereas bent-
core molecules are suitably tilted to match the cross section of
the rodlike units (see Figure 7c). In both families of
compounds, the required tilts of the bent cores are compatible
with the layer spacings measured by XRD.
DISCUSSION
■
As mentioned previously, the Janus-type codendrimers under
investigation can be classified into two different families
according to their mesogenic behavior. On the one hand, for
compounds with a 1:2 bent/rod ratio segregation did not occur
between bent- and rod-shaped cores. On the other hand,
segregation was observed for the group with a 1:1 bent/rod
ratio. This peculiar behavior can be explained in terms of the
different effective cross sections that the cores present. First, we
will assume that a rod-like core has a similar transverse area to
the lateral structures (arms) of a bent-shaped molecule.
However, it is clear that for the geometry of a bent-core
smectic material the arms are tilted with respect to the layer
normal even in the absence of a molecular tilt. This
arrangement increases the effective cross-section of the arm,
as depicted in Figure 7a. The enhancement is given by a factor
1/cosϕ, where ϕ is the angle of the molecular segment with
respect to the smectic layer normal. This angle is related to the
molecular tilt θ according to cosϕ = cosθ sin(γ/2), where γ is
the bend angle (about 120°). For typical values of θ and γ, the
effective area becomes enlarged by a factor close to 2. In this
case the difference between the in-plane areas per molecular
moiety could give rise to conflict when accommodating the
different cores in the mesophase. In the case of the 1:2 bent/
rod ratio, the cross-section of the bent cores, which in principle
is smaller, can be increased to match that of the rod-like cores
simply by means of a suitable molecular tilt. Once the molecule
is tilted, both bent-shaped and rod-shaped core groups will have
the same cross section and these would be be indistinguishable
when packed within the layers − a situation that does not give
rise to core segregation (see Figure 7b). In contrast, in the case
of the 1:1 bent/rod ratio, the cross-section of the bent-core part
is inevitably higher due to the molecular bend. This undesirable
situation can be resolved if core segregation is assumed. The
An apparent exception to this general behavior is the
structure proposed for B2G1-TA-RG2 (Figure 3e), where
there is core segregation for a 1:2 bent/rod ratio compound.
However, arguments based on the calculation of the mass
density of the material allow us to rule out this possibility. The
density of the structure is given by ρ = (2M)/(cA), where M is
the molecular mass and A is the transverse area of 4 calamitic
segments (A = 4π(d/2)2). If we take d = 4.6 Å, the well-known
typical diameter of a calamitic molecule, and c = 134 Å, we
obtain ρ = 1.58 g/cm3, which is rather high for a purely organic
liquid crystal. The alternative structure (Figure 3d) is more
realistic. In this case, ρ = M/(2cA) and c = 134/3 Å = 44.7 Å.
This gives ρ = 1.18 g/cm3, which is more in keeping with the
expected value (in fact, the same density ρ = 1.18 g/cm3 also
results for the structure shown in Figure 4b for a 1:2 bent/rod
ratio dendrimer). We can therefore conclude that nano-
segregation does not occur in this material.
A very interesting feature of the family with the 1:1 bent/rod
ratio is the appearance of true ferroelectricity. There are very
few examples of this kind of behavior in bent-core liquid
crystals6b,40−42 and it is even more unusual if the materials are
composed of achiral molecules, as in the present case. In the
smectic mesophases of bent-core compounds antiferroelec-
tricity is usually favored. However, in the present family of
codendrimers the appearance of the rodlike sublayer partially
inhibits the interaction between bent-core layers and allows the
appearance of ferroelectricity. A similar origin for the
ferroelectricity in bent-core compounds attached to bulky
oligosiloxane units has been proposed.41b In those cases, it is
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Chemistry of Materials
Article
the siloxane groups that are assumed to decouple the bent-core
molecular layers and prevent the appearance of antiferroelec-
tricity.
Analysis Services from the Instituto de Ciencia de Materiales de
́
Aragon (ICMA), Universidad de Zaragoza−CSIC (Spain).
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■
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■
A variety of liquid crystalline phases can be formed by first- or
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ASSOCIATED CONTENT
* Supporting Information
Experimental section: Synthesis of the compounds. DSC and
X-ray data for the dendrons derived from calamitic units. DSC
diagrams and POM for codendrimers, and polarization
switching current curves of representative dendrons and
codendrimers. This information is available free of charge via
the Internet at http://pubs.acs.org/.
■
S
Luchette, P.; Gleeson, J. T.; Sprunt, S.; Jak
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +34 976762277. E-mail: bros@unizar.es.
Notes
■
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors greatly appreciate financial support from the
Spanish Government (MICINN-FEDER projects MAT2009-
14636-C03) and the Aragon-FSE (project E04) and Basque
́
(project GIC10/45) Governments. The authors are also
■
grateful to Juan de la Cierva-MICINN, JAE-DOC−CSIC
́
(N.G.), to the BSCH-UZ and Aragon Government (project
́
E04) (J.V.) and Aragon Government (M.C.) fellowship
programs for support. S.R.C. and G.S.E. thank the MEC of
Spain and the Basque Government for grants. Thanks are given
to Nuclear Magnetic Resonance, Mass Spectra and Thermal
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