New SmCG phases in a hydrogen-bonded bent-core liquid crystal featuring a branched siloxane terminal group

Article abstract of DOI:10.1021/ja205550b

In this study, we synthesized three analogous bent-core molecules, a hydrogen-bonded complex and a covalent-bonded compound with branched siloxane units (H-SiO and C-SiO, respectively) and a hydrogen-bonded complex with an alkyl unit (H-Alk), and investigated the effects of the hydrogen bonding and branched siloxane terminal units on their mesomorphic properties. The covalent-bonded compound C-SiO and the hydrogen-bonded complex H-Alk exhibited typical SmCP phases; in contrast, the hydrogen-bonded complex H-SiO exhibited a series of general tilt smectic (SmCG) phases with highly ordered layer structures (i.e., SmCG₂P_F-USmCG₂P _A-SmCG₂P_F-SmCGP_F upon cooling). During the SmCG-type phase transition process, a 2D-modulated ribbon structure transferred into highly ordered layers via undulated layers, as the hydrogen-bonding strength increased with reduced temperatures. As the SmCG domains were aligned under dc electric fields, a gradual decrease in the leaning angle from ca. 60° to 50° (while the tilt angle kept at ca. 31°) could be determined by in situ wide-angle X-ray scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer structure of the hydrogen-bonded complex H-SiO. In addition, the ferroelectric-(antiferroelectric)-ferroelectric transitions proven by the switching current responses in the SmCG-type phases of H-SiO reveal that the polar switching occurred through collective rotations around the long axis of H-SiO. Therefore, novel SmCG phases with a series of highly ordered 2D-structures were induced by the effects of the hydrogen bonding and branched terminal siloxane unit in the bent-core hydrogen-bonded LC complex H-SiO.

Full text of DOI:10.1021/ja205550b

ARTICLE
pubs.acs.org/JACS
New SmCG Phases in a Hydrogen-Bonded Bent-Core Liquid Crystal
Featuring a Branched Siloxane Terminal Group
Wei-Hong Chen,† Wei-Tsung Chuang,*,‡ U-Ser Jeng,‡ Hwo-Shuenn Sheu,‡ and Hong-Cheu Lin*,†
†Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, 30049, Taiwan
‡National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan
S Supporting Information
b
ABSTRACT: Inthisstudy, wesynthesized three analogous bent-
core molecules, a hydrogen-bonded complex and a covalent-
bonded compound with branched siloxane units (HÀSiO and
CÀSiO, respectively) and a hydrogen-bonded complex with an
alkyl unit (HÀAlk), and investigated the effects of the hydrogen
bonding and branched siloxane terminal units on their meso-
morphic properties. The covalent-bonded compound CÀSiO
and the hydrogen-bonded complex HÀAlk exhibited typical
SmCP phases; in contrast, the hydrogen-bonded complex HÀSiO exhibited a series of general tilt smectic (SmCG) phases with
~
highly ordered layer structures (i.e., SmCG2PFÀUSmCG2PAÀSmCG2PFÀSmCGPF upon cooling). During the SmCG-type phase
transition process, a 2D-modulated ribbon structure transferred into highly ordered layers via undulated layers, as the hydrogen-
bonding strength increased with reduced temperatures. As the SmCG domains were aligned under dc electric fields, a gradual
decrease in the leaning angle from ca. 60° to 50° (while the tilt angle kept at ca. 31°) could be determined by in situ wide-angle X-ray
scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in
the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer
structure of the hydrogen-bonded complex HÀSiO. In addition, the ferroelectricÀ(antiferroelectric)Àferroelectric transitions
proven by the switching current responses in the SmCG-type phases of HÀSiO reveal that the polar switching occurred through
collective rotations around the long axis of HÀSiO. Therefore, novel SmCG phases with a series of highly ordered 2D-structures
were induced by the effects of the hydrogen bonding and branched terminal siloxane unit in the bent-core hydrogen-bonded LC
complex HÀSiO.
’ INTRODUCTION
phases have also displayed antiferroelectric switching (ColobPA,
ColrPA)4 and ferroelectric switching columnar phases (ColobPFE,
ColrPFE).5
Bent-core liquid crystals (LCs) have received great attention
since the discovery, by Niori et al.,1 of their special electro-optical
properties, such as polar switching behaviors, spontaneous
polarized capabilities, and nonlinear optics.2 Several studies have
been made into the effects of the configuration of the bent-core
molecules, including the zenithal constituent, the shape of the
rigid core, the lengths of the flexible chains, and the role of
substituents at selective positions of a rigid core, on the proper-
ties of the LCs.2 Those effects strongly influence the mesophases
of the bent-core molecules, resulting in eight main mesophases,
denoted B1ÀB8,2 with smectic, columnar, or three-dimensional
orders having been discovered to date. Depending upon the
direction of polarization in the continuous layers and the
molecular tilt angle with respect to the layer normal, the bent-
core molecules spontaneously exhibit polar and chiral layers,
leading to SmCP phases (B2) having four layer-type structures
with a ferroelectric or antiferroelectric state, the homochiral
SmCSPF and SmCaPA and racemic SmCaPF and SmCSPA phases.2
As a result of the splay of polarization in smectic layers,3
modulation induces the breaking layers or ribbons to undergo
long-range ordering in the two-dimensional 2D lattices of oblique
(Colob) and rectangular (Colr) phases.2 These modulated smectic
In addition to the typical SmC and modulated SmC phases,
bent-core molecules have the potential to display a rarely
observed related phase: a double-tilted smectic phase having a
chiral C1 symmetry configuration, assigned to a SmCG phase, as
proposed by de Gennes et al.6 Two tilted angles are defined with
respect to the layer normal: the tilt of the molecular plane
(clinicity) and the tilt of the molecular kink direction (leaning).
This double-tilt conformation gives rise to syn-leaning and anti-
leaning structures in addition to syn-clinic and anti-clinic con-
figurations in the SmC phase. On the basis of these conforma-
tional varieties, eight different orientations are possible for
molecular packing in the SmCG phase;7 nevertheless, few
experimental studies have been reported regarding the realiza-
tion of the SmCG phase in bent-core molecules.
Most modulated bilayer structures composed of asymmetric
bent-core molecules have been assumed to feature a double tilt
SmCG2 phase;8,9 nevertheless, unambiguous evidence for the
Received: June 22, 2011
Published: August 26, 2011
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2011 American Chemical Society
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leaning angle has been rare. The SmCG2 phase was suggested to
relate to the deflection of the molecular mass center in con-
secutive layers in symmetric and asymmetric molecular struc-
tures of the bent-shaped molecules.9c Recently, a series of phase
and A12, respectively) and the H-acceptor VPy in THF solution at
40 °C to obtain the self-assembled asymmetric bent-core complexes.
Synthetic procedures for all of the compounds are provided in the
Supporting Information. The homeotropic aligned samples of the bent-
core molecules were obtained by heating small amounts of compounds
from the isotropic state with a very slow cooling rate on a glass substrate
treated with a commercially available homeotropic agent, for the 2D-
XRD patterns. Evidence for hydrogen bonding in these asymmetric
heterodimers was obtained using FTIR spectroscopy (Figure S1). A similarly
structured, covalent-bonded bent-core molecule featuring the same siloxane
unit (CÀSiO) was synthesized as a control. The phase transition tem-
peratures and enthalpies of the three systems, obtained using differential
scanning calorimetry (DSC; Figure S2), are listed in Table 1.
~
transitions SmCG2ÀSmCG2ÀSmC was examined through
X-ray diffraction and construction of electron density maps in
an attempt to elucidate the formation of modulation for the
SmCG2 phase;9b even so, the origin of the SmCG-phase transi-
~
tions was not fully addressed.
Providing an interesting approach toward generating novel
and creative materials through supramolecular chemistry, hydro-
gen-bonded bent-core LCs composed of nonmesomorphic small
molecules is a leading topic in the field of LCs because of their
potential application in biomaterials10 and electro-optical devices.11
Supramolecular interactions between functionalized proton donors
(H-donor) and proton acceptors (H-acceptor) bearing comple-
mentary binding sites can provide new complexes exhibiting
mesomorphic structures. A fundamental understanding of su-
pramolecular self-assembling LC systems will be necessary for
their future scientific and technological development. The nature
of the pendant groups on the bent-cores of hydrogen-bonded
bent-core LCs can affect their electric and optical behavior.12 Lin
et al. have studied how the mesophasic type is influenced by (i)
the hydrogen-bonded site of the core, (ii) the total number of
aromatic units in the hydrogen-bonded core, and (iii) the presence
percentages of hydrogen and covalent bonds in hydrogen-bonded
side-chain copolymers.13 Tschierske et al. studied the properties of
bent-core structures containing siloxane units at one or both ends of
the flexible alkyl chain termini.14 They suggested that the siloxane
units segregate into divided sublayers at the interfaces between
adjacent layers; this microsegregation alters the phase character-
ization of the bent-core molecules. From the viewpoint of
molecular design, the hydrogen-bonded bent-core molecules
combined with the siloxane units might provide interesting
supramolecular LC structures; to the best of our knowledge,
however, such systems have not been investigated previously.
In this study, we synthesized a new hydrogen-bonded complex
and a covalent-bonded compound with branched siloxane units
(HÀSiO and CÀSiO, respectively) and a hydrogen-bonded
complex lacking siloxane units (HÀAlk) to explore the effects of
both hydrogen bonds and branched siloxane units. The complex
of hydrogen-bonded bent-core molecules with the branched
siloxane unit exhibited a series of SmCG-type phase transitions.
We determined the leaning angles of these SmCG-type phases
using synchrotron-based in situ small-angle X-ray scattering (SAXS)
and wide-angle X-ray scattering (WAXS) measurements under dc
electric fields. To probe the origin of the SmCG-type phase tra-
nsitions, we examined the relationship between the variation in
leaning angle and the interplay between the competition of hydro-
gen bonds and microsegregation of the siloxane units. Herein, we
provide a possible explanation for the rare SmCG-type phase
transitions, based on the results obtained from in situ X-ray scattering
and Fourier transform infrared (FTIR) and Raman spectra.
Characterization Methods. 1H NMR spectra were recorded
using a Varian Unity 300 MHz spectrometer, with DMSO-d6 and
CDCl3 as solvents. Mass spectral data were obtained using a Micromass
TRIO-2000 GC/MS system. Elemental analyses were performed using a
Heraeus CHN-OS RAPID elemental analyzer. Mesophasic textures
were characterized using a Leica DMLP polarizing optical microscope
equipped with a Linkam hot stage. FTIR spectra were recorded using a
Thermo Nicolet Nexus 6700 instrument. Raman spectroscopy was
performed using a HeÀNe 633 laser source.
Synchrotron-based small-angle and wide-angle X-ray scattering mea-
surements in conjunction with grazing-incidence scattering (SAXS/
GISAXS and WAXS/GIWAXS, respectively) were performed at the
BL23A SAXS and BL01C2 XRD endstations, respectively, of the
National Synchrotron Radiation Research Center, Hsinchu, Taiwan.16
With the marCCD165 detector used for SAXS and the mar345 image
plate for WAXS, typical scattering patterns were collected from at least
60 to 300 s to obtain sufficient scattering intensities. The beam diameter
of the 10 keV beam (λ = 0.124 nmÀ1) was 0.5 mm. The scattering
vectors (q = 4π/λ sin θ; with scattering angle 2θ) for SAXS and WAXS
were calibrated using two standard samples of silver behenate and
silicon. The three synthesized systems were subjected to homeotropical
alignment on a glass substrate for the GISAXS and GIWAXS measure-
ments. With electric-field alignment, the sample cells for in situ X-ray
scattering measurement consisted of two stainless-steel electrodes
having a thickness of 1 mm separated by a controlled gap of 0.6 mm
(see Figure S3). This electrode configuration was held in the modified
Linkam hot stage. Samples were heated above the isotropic temperature
within the electrodes and held in place by capillary forces. A dc electric
field of 200 V was applied across the electrodes while in the isotropic
phase; the sample was then cooled to within the given temperature
region. The incident X-rays were aligned normal to the direction of the
applied electric field. Two sample alignments of the surface and electric
field, combined with the X-ray scattering measurements, provided views
parallel and perpendicular to the bending plane of bent-core molecules,
respectively. When the molecules are well aligned, the former provided
the tilt angle from the diffuse scattering halo, and the latter provided the
leaning angle of the SmCG phase.
The electro-optical properties were determined in commercially available
indium tin oxide (ITO) cells (mesostate; thickness, 7.5 μm; active area,
1 cm2) featuring rubbed polyimide alignment coatings (parallel rubbing
direction). A digital oscilloscope (Tektronix TDS-3012B) was used in these
measurements; a high-power amplifier connected to a function generator
(GW model GFG-813) with a dc power supply (Keithley 2400) was
used in the dc field experiments. During electro-optical measurements,
the modulation of the texture under the applied electric field was
monitored using polarizing optical microscopy (POM).
’ EXPERIMENTAL SECTION
Synthesis and Sample Preparation. Three structurally similar
molecules were synthesized; their molecular structures are provided in
Scheme 1. Hydrosilylation of the olefinic compounds was performed
using 1,1,1,3,5,5,5-heptamethyltrisiloxane in the presence of Karstedt’s
catalyst.15 The hydrogen-bonded complexes HÀSiO and HÀAlk were
prepared by mixing appropriate molar ratios (1:1) of the H-donor (3SiA
’ RESULTS AND DISCUSSION
To study the effects of the hydrogen bonds and branched
siloxane units on the mesomorphic properties of LC molecules,
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Scheme 1. Chemical Structures of the Hydrogen-Bonded Complex and Covalent-Bonded Compound with Siloxane Units
(HÀSiO and CÀSiO, Respectively) and the Hydrogen-Bonded Complex Lacking a Siloxane Unit (HÀAlk)
Table 1. Phase Transition Temperatures and Enthalpies of
the Hydrogen-Bonded Complex and Covalent-Bonded
Compound with Siloxane Units (HÀSiO and CÀSiO, Re-
spectively) and the Hydrogen-Bonded Complex Lacking
Siloxane Units (HÀAlk)a
with and without siloxane units (56.2 and 32.2 °C for HÀSiO
and HÀAlk, respectively), the bent-core covalent-bonded com-
pound CÀSiO exhibited a broader LC phase range (ca.
123.2 °C) because the covalent-bonded structure bearing a
siloxane unit stabilized the LC phase2a,14 (see Table 1). POM
images revealed characteristic fan textures for HÀAlk, CÀSiO,
and HÀSiO (Figures 1À3, respectively).
phase transition temperature (°C)
compound
and enthalpy (kJ/mol)
Substrate-Oriented SmCG-type Phases. To investigate the
mesophasic types of these analogous bent-core molecules,
well-aligned samples were prepared via homeotropic align-
ment technique (as described in the Experimental Section) to
observe the side-view of the molecular arrangements by
GISAXS/GIWAXS patterns. In Figure 1a, the GIWAXS pat-
tern of the hydrogen-bonded complex HÀAlk reveals layer-
like reflections on meridian, yielding a layer spacing (dlayer) of
5.2 nm that is smaller than the theoretical molecular length of
the complex HÀAlk (L = 6.3 nm), indicating the tilted
arrangement of a smectic structure (SmC phase). The diffuse
scattering halo at a value of q of approximately 14 nmÀ1 (d =
0.45 nm) represents the mean distance between liquid-like
alkyl chains. Two azimuthal maxima among the halo located
out of the equator reveal a tilt angle ω of approximately 30°
HÀSiO
Cr 75.9[22.8],b SmCGPF 86.0, SmCG2PF 97.0,
b
~
USmCG2PA 115.0, SmCG2PF 132.1 [28.6], Iso
HÀAlk
CÀSiO
Cr 96.4 [19.5],b SmCPA 128.6 [30.9],b Iso
Cr 49.3 [10.7],b SmCPF 172.5 [29.7],b Iso
a Mesophasic types and transition temperatures were observed using
XRD and switching current measurements in terms of changes in polar
switching behavior. b Phase transitions were measured from the second
cooling scan of DSC (cooling rate: 5 °C minÀ1).
we used POM and DSC to investigate the compound with
siloxane units (HÀSiO and CÀSiO, respectively) and a hydro-
gen-bonded complex lacking a siloxane unit (HÀAlk). In con-
trast to the mesophasic ranges of the hydrogen-bonded complexes
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(consistent with the tilt angle of 34° calculated using the
equation cos ω = dlayer/L) for the long molecular axes with
respect to the layer normal. In Figure 1b, the POM image
displays the weak birefringence texture of the SmC phase in the
hydrogen-bonded complex HÀAlk. For the covalent-bonded
compound CÀSiO, the layer-like reflections, observed as DebyÀ
Scherrer rings, in Figure 2a reveal a layer spacing of 6.8 nm,
slightly smaller than the molecular length of 7.1 nm, suggesting a
small tilted angle in the SmC phase. The SmC phase of the
covalent-bonded compound CÀSiO, characterized by high visco-
elasticity, was negligibly aligned by the surface and electric-field
alignments, leading to a powder-like form. Figure 2b displays the
high birefringence in the fan-like texture of the covalent-bonded
compound CÀSiO.
Because of a large structure size in the hydrogen-bonded
complex HÀSiO, we present combined GISAXS and GIWAXS
patterns to show the complete reflections in the full q scale.
Figure 3aÀd displays the combined GISAXS and GIWAXS
patterns at representative temperatures, after cooling from the
isotropic temperature (140 °C), for the surface-aligned sample of
the hydrogen-bonded complex HÀSiO; POM images are pro-
vided as insets. Figure 3a observed at 130 °C is the highly ordered
reflections located along the meridional and off-meridional
directions of the GISAXS/GIWAXS patterns, indicating a highly
ordered structure. Using an oblique 2D lattice, with the two
lattice parameters a (=19.1 nm) and c (=12.9 nm) and an oblique
angle β (96°), we can index all reflections in Figure 3a. The d-
spacing of (01) (12.8 nm) corresponds to a bilayer spacing of the
LC domains of the hydrogen-bonded complex HÀSiO (in which
the molecular length is ca. 7.2 nm). Assuming an average distance
of 0.5 nm between molecular planes, the bilayer spacing, and a
density of 1.0 g cmÀ3 for HÀSiO, we have further estimated that
each cross section of the ribbon unit shown in Figure 3a may
consist of ca. 70 bent-core molecules. This large ribbon dimen-
sion is less commonly observed in a bent-core LC.
Figure 1. (a) GIWAXS pattern and (b) POM image of the surface-
aligned sample of the hydrogen-bonded complex HÀAlk at 110 °C.
As the temperature decreased to 110 °C (Figure 3b), the (20)
reflection at the equatorial direction and some higher-order
diffractions at the off-meridian direction disappeared, while the
(11) and (À11) reflections moved toward to the meridian. These
changes signify merging of ordered ribbons into undulated
bilayers. Further cooling to 95 °C, the diminishing (11) and
(À11) reflections were nearly absorbed into the (01) reflection
spot. At 85 °C, approaching the LC phase border, signature
reflections of the bilayer structure were largely gone, leaving
only characteristic reflections from lamellae of characteristic
Figure 2. (a) GIWAXS pattern and (b) POM image of the surface-
aligned sample of the covalent-bonded compound CÀSiO at 100 °C.
Figure 3. Combined GISAXS and GIWAXS patterns of the surface-aligned sample of the hydrogen-bonded complex HÀSiO at various temperatures:
~
(a) modulated ribbon structure (SmCG2) at 130 °C; (b) undulated bilayer structure (USmCG2) at 110 °C; (c) bilayer structure (SmCG2) at 95 °C; and
(d) monolayer structure (SmCG) at 85 °C. Model: Yellow, green, and blue lines indicate oligosiloxane units, alkyl chains, and aromatic cores,
respectively. The light gray part of the inset in (a) shows the molecular organization at a side view along the direction a (right) and front view along the
normal direction of the aÀc plane (left).
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phase (Figure 5). In addition, a higher value centered at q ≈
9 nmÀ1 (or 0.7 nm spacing) is an isotropic halo ring (Figure S4),
likely corresponding to a mean distance between the segregated
siloxane units as also proposed by Tschierske et al.14 Further-
more, the azimuthal intensity distributions of the GIWAXS
diffuse halos at q ≈ 13.5 nmÀ1 can be decomposed into two
humps (Figure 4) contributed respectively by the tilt and leaning
of the hydrogen-bonded complex HÀSiO, because the surface-
aligned sample is 2D powders in which the layers are stacked
along the substrate normal but the azimuthal orientation of the
layers is random. For the first set, the decomposed humps
centered at a constant angle of ca. 31° are insensitive to
temperature change, and the angle is close to the calculated tilt
angle of 29° based on the measured d-spacing and simulated
molecular length. For the second set, the hump centers, corre-
sponding to the mean leaning angles, shift slowly from 60° to 50°,
as temperature decreased from 130 to 85 °C; the result is
consistent with increased hydrogen-bonding strength for a better
alignment (smaller leaning) of HÀSiO along the smectic layer
normal. The mean tilt and leaning angles thus decoupled are
consistent with those obtained in the case with an electric-field
alignment (see next section).
Electric-Field-Oriented SmCG-type Domains. To better
illustrate the tilt and leaning angles of the SmCG-type phases
of the hydrogen-bonded complex HÀSiO, we performed in situ
SAXS and WAXS measurements under electric fields perpendi-
cular to the X-ray beam, as illustrated in Figure S3. In the
corresponding SAXS patterns (Figure 6), all reflections situate
along the equatorial direction, and all peak positions are the same
as those obtained for the surface aligned sample using GISAXS
(Figure 6c), as selectively shown at 95 and 130 °C cases. This
result indicates that the bilayer structures were merely oriented
along the electric-field direction, as a bookshelf structure, but
without local structural changes, such as the tilt angle (as
evidenced by the same bilayer d-spacing with and without
electric field as illustrated in Figure 6c). Furthermore, the
GISAXS result reveals a polarized direction of the bilayer in
the hydrogen-bonded complex HÀSiO along the 2D lattice
plane, as a B1rev,tilted phase for the modulated bilayer structure
Figure 4. Azimuthal scans of the GIWAXS diffuse halos in the SmCG
phases of the hydrogen-bonded complex HÀSiO at various tempera-
tures, and the zero angle corresponding to the layer normal. The profiles
are fitted with two sets of Gaussian curves: the blue curves indicate the
decreased mean leaning angles with decreasing temperature from 130 to
85 °C, whereas the green curves indicate a constant mean tilt angle in
this temperature range. Inset: 2D-GIWAXS pattern obtained at 130 °C.
~
(SmCG2); this finding would frustrate the possibility of a 3D
modulated phase proposed by Szydlowska et al.17 Previously,
Jakli et al.18 suggested that the layer normal of the SmCG phase
could be tilted with respect to the direction of the electric field
under a large electric field. In this case, we found that the layer
normal is only perpendicular to the electric field under a small
applied field (0.33 V μmÀ1) (Figures 6a,b, 7a,b); the small
electric field, nevertheless, could effectively align the smectic
layer domains of the SmCG-type phases. As noted, the applied
small electric field meant to align the layer domains of the
SmCG-type phases, rather than individual HÀSiO molecules
by their long or short axes. The successful electric-field align-
ment can be realized by the largely enhanced peak intensity shown
in Figure 6c (corresponding to more oriented SmCG domains).
Likely, the electric dipole moments, oriented along the mirror
symmetry direction of the back-to-back packed HÀSiO pairs, are
always perpendicular to the smectic layer normal, hence to in-
duce an effective alignment by a small electric field. We empha-
size that the applied small electric field did not change the tilt
angle of the hydrogen-bonded complex HÀSiO in the SmCG-
type phases, a critical point that allowed the decoupling of the
leaning from the tile angle, during the manipulation of the former
via temperature.
monolayer stacking (of a d-spacing of 6.2 nm) of the bent-core
molecules (Figure 3d). Overall, the X-ray scattering data col-
lected during the cooling process detailed a series of phase
transitions of the SmCG-type phase, from modulated bilayers
~
to monolayers (SmCG2ÀUSmCG2ÀSmCG2ÀSmCG), as illu-
strated by the cartoons in Figure 3; these phase transitions were
thermally reversible. These results indicate that the segregation/
aggregation of the oligosiloxane terminal groups of the asym-
metric hydrogen-bonded complex HÀSiO associates closely
with the formation of the bilayer-based SmCG phase, as was
also suggested previously.14d
The corresponding GIWAXS profiles in Figure 4 were mea-
sured at various temperatures for the SmCG-type phases of the
hydrogen-bonded complex HÀSiO. The corresponding 2D-
scattering patterns (for instance, inset of Figure 4 at 130 °C),
in general, display an anisotropic diffuse halo ring at q ≈ 13.5 nmÀ1
(corresponding to a Bragg spacing ∼0.47 nm for the packing of
the bent-core molecules), with emphases on the two discernible
maxima located above the equatorial plane. The locations of the
maxima could associate closely with the tilt and leaning angles of
the bent-core molecules in the smectic layers of the SmCG-type
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Figure 5. Schematic representation of the orientation of the bent-core molecules in the SmCG2 phase.
Figure 6. (a,b) SAXS patterns of the electric field-aligned sample of the hydrogen-bonded complex HÀSiO at (a) 130 and (b) 95 °C. (c) SAXS profiles
from (a) and (b) as compared to GISAXS profiles recorded at the same temperatures.
As expected with the applied electric field for better aligned LC
domains of the SmCG-type phases, the WAXD patterns re-
corded at 130 °C (Figure 7a) and 95 °C (Figure 7b) display more
clearer four-lobe scattering among the diffuse halo at a q value of
13.5 nmÀ1 (d = 0.47 nm), as compared to the sample in Figure 4
only weakly aligned by a surface coating layer. The WAXS
pattern can reveal the leaning angle of the molecules in the
SmCG phase in terms of the azimuthal angle distribution of the
diffuse halo, if the molecules within the oriented ribbons or layers
are perfectly aligned along the electric field. Before turning to a
closer examination of the SmCG-type phase transitions, we wish
to clarify an additional point regarding how the bending angle of
bent-core molecules can be reflected in the azimuthal distribu-
tion of an outer diffuse halo. Takanishi et al.19 verified that the
angle between the nearest peaks of a four-lobe diffuse halo is
relative to the supplementary angle of the molecular bending
angle, as measured using microbeam X-ray scattering (beam size:
3 Â 4 μm2), when the incident beam is normal to the bending
plane of the bent-core molecules and the beam size is sufficiently
small to detect a single LC grain.
of the diffuse halo. Considering our experimental conditions and
the bilayer packing structure in the hydrogen-bonded complex
HÀSiO, we believe that the four-lobe diffuse halo is character-
ized by the projection of the bent-core molecules as a V-shape
arrangement along the electric field on the basis of the suppo-
sedly neglected molecular bend. The insets of Figure 7a and b
present possible bilayer packing structures satisfying the condi-
tion for a syn-clinic and anti-leaning SmCG2 phase. Hence, the
included angle between the layer normal direction and the
nearest one of an azimuthal maximum (Figure 7c) can be as-
signed as arising from the leaning angle of the bent-core mole-
cules in the SmCG-type phase, if the molecular bending plane
were perfectly parallel to the electric field and normal to the
incident beam. Although the layer structures were oriented well
by the electric field, it was difficult to avoid the rotation of layers
around the layer stacking direction in the partial LC domains, due
to the nonhomogeneous distribution of the electric field and/or
defect domains. With rotation around the columnar axis, the
~
modulated bilayer structure (SmCG2) lacks the layer rotation
effect. As a result, the azimuthal scans of the diffuse halo broadened
toward the meridian at the temperature below 110 °C for the
phases of undulated bilayer, bilayer, and monolayer structures
(Figure 7c). Hence, we need to consider the layer-rotation effect
for the temperature below 110 °C. Figure 7c displays the
azimuthal scans of the temperature-dependent four-lobe diffuse
Because we performed the SAXS and WAXS experiments of
the electric-field sample cell at a beam diameter of 0.5 mm and a
sample thickness of 1 mm, all X-ray data in this study should be
average values of the LC polygrains, with the low resolution in
the outer diffuse halo resulting in the broad azimuthal distribution
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Figure 7. (a,b) WAXS patterns of the electric field-aligned sample and schematic representations of the molecular orientations of the hydrogen-bonded
complex HÀSiO at (a) 130 and (b) 95 °C. (c) Azimuthal scans of the diffuse halos at various temperatures, fitted with two sets of Gaussian curves (green
and blue curves denoting the contribution from the tilt and leaning angles, respectively).
Table 2. Crystallographic Unit Cell Parameters and Tilt and
Leaning Angles of the Hydrogen-Bonded Complex HÀSiO at
Various Temperatures
bent-core LCs is usually produced through the formation of a
polarization splay.3,9 In the hydrogen-bonded complex HÀSiO,
the modulation can be attributed not so much to the polarization
splay as to the larger leaning angle, because the splayed angle of
the polar direction between two adjacent molecules was very
small (<5°) within the cross section of the bulky ribbons, which
comprised approximately 70 molecules. From the mostly un-
changed bilayer spacing (or the a-axis length) during the phase
transitions, we speculate that the changing of domain wall
thickness within the modulated boundaries or the interdigitation
of terminal alkyl chains between adjacent bilayers could com-
pensate the changes in bilayer spacing caused by the changes of
the leaning angle in the SmCG-type phase transitions (as
illustrated in the insets of Figure 7a and b).
In comparison with the SmC phase of the hydrogen-bonded
complex HÀAlk, the WAXS pattern recorded under the electric
field displayed relatively weaker four-lobe diffuse scattering
(Figure 8a). In Figure 8b, the intensity profile obtained from
the azimuthal scan of the diffuse halo could also be decomposed
into two sets of Gaussian humps, with the hump centered at 90°
revealing a zero leaning of HÀAlk, and the other set of humps
indicating a tilt angle of about 30°, which is in agreement with
that obtained by GIWAXS (Figure 1).
Structural Effects of Hydrogen Bonding and Siloxane
Groups on the SmCG Phases. Unlike the hydrogen-bonded
complex HÀAlk and covalent-bonded compound CÀSiO, the
hydrogen-bonded complex HÀSiO, featuring both hydrogen
bonds and branched siloxane units, was the only system exhibit-
ing the bilayer structure with complicated SmCG-type phase
transitions. This situation seems to imply that interplay between
the stability of the hydrogen bonds and the microsegregation
of the siloxane units has an important effect on the molecular
packing of the bilayer structure. It is necessary, at this point, for
us to clarify the relationship between the SmCG-type phase
~
SmCG2
USmCG2
SmCG2
SmCG
temp (°C)
a (nm)
130
120
18.6
110
100
95
85
19.1
12.9
96°
31°
60°
c (nm)
12.6
96°
31°
57°
12.3
12.4
12.4
6.2
β
ω (tilt angle)
j (leaning angle)
31°
56°
31°
54°
30°
52°
30°
50°
halos from 0 to 180°; the profiles are decomposed into two sets of
humps contributed, respectively, by the tilt and leaning angles,
following the same procedure as that done previously. The thus
extracted leaning angle decreased from approximately 60° to 50°
upon decreasing temperature from the modulated bilayer
~
(SmCG2) at 130 °C to monolayer (SmCG) at 85 °C. On the
other hand, the extracted constant tilt angle, ca. 31°, is the same
as that extracted without electric fields previously. Table 2 lists
the leaning angles thus obtained.
In Table 2, the decrease in the leaning angle from 60° to 50°
that accompanied the rich phase transitions reveals that the phase
transitionsduringthe coolingprocessfollowed the phase sequence
~
IsoÀSmCG2ÀUSmCG2ÀSmCG2ÀSmCG in the hydrogen-
bonded complex HÀSiO. A large enthalpy change of 14.4 kJ gÀ1
~
occurred during the IsoÀSmCG2 transition, corresponding to a
first-order phase transition; in contrast, the subphase transitions
~
SmCG2ÀUSmCG2ÀSmCG2ÀSmCG, which proceeded without
enthalpy formation, may be assigned to a second-order phase
transition (Figure S2). These findings suggest that the changes in
the leaning angle induced a break in chiral symmetry, leading
to the subtle phase transitions. A modulated layer structure of
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Figure 8. (a) WAXS pattern and (b) azimuthal scan of the diffuse halos (green curves denoting the contribution from the layer rotation effect; blue
curves indicating the molecular orientation) of the hydrogen-bonded complex HÀAlk at 110 °C under an electric field.
Figure 9. (a) FTIR spectra of the hydrogen-bonded complex HÀSiO at various temperatures. Two OÀH bands appear at 2500 and 1940 cmÀ1. The
CdO band, influenced by the formation of hydrogen bonds between the pyridyl and carboxylic acid groups, appears in the range 1780À1640 cmÀ1. (b)
Temperature-dependent Raman spectra of the characteristic peaks for SiÀOÀSi symmetric stretching bands.
transitions of the hydrogen-bonded complex HÀSiO in terms of
the stability of the hydrogen bonds and the microsegregation.
Figure 9a and b presents FTIR and Raman spectra, respectively,
recorded at various temperatures corresponding to the Iso (140 °C),
orientation of hydrogen-bonded complex HÀSiO. Dependence
of phase behavior on hydrogen bonds has been observed in other
supramolecular LC systems.20 On the basis of the FTIR and
Raman spectra, we suggest that the balance between the hydro-
gen bonding and segregation of the siloxane units resulted in the
changes of the leaning angles in the SmCG-type phase transitions
of HÀSiO. Correspondingly, measured FTIR and Raman spec-
tra for compounds CÀSiO and HÀAlk (Figures S5 and S6)
support the hydrogen bonding and influence on the fine struc-
tural changes of the SmCG phase in the hydrogen-bonded
complex HÀSiO. At high temperatures, hydrogen bonding was
weakened, and a stronger segregation of siloxane units resulted in
larger leaning angles. A larger leaning angle would cause the
molecular mass center to deviate substantially from the central
position of the layer, and hence to induce the modulation or
~
SmCG2 (130 °C), SmCG2 (90 °C), SmCG (80 °C), and crystalline
(RT) phases of the hydrogen-bonded complex HÀSiO. Thesignals
for the OÀH bonds appeared at 1940 and 2500 cmÀ1 and the
CdO band shifted from 1687 to 1693 cmÀ1, confirming the
formation of hydrogen bonds between the carboxylic acid units
of 3SiA and the pyridyl groups of VPy.20 When the system
approached the isotropic phase, weakening of the hydrogen
bonds occurred: the signals of the OÀH bonds weakened and
broadened, and the CdO band shifted to higher wavenumber.
On the other hand, the characteristic peak at 500 cmÀ1 for
SiÀOÀSi symmetric stretching in the Raman spectrum21 shifted
slightly to higher wavenumber at the higher temperature, pre-
sumably because of relatively enhanced microsegregation of the
siloxane units upon weakening of the hydrogen bonds.
~
undulation layers (SmCG2 and USmCG2). At lower tempera-
tures, the stronger hydrogen-bonding strength suppresses the
leaning and leads to the transition toward the monolayer SmCG
phase. The transformation from the bilayer SmCG2 to monolayer
Both the FTIR and the Raman spectral data suggest that the
existence of the hydrogen bonds strongly affected the molecular
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Figure 10. Switching current response curves of the hydrogen-bonded complex HÀSiO under applied triangular waves (Vpp = 300 V; f = 100 Hz; in
parallel rubbing cells: thickness, 7.5 μm) at (a) 130, (b) 113, (c) 103, (d) 97, (e) 95, and (f) 85 °C.
~
SmCG phase at low temperatures indicates a favorable monolayer
packing of the hydrogen-bonded complex HÀSiO.
Consequently, the SmCG2PF phase, as the highest-temperature
mesophase, of the hydrogen-bonded complex HÀSiO possessed
a general tilted bilayer modulated smectic phase with a longer
modulation periodicity in the oblique lattice (as determined
through X-ray measurements) and ferroelectricity (as deter-
mined though switching current measurements). Figure 11 pre-
sents POM photos images of the circular domains in all of the
mesophases (i.e., the SmCG phases) of the hydrogen-bonded
complex HÀSiO under dc electric fields of +50 and 0 V. The
circular domains of the mosaic-like texture in the hydrogen-
Electro-optical Properties. Combining the switching current
responses with the XRD data obtained at various temperatures
facilitates the characterization of the mesomorphic properties of
these bent-core LCs. Thus, to prove the polar switching proper-
ties of the three analogous compounds (HÀSiO, CÀSiO, and
HÀAlk), we employed the triangular wave technique22 to
measure the switching current behavior (including verifications
of ferroelectricity, antiferroelectricity, and spontaneous polariza-
tion by repolarization current peaks) in parallel rubbing cells
having a thickness of 7.5 μm. For the hydrogen-bonded complex
HÀSiO, three modes appeared for the polar switching currents
during the cooling process.23 In the highest-temperature LC
phase range (identified as the modulated bilayer structure in the
X-ray measurements), a single repolarization current peak was
observed at 132 °C during the half period of the applied triangle
voltage (Figure 10a); this clearly indicates a ferroelectric switch-
ing process. In addition, the modified triangular wave could also
verify the ferroelectric switching process further (see Figure S7a).
~
bonded complex HÀSiO at 130 °C, typical for the SmCG2PF
phase, did not alter upon changing of the applied voltage be-
cause of an unfavorable cone-like rotation in the modulations
(Figure 11a and b). When we decreased the temperature of the
hydrogen-bonded complex HÀSiO gradually to 113 °C, two
repolarization peaks steadily appeared (Figure 10b). Further-
~
more, the initial peak intensity of the SmCG2PF phase decreased
and the newly evolved peak increased its intensity upon decreas-
ing temperature (to 103 °C in Figure 10c and to 97 °C in
Figure 10d). Therefore, the switching current behavior, with two
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Figure 11. Circular domains of the hydrogen-bonded complex HÀSiO measured under various dc electric fields upon cooling (in parallel rubbing cells:
thickness, 7.5 μm): (a) +50 V at 130 °C; (b) 0 V at 130 °C; (c) +50 V at 105 °C; (d) 0 V at 105 °C; (e) +50 V at 85 °C; and (f) 0 V at 85 °C.
repolarization peaks present in the range 113À97 °C, confirmed
an antiferroelectric switching process. As a result, the USmCG2-
PA phase, as the second second-highest-temperature mesophase,
of the hydrogen-bonded complex HÀSiO possessed a general
tilted bilayer undulated smectic phase (as determined through
X-ray measurements) and antiferroelectricity (as determined
through switching current measurements). The nonvariations
of the circular domains were preserved under dc electric fields of
+50 and 0 V (see Figure 11c and d),24 most likely the result of a
modulated or an undulated smectic phase possessing a long
undulation or modulation periodicity. Further cooling to 95 and
85 °C by applying a triangular wave (see Figure 10e and f,
respectively) and a modified triangular wave (see Figure S7b)
resulted in only one sharp peak for the hydrogen-bonded com-
plex HÀSiO, indicating a ferroelectric switching process. Ac-
cordingly, the SmCG2PF and SmCGPF phases, as the lowest-
temperature mesophases, of the hydrogen-bonded complex
HÀSiO possessed respectively general tilted bilayer and mono-
layer smectic phases (as determined through the X-ray measure-
ments) as well as ferroelectricity (as determined through switch-
ing current measurements). The extinction crosses were all
parallel to the polarizer and analyzer (Figure 11e and f), indicat-
ing an overall anti-clinic organization of the molecules.25 In
general, the presence of the circular domains grown under dc
fields of +50 V indicates that none of the variations of the
extinction crosses were caused by the molecular rotations around
the long axis26 in the studied temperature range, most likely
because of the steric effects induced by the siloxane units (rather
than the aromatic core) of HÀSiO. Therefore, the branched
siloxane terminus of HÀSiO offered sufficient free space for
rapid reorientation of the molecules around their long axes,
thereby restricting the molecular rotation on a cone. Along with
the smectic layered structure confirmed by the X-ray measure-
ments, the antiferroelectric formation suggested by the two-peak
switchingcurrentresponse inFigure12aproves thatthe hydrogen-
bonded complex HÀAlk possessed the SmCPA phase.27 On the
other hand, the SmCPF structure (i.e., the B2 phase) of the
covalent-bonded compound CÀSiO was characterized by a one-
~
peak switching current response in Figures 12b and S7c, SmCG2,
and a smectic layered structure obtained from the X-ray mea-
surements. POM textures for HÀAlk at 110 °C (Vpp = 300 V; f =
100 Hz) and CÀSiO at 150 °C (Vpp = 100 V; f = 100 Hz) were
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Figure 12. Switching current response curves under applied triangular waves (in parallel rubbing cells: thickness, 7.5 μm) for the (a) hydrogen-bonded
complex HÀAlk at 110 °C (Vpp = 300 V; f = 100 Hz) and (b) covalent-bonded compound CÀSiO at 150 °C (Vpp = 100 V; f = 100 Hz); POM textures of
the corresponding switching current measurements for (c) HÀAlk at 110 °C (Vpp = 300 V; f = 100 Hz) and CÀSiO at 150 °C (Vpp = 100 V; f = 100 Hz).
also shown in Figure 12c and d. With these results, we have
demonstrated the interplay between the hydrogen bonding and
segregation of siloxane terminal groups in the SmCG phases
of the three mutually contrasting bent-core LCs of HÀSiO,
HÀAlk, and CÀSiO.
competition between the hydrogen-bonded linkages and the
microsegregation of the terminal siloxane units. Therefore, for
the hydrogen-bonded complex HÀSiO, the effects of the
hydrogen bonding and branched siloxane unit induced the
bent-core LC molecules to undergo self-assembly to form a
series of highly ordered 2D structures. Supramolecular design
with hydrogen bonds and terminal siloxane units is interesting
not only from theoretical viewpoints but also for the potential
applications of structurally designed LC templates.
’ CONCLUSIONS
In this study, we investigated the local molecular packing
within the mesophase structures of three analogous bent-core
molecules, the hydrogen-bonded complex and the covalent-
bonded compound with branched siloxane units (HÀSiO and
CÀSiO, respectively) and the hydrogen-bonded complex lack-
ing siloxane units (HÀAlk). Whereas the hydrogen-bonded
complex HÀAlk and the covalent-bonded compound CÀSiO
exhibited SmCPA and SmCPF phases, respectively, the hydrogen-
bonded complex HÀSiO exhibited various novel SmCG-type
phases. The leaning angles, relative to the molecular arrange-
ments, in the SmCG2-type phases resulted in wide-angle diffuse
halos evident though in situ X-ray scattering under dc electric
fields. This approach is a simple and direct method of probing
SmCG-type phases when the differences between tilt and leaning
angles are sufficiently large to be recognized by the azimuthal
angle distributions of outer diffuse halos. For the hydrogen-
bonded complex HÀSiO, modulated ribbons merged gradually
’ ASSOCIATED CONTENT
S
Supporting Information. Complete refs 3 and 16; and
b
detailed synthesized schemes, figures of FTIR, Raman, DSC, and
SAXS, WAXS setup under an electric field, and the modified
triangle method. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
weitsung@nsrrc.org.tw; linhc@mail.nctu.edu.tw
’ ACKNOWLEDGMENT
~
into a monolayer, resulting in the phase sequence SmCG2PFÀ
USmCG2PAÀSmCG2PFÀSmCGPF during the cooling process.
We also determined the SmCG-type phase transitions of the
hydrogen-bonded complex HÀSiO as a function of tempera-
ture in terms of its leaning angles, which were related to the
We thank Dr. C. J. Su for help with the SAXS/GISAXS
measurements and the National Science Council of Taiwan
(ROC) for financial support through NSC97-2113-M-009-
006-MY2 and NSC99-2218-E-213-001-MY3.
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