3-Hydroxycinnamic acid-a new central core for the design of bent-shaped liquid crystals

Article abstract of DOI:10.1039/c3tc30664j

The synthesis and mesomorphic properties of a new series of bent-shaped liquid crystals based on 3-hydroxycinnamic acid are presented. Prepared compounds differ in the orientation of an ester linking group in one of the side arms. Additionally, the mesomorphic properties have been tuned by employing either an alkoxy or carboxylic terminal group and by varying the length of terminal alkyl chains. Texture observation and calorimetric studies, as well as electro-optics, X-ray and dielectric measurements have been performed. Influence of the ester linking group orientation and the length of the terminal alkyl chain on mesomorphic behaviour has been established. Out of 12 studied compounds only one formed lamellar phases, namely the B₂-B₅ phase sequence, while all other exhibited columnar phases, either B₁ or B_1Rev type, depending on the direction of the ester group. Structural properties of the columnar phases have been studied in detail using X-ray diffraction techniques for pure materials as well as for binary mixtures of compounds forming the B₁ and B_1Rev phase. All results are discussed with respect to the molecular structure.

Full text of DOI:10.1039/c3tc30664j

Journal of
Materials Chemistry C
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PAPER
3-Hydroxycinnamic acid – a new central core for the
design of bent-shaped liquid crystals†
Cite this: J. Mater. Chem. C, 2013, 1,
4962
a
a
a
b
Michal Kohout, Jirı Tuma, Jirı Svoboda, Vladimıra Novotna, Ewa Goreckac
*
´
ˇ´ ˚
ˇ´
´
and Damian Pociechac
The synthesis and mesomorphic properties of a new series of bent-shaped liquid crystals based on
3-hydroxycinnamic acid are presented. Prepared compounds differ in the orientation of an ester linking
group in one of the side arms. Additionally, the mesomorphic properties have been tuned by employing
either an alkoxy or carboxylic terminal group and by varying the length of terminal alkyl chains. Texture
observation and calorimetric studies, as well as electro-optics, X-ray and dielectric measurements have been
performed. Influence of the ester linking group orientation and the length of the terminal alkyl chain on
mesomorphic behaviour has been established. Out of 12 studied compounds only one formed lamellar
phases, namely the B2–B5 phase sequence, while all other exhibited columnar phases, either B1 or B1Rev
type, depending on the direction of the ester group. Structural properties of the columnar phases have
been studied in detail using X-ray diffraction techniques for pure materials as well as for binary mixtures of
compounds forming the B1 and B1Rev phase. All results are discussed with respect to the molecular structure.
Received 10th April 2013
Accepted 11th June 2013
DOI: 10.1039/c3tc30664j
www.rsc.org/MaterialsC
bent-shaped mesogens.5,9,10 Most of the studied compounds
possessed the cinnamate connecting unit in the outer position
1 Introduction
of the lengthening arms.9 On the other hand, an example of a
material with the cinnamate unit joined to the central core5 and
some series of compounds with a reversed orientation of the
cinnamate unit in the outer position11 have been reported. Their
mesomorphic properties have also been modied by the lateral
substitution of a cinnamate double bond by methyl9,10 and
cyano5 groups. The role of ester linking groups has been
established for bent-shaped compounds with benzene12 and
naphthalene13 central units and it has been found that the ester
orientation strongly inuences the mesomorphic properties.
The most studied mesophase of bent-shaped materials is a
lamellar B2 phase, in which molecules are closely packed in
layers and tilted with respect to the layer normal. For the B2
phase the notation SmCP has been oen utilized with the index
S or A at the letter C, which describes the synclinic or anticlinic
orientation of the tilt in neighbouring layers, respectively. Letter
P stands for a polarization state and the corresponding
subscript index F or A expresses ferroelectricity or anti-
ferroelectricity of the lamellar system,3 respectively. The anti-
ferroelectric type of switching is preferably observed in the B2
phases. Additionally, another switchable smectic phase with
tilted molecules has been found and, according to the nomen-
clature, designated B5. The B5 phase exhibits a long range order
(2D lattice) within the layers, but no positional correlations
between layers.2,14 Until now only a few examples of the B5 phase
have been presented and properly characterized by X-ray
diffraction.
Bent-shaped mesogens have become an integral part of liquid
crystal research in the past few years. Since the discovery of
polar chiral phases and ferroelectric switching from achiral
bent-shaped mesogens1 detailed studies on the structure–
property relationships have been published and summarized in
various reviews.2,3 Recently, functionalized bent-shaped mate-
rials feasible for electro-optical applications have also been
developed.4,5 This required designing liquid crystalline
compounds, which possess additional functional groups (e.g. a
double bond) in their molecular structure. The double bond of
cinnamic acid has already been of interest in the eld of liquid
crystals6–8 as it enables modication of their mesomorphic
behaviour by illumination. This approach was utilized to double
bond isomerisation and thus to switching the mesomorphic
properties,7 and to photodimerisation and cross-linking the
molecules in the liquid crystalline state.8 The cinnamate
structural motif was also occasionally applied in the design of
aDepartment of Organic Chemistry, Institute of Chemical Technology, CZ-166 28
Prague 6, Czech Republic. E-mail: Jiri.Svoboda@vscht.cz; Fax: +420 220444182; Tel:
+420 220444288
bInstitute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, CZ-182
21 Prague 9, Czech Republic. E-mail: novotna@fzu.cz; Fax: +420 286890527; Tel: +420
266053111
cLaboratory of Dielectrics and Magnetics, Chemistry Department, Warsaw University,
Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland. E-mail: pociu@chem.uw.edu.pl; Fax:
+48 228221075; Tel: +48 228221075
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc30664j
4962 | J. Mater. Chem. C, 2013, 1, 4962–4969
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Several columnar mesophases were found for the bent-
Journal of Materials Chemistry C
The sequence of phases and phase transition temperatures
shaped mesogens that are built from the layer fragments were determined from optical textures and their changes were
(molecular blocks) arranged in a two dimensional (2D) lattice. observed under a polarizing optical microscope (Nikon Eclipse
First, the columnar B1 phase has been described with a density E600Pol). The Linkam LTS E350 heating/cooling stage with a
modulation in the plane parallel to the polarization vector, P. In TMS 93 temperature programmer was used for the temperature
the B1 phase no electro-optic response and switching current control, which enabled temperature stabilization within ꢀ0.1 K.
were observed. When the density modulation plane is perpen- The planar samples for the texture observation and electro-
dicular to the P vector, the nomenclature of the B1Rev phase was optical studies were made from glasses with ITO transparent
proposed.15 These phases can switch under the electric eld and electrodes (5 ꢁ 5 mm2), separated by mylar sheets dening the
exist in different variants: with tilted or non-tilted molecules cell thickness. They were lled in the isotropic phase by capil-
with respect to the layer fragment normal.
lary action. Free-standing lms were prepared by spreading a
In this paper we report on synthesis and mesomorphic compound in a liquid crystalline phase across a hole in the
properties of new bent-shaped materials based on a 3-hydroxy- metallic plate.
cinnamic acid central core. Such a molecular core is utilized in
For all compounds differential scanning calorimetry (DSC)
the design of the bent-shaped mesogens for the rst time. With was carried out by employing a Perkin-Elmer Pyris Diamond
respect to analogous compounds based on the naphthalene calorimeter. The samples of about 2–5 mg, hermetically sealed
core,13 in this new non-symmetrical central unit one of the fused in aluminium pans, were placed in a nitrogen atmosphere in
benzene rings is reduced to a single double bond. As a conse- the calorimeter working place. The calorimeter data were cali-
quence of a lower rigidity and a change of the packing mode one brated on extrapolated onset temperatures and enthalpy
can expect lower transition temperatures and different meso- changes of water, indium and zinc. The measurements were
morphic properties in comparison with naphthalene-core bent- performed on cooling/heating runs at a rate of 5 K minꢂ1
shaped compounds. Three series of new materials were syn- The switching properties were studied with driving voltage
.
thesised differing in the number and orientation of ester groups from a Phillips generator PM 5191, which is accompanied by a
in the lengthening arms. The properties of new compounds linear amplier, providing the maximum amplitude of 120 V.
were investigated by differential scanning calorimetry (DSC), The switching current proles vs. time were recorded using a
polarizing optical microscopy and selected samples were Tektronix memory oscilloscope DPO4034. Dielectric properties
studied by X-ray diffraction and dielectric measurements. were studied using a Schlumberger 1260 impedance analyser.
Structural parameters of different columnar phases were ana- The frequency dispersions were measured in the range of 10 Hz
lysed and compared. Binary mixtures of homologues with to 10 MHz at a stabilized temperature during the cooling
reversed orientation of the ester group revealing different types process at a rate of about 0.2 K minꢂ1. The dispersion data
of the columnar phase were prepared and their parameters were were analyzed using the Cole–Cole formula (1) in a generalized
studied.
form. For the frequency dependent complex permittivity 3*( f ) ¼
30 ꢂ i300 we have utilized:
ꢀ
ꢁ
D3
1 þ ðif =frÞð1ꢂaÞ
s
3* ꢂ 3N
¼
ꢂ i
þ Af m
;
(1)
2p30 f n
2 Experimental
where fr is the relaxation frequency, D3 is the dielectric
strength, a is the distribution parameter of the relaxation, 30 is
the permittivity of vacuum, 3N is the high frequency permit-
tivity and n, m, A are the parameters of tting. The second and
the third terms of the equation (in brackets) are used to
eliminate a low frequency contribution from d.c. conductivity
s and a high frequency contribution due to the resistance of
the electrodes, respectively. The real, 30, and imaginary, 300,
parts of permittivity were simultaneously tted to (1) and the
temperature dependences or fr and D3 were plotted and
analyzed.
The X-ray diffraction (XRD) studies were performed using a
Bruker Nanostar system (CuKa radiation, cross-coupled Goebel
mirrors, three pin-hole collimation, Vantec 2000 area detector,
MRI TCPU H heating stage) and a Bruker GADDS system (CuKa
radiation, Goebel mirror, point collimator and Vantec 2000 area
detector, homemade heating stage). In both systems the
temperature stability was 0.1 K. Powder samples (for Nanostar)
were prepared in thin-walled glass capillaries (1.5 mm diam-
eter), and partially oriented samples for experiments in reec-
tion were prepared as droplets on a heated surface.
The synthetic pathway for the target compounds I–III is shown
in Scheme 1. First, the hydroxylic group of the starting
3-hydroxycinnamic acid was protected with the tert-butyl-
(dimethyl)silyl group (TBDMS) to yield the protected acid 1. In
the next step, the rst lengthening arm was introduced in a
N,N0-dicyclohexylcarbodiimide (DCC) mediated esterication of
acid 1 with the corresponding hydroxy esters 2A–L in the pres-
ence of N,N-dimethylaminopyridine (DMAP) as a catalyst.
Subsequently, the protecting silyl group in the formed inter-
mediate 3 was removed by means of tetrabutylammonium
uoride (TBAF) in wet tetrahydrofuran (THF) and the released
hydroxylic group in esters 4 was nally acylated by acid chlo-
rides 5A–D of the second lengthening arm, giving rise to the
target compounds I–III (Scheme 1).
Syntheses of the lengthening arms of the target materials,
4-alkoxyphenyl 4-hydroxybenzoates (2A–D), 4-hydroxyphenyl
4-alkoxybenzoates (2E–H), alkyl 4-[(4-hydroxybenzoyl)oxy]-
benzoates (2I–L) and 4-[(4-alkoxybenzoyl)oxy]benzoic acid
chlorides (5A–D) (A, E, I with R ¼ octyl, B, F, J with R ¼ decyl, C,
G, K with R ¼ dodecyl, and D, H, L with R ¼ tetradecyl), were
described previously.13a,16
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Scheme 1 The synthetic route and designation for compounds. Final compounds from series I, II and III were denoted with additional letters, a for R ¼ C8H17, b for
R ¼ C10H21, c for R ¼ C12H25 and d for R ¼ C14H29
.
additional ester group has been attached in the identical
orientation, so there are ve ester functionalities in total (see
Scheme 1). DSC studies were performed for all compounds; the
phase transition temperatures and associated enthalpy changes
are summarized in Table 1. The DSC thermograms are shown in
Fig. 1a–c for materials Ib, IIb and IId, respectively. For all
studied materials, at least one enantiotropic mesophase has
been observed. Phases and phase sequences have been identi-
ed by texture observation under the polarizing optical
3 Results
With respect to the number and orientation of the ester linking
groups in the lengthening arms the materials are divided into
three series denoted I, II and III. The le arm of compounds is
identical and possesses two uniformly oriented ester units.
Series I and II possess two ester groups in the right arm and
differ in their orientation. Series III resembles series I con-
cerning the ester orientation in the right arm, to which an
4964 | J. Mater. Chem. C, 2013, 1, 4962–4969
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Table 1 Phase transition temperatures, Ttr, in ꢃC and the corresponding enthalpies, DH, were detected on the second cooling, melting points, M.p., on the second
heating. All changes were performed at a rate of 5 K minꢂ1. The enthalpies are presented in brackets in kJ molꢂ1. The position of the alkyl chain R is shown in Scheme 1
R
M.p. (DH)
Tcr (DH)
M2
Ttr (DH)
M1
Ttr (DH)
Iso
Ia
Ib
Ic
Id
IIa
IIb
IIc
IId
IIIa
IIIb
IIIc
IIId
C8H17
122 (+32.8)
121 (+29.0)
134 (+47.3)
117 (+38.0)
87 (+32.9)
101 (+30.7)
92 (+18.5)
93 (+23.7)
108 (+42.4)
102 (+27.5)
106 (+33.2)
104 (+30.4)
105 (ꢂ27.5)
94 (ꢂ24.1)
97 (ꢂ26.3)
100 (ꢂ31.0)
67 (ꢂ20.0)
76 (ꢂ23.5)
83 (ꢂ18.8)
76 (ꢂ25.9)
102 (ꢂ39.0)
98 (ꢂ27.0)
102 (ꢂ32.5)
93 (ꢂ29.8)
B1Rev
B1Rev
B1Rev
B1Rev
B1
B1
B1
B2
B1Rev
B1Rev
B1Rev
B1Rev
159 (ꢂ21.3)
159 (ꢂ24.1)
158 (ꢂ22.3)
157 (ꢂ22.7)
147 (ꢂ21.8)
139 (ꢂ23.8)
135 (ꢂ21.0)
136 (ꢂ13.5)
153 (ꢂ15.1)
152 (ꢂ7.2)
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
C
C
10H21
12H25
C14H29
C8H17
C
C
10H21
12H25
C14H29
C8H17
C10H21
C12H25
C14H29
B5
100 (ꢂ0.6)
156 (ꢂ13.2)
152 (ꢂ10.1)
Enantiotropic nematic phases were identied for hydroxy
derivatives 4E–J and the SmA phase for compounds 4K–L (see
Table S1 in the ESI†).
For all compounds of the series I the columnar phase of B1Rev
type was identied regardless of the length of the terminal alkyl
chain. Typical texture of the B1Rev phase is shown in Fig. 2a for
Ia. Under an applied electric eld only the birefringence irre-
versibly changed, as shown in Fig. 2b. For the materials of series
II with shorter terminal alkyl chains (C8H17–C12H25) the
columnar B1 phase has been identied. Typical mosaic textures
of this phase are shown in Fig. 2c for compounds IIa. Materials
of series III showed uniform mesomorphic behaviour and for all
lengths of the terminal alkyl chain the columnar B1Rev phase
was found. Physical properties of the B1Rev phase, observed in
series III, are very similar to those of compounds I.
Fig. 1 Thermographs recorded for compounds (a) Ib, (b) IIb and (c) IId during
the second heating and cooling runs (the upper and lower curves in each figure,
resp.). Measurements were recorded at the rate of 5 K minꢂ1. The slopes are
adjusted for convenience; mesophases are indicated. In the inset of (c) the B2–B5
phase transition (marked by an arrow in the cooling curve) is presented in an
enlarged view.
microscope and conrmed by X-ray measurements (see the
following paragraph). Switching properties and dielectric
measurements have been studied as well.
We have also studied the intermediate silyl derivatives 3 and
hydroxy derivatives 4 and found that 3E–H and 4E–L exhibited
mesomorphic properties. Mesophases identication, phase
transition temperatures and corresponding enthalpies are
summarized in Table S1 in the ESI.† For silylated compounds
3E–H a monotropic nematic phase was found. The homologue
with the longest terminal chain 3H exhibited polymorphism:
Fig. 2 Planar texture of compound Ia in the B1Rev phase at 140 ꢃC (a) before
application of the electric field and (b) after switching off the field at about
50 V mmꢂ1. The width of each figure is 150 mm. (c) Mosaic texture of the B1 phase
the SmC phase was observed below the nematic phase. for IIa. The width of the figure corresponds to 300 mm.
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Two switchable phases have been observed for a homologue
IId with the longest (C14H29) chain. For the higher temperature
phase, which can be identied with a B2 phase, the antiferro-
electric character has been proved by the switching current
prole (Fig. 3). From the planar textures and their behaviour
(Fig. 4) under the applied electric eld we can establish the
character of the B2 phase. Without electric eld the overall
extinction position is parallel to the smectic layer normal, which
evidences anticlinicity (Fig. 4a). Under the weak electric eld the
birefringence slightly increases and at a threshold value of
about 5 V mmꢂ1 it abruptly grows up and the extinction brushes
become inclined about 45 degrees from the layer normal within
the whole sample (see Fig. 4b). The birefringence of the
switched texture increases further with electric eld up to the
saturation at 10 V mmꢂ1 (Fig. 4c). We consider that the ground
state corresponds to the SmCAPA (antiferroelectric) phase and
transforms to the SmCSPF (ferroelectric) phase under the
applied electric eld.
Another phase has been found below the B2 phase on cooling
for IId. Based on X-ray analysis, we attributed this phase to a B5
phase with respect to the nomenclature. The structural character
of the observed B5 phase will be described later. The B2–B5 phase
transition is accompanied by a small enthalpy change. Addi-
tionally, only a slight modication of textures has been observed
in the planar (book-shelf) geometry as well asin the free-standing
lm. The antiferroelectric switching has been detected in the B5
phase under an applied electric eld, and the observed electro-
optical response has been found comparable to that of the B2
phase. We have measured the dielectric properties of compound
IId in dependence on frequency and temperature within the
temperature interval of the studied mesophases. One distinct
mode has been observed in both mesophases and attributed to a
collective dipolar character, as it completely disappears in the
isotropic, as well as in the crystalline phases. The imaginary part
of the permittivity, 300, is presented versus frequency and
temperature in Fig. 5. One can see a jump-like change of the
mode strength at the B2–B5 phase transition. The temperature
dependence of the tting parameters for the dielectric spec-
Fig. 4 Planar texture of compound IId in the SmCAPA phase (a) without electric
field (after its application), under the electric field of about (b) 5 V mmꢂ1 and (c)
10 V mmꢂ1. Crossed polarizers are parallel to the microphotograph edge. The
width of the figure is about 150 mm.
troscopy data is presented in Fig. S2.† The relaxation frequency, 40 kHz just before crystallization. A small jump-down change is
fr, decreases from 140 kHz, at the Iso–B2 phase transition, to detected on fr(T) at the B2–B5 phase transition. The dielectric
strength is practically constant within the mesophase tempera-
ture range and can be evaluated as D3 ¼ 4.5 in the B2 phase and
D3 ¼ 6 in the B5 phase. Temperature dependence of fr in a log-
arithmic scale versus reciprocal temperature in Kelvins, 1/T, is
shown in Fig. 6. The linear dependence can be demonstrated in
both mesophases with a clear change of the slope at the B2–B5
phase transition. It is evident that both dependences full
the Arrhenius law. We have calculated the activation energy,
DEA, and found DEA ¼ 56 kJ molꢂ1 in the B2 phase and DEA ¼
84 kJ molꢂ1 in the B5 phase.
Structures of mesophases were determined by X-ray diffrac-
tion studies. The X-ray pattern recorded for compound IId at
120 ꢃC exhibits a series of sharp, commensurate small angle
reections and a diffuse scattering maximum in the wide angle
region (Fig. 7). Such a pattern is characteristic for a lamellar
structure with the liquid-like molecular packing within the
layers, so it conrms the identication of the smectic B2 phase.
Fig. 3 Switching current profile for IId at a frequency of 10 Hz in the SmCAPA
phase at T ¼ 110 ꢃC.
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Fig. 5 3-Dimensional plot of the imaginary part of permittivity, 300, versus
temperature and frequency for IId.
Fig. 7 2D X-ray patterns for the partially oriented sample of compound IId in (a)
the B2 phase at T ¼ 120 ꢃC and (b) the B5 phase at T ¼ 96 ꢃC. (c) The intensity
profile versus the scattering angle at T ¼ 120 ꢃC and T ¼ 96 ꢃC and (d) the wide
angle range for T ¼ 96 ꢃC shown in an enlarged view (arrows mark sharp signals
appearing in the B5 phase).
ꢃ
below the B2–B5 phase transition at T ¼ 100 C two additional
sharp peaks appeared in the wide angle region, overlapped by
the broad diffuse maxima (Fig. 7b and c). In the B5 phase, the
diffuse scattering remains and evidences still a high amount of
disorder. Two additional maxima visible in the wide-angle
range indicate a long-range in-plane positional correlation
within the smectic layers. We observed two kinds of scattering
in the B5 phase: ordered in a two-dimensional lattice (the
molecules from different layers are not correlated) and disor-
dered centers, which give a broad diffuse halo. Such behavior is
Fig. 6 The relaxation frequency, fr, in a reciprocal temperature scale for IId.
The layer spacing, d, calculated from the position of the X-ray characteristic for the B5 phase.14
peaks, slightly increases on cooling (Fig. S3†). This effect can be
Except for IId, all other compounds exhibited columnar
caused by a gradual stretching of the terminal chains and/or phases with 2D periodic density modulations. Two types of the
increase of the orientational order of the molecules. By columnar phases were observed and distinguished by their XRD
comparing the measured layer thickness with the estimated patterns. Low angle diffractograms for compounds IIa–c
˚
molecular length l z 60 to 61 A, we can evaluate the tilt angle exhibited only two incommensurate sharp reections (Fig. 8a)
value, approximately 44–45 degrees, which is in agreement with that can be indexed as (11) and (02) assuming centred rectan-
the value determined from the rotation of extinction brushes in gular unit cells. Such a pattern is typically attributed to the B1
the fan-shaped texture of the sample exposed to electric eld phase. XRD patterns registered for all compounds of series I
(see Fig. 4). At the B2–B5 phase transition all parameters of the and III were much richer (Fig. 8a), the signals could be indexed
X-ray pattern slightly change and, namely, the full-width at a with a centred oblique unit cell; such diffractograms are typi-
half maximum, FWHM, exhibits a clear anomaly (Fig. S3†). The cally ascribed to B1Rev type phases. In all cases the crystallo-
slope of the linear d(T) dependence, which corresponds to the graphic lattice parameters were weakly temperature dependent,
thermal expansion coefficient, a, changes at the B2–B5 phase the parameter related to layer fragment thickness grows on
ꢂ1
ꢂ1
˚
˚
transition; a ¼ ꢂ0.02 A K and ꢂ0.04 A K in the B2 and B5 cooling, while the behaviour of the other unit cell dimension
phases, respectively. On the X-ray diffraction (XRD) pattern related to the molecular block size depends on the phase type,
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Fig. 9 Optical textures of the B1 and B1Rev phases taken for contact cell prepared
with two mixtures of Ia and IIa compounds close to the critical concentration, a
clear phase boundary is seen.
B1Rev phase on approaching the critical concentration and the
absence of this signal in the B1 phase formed for mixtures above
the critical concentration suggests that the phases differ in the
defect separating neighbouring blocks. In the B1rev phase the
regions between blocks are lled with the molecules that
strongly splay their polarization vectors15d and in the B1 phase
the blocks are attached to each other. Apparently, in the binary
mixtures the width of the ‘defect’ region between the blocks
decreases with the growing concentration of the IIa component.
4 Discussion and conclusions
Fig. 8 (a) X-ray intensity versus the scattering angle for compounds Ia and IIa
and their mixtures. (b) Evolution of the crystallographic unit cell parameters a and
b with changing the composition of the mixtures of Ia and IIa compounds. Lines
are guides for the eye.
Herein we report for the rst time bent-shaped materials based
on a 3-hydroxycinnamic acid central core. We have synthesized
three series (denoted I, II and III) and modied the ester-group
orientation and the length of the terminal alkyl chain. Regard-
less of the length of the terminal chain, the transition temper-
atures of compounds I are nearly identical. All compounds I
in the B1 phase (compounds of series II), the size of the column exhibit the formation of the polar columnar B1Rev phase. The
decreases on cooling, while in the B1Rev phase (compounds of same behaviour was observed for compounds III with ve
series I and III) the column size increases on cooling (Fig. S4†). uniformly oriented ester groups. The presence of an additional
In order to check the phase identication, the miscibility ester group in the terminal chain caused only a slight decrease
experiments were conducted. Analogous compounds from in the clearing temperatures, by 5–7 ꢃC. The change of the
series I and II, having the same length of the terminal chains, orientation of the outer ester group in the lengthening arm in
have been chosen, differing only in the orientation of one ester compounds II led to a decrease of both the melting and the
group in the mesogenic core. A very similar chemical structure clearing temperature. At the same time, the effect of the
of compounds Ia and IIa gives the opportunity to study mixtures terminal chain length on mesomorphic properties is well
with a very small two-phase region. The parameters of the documented for substances II. The temperature interval of the
crystallographic unit cell of the B1Rev phase change continu- columnar mesophases dropped from the width of 80 ꢃC for IIa
ously when component IIa is added to the Ia matrix (Fig. 8b), the to 52 ꢃC for IIc. Finally, for compound IId with the longest
cell inclination angle approaches 90 degree, the cross-section of terminal chain two lamellar phases (B2 and B5) appeared
the molecular blocks decreases and the intensity of signals (20) instead of the B1 phase. The formation of lamellar phases for
and (31) decreases. At the critical concentration, ꢅ30% of compounds with longer alkyl terminal chains has already been
compound IIa, the crystallographic unit cell for both phases B1 reported earlier for various bent-shaped mesogens.13,16 The B5
and B1Rev becomes identical, or nearly identical. However, the phase occurrence is rather unique,11,14 because in smectic
contact cell, prepared with two mixtures close to the critical phases of bent-shaped mesogens a short range positional order
concentration, still shows the phase boundary, the textures are in layers is usually observed.
different (Fig. 9). Based on this experiment, the question arises:
When compared to the previously studied naphthalene
what is the difference between these two phases? Both phases based mesogens,11,13 we can claim that the substitution of one of
are made of smectic layer fragments being vertically shied one the benzene rings for a trans-double bond in studied materials
against another. The lowering intensity of the (20) signal in the I–III generally results in a decrease of the p–p interactions of
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the aromatic rings. It leads to a lower rigidity of the central unit,
thus to a decrease of the clearing temperatures of compounds
I–III. The mesomorphic behaviour of compounds I–III is similar
to that of the naphthalene based mesogens, where predomi-
nantly various columnar phases were detected, and only for the
homologues with long terminal chains a lamellar B2 phase
appeared. It is also worth noting that some intermediates, silyl
derivatives 3E–3H and hydroxy derivatives 4E–4L, exhibited the
mesomorphic behaviour, and calamitic mesophases (nematic,
SmA and SmC phases) were observed.
8 (a) P.-C. Yang and J.-H. Liu, J. Polym. Sci., Part A: Polym.
Chem., 2008, 46, 1289–1304; (b) A. Bobrovsky, V. Shibaev,
´
ˇ
´
´
V. Hamplova, M. Kaspar, V. Novotna, M. Glogarova and
E. Pozhidaev, Liq. Cryst., 2009, 36, 989–997; (c) C. Keith,
A. Lehmann, U. Baumeister, M. Prehm and C. Tschierske,
So Matter, 2010, 6, 1704–1721.
9 (a) R. Amarantha Reddy, B. K. Sadashiva and S. Dhara, Chem.
´
Commun., 2001, 1972–1973; (b) E. Matyus and K. Fodor-
Csorba, Liq. Cryst., 2003, 30, 445–450; (c) H. N. Shreenivasa
Murthy and B. K. Sadashiva, Liq. Cryst., 2004, 31, 1347–
1356; (d) R. Amarantha Reddy, B. K. Sadashiva and
V. A. Raghunathan, Chem. Mater., 2004, 16, 4050–4062.
To summarize, the studied cinnamic acid-based bent-
shaped materials with the uniform orientation of ester groups
(series I and III) prefer the formation of the columnar phases of 10 (a) R. Amarantha Reddy, B. K. Sadashiva and U. Baumeister,
the B1Rev type. For compounds IIa–IIc the B1 phase has been
detected. Both columnar mesophases, the B1Rev and the B1
phase, have been characterized by X-ray measurements. It has
been demonstrated that the structural parameters of B1Rev and
J. Mater. Chem., 2005, 15, 3303–3316; (b) W. Weissog,
U. Dunemann, S. Findeisen-Tandel, M. G. Tamba,
H. Kresse, G. Pelzl, S. Diele, U. Baumeister, A. Erermin,
S. Stern and R. Stannarius, So Matter, 2009, 5, 1840–1847.
¨
B1 phases can be continuously tuned when preparing binary 11 G. Pelzl, M. G. Tamba, S. Findeisen-Tandel, M. W. Schroder,
mixtures of homologues I and II. For material IId with the tet-
radecyl terminal alkyl chain the presence of two polar lamellar
switchable phases (B2 and B5) has been proved.
U. Baumeister, S. Diele and W. Weissog, J. Mater. Chem.,
2008, 18, 3017–3031.
ˇ
¨
12 (a) W. Weissog, G. Nauman, B. Kosata, M. W. Schroder,
A. Eremin, S. Diele, Z. Vakhovskaya, H. Kresse,
R. Friedmann, S. A. R. Krishnan and G. Pelzl, J. Mater.
Chem., 2005, 15, 4328–4337; (b) S. Ananda Rama Krishnan,
W. Weissog, G. Pelzl, S. Diele, H. Kresse, Z. Vakhovskaya
and R. Friedemann, Phys. Chem. Chem. Phys., 2006, 8,
1170–1177.
Acknowledgements
This work was supported by the Czech Science Foundation
(projects P204/11/0723 and CSF 13-14133S).
´
13 (a) M. Kohout, J. Svoboda, V. Novotna, D. Pociecha,
References
´
M. Glogarova and E. Gorecka, J. Mater. Chem., 2009, 19,
´
3153–3160; (b) M. Kohout, J. Svoboda, V. Novotna,
1 (a) T. Niori, T. Sekine, J. Watanabe, T. Furukawa and
H. Takezoe, J. Mater. Chem., 1996, 6, 1231–1233; (b)
D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
´
M. Glogarova and D. Pociecha, Liq. Cryst., 2010, 37, 987–
996; (c) M. Kohout, J. Svoboda, V. Novotna and
´
¨
D. Pociecha, Liq. Cryst., 2011, 38, 1099–1110.
E. Korblova and D. M. Walba, Science, 1997, 278, 1924–1927.
14 (a) R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem.,
2 G. Pelzl, S. Diele and W. Weissog, Adv. Mater., 1999, 11,
707–724.
´
2006, 16, 907–961; (b) H. Nadasi, W. Weissog, A. Eremin,
´
G. Pelzl, S. Diele, B. Das and S. Grande, J. Mater. Chem.,
2002, 12, 1316–1324.
15 (a) J. Szydlowska, J. Mieczkowski, J. Matraszek, D. W. Bruce,
E. Gorecka, D. Pociecha and D. Guillon, Phys. Rev. E: Stat.,
Nonlinear, So Matter Phys., 2003, 67, 031702; (b)
3 (a) W. Weissog, H. Nadasi, U. Dunemann, G. Pelzl, S. Diele,
A. Eremin and H. Kresse, J. Mater. Chem., 2001, 11, 2748–
2758; (b) R. Amarantha Reddy and C. Tschierske, J. Mater.
Chem., 2006, 16, 907–961; (c) H. Takezoe and Y. Takanishi,
Jpn. J. Appl. Phys., 2006, 45, 597–625; (d) W. Weissog,
H. N. Shreenivasa Murthy, S. Diele and G. Pelzl, Philos.
Trans. R. Soc., A, 2006, 364, 2657–2679; (e) U. Dunemann,
M. W. Schroeder, R. Amarantha Reddy, G. Pelzl, S. Diele
and W. Weissog, J. Mater. Chem., 2005, 15, 4051–4061.
4 J. Etxebarria and M. Blanca Ros, J. Mater. Chem., 2008, 18,
2919–2926.
´
´
E. Gorecka, N. Vaupotic, D. Pociecha, M. Cepic and
J. Mieczkowski, ChemPhysChem., 2005, 6, 1087–1093; (c)
E. Gorecka, N. Vaupotic and D. Pociecha, Chem. Mater.,
2007, 19, 3027–3031; (d) N. Vaupotic, M. Copic, E. Gorecka
and D. Pociecha, Phys. Rev. Lett., 2007, 98, 247802.
´
´
16 (a) J. Svoboda, V. Novotna, V. Kozmik, M. Glogarova,
W. Weissog, S. Diele and G. Pelzl, J. Mater. Chem., 2003,
5 N. Boiko, X. Zhu, A. Bobrovsky and V. Shibaev, Chem. Mater.,
2001, 13, 1447–1452.
´
ˇ
13, 2104–2110; (b) V. Kozmık, M. Kuchar, J. Svoboda,
´
´
V. Novotna, M. Glogarova, U. Baumeister, S. Diele and
6 H. Kihara and N. Tamaoki, Macromol. Rapid Commun., 2006,
27, 829–834.
´
G. Pelzl, Liq. Cryst., 2005, 32, 1151–1160; (c) V. Kozmık,
´ˇ
´
´
ˇ
´
A. Kovarova, M. Kuchar, J. Svoboda, V. Novotna,
7 (a) I. C. Pintre, J. L. Serrano, M. Blanca Ros, J. Martinez-
Perdiguero, I. Alonso, J. Ortega, C. L. Folcia, J. Etxebarria,
R. Alicante and B. Villacampa, J. Mater. Chem., 2010, 20,
2965–2971; (b) H. Kihara and N. Tamaoki, Liq. Cryst., 2007,
34, 1337–1347.
M. Glogarova and J. Kroupa, Liq. Cryst., 2006, 33, 41–56; (d)
´
´ˇ
V. Kozmık, P. Polasek, A. Seidler, M. Kohout, J. Svoboda,
´
´
V. Novotna, M. Glogarova and D. Pociecha, J. Mater. Chem.,
2010, 20, 7430–7435.
This journal is ª The Royal Society of Chemistry 2013
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