Bent-core mesogens with thiophene units

Article abstract of DOI:10.1039/c0jm01919d

Bent-core mesogens with thiophene and 2,2′-bithiophene units at the periphery of the aromatic core were synthesized and investigated by polarizing microscopy, XRD and electrooptical methods. Resorcinol, 2,7-naphthalenediol and 3,4′-biphenyldiol were used as bent units, phenyl thiophene-2-carboxylates form the rod-like wings and n-alkyl chains as well as olefin- and silylated-terminated alkyl chains were attached as flexible end-groups. A broad variety of different mesophase structures, incorporating ferroelectric and antiferroelectric switching smectic as well as modulated smectic phases, was obtained. Compared with the corresponding benzoates, these thiophene derived compounds form nearly identical LC phase structures, but have significantly reduced transition temperatures, which make them advantageous for applications.

Full text of DOI:10.1039/c0jm01919d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Bent-core mesogens with thiophene units†
*
Karina Geese, Marko Prehm and Carsten Tschierske
Received 16th June 2010, Accepted 17th August 2010
DOI: 10.1039/c0jm01919d
Bent-core mesogens with thiophene and 2,20-bithiophene units at the periphery of the aromatic core
were synthesized and investigated by polarizing microscopy, XRD and electrooptical methods.
Resorcinol, 2,7-naphthalenediol and 3,40-biphenyldiol were used as bent units, phenyl thiophene-2-
carboxylates form the rod-like wings and n-alkyl chains as well as olefin- and silylated-terminated alkyl
chains were attached as flexible end-groups. A broad variety of different mesophase structures,
incorporating ferroelectric and antiferroelectric switching smectic as well as modulated smectic phases,
was obtained. Compared with the corresponding benzoates, these thiophene derived compounds form
nearly identical LC phase structures, but have significantly reduced transition temperatures, which
make them advantageous for applications.
these compounds a broad variety of different mesophase struc-
1. Introduction
tures, including different polar smectic as well as modulated
smectic phases, were obtained. The mesophases and transition
temperatures of these thiophene-2-carboxylates were compared
with corresponding benzoates (B), if possible.
Bent-core mesogens have attracted considerable attention in the
recent years as they can form ferroelectric and antiferroelectric
smectic and columnar liquid crystalline (LC) phases and show
spontaneous symmetry breaking.1,2 Several applications of these
materials have been proposed, for example as materials for new
generations of fast switching displays,3 NLO applications4 and as
pyroelectric materials.5 However, a major disadvantage of bent-core
mesogens is provided by their high transition temperatures which
often inhibit practical applications. Herein we report new bent-core
mesogens incorporating thiophene units. Previous attempts to use
thiophenes as building blocks of bent-core mesogens were mainly
focussed on their use as bent unit, but the bending angle provided by
this unit (158ꢀ) is too large to yield the desired polar order as typical
for bent-core mesogens.6,7 In this article the 2,5-substituted thio-
phene units and the 5,50-substituted 2,20-dithiophene units were
introduced as structural units at the periphery of the bent core. One
target was to reduce the transition temperatures. In addition,
p-conjugated thiophene-containing bent-core molecules could be of
interest on their own, for example, as potential organic electronic
materials.8,9 Especially, the more dense packing compared to usual
rod-like liquid crystals could be an advantage for these applications.
Moreover, S-atoms are useful probes for resonant X-ray scattering10
and hence, investigation of thiophene based bent-core mesogens
could provide additional insights into the often complex structures
of their liquid crystalline phases.
2. Results and discussion
2.1 Synthesis
The synthesis of the thiophene containing bent-core mesogens T is
described in Scheme 1. The 5-alkylthiophene-2-carboxylic acids
were synthesized in two steps. The 2-alkylthiophenes were
obtained in the first step by a metallation–alkylation procedure
from thiophene and appropriate alkyl bromides.11 The carboxyl
group was introduced in a second step by metallation of the
resulting 2-alkylthiophenes, followed by reaction with CO2.12 In
an analogous way 5-(10-undecenyl)-thiophene-2-carboxylic acid
was synthesized.13 The use of a metallation reaction instead of the
often used metal–halogen exchange reaction14 with
2-bromothiophenes provides the advantage that alkyl chain
scrambling is excluded.15 50-Dodecyl-2,20-dithiophene-5-
carboxylic acid16 was obtained by Kumada coupling of 2-bromo-
5-dodecylthiophene17 with thienyl magnesium bromide,14
followed by carboxylation as described above. Esterification of the
thiophene-2-carboxylic acids and 50-dodecyl-2,20-dithiophene-5-
carboxylic acid with appropriate divalent phenols 4–6, incorpo-
rating different types of bent-core units18,19 (see ESI†), yielded
compounds T and the bithiophene TT-Res-12, respectively. The
olefin terminated compound T-Bip-en (R ¼ –(CH2)9–CH]CH2)
was hydrosilylated20,21 in a next step with 1-H-1,1,2,2,3,3,3-hep-
tamethyltrisiloxane to give the silylated compound T-Bip-Si
(R ¼ –(CH2)11–(SiMe2O)2SiMe3). Compounds were purified by
column chromatography (silica gel, eluent: chloroform) and by
repeated crystallization from ethyl acetate. The experimental
procedures and analytical data are reported in the ESI†.
Therefore, a series of thiophene based bent-core mesogens T
with a wide structural diversity, including resorcinol-(T-Res-12),
naphthalene-(T-Nap-12) and biphenyl-derivatives (T-Bip-n), was
synthesized and investigated. Also a bithiophene derivative
(TT-Res-12) as well as a compound with olefinic (T-Bip-en) and
one with silylated (T-Bip-Si) end groups was prepared. With
Institute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt-Mothes-Str. 2, D-06120, Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 552 7346;
Tel: +49 345 552 5664
2.2 Mesomorphic properties
† Electronic supplementary information (ESI) available: Synthesis,
analytical data and additional experimental data (DSCs, textures,
switching current curves). See DOI: 10.1039/c0jm01919d
Phase transitions were determined by polarizing optical micros-
copy (Optiphot 2, Nikon) in conjunction with a heating stage
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a drop of the compound on a glass surface. The X-ray beam was
applied parallel to the substrate. Electrooptical experiments have
been carried out using a home built electrooptical setup in
commercially available ITO coated glass cells (E.H.C., Japan)
with a measuring area of 1 cm2.
2.2.1 5-Alkylthiophene-2-carboxylates with resorcinol, 2,7-
naphthyl and 3,40-biphenyl bent cores. At first, the influence of the
central bent-core unit was investigated with compounds having
a chain length of n ¼ 12 (Table 1). Thiophenene-2-carboxylates
with a resorcinol core (T-Res-12) or a naphthalene-2,7-diol core
(T-Nap-12) do not show LC properties.22 In contrast, for the
thiophene compounds T-Bip-n with a 3,40-disubstituted biphenyl
based bent core (Table 2), LC phases were observed and the
mesophase types are similar to those observed for the corre-
sponding benzoates (e.g. B-Bip-8 and B-Bip-12,19 Table 3),
forming a B1 type rectangular columnar phase (Colrec) for the
short chain compound (n ¼ 8) and SmCPA phases for
Table 2 Phase transitions observed for the thiophene derivatives T-Bip-
8 to T-Bip-16 and T-Bip-ena
Scheme 1 Synthesis of the compound under discussion (for T-Res-n,
TT-Res-n, T-Nap-n and T-Bip-n: CnH2n+1 for T-Bip-en:
R
¼
;
R ¼ (CH2)9–CH]CH2); reagents and conditions: (i) (1) BuLi, THF, 0 ꢀC,
20 min and (2) R–Br, reflux, 4 h;11 (ii) (1) NBS, CHCl3, AcOH, RT, 24 h17
and (2) 2-thienyl–MgBr, Ni(dppp)Cl2, Et2O, reflux, 3 h; (iii) (1) BuLi,
Et2O, 0 ꢀC, 10 h, and (2) CO2, 0–20ꢀ C, 10h;12 and (iv) (1) SOCl2, reflux, 2
h and (2) 4–6, DMPA, CH2Cl2, reflux, 5 h for the synthesis of T-Bip-Si
Comp.
–R
T/ꢀC, [DH/kJ molꢁ1
]
T-Bip-8
–C8H17
Cr 87 [21.9] Colrec 110 [16.2] Iso
Cr 64 [6.5] SmCaPA 104 [17.7] Iso
Cr 88 [36.7] SmCaPA 121 [21.2] Iso
Cr 101 [67.4] SmCaPA 124 [25.5] Iso
T-Bip-en
T-Bip-12
T-Bip-16
–(CH2)9CH]CH2
–C12H25
–C16H33
(R
¼
–(CH2)11–(SiMe2O)2SiMe3). T-Bip-en was hydrosilylated:
H–(SiMe2O)2SiMe3, Karstedt’s catalyst (2% Pt), toluene, 20 ꢀC, 24 h.
a
Transition temperatures and corresponding enthalpy values were
determined by DSC (first heating scan, peak temperatures, 10 K
(FP 82 HT, Mettler) and by differential scanning calorimetry
(DSC-7, Perkin Elmer). The observed mesophases, the phase
transition temperatures and corresponding transition enthalpies
are collated in Tables 1 and 2, and for TT-Res-12 and T-Bip-Si
these data are shown in Fig. 3a and 4, respectively. The assign-
ment of the mesophases was made on the basis of optical textures
and by X-ray diffraction (XRD). Investigations of surface
aligned samples were performed using a 2D detector (HI-Star,
Siemens). Uniform orientation was achieved by slowly cooling
minꢁ1); abbreviations: Colrec
rectangular 2D lattice (B1rev phase), SmCaPA antiferroelectric
¼
modulated smectic phase with
¼
switching tilted smectic phase with anticlinic interlayer correlation, as
observed in electric field experiments;26 for the other abbreviations, see
Table 1.
Table 3 Phase transitions observed for the benzoates B-Bip-8 and B-
Bip-12a
19
Table 1 Comparison of compounds T-Res-12 and T-Nap-12a
Comp.
–R
T/ꢀC [DH/kJ molꢁ1
]
Comp.
BC
T/ꢀC, [DH/kJ molꢁ1
]
B-Bip-8
B-Bip-12
–C8H17
–C12H25
Cr 85 [12.0] SmX 86 [6.3] SmCPA
(147 [3.4] Colrec)b 152 [16.2] Iso
Cr 89 [10.3] SmX 78 [6.3] SmCPA
156 [20.7] Iso
T-Res-12
T-Nap-12
1,3-Phenylene
2,7-Naphthalene
Cr 97 [81.6] Iso
Cr 160 [45.1] Iso
a
Determined by DSC (first heating scan, peak temperatures, 10 K
minꢁ1); abbreviations: Cr ¼ solid crystalline phase and Iso ¼ isotropic
liquid.
a
SmX ¼ smectic phase with unknown structure, for other abbreviations,
b
see Table 2. Colrec phase only observed on cooling from Iso.
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compounds with longer chains (n ¼ 12 and 16). The tendency to
form B1 type phases is a bit stronger for the thiophenes as T-Bip-
8 has the B1 phase over the whole temperature region whereas
for B-Bip-8 it is only observed on cooling in a certain temperature
range beside a SmCPA phase.
a non-tilted B1rev phase (see Fig. 1c),25 the same as observed for
the related benzoate B-Bip-8 upon cooling the isotropic liquid.19
Compounds T-Bip-12 and T-Bip-16 with longer alkyl chains
form smectic phases with textures as typical for SmCPA phases
(B2-phases, see Fig. 2a and S2b†). Electrooptical investigations
confirm antiferroelectric switching with spontaneous polariza-
tion of PS ¼ 460 and 790 nC cmꢁ1 for compounds T-Bip-12 and
T-Bip-16, respectively (Fig. 2b and S2a†). In circular domains the
position of the extinction crosses is parallel to polarizer and
Hence, it appears that also in the case of thiophene derivatives
the biphenyl central core is the optimal choice for the design of
bent-core mesogens.19,23,24 For these less symmetric compounds
with six aromatic rings in the bent core the clearing temperatures
and to some extend also the melting temperatures of the thio-
phene derivatives are reduced compared to the related benzoates,
but the phase type remains essentially the same.
The B1 phase of T-Bip-8 has a rectangular lattice as indicated
by the typical texture (Fig. 1a) and confirmed by XRD of surface
aligned samples (Fig. 1b). The lattice parameters calculated from
the small angle diffraction pattern are a ¼ 3.5 and b ¼ 4.5 nm at
T ¼ 100 ꢀC; the diffuse wide angle scattering is perpendicular to
b, which means that b is in a direction perpendicular to the
modulated layers and a/2 corresponds to the diameter of each
ribbon. In this phase the polar direction of the molecules is
perpendicular to the 2D lattice and antiparallel in adjacent
ribbons. Hence, this B1 type banana phase can be classified as
Fig. 2 SmCaPA phase of compound T-Bip-16. (a) Texture as observed
between crossed polarizers at T ¼ 110 ꢀC; (b) switching current curve
under an applied triangular wave voltage (114 ꢀC, 120 Vpp, 10 Hz, 5 kU,
10 mm ITO cell, EHC, Japan) indicating antiferroelectric switching; (c)
circular domains grown at 115 ꢀC under a DC field of 100 V (left), after
switching off the field (middle) and upon application of a DC field with
opposite direction (right), indicating an antiferroelectric switching on
a cone; and (d) sketch showing the distinct phase structures and the
reorganization of the molecules during this switching process.
Fig. 1 (a) Texture as observed between crossed polarizers and (b) XRD
pattern of an aligned sample of the Colrec phase (B1rev phase) of
compound T-Bip-8 at T ¼ 100 ꢀC, and (c) model showing the organi-
zation of the molecules.
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analyzer and together with the low birefringence this indicates
a homogeneously chiral SmCaPA ground state structure under
these conditions.26 Upon application of an electric field, flipping
of the molecules takes place by rotation on a cone which leads to
field induced SmCsPF structures with high birefringence and
extinction crosses inclined with the directions of polarizer and
analyzer (see Fig. 2c and d). The same kind of switching was
observed for the SmCPA phase of the olefin T-Bip-en
(see Fig. S3†).
the tilt angle, as determined from the maxima of the position of
the wide angle scatterings with respect to the equator, is about
40ꢀ to 41ꢀ. A slightly unequal intensity of the wide angle scat-
tering in the XRD pattern of the aligned sample indicates a pre-
dominating synclinic SmCsPA ground state structure (Fig. 3d) in
this case.
This is the first example of a dithiophene base bent-core
mesogen. Compared with the related biphenyl derivatives BB-
Res-12 (X ¼ H),27 having exactly the same alkyl chain length, the
clearing temperature is dramatically decreased whereas a slight
increase in the melting point is observed. Nevertheless, the
dithiophene unit seems to be a valuable tool to shift the meso-
phases into a temperature range more appropriate for investi-
gation and applications; the effect is comparable with the
addition of two lateral substituents, such as Cl or methoxy, into
the rod-like wings of the biphenyl derivative BB-Res-12 (X ¼ Cl
and OMe, see Fig. 3e).27
2.2.2 Compound TT-Res-12 comprising bithiophene units. A
SmCPA phase with a texture identical to the one shown in Fig. 2a
was also observed for the resorcinol bisbenzoate TT-Res-12
incorporating two dithiophenecarboxylate units (see Fig. S4†).
Also for this LC phase antiferroelectric switching was observed
(PS ¼ 700 nC cmꢁ2 at T ¼ 130 C). The XRD pattern of this
ꢀ
compound is shown in Fig. 3b, c. In the small angle region there
is a sharp layer reflection together with the second and third
harmonics indicating a well defined layer structure with relatively
sharp interfaces. The layer distance is d ¼ 4.3 nm (see Table S2†),
2.2.3 The silyl substituted compound T-Bip-Si. Compound
T-Bip-Si, silylated at both ends, was of special interest as it is
known that silyl groups can largely modify the mesomorphic
properties due to layer decoupling and steric effects.21,28,29 This
leads to ferroelectric switching smectic phases and to several
distinct types of polar and non-polar modulated smectic phases
(columnar phases, Col) as, for example, observed for the related
benzoate B-Bip-Si (see Fig. 4a).29a
Also compound T-Bip-Si has a rich phase sequence as indi-
cated by electrooptical investigations (Fig. 5a, c and e). However,
in the textures (Fig. 5b, d and f), in the DSC curves (Fig. S5†) and
in the XRD patterns (Fig. 4b–e, S6 and Table S3†) the phase
transitions are difficult to recognize. The XRD patterns of
surface aligned samples (Fig. 4b–e) are characterized by two
diffuse wide angle scatterings, the one at d ¼ 0.68 nm forms
a nearly closed ring and is attributed to the mean distance
between the siloxane units. The second one at d ¼ 0. 48 nm has
maxima at preferred directions inclined between equator and
meridian and is attributed to the mean distances between the
alkyl chains and between the aromatic cores of the molecules.
From the position of the diffuse wide angle scattering an unusual
strong tilt of about 54ꢀ can be calculated, which does not
significantly change with temperature. In the small angle region
only a layer reflection at d ¼ 4.6 nm, together with the second and
fourth order, can be observed at all temperatures, though the
shape and intensity of these reflection slightly changes with
temperature.
In contrast to the XRD results which would suggest a layer
structure at all temperatures, the texture observed between
crossed polarizers (upon cooling from the isotropic liquid state)
is untypical for smectic phases and is more similar to the
appearance of a columnar (modulated smectic) phases composed
of high birefringent (homogeneously aligned) and low birefrin-
gent (homeotropically aligned) areas with spherulitic texture
(Fig. 5b). The texture changes only slightly in the whole
temperature range; mainly the spherulitic texture in the low
birefringence regions becomes more Schlieren-like at lower
temperature, whereas no significant change can be observed in
the highly birefringent areas (Fig. 5d and f).
Fig. 3 (a) Structure, transition temperatures (ꢀC) and corresponding
enthalpy values (in square brackets, DH/kJ molꢁ1) as determined by DSC
(first heating scan, peak temperatures, 10 K minꢁ1) of compound TT-Res-
12; XRD pattern of a surface aligned sample of the SmCsPA phase of
compound TT-Res-12 at T ¼ 135 ꢀC: (b) original pattern and (c) pattern
obtained after subtraction of the diffraction pattern of the isotropic
liquid T ¼ 150 ꢀC; (d) shows the synclinic and antiferroelectric organi-
zation in the SmCsPA phase; and (e) transition temperatures of related
biphenyl derivatives BB-Res-12 with and without additional lateral
substituents.27
Also in the DSC heating curve beside the melting transition at
T ¼ 64 ꢀC only the clearing transition at T ¼ 115 ꢀC with
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Fig. 4 (a) Structure and transition temperatures of compound T-Bip-Si and comparison with the related biphenyl derivative B-Bip-Si;29a transition
temperatures (ꢀC) and corresponding enthalpy values (in square brackets, DH/kJ molꢁ1) were determined by DSC (first heating scan, peak temperatures,
10 K minꢁ1); abbreviations: Colob ¼ non-polar modulated SmC phase, ColobPA ¼ antiferroelectric switching Colob phase, and SmCPFE ¼ ferroelectric
switching SmC phase, for other abbreviations, see Tables 1 and 2; XRD patterns of a surface aligned sample of compound T-Bip-Si in the temperature
range of the modulated smectic phase at T ¼ 104 ꢀC: (b) original pattern and (c) pattern after subtraction of the diffraction pattern of the isotropic liquid
at T ¼ 120 ꢀC; in the temperature range of the SmCPFE phase at T ¼ 80 ꢀC: (d) original pattern and (e) pattern after subtraction of the diffraction pattern
of the isotropic liquid at T ¼ 120 ꢀC.
a relatively high transition enthalpy (17.9 kJ molꢁ1) can be
observed clꢀearly. However, on cooling there are broad features
switching is not continuous but discontinuous, i.e. the peaks do
ꢀ
not move upon approaching 96 C, but instead at this temper-
ꢀ
around 95 C and around 87 C with very low enthalpy values
ature the two well separated peaks disappear and the two less
separated peaks appear. Below T ¼ 87 ꢀC there is only one sharp
peak (Fig. 5e) which cannot be splitted by applying a modified
triangular wave voltage with an additional relaxation time at 0 V
(Fig. S7†). This indicates a bistable (ferroelectric) switching
process; the spontaneous polarization is further increased to
630 nC cmꢁ2. The transition from the antiferroelectric switching
to single peak switching seems to be continuous, as upon
approaching this transition the intensity of the peak occurring at
higher voltage decreases whereas simultaneously the peak at
lower voltage increases. This could be interpreted as an increase
of the size of the ferroelectric clusters, leading to surface stabi-
lized ferroelectric switching below this transition, similar to the
observations made with the related compound B-Bip-Si for
which this process was investigated by dielectric investigations.29a
Investigations of circular domains, grown under a DC field,
indicate switching around the long axis at all temperatures
(Fig. 6). This is most likely due to the steric effect provided by the
bulky trisiloxane units which separate the aromatic cores of the
molecules and thus provide sufficient free space for fast
(see Fig. S5†).
Only by means of electrooptical investigations strong changes
can be observed at these temperatures. At high temperature
between 96 and 114 ꢀC two well separated repolarization current
peaks can be observed in one period of an applied triangular wave
voltage (Fig. 5a), clearly indicating an antiferroelectric switching
process with a spontaneous polarization of PS ¼ 180 nC cmꢁ2
.
A relatively high threshold of 150 V (10 mm cell) is required for
switching and this switching takes place by collective rotation of
the molecules around their long axis (chirality flipping) as indi-
cated by the investigation of circular domains (Fig. 6a), where the
position of the extinction crosses does not change during the
switching process.
ꢀ
In the temperature region between 87 and 96 C two polari-
zation current curves are retained, but these are closer to each
other (Fig. 5c) indicating a reduction of the energy difference
between the antiferroelectric and ferroelectric states. Moreover,
the spontaneous polarization is increased to 430 nC cmꢁ2. The
transition between these two distinct modes of antiferroelectric
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Fig. 5 Textures as grown from the isotropic liquid state without applied electric field (right) and switching current response curves (left) as observed for
compound T-Bip-Si at different temperatures (10 Hz, 10 mm non-coated ITO-cell): (a) antiferroelectric switching at T ¼ 105 ꢀC (PS ¼ 180 nC cmꢁ2);
(b) texture at T ¼ 110 ꢀC; (c) antiferroelectric switching with only small splitting of the polarization current peaks at T ¼ 92 ꢀC (PS ¼ 430 nC cmꢁ2);
(d) texture at the same temperature; (e) ferroelectric switching smectic phase at T ¼ 71 ꢀC (PS ¼ 620 nC cmꢁ2); and (f) texture at T ¼ 82 ꢀC.
reorientation of the molecules around their long axes, but in
addition also layer modulations should contribute to inhibition
of rotation on a cone30 and this effect increases with rising
temperature, as the density of the modulations is also increased
(see below).
At low temperature (below 87 ꢀC) the extinction crosses are all
parallel to polarizer and analyzer indicating an overall anticlinic
orgꢀanization of the molecules (Fig. 6c).31 Above a temperature of
87 C beside the low birefringent domains also highly birefrin-
gent synclinic domains occur (Fig. 6b) and the tendency to form
synclinic domains strongly increases with temperature. In the
high temperature region synclinic domains are dominating
(Fig. 6a) and some anticlinic domains, observed under the
applied electric field, become instable at 0 V and relax to a syn-
clinic structure by realignment (see Fig. S8†).
unfavourable interfaces for anticlinic packing, hence favouring
synclinic organization and also contribute to a preference of
rotation around the long axis over the rotation on a cone.30 Layer
modulation is also considered as responsible for the transition
from (surface stabilized) ferroelectric switching in the smectic
phase below 87 ꢀC to antiferroelectric switching above 87 ꢀC, as
ferroelectric correlation is unfavourable in the ribbon structures
of modulated smectic phases (see Fig. S9†).29a Hence, the phase
assigned as SmCPA is most likely a modulated or undulated
smectic phase with a long undulation/modulation periodicity and
under an electric field these undulations/modulations can be
largely removed (see Fig. 7). The phase at high temperature,
assigned as ColobPA is a modulated smectic phase with a shorter
modulation periodicity which is retained under the applied
electric field. The assignment ColobPA is used as the mesophase
of T-Bip-Si is very similar to the ColobPA phase of B-Bip-Si for
which the oblique lattice was proven by XRD. However,
compared to the benzoate B-Bip-Si29a the strength of layer
modulation seems to be weaker, i.e. the modulation period seems
to be longer in the case of the thiophene compound T-Bip-Si.
All these observations indicate a destabilization of anticlinic
organization with rising temperature. This is in line with the
presence of layer modulation in the two LC phases at higher
temperature and an increase of the strength of layer modulation
with temperature (see Fig. 7). These modulations provide
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B-Bip-Si,29a is missing in the phase sequence of T-Bip-Si which
means that the modulation density does in this case not become
large enough to suppress AF switching completely. Comparing
compounds T-Bip-Si and B-Bip-Si (Fig. 4a) shows once again
that the fundamental phase structures are retained by replacing
benzene rings in the rod-like wings of bent-core mesogens by
2,5-disubstituted thiophenes.
3. Conclusions
Thiophene rings were introduced into a variety of distinct types
of bent-core mesogens, differing in the structure of the bent unit,
the number of thiophene units and the kind of end-functionali-
zation of the flexible alkyl chains. The main effect of replacing
the peripheral benzene rings by thiophenes is a reduction of
phase transitions. This is observed for all mesophases and in
many cases also for the melting points. In this respect this
exchange is important for practical applications of bent-core
mesogens as high transition temperatures represent a major
problem for their investigation and application. Moreover, these
compounds are of significant interest for resonant X-ray scat-
tering, as heavy probe atoms were introduced without modifying
the phase structure. A variety of different phase structures is
provided here and especially the investigation of the silylated
compounds, where structures composed of layer stacks were
proposed previously,21,28,29 could contribute to a better under-
standing of their nanoscale structures. Chlorine, which is
a possible alternative probe for resonant X-ray scattering,10b is
less advantageous for this purpose, as it has to be introduced as
a lateral substituent, which in most cases leads to a drastic change
of the LC phase structures of bent-core mesogens.1c,e,32 Though
the use of thiobenzoate units,33,10b which are nearly isosteric with
the benzoate groups, is another alternative, thiobenzoates are
more reactive than usual benzoate groups and also catalytic
hydrosilylation would in this case be difficult or impossible to
perform. Beside this scientific application, oligothiophenes
incorporated into the structure of bent-core mesogens provide
the possibility to introduce flat p-conjugated aromatic systems
into bent-core molecules.34 Such materials could be of interest for
future application as organic electronic materials.8,35 The denser
packing of the aromatic cores in the mesophases of bent-core
mesogens compared to rod-like mesogens, leading to polar order
and sharper aromatic–aliphatic interfaces (as indicated by the
larger number of higher order layer reflections in the XRD
patterns), is promising for such applications. Especially higher
ordered smectic mesophases of bent-core mesogens, such as B5
phases,1c,36 and also soft crystalline lamellar phases37 could
possibility lead to useful charge carrier materials, as the addi-
tional in-plane order could improve overlapping of the conju-
gated p-systems of neighbouring molecules. The LC bithiophene
derived compound TT-Res-12 has shown that it is possible to
incorporate oligothiophenes into the basic structure of bent-core
molecules and this concept should be extended further by using
longer oligothiophenes.
Fig. 6 Optical observation of the switching process in circular domains
grown in the distinct mesophases of T-Bip-Si under a DC electric filed in
10 mm ITO cells: (a) at T ¼ 105 ꢀC, (b) at T ¼ 92 ꢀC and (c) at T ¼ 71 ꢀC;
polarizers are horizontal and vertical (slightly inclined by about 5ꢀ in (b)
and (c)); (d) shows the switching of the molecules by rotation around the
long axis (chirality flipping).
Fig. 7 Schematic sketch showing the dependence of the phase structure
of compound T-Bip-Si on temperature (T) and the strength of the applied
electric field (E) (polar order of the molecules is not considered for
clarity); the presence of an undulated smectic C phase (USmC) was not
proven, but this phase or a ColobPA phase with very long modulation
period might be the intermediate structure showing antiferroelectric
switching.31
This might be the reason that by using usual XRD techniques no
2D lattice can be detected in the diffraction patterns. Also the
fact that anticlinic domains are not completely suppressed in the
high temperature phase (see Fig. S8†) and still can be observed if
a sufficiently strong electric field is applied suggests that the
applied field can still reduce the number of modulations signifi-
cantly. Moreover, a non-switching Colob phase, as observed for
Acknowledgements
This work was supported by the Fonds der Chemischen
Industrie.
9664 | J. Mater. Chem., 2010, 20, 9658–9665
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22 Selected examples of previously reported bent-core mesogens with
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