Bent-core mesogens with an aromatic unit at the terminal position

Article abstract of DOI:10.1039/c6nj03908a

Bent-core liquid crystals with a naphthalene central unit and an aromatic ring at the terminal position of molecular tails were synthesised with the aim of enhancing nanosegregation. It was found that the length of the spacer between the rigid core and the terminal aromatic moiety had a profound influence on the liquid crystal polymorphism. The homologues with short spacers exhibited nematic and columnar phases, whereas the homologue with long spacers exhibited a tilted lamellar phase with a liquid-like in-plane order, indicating an unusual morphology of the densely packed toroidal objects. The morphology can be changed to twisted ribbons by small additives adsorbed on the membrane surface. This is the first example of twisted ribbons constructed by a lamellar system with no long-range in-plane order.

Full text of DOI:10.1039/c6nj03908a

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Bent-core mesogens with an aromatic unit at the
terminal position†
Cite this:DOI: 10.1039/c6nj03908a
a
b
Kvetoslava Bajzıkova,
Damian Pociecha
Jirı Svoboda, *a Vladimıra Novotna,
*
´
´
ˇ´
´
´
c
c
and Ewa Gorecka
Bent-core liquid crystals with a naphthalene central unit and an aromatic ring at the terminal position of
molecular tails were synthesised with the aim of enhancing nanosegregation. It was found that the
length of the spacer between the rigid core and the terminal aromatic moiety had a profound influence
on the liquid crystal polymorphism. The homologues with short spacers exhibited nematic and columnar
phases, whereas the homologue with long spacers exhibited a tilted lamellar phase with a liquid-like
in-plane order, indicating an unusual morphology of the densely packed toroidal objects. The morphology
can be changed to twisted ribbons by small additives adsorbed on the membrane surface. This is the first
example of twisted ribbons constructed by a lamellar system with no long-range in-plane order.
Received 13th December 2016,
Accepted 28th April 2017
DOI: 10.1039/c6nj03908a
rsc.li/njc
One of the most intriguing phases formed by bent-core molecules
is the dark-conglomerate phase (DC).8–12 The DC phase is a
Introduction
The unusual properties of bent-core liquid crystals (BC) were smectic mesophase, but its texture is optically isotropic with
first described in 1996;1 since then, significant efforts have been optically active domains that are easily detectable via decrossing
devoted to establishing the molecular structure–mesomorphic of the polarizers. It is generally accepted that the DC phase is
property relationship for this group of materials. For bent-core formed from strongly deformed membranes (layer stacks) with
mesogens, much richer polymorphism of mesophases has been local saddle-splay curvature (sponge morphology).13
observed as compared to rod-like molecules.2 Besides the
Introduction of functional groups of various shapes and
nematic phase, several smectic (lamellar) phases, mainly tilted polarities into the terminal chains was utilized to enhance the
smectic phases (SmCP) in which a close packing of the bent-core nanosegregation and change the self-assembly process as
molecules results in a polar order, have been described and well as the mesomorphic behaviour of the liquid crystalline
studied. The SmCP phases can be distinguished as antiferro- materials.14 For example, in the field of rod-like mesogens, the
electric (A) and ferroelectric (F) according to their polar order bulky tert-butyl and cycloalkyl,15–19 alkylsilyl, oligosiloxane and
and as synclinic (S) and anticlinic (A) according to the orientation carbosilane,15,19,20 semifluoroalkyl, polyfluoroalkyl, and aryl
of the tilt in the neighbouring layers.3,4 Bent-core mesogens can chains18–22 were introduced to tune the mesomorphic properties
also form phases with a structure built of smectic layer fragments of the LCs. Few examples of benzene-terminated rod-like materials
(columns) arranged into a 2D lattice; these columnar phases have have also been investigated.19,23,24 Analogously, the role of different
been labelled as B1-type.5,6 There are two general types of B1 polar and bulky groups at the terminal position of the bent-core
phases in which layer fragments are either directly connected to materials has been studied: e.g. linear alkyl, benzyl and perfluoro-
each other (B1) or through a defect region (B1Rev);7 the phases phenyl esters,25–28 bulky adamantane,29 fullerene,30 alkylsilyl,
differ in their electrooptical response: no switching is observed for oligosiloxane and carbosilane units,31–40 and semi- and poly-
B1, whereas B1Rev is usually sensitive to an applied electric field. fluorinated chains28,41–43 have been carefully investigated to
recognize the structure–property relationship.
Herein, we report the synthesis and physical properties of a
series of bent-core compounds terminated with an arylalkyl,
a Department of Organic Chemistry, University of Chemistry and Technology,
CZ-166 28 Prague 6, Czech Republic. E-mail: Jiri.Svoboda@vscht.cz;
heteroarylalkyl, and aryloxyalkyl chains. It was expected that
the bulky aryl group at the terminal position should prevent
interdigitation of the terminal chains between neighbouring
layers, and the decoupled layers should be less stiff and therefore
more susceptible to deformation. In addition, the compounds
end-capped with a reactive aromatic and/or properly substituted
Fax: +420 220444182; Tel: +420 220444288
b Institute of Physics, Czech Academy of Sciences, Na Slovance 2,
CZ-182 21 Prague 8, Czech Republic. E-mail: novotna@fzu.cz;
Fax: +420 286890527; Tel: +420 266053111
c Laboratory 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/c6nj03908a aromatic group can be utilized in the future for the preparation of
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Fig. 1 General structure of the designed materials.
new types of metallic nanoparticles grafted with organometallic
mesogenic ligands.44–46 To optimize the mesomorphic properties,
the general molecular structure (Fig. 1) has been modified by
changing the type of the linking group (Z = ester and azo) in the
lengthening arm, the length of the arylalkyl (Y = CH2) and
aryloxyalkyl (Y = O) terminal chains (n = 0–5, 7), the type of the
aromatic unit (Ar = benzene and thiophene), and the character of
the terminal chain (R = alkyl, arylalkyl, and perfluoroalkyl).
For the design of new materials, we utilized our knowledge
about the mesomorphic properties of the previously studied
naphthalene-based mesogens.47–53 As a reference compound,
we chose a material with two dodecyloxy terminal chains,48
which was later modified by a nonafluorododecyloxy chain53
and the outer ester linkage was also replaced by an azo
moiety.54 All these compounds exhibited SmCP type phases
in a broad temperature range. For the compounds presented
herein, the original dodecyl chain in the model compounds was
primarily replaced by a phenyloctyl or 2-thienyloctyl unit to
keep the length of the terminal chain unchanged. Furthermore,
the second terminal alkyl chain R was also substituted by
phenyloctyl or polyfluoroalkyl chains and the ester linkage Z
was substituted by an azo group. Based on the preliminary
results, the studied series was extended by modifying the length
of the phenylalkyl chain (n), introduction of oxygen in the
vicinity of the phenyl moiety (Y = O and phenoxyalkyl), and
phenyl substitution (Br, COOH).
Results and discussion
Fig. 2 DSC plots for selected compounds: (a) Ib, (b) Ic, (c) If, and (d) IIIa,
obtained during the second heating (upper) and cooling (lower curve) at a
rate of 5 K minÀ1. Arrows mark the phase transitions, which are not clearly
seen at this scale; phases are indicated.
Experimental details are described in ESI.† For all the studied
compounds, liquid crystalline phases were tentatively identified by
their optical textures via a polarising microscope and confirmed by
X-ray diffraction studies. The phase transitions and enthalpy
changes were determined from the DSC measurements. DSC and Ie with an odd number of carbon atoms in the linkage, a
thermographs for the selected compounds from all series are sequence nematic–columnar phase was observed at rather high
presented in Fig. 2. Chemical formulae of the studied compounds transition temperatures, whereas for the materials Ib and Id
are inserted into the headings of the corresponding tables.
with an even number of carbon atoms, direct transition from
Compounds of series I (Table 1) differ in the length of the the isotropic liquid to a columnar phase was found at substantially
linkage between the terminal phenyl ring and mesogenic core. lower temperatures (Fig. 3). The nematic phase showed a typical
A strong odd–even effect was observed regarding the clearing marble-like texture55 (Fig. 4a), whereas a mosaic texture was
temperature and the phase sequence. For the materials Ia, Ic, observed for the columnar phase (Fig. 4b). The columnar phase
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Table 1 Phase transition temperatures, Ttr, in 1C, and enthalpies, DH, in kJ molÀ1, were detected during the second cooling. Melting points, m.p., were
obtained on the second heating. All thermographs were obtained at a rate of 5 K minÀ1
n
m.p. (DH)
Tcr (DH)
M2
Ttr (DH)
M1
Ttr (DH)
Iso
Ia
Ib
Ic
Id
Ie
If
1
2
3
4
5
8
171 (+27.7)
132 (+12.1)
139 (+30.1)
126 (+19.4)
143 (+35.3)
134 (+17.9)
146 (À28.1)
121 (À16.5)
128 (À14.6)
116 (À17.2)
131 (À21.1)
116 (À7.7)
B1rev
—
B1rev
—
B1rev
—
168 (À10.7)
N
B1rev
N
B1rev
N
DC
181 (À0.3)
136 (À11.0)
168 (À0.4)
139 (À13.4)
155 (À0.4)
138 (À12.6)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
—
153 (À10.9)
—
141 (À10.7)
—
Fig. 3 Phase diagram for compounds Ia–e.
in all compounds, Ia–Ie, was identified as a broken-layer B1rev type.
Under an applied electric field, the B1rev textures revealed changes
in birefringence, but no switching of the extinction position was
observed. For all the materials exhibiting the B1rev phase, in the
X-ray patterns, few incommensurate Bragg reflections were
obtained at a low angle range and a diffused signal was observed
in the high angle region (Fig. 5a). The best fitting of the signal
positions was obtained via speculating an oblique 2D lattice (with
z = 2). The crystallographic unit cell parameter b steadily increased
by B1 Å with elongation of the molecular arm by each methylene
group (Table 2). Note that the parameter b is considerably smaller
than the molecular length, suggesting that molecules in the layer
Fig. 4 Texture of Ia in (a) the nematic phase at T = 172 1C and (b) the
columnar B1 phase at T = 164 1C.
fragments, forming the 2D columnar phase, are strongly tilted. samples, are presented for different studied compounds (Fig. S1,
The inclination angle of the crystallographic unit cell g B 100 deg, ESI†).
as well as the sizes of the layer blocks (unit cell parameter a B 28 Å,
For the longest studied homologue of series I, with n = 8 (If),
corresponding to about 6 molecules in the block cross section) another mesophase has been observed, with optical textures
were only weakly dependent in the homologue series (Table 2). typical for a lamellar DC phase.8–12 The X-ray pattern confirmed
The speculated crystallographic lattice was confirmed by azimuthal the simple lamellar structure of the phase, and only commensurate
signal positions in the 2D X-ray patterns for partially aligned peaks at low angle range were detected (Fig. 5b). The layer thickness
samples. The ESI X-ray patterns, obtained for partially oriented determined from the signal position was much smaller than the
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Table 2 Cell parameters measured by X-ray for selected compounds at
temperatures T
T/1C
a/Å
c/Å
b/1
Ia
165
25.6
30.0
103.8
Ib
133
26.9
30.9
104.4
Ic
148
140
130
28.3
28.2
28.3
31.6
31.7
31.7
103.4
103.6
103.8
Id
135
125
27.1
27.4
32.7
32.7
102.4
102.3
Ie
137
28.8
33.1
40.2
103.0
92.3
If
Fig. 5 X-ray patterns for the compounds (a) Id in the B1rev phase and (b) If
in the DC phase (T = 135 1C) and glassy state of the DC phase (T = 30 1C).
135
130
125
d = 36.4
d = 36.4
d = 36.3
IIa
180
molecular length, suggesting tilted arrangement of the molecules
in the layers (tilt angle B40–45 deg). In the high diffraction angle
29.1
region, a diffused maximum corresponding to an average IId
150
d = 38.8
d = 38.3
distance between molecules was present, typical for LC mesophases
130
with short-range molecular ordering in the layers. Optical textures of
the lamellar phase evidenced nearly zero birefringence, indicating IIf
that the orientation of the molecules in the phase was space-
135
d = 36.1
d = 38.9
24.9
averaged over distances smaller than the wavelength of visible
IIg
light; however, the textures show large (micron-size) optically 145
active domains with right- and left-handed optical rotatory
IIIc
120
power (Fig. 6), characteristic for dark-conglomerate (DC) phase.8–12
40.5
89.1
The optical activity was nearly temperature independent,
B0.25 degree per mm. The application of an electric field induced
a reversible change to the birefringent grainy texture. Apparently, the studied materials, doping with a small amount of nematic
the electric field straightened the layers. Rapid cooling of the compound (B10 wt% of 6CHBT, see Fig. S5, ESI†) influenced the
sample did not change the texture, and the boundaries of the sample morphology. In mixtures, the toroidal-like objects were
optically active domains were preserved. The X-ray pattern of replaced with irregular sponge with some twisted ribbons (Fig. 7c
the temperature-quenched sample, obtained at room temperature, and Fig. S6, ESI†), similar to those found in the B4 phase.57 The
shows that the high angle signal is broad although weakly split, X-ray studies showed that although the morphology of the
evidencing the glassy state of the DC phase, with short positional system changed, the crystallographic structure of the DC phase
correlations inside the layers. Temperature quenching of the remained unchanged under doping, the layer thickness only
sample prevents recrystallization, and the sample remains in slightly expanded (by a few% in B20 wt% mixture), and the
the DC phase.
in-plain order stayed short-range. This clearly shows that
To obtain some insight into the nature of the DC phase, its despite the liquid crystalline smectic order of the DC phase,
morphology was investigated via AFM (Fig. 7). The AFM images the components of the mixture segregate at nanoscale, and only
obtained for thermally quenched samples revealed characteristic, a very limited number of nematogenic molecules are incorporated
densely packed toroidal-like objects with the diameter B150– in the membranes of the dark conglomerate phase and most
250 nm, built of 4–5 short, connected tubules with the diameter probably in the regions between smectic layers. This finding is
B50–80 nm, and the tubules were made of wrapped layers. At contrary to the common behaviour of the LC mixtures, for
higher magnification (Fig. 7b), it was possible to detect that the which it was observed that even chemically different mesogenic
central part of the torus was filled with cylindrically folded layers. molecules (rod and bent-core) can form homogeneous smectic
The morphology of the DC phase resembles focal conic domains, structures with the layer thickness linearly related to the
interconnected in 3D space. It was recently shown that the mixture concentration.58 For the studied system, demixing of
morphology of the lamellar crystals (B4 phase) can be changed the materials was observed despite the fact that the membranes
in the presence of some additives in the system.56 Moreover, for were built of layers having liquid like in-plane order. It seems
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Fig. 6 Texture of If observed using a polarizing microscope; polarizers
are de-crossed by an angle of about 6 degrees (a) clockwise and (b)
anticlockwise. The position of polarizers is schematically depicted and the
width of the image corresponds to 150 mm.
that the formation of toroidal or twisted nanofilament objects does
not require crystalline layers, like in the B4 phase.58 Moreover, all
morphologies (toroidal, sponge, and twisted ribbons) are driven by
the same tendency towards saddle-splay deformation of membranes.
The energy balance between different morphologies is rather subtle
and might be changed via the adsorption of the dopant molecules on
the membrane surface or via weakening of the layer coupling. In both
cases, the toroidal objects are replaced by objects, such as
membranes or ribbons, with higher surface to volume ratio.
Molecular structures with the shortest and longest linkages
between the terminal aryl ring and mesogenic core (n = 1 and
n = 8, respectively) were subjected to further modifications, see
Table 3. Exchanging of the alkyl terminal chain by a partially
fluorinated chain of the same length (compounds IIa and IId)
resulted in an increase of the clearing temperature by B20 K,
and the phase sequence was not affected. Replacement of an
ester linkage at the Z position in the molecular arm (Table 3)
Fig. 7 AFM images: (a and b) for compound If obtained at room temperature
for a sample quenched from the DC phase (figures differing in resolution);
(c) for mixture of compound If with 10 wt% of nematogen 6CHB; twisted
ribbons are clearly visible in the left part of the image.
with an azo moiety (compounds IIb and IIe) caused an increase Although the presence of sulphur in the thiophene ring introduced
in the melting temperature; this prevented observation of the an additional dipole moment (B0.5 D), mesomorphic properties
columnar phase in IIb and resulted in a change in the liquid of the materials IIf–g were similar to their analogues with a phenyl
crystalline phase in the case of compound IIe – columnar phase ring in the terminal chain. Only in the case of the azo derivative
was found instead of the DC lamellar phase formed by an IIh, there was a change in the type of LC phase formed; the
analogous compound If.
nematic phase was observed instead of the columnar phase found
Additionally, another type of aryl ring at the terminal position in IIe. Material IIc, in which phenyl rings were applied at the ends
was tested; for materials IIf–h, a thiophene unit was applied. of both terminal chains, showed no liquid crystalline phases.
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Table 3 Phase transition temperatures, Ttr, and temperature of crystallization, Tcr, in 1C detected during the second cooling, and the corresponding
enthalpies, DH, are in brackets in kJ molÀ1. Melting points, m.p., have been obtained during the second heating. All thermographs were obtained at a rate
of 5 K minÀ1. Ph = phenyl, Th = 2-thienyl
n
Z
Ar
R
m.p. (DH)
Tcr (DH)
M2
Tcr (DH)
M1
Ttr (DH)
Iso
IIa
IIb
IIc
IId
IIe
IIf
1
1
8
8
8
8
8
8
OOC
NQN
OOC
OOC
NQN
OOC
OOC
NQN
Ph
Ph
Ph
Ph
Ph
Th
Th
Th
(CH2)8C4F9
C12H25
(CH2)8Ph
(CH2)8C4F9
C12H25
C12H25
(CH2)8C4F9
C12H25
192 (+43.2)
182 (+56.2)
123 (+25.4)
140 (+22.2)
148 (+31.7)
120 (+22.1)
142 (+74.7)
146 (+35.2)
177 (À35.1)
176 (À52.3)
113 (À25.3)
118 (À21.0)
135 (À20.4)
119 (À10.1)
121 (À68.1)
132 (À17.3)
B1Rev
—
—
—
—
—
—
—
189 (À8.6)
N
N
—
DC
B1Rev
DC
DC
N
202 (À0.6)
188 (À0.4)
—
161 (À13.5)
139 (À9.5)
139 (À15.2)
157 (À15.1)
138 (À6.7)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
IIg
IIh
Table 4 Phase transition temperatures, Ttr, in 1C, and enthalpies, DH, in kJ molÀ1, were obtained during the second cooling. Melting points, m.p., were
obtained during the second heating. All thermographs were obtained at a rate of 5 K minÀ1
X
m.p. (DH)
Tcr (DH)
M2
Ttr (DH)
M1
Ttr (DH)
Iso
IIIa
IIIb
IIIc
IIId
H
Br
128 (+27.9)
130 (+33.4)
111 (+17.1)
195 (+11.9)
121 (À16.8)
116 (À19.2)
104 (À14.7)
171 (À24.1)
B1Rev
B1Rev
B1Rev
—
129 (À9.4)
127 (À6.4)
121 (À8.3)
—
N
N
N
—
140 (À0.3)
155 (À0.4)
127 (À0.5)
—
ꢀ
ꢀ
ꢀ
ꢀ
PhCH2OOC
HOOC
All materials forming the DC phase had morphologies similar to form strong hydrogen bonds, is not mesogenic. Mesophases
those observed for the If compound, i.e. densely packed toroidal formed by the compounds of series III exhibit typical optical
objects were visible in their AFM images.
textures, and examples are presented in ESI† (Fig. S3 for IIIb). The
Finally, a methylene group in the vicinity of the terminal B1Rev phase show characteristic colour domains, which do not
phenyl group was substituted for oxygen in the materials III reveal any electrooptical response. Crystallographic unit cell
(Table 4). In addition, some functionality was introduced into parameters for the selected compounds of series II and III are
the phenyl ring, which could be utilized in the planned design presented in Table 2.
of the organometallic ligands. In comparison with If, the
presence of oxygen and thus formation of an additional dipole
in the terminal chain of IIIa resulted in a change of mesomorphic
behaviour and a sequence of N–B1Rev phases was observed. The
Conclusions
transition was strongly first order; therefore, as expected, only The bent-core compounds with an aromatic and/or substituted
weak cybotactic groups were observed in the nematic phase via aromatic unit placed at the end of the terminal chain, exhibiting
the X-ray method (Fig. S2, ESI†). Moreover, similar observation nematic–columnar phase sequence, were prepared and studied.
was obtained for the bromophenyl substituted material IIIb. A strong even–odd effect was observed with respect to the length
Compound IIIc, although having a bulky group in the terminal of the alkyl linkage in the terminal chain, which was much more
position, exhibits the mesomorphic behaviour. On the other pronounced compared to that of the previously studied bent-core
hand, material IIId with a free carboxylic group, which can easily compounds with simple alkyl/alkyloxy terminal chains. It is evident
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ring deviates from linearity, which influences the process
of self-assembly and therefore isotropization temperatures.
Surprisingly, crystallographic lattice parameters of the columnar
¨
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bent-core mesogens prevents interdigitation of the molecules
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broken layer columnar phases or a lamellar phase with complex
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toroidal objects are formed and the morphology can be changed by
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objects are replaced by sponge or twisted ribbons, without changes
in the smectic layer structure. Previously, the formation of twisted
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Acknowledgements
This project was supported by the Czech Science Foundation
(projects No. 15-02843S and 16-12150S).
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