Laterally connected bent-core dimers and bent-core-rod couples with nematic liquid crystalline phases

Article abstract of DOI:10.1039/c1jm13649f

First examples of laterally connected bent-core dimesogens and liquid crystalline dimesogens involving bent-core units and rod-like units are reported. Four distinct types of dimesogens have been synthesized and investigated by polarizing microscopy (PM),

Full text of DOI:10.1039/c1jm13649f

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PAPER
Laterally connected bent-core dimers and bent-core-rod couples with nematic
liquid crystalline phases†
*
Govindaswamy Shanker, Marko Prehm and Carsten Tschierske
Received 31st July 2011, Accepted 6th September 2011
DOI: 10.1039/c1jm13649f
First examples of laterally connected bent-core dimesogens and liquid crystalline dimesogens involving
bent-core units and rod-like units are reported. Four distinct types of dimesogens have been synthesized
and investigated by polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray
diffraction (XRD) studies. Two of them combine two bent-core mesogens derived from 4-
cyanoresorcinol either in a lateral–lateral or lateral–terminal topology, the other two types combine a 4-
cyanoresorcinol based bent-core mesogen with a rod-like mesogen in a lateral–lateral or lateral–terminal
manner. All compounds, except one, exhibit nematic phases composed of SmC-type cybotactic clusters
(NcybC) with wide phase ranges and low crystallization temperatures. Two compounds show additional
low temperature phases, an SmC phase and a rectangular columnar phase were observed.
Mesogenic dimers incorporating bent-core units could possibly
1. Introduction
provide a compromise as these compounds combine features of
polymers/oligomesogens with those of low molecular mass
mesogens, i.e. they interconnect and preorganize individual
mesogenic units covalently, but still have relatively low molec-
ular masses and viscosities. Previously reported dimers incor-
porating bent-core units represented exclusively end-to-end
connected dimesogens (Scheme 1, left),17 either composed of two
bent-core mesogenic segments18 or combining a rod-like unit
with a bent-core unit.19 Here we report the first examples of side-
by-side and side-to-end connected bent-core dimesogens as well
as related dimesogens composed of a bent-core unit combined
with a laterally attached rod-like mesogenic core (Scheme 1,
right). As bent-core mesogenic unit a 4-cyanoresorcinol bisben-
zoate core was chosen, because this unit is known to favour
formation of nematic phases.10,20,21 The flexible spacer units
consist of alkyl chains which, for synthetic reasons, contain a
bis(dimethylsiloxane) unit in the middle.
Liquid crystalline (LC) materials1 are of great importance for
different types of functional materials. Besides the well-known
display applications also several new areas are rapidly emerging,
as for example in charge carrier materials2 and in biosensor
applications3 to mention only two. Among the distinct types of
LC materials ranging from rod-like via disc-like to globular,4
bent-core mesogens have attracted special attention due to their
unique properties,5 as for example, spontaneous polar order5 and
achiral symmetry breaking with formation of chiral superstruc-
tures despite the molecules being achiral.6 In recent years the
possibility of phase biaxiality in the nematic phases (Nb phases)
of bent core molecules, caused by the restricted rotation of these
molecules around the long axis, was discussed intensively.7–10 Nb
phases have attracted a lot of attention since their theoretical
prediction11 and their first observation in lyotropic LC systems.12
These LC phases would be of fundamental interest for general
soft matter physics as well as for use in display applications with
a magnitude faster expected switching times than the existing
uniaxial nematics.13 Though there have been stimulating simu-
lation results14 and several different synthetic approaches to
discover the illusive biaxial nematic phases by designing different
kinds of molecules,8 solid proof was only provided for polymeric
LC15 and laterally connected oligomesogen16 in temperature
ranges close to the glass transition temperature. However, the
high viscosity of these polymers and oligomers under these
conditions is disadvantageous for investigations and application.
Institute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle/Saale, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 0 345 55 27346
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c1jm13649f
Scheme 1 Different types of bent-core dimesogens.
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mesophases was made on the basis of combined results of optical
textures and X-ray diffraction (XRD). Investigations of oriented
samples were performed using a 2D detector (H1-Star, Siemens
AG). Uniform orientation was achieved by alignment in
a magnetic field (B z 1T) using thin capillaries. The orientation
once achieved is maintained by slow cooling (0.1 K minꢀ1) in the
presence of the magnetic field. Electro-optical experiments have
been carried out using a home built electro-optical setup in
commercially available ITO coated glass cells (E.H.C., Japan,
polyimide coated for planar alignment, antiparallel rubbing,
thickness 6 mm and measuring area 1 cm2).
2. Experimental
2.1. Synthesis
As shown in Scheme 2, the bent-core dimesogens BB1 and BB2
and the bent-core-rod couples BR1–BR4 were synthesized by
hydrosilylation of the bent core compounds B1–B3 with 1,1,3,
3-tetramethyldisiloxane using Karstedt’s catalyst.17,22 The inter-
mediate H-silanes Si1–Si3 were isolated and immediately used23
for the second hydrosilylation step with the olefin-functionalized
bent-core mesogens B1 and B3 to give the bent-core dimesogens
BB1 and BB2, or they were used in hydrosilylation reactions with
the laterally olefin-functionalized rod-like molecules R1 and R2
to give the bent-core-rod couples BR1–BR4. All dimesogens were
purified by column chromatography and the structures were
3. Results and discussion
1
confirmed by H NMR and 13C NMR (see Fig. S1 and S2†).
3.1. Mesophases and transition temperatures
Details of the synthetic procedures and analytical data are
collated in the ESI†. The olefin substituted bent-core mesogen B3
will be reported separately24 and the naphthalene based rod-like
The detailed structures, phase transition temperatures, transition
enthalpies and phase sequences of the synthesized dimesogens
are summarized in Fig. 1–3 and Table 1. All compounds exhibit
liquid crystalline behaviour except the dimesogen BR3 which has
a naphthalene based rod-like unit connected to the bent-core
moiety in a lateral–lateral mode (Table 1). A short monotropic
nematic phase range was observed for the lateral–lateral con-
nected bent-core dimer BB1 (Table 1). All other dimesogens
show broad regions of nematic phases. On cooling from the
isotropic liquid state these LC phases appear with a schlieren
unit R225 was kindly provided by F. Brommel and A. Hoffmann,
€
University Freiburg.
2.2. Investigations
The obtained dimers were investigated by PM (Optiphot 2,
Nikon) in conjunction with a heating stage (FP82HT, Mettler)
and by DSC (DSC-7 Perkin Elmer). The assignment of the
Scheme 2 Synthesis of compounds BB1, BB2 and BR1–BR4; reagents and conditions: (i) H–Me2SiOMe2Si–H, Karstedt’s catalyst, dry toluene, T ¼
25 ꢁC, 12 h, (ii) Karstedt’s catalyst, dry toluene, T ¼ 25 ꢁC, 12 h.22
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Fig. 2 Structures of dimesogens BR1 and BR2 combining a rod-like and
a bent-core unit.
Fig. 1 Structures of the bent-core dimesogens BB1 and BB2.
Fig. 3 Structures of dimesogens BR3 and BR4 combining a naphthalene based rod-like unit with a bent-core unit.
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Table 1 Phase transition temperatures (T/ꢁC), associated enthalpy
values [DH/kJ molꢀ1] and crystallization temperatures (TCr/ꢁC) of the
bent-core dimesogens BB1, BB2 and bent-core-rod couples BR1–BR4a
Comp.
Phase transitions
Tcr
41
35
<20
<20
<20
<20
BB1
BB2
BR1
BR2
BR3
BR4
Cr 84 [54.9] (N 68 [0.2]) Iso
Cr 94 [63.2] (SmC 44 [5.2]) NcybC 117 [1.1] Iso
Cr 53 [22.8] NcybC 71 [0.6] Iso
Cr 51 [11.0] (Colrec 47 [5.4]) NcybC 100 [0.5] Iso
Cr 40 [18.3] Iso
Cr 32 [15.4] (M 22 [3.2]) NcybC 84 [1.4] Iso
Fig. 4 Textures of BR2 as observed between crossed polarizers (a) NcybC
at T ¼ 99 ꢁC and (b) in the Colrec phase at T ¼ 40 ꢁC.
a
Peak temperatures in the DSC thermograms obtained during first
heating and cooling cycles at 10 K minꢀ1, monotropic phase transitions
and crystallization temperatures were taken from the corresponding
cooling scans (10 K minꢀ1); Abbreviations: Cr ¼ crystalline solid; N ¼
nematic phase; SmC ¼ smectic C phase; NcybC ¼ nematic phase
composed of SmC type cybotactic clusters; M ¼ mesophase with
3.2. XRD studies
XRD studies were carried out with magnetically aligned samples
of the dimesogens BB2, BR1, BR2 and BR4. In the temperature
range of the nematic phases all XRD patterns show a diffuse
scattering in the wide angle region with a maximum at d ¼ 0.46–
0.47 nm, indicating fluid LC phases. The maximum is located on
the equator (Fig. 5a and 6a and c) indicating an alignment of the
molecules with the long axes of the aromatic cores parallel to the
magnetic field direction. The diffuse scattering in the small angle
region indicates that there is only short range periodicity as
typical for nematic phases. The intensity of the small angle
scattering is in the same range or larger than the intensity of the
wide angle scattering (Iw # Is), indicating the presence of cybo-
tactic groups.26 This scattering is smeared out to dumbbell-sha-
ped streaks parallel to the equator with four distinct maxima
located beside the meridian (see Fig. 5c and 6d). This shape of the
small angle scattering indicates cybotactic nematic phases
formed by tilted (SmC-like) clusters (NcybC phases).10
unknown structure; Colrec
¼
rectangular columnar phase; Iso
¼
isotropic liquid; for BR1–BR4 compounds no crystallization peak was
observed by DSC down to ꢀ20 ꢁC.
texture containing p/2 and p/4 disclinations and marble domains
as typical for nematic phases. A typical texture is shown in
Fig. 4a for the nematic phase of compound BR2 as example.
For compound BR1 (Table 1), combining a rod-like phenyl-
benzoate unit with the bent-core unit in a side-by-side fashion via
a relatively short spacer unit, the nematic phase can be over-
cooled down to ꢀ20 ꢁC. No transition to a smectic or columnar
phase was observed, despite the incorporation of a bis(dime-
thylsilyl)oxy unit which is expected to have a certain tendency for
micro-segregation.4,17 All other mesogenic compounds with
longer spacer units can also be cooled down below room
temperature without crystallization, but in these cases additional
phase transitions were observed. For BR2, the low temperature
phase is a rectangular columnar phase as evidenced by XRD (see
below) and also by DSC thermograms (Fig. S3a†). The lateral–
terminal connected bent-core dimer BB2 (Table 1) exhibits
a short range SmC phase (Fig. S3b†) on cooling before
crystallization.
For compound BB2 (Fig. 5) the correlation length, estimated
according to xk,t ¼ 2/Dq from the full width at half maximum
(Dq)27 of the small angle scatterings in the transversal direction is
xk ¼ 2.7 nm and in longitudinal direction it is xt ¼ 3.2 nm. The
dimensions of the cybotactic clusters (Lk,t) can be approximated
to Lk,t z 3xk,t,
28 leading to the values L z 8 nm and Lt z 3
k
to 4 nm for the cluster size. Accordingly, the cybotactic clusters
in the NcybC phase of BB2 are composed on average of about 2
layers and about 7–8 aromatic cores are arranged in the cross-
section.26 Similar values were obtained for compounds BR2 and
BR4 (Fig. S5†) (see Table 2). Only for BR1 (Fig. S4†) the cluster
size is smaller than for those of the other compounds (thickness
of only one layer). The temperature dependence of the cluster size
was investigated for compound BR4 and, as expected, the size
decreases with rising temperature.10 The splitting of the small
The highest stability of the nematic phase was observed for
compounds BB2, BR2 and BR4 in which the bent-core unit is
coupled at the end with a laterally attached rod-like unit or
a laterally attached second bent-core unit. In contrast, lateral
attachment of the bent-core unit is generally unfavourable for
mesophase formation giving crystalline material (BR3) or lower
transition temperatures (BB1 and BR1) and shorter temperature
ranges of the N phases.
Fig. 5 (a) Diffraction pattern of a magnetically aligned sample (direction of the magnetic filed is shown as white arrow) of BB2 at 80 ꢁC after
subtraction of the scattering pattern in the Iso phase at T ¼ 120 ꢁC (the inset shows the scattering in the small angle region); (b) q-scans at 100 ꢁC and
80 ꢁC; (c) c-scans over the diffuse small angle scattering (for 2q ¼ 1.5–4.5ꢁ), Irel ¼ I(T)/I(120 ꢁC, Iso).
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monomeric compound B7 (d ¼ 3.8 nm)10 results from the much
larger tilt of the dimesogen, which is nearly double compared to
B7. For the other dimesogens it is more difficult to evaluate the
ratios between d-value and molecular length, as the length of the
two aromatic units is different and for dimesogens with
a connection via the end of the bent core unit it is also difficult to
estimate the effective molecular size as one of the terminal chains
simultaneously acts as a spacer unit and this should have
a significant effect on chain conformation.
As shown in Fig. 6a–d for the XRD patterns of compound
BR2 the intensity of the small angle scattering increases as the
temperature is reduced, indicating increasing cluster size at lower
temperature (see also Table 2). At T ¼ 47 ꢁC a phase transition
takes place (see Fig. 4b for the textural change) and below this
temperature the XRD pattern is significantly changed (Fig. 6e–
g). The wide angle scattering remains diffuse, indicating that the
low temperature phase is also a fluid LC phase. However, the
diffuse small angle scattering becomes a sharp Bragg peak and an
additional reflection occurs on the meridian. This diffraction
pattern can be indexed on a centred rectangular lattice (c2mm)
with the parameters a ¼ 5.05 and b ¼ 5.81 nm (T ¼ 40 ꢁC). The
diffuse wide angle scattering does not split at this phase transi-
tion and remains centered on the equator. Therefore, the
aromatic cores should be organized with their long axes parallel
to the direction b (Fig. 6h). This is also in agreement with the
increase of the d-value for the small angle scattering from d ¼ 4.5
nm in the nematic phase to d ¼ 5.81 nm in the low temperature
phase. This alignment of the cores and the presence of a rectan-
gular 2D lattice indicate that the molecules are organized
perpendicular to the (broken) layers of a modulated smectic
phase. The parameter a/2 corresponds to the lateral distance
between ribbon-like blocks of molecules and indicates that the
diameter of each ribbon is about 2.5 nm, which correspond to
roughly five aromatic units arranged side by side. This is less than
half the value of the estimated lateral dimension Lt ¼ 6.6 nm of
the cybotactic clusters in the nematic phase at T ¼ 60 ꢁC. Hence,
at this transition the cybotactic clusters of the NcybC phase
change their shape and form quasi-infinite ribbons which orga-
nize on a c2mm type 2D lattice (see Fig. 6h).
It is a bit surprising that the relatively strong tilt in the SmC-
clusters is completely removed at this phase transition.29 It is not
clear if in the ribbons the molecules adopt polar order along
direction c, with antiparallel alignment of the polar direction
between adjacent ribbons, as typically observed for B1rev
Fig. 6 XRD investigation of an aligned sample of compound BR2 (a
and b) NcybC phases at T ¼ 90 ꢁC; (c and d) at T ¼ 60 ꢁC and (e and f)
Colrec phase T ¼ 40 ꢁC; (a, c and e) show the complete diffraction
patterns, (b, d and f) the small angle region; (g) q-scan of diffraction
pattern shown in (f), and (h) model showing the proposed organization of
bent-core units (rotationally disordered around their long axes) on
a rectangular lattice.
Table 2 XRD data of the nematic phases of compounds BB2, BR1, BR2
and BR4 and the reference compound B710 having the same structure of
the bent-core mesogen as used in compound BB1, but with two C7H15
chains instead of the OC6H13 chains
angle scattering Dc/2 is between 35ꢁ (BR4) and 50ꢁ (BR1) indi-
cating that there is a significant tilt of the molecules in the SmC-
like clusters.10 The position of the maximum of the small angle
scattering is between d ¼ 3.2 nm for the bent-core dimesogen
BR1 and d ¼ 4.6 for the mixed dimesogen BR2. The value of d ¼
3.2 nm is in good agreement with the distances typically observed
for the small angle reflection in the nematic phases of comparable
single mesogens (e.g. compound B7 in Table 2). The difference
between the d-values of the dimesogen BR1 (d ¼ 3.2 nm) and the
Comp.
T/ꢁC
d/nm
Dc/2/ꢁ
d/nm
Lt/nm
L /nm
k
BR4
BR4
BR4
BR2
BR2
BR1
BB2
B710
80
60
40
90
60
65
80
70
4.1
4.5
4.4
4.6
4.5
3.2
4.2
3.8
35
38
39
42
44
50
39
26
0.47
0.46
0.46
0.47
0.47
0.47
0.46
0.45
5.1
6.5
7.2
5.8
6.6
3.0
3.5
3.7
6.6
9.0
10.3
9.3
8.8
3.5
8.2
14.0
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This would mean that the laterally connected bent-core dimes-
ogens reported herein do not provide that degree of coupling
which would be required for phase biaxiality to occur. Probably,
by reduction of the spacer length and spacer flexibility a stronger
coupling could be achieved, which is presently under
investigation.
Acknowledgements
This work was supported by the EU within the FP7 funded
Collaborative Project BIND (Grant No 216025); we are grateful
€
to F. Brommel and A. Hoffmann, University Freiburg for
Fig. 7 Electrohydrodynamic instabilities as seen for the nematic phase
of BR2 in a 6 mm polyimide coated ITO cell (a) at T ¼ 75 ꢁC, 10 Hz,
160 Vpp and (b) at T ¼ 75 ꢁC, 10 Hz, 200 Vpp.
providing us compound R2.
phases,30 or if the bent aromatic cores are rotationally disordered
around their long axes. As in electro-optical investigations no
polarization current peak could be recorded (see below) and in
compound BR2 the bent-core mesogens are diluted in the ratio
1 : 1 with rod-like mesogens it appears more likely that polar
order is absent (see Fig. 6h).
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First examples of laterally connected bent-core dimesogens and
dimesogens combining a bent-core unit with a rod-like unit have
been synthesized and investigated. These dimesogens have broad
nematic phase ranges (enantiotropic and monotropic) and low
crystallization tendency if they are lateral–terminal connected,
whereas a lateral–lateral connection reduces the nematic phase
stability. Compounds with short end-chains and short spacer
units can provide nematic phases at ambient temperature, which
do not show transitions to other LC phases down to ꢀ20 ꢁC,
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174 | J. Mater. Chem., 2012, 22, 168–174
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