Intercalated liquid-crystalline phases formed by symmetric dimers with an α,ω-diiminoalkylene spacer

Article abstract of DOI:10.1039/b612517d

Four series of symmetric, dimeric molecules with flexible α,ω-diiminoalkylene spacers have been synthesised and characterised by polarising optical microscopy, differential scanning calorimetry, and X-ray diffraction. The effects on the mesomorphism of th

Full text of DOI:10.1039/b612517d

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www.rsc.org/materials | Journal of Materials Chemistry
Intercalated liquid-crystalline phases formed by symmetric dimers with an
a,v-diiminoalkylene spacer{
a
a
b
b
c
d
ˇ
Maja Sepelj, Andreja Lesac,* Ute Baumeister, Siegmar Diele, H. Loc Nguyen{ and Duncan W. Bruce*
Received 30th August 2006, Accepted 11th December 2006
First published as an Advance Article on the web 9th January 2007
DOI: 10.1039/b612517d
Four series of symmetric, dimeric molecules with flexible a,v-diiminoalkylene spacers have been
synthesised and characterised by polarising optical microscopy, differential scanning calorimetry,
and X-ray diffraction. The effects on the mesomorphism of the various alkylene spacer (number
of methylene units m = 5 to 8) as well as the terminal alkoxy chain length (n = 4 to 14) on the
mesomorphic properties have been studied. The compounds with an even-numbered spacer
display, in addition to a nematic phase, various smectic phases whose nature depends on the ratio
˜
between the length of the terminal chain and the length of the spacer (n/m). A modulated SmA phase
was found for the ratio n/m = 1 to 1.6 and a SmC/crystal G polymorphism emerges at n/m . 1.7. In
contrast, all liquid-crystalline dimers with odd-numbered spacers display an intercalated B6 phase
regardless of the chain length. The preference for intercalated structures is discussed based on
dipolar considerations following electronic structure calculations using a model molecule.
Introduction
Dimeric liquid crystalline compounds, in which two mesogenic
units are linked by a flexible spacer, have attracted consider-
able attention due to their ability to serve as model compounds
for liquid-crystalline polymers and their rich and unusual
mesomorphism, which differs from that of the corresponding
monomers.1–4 The conformation and the mesomorphic beha-
viour of such compounds are influenced significantly by the
spacer parity,3–5 which affects the mutual orientation of the
mesogenic units, resulting in a linear conformation for an even
number and a bent-shaped motif for an odd number of
methylene units if an all-trans spacer conformation is assumed.
In addition to nematic, smectic A and smectic C phases have
been frequently observed for the majority of dimers. Within
the smectic phase, dimeric molecules can be considered as
adopting one of two plausible arrangements (Fig. 1).6 In the
first (Fig. 1a) the molecules intercalate forming a phase in
which terminal chains and the spacers are mixed randomly and
the layer spacing is approximately half the molecular length. In
the second arrangement (Fig. 1b), the molecules are segregated
Fig. 1 Diagram to show the two possible arrangements of the dimeric
mesogens into layers: (a) intercalated and (b) non-intercalated.
a
-
´
ˇ
Ruder Bosˇkovic Institute, Bijenicka cesta 54, 10000, Zagreb, Croatia.
E-mail: alesac@irb.hr; Fax: +385 14680108
forming three distinct regions containing mesogenic cores,
terminal chains and spacers giving rise to, in effect, a smectic
phase in which the layer spacing corresponds to the molecular
length. Generally the intercalated structures are typical for
non-symmetric dimers (two different mesogenic groups) while
the vast majority of symmetric dimers display phases with
layer spacings approximately equal to the molecular length.
However, Watanabe et al.7–9 observed both types of smectic
structures for symmetric dimers: an intercalated arrangement
when the length of the terminal chains (n) is smaller than, or
comparable to, the length of the spacer (m), and a non-
intercalated arrangement when the terminal chains are longer
than the spacer. The latter situation becomes very interesting
bInstitute of Physical Chemistry, Martin-Luther-Universita¨t Halle-
Wittenberg, Mu¨hlpforte 1, 06108, Halle, Germany
cDepartment of Chemistry, University of Exeter, Stocker Road, Exeter,
UK EX4 4QD
dDepartment of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: d.bruce@exeter.ac.uk; Fax: +44 1904 432516
{ Electronic supplementary information (ESI) available: X-ray pat-
terns of 8-12 (Fig. S1) and 6-12 (Fig. S2); x-scans for the 2D X-ray
patterns of the SmC phase of 8-14 (Fig. S3); short take of the powder-
like X-ray pattern for the lower temperature monotropic phase of 8-14
(Fig. S4); temperature dependence of h and FWHM of 7-4, and
x-scans for the 2D X-ray patterns of 7-4 (Fig. S5); temperature
dependence of d-values for 7-10 (Fig. S6); colour version of Fig. 4
(Fig. S7). See DOI: 10.1039/b612517d
{ Present address: Department of Chemistry, University Science
Laboratories, South Road, Durham, UK DH1 3LE.
1154 | J. Mater. Chem., 2007, 17, 1154–1165
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particularly when it arises from dimers with an odd-numbered
spacer due to potential antiferroelectric switching properties.10
Recently, we described the properties of a range of odd- and
even-membered dimeric molecules in which a salicylaldimine
core was linked to the alkylene spacer via an imino group.11
The mesomorphic behaviour of the even members was charac-
teristic for symmetric dimers and, depending on the n/m ratio,
they exhibited a nematic and a non-intercalated SmC phase. In
contrast, the dimers with an odd-numbered alkylene spacer
chain having the same n/m ratio displayed columnar rectan-
gular phases instead of the non-intercalated SmC phase. In
continuation of our investigation of symmetric liquid-crystal-
line dimers, here we report the mesomorphic properties of a
new series of dimeric molecules in which the salicylaldimine
core has been replaced by the benzoyloxybenzylidene group.
The absence of intramolecular hydrogen bonding both reduces
the molecular width and increases the flexibility of the mole-
cule, the latter allowing better inter-mesogen interactions
which, in the case of materials with odd-numbered spacers,
might lead to the non-intercalated structures with antiferro-
electric properties.
scanning calorimetry and X-ray diffraction. The phase
transition temperatures and the corresponding enthalpy
changes are collected in Tables 1 and 2.
Mesomorphism of 6-n and 8-n. The data listed in Table 1
reveal that all the dimers with an even-numbered methylene
spacer display mesomorphism in accordance with their overall
linear shape. In both series, the lower homologues (n = 4, 6)
exhibit solely a nematic phase, identified by its characteristic
schlieren and marbled textures. Increasing the chain length
resulted in the additional appearance of various smectic
phases. The compounds with intermediate chain lengths
showed a smectic A-type phase, established from optical
textures obtained on slow cooling of the preceding nematic
phase. The texture obtained from the schlieren nematic
displayed both focal conic and homeotropic domains. In
addition, contact preparation between the smectic A phases of
ethyl-(4-ethoxybenzal)-49-aminocinammate12 and compounds
6-8, 8-8 and 8-10 respectively, confirmed complete miscibility
of the two phases. Compounds 6-10 and 8-12 represent
the highest homologues that display a smectic A phase. On
cooling 6-10, the monotropic phase below the nematic phase
displayed a fan-like texture, which closely resembles that of a
SmA phase (Fig. 2).
Results and discussion
Synthesis
The presence of a smectic A phase is supported by X-ray
measurements. The two-dimensional diffraction patterns
obtained from samples aligned in a magnetic field clearly
showed a two-dimensional modulation of the centred rectan-
gular type with unit cell parameters a = 4.3 nm corresponding
to the periodicity of the modulation perpendicular to the layer
normal and b = 5.7 nm corresponding to a layer distance that
is in excellent agreement with the molecular length of 5.8 nm as
determined from CPK models for a fully stretched molecule
(terminal chains and spacer in all-trans configuration) measur-
ing along a line connecting the centres of the rings. Thus, the
The synthesis of all the compounds was carried out following
Scheme 1. All Schiff bases were prepared in good yields of
60 to 80% by addition of a,v-diaminoalkane to the corre-
sponding aldehyde 3a–f according to the procedure described
previously.11 The dimers were identified by H and 13C NMR
1
spectroscopy, while purity was established by elemental
analysis.
Mesomorphism
˜
phase is of the type SmA, namely a modulated SmA phase
The mesomorphic behaviour of the new compounds was
investigated by polarising optical microscopy, differential
(Fig. 3b, Table 3). Estimating the molecular volume in the
Scheme 1 Synthetic route for the compounds m-n (m = 5,6,7,8, n = 4 to 14).
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Table 1 Transition temperatures (uC), enthalpies (kJ mol21) in italics and the dimensionless value of DS/R in square brackets for compounds 6-n
and 8-n
˜
SmA
Dimer
6-4
Cr
N
G
SmB
SmC
N
N
Iso
164.4
56.5[16]
141.4
47.4[14]
133.2
51.1[15]
119.6
51.4[16]
102.7
38.8[12]
113.8
64.3[20]
128.4
15.4[5]
135.8
51.6[15]
128.3
33.5[10]
117.2
37.9[12]
111.4
74.7[23]
98.5
45.6[15]
182.0
9.5[3]
164.2
8.7[2]
154.3
9.5[3]
144.6
10.8[3]
138.4
11.5[3]
N
N
N
N
N
N
N
N
N
N
N
N
6-6
N
N
6-8
N
(N
(N
133.2)
N
20.9[0.3]
116.2)
20.2[0.1]
6-10a
6-12b
6-14
8-4c
8-6
N
N
N
(N
(N
94.3)
N
N
123.9
2.8[1]
135.1
22.4[7]
N
26.7[2]
94.4)
26.1[2]
N
N
N
N
N
N
N
N
164.7
9.5[3]
151.2
9.4[3]
138.9
7.8[2]
130.8
10.5[3]
124.2
12.1[3]
118.9
11.0[3]
N
8-8d
8-10
8-12
8-14e
a
N
N
N
N
132.9
0.4[0.1]
125.3
1.2[0.4]
107.2)
20.4[0.1]
N
N
(N
96.1
20.2[0.1]
N
(N
94.4)
27.3[2]
N
114.9
3.0[1]
Cr–Cr transition at 114 uC, DH = 0.8 kJ mol21
.
Cr–Cr transition at 79 uC, DH = 59.7 kJ mol21
.
Cr–Cr transition at 124 uC, DH =
b
c
27.6 kJ mol21
.
Cr–Cr transition at 114 uC, DH = 0.5 kJ mol21
.
Cr–Cr transition at 91 uC, DH = 85.5 kJ mol21
.
d
e
phase according to Immirzi and Perini13 gives a value of about
1.23 nm3. Assuming a unit cell volume of 11.4 nm3 calculated
from the two-dimensional modulation parameters with the
third dimension taken as the average repeating distance for
neighbouring molecules estimated at 0.47 nm from the outer
scattering, then there are, on average, 9.2 molecules in the unit
cell, i.e. between 4 and 5 molecules within one block. These
values correspond to a packing coefficient k = 0.7, a medium
value in the crystalline state.13,14 For the isotropic liquid with
k # 0.55,14 3.6 molecules would fill the same volume of one
˜
block. Since the exact density of the compound in the SmA
phase is not known we assume an intermediate value of about
four molecules in the building block and eight molecules per
unit cell. Approximately the same number results from a
purely two-dimensional estimation. Thus, if the molecules are
considered to be oriented parallel to the layer normal (in this
case: along the b-axis) and the average lateral repeat distance is
0.47 nm (see above), then about 9.1 molecules are needed to fill
the cell in the a-direction. The extension of the reflections
Table 2 Transition temperatures (uC), enthalpies (kJ mol21) in italics
and the dimensionless value of DS/R in square brackets of homologues
series 5-n and 7-n (B69 denotes low temperature B6 phase)
DimerCr
B69
B6
Nx
N
Iso
5-4
N
N
N
N
N
N
N
N
N
N
N
N
114.0
49.5[15]
123.5
63.8[19]
94.2
56.8[19]
88.5
54.4[18]
96.2
78.6[26]
101.2
104.0[33]
112.2
45.1[14]
96.4
32.1[10]
111.2
53.5[17]
100.1
40.6[13]
90.9
63.7[21]
73.3
58.4[19]
(N 99.1
N 102.0)
20.2[0.1]
N
N
N
N
N
N
N
N
N
N
N
N
26.4[2]
(N 116.9)
211.2[3]
121.0
5-6
5-8
N
N
16.4[5]
109.5
15.5[5]
5-10
5-12
5-14
7-4
(N 96.1)
213.8[5]
(N 84.4
N
96.6)
N 115.0
22.5[0.1] 20.5[0.2] 0.4[0.1]
7-6a
7-8
N
N
N
114.7
12.5[4]
119.5
17.1[5]
110.1
7-10
7-12
7-14b
a
(N 93.7)
20.2[0.1] 17.6[6]
N
99.3
16.0[5]
b
Cr–Cr transition at 87.8 uC, DH
=
1.7 kJ mol21
.
Cr–Cr
˜
Fig. 2 Photomicrograph of the fan-like texture of the smectic A
transition at 91 uC, DH = 8.0 kJ mol21
.
phase of 6-10 obtained at 112 uC.
1156 | J. Mater. Chem., 2007, 17, 1154–1165
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Table 3 X-Ray data from Guinier powder patterns and from aligned
samples (hobs: experimental scattering angle; dobs: experimental d
spacing; n/hk: assigned order of reflection/indices for 2D phases.
Parameters: lattice parameters or d values with an error of the
calculated parameters in the order of 0.1 nm, L: molecular lengths
measured from CPK models, fully extended chains, zigzag shape for
compounds with even-membered spacers, banana shape for those with
odd-membered ones)
Compound T/uC Phase hobs/u dobs/nm n/hk Parameter/nm L/nm
˜
108 SmA 1.296 3.41
6-10
8-12
6-12
8-14
11 a = 4.27
02 b = 5.66
11 a = 5.20
02 b = 6.30
5.8
6.5
6.3
6.9
1.563 2.83
105 SmA 1.100 4.01
˜
1.400 3.15
110 SmC 1.112 3.97
2.225 1.98
110 SmC 0.988 4.47
1.975 2.24
1
2
1
2
1
d = 3.97
d = 4.47
d = 2.34
5-8
7-4
105 B6
100
88 Nx
100 B6
85 B69
96 B6
1.888 2.34
2.081a 2.12a
2.148a 2.06a
4.6
3.9
N
7-10
1.70
1.68
1.60
2.59
2.62
2.76
1
1
1
d = 2.59
d = 2.62
d = 2.76
5.0
5.7
7-12
a
For the maximum of the diffuse scattering in the small angle
region.
˜
For compound 8-12 the SmA phase was established based
on a fan-shaped and a homeotropic texture. On cooling from
˜
the schlieren texture of the nematic phase, the SmA phase
appeared as a fan-shaped texture. Further cooling results in a
˜
SmA to SmB transition, which can be recognised by the loss of
fine details on the fluid fan texture on re-heating (Fig. 4).
As found already for compound 6-10, X-ray measurements
˜
on 8-12 confirmed the formation of the SmA phase, the
molecular length of about 6.5 nm as measured from CPK
models being in good agreement with the cell parameter b =
6.3 nm in the direction of the long molecular axes (Fig. S1{
and Table 3). From the same calculations as in the case of 6-10
an average number between 9.0 and 11.4 molecules per three-
dimensional unit cell (molecular volume 20.0 nm3 and 15.7 nm3
for k = 0.55 and 0.7, respectively, and d = 0.48 for the wide-
angle scattering) and 10.8 molecules per two-dimensional unit
cell results for 8-12. On cooling in real time an image of a
pattern that could be attributed to a more ordered mesophase
was observed for
a short time on the monitor, but
unfortunately, the alignment was lost and no recording was
possible, because the sample rapidly crystallised, so a unique
determination of the phase type was not possible from the
X-ray patterns.
Fig. 3 X-Ray patterns of 6-10 aligned in the magnetic field on cooling
Compounds 6-12 and 8-14 display three different phase
transitions. Enantiotropic N and SmC phases were identified
according to their characteristic schlieren textures and, during
transition from N to SmC, a typical fingerprint texture was
observed. X-Ray measurements performed on both com-
pounds showed the scattering typical of a nematic phase
with cybotactic SmC groups on cooling when aligned in the
magnetic field (Fig. 5, Fig. S2{). The measurements at 100 uC
confirmed the presence of a SmC phase with molecules tilted
strongly with respect to the layer normal (tilt angle 45u for
8-14, see Fig. S3{).
from the isotropic liquid: a) nematic phase with cybotactic groups at
˜
130 uC; b) SmA phase at 108 uC. c) Proposed molecular packing mode
˜
in the SmA phase of the dimers.
perpendicular to the layer normal shows that the correlation
length in the a-direction is much smaller than that for the
layers themselves, so that the order is probably only short-
range. A model of the molecular packing in such a phase is
given in Fig. 3c. The diffuse, small-angle scattering in the
preceding nematic phase (Fig. 3a) can be attributed to
cybotactic groups representing fluctuating arrays of molecules
with a short-range order of the same type as the long-range
Further cooling resulted in the appearance of a highly
ordered monotropic phase whose texture depended strongly on
˜
order in the SmA phase.
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Fig. 4 Photomicrographs of compound 8-12 obtained on cooling: (a)
˜
fan-shaped texture of the SmA phase at 106 uC; (b) fan-shaped texture
of the SmB phase at 96 uC.
Fig. 5 X-Ray patterns of 8-14 aligned in the magnetic field on cooling
from the isotropic liquid: a) nematic phase with cybotactic groups of
the SmC type at 115 uC; b) SmC-type phase at 100 uC, the sample is
fibre-like disordered around an axis parallel to the meridian of the
pattern with one preferred orientation.
the texture of the preceding SmC phase. Thus, on cooling, the
monotropic phase appeared either as a mosaic texture if from a
schlieren SmC texture (Fig. 6a and b) or as an arced, broken
fan texture, if from a broken fan-shaped texture (Fig. 6c and d).
As in the cases of 6-10 and 8-12, rapid crystallisation prevented
a clear recording of the X-ray diffraction pattern of these
phases, but the textural behaviour in conjunction with the
image of a highly ordered smectic phase observed for a short
time during X-ray investigation (see Fig. S4{) implies the
presence of a crystal G mesophase.
and 7-n) show completely different mesomorphic behaviour
(Table 2).
In both series, 5-n and 7-n, a nematic phase was observed
only for the shortest homologues, 5-4 and 7-4. Microscopic
observation of 5-4 revealed the presence of the nematic phase
characterised by schlieren and marble textures. On further
cooling the schlieren texture turned into fans without features
characteristic of a N-to-SmA or a N-to-SmC transition.
Applying mechanical stress resulted in a schlieren texture with
two and four brushes. Similar textural characteristics were
observed for the phases displayed by the higher homologues of
both series and the miscibility studies performed by contact
preparation reveal that they displayed the same phase.
However, as the higher homologues do not exhibit a nematic
phase this new liquid-crystalline phase separates out from
the isotropic liquid in the form of baˆtonnets which then
coalescence and form a focal conic fan texture (Fig. 7a).
Compound 6-14, in which the lengths of the chains are more
than twice the length of the spacer, is the only compound that
does not show a nematic phase. Instead, a direct transition
from the isotropic liquid to a smectic C phase was observed.
On cooling to a lower-temperature monotropic phase the
schlieren texture transformed into a mosaic-like one while the
broken fans became striped. The phase is again assigned as
crystal G based on direct analogy with the textural behaviour
displayed by compounds 6-10 and 8-14.
Mesomorphism of 5-n and 7-n. Comparing to the even dimers
the compounds with odd-numbered spacers (compounds 5-n
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Fig. 6 Photomicrographs of compound 8-14 obtained on cooling: (a) schlieren texture of the SmC phase at 108 uC; (b) mosaic texture of the
crystal G mesophase at 94 uC; (c) broken-fan texture of SmC phase at 96 uC; (d) arced-broken fan texture of the crystal G phase at 94 uC.
Shearing this specimen led again to a grey, schlieren-like
texture with singularities of S = ¡1 and ¡K (Fig. 7b) which
have also been observed in antiferroelectric SmC,15 inter-
calated SmC phases of mesogenic twins2,4,7,16–18 or the B6
phase of bent-core mesogens.19,20 The occurrence of such
singularities is attributed to an opposite tilt direction of the
mesogenic groups between adjacent layers.2,16,21
showed that it was impossible to obtain a homeotropic texture
either by shearing or by surface treatment, which implies a
phase with tilted molecules, which can be characterised as a
SmCc phase.22 Furthermore, the miscibility studies performed
between the derivative of 1,3-diaminobenzene 1S, for which a
tilted, intercalated B6 phase was established,18 and homologue
7-8 showed continuous miscibility across the full composition
range between 1S and 7-8 for the B6 phase, confirming that the
odd dimers, indeed, display the B6 phase, Fig. 9.
In order to clarify the structure of this phase X-ray
measurements were performed on compounds 5-8 and 7-10.
They showed very similar patterns for both compounds, as
expected. Thus, layer reflections for the liquid-crystalline
phase observed in the Guinier powder patterns at 2.34 nm
for 5-8 and at 2.6 nm for 7-10 correspond to half the molecular
length, suggesting an intercalated structure like B6. The B6
phase as described up to now in the literature is found to be
tilted with a tilt angle of about 10–30u (tilt direction is
perpendicular to the plane described by the bent-core
molecules in the smectic layer).19,20 Therefore aligned samples
have been investigated, too (Fig. 8). In contrast to the
alignment in the smectic phases of the even-membered
compounds, the molecules adopted the so-called bookshelf
orientation in this phase when surface aligned, which means
that the layer normal is perpendicular to the meridian of the
pattern and the layer reflections lie at the equator. The
partially aligned samples showed the tilt angle to be small,
below 15u. According to these X-ray patterns no unique choice
can be made between the two possibilities of a small tilt or an
orthogonal alignment, respectively. Microscopic observation
Thus, the B6 phase is observed for all liquid-crystalline
dimers with an odd-numbered alkylene spacer, the only
exceptions being the highest homologues which show only a
transition from crystal to isotropic liquid.
In addition to the already established nematic and B6
phases, two homologues of series 7-n display peculiar phase
behaviour. Thus, the shortest homologue 7-4 has three
different phases, displaying an enantiotropic nematic and
two monotropic phases. On cooling from the isotropic liquid,
the nematic phase showed typical marbled and homeotropic
textures. Below this nematic were two monotropic phases and
the transition to the higher-temperature of these was
accompanied by a change in texture to a fan-like arrangement
(Fig. 10a) that changed into oily streaks on shearing the
sample (Fig. 10b). However, the intriguing parts of the fan-like
texture are spiral domains whose origin we cannot currently
explain (Fig. 10c), although we might speculate on whether
this effect is related to escape from polar order, given the
discussion below on the fairly strongly dipolar nature of these
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Fig. 7 Photomicrographs of compound 5-12: (a) fan-like texture
of the B6 phase; (b) schlieren-like texture of the B6 phase obtained
after shearing.
compounds. We also note in passing that Go¨rtz and Goodby23
recently reported spontaneous chiral separation in a phase
below the nematic of some bent-core oxadiazole mesogens
(although the optical textures were quite different) and that
spontaneous symmetry breaking is a well-known phenomenon
in certain phases of bent-core materials.22
Fig. 8 X-Ray patterns of 5-8 and 7-10 partially aligned at the sample/
air interface in the B6-type phase on cooling: a) 5-8 at 111 uC; b) 7-10
at 100 uC.
texture with two and four brushes was observed, Fig. 10e. We
believe, therefore, that the low-temperature, monotropic
phase is the B6 phase, identical to that displayed by the higher
homologues of the same series. It is then interesting to
speculate on whether the monotropic Nx phase can be
regarded as a precursor of the B6 phase, in which bundles of
intercalated, bent-shaped molecules without long-range order
are formed; this would be in line with the small enthalpy (DH =
22.47 kJ mol21) for the Nx-to-B6 transition.
The X-ray patterns show only diffuse scattering between
y107 and y85 uC in a manner typical of nematic phases
(Fig. 11). The temperature dependence of the position and
width of the small-angle scattering measured on a powder-like
sample with a position-sensitive detector gave only a weak
indication of a phase transition between 92 and 96 uC on
cooling (Fig. S5{), but the intensity and the position of this
scattering with a maximum at d = 2.1 nm in the lower-
temperature nematic phase (Fig. 11b) implies a correlation of
the electron density maxima approximately orthogonal to the
lateral distances of the molecules with short-range character.
Although the textural behaviour resembles closely that
observed previously for the Ncol phase,24,25 the d-value is only
about half the molecular length, and the simple model of an
Ncol phase of bent-core mesogens cannot be applied.
Compound 7-10 is the only dimer with an odd number of
methylenes in the spacer to display smectic polymorphism
and the DSC trace of 7-10 revealed the presence of two
mesophases. Combination of the microscopic observations
and X-ray measurements revealed that the high-temperature,
enantiotropic mesophase is the B6 phase (Fig. 8b). The low-
temperature monotropic phase occurred during the cooling
cycle at 93.7 uC and the transition was accompanied by a small
change in enthalpy of 20.24 kJ mol21. The phase showed a
focal-conic fan texture crossed with light bands and a schlieren
texture (Fig. 12c and d). The X-ray measurements did not
show any significant change in the temperature dependence of
The lower monotropic phase existed over a short tempera-
ture range and, again, during the X-ray investigation the
sample crystallised. On cooling from Nx to this phase, very
small fans developed that almost looked like a mosaic texture,
Fig. 10d. However, on shearing the sample a schlieren-like
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the quasi-long range order from a sinusoidal modulation of the
electron density along the layer normal found in the higher
temperature phase (where only the first order is observed), to
more strictly defined layers with a more pronounced segrega-
tion of the aromatic and aliphatic parts of the molecules within
the intercalated structure. The small enthalpy change of
20.24 kJ mol21 is in agreement with such a small change in
the molecular arrangement and its non-zero value allows for a
transition with no change in overall symmetry. In Table 2, the
high- and the low-temperature mesophases are assigned as B6
and B69 respectively.
The results presented above show a remarkable effect of the
imino linkage group as well as of the chain length and the
spacer parity on the mesomorphic behaviour. The compounds
with an even number of methylene units in the spacer display
mesomorphism in accordance with their overall linear shape.
A comparison of the mesomorphic behaviour of the series 6-n
and 8-n (Table 1) reveals that the nature of the phase formed
strongly depends on the ratio between the length of the
terminal chain and the length of the spacer (n/m).
Thus, all the compounds of the series 6-n and 8-n, except
6-14 for which n/m . 2, show nematic phases. Among these,
for the compounds with n/m ¡ 1 only the nematic phase was
observed. Increasing the terminal alkoxy chain resulted in the
appearance of an additional smectic phase and the smectic
˜
polymorphism. Thus, the SmA phase was found for the ratio
Fig. 9 Miscibility phase diagram between 1S and 7-8.
1 ¡ n/m ¡ 1.6, while a further increase in the ratio (n/m . 1.7)
resulted in the observation of SmC and crystal G phases. An
intercalated SmAc phase in which the molecules are arranged
in such a way that the spacer and terminal chains are mixed
randomly was observed for the even symmetric dimers having
terminal chains equal to or shorter than the spacer.7 The
presence of an intercalated layer structure has been explained
the layer distance on cooling, including at the transition from
the B6 to the lower-temperature mesophase and indeed
until the sample crystallised (Fig. S6{). On the other hand,
the appearance of second- and third-order layer reflections in
the lower temperature phase (Fig. 13) suggests a change in
Fig. 10 Photomicrographs of compound 7-4: (a) fan-like texture of the monotropic high-temperature phase; (b) oily streaks texture of the
monotropic high-temperature phase obtained after shearing the fan-like texture; (c) texture of the monotropic high-temperature phase containing
spiral domains with both handednesses; (d) fan-like texture of the monotropic low-temperature phase; (e) schlieren-like texture of the monotropic
low-temperature phase.
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expectations, the mesomorphic behaviour of these dimers
appears almost insensitive to the chain length and all except
the highest homologues display the intercalated B6 phase. This
phase persists until the chains become nearly twice the length
of the spacer and then, instead of forming the segregated
non-intercalated structures, the liquid crystallinity is lost. In
addition to the B6 phase, only the lowest odd homologues
display a nematic phase.
A comparison of the thermodynamic data between the
lowest homologues (n = 4) of each series reveals a strong odd–
even effect. According to the model developed by Luckhurst
et al.,26 the magnitude of the effect depends strongly on the
nature of the link between the mesogenic unit and the alkylene
spacer and, with increasing molecular biaxiality, the value of
DSNI/R decreases. The very tiny values of DSNI/R observed
here point to the presence of a highly bent conformer whose
geometry is imposed by the imino linkage group and odd
parity of the spacer. Therefore, the dimers with odd-numbered
spacers described here can be regarded as true, bent-shaped
molecules that display a ‘B-type’ phase characteristic for bent-
core molecules containing a rigid angular central unit.
The results presented above show that the dimers described
here display strong affinity for intercalated structures regard-
less of the spacer parity and it has been suggested that a
specific, dipolar interaction is required between two mesogenic
groups in order to stabilise the intercalated arrangement.27 In
consideration of this hypothesis, we carried out gas-phase
electronic structure calculations using density functional
theory28 with the B1LYP functional29,30 and the 6-31G(d,p)
basis set on the model compound, M (Fig. 14), that represents
one half of a dimer.
Since the most critical fragment in the structure is the ester
linkage, we performed a calculation of the energy as a function
of the dihedral angle (w) C1–C2–O3–C4. The results revealed
that the molecule has a great degree of rotational freedom
having the rotational barriers within 5 kJ mol21. Rotation
around the C2–O3 bond gives two shallow minima that differ
in energy by only 1.3 kJ mol21. Both minima correspond to the
geometry in which the phenyl ring can make any angle within
30u with the plane of the benzoate group without significant
increase in energy. However, according to the energy content,
the conformer having the dihedral angle w = 180u is more
stable than the conformer where w = 10u. As a result of
different dihedral angles, the geometries of the conformers can
be distinguished by the directions of the carbonyl and imino
groups relative to one another and these can plausibly be
assigned as anti for the former and syn for the latter. The
calculation of dipole moment for each conformer revealed that
the total dipole moment for the twisted, syn conformer (3.34 D)
is much higher than for the planar, anti conformer (2.69 D). In
addition, the results shows that the dipole origin is in the mass
centre of the molecule and points toward the imino linkage
group at an angle of 34u, as a consequence of which the
strongest contribution is along the x-axis, which is parallel to
the principal axis of the molecule (Fig. 15).
Fig. 11 X-Ray patterns of 7-4 aligned in the magnetic field on
cooling from the isotropic liquid: a) N phase at 100 uC, b) the lower
temperature nematic phase at 90 uC.
previously in terms of either an electrostatic, quadrupolar
interaction, an increase in entropy gained from the homo-
geneous mixing of the mesogenic moieties, or by excluded-
volume or space-filling constraints.4 In our case, the length of
the terminal chains exceeds the length of the spacer, and the
splitting of the layers into blocks with a width of only a few
molecules in a non-intercalated SmA arrangement and SmAc
interfaces implies the presence of strong, lateral interactions
between the mesogenic units that compete with unfavourable,
space-filling interactions caused by a mismatch in the lengths
of the chains and the spacer. Increasing n/m beyond 1.7
enhances unfavourable interactions caused by the inability of
the spacer to accommodate two, long chains and destabilises
the intercalated arrangement. As a result, the molecules
segregate into a non-intercalated smectic phase containing
three, distinct regions: mesogenic units, spacers and terminal
alkoxy chains.
From the experimental data for the odd dimers (5-n and
7-n), the layer distance that corresponds to 1/2L of the highly
bent conformer and very tiny values of DSNI/R point strongly
to the presence of the syn conformer in the liquid-crystalline
The tendency toward intercalation is even more pronounced
for the dimers with an odd-numbered spacer. Contrary to
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Fig. 12 Photomicrographs of compound 7-10: (a) fan-like texture of the B6 phase; (b) schlieren-like texture of the B6 phase; (c) focal-conic fan
texture of the monotropic B69 phase; (d) schlieren-like texture of the monotropic B69 phase.
state and, consequently, to stronger dipole–dipole interactions.
Taking into account more than one molecule in the side-by-
side arrangement, the electrostatic repulsion between two
dipoles is minimised by their antiparallel orientation.
Application of this model to the layer arrangement of the
dimeric molecules indicates strongly that the compensation
of the dipoles is a driving force for the formation of an
intercalated structure.
Fig. 14 Half-dimer molecule M used in the DFT calculations.
Fig. 15 Ball-and-stick presentation of the syn conformer obtained
Fig. 13 X-Ray pattern of 7-10 partially aligned at the sample/air
interface in the low-temperature B69 phase at 81 uC on cooling.
from electronic structure calculations showing the projection of the
dipole moment (arrow).
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MB 102 spectrophotometer; absorption maxima are given
in cm21. Phase transition temperatures and textures were
determined using an Olympus BX51 polarizing microscope
equipped with a Linkam TH600 hot stage and a PR600
temperature controller. Enthalpies of transition were deter-
mined from thermograms recorded on a Perkin-Elmer DSC-7
differential scanning calorimeter operated at scanning rates of
5 to 10 K min21. Powder X-ray patterns were obtained with a
Guinier film camera (HUBER Diffraktionstechnik, Germany)
using quartz-monochromated CuKa radiation and, for the
small-angle region, with a modified Kratky camera and a one-
dimensional position-sensitive detector (M. Braun, Germany)
using Ni-filtered CuKa radiation from samples in glass
Conclusion
Here we have described the synthesis and the mesomorphic
properties of four series of dimeric molecules in which the
benzoyloxybenzylidene groups are linked to an alkylene spacer
via an imino group. In order to study structure–property
relationships, the lengths of the alkylene spacers (m = 5 to 8) as
well as those of the terminal alkoxy chains (n = 4 to 14) have
been varied. The results show that both the spacer parity and
the imino linkage have a great impact on the mesomorphic
behaviour. Furthermore, the influence of the chain length on
the liquid-crystalline properties is much more pronounced for
the even compared to the odd dimers. The even dimers exhibit
both modulated and non-intercalated structures depending on
the n/m ratio, while the odd ones display an intercalated B6
phase with the exception of homologues with n = 14.
Quantum chemical calculations on the model compound
suggest that the compensation of the dipoles is responsible
for forming the intercalated structures. The presence of the
intercalated phase in the dimers of both spacer parities at high
n/m ratios indicates that the antiparallel arrangement of the
molecules within the phase is facilitated by strong, dipole–
dipole interactions between mesogenic units, which appear to
prevail over unfavourable, space-filling interactions caused by
a mismatch in the lengths of the chains and the spacer.
Furthermore, the imino linkage group coupled with an odd
spacer parity gives rise to the highly bent geometry of the
molecules, causing the odd dimers to behave like true bent-
core mesogens.
capillaries (diameter
1 mm) mounted in temperature-
controlled heating stages. Patterns of aligned samples on a
glass plate on a temperature-controlled heating stage (align-
ment at the sample/glass or at the sample/air interface, sample
fibre-like disordered around an ‘-fold axis perpendicular to
this interface) were obtained with a two-dimensional detector
(HI-STAR, Siemens).
General procedure for the preparation of imines
A solution of the appropriate a,v-diamine (1 mmol) in ethanol
(absolute, 5 cm3) was added dropwise to a hot solution of the
corresponding aldehyde (3a–f; 2 mmol) in ethanol (absolute,
25 cm3). The reaction mixture was heated under reflux for 1 h
under argon and then cooled to room temperature. The
crystalline product, which precipitated, was separated by
filtration. The pure product was obtained after several
crystallisations from acetone or toluene in yields of 60 to 80%.
Analytical data for the new dimeric compounds are collected
in Table 4. Spectroscopic data are given for one imine
homologue, since the only significant difference in the NMR
spectra relates to the number of carbons in the alkoxy chains
and alkylene spacer.
Thus, our studies reveal that the imino-linkage group takes a
significant part in the generation of dipolar interactions
favouring intercalated structures and, in conjunction with the
spacer parity, dictating the geometry of the dimer. As such, it
has an important roˆle in the determination of the liquid-
crystalline behaviour.
Experimental
N,N9-Bis[49-(40-decyloxybenzoyloxy)benzylidene]heptane-
1,7-diamine 7-10. IR (KBr) n(cm21): 2921, 1725, 1648, 1607,
1262; 1H-NMR (CDCl3) d: 0.89 (t, J = 6.6 Hz, 6H), 1.04–1.85
(m, 42H), 3.62 (t, J = 6.6 Hz, 4H), 4.05 (t, J = 6.6 Hz, 4H
–OCH2), 6.98 (d, J = 8.7 Hz, 4H Ar), 7.26 (d, J = 8.4 Hz, 4H
Ar), 7.79 (d, J = 8.4 Hz, 4H Ar), 8.14 (d, J = 8.7 Hz, 4H Ar),
8.28 (s, 2H CHLN); 13C-NMR (CDCl3) d: 14.01, 22.57, 25.85,
All the solvents were either puriss p.a. quality or distilled over
appropriate drying reagents.31 All the other reagents were
used as purchased from Aldrich. 1H- (300 MHz) and 13C-
(75.5 MHz) NMR spectra were recorded on a Bruker AV 300
instrument in CDCl3 with SiMe4 as internal standard unless
stated otherwise. Infrared spectra were recorded on a Bomem
Table 4 Analytical data for the new, dimeric compounds
Analysis: Calc (Found) (%)
Analysis: Calc (Found) (%)
Dimer
C
H
N
Dimer
C
H
N
5-4
5-6
5-8
5-10
5-12
5-14
7-4
7-6
7-8
74.3 (74.2)
75.2 (75.3)
75.9 (75.9)
76.6 (76.3)
77.2 (77.4)
77.7 (77.7)
74.8 (74.6)
75.6 (75.7)
76.3 (76.6)
76.9 (77.1)
77.4 (77.6)
77.9 (78.1)
7.0 (7.3)
7.6 (7.7)
8.1 (8.1)
8.5 (8.4)
8.9 (9.1)
9.2 (9.1)
7.3 (7.5)
7.8 (7.8)
8.3 (8.5)
8.7 (8.9)
9.0 (9.2)
9.3 (9.5)
4.2 (4.3)
3.9 (4.0)
3.6 (3.6)
3.4 (3.4)
3.1 (2.9)
3.0 (3.0)
4.1 (4.1)
3.8 (3.5)
3.5 (3.4)
3.3 (3.1)
3.1 (3.1)
2.9 (3.0)
6-4
6-6
6-8
6-10
6-12
6-14
8-4
8-6
8-8
74.5 (74.6)
75.4 (75.3)
76.1 (75.9)
76.7 (76.7)
77.3 (77.2)
77.8 (77.8)
75.0 (74.9)
75.8 (76.0)
76.4 (76.6)
77.0 (77.1)
77.6 (77.3)
78.0 (78.0)
7.2 (7.3)
7.7 (7.8)
8.2 (8.2)
8.6 (8.8)
9.0 (9.0)
9.3 (9.2)
7.4 (7.6)
8.0 (7.9)
8.4 (8.6)
8.8 (9.0)
9.1 (9.3)
9.4 (9.7)
4.1 (4.2)
3.8 (3.9)
3.6 (3.6)
3.3 (3.5)
3.1 (3.2)
2.9 (3.0)
4.0 (4.0)
3.7 (3.6)
3.4 (3.4)
3.2 (3.3)
3.0 (3.0)
2.8 (2.6)
7-10
7-12
7-14
8-10
8-12
8-14
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163.49, 164.55.
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Acknowledgements
The authors would like to thank the UK EPSRC (HLN)
and the MZOS Republic of Croatia for financial support,
and the Royal Society of London for the provision of a
Collaborative Grant that facilitated the collaboration
-
between AL and DWB. We are also grateful to Dr Nada
-
´
´
Dosˇlic (Ruder Bosˇkovic Institute, Zagreb) for performing the
electronic structure calculations and Professor John Goodby
(University of York) for helpful discussions concerning optical
textures.
ˇ ´
25 A. Lesac, H. L. Nguyen, S. Narancic, U. Baumeister, S. Diele and
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26 A. Ferrarini, G. R. Luckhurst, P. L. Nordio and S. J. Roskilly, Liq.
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