Synthesis and mesophase behaviour of ionic liquid crystals

Article abstract of DOI:10.1039/b705519f

N-Alkyl-pyridinium derivatives 1-3 and N-alkyl-stilbazolium halides 4-6 with hydroxy, methoxy and hydrogen 4′-substituents and long non-branched alkyl chains (n = 14, 16, 18, 20, 22) were synthesized and characterized by polarizing microscopy, differentia

Full text of DOI:10.1039/b705519f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and mesophase behaviour of ionic liquid crystals{
D. Ster,a U. Baumeister,b J. Lorenzo Chao,b C. Tschierskea and G. Israel*a
Received 11th April 2007, Accepted 24th May 2007
First published as an Advance Article on the web 8th June 2007
DOI: 10.1039/b705519f
N-Alkyl-pyridinium derivatives 1–3 and N-alkyl-stilbazolium halides 4–6 with hydroxy, methoxy
and hydrogen 49-substituents and long non-branched alkyl chains (n = 14, 16, 18, 20, 22) were
synthesized and characterized by polarizing microscopy, differential scanning calorimetry and
X-ray measurements. The compounds exhibit liquid crystalline SmA phases when heated above
their melting point. Phenyl substitution in the 3- and 4-position of the pyridinium ring causes a
large tendency to decrease the clearing temperature in comparison to the 4-methyl substituted
N-alkyl-pyridinium salts. By elongation of the pyridinium ring to the 49-substituted stilbazolium
unit the clearing points of the new compounds 4–6 increase drastically up to temperatures
¢200 uC in which partial decomposition of the compounds sets in. The length of the alkyl chains
and the type of counter ions have large influences on the stability of the mesophase. Elongation of
the alkyl chain length n increases the temperature range of the liquid crystalline phase. The
counter ions increase the stability of the SmA phase in order Cl2 . Br2 . I2 . BPh42. When
2
CH3–C6H5SO3 is introduced as an anion no liquid crystalline phase can be observed. UV/Vis
measurements indicate the presence of a charge-transfer complex between the pyridinium cation
and the iodide anion. Differences in the liquid crystalline behaviour of N-alkyl-49-substituted
stilbazolium halides 4–6 compared with N-alkyl-3- and -4-substituted-pyridinium derivatives 1–3
may be explained by additional intramolecular charge-transfer and resulting strong dipole–dipole
interactions between stilbazolium compounds.
1-alkyl-3-methyl-imidazolium
salts
having
chloride,
1. Introduction
tetrachlorocobaltate(II) and tetrachloronickelate(II) counter
ions.7 Most of the investigations were carried out on
Ionic liquid crystals (LCs) can be considered as materials
combining the properties of liquid crystals and ionic liquids.
The ionic character means that some of the properties of ionic
LCs may differ significantly from those of conventional liquid
crystals.1 The driving forces for the formation of ionic LCs are
hydrophobic interactions of the alkyl groups and ionic, dipole–
dipole, cation–p interactions as well as p–p stacking of the core
groups. Therefore, the properties and stabilities of LC phases
should depend on the relative contributions of these interac-
tion forces, depending on the structures of the individual
molecules from each series. From the literature is known that
N-alkyl-4-methyl-pyridinium,2 1-n-alkyl-4-cyano-pyridinium
bromides,3 hexafluorophosphate salts of alkyl-substituted
imidazolium, pyridinium and 3- and 4-methyl-pyridinium
cations4 as well as 1-alkyl-pyridinium bromides5 exhibit SmA
mesophases. Tabrizian et al.6 compared the thermotropic
behaviour of 1-n-alkyl-(4-methyl or 4-tolyl)-pyridinium bro-
mides with alkyl chain lengths from 12 to 22. The clearing
temperatures are similar in both cases, when n = 22, indicating
that the ionic forces are primarily responsible for maintaining
the layer structure of the compounds up to high temperature.
Smectic phases were found also for N-alkyl-pyridinium and
pyridinium derivatives with
a
single aromatic core.
Therefore, we studied the effect of the elongation of the
aromatic core in order to get a deeper insight into the
relationship between molecular p-electron structure and
supramolecular behaviour.
Here we report four series of LC molecules including
N-alkyl-4-methyl-pyridinium halides 1a–g, N-alkyl-4-phenyl-
pyridinium salts 2a–j, N-alkyl-3-phenyl-pyridinium salts 3a–f
and the vinylene-conjugated N-alkyl-49-substituted-stilbazo-
lium halides 4a–g, 5a–f and 6a–f (Scheme 1). The compounds
were synthesized in order to study the influence of the
aInstitute of Chemistry (OC), Martin-Luther-University Halle-
Wittenberg, Kurt-Mothes-Str. 2, D-06120 Halle (Saale), Germany
bInstitute of Chemistry (PC), Martin-Luther-University Halle-
Wittenberg, Mu¨hlpforte 1, D-06108 Halle (Saale), Germany
{ Electronic supplementary information (ESI) available: X-ray data;
concentration dependence of absorbance and Lambert–Beer plot for
2g. See DOI: 10.1039/b705519f
Scheme 1 Structures of the N-alkyl-3- and -4-substituted-pyridinium
salts 1–3 and N-alkyl-49-substituted-stilbazolium salts 4–6; n = number
of C atoms in the non-branched alkyl chain.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3393–3400 | 3393

View Article Online
Fig. 1 a) Fan-shaped texture in the smectic A phase of 2e at 152 uC on cooling. b) Grainy texture in mesophase M of compound 2c at 100 uC on
cooling. c) Transition temperatures T (uC) as a function of the alkyl chain length n for N-alkyl-4-phenyl-pyridinium bromides 2a, 2c, 2e, 2g.
alkyl-chain length, the counter ions and the different
substituents at the pyridinium ring on the LC behaviour.
Compared with the pyridinium derivatives 1–3, stilbazolium
compounds 4–6 should exhibit much stronger dipole–dipole
interactions, in addition to the ionic forces.
iodides have similar melting temperatures and generally lower
clearing points with increasing alkyl chain length n.
The anion effect was investigated in more detail for salts 2f–j
having 22 carbon atoms in the lateral chain. A large influence
on the temperature range of the SmA phase was observed
when replacing the bromide by iodide, chloride and tetra-
phenylborate. Compound 2j with p-toluenesulfonate as anion
exhibits no liquid crystalline phase. The melting temperatures
of the salts with bromide and iodide anions are similar but the
2. Results
Measurements by polarizing microscopy on the N-alkyl-3- and
-4-substituted-pyridinium salts of type 1–3 and N-alkyl-
49-substituted-stilbazolium salts 4–6 show focal-conic fan
textures with homeotropic regions characteristic for a smectic
A phase, SmA (Fig. 1a). In the case of 1–3, the low clearing
enthalpies DHc between 0.05 and 1.2 kJ mol21 are also in line
with the presence of a SmA phase. The salts 4–6 have stronger
dipolar interactions inducing an increase of DHc between 0.3
clearing temperature decreases in the order Cl2 . Br2 . I2
.
BPh42, with increasing ion radius rX2 (Table 2). Replacing the
bromide with chloride leads to a decrease of the melting point
and a strong increase in the clearing point of the salt 2f. For
this compound at temperatures higher than 200 uC partial
decomposition is observed.
Compound 2g shows the additional mesophase M from
98 uC to 109 uC, which is observed only for the two Br2
compounds 2c and 2e bearing different chain lengths (see
Fig. 1c).
and 15.7 kJ mol21
.
N-Alkyl-4-phenyl-pyridinium salts 2a–j
The formation of a smectic A phase was confirmed by X-ray
investigations on aligned samples of compounds 2e and 2i. The
diffraction patterns show first and second order layer
The salts 2a and 2b with n = 16 do not form any LC phases.
The stability of the liquid-crystalline phase of compounds 2d–i
increases markedly with increasing alkyl chain length n (see
Table 1). The phase diagram of the salts with a bromide anion
is presented in Fig. 1c. Elongating the chain has no large
influence on the melting temperature of the compounds, while
the clearing temperature increases drastically. The SmA
temperature range DT increases from 22 uC up to 70 uC for
the bromides 2e (n = 20) and 2g (n = 22).
Table 1 Transition temperatures T (uC) and enthalpies DH (kJ mol21
from differential scanning calorimetry for N-alkyl-4-phenyl-pyridi-
)
nium salts 2a–j; DT (uC) = temperature range of the smectic A phase
Phase transitions T/uC
Comp. n X2
DH/kJ mol21
DT/uC
—
Salts 2c, 2e and 2g do show an unknown mesophase which is
named M in this paper. Differential scanning calorimetry (DSC)
of compounds 2e and 2g shows on heating first a large-enthalpy
transition (melting point) followed by two transitions with a
smaller enthalpy change indicating the presence of two
mesophases (Table 1). On cooling from the isotropic liquid,
first the formation of a fan-texture with homeotropic regions is
observed under the polarising microscope, an indication of a
smectic A phase. During the second phase transition the
viscosity of the sample increases and a grainy texture is formed,
the so-called M phase (see Fig. 1b). Only a small transition
enthalpy for the formation of the M phase was detected by DSC
on cooling. This mesophase M, appearing also for compound 2c,
could not be identified by X-ray methods up to now, therefore
the exact phase type cannot be assigned yet.
2a
2b
2c
2d
2e
2f
16 Br2
16 I2
Cr 86 Iso
49
Cr 83 Iso
41
—
18 Br2
18 I2
Cr 89
50.5
M
114 Iso
0.2
—
Cr 90 SmA 98 Iso
51.9 0.05
Cr 95 134 SmA 156 Iso 22
46.8 1.1 0.12
Cr 72 SmA 213 Iso
53.6 1.9
Cr 98 109 SmA 179 Iso 70
56.9 0.3 0.7
Cr 95 SmA 161 Iso
66.3 0.7
22 B(C6H5)4 Cr 78 SmA 151 Iso
67.2 1.2
8
20 Br2
22 Cl2
22 Br2
22 I2
M
141
2g
2h
2i
M
66
73
—
2
2
2j
22 C7H8SO3 Cr 94 Iso
52.6
Table 1 also contains thermal data for N-alkyl-4-phenyl-
pyridinium iodides 2b, d, h. Compared with the bromides,
3394 | J. Mater. Chem., 2007, 17, 3393–3400
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Table 2 Influence of the size of the counter ion on the mesophase stability. rX2 (nm) = ionic radius of Cl2, Br2, I2, BPh42;8 T (uC) = transition
temperature; DH (kJ mol21) = transition enthalpy; Cr = crystalline state; SmA = smectic A phase; Iso = isotropic liquid state, d (nm) = layer
distance at different temperatures (T (uC), given in parentheses)
2f
2g
2h
2i
X2 Cl2
Br2
0.196
I2
0.222
BPh4
0.421
2
2
rX
T
DH
d
0.181
Cr 72
53.6
SmA 213 Isoa Cr 98
M
109 SmA 179 Iso Cr 95
SmA 161 Iso Cr 78
0.7
SmA 151 Iso
1.2
1.9
56.9
3.76 (130)
3.71 (150)
0.3
0.7
66.3
3.95 (130)
3.86 (150)
67.2
—
—
3.7 (150)
a
Partial decomposition observed.
3.7 nm at 150 uC, with its much larger tetraphenylborate10
counter ion which has a tetrahedral arrangement of the phenyl
units. The comparatively small layer distance may be explained
assuming a shortening of the effective chain length by a
stronger disorder of the alkyl chains to fit the large space
occupied by the anions. One hint of such stronger disorder
may be the above-mentioned slightly enhanced D = 0.54 nm of
the outer diffuse X-ray scattering for 2i.
reflections on the meridian in the small angle region along with
a diffuse outer scattering with a maximum of the intensity on
the equator (Fig. 2a and b, and S1 and S2 in the ESI{). This
indicates a layer structure with liquid-like distributed lateral
distances of the molecules, the long axes of the molecules being
parallel to the layer normal. The maximum of the outer diffuse
scattering of 2e at D = 0.45 nm, corresponds to the average
lateral distance of the molecules of about 0.5 nm and is
typically found for alkyl chains.9 In case of 2i this value is
slightly enhanced to 0.48 nm corresponding to an average
lateral distance of the chains of about 0.54 nm, which may be
explained by packing requirements described below.
N-Alkyl-3-phenyl-pyridinium salts 3a–f
Changing the phenyl group from the para to the meta position,
as shown for compounds 3a–f, may influence the stability of
the mesophase. The transition temperatures for compounds of
type 3 are summarized in Table 3. Comparable to the para-
substituted compounds 2, the salts containing alkyl chains with
n ¢ 18 do form smectic A phases. No M phases were detected.
By increasing the alkyl chain length to carbon number n = 22
the melting temperatures become lower than in the case of the
para-substituted compounds 2a–j, but the SmA temperature
range become larger. The influence of the alkyl chain length on
the transition temperatures of the bromides 3a, c, e and f is
summarized in Fig. 3a.
The layer spacing for 2e is calculated as 3.55 nm at 130 uC.
Since the long axes of the molecules are, on average, parallel to
the layer normal, this distance is the length of the building unit
of the layers. A CPK model for the molecular packing in the
smectic A phase of compound 2e shows a head-to-tail
arrangement of the molecules within the layer, and the
bromide anion is localized between the aromatic rings
(Fig. 2c). The thickness of the layer d calculated from this
model is 3.55 nm, in agreement with the value obtained by
X-ray measurements. Upon increasing the number of carbon
atoms n in the alkyl chain from 20 to 22, the layer distance
determined by the X-ray measurements increases to 3.76 nm
for compound 2g at 130 uC, an expected enhancement for an
elongation of the chains by two methylene groups. When the
bromide ion is replaced by iodide, compound 2h, the thickness
of the layer is 3.95 nm at the same temperature (for
temperature dependence of the d values see ESI{ Table S1).
Obviously, the packing of the chains, most likely the degree of
intercalation, is modified to accommodate the slightly bigger
iodide ions. In the case of compound 2i the layer distance d is
The X-ray diffraction pattern of an aligned sample of 3e in the
SmA phase at 129 uC (Fig. 3b) closely resembles the
corresponding one of 2e (Fig. 2a). The layer distance d amounts
to 3.9 nm and the outer diffuse scattering shows its maximum at
D = 0.46 nm. Upon increasing the length of the alkyl chains to 22
carbon atoms, the thickness of the layer decreases to 3.8 nm.
In order to compare the influence of the substituents at the
pyridinium ring on the stability of the SmA phase, we
synthesized the N-alkyl-4-methyl-pyridinium halides 1a–g with
alkyl chains with n ¢ 14. The transition temperatures and
Fig. 2 2D X-ray diffraction patterns of the smectic A phase for surface-aligned samples of a) 2e at 149 uC and b) 2i at 150 uC on cooling (the
scattering of the isotropic liquid has been subtracted to enhance the visibility of the anisotropic distribution of the outer diffuse scattering); c) CPK
model of compound 2e, assuming the all-trans conformation of the alkyl chains.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3393–3400 | 3395

View Article Online
Table 4 Transition temperatures T (uC) and enthalpies DH (kJ mol21
)
Table 3 Transition temperatures T (uC) and enthalpies DH (kJ mol21
from differential scanning calorimetry for N-alkyl-3-phenyl-pyridi-
nium halides 3a–f. DT (uC) = temperature range of the smectic A phase
)
from differential scanning calorimetry for N-alkyl-4-methyl-pyridi-
nium halides 1a–g. DT (uC) = temperature range of the smectic A
phase. The values in parentheses correspond to those reported in
reference 6
Phase transitions T/uC
Comp.
3a
n
X2 DH/kJ mol21
DT/uC
—
Phase transitions T/uC
16 Br2 Cr 99
28.7
Cr 77
30.6
18 Br2 Cr 45
31.4
Cr 44
27.1
20 Br2 Cr 54
33.4
22 Br2 Cr1 67
40.9
Iso
Comp.
1a
n
X2 DH/kJ mol21
DT/uC
—
14 Br2 Cr 77
Iso
3b
16 I2
Iso
—
25.4
16 Br2 Cr 85 (84)
39.1 (44)
1b
SmA 119 (109) Iso
0.3 (0.3)
34
3c
SmA 82
Iso
Iso
37
0.34
18 I2
SmA 65
1.0
SmA 131 Iso
0.8
21
1c
16 I2
Cr 67
11.1
SmA 104
0.3
SmA 168 (158) Iso
0.8 (0.7)
Iso
37
3d
1d
18 Br2 Cr 82 (92)
38.8 (59)
86
3e
77
1e
18 I2
Cr 84
44.6
20 Br2 Cr 84
51.6
22 Br2 Cr 91 (94)
75.4 (69)
SmA 142
0.6
SmA 197
0.9
SmA 207 (206) Iso 116
1.1 (2)
Iso
58
3f
Cr2
106 SmA 149 Iso 43
41.8 1.1
1f
Iso 113
1g
enthalpies are presented in Table 4 showing that compounds 1
exhibit more stable mesophases (n ¢ 16, larger DT values than
2 and the related compounds 3). Compounds of type 1 show
focal conic textures characteristic of a SmA phase under
polarizing optical microscopy. The packing of the molecules is
considered to be interdigitated, with the anions sandwiched
between the pyridinium rings.2,3,6 The values in Table 4
agree to a great extent with those from known compounds 1b,
d and g.6
acceptor.11 This ion pairing in solution will support the
suggested structure in the CPK models in Fig. 2c and 3c with
the localization of X2 near the pyridinium ring. The
concentration dependence experiment of N-octadecyl-4-
phenyl-pyridinium iodide 2d presented in Fig. 4 shows a shift
of the maximum of the absorbance from 305 nm at
concentration of 1 6 1025 mol l21 to 297 nm at 7 6
1025 mol l21 . This may confirm the existence of a second
charge-transfer band which may be assigned to a electronic
transition from the highest occupied molecular orbital
(HOMO) of the iodide ion to two closely located vacant
molecular orbitals in the pyridinium ion.12 When the iodide
ion was replaced by the more electrophilic bromide, the
charge-transfer band was no longer present, and a linear
correlation of Lambert–Beer plot indicated that no other light
absorbing species are formed in solution (see ESI{ Fig. S3).
External charge-transfer of N-alkyl-3- and -4-substituted-
pyridinium salts 1–3
Compounds 1–3 were investigated by UV/Vis spectroscopy
and show deviations from the Lambert–Beer law indicating the
presence of new species in solution in the case of iodides.
To gain a better understanding of this effect the spectral
properties of N-octadecyl-4-phenyl-pyridinium iodide 2d in
1,2-dichloroethane were investigated (see Fig. 4). The UV/Vis
spectrum shows two absorption bands at 219 nm and 305 nm
and with increasing concentration a new one at 370 nm. A plot
of the maximum absorbances at 305 and 370 nm versus
concentration shows deviations from the Lambert–Beer law
which indicates that additional ion pairs are formed in
solution. The long wavelength absorption corresponds to a
charge-transfer band which arises from an ion pair between the
iodide anion as electron donor and the pyridinium ring as
N-Alkyl-49-substituted-stilbazolium halides 4–6
In comparison to the simple ionic compounds 1, 2 and 3 the
stilbazolium compounds 4, 5 and 6 exhibit both ionic and
appreciable dipolar interaction forces. This can be easily
explained by the chemical structure allowing strong mesomeric
interactions between the methoxy and hydroxy groups as
Fig. 3 a) Transition temperatures of compounds 3a, c, e, f as a function of the chain length n. b) 2D X-ray diffraction pattern of the smectic A
phase for the surface-aligned sample 3e at 129 uC on cooling (the scattering of the isotropic liquid has been subtracted to enhance the visibility of
the anisotropic distribution of the outer diffuse scattering). c) CPK model of compound 3e.
3396 | J. Mater. Chem., 2007, 17, 3393–3400
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Fig. 4 a) Concentration dependence of the absorbance of N-octadecyl-4-phenyl-pyridinium iodide 2d in 1,2-dichloroethane, l (path length of the
quartz cell) = 1 cm. b) Lambert–Beer plot.
electron donors and the quaternary ammonium group as an
acceptor (intramolecular charge-transfer). Therefore, it was of
interest to investigate the LC properties of the stilbazolium
halides with hydroxy, methoxy and, for comparison, hydrogen
substituents at the donor position 49 and long alkyl chains at
the quaternary ammonium group, compounds 4–6. The
influence of these structural changes, including the counter
ions, on the mesophase behaviour of the compounds 4–6 was
investigated by polarizing microscopy, differential scanning
calorimetry and X-ray diffraction.
mesophase is shifted to lower temperatures, at which less
decomposition effects can be expected and X-ray investiga-
tions are possible. Powder patterns of N-octadecyl-49-hydroxy-
stilbazolium iodide 4e have been recorded, from which a layer
distance d of 3.85 nm is obtained for the SmA phase at 160 uC.
A similar supramolecular arrangement can be expected from
the molecular packing model presented in Fig. 5b showing a
head-to-tail orientation of the stilbazolium unit with the anion
sandwiched between the aromatic rings. Thus this model is
assumed as representative for compounds of type 4.
Investigations by polarizing microscopy on compounds 4
with n ¢ 14 show the formation of focal-conic fan and
homeotropic textures characteristic for smectic A phases. The
influence of the length of the hydrocarbon chains attached to
the quaternary ammonium group on the transition tempera-
tures of the mesophases for 4a–g is shown in Table 5. An
increase in the clearing point was observed with increasing
length of the alkyl chain. At temperatures higher than 200 uC
the compounds may partially decompose as can be derived
from decreasing clearing points after heating the substances
several times into the isotropic liquid.
A growing of baˆtonnets (see Fig. 5a) followed by the
formation of a fan-shaped texture is observed on cooling by
polarizing microscopy. Partial decomposition is not only
evident from the microscopic observations of gas bubbles in
the melt, but also by the lowering of the clearing point in the
second and third cooling. Due to decomposition of the
samples, the transition temperatures are given for the first
heating, but the SmA texture was observed by polarizing
microscopy even at higher temperatures. In all cases the
stilbazolium compounds exhibit a tendency to supercool
before crystallization or to form glasses.
When the counter ion is changed from bromide to iodide
both the melting point and the clearing point decrease
drastically meaning that the stability of the smectic A phase
is influenced by the size of the anion. The stability range of the
Methoxy substituted stilbazolium halides 5 show large
temperature ranges of the LC phases (see Table 6). The
compounds with n ¢ 16 have relatively low melting points
with small increase by increasing alkyl chain length. Upon
cooling from the isotropic liquid the formation of a fan-shaped
texture is observed by polarizing microscopy.
Table 5 Transition temperatures T (uC) and enthalpies DH (kJ mol21
)
from differential scanning calorimetry of N-alkyl-49-hydroxy-stilbazo-
lium halides 4a–g. DT (uC) = temperature range of the smectic A phase
Compounds 6a–f with n ¢ 18 show, under polarizing
microscopy, focal conic textures characteristic for SmA.
Substitution of methoxy group by hydrogen increases the
Phase transitions T/uC
Comp.
4a
n
X2
DH/kJ mol21
DT/uC
13
14 Br2
16 Br2
16 I2
Cr
Cr
Cr
Cr
Cr
Cr
Cr
173
7.5
177
12.0
143
2.3
172
22.6
130
17.6
172
14.7
176
17.3
Sm
186
0.3
221
2.0
181
9.5
230
0.9
Iso
4b
SmA
SmA
SmA
SmA
Isoa
Iso
¢44
38
4c
4d
18 Br2
18 I2
Isoa
Iso
¢58
54
4e
184
6.81
260
4f
20 Br2
22 Br2
SmA
2.6
SmA
Isoa
Isoa
¢88
¢68
4g
245
3.5
Fig. 5 a) Texture of N-octadecyl-49-hydroxy-stilbazolium iodide 4e
at 164.3 uC. b) CPK model of N-eicosyl-49-hydroxy-stilbazolium
bromide 4f.
a
Partial decomposition observed.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3393–3400 | 3397

View Article Online
SmA phases upon heating above their melting point. The
formation of layered structures is attributed to the attachment
of the aliphatic chains to an ionic group. The length of
the alkyl chains n plays an important role in the stability of
the pyridinium salts. By increasing the length of the alkyl
chain for compounds of type 1–3 the melting points
remain nearly constant and the clearing temperatures increase.
This may be explained by increasing hydrophobic stabilization
of the aromatic unit by alkyl groups. Compounds of type 3
show a shorter temperature range of the SmA phase with
increasing length of the alkyl chain from 20 to 22 carbon
atoms. This result was confirmed by X-ray investigations
where we found also that the layer distance d decreases from
3.9 nm to 3.8 nm upon increasing the alkyl chain by two
carbons. Interestingly, compounds 6 do not show a decrease of
the layer distance upon increasing the chain length from 20 to
22 carbons.
Table 6 Transition temperatures T (uC) and enthalpies DH (kJ
mol21) from differential scanning calorimetry of N-alkyl-49-methoxy-
stilbazolium halides 5a–f. DT (uC) = temperature range of the smectic
A phase
Phase transitions T/uC
Comp.
5a
n
X2
DH/kJ mol21
DT/uC
¢16
14 Br2
16 Br2
18 Br2
18 I2
Cr
Cr
Cr
Cr1
Cr
Cr
231
4.2
54
9.4
63
15.7
67
10.4
68
SmA
SmA
SmA
Cr2
247
6.97
227
2.83
255
5.9
245
15.7
227
2
Isoa
Isoa
Isoa
Isoa
Isoa
Isoa
5b
¢173
¢192
—
5c
5d
5e
20 Br2
22 Br2
SmA
SmA
¢159
¢139
16.8
75
5f
214
2.1
18.5
a
Partial decomposition observed.
The methyl group in the para-position of the pyridinium
ring, compounds 1, induces a large stability of the mesophase.
When the aromatic core is extended by replacing the methyl
group with a phenyl substituent, compounds 2, the clearing
temperatures decrease indicating that an aromatic core
destabilizes the mesophase. Additionally, it leads to the
formation of an ordered intermediate mesophase M. The M
phase was observed only in the case of N-alkyl-4-phenyl-
pyridinium salts having bromide as counter ion. The more
bulky 3-phenyl-pyridinium head group in 3 lowers the melting
points drastically and reduces the temperature range of LC
phases. The temperature range of the SmA phase increases
upon attaching substituents to the pyridinium unit in the order
4-methyl (1) . 3-phenyl (3) . 4-phenyl (2).
melting points resulting in a decreasing of the mesophase
temperature range (see Table 7).
Compound 6d with iodide anion does not form LC phases.
Compared with series 2, compounds 6 have only the
additional CLC double bond between the aromatic rings,
which may indicate that series 2 and 6 should have similar
liquid crystalline behaviour. Both of the series display SmA
phases on heating. Compounds 2 with bromide anion show an
additional phase M, which is not present in the case of salts 6.
Compared with bromides 2, the SmA temperature range is
larger in the case of salts 6. Also, a relatively large increase of
the melting and clearing points was observed for salts 6,
leading to a partial decomposition at temperatures higher than
200 uC. The increase of the transition temperatures may be
attributed to the elongation of the aromatic core by a CLC
double bond which will induce additional p–p interactions in
the case of 6.
By changing the anion from iodide to bromide and then to
chloride, compounds 2f–h, the tendency to form LC phases
increases. Reducing the size of the anion leads to a better
stabilization of the layers, also supported by observations on
CPK models, because an anion with a smaller size is readily
accommodated near the pyridinium cation. When the halides
are replaced with tetraphenylborate, which is composed of
four phenyl units arranged tetrahedrally and has a larger size
than the halides, the clearing temperature decreases. For
instance, upon increasing the size of the counter ion from
iodide (rI2 = 0.22 nm, rI2 = ionic radius of iodide) to
tetraphenylborate (rBPh42 = 0.42 nm, rBPh42 = ionic radius of
tetraphenylborate) the clearing temperature decreases by 10 uC.
Therefore, we can conclude that the increase in the clearing
points is caused by the decrease in the size of the anion in the
order chloride . bromide . iodide . tetraphenylborate (see
Table 2). The presence of the flat anion p-toluenesulfonate,
compound 2j, indicates that the tendency to form a SmA phase
is suppressed.
Kosaka et al.13 investigated N-alkyl-stilbazolium com-
pounds having the electron accepting NO2 group at the
49-position of the stilbazolium head group. These compounds
containing alkyl chains up to ten carbons and bromide as
counter ion show a relatively narrow temperature range of the
mesophase. Replacing the bromides with chlorides the
mesophase stability increases and the compounds will ther-
mally decompose. Upon using an dialkylamino group at the
49-position in the stilbazolium core and alkyl chains with 14 to
3. Discussion
N-Alkyl-3- and -4-substituted-pyridinium salts 1–3 and
N-alkyl-49-substituted-stilbazolium compounds 4–6 display
Table 7 Transition temperatures T (uC) and enthalpies DH (kJ
mol21) from differential scanning calorimetry of N-alkyl-stilbazolium
halides 6a–f
Phase transitions T/uC
Comp.
6a
n
X2
DH/kJ mol21
DT/uC
—
14 Br2
16 Br2
18 Br2
18 I2
Cr
Cr
Cr
Cr
Cr
Cr
209
2.9
200
2.2
153
4.8
189
10.6
141
Isoa
Iso
6b
—
6c
SmA
Iso
217
2.5
Isoa
¢64
—
6d
6e
20 Br2
22 Br2
SmA
2.6
SmA
216
0.3
247
1.3
Isoa
Isoa
¢75
¢98
6f
149
4.4
a
Partial decomposition observed.
3398 | J. Mater. Chem., 2007, 17, 3393–3400
This journal is ß The Royal Society of Chemistry 2007

View Article Online
18 carbon atoms attached to the pyridinium ring, the
temperature range of the mesophase decreases drastically.14
These examples indicate the influence of the 49-substitution
at the stilbazolium group on the LC properties. N-Alkyl-
49-substituted-stilbazolium salts 4–6, which form also SmA
phases, have the electron donor hydroxy and methoxy group
at the 49-position and the electron accepting unit with
character at the pyridinium group inducing strong dipole–
dipole interactions in addition to the ionic forces. The DSC
measurements of compounds 4–6 show peaks corresponding to
crystal–crystal, crystal–mesophase and mesophase–isotropic
liquid transitions (Tables 5–7), with significant changes in
enthalpies DH. For instance, in the case of 6f, DH decreases
from 4.4 kJ mol21 for the crystal–melting transition to
1.3 kJ mol21 for the SmA phase–isotropic transition. Above
200 uC an increase of the baseline was observed due to partial
decomposition. It is found that the substituents in the
49-position increase the stability of the mesophase in the order
OCH3 . OH . H.
with Guinier equipment (film camera and goniometer, both by
Huber) with samples in a temperature-controlled heating stage
using quartz-monochromatized CuKa radiation (calibration
of the film patterns with the powder pattern of Pb(NO3)2).
Two-dimensional patterns for aligned samples on a tempera-
ture controlled heating stage were recorded with a 2D detector
(HI-STAR, Siemens). The substances had to be heated, for
orientation, into the isotropic liquid during the sample
preparation for the X-ray investigations. Furthermore, the
samples for the Guinier measurements were sealed in glass
capillaries (1 mm), those for the 2D measurements were drops
on a glass plate aligned at the sample/glass or at the sample/air
interface. Taking into account the influences of water and
partial decomposition (see above), the transition temperatures
and even the phases observed under the conditions of the
different X-ray measurements may differ from each other and
from those found by the other methods. The UV/Vis
measurements were made on a UV 3101 PC spectrometer
(Shimadzu Co.).
The stilbazolium compounds 4–6 show higher temperatures
of the SmA–isotropic liquid transition in comparison to the
pyridinium salts of type 1–3. Thus, thermal decomposition of
the salts upon heating above 200 uC has a major influence on
the stability of their mesophases. Therefore, besides the ionic
interactions, the liquid crystalline behaviour is influenced, in
the case of salts 4 and 5, also by dipolar interactions by
introducing hydroxy and methoxy groups in the 49-position of
the stilbazolium unit. Generally, the salts with bromide anion
show an increase of the mesophase temperature range DT with
increasing alkyl chain length n.
General procedure for preparation of compounds 1–3
The following procedure was used generally for the prepara-
tion of N-alkyl-3- and -4-substituted-pyridinium salts 1–3.
Preparation of 2e: 4-Phenyl-pyridine (3.22 mmol, 0.5 g) and
1-bromoeicosane (3.22 mmol, 1.17 g) were heated in 10 ml of
dried toluene for 10 h at 100–110 uC. After cooling to room
temperature, the crude product was purified by washing with
diethyl ether and recrystallization from ethanol to give a white
powder of N-eicosyl-4-phenyl-pyridinium bromide. Yield:
61.80% (1.99 mmol).
N-Eicosyl-4-phenyl-pyridinium bromide 2e: 1H NMR
(CDCl3, J/Hz, 400 MHz) d = 9.44 (d, J = 6.64, 2 H, Ar-H),
8.23 (d, J = 6.64, 2 H, Ar-H), 7.77 (m, 2 H, Ar-H), 7.56–7.51
(m, 3 H, Ar-H), 4.90 (t, J = 7.3, 2 H, CH2), 2.02–1.99 (m, 2 H,
CH2), 1.37–1.18 (m, 34 H, CH2), 0.84 (t, J = 7.8, 3 H, CH3).
13C NMR (CDCl3, 100 MHz) d = 156.24, 145.02, 133.53,
132.34, 129.89, 127.79, 124.95, 61.25, 31.9, 31.86, 29.68, 29.63,
29.59, 29.50, 29.36, 29.33, 29.09, 26.14, 22.66, 14.07. Anal.
calc. for C31H50BrN?H2O (Calc.) C: 69.64%, H: 9.80%, Br:
14.94%, N: 2.62%, (Found) C: 69.57%, H: 9.79%, Br: 15.01%,
N: 2.62%.
N-Eicosyl-3-phenyl-pyridinium bromide 3e: 1H NMR
(CDCl3, J/Hz, 400 MHz) d = 9.54 (s, 1 H, Ar-H), 9.34 (d,
J = 6.02, 1 H, Ar-H), 8.55 (d, J = 8.09, 1 H, Ar-H), 8.14–8.11
(m, 1 H, Ar-H) 7.84 (d, J = 6.84, 2 H, Ar-H), 7.53–7.44 (m, 3 H,
Ar-H), 5.09 (t, J = 7.3, 2 H, CH2), 2.03–1.97 (m, 2 H, CH2),
1.36–1.17 (m, 34 H, CH2), 0.84 (t, J = 6.7, 3 H, CH3).13C
NMR (CDCl3, 100 MHz) d = 143.1, 142.6, 142.3, 141.2, 132.6,
130.6, 129.8, 128.4, 127.7, 62.3, 32.9, 32.2, 31.9, 29.7, 29.7,
29.6, 29.6, 29.5, 29.4, 29.4, 29.2, 28.8, 28.2, 26.1, 22.7, 14.1.
Anal. calc. for C31H50BrN?0.5H2O (Calc.) C: 70.77%, H:
9.70%, N: 2.66%, (Found) C: 70.78%, H: 10.12%, N: 2.02%.
Preparation of 2f–j: Compound 2h was prepared by a
modified ion-exchange procedure15 using Dowex 1 6 8-400,
ion exchange resin (Acros), N-docosyl-4-phenyl-pyridinium
bromide 2g, NaI and methanol: 10 g of Dowex resin was
suspended in 100 ml water for 2 h. After removing the
water, the resin was washed with NaI (or NaCl, NaB(C6H5)4,
4. Experimental
The reagents were purchased from Aldrich, Acros and Fluka.
All the solvents were dried using general standard procedures.
The investigated compounds were dried for at least two days
under vacuum (2 Torr) at temperatures below the melting
points or maximum 100 uC for high melting salts and using
P2O5 as drying agent. 1H and 13C NMR spectra were recorded
using Varian Unity 200 or 400 spectrometers. The calorimetric
measurements were made using
a Perkin-Elmer DSC-7
differential scanning calorimeter with a heating–cooling rate
of 10 uC min21. In the case of pyridinium salts 1–3, the
transition temperatures, which are summarized in Tables 1–4,
correspond to the second heating. Due to a partial decom-
position of the stilbazolium salts, the transition temperatures
presented in Tables 5–7 correspond to the first heating. The
samples were heated to ca. 120 uC before DSC measurements
to remove traces of water. Thermogravimetric measurements
were made using a STA 409 instrument (Netzsch) and show
that the compounds start to decompose at temperatures above
200 uC. Salts 2e and 5e show by thermogravimetric analysis a
loss of weight at 100 uC which corresponds to 0.8 mol water
(2.78%) for salt 2e and 0.16 mol water (0.52%) for salt 5e. ESI-
MS spectra were obtained using a Finnigan LQC spectrometer
(Thermo Electron Corp.). The microscopic investigations were
performed by a Nikon Optiphot 2 polarizing microscope
which was connected with a Mettler FP 82 HT heating stage.
X-Ray investigations on powder-like samples were carried out
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3393–3400 | 3399

View Article Online
N-Octadecyl-stilbazolium bromide 6c: 1H NMR (CDCl3,
J/Hz, 400 MHz) d = 9.15 (d, J = 6.64, 2 H, Ar-H), 8.07 (d, J =
6.84, 2 H, Ar-H), 7.71 (d, J = 16.39, 1 H, CH), 7.68–7.60 (m,
2 H, Ar-H), 7.42–7.38 (m, 3 H, Ar-H), 7.16 (d, J = 16.86, 1 H,
CH), 4.77 (t, J = 7.3, 2 H, CH2), 1.98–1.88 (m, 2 H, CH2),
1.28–1.19 (m, 30 H, CH2), 0.84 (t, J = 6.8, 3 H, CH3). 13C
NMR (CDCl3, 100 MHz) d = 153.41, 144.35, 142.16,
134.48, 130.98, 129.18, 128.43, 124.25, 122.21, 61.08, 31.96,
31.78, 29.74, 29.70, 29.64, 29.55, 29.41, 29.39, 29.13, 26.18,
22.72, 14.14. Anal. calc. for C31H48BrN?0.5H2O (Calc.) C:
71.12%, H: 9.36%, N: 2.67%, (Found) C: 71.02%, H: 9.62%, N:
2.52%.
CH3–C6H5–SO3H) solution three times. The resin was kept in
contact for 2 days with the third part of NaI solution and then
filtered. The ratio of 2g/NaI was 1/10. 100 mg of salt 2g
(0.18 mmol) was dissolved in 10 ml of methanol and mixed
with resin. After keeping the resin in contact with the salt 2g
for two days, the product was extracted with methanol and
washed with water to remove the excess of NaI. The solvent
was evaporated by distillation and pure 2h was obtained.
Compounds 2f–j were investigated by ESI-MS and no bromide
ion was identified in their spectra, which indicates that the
exchange procedure was complete.
General procedure for the preparation of compounds 4–6
Acknowledgements
The following procedure was used generally for the prepara-
tion of N-alkyl-49-substituted-stilbazolium salts 4–6.
Financial
support
of
this
work
by
Deutsche
Preparation of 4d: 1-Octadecyl-4-methyl-pyridinium bro-
mide (0.94 mmol, 0.4 g), 4-hydroxy-benzaldehyde (0.94 mmol,
0.11 g), piperidine (0.12 ml) and dry ethanol (5 ml) were heated
under reflux for 8 h. Upon cooling the reaction to room
temperature the product precipitated. The resulting red preci-
pitate was filtered, washed with diethyl ether and recrystallized
from ethanol to give 60% (0.56 mmol), red powder.
Forschungsgemeinschaft (GRK 894) is gratefully acknowl-
edged. We would like to thank Dr T. Mu¨ller for the
thermogravimetric analysis and Dr R. Kluge for the ESI-MS
measurements.
References
N-Octadecyl-49-hydroxy-stilbazolium bromide 4d: 1H NMR
(DMSO, J/Hz, 400 MHz) d = 8.77 (d, J = 6.64, 2 H, Ar-H),
8.06 (d, J = 6.84, 2 H, Ar-H), 7.89 (d, J = 16.18, 1 H, CH), 7.56
(d, J = 8.71, 2 H, Ar-H), 7.18 (d, J = 16.18, 1 H, CH), 6.78 (d,
J = 8.71, 2 H, Ar-H), 4.40 (t, J = 7.3, 2 H, CH2), 1.87–1.83 (m,
2 H, CH2), 1.24–1.20 (m, 30 H, CH2), 0.83 (t, J = 6.8, 3 H,
CH3). 13C NMR (DMSO, 100 MHz) d = 160.5, 153.3, 143.6,
141.4, 130.2, 125.8, 122.9, 119.1, 116.1, 59.3, 31.2, 30.3, 28.9,
28.7, 28.6, 28.5, 28.2, 25.3, 21.9, 13.8. Anal. calc. for
C31H48BrNO (Calc.) C: 70.17%, H: 9.12%, N: 2.64%,
(Found) C: 70.73%, H: 9.45%, N: 2.64%.
N-Eicosyl-49-methoxy-stilbazolium bromide 5e: 1H NMR
(CDCl3, J/Hz, 400 MHz) d = 9.04 (d, J = 6.02, 2 H, Ar-H),
8.00 (d, J = 6.02, 2 H, Ar-H), 7.67 (d, J = 15.97, 1 H, CH), 7.57
(d, J = 8.71, 2 H, Ar-H), 7.00 (d, J = 15.82, 1 H, CH), 6.90 (d,
J = 8.71, 2 H, Ar-H), 4.71 (t, J = 7.01, 2 H, CH2), 3.82 (s, 3 H,
OCH3), 1.95–1.91 (m, 2 H, CH2), 1.28–1.18 (m, 34 H, CH2),
0.84 (t, J = 6.8, 3 H, CH3). 13C NMR (CDCl3, 125 MHz) d =
162.1, 153.8, 144.1, 142.0, 130.4, 127.3, 123.7, 119.7, 114.6,
60.8, 55.4, 31.9, 31.8, 31.7, 29.6, 29.6, 29.6, 29.5, 29.3, 29.3,
29.0, 26.1, 22.6, 14.1. Anal. calc. for C34H54BrNO?0.2H2O
(Calc.) C: 70.79%, H: 9.44%, Br: 13.88%, N: 2.43%, (Found)
C: 70.51%, H: 9.63%, Br: 13.95%, N: 2.39%.
1 K. Binnemans, Chem. Rev., 2005, 105, 4148.
2 C. G. Bazuin, D. Guillon and A. Skoulios, Liq. Cryst., 1986, 1(2),
181.
3 D. P. Jackson and B. M. Fung, Mol. Cryst. Liq. Cryst., 1997, 303,
73.
4 C. M. Gordon, J. D. Holbrey, A. R. Kennedy and K. R. Seddon,
J. Mater. Chem., 1998, 8, 2627.
5 G. A. Knight and B. D. Shaw, J. Chem. Soc., 1938, 682.
6 M. Tabrizian, A. Soldera, M. Couturier and C. G. Bazuin,
Liq. Cryst., 1995, 18(3), 475.
7 C. J. Bowlas, D. W. Bruce and K. R. Seddon, Chem. Commun.,
1996, 1625.
8 Y. Marcus, J. Chem. Soc., Faraday Trans., 1991, 87(18), 2995.
9 The average lateral distance a of the molecules is calculated
according to a = 1.12 6 d from the relation for randomly packed
long cylinders as usually applied for calamitic liquid crystals, see
e.g. M. J. Seddon, in Handbook of Liquid Crystals, ed. D. Demus,
J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH
Verlag GmbH, Weinheim, 1998, vol. 1, p. 648.
10 T. Nhujak and D. M. Goodall, Electrophoresis, 2001, 22, 117.
11 E. M. Kosower, J. Am. Chem. Soc., 1958, 80, 3253.
12 R. A. Mackay, J. R. Landolph and E. J. Poziomek, J. Am. Chem.
Soc., 1971, 93, 5026.
13 Y. Kosaka, T. Kato and T. Uryu, Liq. Cryst., 1995, 18(5), 693.
14 K. Binnemans, C. Bex and R. Van Deun, J. Inclusion Phenom.
Macrocyclic Chem., 1999, 35, 63.
15 R. Matissek, Lebensmittelanaytik: Grundzu¨ge, Methoden,
Anwendungen, Springer, Berlin, 2006, p. 187.
3400 | J. Mater. Chem., 2007, 17, 3393–3400
This journal is ß The Royal Society of Chemistry 2007