Hydrogen-bonded ionic liquid crystals: pyridinylmethylimidazolium as a versatile building block

Article abstract of DOI:10.1016/j.tetlet.2010.01.045

Utilizing hydrogen bonding between pyridine and benzoic acids, hydrogen-bonded ionic liquid crystals were prepared from pyridinylmethylimidazolium and alkoxy-substituted benzoic acids. Depending on the number of alkoxy substituents, smectic C (SmC), rectangular columnar (Col_r), and cubic (Cub) phases were obtained from mono-, di-, and tri-substituted benzoic acids, respectively. These phases were investigated with X-ray diffraction, differential scanning calorimeter, and polarized optical microscopy.

Full text of DOI:10.1016/j.tetlet.2010.01.045

Tetrahedron Letters 51 (2010) 1508–1511
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Hydrogen-bonded ionic liquid crystals: pyridinylmethylimidazolium
as a versatile building block
*
Shigeo Kohmoto , Yukiko Hara, Keiki Kishikawa
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Utilizing hydrogen bonding between pyridine and benzoic acids, hydrogen-bonded ionic liquid crystals
were prepared from pyridinylmethylimidazolium and alkoxy-substituted benzoic acids. Depending on
the number of alkoxy substituents, smectic C (SmC), rectangular columnar (Colr), and cubic (Cub) phases
were obtained from mono-, di-, and tri-substituted benzoic acids, respectively. These phases were inves-
tigated with X-ray diffraction, differential scanning calorimeter, and polarized optical microscopy.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 21 November 2009
Revised 8 January 2010
Accepted 14 January 2010
Available online 18 January 2010
Recently, considerable attention has been focused on ionic li-
quid crystals.1 Ionic liquid crystals can be considered as materials
that possess both properties of liquid crystals and ionic liquids.2
Among the ionic liquids, imidazolium-based ionic liquids are the
most intensively studied ones as environmentally benign solvents
because of their thermal stability, non-flammability, very low va-
por pressure, and reusability. As well as in ionic liquid, imidazoli-
um moiety is commonly utilized in the field of ionic liquid
crystals as an ionic mesogen.3 They have a potential application
to ion conductive materials.4 Owing to the ionic interactions, ionic
parts assemble to result in a well-microsegregated ionic phase
from alkyl chain parts. A variety of ionic liquid crystals; smec-
tic,3a,c,5 discotic,3b,6 and cubic7 mesophases, have been generated
depending on the choice of molecular shapes. We are interested
to prepare ionic liquid crystals in a supramolecular way using
hydrogen bonding. The advantage of supramolecular assembling8
is an easy preparation of building blocks and the diversity of supra-
molecules created because of a variety of combinations of building
blocks. Pioneering work by Fréchet and Kato9 showed that the bin-
ary complexes of carboxylic acids and pyridine derivatives gave
stable mesophases and many examples have been issued.10 Inter-
molecular hydrogen bonds between two dissimilar mesogens cre-
ated supramolecular liquid crystals.
tor and a donor, respectively, can be self-assembled by hydrogen
bonding. The ionic parts aggregate by ionic interactions. Thus,
the hydrogen-bonded ionic layers can be created, which affords
smectic, columnar and cubic phases depending on the type of car-
boxylic acids employed.
For our study, we prepared pyridinylmethylimidazolium chlo-
ride 113 as an ionic hydrogen-bonding donor. The molecule pos-
We have planned to combine the aforementioned ionic interac-
tion and hydrogen bonding as assembling forces to create supra-
molecularly assembled hydrogen-bonded ionic liquid crystals.
Prior to our work, hydrogen-bonded ionic liquid crystals possess-
ing pyridinium moiety as an ionic part have been studied.11 How-
ever, the examples were limited to smectic phases except one
example of a columnar phase which was not defined well.12 Our
concept is depicted in Figure 1. An ionic mesogen part and a flex-
ible alkyl chain part possessing a hydrogen-bonding proton accep-
Figure 1. Schematic representation of the supramolecularly hydrogen-bonded
ionic liquid crystals. An assembly of an ionic core possessing a hydrogen-bonding
proton acceptor and a hydrogen-bonding proton donor with long alkoxy chains
results in the specific generation of mesophases, Sm, Col, or cubic phases depending
on the number of alkoxy substituents employed, respectively.
* Corresponding author. Tel.: +81 43 290 3420; fax: +81 43 290 3422.
E-mail address: kohmoto@faculty.chiba-u.jp (S. Kohmoto).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.045

S. Kohmoto et al. / Tetrahedron Letters 51 (2010) 1508–1511
1509
Figure 2. Pyridinylmethylimidazolium salts (ionic cores with hydrogen-bonding
proton acceptor) and alkoxybenzoic acids (hydrogen-bonding proton donor)
examined.
Table 1
Phase transition temperatures [°C] and enthalpies [kJ molÀ1
complexes of 1 with benzoic acid derivativesa
] of supramolecular
Complex Phase transitions (onset [°C]) and transition enthalpies (given in
parentheses) [kJ/mol]b
1Á2a
1Á2b
1Á2c
1Á3a
1Á3b
1Á3c
1Á4a
1Á4b
Heating Cr 90/(16.7) SmC 117/(0.7) Iso
Cooling Iso 114/(À1.0) SmC 73/(À15.7) Cr
Heating Cr 91/(22.7) SmC 117/(1.0) Iso
Cooling Iso 114/(À1.5) SmC 87/(À3.7) Cr1 75/(À16.4) Cr2
Heating Cr 92/(44.9) SmC 118/(1.3) Iso
Cooling Iso 116/(À2.3) SmC 89/(À12.6) Cr1 75/(À24.0) Cr2
Heating Cr 104/(—) Colr 128/(—) Iso
Cooling Iso 126/(—) Colr 97/(—) Cr
Heating Cr 106/(—) Colr 135(—) Iso
Cooling Iso 125/(—) Colr 102/(—) Cr
Heating Cr 123 Iso
Cooling Iso 105 Cr
Heating Cr 98/(—) Iso
Cooling Iso 87/(—) Cub 38/(—) Cr
Heating Cr 90/(—) Iso
Cooling Iso 72/(—) Cub 36/(—) Cr
sesses an imidazolium moiety as an ionic core and a pyridiylmeth-
yl moiety as a hydrogen-bonding site. Alkoxybenzoic acid deriva-
tives 2–4 were used as hydrogen-bonding proton donors (Fig. 2).
Hydrogen-bonded supramolecular complexes were prepared by
dissolving 1 and alkoxybenzoic acid derivatives (2, 3, and 4) in a ra-
tio 1:1 in methanol. After the removal of the solvent by evapora-
tion the residue was dried in vacuum. Liquid crystallinity of the
supramolecular complexes was investigated by polarized optical
microscopy (POM) and X-ray diffraction (XRD).
Table 1 summarizes their phase transition behaviors. The com-
plexes of 1 with monoalkoxybenzoic acid derivatives 2 showed
enantiotopic SmC phases. Their POM observation showed schlieren
textures having four-brushes. The textures observed for 1Á2a, 1Á2b,
and 1Á2c by POM are shown in Figure 3a–c, respectively. Hydro-
gen-bonded dimeric forms of n-alkoxybenzoic acids were reported
to show the liquid crystallinity.14 It is also known that they exhibit
the liquid crystallinity with non-mesogenic aromatic acids.15 Since
imidazolium 1 is non-mesogenic, the newly observed mesomor-
phism is due to the resulting hydrogen-bonded complexes. The
phase behaviors and the textures of the complexes of 1 with 2a–
c were different from those of 2a–c, which also indicated that
the observed SmC phases originated in the resulting complexes.
No phase separation was observed. Their phase structures were
confirmed by XRD analysis. Figure 3d shows the XRD pattern of
the complex 1Á2a at 120 °C. Reflections corresponding to (1 0 0)
and (2 0 0) observed for 1Á2a indicate it is a Sm phase. Its layer dis-
tance (d = 31.1 Å) is longer than the calculated length (24 Å) of the
complex. Therefore, the layers can be created by the dimeric form;
the association of the two complexes of 1Á2a via ionic interaction
Figure 3. Optical textures and XRD patterns of the complexes. POM microphoto-
graphs of (a) 1Á2a, (b) 1Á2b, and (c) 1Á2c on the cooling scans at 103, 105, and 100 °C,
respectively, and XRD patterns of (d) 1Á2a, (e) 1Á2b, and (f) 1Á2c at 120, 110, and
100 °C, respectively. The d-spacings are indicated in Å. The numbers in parentheses
are the Miller indices of the peaks.
between imidazolium moieties. From the XRD analysis and the
POM observation, its phase is determined to be a SmC phase. Sim-
ilarly, the mesophases derived from 1Á2b and 1Á2c are SmC phases.
The complexes of 1 with dialkoxybenzoic acids 3 showed enan-
tiotropic Colr phases in the cases with 3a and 3b. A fan-like focal-
conic texture was observed for 1Á3a and 1Á3b (Fig. 4a and b). The
complex with a long alkyl chain, 1Á3c, did not show liquid

1510
S. Kohmoto et al. / Tetrahedron Letters 51 (2010) 1508–1511
Figure 6. The plot of 1/dhkl versus (h2 + k2 + l2)1/2 for assigned XRD hkl of 1Á4a.
The way of assembling of ionic cores in ionic liquid crystals is
important to obtain stable mesophase. Two models of assembling
of ionic cores, an ionic bilayer structure16 and a sandwich struc-
ture,17 were proposed for the mesophase of the short-chain sodium
alkanoate.1a In the former model, the sodium ions and carboxylate
groups are arranged in a double layer. Each sodium ion is sur-
rounded by carboxylate groups and vice versa to neutralize electri-
cally. In the latter model, a layer of sodium ions is sandwiched with
the layers of carboxylate groups located above and below the layer
of sodium ions. Because of the strong electrostatic repulsion be-
tween the sodium ions the sandwich structure is less stable than
the bilayer structure. Similar bilayer structures were observed in
single crystal X-ray structures of several liquid crystalline imidazo-
lium salts exhibiting Sm phases.3a,5a,e,18 In our hydrogen-bonded
pyridinylmethylimidazolium, ionic cores could be assembled to
create a similar bilayer structure in a Sm phase. Imidazolium and
chloride ions might be located alternately and circularly to create
an ionic core in a discotic assembly.
The hydrogen bonded complex of 1 with the benzoic acids with
three alkoxy groups 4 exhibited cubic phases. No texture was ob-
served by POM, which was typical of a cubic phase. The XRD of
1Á4a (Fig. 5) showed several reflections which were assigned as
(2 0 0), (2 2 1), (3 1 0), (3 1 1), (2 2 2), (3 2 1), and (4 0 0) from the
good fit of the linear plot of 1/dhkl versus (h2 + k2 + l2)1/2 for the ob-
served diffraction peaks (Fig. 6). This XRD pattern agrees with
either of the pattern of the micellar cubic phase with a Pm3m lat-
tice or the bicontinuous cubic phase with a Pn3m lattice. The num-
ber of molecules in a unit cell (Z) is calculated to be ca. 350 from
the lattice constant (a = 67.4 Å) with the assumption that the den-
sity of the complex is 1 g/cm3. Complex 1Á4b did not give analyz-
able XRD, probably, due to disorder.
Figure 4. Optical textures and XRD patterns of the complexes of 1 with dialkoxy-
benzoic acids. POM microphotographs of (a) 1Á3a and (b) 1Á3b on the cooling scans
at 108 and 115 °C, respectively, and XRD patterns of (c) 1Á3a and (d) 1Á3b at 115 °C,
respectively. The d-spacings are indicated in Å. The numbers in parentheses are the
Miller indices of the peaks.
crystallinity. The XRD of 1Á3a (Fig. 4c) at 115 °C showed two sharp
reflections with d-spacings of 39.7 and 27.1 Å corresponding to
(2 0 0) and (1 1 0) together with two weak peaks corresponding
to (3 1 0) and (2 2 0), respectively, which is a typical pattern of a
rectangular columnar (Colr) phase with a C2/m lattice. The number
of complexes in a unit cell (Z) with lattice constants of a = 79.4 Å
and b = 28.8 Å is calculated to be ca. 8 with the assumption that
the density and the lattice constant c of the complex are 1 g/cm3
and 3.5 Å, respectively. Therefore, approximately the four com-
plexes of 1Á3a create one disc structure via ionic interaction as
shown in Figure 1. The complex 1Á3b showed a similar XRD pattern
and it was determined to be a Colr phase (Fig. 4d). Because of the
large volume of the alkyl chain part in 3c, 1Á3c cannot form a
columnar array.
We have demonstrated a new methodology to create ionic li-
quid crystals in a supramolecular way. Pyridinylmethylimidazoli-
um is developed as an ionic building block. Hydrogen bonding
between the pyridine moiety of the imidazolium and benzoic acids
generated stable liquid crystal phases. The substitution patterns of
benzoic acids diversify the way of self-assembly of the hydrogen-
bonded complexes, which reflects to the morphology of mesopha-
ses generated.
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