Thermotropic ionic liquid crystals by molecular assembly and ion pairing of 4,4′-bipyridinium derivatives and tris(dodecyloxy)benzenesulfonates in a non-polar solvent

Article abstract of DOI:10.1039/c2jm31830j

The present work reports a simple and versatile pathway to incorporate redox-active 4,4′-bipyridinium ("viologen") units with two, three, or six charges into dynamically ordered liquid crystalline structures by ionic interactions. A detailed analysis of the structure-property relationship was supported by molecular modelling experiments. Three redox-active 4,4′-bipyridinium ("viologen") cores, 1,1′-dimethyl (4-pyridin-4-ylpyridinium) di- (MV²⁺), 1,1′,1′′- [benzene-1,3,5-triyltris(methylene)]tris(4-pyridin-4-ylpyridinium) tri- (V ³⁺), and 1,1′,1′′-[benzene-1,3,5- triyltris(methylene)]tris[(methyl) 4-pyridin-4-ylpyridinium] hexa- (MV ⁶⁺) cationic halides, were combined with an anionic caesium 3,4,5-tris(dodecyloxy)benzene sulfonate (DOBS) under phase transfer conditions to yield complexes of constant stoichiometry. The formed complexes, MV ²⁺(DOBS)₂, V³⁺(DOBS)₃ and MV ⁶⁺(DOBS)₆, were characterized by NMR spectroscopy. Depending on the molecular structure, significantly different characteristics in electrochemical behaviour of ionic liquid crystals (ILCs) with two DOBS anions and a rod-like core (MV²⁺) were observed compared with ILs with three and six DOBS anions and y-shaped cores (V³⁺), and (MV⁶⁺) cations respectively. We report for the first time a new electroactive thermotropic liquid crystal, MV²⁺(DOBS)₂, which changes colour from yellow to blue-green above 75 °C typically for redox units in the reduced state. The reduced states of the MV²⁺(DOBS)₂ complex at E = -2 V exhibit reversible redox waves and have influence on molecular arrangement in the crystal structure as seen by polarisation microscopy. Our ionic liquid crystals have potential applications, due to their low melting temperature, wide temperature range, ionic conductivity, and thermal- and electro-chromism, for instance, for the design of electrochromic displays.

Full text of DOI:10.1039/c2jm31830j

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PAPER
Thermotropic ionic liquid crystals by molecular assembly and ion pairing
of 4,40-bipyridinium derivatives and tris(dodecyloxy)benzenesulfonates in
a non-polar solvent†
*
Simona Asaftei, Marius Ciobanu, Ana Maria Lepadatu, Enfeng Song and Uwe Beginn
Received 23rd March 2012, Accepted 30th April 2012
DOI: 10.1039/c2jm31830j
The present work reports a simple and versatile pathway to incorporate redox-active 4,40-bipyridinium
(‘‘viologen’’) units with two, three, or six charges into dynamically ordered liquid crystalline structures
by ionic interactions. A detailed analysis of the structure–property relationship was supported by
molecular modelling experiments. Three redox-active 4,40-bipyridinium (‘‘viologen’’) cores, 1,10-
dimethyl (4-pyridin-4-ylpyridinium) di- (MV2+), 1,10,100-[benzene-1,3,5-triyltris(methylene)]tris(4-
pyridin-4-ylpyridinium) tri- (V3+), and 1,10,100-[benzene-1,3,5-triyltris(methylene)]tris[(methyl)
4-pyridin-4-ylpyridinium] hexa- (MV6+) cationic halides, were combined with an anionic caesium 3,4,5-
tris(dodecyloxy)benzene sulfonate (DOBS) under phase transfer conditions to yield complexes of
constant stoichiometry. The formed complexes, MV2+(DOBS)2, V3+(DOBS)3 and MV6+(DOBS)6, were
characterized by NMR spectroscopy. Depending on the molecular structure, significantly different
characteristics in electrochemical behaviour of ionic liquid crystals (ILCs) with two DOBS anions and
a rod-like core (MV2+) were observed compared with ILs with three and six DOBS anions and y-shaped
cores (V3+), and (MV6+) cations respectively. We report for the first time a new electroactive
thermotropic liquid crystal, MV2+(DOBS)2, which changes colour from yellow to blue-green above
75 ꢀC typically for redox units in the reduced state. The reduced states of the MV2+(DOBS)2 complex at
E ¼ ꢁ2 V exhibit reversible redox waves and have influence on molecular arrangement in the crystal
structure as seen by polarisation microscopy. Our ionic liquid crystals have potential applications, due
to their low melting temperature, wide temperature range, ionic conductivity, and thermal- and electro-
chromism, for instance, for the design of electrochromic displays.
liquid (IL) has yielded a new class of soft materials called ionic
Introduction
liquid crystals (ILCs).29 Ionic liquid crystals are self-organizing
systems that offer great advantage over isotropic systems, and
have also been used as an anisotropic medium with many
applications, e.g. in electrochemistry with dye-sensitized solar
cells.30 Bis-quaternized 4,40-bipyridinium units, so-called ‘‘viol-
ogens’’,31 are building blocks of special importance, because they
allow generating redox-active supramolecular assemblies that
may be usable to obtain new ordered functional materials with
electroactive properties such as electrochromism,27 electrical
conductivity,24 photochromism,32,33 thermochromism,34 electron
transfer,35 and charge transfer abilities.36,37 The synthesis of
liquid-crystal ‘‘viologens’’ makes it possible to expand the field of
application of viologens. First thermotropic liquid crystalline
viologen compounds with electrochromic properties were
described by the group of Kominami.38 Discotic molecules such
as ‘‘viologens’’ usually form columnar mesophases, the molecules
are stacked on top of each other into columns, which are
arranged in either hexagonal Colh or rectangular Colr order.39,40
Recently Kato and co-workers23 reported the first example of
columnar liquid crystals incorporating a redox-active viologen
The ‘‘construction’’ of molecular assemblies exhibiting dynamic
functions by means of small building units, and exploitation of
suitable intramolecular interactions such as ionic interactions,1
hydrogen bonding,2,3 charge transfer interactions,4 p–p inter-
actions,5,6 as well as van der Waals interactions,7 is a promising
approach toward the development of new functional soft mate-
rials. The supramolecular liquid crystals concept has been
introduced into the design of columnar materials to induce
functions such as ion transportation,8–13 sensor and separation
membranes,14,15 electrolytes,16,17 electron transporting,18,19
light-emitting diodes (LEDs) and non-linear optics (NLO),20,21
photoluminescence,22 electrochromism and electrical conduc-
tivity.23–28 The combination between liquid crystal (LC) and ionic
Organic Materials Chemistry, Bioorganic Chemistry, Institute of
€
€
Chemistry, University of Osnabruck, Barbarastr. 7, D-49076 Osnabruck,
Germany. E-mail: sasaftei@uos.de; Fax: +49-541-969 237; Tel: +49-
541-969 2802
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm31830j
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unit. They exhibit rectangular or hexagonal columnar phases,
which are stable up to 160 ꢀC. The group of Saielli41 showed that
the symmetrically substituted viologen salts with sufficiently long
alkyl chains exhibit a smectic mesophase of type B (SmB) over
a wide temperature range from 0 ꢀC to 140 ꢀC. The SmA phase of
such systems often requires a minimum length of chain, usually
C12, and is strongly dependent on the counterion. Because of
a rigid anisotropic molecular core no nematic mesophase is
known for such systems.29 More specifically, by the induction of
asymmetry in molecular structure by changes in the alkyl side-
chains attached to the viologen core, the solid–liquid crystal
transition temperatures can be lowered more than in a symmet-
rical structure.41 Moreover, viologen dications are rod-like
molecules with a high structural similarity to typical rod-like
mesogenic dipolar molecules such as biphenyl-based imidazo-
lium or phenanthrolinium salts.42,43 Replacement of one tail with
a threefold substituted benzyl group gives a y-shaped ionic liquid
crystal that forms columnar (Col) mesophases.16,44 The viologen-
based compounds are an alternative to classical ionic organic
mesogenic cores such as pyridinium or imidazolium salts because,
due to the molecular shape of the viologen, they can be used as
photochromic and electrochromic additives to a liquid-crystal
matrix to make a ‘‘task-specific’’ ionic liquid with one or more
functionalities.45,46 These functional groups bring new chemical or
physical properties to the ionic liquid or improve the performance
when built up in a device or process. Understanding correctly the
physical properties of ionic liquids via computer simulation is an
important predictive tool in this field, for their potential techno-
logical applications. Given this background, it is not surprising
that recent intensive efforts have been directed towards the
development of a transferable all-atom molecular force field.47–51
With this application one can simulate different interactions
present in ionic liquids (coulombic and van der Waals forces,
polarization, charge delocalisation) to understand the molecular
structure and be able to predict their various properties.
branching unit’’ in the divergent synthesis of dendrimers,53 was
synthesised according to the procedure described in the litera-
ture54 by reaction of 1,3,5-tris(bromomethyl)benzene55 with an
excess of 4,40-bipyridine in MeCN. The compound MV6+I6
(1,10,100-[benzene-1,3,5-triyltris(methylene)]tris(10-methyl-4,40-
bipyridinium) hexaiodide) was obtained in 22% yield by treating
the precursor V3+(PF6)3 with an excess of methyliodide in MeCN
(Scheme 1b). The caesium 3,4,5-tris(dodecyloxy)benzenesulfo-
nate was prepared according to a literature procedure12 by
alkylation of 3,4,5-trihydroxybenzenesulfonic acid with an excess
of 1-bromododecane in DMF (Scheme 1c). The structure of the
compounds was confirmed by 1H- and 13C-NMR, and their
purity was checked by NMR and TLC to exceed 99%. Synthesis
details are described in the ESI†.
Complexes from the cationic viologen cores and the anionic
cunitic ligand were prepared by phase transfer reactions. After
dissolving the viologen compounds in water their solution was
intensively mixed with a solution of the anionic ligand in chlo-
roform. It was observed that the viologen cores became trans-
ferred to the organic phase, since the intensive colour of the
viologen moved from the aqueous to the chloroform phase. After
21 h of stirring the organic phase was separated, dried, and
investigated by NMR spectroscopy.
The NMR spectroscopic investigation showed that the
molecules were combined in different molar ratios
(CsDOBS : MV2+I2 ¼ 2 : 1, CsDOBS : V3+Br3 ¼ 3 : 1, and
CsDOBS : MV6+I6 ¼ 6 : 1) by means of phase transfer reactions,
dependent on the viologen : sulfonate ratio, and are almost
independent of the compositions and phase volume ratio of both
the aqueous and the organic phase. The molecular structures
have implications for the organisation in the material and
increase of the number of cunitic ligands (DOBSꢁ)12 has an
influence on the physicochemical properties such as electric
conductivity and redox activity.
Fig. 1 depicts typical NMR spectra of reaction products
obtained from the organic phase ([Viologen] ¼ [CsDOBS] ¼ 0.01
M, Vviologen-solution ¼ VDOBS-solution ¼ 15 mL). In any case the
characteristic signals of the respective viologen core as well as the
signals of the anionic ligand were found (see Experimental
section). The chemical shifts of the protons of the caesium
amphiphile CsDOBS did not strongly change upon complexa-
tion, i.e. the position of its aromatic proton signal changed
of +0.04 ppm in compound MV2+(DOBS)2, while with
compounds V3+(DOBS)3 and MV6+(DOBS)6 the shifts were
+0.21 ppm and +0.11 ppm, respectively. In the 1H-NMR spectra
of the viologen core of compounds MV6+(DOBS)6 a bipyr-
idinium proton ratio of 6 : 12 : 6 and for V3+(DOBS)y a bipyr-
idinium proton ratio of 6 : 6 : 6 : 6 were observed, while the
compounds MV2+(DOBS)x showed an intensity ratio of 4 : 4
(Fig. 1a–c) respectively. The signals 1 to 4 shown in Fig. 1a–c
belong to bipyridinium units and signal 5 belongs to the aromatic
unit of the DOBS molecule. The singlet-signal of the aromatic
protons of the DOBS ligand (7.0, ., 7.25 ppm) was well sepa-
rated from the signals of the viologen cores (7.6, ., 10.3 ppm)
(Fig. 1a–c, number 5). The integration of these viologen and
DOBSꢁ signals allowed us to calculate the viologen : sulfonate
ratio of the generated substances.
In the present work, we report an easy, optimized method to
design new ‘‘viologen’’-based electroactive thermotropic liquid
crystals. The bulk solid state behaviour of such complexes,
obtained by ionic association of the donor compound (caesium
3,4,5-tris[dodecyloxy]benzenesulfonate (CsDOBS)) and two
types of acceptor compounds – rigid rod (1,10-dimethyl-4,40-
bipyridinium diiodide (MV2+I2)) and planar y-shaped (10,100-
[benzene-1,3,5-triyltris(methylene)]tris(4-pyridin-4-ylpyridinium)
tribromide (V3+Br3) or 1,10,100-[benzene-1,3,5-triyltris(methylene)]-
tris[(methyl) 4-pyridin-4-ylpyridinium] hexaiodide (MV6+I6))
(Chart 1), was studied.
Results and discussion
Synthesis
The viologen salts MV2+I2, V3+Br3, and MV6+I6 were obtained by
nucleophilic substitution (SN2) of 4,40-bipyridine with an excess
of methyl halide in acetonitrile (MeCN). 1,10-Dimethyl 4,40-
bipyridinium iodide (MV2+I2) was synthesized according to
a literature procedure52 in moderate yield (35%) from 4,40-
bipyridine using a two-fold excess of methyl iodide (Scheme 1a).
The tricationic viologen V3+Br3, consisting of a mesityl core
bearing three 4,40-bipyridine units, well-known as a ‘‘preformed
Obviously an ion exchange reaction took place, where the
viologen units were transferred from the aqueous phase into
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Chart 1 Molecular structures of the viologen units with two (MV2+), three (V3+) and six (MV6+) positive charges, as well as the anionic cunitic ligand
(CsDOBS) used in this study.
Scheme 1 Synthesis of the viologen cores MV2+I2 (a), V3+Br3 and MV6+I6 (b), and the anionic cunitic ligand CsDOBS (c).
the organic phase. Electroneutrality demands that an equivalent
amount of caesium ions moved from the chloroform phase into
the water phase. The effect of the DOBS : viologen molar ratio as
applied during the exchange reaction on the composition of the
reaction product was investigated with the example of the
V3+(DOBS)y system. In a series of seven experiments equal
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Fig. 1 1H-NMR spectra of the compounds MV2+(DOBS)2 (A), MV3+(DOBS)3 (B) and MV6+(DOBS)6 (C) as obtained from CsDOBS:viologen ¼
1 mol : 1 mol reaction mixtures.
volumes of aqueous and chloroform phases were mixed. While
the initial concentration of viologen in the aqueous phase was
kept constant ([viologen] ¼ 0.01 M) the sulfonate concentration
in the chloroform phase was varied between 0.025 M and 0.04 M,
respectively. After isolation of the compounds from the organic
phase their compositions were calculated from the intensity
ratios of the integrated signals at d ¼ 7.2 ppm (DOBSꢁ, two
aromatic protons) and at d ¼ 8.9 ppm (V3+, three aromatic
protons) according to eqn (1):
NDOBS 3 A7:2ppm
¼
y ¼
(1)
NV3þ
2 A8:9ppm
NDOBS/NV3+ ¼ molar ratio of DOBS : V3+ - units in a complex,
A7.2ppm ¼ integrated 1H-NMR signal at d ¼ 7.2 ppm, A8.9ppm
¼
1
integrated H-NMR signal at d ¼ 8.9 ppm.
As depicted in Fig. 2 the measured molar composition y of the
obtained V3+(DOBS)y compounds does not equal the initial
CsDOBS : V3+Br3 ratio used in the reaction mixture. In fact the
compounds composition was fairly constant and close to y ¼ 3 as
long as the CsDOBS : V3+Br3 ratio was below 2.0, indicating the
formation of a stoichiometric 3 : 1 complex V3+(DOBS)3. When
a threefold excess of CsDOBS over V3+Br3 was applied, the
isolated compound contained about 5.2 DOBSꢁ units per viol-
ogen core, and with a fourfold excess it was composed of
6.5 DOBSꢁ molecules per V3+-core. This result demonstrates
a strong affinity between the organic ions to form a well defined
supramolecular unit by means of ion-pairing, and entropy gain
by releasing small ions (Cs+) into the aqueous phase.
Fig. 2 Study of combination of V3+ and DOBS by several molar ratios at
0.01 M concentration.
Obviously the organic phase tends to take up a sufficient
amount of the organic cation to exchange all inorganic anions
against the organic counterpart, i.e. the viologen transfer
proceeds until each cationic group of the respective viologen core
is paired with one DOBSꢁ counterion. However, the mutual
attraction is not strong enough to completely deplete the aqueous
phase of viologen cations, since the ‘‘ideal’’ 3 : 1 composition in
the reaction mixture does not result in the formation of the
stoichiometric V3+(DOBS)3 compound, leaving an ‘‘emptied’’
aqueous phase behind.
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Table 1 Phase transitions of compounds MV2+(DOBS)2, V3+(DOBS)3,
and MV6+(DOBS)6 during the 1st heating run (DSC onset values, dT/dt ¼
Subsequent to this experiment three DOBS:viologen
complexes (MV2+(DOBS)2, V3+(DOBS)3, and MV6+(DOBS)6)
have been prepared from initially equimolar sulfonate : viologen
ratios in the reaction mixtures (1H-NMR: cf. Fig. 1). All
complexes derived from the bi-, tri- and hexa-cationic cores were
characterized by a complete exchange of caesium against viol-
ogen cations, resulting in the sulfonate : viologen compositions.
MV2+(DOBS)2, V3+(DOBS)3, and MV6+(DOBS)6 being totally
neutralized with sulfonate ions.
10 K minꢁ1
Sample
)
Phase transitiona
MV2+(DOBS)2
V3+(DOBS)3
MV6+(DOBS)6
Cr. 64.7 ꢀC (91.8 kJ molꢁ1) F 270 ꢀC, Ib
Cr. 36.5 ꢀC (98.8 kJ molꢁ1) F 230 ꢀC, Ib
Cr. 39.0 ꢀC (77.7 kJ molꢁ1) F 230 ꢀC, Ib
a
Transition temperatures (ꢀC) and enthalpies (kJ molꢁ1, in parentheses)
b
determined by DSC during the 1st heating run. Isotropisation
The caesium content of the compounds was controlled inde-
pendently by elemental analysis (cf. Experimental section).
temperatures were determined by thermo-optical analysis; Cr ¼ crystal
phase, F ¼ mesophase, I ¼ isotropic phase.
Thermal analysis
and 80 ꢀC (V3+(DOBS)3 and MV6+(DOBS)6), respectively, prior
to cooling and re-ꢀheating to these temperatures. MV2+(DOBS)2
Thermal characterisation was carried out by a combination of
DSC and hot-stage thermo-optical polarised microscopy
(TOPM). Transition temperatures were all determinated by DSC
at a heating/cooling rate of 10 K minꢁ1. The DSC measurement
program consisted of several subsequent heating and cooling
runs to detect the occurrence of reversible and irreversible
thermal transitions. The measured DSC thermograms are
depicted in Fig. 3.
ꢀ
crystallized at 30 C, and melted at 44 C, while the other two
compounds vitrified upon cooling, forming mesoglasses that
softened during the subsequent heating runs (V3+(DOBS)3: ꢁ25
ꢀC, MV6+(DOBS)6: ꢁ20 C).
ꢀ
The optical texture between crossed polarizers of compound
MV2+(DOBS)2 (Fig. 4d) consists of large domains (ꢂ100 mm),
the texture in the liquid crystal phase and the specific types of
defects formed by its mesophase exhibit features typical for
hexagonal columnar arrangements. In Fig. 3d–f, a so-called
‘‘developable domain’’56 type of defect was detected, which can be
a signature of a hexagonal columnar phase.57
During the first heating scan all three compounds exhibited an
intensive endothermic signal at 70 ꢀC, (MV2+(ꢀDOBS)2: 65 ꢀC,
V3+(DOBS)3: 36.5 C, and MV6+(DOBS)6: 39 C, cf. Table 1),
ꢀ
and above 200 ꢀC decomposition took place. Thermo-optical
The textures of compounds V3+(DOBS)3 and MV6+(DOBS)6
are similar to each other, and distinctly different from that of
compound MV2+(DOBS)2. The finely grained ‘needle’-like
texture can be characteristic for columnar phases; however
the exact phase type, i.e. hexagonal vs. rectangular and ordered
vs. disordered, cannot surely be deduced from the present
data.
analysis in polarized light revealed the presence of textures and
ꢀ
the presence of a liquid phase, at 100 C; hence the endotherms
represent the melting transitions into mesophases. ꢀThe meso-
phases were found to become isotropic around 230 C, accom-
panied by decomposition of the substances.
To avoid decomposition reactions in subsequent DSC exper-
iments, the compounds were heated to 100 C (MV2+(DOBS)2)
ꢀ
Fig. 3 DSC thermograms of compounds MV2+(DOBS)2 (a), V3+(DOBS)3 (b), and MV6+(DOBS)6 (c) (black line 1st-, red line 2nd-, and blue line 3rd-
heating and cooling cycles), and (d–f) optical textures between crossed polarisers at 100 ꢀC.
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Fig.
4
Current versus temperature of (A) MV2+(DOBS)2, (-)
V3+(DOBS)3 and (:) MV6+(DOBS)6 melts, at constant voltage.
Fig. 5 Arrhenius plots of ionic conductivity vs. reciprocal temperature
for (A) MV2+(DOBS)2, (-) V3+(DOBS)3 and (:) MV6+(DOBS)6 melts
at constant voltage.
Ionic conductivity and electrochemistry
The quaternized 4,40-bipyridinium units called ‘‘viologen’’ offer
a suitable redox substrate for heterogeneous electron transfer
studies because they have low redox potential with a significant
degree of reversibility. The reduction of a viologen unit is a two
step process, consisting of the reduction of a viologen dication to
a radical monocation, and the subsequent reduction into
a neutral species. The colouring/bleaching reaction of the chro-
mophoric viologen group by the direct current is shown in
Scheme 2.
The MV2+(DOBS)2, V3+(DOBS)3 and MV6+(DOBS)6
complexes as a solid solvent may promote a partial dissociation
of the viologen salt (VAn2) into divalent cations (V2+) and anions
(An). When a divalent cation V2+ obtains an electron from the
cathode, it is reduced to a radical cation V+_, and it is accompa-
nied by an anion migrating toward the anode in order to keep
electro-neutrality of the electroactive film in an EDC. Because
the bleached state of viologen (dication) is reduced to a colored
state (radical cation) as a result of electron transfer, the fast
redox reaction of the viologen species promotes a rapid EDC
response time. These cation radicals exhibit a bright bluish violet
colour and they could be reduced further to a neutral group (V0)
upon applying a higher voltage. In other words, high device ionic
conductivity increases the electron transfer rate by ions in the
device.
energy for charge transport at a potential of ꢁ2 V is 56 kJ molꢁ1
for MV2+(DOBS)2, 50 kJ molꢁ1 for V3+(DOBS)3 and 42 kJ molꢁ1
for MV6+(DOBS)6 respectively. Coulombic work terms are
absent, the melt is undiluted and the complexes are forced into
intimate contact with the electrodes. The DSC curves illustrate
that the mesophase transition temperature are 36.5 ꢀC
(V3+(DOBS)3), 39 ꢀC (MV6+(DOBS)6), 64.7 ꢀC (MV2+(DOBS)2)
respectively and the crystal isotropisation temperature (deter-
mined by PL-microscopy) takes place above 230 ꢀC for all three
compounds. Fig. 5 shows an Arrhenius plot that suggests that the
ion transport behaviour is strongly dependent on temperature
and molecular arrangement in the complexes.
Cyclic voltammetry (CV)
Current–voltage responses of EDC
5 mm thin films of
MV2+(DOBS)2, V3+(DOBS)3 and MV6+(DOBS)6 respectively at
75 ꢀC with a slow sweep 10 mV sꢁ1 are displayed in Fig. 6.
The voltammetric wave shape indicates that the interfacial
The conductivity
s
measurements of MV2+(DOBS)2,
V3+(DOBS)3 and MV6+(DOBS)6 melts were performed by
applying a potential of ꢁ2 V in the temperature range between 30
ꢀ
and 75 C. The viscosity of the films decreased with increasing
temperature and in the mesomorphic state the complexes began
to transport electrical charge. On the other hand, the viologen
units act as electroactive groups and ion sources to produce an
electrochromic function as ion carriers. All these compounds
exhibited the behaviour of 2nd order conductors, the current
growing with temperature (Fig. 4).
The logarithm of ionic conductivity measured in the range
from 30 to 75 ꢀC is depicted in Fig. 5. The activation barrier
Fig. 6 Cyclic voltammogram of EDC; thickness cca. 5 mm; area ꢂ 0.042
Scheme 2 Dissociation and redox reactions of viologen species.
This journal is ª The Royal Society of Chemistry 2012
cm2; scan rate 10 mV sꢁ1 at 75 ꢀC.
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electrolysis is occurring, which in turn means that the ionic
conductivity of the melt phase is substantial. The MV2+(DOBS)2
shows an intensive blue colour in the reduced state and
a reversible cathode peak current in the cyclic voltammogram
appears at ꢁ500 mV. In the MV6+(DOBS)6 complex, neither
a blue colour nor reduction cathode peak current could be
observed. The second reduction step is, on average, less affected
by the number of amphiphilic chains. The V3+(DOBS)3 is not
expected to become coloured or to exhibit reduction cathode
peak current, because the monopyridinium cannot reach the
cation radical state. We assume that the electrochromic colour-
ing is concerned with the specific microenvironment of the viol-
ogen chromophoric group. When a high amount of amphiphilic
species are employed e.g. device MV6+(DOBS)6, the ionic
conductivity is relatively low, two times less than in the case of
MV2+(DOBS)2, because of non-conductivity of the alkyl chain
and the low concentration of the electrolyte in the device.
Moreover the reduced states of viologen units in the
MV2+(DOBS)2 complex at E ¼ ꢁ2 V have influence on molecular
arrangement in the crystal structure which is observed in the
texture of the liquid crystal by polarisation microscopy (Fig. 7a
and b). The SmA phase can be obtained by cooling either
a nematic or the isotropic phase. Two important microscopic
textures of SmA are the homeotropic texture and the focal-conic
fan texture. Observation using POM shows a black area of
homeotropic texture except for the area around air bubbles.58
The common texture for SmA is the focal conic fan, which is
formed upon cooling the isotropic phase (Fig. 7b).58
The temperature effect on the electrochemical properties of
MV2+(DOBS)2, V3+(DOBS)3 and MV6+(DOBS)6 melts
The samples exhibit in the oxidised states an absorption
maximum around 400 nm, which relates to a yellow colour.
When 0.5 mm thin films of the complexes confined between
microscope glass slides were only heated to 75 ꢀC without
application of an electric voltage, a strong blue colour was
observed with MV2+(DOBS)2 after a short time, with an
absorption maximum at 600 nm, while the MV6+(DOBS)6
sample did not exhibit any colour at the same temperature
(Fig. 8). As expected the V3+(DOBS)3 did not become colorized
because the state of V+_ cannot be reached. In the case of
MV2+(DOBS)2 by increase of temperature over 75 ꢀC the
electron
transfer
process
from
3,4,5-tris(dodecyloxy)
Fig. 7 Optical textures between crossed polarisers in oxidized (a, c and e) 0 V and reduced (b, d and f) states ꢁ2 V of MV2+(DOBS)2, V3+(DOBS)3 and
MV6+(DOBS)6 melt by T ¼ 65 ꢀC, between two ITO glass slides.
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benzenesulfonate (DOBS) moieties to viologen units is facili-
tated, required for colouring the radical cation species V+_. The
thermal effect is clearly observed offering interesting evidence of
structure dependent behaviour at the molecular level (Fig. 8).
Moreover a pairing of two radical-cation viologen units V+_–V+_
occurs, confirmed by an absorption maximum at 900 nm. This is
influenced by the possibility of the 3,4,5-tris(dodecyloxy)benze-
nesulfonate (DOBS) moieties to adopt the necessary conforma-
tions to keep the two units close to each other. For an isotropic
solution the dimerisation of the viologen radical cations to form
a dimer through spin-pairing has been well documented in the
literature.27,28 Free energies of dimerisation for radical cations of
viologen bromides in water were in the range from ꢁ20 to ꢁ30 kJ
molꢁ1. This observation also suggested that the two units were
closely stacked together because the pairing is unlikely for
charged dications by large electrostatic repulsion and it is
expected to be weak for neutral reduced species. Also in this case,
the observed electrochromic effects of the MV2+(DOBS)2
compound may be concerned with the microenvironment of the
chromophoric groups, its number of amphiphilic units and the
molecular structure.
simulation. The electrostatic and the van der Waals (VDW)
interactions are two competitive interactions in the ILCs. The
VDW interactions are only significant when the nonpolar groups
come very close while the electrostatic interactions dominate the
local behavior of the charged groups (Fig. 10a and b, red color:
viologen – cations).
The electrostatic interactions on the nonpolar tail groups are
not so strong in the MV2+(DOBS)2 complex compared with the
collective VDW interaction in the MV6+(DOBS)6. In the
MV2+(DOBS)2 complex the alkyl chain moieties are three fold
fewer than in the MV6+(DOBS)6 and the amphiphilic character is
not apparent, the cations distribute almost uniformly in the
space, with the diffusive behavior of the ions (Fig. 9a and b, red
color: viologen – cations). With an increase of the number of
amphiphilic units in the MV6+(DOBS)6 complex the heteroge-
neously distributed tail domains result in effective binding
energies between cations, and the collective VDW interactions of
the amphiphilic part of complexes are strong so the ions move
resulting in a very slow diffusion (Fig. 9a and b, yellow color:
alkyl chain). This explains the experimentally observed ion/
electron diffusion decrease with the increase of number of
amphiphilic moieties in the structure.
On the other hand we found a new very interesting electro-
active thermotropic liquid crystal: MV2+(DOBS)2 which changed
the color from yellow to blue-green above 75 ꢀC with an
absorption maximum at 600 nm. The colour induced by
temperature is maintained above 98% at 25 ꢀC after one day.
The peculiar behaviour of the MV2+(DOBS)2 complex can be
explained by tail aggregation and spatial heterogeneity discov-
ered earlier59–61 for ILs and supported also by molecular dynamic
Fig. 10 depicts the proposed molecular arrangements in the
mesophases of the three viologen:sulfonate complexes
MV2+(DOBS)2, V3+(DOBS)3 and MV6+(DOBS)6. The model of
the MV2+(DOBS)2 complex was developed in analogy to the
supramolecular mesogens consisting of bipyridine cores and
tris(alkoxy)benzoic acid ligands,62 while the proposed model of
the V3+(DOBS)3 and MV6+(DOBS)6 complex is based on the
‘‘core–shell’’ columnar systems made up of benzene-tris(imida-
zoline) cores, surrounded by tris(alkoxy)benzoic acid moieties.63
These supramolecular complexes obtained by ionic association
may be useful as a model system for the creation and study of
new classes of molten materials with well-defined structural
changes programmed by a mesogenic core and monitored by
electrochemical signals. The investigation of the structural
behavior of such type of ILCs is therefore of future interest.
Experimental section
Compounds: MV2+I2, V3+Br3, MV6+I6 and CsDOBS were
prepared according to published procedures12,52–55 (see ESI†) and
characterized by NMR, IR and elemental analysis.
Molecular Characterization
1H-NMR (500 MHz) and 13C-NMR (125 MHz) spectra were
recorded on a Bruker AMX-500 spectrometer, using tetrame-
thylsilane (TMS) as an internal standard and deuterated chlo-
roform or deuterated water at 20 ꢀC as a solvent. Elemental
analyses were made on a VarioMICRO cube device and IR
analysis on a Bruker FT-IR Spectrophotometer Vertex 70.
DSC
Differential scanning calorimetry (DSC) measurements were
performed on samples of about 4–12 mg. Phase transitions were
measured on a Netzsch DSC 204 ‘Phoenix’ differential scanning
calorimeter. The heating and cooling rates were 10 ꢀC minꢁ1
.
Fig. 8 Vis absorption spectra of MV2+(DOBS)2 and MV6+(DOBS)6
melts at T ¼ 75 ꢀC, between two microscope glass slides.
Indium, tin and cyclohexane were used as calibration standards.
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Fig. 9 (a) Top views (tube models), and (b) side views (CPK models) of single columns, each consisting of 16 stacked molecular units of MV2+(DOBS)2
(left), V3+(DOBS)3 (middle), and MV6+(DOBS)6 (right) as obtained from force field modeling calculations (MacroModel 8.0, OPLS-2005, PCRG (g <
10ꢁ3 kJ Aꢁ1) in vacuo). Red color: viologen – cations, yellow color: DOBS – anions. The cycles in blue illustrate the approximate positions of the isolated
tail domain-spatial heterogeneities.
Fig. 10 Schematic sketch of the proposed molecular arrangement in the columnar mesophases of the viologen:sulfonate complexes.
14434 | J. Mater. Chem., 2012, 22, 14426–14437
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Thermo-optical polarizing microscopy
over anhydrous Mg2SO4. After evaporation of the sol the
ꢀ
resulting solid was dried for 12 h under vacuum at 45 C. The
corresponding amount of MV2+I2, V3+Br3, MV6+Br6 and
A Zeiss AXIOPLAN 2 polarising microscope was used, equip-
ped with a Mettler FP 80 hot stage. Pictures were taken using
a digital Zeiss AxioCam MRC4 camera with a resolution of 4
megapixels in combination with Zeiss AxioVision software.
CsDOBS respectively, as well as obtained products, are listed in
1
Table 2. MV2+(DOBS)2: H-NMR (500 MHz, CDCl3, 20 ꢀC,
TMS; d (ppm)): 9.03 (d, 4H, J ¼ 6.0 Hz), 8.61 (d, 4H, J ¼ 6.0 Hz),
7.01 (s, 4H, aromatic), 4.44 (s, 6H), 3.92 (t, 8H, OCH2, J ¼ 6.2
Hz), 3.86 (t, 4H, OCH2, J ¼ 6.7 Hz), 1.73–1.65 (m, 12H,
OCH2CH2), 1.40–1.36 (m, 12H, O(CH2)2CH2), 1.22 (overlapped
peaks, 96H, O(CH2)3(CH2)8), 0.84 (t, 18H, CH3, J ¼ 6.7 Hz).
UV-VIS spectroscopy
VIS-spectra are performed with a Hewlett-Packard 8453 UV-
visible diode-array spectrophotometer, Agilent, Germany. The
reference for VIS spectra of compounds was the two microscope
glass slides placed one upon the other in air.
13C-NMR (125 MHz, CDCl3, 20 C, TMS; d (ppm)): 152.99(s),
ꢀ
148.62(s), 147.26(d), 141.11(s), 139.50(s), 127.20(d), 104.76(d),
73.68(t), 69.56(t), 49.16(q), 32.15(t), 30.59(t), 29.87(t), 29.73(t),
26.44(t), 22.89(t), 14.28(q). IR (ATR, cmꢁ1): 3114 (br), 3060(m),
2918(s), 2850(s), 1643(m), 1583(m), 1494(m), 1465(m), 1420(m),
1381(m), 1314(s), 1184(s), 1108(s), 1045(s), 837(m), 719(w),
643(m), 599(w), 540(w). Elemental analysis: calculated for
C96H168N2O12S2 (wt%), C 71.77, H 10.54, N 1.74; found, C
70.80, H 11.28, N 1.61.
V3+(DOBS)3: 1H-NMR (500 MHz, CDCl3, 20 ꢀC, TMS;
d (ppm)): 10.08 (d, 6H, J ¼ 6.5 Hz), 8.88 (s, 3H), 8.80 (d, 6H, J ¼
6.0 Hz), 8.16 (d, 6H, J ¼ 6.5 Hz), 7.53 (d, 6H, J ¼ 5.5 Hz), 7.20 (s,
6H, aromatic), 5.98 (s, 6H), 3.97–3.91 (m, 18H, OCH2), 1.76–
1.68 (m, 18H, OCH2CH2), 1.45–1.35 (m, 18H, O(CH2)2CH2),
1.23 (overlapped peaks, 144H, O(CH2)3(CH2)8), 0.84 (t, 27H,
Electrochemistry
CVs were measured with the potentiostat PGSTAT 302N from
AUTOLAB, controlled by a PC running under GPES for
Windows, version 4.9 (ECO Chemie B.V.). For electrochemical
characterizations of compounds (cyclic voltammogram and
electric conductivity) an electrochromic display device (EDC) –
two electrode system – without an electrolyte was used.
Electrochromic display device (EDC) fabrication
Two glass slides (60 mm ꢃ 70 mm) coated with ITO (In-doped
SnO2, 20 U cmꢁ2) from BTE Bedampfungstechnik (Elsoff, Ger-
many) were used as received. The glasses were cleaned according
to the method reported by Hillebrandt et al.64 prior to use. A
droplet of materials (cca. 4 mg) MV2+(DOBS)2, V3+(DOBS)3 and
MV6+(DOBS)6, respectively, were placed, heating and pressing in
between two glass pieces of ITO with a ‘‘Sellotape’’ film as
spacers (Fig. 11). The electroactive films on ITO with an
approximate area of 0.42 cm2 and a thickness of about 5 mm act
as the working electrode and counter electrode, respectively.
CH3, J ¼ 3.5 Hz). 13C-NMR (125 MHz, CDCl3, 20 C, TMS;
ꢀ
d (ppm)): 154.28(s), 153.20(s), 151.58(d), 147.00(d), 141.08(s),
140.96(s), 140.02(s), 136.04(s), 133.57(d), 126.55(d), 121.50(d),
105.01(d), 73.71(t), 69.59(t), 63.24(t), 32.12(t), 30.55(t), 29.88(t),
29.62(t), 26.45(t), 22.86(t), 14.27(q). IR (ATR, cmꢁ1): 3451 (br),
3055(m), 2918(s), 2850(s), 1643(m), 1585(m), 1494(m), 1462(m),
1418(m), 1382(m), 1313(s), 1184(s), 1109(s), 1041(s), 813(m),
719(w), 643(m), 601(w), 535(w). Elemental analysis: calculated
for C165H264N6O18S3 (wt%), C 72.96, H 9.80, N 3.09; found, C
71.61, H 10.32, N 2.84.
1
MV6+(DOBS)6: H-NMR (500 MHz, CDCl3, 20 ꢀC, TMS;
Synthesis of the viologen:sulfonate complexes MV2+(DOBS)2,
V3+(DOBS)3, and MV6+(DOBS)6
d (ppm)): 10.29 (d, 6H, J ¼ 6.0 Hz), 8.98 (s, 3H), 8.87 (d, 12H, J ¼
7.0 Hz), 8.66 (d, 6H, J ¼ 6.5 Hz), 7.16 (s, 12H, aromatic), 6.02 (s,
6H), 4.25 (s, 9H), 3.99 (t, 24H, OCH2, J ¼ 6.0 Hz), 3.91 (t, 12H,
OCH2, J ¼ 6.5 Hz), 1.77–1.67 (m, 36H, OCH2CH2), 1.44–1.38
(m, 36H, O(CH2)2CH2), 1.23 (overlapped peaks, 288H,
O(CH2)3(CH2)8), 0.85 (t, 54H, CH3, J ¼ 3.2 Hz). 13C-NMR (125
MHz, CDCl3, 20 ꢀC, TMS; d (ppm)): 153.14(s), 148.91(s),
148.48(s), 147.23(d), 147.07(d), 140.72(s), 139.83(s), 137.28(s),
132.94(d), 128.67(d), 127.53(d), 104.87(d), 73.70(t), 69.60(t),
63.70(t), 48.65(q), 32.15(t), 30.59(t), 29.87(t), 29.73(t), 26.44(t),
22.89(t), 14.28(q). IR (ATR, cmꢁ1): 3421 (br), 3058(m), 2919(s),
2851(s), 1639(m), 1583(m), 1495(m), 1464(m), 1421(m), 1381(m),
1313(s), 1187(s), 1109(s), 1044(s), 836(m), 722(w), 659(m),
At ambient temperature an aqueous 0.01 M solution of MV2+I2,
V3+Br3, or MV6+Br6, respectively, was mixed together with an
equal corresponding volume of chloroform solution 0.01 M of
CsDOBS (1 : 1 molar ratio). The mixtures were intensively stir-
red at room temperature for 12 hours. Subsequently, the organic
phase was separated, thoroughly washed with water and dried
Table 2 The corresponding amount of ‘‘viologen’’ cores (Vn+) and cunitic
ligand/Cs(DOBS)
V
(mg (mmol))
n+ (n ¼ 2, 3 and 6)
Cs(DOBS)
(mg (mmol))
Yield of ionic
complex
Ionic complex
Fig. 11 Electrochromic display device (EDC) construction of
MV2+(DOBS)2 (a), V3+(DOBS)3 (b) and MV6+(DOBS)6 (c) based all-
solid-state conductivity measurement set-up: (1) glass; (2) solid sample;
(3) SnO2 electrode.
MV2+(DOBS)2
V3+(DOBS)3
MV6+(DOBS)6
44 (0.1)
149 (0.18)
70 (0.05)
84 (0.1)
152 (0.18)
42 (0.05)
76 mg (94%)
133 mg (82%)
34 mg (83%)
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599(w), 540(w). Elemental analysis: calculated for C294H504
-
very simple to obtain, and has only one layer of film which
maintains the oxidation and reduction, respectively, at the same
time on two electrodes. Thus, the EDC functions as a double-
faced display. One of the advantages of EDC over liquid crystal
display (LCD) is the storage properties of EDC in an open
circuit. The generated complexes may be addressed as potential
anisotropic, electroactive, electron-, and ion conductive liquid
crystalline materials. The confinement of the electroactive unit in
the centre of the columns, as well as its arrangement in molecular
defined distances along the cylinder axis, may lead to the
formation of a ‘‘molecular electric wire’’. The existence of such
systems has already been demonstrated with ion conducting
columnar phases.16 The ionic liquid crystals presented have
potential applications, due to their low melting temperature,
wide temperature range, ionic conductivity, and thermal- and
electro-chromism.
N6O36S6 (wt%), C 72.19, H 10.39, N 1.72; found, C 70.99,
H 10.37, N 1.31.
The caesium content of the compounds was independently
measured by elemental analysis, while virtually no caesium
remained in the compounds MV2+(DOBS)2, ([Cs+] < 28 ppm),
V3+(DOBS)3 ([Cs+] < 3 ppm), and MV6+(DOBS)6 ([Cs+] < 4 ppm).
Molecular modelling
Molecular modelling was performed with MacroModel 8.0
(Schrodinger LLC.),65 using the ‘‘Optimized Potential for Liquid
€
Simulations’’ (OPLS-2005) force field66–68 in a vacuum. Electro-
static interactions were treated by a distance-independent
dielectric constant of 3 ¼ 1.0. The electric charges of the atoms
were attributed by the force field definition. The cutoff distance
of interactions was 0.7 nm for van der Waals, 1.2 nm for elec-
trostatic and 0.4 nm for hydrogen bond interactions. Structure
optimization of the columnar structures was performed by
application of the Polak–Ribier algorithm69 to regular stacks of
pre-optimized viologen:alkoxysulfonate layers until a final
Further investigation will include X-ray diffraction analysis to
learn more about the structure of such crystalline phases.
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was optimized as previously mentioned. In order to model the
interaction between the cationic ‘‘viologen’’ units and cunitic
ligands (DOBSꢁ), 16 stacked molecular units have been used.
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