Towards room-temperature ionic liquid crystals

Article abstract of DOI:10.1039/c2ta00133k

Ionic liquid crystals (ILCs) with displaying birefringence have great potential in various (bio)sensor schemes. So far, their high transition temperatures prevent their application. We demonstrate in a novel series of ILCs, based on archetypical mesogens, how to reduce clearing temperatures and we explain our results qualitatively.

Full text of DOI:10.1039/c2ta00133k

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Towards room-temperature ionic liquid crystals†
*
Alexandra Alvarez Fernandez, Laurens T. de Haan and Paul H. J. Kouwer
Cite this: J. Mater. Chem. A, 2013, 1,
354
Ionic liquid crystals (ILCs) with displaying birefringence have great potential in various (bio)sensor schemes.
So far, their high transition temperatures prevent their application. We demonstrate in a novel series of
ILCs, based on archetypical mesogens, how to reduce clearing temperatures and we explain our results
qualitatively.
Received 30th August 2012
Accepted 4th October 2012
DOI: 10.1039/c2ta00133k
www.rsc.org/MaterialsA
Since the middle 1980’s the expanding class of ionic liquid relationships with a focus on transition temperatures, but have
crystals (ILCs) has been described from many different failed to make the nal step towards application requirements.
perspectives: a merge between ionic liquids (ILs) and liquid In this study, we have combined several “classical” strategies to
crystals (LCs), ionic salts displaying a LC phase, or ion give truly room temperature applicable materials while main-
conductive anisotropic materials.1 The versatility in the deni- taining a high birefringence. The strategies originate from the
tion of these materials is reected in the wide range of appli- eld of liquid crystals and ionic liquids.
cations, ranging from typical IL applications that benet from
Three molecular parameters were studied, all inspired by
anisotropy2 to characteristic LC applications that require results from either classical LCs or ILs. The rst is the intro-
charged species.3 For instance, the bio-compatibilisation of duction of a cis fatty acid tail as a exible chain to the core. The
classical LCs with their characteristic electro-optical switching bent-shaped tail hinders their close packing, already reducing
properties opens up a new avenue of applications in biomedical transition temperatures as observed in liquid crystalline and
areas.3a
non-liquid crystalline materials.8 In order to study the thermal
Recent experiments in enzyme catalysis using ILs as solvent behaviour induced by the lateral kink, reference compounds 2f
showed that materials built from N-alkylpyridinium and and 3f (containing an octadecyl tail) were synthesised and
N-alkylimidazolium can successfully host or even accelerate respectively compared with oleyl analogues 2b and 3b. A
reactions.4 Analogously, in the design of novel (potentially second, commonly applied strategy to reduce clearing temper-
biocompatible) ILCs, the 1,3-substituted imidazolium salts have atures in LCs is the introduction of (small) lateral substituents
been introduced as a promising core in many studies. Different to the core. By placing the substituents on the 2-position of the
strategies, targeted to understand self-organization and binding imidazolium group, we anticipated to inuence the size and
interactions5–7 that induce and stabilize ILC mesophases, have conformation of the cores as well as their inter and intra-
been reported. The common approach is the modication of molecular interactions. The experience from ILs was used to
the side chains5 (changing the number, length and branching demonstrate the potential of the third parameter selected, the
pattern), and the introduction of, for instance, vinyl, hydroxyl, anion, to optimise the properties of materials, including the
and amide groups.6 Although their thermal properties are reduction of the transition temperatures. The studied materials
promising, the low birefringence of these materials disqualies are summarised in Fig. 1.
them for most optical applications. The approach towards high
birefringent ILCs has already been published: the synthesis of a
core where an imidazolium group was coupled to an electron-
rich biphenyl group.7 However, strong ionic and p–p interac-
tions associated with an ionic rigid core yield materials of high
clearing temperatures and corresponding high viscosities in
particular at room temperature. Previous studies using this
design principle have addressed various structure–property
Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg
135, 6525 AJ Nijmegen, The Netherlands. E-mail: p.kouwer@science.ru.nl
† Electronic supplementary information (ESI) available: Synthetic procedures and
Fig. 1 General structure of the investigated ILCs, changing the lateral substit-
uent R, the side chain R0 and the counter ion X.
characterisation as well as details of the modeling procedures. See DOI:
10.1039/c2ta00133k
354 | J. Mater. Chem. A, 2013, 1, 354–357
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smectic A (SmA) phases (Fig. 2) for all the samples between
crystalline (Cr) and isotropic (Iso) phases. An example of the
texture and two DSC traces are shown in Fig. 2.
ꢀ
The phase behaviour of 2, ILCs with the BF4 anion, is
summarized in Fig. 3. In general, the clearing temperatures are
lower than expected for molecules containing such a rigid core.7
This result is attributed to our rst design approach: the kink in
the terminal chain induced by the cis fatty acid tail. By
comparing compounds 2b and 2f a reduction of around 50
degrees in the clearing temperature can be related to the kink in
the side chain. The effect of the lateral substituent R on the
mesomorphic behaviour is evident: a decrease of the clearing
temperatures was observed by increasing the length of R. In
conventional mesogens based on phenyl esters, this trend has
been experimentally observed and explained in terms of steric
factors where the lateral substituent effectively changes the
length-to-breath ratio of the mesogens and, as a result, changes
the stability of the mesophase.9
The same tendency is observed for 2. In molecules with such
strongly interacting moieties, however, taking only steric factors
into account may lead to a signicant oversimplication. In the
planar conformation, with the imidazolium and the adjacent
phenyl group in the same plane, attractive p–p interactions will
play a prominent role. We anticipated that the bulky R will also
prevent the formation of the planar conformation and in this
way contribute as well to the decrease of the transition
temperatures.
The most straightforward approach to quantify these inter-
molecular interactions is to calculate the rotational energy
barriers10 between the imidazolium and phenyl groups of our
mesogens, see Fig. 4.
Simple modelling studies clearly show an increase in the
rotational energy barrier when the size of the lateral substituent
R is increased. For R ¼ H, the energetic penalty of the at
conformation is relatively small, which allows for intermolec-
ular ion-dipole and p–p interactions. Unexpectedly, a signi-
cant barrier was already found for the sterically undemanding
methyl group. The bulky isopropyl group gives the largest
Scheme 1 Synthesis of ILCs 1–3. Key: (i) (R ¼ H, CH3, C2H5) K2CO3, CuI, N,N-
dimethylglycine, DMSO, 48–72 h at 110 ꢁC; (ii) (R ¼ n-C3H7, i-C3H7) Cs2CO3, Cu2O,
4,7-dimethoxy-1,10-phenanthroline, NMP, 5 days reflux; (iii or v) NaI, acetone, rt;
(iv or vi) PTSA, toluene, 24 h, reflux (vii or viii) toluene, 80 ꢁC, 24 h; (ix) NaBF4 (aq.)
multiple washes; and (x) AgNTf2, methanol.
Compounds 1–3 were synthesised in ten steps starting from
the substituted biphenyl7 4 (see Scheme 1); the details are
described in the ESI.† The Ullmann coupling (step i), the key-
step in the protocol, was carried out using different ligands for
the copper catalyst depending on the lateral substituent R. The
oleyl‡ and octadecyl tails were functionalised with chloroacetic
acid and, aer being converted to the corresponding iodide,
used to alkylate the imidazole group, yielding 1. Ion exchange of
1 with NaBF4 or AgNTf2 gave the target compounds 2 and 3.
The mesophase behaviour of 2 and 3 was investigated using
optical polarized microscopy (OPM) and differential scanning
calorimetry (DSC).§ OPM analysis showed the formation of
Fig. 2 Polarised optical microscopy image of (a) 2a at T ¼ 176 ꢁC (SmA) and (b)
DSC traces of monotropic ILC 3d and room temperature ILC 3e. The arrows
indicate SmA-I transitions. Note that most materials we characterised display an
enantiotropic SmA phase that appears in DSC in both the heating and the cooling
run. Compound 3e is monotropic, only exhibiting the SmA phase on cooling as a
result of supercooling the Cr phase.
ꢀ
Fig. 3 Mesomorphic properties of series 2: ILCs with BF4 as a counter ion,
‡ The oleyl alcohol used was of technical grade and consisted of approximately
15% trans and 85% cis double bonds. This ratio was carried over to subsequent
products.
different lateral substituents R ¼ H, CH3, C2H5, n-C3H7 and i-C3H7 and a reference
compound 2f containing R0 ¼ C18H37 as a side chain. The crystal phase of 2b–d is
supercooled as a result of the slow crystallisation process. These materials,
instead, show a glass transition at room temperature. Such a behaviour was
observed before.7
§ In our experience, the thermal results of ILCs with the iodide counter ions (1)
provided unreliable data as a result of decomposition.
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3d and 3e was reproduced. It supports our previous conclusion
that the length of the lateral substituents is theꢀdominating
factor. Until now, the high effectiveness of NTf2 remains a
matter of speculation.
Two effects can easily be regarded: the high electronegativity
of the uorine atoms redistributes the negative charge over the
entire anion; and the anion is much larger than many of the
other (spherical) anions. Currently, we are investigating the role
of the counter ion in more detail, separating the effects of its
size, shape and charge distribution.
Conclusions
Fig. 4 Calculated energy barriers (HF 13G*) of the C1–N1 bond rotation for
different lateral substituents R incorporated in the imidazolium molecule dis-
played right.
The availability of room temperature anisotropic ionic mate-
rials with potential optical applications is restricted by the
rigidity of the core combined with its strong electrostatic
interactions. We have presented a number of design principles
to reduce clearing temperatures, aimed at hindering molecular
packing in the system: (a) introducing a kink in the terminal
chain; (b) attaching lateral substituents; and (c) making use of
different counter ions. Experimentally, the sum of these three
strategies led to a reduction of nearly two hundred degrees in
the clearing temperatures, yielding highly birefringent room
temperature ILCs. We are now studying the application of these
novel materials as anisotropic biocompatible solvents.
barrier. The reduction in conformations combined with the
classical steric effects described by Weissog9 explains the large
impact of the lateral substituents on the clearing temperature.
Evaluation of 2d and 2e allows us to compare the two effects
directly: in 2d we expect a stronger contribution from the clas-
sical steric effects and vice versa. Experimentally, a much lower
clearing temperature is observed for 2d, indicating that the
length of the n-propyl group is the dominating factor.
In order to reproduce the trend observed in Fig. 3 and to
drive our ILCs towards lower clearing temperatures, the counter
ion was exchanged. We selected the triimide (NTf2ꢀ) anion
because of its excellent behaviour in well-known and already
commercialised ILs,11 and in ILCs12 (low viscosity, low melting
temperatures). The results of seriesꢀ3 are summarized in Fig. 5.
Upon ion exchange from BF4 to NTf2ꢀ, a decrease of
roughly one hundred degrees in the clearing temperature of the
material was observed, yielding truly room temperature
processable materials. In line with the results of ILs,13 we
noticed the high uidity of these materials in comparison with 2
during the microscopy experiment. Fig. 5 clearly highlights the
role of the Coulombic interactions as a key method to drasti-
cally manipulate the transition temperatures of ILCs. The same
trend related to the nature of the substituent R for compounds
Acknowledgements
Paul Schlebos is acknowledged for the molecular modelling
studies and the EU ITN 2007-215851 “Hierarchy” for nancial
support.
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