Synthesis and ionic properties of nematic compounds bearing an ether- crown moiety: An NMR approach

Article abstract of DOI:10.1039/b000501k

Thermotropic ionic nematics with moderate transition temperatures are obtained by dissolving alkali metal salts within nematogens bearing a crown ether moiety; in these media, the ions show a significant 'apparent ionic order' as measured by quadrupolar N

Full text of DOI:10.1039/b000501k

Synthesis and ionic properties of nematic compounds bearing an ether-crown
moiety: an NMR approach†
K. Leblanc,a P. Berdagué,a J. Rault,b J.-P. Baylea and P. Judeinstein*a
a R.M.N. en Milieu Orienté, ESA 8074, Bâtiment 410, Université Paris-Sud, 91405 Orsay, France.
E-mail: pjudeins@icmo.u-psud.fr
b Laboratoire de Physique des Solides, LA 2, Bâtiment 510, Université Paris-Sud, 91405 Orsay, France
Received (in Oxford, UK) 13th January 2000, Accepted 25th May 2000
Published on the Web 26th June 2000
Thermotropic ionic nematics with moderate transition
temperatures are obtained by dissolving alkali metal salts
within nematogens bearing a crown ether moiety; in these
media, the ions show a significant ‘apparent ionic order’ as
measured by quadrupolar NMR spectroscopy.
Functionalized liquid crystals, which show the ordered struc-
tures of solids and the mobility of liquids, provide a combina-
tion of very innovative properties for numerous applications.1 It
is with this intent that grafting of poly(ethylene oxide)2–6 or
crown ether moieties has been already researched.6–10 These
segments are intriguing because they can dissolve organic
Fig. 1 Nematic range for CBSn and CHAn; transition temperatures
measured by DSC (10 °C min21) (Cr: crystalline, N: nematic, I:
molecules and ionic compounds.11 However, when these polar
units are linked to nematogen cores, microsegregation occurs
between the different segments and leads to highly ordered self-
organized phases.12 The nematic phase is most suitable for
NMR studies owing to its easy orientation in the magnetic
field.
In this work we show that liquid crystals CBSn, CHAn and
CRA6 containing lateral chains and terminal crown ethers
display a nematic phase at moderate temperatures over a wide
temperature range, even when significant amounts of salt are
dissolved within the phase. The cation binding ability of these
new nematogens was analysed by thermal analysis and NMR
studies. The three series of compounds were synthesized
following standard steps which have already been re-
ported.4,13
isotropic).
and the isotropic–nematic transitions.13 Lenghtening of the
rigid core in the CHAn series leads to the formation of
enantiotropic nematophases at fairly high temperatures relative
to the CBSn series.
To our knowledge, molecules of the CHAn series are the first
nematogens containing a crown ether moiety which show a
large liquid crystalline range and reasonably low transition
temperatures. This can be explained by the presence of three
alkyl chains which increases the fraction of the mobile
components which perturb the molecular cooperative packing
by decreasing the segregation between aromatic, polyether and
aliphatic fragments.13 This effect has been also observed in
polycatenar compounds.14 In order to decrease the transition
temperatures we have introduced lateral substituents. The
liquid-crystalline properties of the CRA6 compounds are listed
in Table 1. As expected, the presence of lateral substituents
substantially decreases the transition temperatures.
Crown ether derivatives are widely studied for their ability to
dissolve salts by complexing cations leading to ‘host–guest’
entities.11 Consequently, Li+ and Na+ salts were introduced
inside the CHA4 compound. Measured quantities of salt and
nematogen were dissolved in CHCl3–THF and heated for 1 h.
Then the solvents were removed in vacuo and further dried (0.1
mm Hg, 70 °C, 3 h). The mixtures were then studied by DSC
and polarizing microscopy. The phase diagrams are shown in
Fig. 2 as a function of the molar ratio x (x = cation/crown ether
moiety). CHA4 mixed with LiBF4 or NaCF3SO3 behave
similarly with a nematic phase observed for x up to 0.5, and a
more ordered smectic phase for higher values of x.
The transition temperatures of the compounds CBSn and
CHAn are plotted in Fig. 1. The CBSn compounds show
monotropic behaviour while all compounds in the CHAn and
CRA6 families show an enantiotropic nematophase over a wide
temperature range. As expected, the mesogenic behaviour is
very sensitive to the properties of the rigid core. The CBSn
series is based on three conjugated aromatic units. The balance
between the mobility induced by the three alkoxy chains, the
rigidity of the conjugated core and the relative rigidity of the
crown ether leads to compounds presenting monotropic behav-
iour with a large temperature difference between the melting
This molecule shows an unexpected nematic phase for a wide
range of x, whereas molecules described elsewhere in the
literature always show very ordered phases, even for low values
of x. These ordered phases are generally explained by the
Table 1 Nematic range for CRA6 compounds
R
H
Cr
•
•
•
•
N
•
•
•
•
I
•
•
•
•
131.7
137.5
120.7
75.9
163.9
159.8
129.3
88.4
158.8
157.5
126.1
83.7
65
119
< 30
< 30
Me
OMe
OC6H13
† Electronic supplementary information (ESI) available: synthesis of CBSn
and CRA6. See http://www.rsc.org/suppdata/cc/b0/b000501k/
DOI: 10.1039/b000501k
Chem. Commun., 2000, 1291–1292
This journal is © The Royal Society of Chemistry 2000
1291

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Fig. 2 Phase diagrams for CHA4 alkali salt mixtures.
Fig. 3 7Li and 11B NMR spectra of LiBF4 (0.2 mol dm23) dissolved in
CMeA6, in the isotropic and nematic state. Spectra are shown to an absolute
scale.
stacking of two molecules sharing one cation or by the
segregation of the ionic motives in columnar mesostructures.6
In these new systems, the observation of nematic phases over a
large compositional range (x) can be explained by the three
alkyl chains which prevent ionic interactions between cations,
counter anions and molecules. The size of the crown cavity is
larger than Li+ but fits Na+ ions well, while larger cations such
as Cs+ do not fit11 and attempts to obtain mesogenic phases with
this ion failed.
Further studies are in progress to understand the orientation
inside these systems and to correlate these values with ion-
crown complexation and ion-pairing.
1
Notes and references
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when ions are dissolved in anisotropic media, host–guest
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a partially ordered efg of the cation which leads to 2I equally
spaced lines. The existence of a quadrupolar splitting indicates
that ions are effectively distorted and oriented inside anisotropic
media. The measured quadrupolar splitting, Dn(Qion), represents
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3
7Li and 11B (I = ) NMR spectra obtained for CRA6 (R = Me)
2
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Chem. Commun., 2000, 1291–1292