4′-(2-(2-Ethoxyethoxy)ethoxy)biphenyl-4-carboxylic acid-a polar smectogen for amphipathic liquid crystals

Article abstract of DOI:10.1016/j.tetlet.2009.07.002

We describe a new calamitic smectogen and some of its esters with a three-block molecular architecture (polar oxyethylene chain, rigid biphenyl and non-polar aliphatic chain) which were characterised by a combination of optical, thermical, X-ray and computational techniques. The title compound is a valuable smectogenic building block for amphipathic functional organic materials.

Full text of DOI:10.1016/j.tetlet.2009.07.002

Tetrahedron Letters 50 (2009) 5231–5234
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Tetrahedron Letters
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40-(2-(2-Ethoxyethoxy)ethoxy)biphenyl-4-carboxylic acid—a polar smectogen
for amphipathic liquid crystals
Rosana S. Montani a, Claudia M. Hegguilustoy a, Pablo G. Del Rosso a, Bertrand Donnio b, Daniel Guillon b,
Raúl O. Garay a,
*
a INQUISUR, Departamento de Química, Universidad Nacional del Sur, CP 8000 Bahía Blanca, Argentina
b Institut de Physique et Chimie des Matériaux de Strasbourg, CNRS-UDS (UMR7504), BP 43, F-67034 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe a new calamitic smectogen and some of its esters with a three-block molecular architecture
(polar oxyethylene chain, rigid biphenyl and non-polar aliphatic chain) which were characterised by a
combination of optical, thermical, X-ray and computational techniques. The title compound is a valuable
smectogenic building block for amphipathic functional organic materials.
Received 16 June 2009
Revised 29 June 2009
Accepted 1 July 2009
Available online 5 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Linear polyphilic molecules1,2 self-assemble in a variety of well-
defined anisotropic mesophases, including lamellar liquid crystal-
line phases whose occurrence led to improvements in the perfor-
mance of molecular organic materials.3 In this context, the
replacement in calamitic (i.e., rod shaped) mesogens of apolar
methylene terminal chains by polar oxyethylene moieties leads
to significant changes in their aggregation behaviour in the bulk,
at surface or phase boundaries as well as in their dipole moment
and ionophore activity; thus broadening their potential for applica-
tions in a wide range of fields, ranging from selectors in liquid
chromatography4 to nanowires or anisotropic materials for electro
or electro-optical devices.5,6 However, oligo(oxyethylene) chains
usually decrease the clearing temperatures of the mesophases of
small calamitic molecules and strongly perturb lamellar structures
such as the smectic phases.1,3 This behaviour has been customarily
ascribed to the tendency of oxyethylene units to adopt the pre-
ferred gauche conformations resulting in helical chains with larger
lateral mean area per chain and shorter length than linear aliphatic
chains of the same length. Nevertheless, the search for smectogenic
oxyethylene-containing mesogens is of interest since they are ex-
pected to show both lamellar structures and enhanced properties.
Herein we describe a new calamitic smectogen, 1a, and some of its
esters with a three-block molecular architecture (polar oxyethyl-
ene chain, rigid biphenyl and non-polar aliphatic chain) which
were characterised by a combination of optical, thermical, X-ray
diffraction, crystallographic and computational techniques.
synthetic procedures. But all attempts to obtain the dimers 5a–d
by direct esterification from the acid 1a using DCC/DMAP as well
as by condensation of the acid chloride of 1a with the diphenol
in the presence of an organic base using different conditions failed
in our hands. Finally, we succeeded by employing a direct conden-
sation which consists in the in situ treatment of the acid with a
cold mixture of SOCl2/pyridine followed by the addition of the
diphenol, a procedure that has been used to obtain high-molecular
weight phenolic polyesters.7
The liquid crystalline behaviour of 1a was initially investigated
by polarising optical microscopy (POM) and differential scanning
calorimetry (DSC). Transition temperatures and associated
enthalpies obtained from DSC thermograms of compound 1a and
its ester derivatives 3a-b and 5a-b are shown in Table 1. The DSC
thermogram of 1a shows three types of phase transition in both
heating and cooling cycles with rather large enthalpy changes
while the textures observed by POM confirmed that 1a exhibits
enantiotropic smectic and nematic phases. But the focal-conic fan
textures observed by POM as well as the interlamellar d-spacing
of 30.8 Å, which was measured from the powder X-ray diffraction
(XRD) pattern recorded at 187 °C just before sample crystallisation,
were compatible with layered structures in which the molecular
long axes of the mesogens could adopt a disposition either parallel,
a smectic A phase (SmA), or tilted (SmC) with respect to the normal
to the layer planes.
In order to shed light upon this matter, the experimental obser-
vations were complemented with theoretical calculations at the
MP3 MO level. Since the acid 1a can be considered as a dimeric cal-
amitic mesogen formed by hydrogen-bonding association,8 the
molecular length L of the dimer of 1a was calculated with the oxy-
ethylene tail in an all-anti fashion as well as with gauche conforma-
tions. Remarkably, the latter dimer conformation reached the
The synthesis of the acid 1a and its esters is outlined in Scheme
1. The use of the bromide instead of the parent tosylate signifi-
cantly improved the synthesis of 1a in terms of yield and ease of
workup. Most of the compounds were prepared by well-known
* Corresponding author. Tel.: +54 291 4595101; fax: +54 291 4595157.
E-mail address: rgaray@criba.edu.ar (R.O. Garay).
global minimum without
a need to restrain any geometric
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.002

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R. S. Montani et al. / Tetrahedron Letters 50 (2009) 5231–5234
Table 1
Phase transition temperatures and enthalpies [in square brackets] for 1a and its ester derivatives 3a–b and 5a–b
Transition temperatures/°C [D ]
H/kJ molꢀ1
Cr
SmX
SmB
SmC
SmA
N
I
1a
3a
3b
5a
5b
d
d
d
d
d
185[14.6]
90[48.2]
67[36.1]
198[81.6]
195[70.4]
d
d
d
190[5.35]
119a
d
202[2.42]
d
d
d
d
d
d
d
157[5.61]
185[6.90]
211[1.90]
251[1.60]
d
109[3.00]
151a
d
d
d
281[7.30]
303[8.50]
d
a
Transition only observed by POM.
OH
O
O
i, ii
O
Br
+
HO
O
O
OCH3
iii, iv
X
O
X
X
OH
Br (CH2)m Br
+
HO
OH
OH
+ H C (CH )
Br
2 11
HO
OH
3
1a; X = O
1b; X = CH2
v
vi
vii, viii
HO
O
(CH2)4
O
HO
OC12H25
4
2
O
O
X
X
O
O
X
X
X
O
O
X
X
O
O
(CH2)m
3a; X = O
3b; X = CH2
O
O
O
5a; X = O
5b; X = CH2
Scheme 1. Reagents and conditions: (i) NaOH/TsCl (85%); (ii) LiBr/acetone/reflux (91%); (iii) K2CO3/DMF, 100 °C (84%); (iv) KOH/MeOH–H2O, reflux (70%); (v) K2CO3/
cyclohexanone, reflux (25%); (vi) SOCl2, reflux; (vii) pyridine, 125 °C (84%); (viii) SOCl2-pyridine, 4 °C.
parameter, thus the calculated lengths were 45.9 Å and 35.2 Å,
respectively. To explain the observed value of 30.8 Å to our point
of view it seems reasonable to consider that the layer is formed
by dimers with gauche conformations that adopt an average tilt an-
gle of 28°.
The best evidence for this model was given by the single-crystal
X-ray structure of 1a that we described in detail in another report,9
which manifests two highly revealing features as can be seen in
Figure 1. Firstly, it displays a smectic-type layered structure
formed by dimers with head-to-head hydrogen-bonding interac-
tions showing a strong micro-segregation between the aromatic
moieties and the oxyethylene chains. Secondly, as a result of the
occurrence of gauche conformations around the C14–C15 and the
C17–C18 bonds, the terminal tails in the dimers are protruding
outwards so that the effective molecular length in the crystalline
phase along the direction of the dimer long axis is ca. 34 Å, in
excellent agreement with that estimated from PM3 calculations.
Figure 1 also shows the dimer conformation observed in the crys-
tallographic study along with the calculated one.
The DSC thermogram of the monoester 3a with a three-block
molecular architecture (oxyethylene chain, biphenyl and aliphatic
chain) only showed two distinct phase transitions in both heating
and cooling cycles, corresponding to the melting and clearing tran-
sitions. POM observations showed that 3a exhibits a SmA phase
with focal conic and homeotropic textures at higher temperatures
and a SmC phase with Schlieren textures at lower temperatures,
with a SmA–SmC transition at about 119 °C that was not detected
by DSC. This was confirmed by the X-ray patterns recorded be-
tween 105 °C and 150 °C, which are characteristic of SmA or SmC
Figure 1. (a) Packing diagram of 1a viewed down the c axis. (b) X-ray crystal
phases. They contain a very strong first-order reflection and a weak
structure. (c) PM3 gas-phase structure.

R. S. Montani et al. / Tetrahedron Letters 50 (2009) 5231–5234
5233
second-order reflection in the small angle region corresponding to
a
b
c
the layer stacking in addition to a diffuse reflection in the wide
region occurring from lateral distances between molecules, while
the layer spacing dependence upon temperature exhibits the usual
shape: an increase with temperature in the SmC phase, related to
the tilt angle variation (17° at 105 °C), and a very slight decrease
in the SmA phase, related to the nematic order parameter decrease
(Fig. 2b).
For this mesogen, a direct comparison can be made between the
XRD interlamellar distance of 39.9 Å observed at 120 °C in the SmA
phase and the PM3 calculated L-values. So, the molecular length L
of 3a was calculated both with the oxyethylene chain in an all-anti
conformation and with gauche conformations while the methylene
chain always was in an all-anti fashion. Accordingly, the calculated
lengths were 43.7 Å and 38.8 Å, respectively. Again, the likeness
between the experimental d-value and the calculated L-value indi-
cates that the oxyethylene chains are in a predominantly gauche
fashion within the SmA phase, a conclusion that can be extended
on thermodynamical grounds to the lower temperature SmC
phase. In addition, the calculation of the molecular coverage S of
3a of the smectic layer10 by using the molecular volume11 and
the interlamellar distance gives a value for the molecular area
S = V/d of 24 Å2, a value close to the cross-sectional area of one aro-
matic core (ca. 22 Å2). Hence, the efficient packing of the aromatic
core in the smectic layer is the dominating ordering force despite
the laterally protruding oxyethylene chains which could be inter-
twined within the layer just as in the crystalline phase of 1a to
maximise the filling of space.
Figure 3. Schematic representations of the SmA phase of 3a.
in the X-ray pattern which could be associated to any related in-
plane periodicity. Moreover, this organisation is not likely to oc-
cur due to the expected micro-segregation between aliphatic and
oxyethylene chains which are known to exhibit different amphi-
pathic properties. This later argument can also be used to rule
out a single-layer organisation formed by mesogens pointing all
upwards (Fig. 3b). The arrangement shown in Figure 3c has the
polar and non-polar tails segregated into dipolar interactions that
results in a smectic layer periodicity near 80 Å. However, only a
sharp reflection corresponding to d = 39.1 Å was again observed
in low angle part of the diffraction patterns obtained with a Gui-
nier camera that could record periodicities up to 90 Å. Likely, the
organisation is locally of the double layer type, but with a corre-
lation distance (d/2 = L) associated to the distinction of the ali-
phatic and oxyethylene sublayers shorter than the overall
correlation length d associated to the smectic organisation. This
would explain the absence of the reflection associated to the
double layer and be satisfactory upon the expected micro-segre-
gation of aliphatic and oxyethylene tails.
There are different possibilities to consider for the layer stack-
ing of 3a. Although the arrangements shown in Figure 3a and b
agree with the observed periodicity, a local packing with the
mesogens pointing upwards and downwards (Fig. 3a) is not con-
sistent with the existence of the SmC phase and there is no signal
a
The comparison between the mesomorphic behaviours of the
calamitic 3a and dimer 5a with those of their fully aliphatic ana-
logues 3b and 5b can be done to assess the effect of the oxyethyl-
ene terminal tail on the smectic phase stabilities. Indeed, the data
given in Table 1 show that the global mesomorphic behaviour of
the two pairs of compounds does not depend on the terminal chain
nature but rather on the overall molecular architecture. Thus, both
3a and 3b show SmC and SmA phases with comparable transition
enthalpies, while both 5a and 5b show non-tilted smectic phases
(SmB with mosaic textures containing homeotropic domains/
SmA with focal conic and homeotropic textures) and nematic
phase with comparable transition enthalpies. Despite the fact that
the smectic phase stability is somewhat reduced, 3a still shows an
appreciable smectic range. But the presence of two oxyethylene
tails in the dimer 5a significantly decreases the smectic range,
most probably due to its symmetric molecular design (polar/ri-
gid/non-polar/rigid/polar).
Therefore, although we are aware that the real conformations
could be changed by the neighbourhood molecules, and therefore
the application of calculated conformations should be handled
with care, the combination of experimental and modelling infor-
mation on these three-block polyphilic molecules indicates that a
substantial fraction of gauche conformations persist in the oxyeth-
ylene chains up to the isotropisation temperatures; and that their
presence does not prevent the occurrence of lamellar organisations
provided that amphipathicity is preserved in the molecular design.
In this context, 1a is a valuable smectogenic building block for
amphipathic functional organic materials.
0
5
10
15
2θ (°)
20
25
30
b
40.0
39.5
39.0
38.5
38.0
SA
SC
100
110
120
130
140
150
Temperature (ºC)
Figure 2. (a) X-ray pattern of 3a in the SmA phase at 130 °C. (b) X-ray Temperature
dependence on heating of the lamellar distance of 3a.

5234
R. S. Montani et al. / Tetrahedron Letters 50 (2009) 5231–5234
3. (a) Méry, S.; Haristoy, D.; Nicoud, J. F.; Guillon, D.; Diele, S.; Monobe, H.;
Acknowledgements
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Financial support from SGCyT-UNS and SPU-MCYT is acknowl-
edged. C.M.H. thanks SGCYT-UNS for a fellowship. R.O.G. is mem-
ber of the research staff of CONICET.
5. Fütterer, T.; Hellweg, T.; Findenegg, G.; Frahn, J.; Schlütter, D.; Böttcher, C.
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11. The molecular volume was calculated with the assumption of additivity of the
partial volume of the constituent parts of 3a. (see Ref. 10). The crystallographic
data provide a V = 425 Å3 for 1a which is the oxyethylene-biphenyl moiety of
3a. The phenoxy volume plus the 12-carbon tail volume amounts to 489 Å3.
After the correction for expansion of the methylenes with temperature, the
volume of 3a at 120 °C is 953 Å3.
Supplementary data
Experimental and computational details, full synthetic proce-
dures and 1H NMR and 13C NMR characterisation are given in the
Supplementary data. DSC and selected X-ray patterns and textures
of 1a and 3a are also included. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.07.002.
References and notes
1. Tschierske, C. J. Mater. Chem. 1998, 8, 1485–1508.
2. Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671.