Building liquid crystals from the 5-fold symmetrical pillar[5]arene core

Article abstract of DOI:10.1039/c2cc33746k

Comparison of the liquid-crystalline properties of a pillar[5]arene core functionalized with 10 mesogenic cyanobiphenyl units with those of a corresponding model compound revealed the strong influence of the macrocyclic pillar[5]arene core on the mesomorphic properties.

Full text of DOI:10.1039/c2cc33746k

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Building liquid crystals from the 5-fold symmetrical pillar[5]arene core †
Iwona Nierengarten,a Sebastiano Guerra,b Michel Holler,a Jean-François Nierengarten,*a and Robert
Deschenaux*b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
50 properties with those of a corresponding model compound
revealed the dramatic influence of the macrocyclic
pillar[5]arene core on the mesomorphic properties.
Comparison of the liquid-crystalline properties of
a
pillar[5]arene core functionalized with 10 mesogenic
cyanobiphenyl units with those of a corresponding model
compound revealed the strong influence of the macrocyclic
The synthetic approach to the pillar[5]arene derivative rely
upon the copperꢀcatalyzed alkyneꢀazide cycloaddition
55 (CuAAC) reaction used to introduce the mesomorphic
subunits on both rims of the macrocyclic core. This
methodology has proven to be a powerful procedure for the
grafting of multiple mesogens onto a compact multifunctional
core unit6 and clickable pillar[5]arene building blocks are
60 easily available.7 As shown in Scheme 1, pillar[5]arene
derivative 3 was obtained in two steps from compound 1.
10 pillar[5]arene core on the mesomorphic properties.
Pillar[n]arenes are unique tubularꢀshaped macrocyclic
compounds made of 1,4ꢀdisubstituted hydroquinone subunits
linked by methylene bridges in their 2,5ꢀpositions.1 They are
usually prepared from 1,4ꢀdialkoxybenzene derivatives and
15 paraformaldehyde in the presence of a Lewis acid catalyst.2
Owing to the reversibility of the FriedelꢀCrafts reaction, the
cyclooligomerization is thermodynamically driven thus
allowing to obtain pillar[n]arenes in high yields.3 Depending
on the solvent and/or the Lewis acid catalyst, the major
20 cyclooligomerization product is either the cyclopentamer (n =
5) or the cyclohexamer (n = 6).4 Whereas significant research
efforts have been devoted to the study of inclusion complexes
obtained from pillar[5]arenes,1 their tubular shape has not
been exploited so far. With its unique pentagonal rigid
25 structure, the pillar[5]arene moiety appears to be an attractive
core unit for the preparation of novel liquidꢀcrystalline
materials with unconventional shape. It is known that isolated
molecules with point groups displaying 5ꢀfold symmetry must
reduce their symmetry when forming crystalline monolayers.5
30 Thus, pentagonꢀshaped subunits within closely packed
smectic layers may result in orientational and/or positional
disorder. As a result, the crystalline phase may be destabilized
but, at the same time, the intermolecular interactions between
neighboring pentagonꢀshaped moieties should contribute to
35 stabilize the liquidꢀcrystalline phase. With this idea in mind,
R
R
R
R
R
Br
O
O
O
O
O
CH2O / BF3.Et2O
O
2 R = Br
3 R = N3
NaN3 / DMF
(97%)
O
O
ClCH2CH2Cl
(40%)
O
R
O
O
O
R
R
R
Br
R
1
R
R
R
N
R
R
N
N
N
N
N
N
N
N
N
N
N
N
O
N
N
O
O
R
H
O
O
4a-c
O
O
CuSO4.5H2O
O
sodium ascorbate
CH2Cl2 / H2O
(60-88%)
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
R
we have prepared
a pillar[5]arene core decorated with
N
R
N
R
cyanobiphenyl moieties. Comparison of its liquidꢀcrystalline
R
H
R
5a-c
H
MeO
40
a
4a
4b
Laboratoire de Chimie des Matériaux Moléculaires, Université de
Strasbourg et CNRS, Ecole Européenne de Chimie, Polymères et
Matériaux, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
E-mail: nierengarten@unistra.fr
NC
O
O
H
O (CH2)10
O
4c
45 b Institut de Chimie, Université de Neuchâtel, Av. de Bellevaux 51, 2000
Neuchâtel, Switzerland
O
E-mail: robert.deschenaux@unine.ch
† Electronic Supplementary Information (ESI) available: experimental
procedures and spectroscopic data. See DOI: 10.1039/b000000x/
Scheme 1. Synthesis of clicked pillar[5]arene derivatives 5aꢀc.
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Treatment of 1 and paraformaldehyde with BF3.Et2O in 1,2ꢀ
The liquidꢀcrystalline and thermal properties of compounds
5c and 7 were investigated by polarized optical microscopy
(POM) and differential scanning calorimetry (DSC). On
dichloroethane gave pillar[5]arene 2 in 40% yield.8 Under
these conditions, no traces of the corresponding pillar[6]arene
derivative could be detected. Subsequent reaction with sodium 50 heating, model compound 7 melted at 145 °C (ꢀH = 127 kJ
5
azide in DMF at room temperature gave clickable building
block 3 in 97% yield.9 Owing to the high number of azide
residues, compound 3 was handled with special care: upon
evaporation, compound 3 was not dried under high vacuum
molꢀ1) from the crystalline state into the isotropic liquid (Fig.
1). The formation of a monotropic smectic A phase (focalꢀ
conic fan texture) was observed during the cooling run
followed by the crystallization of the sample (I → SmA:
and the use of metallic spatula avoided. Furthermore, this 55 149 °C, ꢀH = 5.8 kJ molꢀ1; SmA → Cr: 125 °C, ꢀH = 119 kJ
10 compound was always prepared on small scales (< 500 mg).
The functionalization of 3 with terminal alkynes under the
typical CuAAC conditions used for the functionalization of
multiꢀazide cores10 (CuSO4.5H2O, sodium ascorbate,
molꢀ1). On the other hand, pillar[5]arene 5c gave rise to an
enantiotropic smectic A phase (focalꢀconic fan texture and
homeotropic areas) (Fig. 2A) over a broad temperature range
(Tg: 32 °C; SmA → I: 201 °C, ꢀH = 53 kJ molꢀ1) (Fig. 2B).
CH2Cl2/H2O) was first attempted from commercially available 60 The formation of a smectic A phase for both compounds is in
15 alkynes 4a-b. The clicked derivatives 5a and 5b were thus
obtained in 60 and 86% yield, respectively. The structure of
compounds 5a-b was confirmed by their 1H and 13C NMR
agreement with the nature and structure of the cyanobiphenyl
mesogenic units.11
1
spectra as well as by mass spectrometry. Inspection of the H
NMR spectra clearly indicates the disappearance of the CH2ꢀ
20 azide signal at δ 3.67 ppm. IR data also confirmed that no
azide residues (2089 cmꢀ1) remain in the final products (Fig.
S1). Importantly, the 1H NMR spectra of 5a-b show the
typical singlet of the 1,2,3ꢀtriazole unit at δ 7.92 and 7.83
ppm, respectively (Fig. S2). The reaction conditions used for
25 the preparation of 5aꢀb from 4aꢀb were then applied to the
mesomorphic subunit 4c (Cr → SmA: 114°C; SmA → N:
156°C; N → I: 158°C) bearing a terminal alkyne function
(Fig. S4). A mixture of 3 (1 equiv.), 4c (11 equiv.),
CuSO4ꢁ5H2O (0.1 equiv.) and sodium ascorbate (0.3 equiv.)
30 in CH2Cl2/H2O (1:1) was vigorously stirred at room
temperature for 12 h. After workꢀup and purification by
column chromatography on SiO2 followed by gel permeation
chromatography (Biobeads SXꢀ1, CH2Cl2), compound 5c was
obtained in 81% yield. The structure of compound 5c was
35 confirmed by 1H and 13C NMR spectroscopy. In particular, the
1H NMR spectrum of 5c (Fig. S5) is in full agreement with its
D5hꢀsymmetrical structure and shows the expected signals for
the 10 equivalent mesogenic subunits.
65 Fig. 1. Differential scanning thermogram of compound 7 registered
during the second heatingꢀcooling cycle.
(A)
Finally, the model compound 7 bearing two mesomorphic
40 groups was prepared by following a similar synthetic route
(Scheme 2). Treatment of 1 with sodium azide in DMF gave 6
(92%). Reaction of the latter with 4c under CuAAC conditions
afforded compound 7 in 91% yield.
!
O
1 R = Br
6 R = N3
R
NaN3 / DMF
(92%)
(B)
R
O
4c / CuSO4.5H2O / sodium ascorbate
CH2Cl2 / H2O (91%)
O
N
N
N
N
R
N
R
N
O
7
O
O
CN
R
=
O (CH2)10
O
O
Fig. 2. (A) Thermal polarized optical micrograph of the texture
70 displayed by compound 5c in the smectic A phase at 183°C. (B)
Differential scanning thermogram of compound 5c registered during
the second heatingꢀcooling cycle.
45
Scheme 2. Synthesis of model compound 7.
2
|
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Based on previous studies of liquid crystalline compounds
prepared with similar cyanobiphenyl mesogenic moieties11
This research was supported by the University of
Strasbourg, the CNRS and the Swiss National Science
and on the calculated structure of compound 5c (Fig. S11), a 45 Foundation (Grant No. 200020ꢀ129501). We further thank M.
5
possible model for the supramolecular organization of 5c
within the smectic A phase can be proposed. As shown in Fig.
3, the pillar[5]arene units form the central sublayer of the
lamellar phase and the cyanobiphenyl mesomorphic units are
oriented upward and downward with respect to the plane
Schmitt for the NMR measurements.
Notes and references
1
2
P. J. Cragg and K. Sharma, Chem. Soc. Rev., 2012, 41, 597ꢀ607.
(a) T. Ogoshi, S. Kanai, S. Fujinami, T.ꢀa. Yamagishi and Y.
Nakamoto, J. Am. Chem. Soc., 2008, 130, 5022–5023; (b) T.
Ogoshi, T. Aoki, K. Kitajima, S. Fujinami, T.ꢀa. Yamagishi and Y.
Nakamoto, J. Org. Chem., 2011, 76, 328ꢀ331; (c) D. Cao, Y. Kou,
J. Liang, Z. Chen, L. Wang and H. Meier, Angew. Chem. Int. Ed.,
2009, 48, 9721ꢀ9724; (d) Y. Kou, H. Tao, D. Cao, Z. Fu, D.
Schollmeyer and H. Meier, Eur. J. Org. Chem., 2010, 6464–6470;
(e) Y. Ma, Z. Zhang, X. Ji, C. Han, J. He, Z. Abliz, W. Chen and
F. Huang, Eur. J. Org. Chem., 2011, 5331ꢀ5335.
10 containing the pillar[5]arene units.
50
55
60
65
70
75
80
85
3
4
5
6
7
M. Holler, N. Allenbach, J. Sonet and J.ꢀF. Nierengarten, Chem.
Commun., 2012, 48, 2576ꢀ2578.
H. Tao, D. Cao, L. Liu, Y. Kou, L. Wang and H. Meier, Sci. China
Chem., 2012, 55, 223ꢀ228.
T. Bauert, L. Merz, D. Bandera, M. Parschau, J. S. Siegel and K.ꢀ
H. Ernst, J. Am. Chem. Soc., 2009, 131, 3460ꢀ3461.
S. Mischler, S. Guerra and R. Deschenaux, Chem. Commun., 2012,
48, 2183ꢀ2185.
For examples of CuAAC reactions with clickable pillar[5]arene
derivatives, see: (a) T. Ogoshi, R. Shiga, M. Hashizume and T.ꢀa.
Yamagishi, Chem. Commun., 2011, 47, 6927ꢀ6929; (b) H. Zhang,
N. L. Strutt, R. S. Stoll, H. Li, Z. Zhu and J. F. Stoddart, Chem.
Commun. 2011, 47, 11420ꢀ11422; (c) N. L. Strutt, R. S. Forgan, J.
M. Spruell, Y. Y. Botros and J. F. Stoddart, J. Am. Chem. Soc.
2011, 133, 5668ꢀ5671.
Fig. 3. Postulated supramolecular organization of 5c within the
smectic A phase.
15
8
9
Y. Ma, X. Ji, F. Xiang, X. Chi, C. Han, J. He, Z. Abliz, W. Chen
and F. Huang, Chem. Commun., 2011, 47, 12340ꢀ12342.
Compound 3 has been already prepared by following another
synthetic route, see : X.ꢀB. Hu, L. Chen, W. Si, Y. Yu and J.ꢀL.
Hou, Chem. Commun., 2011, 47, 4694ꢀ4696.
Comparison of the thermal behavior of 5c and 7 reveals
interesting features. Model compound 7 was isolated in a
crystalline form suggesting favorable intermolecular π−π
interactions between the central aromatic parts of neighboring
20 molecules. By DSC, both the melting point and crystallization
temperature were detected during the heatingꢀcooling cycles.
10 (a) J. Iehl, R. Pereira de Freitas, B. DelavauxꢀNicot and J.ꢀF.
Nierengarten, Chem. Commun., 2008, 2450ꢀ2452; (b) J. Iehl and
J.ꢀF. Nierengarten, Chem. Eur. J., 2009, 15, 7306ꢀ7309; (c) J. Iehl
and J.ꢀF. Nierengarten, Chem. Commun., 2010, 46, 4160ꢀ4162; (d)
S. Cecioni, V. Oerthel, J. Iehl, M. Holler, D. Goyard, J.ꢀP. Praly,
A. Imberty, J.ꢀF. Nierengarten and S. Vidal, Chem. Eur. J., 2011,
17, 3252ꢀ3261; (e) M. Durka, K. Buffet, J. Iehl, M. Holler, J.ꢀF.
Nierengarten and S. P. Vincent, Chem. Eur. J., 2012, 18, 641ꢀ651.
11 S. Campidelli, J. Lenoble, J. Barberá, F. Paolucci, M. Marcaccio,
D. Paolucci and R. Deschenaux, Macromolecules, 2005, 38, 7915ꢀ
7925.
As
a
consequence of the strong intermolecular π−π
interactions, the crystalline phase is stabilized and only a
monotropic mesophase could form. In contrast, by linking
25 together five linear subunits through the central macrocyclic
pillar[5]arene core, the crystallization is prevented. Indeed,
compound 5c was isolated as a glass. At the same time, the
broad enantiotropic mesophase observed for 5c suggests also
that intermolecular π−π interactions between neighboring
30 pillar[5]arene cores play an important role in the stabilization
of the smectic A phase, this is in perfect agreement with the
proposed model depicted in Fig. 3.
In conclusion, we have reported the first example of liquidꢀ
35 crystalline pillar[5]arene derivative. Interestingly, the
macrocyclic core unit is capable of providing at the same time
orientational and/or positional disorder within smectic layers
to prevent crystallization and intermolecular π−π interactions
between neighboring cores to stabilize the liquidꢀcrystalline
40 smectic A phase.
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