Liquid crystalline non-covalent supermolecules of a styrylstilbazole ligand

Article abstract of DOI:10.1039/b103563k

Dimerization of a new styrylstilbazole ligand through noncovalent interactions leads to polycatenar calamitic supramolecular structures with liquid crystalline properties.

Full text of DOI:10.1039/b103563k

Liquid crystalline non-covalent supermolecules of a styrylstilbazole
ligand
Jean-François Eckert, Urszula Maciejczuk, Daniel Guillon and Jean-François Nierengarten*
Institut de Physique et Chimie des Matériaux de Strasbourg, Groupe des Matériaux Organiques,
Université Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg Cedex, France.
E-mail: niereng@ipcms.u-strasbg.fr; Fax: +33 388 10 72 46; Tel: +33 388 10 71 63
Received (in Cambridge, UK) 26th February 2001, Accepted 1st June 2001
First published as an Advance Article on the web 21st June 2001
Dimerization of a new styrylstilbazole ligand through non-
covalent interactions leads to polycatenar calamitic supra-
molecular structures with liquid crystalline properties.
3 are reported in Fig. 1. Compound 3 exhibits a strong
absorption (lmax 367 nm). Interestingly, addition of
=
increasing amounts of TFA to a solution of 3 causes dramatic
changes in the absorption spectrum, the peak being completely
shifted at ca. 10 equivalents of acid added. Isosbestic points are
maintained at 313 and 388 nm indicating that a single chemical
process occurs, i.e. protonation of the pyridine ring. The
initially pale yellow solution becomes orange–yellow at the end
of titration. The addition of acid also leads to changes in the
emission properties (lexc = 313 nm, isosbestic point). The
strong blue luminescence of the ligand (lmax = 501 nm)
progressively disappears and a very weak red-shifted emission
(lmax = 615 nm) corresponding to the luminescence of
protonated 3 is detected at the end of the titration (Fig. 1).
Addition of a base (DBU) restores the initial absorption and
luminescence properties. Therefore ligand 3 also presents
characteristic features that make it an interesting building block
for the preparation of new molecular switches.9
The observation of 3 under optical microscopy does not
reveal any liquid crystalline properties and only shows a
melting point at 83 °C. It is well known that metal complexes of
polycatenar stilbazole derivatives show mesomorphic proper-
ties.3,4 Ligand 3 is also a suitable building block for the
preparation of related metallomesogens. The trans-Pd(II) com-
plex of 3 has been prepared according to the procedure
developed by Bruce and co-workers.3 Treatment of 3 with
PdCl2(CH3CN)2 in a mixture of CH2Cl2–CH3CN 1+1 afforded
PdCl2(3)2 in 95% yield.‡
The organization of a molecular assembly into a liquid
crystalline phase requires a microphase separation resulting
from the amphipathic character of two chemically incompatible
parts in the molecules, in general a rigid core and paraffinic
chains. The self-assembly of building blocks through non-
covalent bonds is also able to generate such a structural
segregation, thus allowing molecular units without intrinsic
mesomorphic properties to form supramolecules presenting
liquid crystalline properties.1–7 The most widely used inter-
actions to obtain self-assembled structures with mesomorphic
properties are coordination to a metal center2–4 (metal-
lomesogens) or hydrogen bonds.5–7 The use of weaker inter-
actions such as van der Waals or hydrophobic forces have been
probed to a much lesser degree.8 Here we report on the synthesis
of a calamitic polycatenar non-mesogenic pyridine containing
ligand, and the preparation of supramolecular mesogens by
using coordination to a metal and ionic bonds, but also van der
Waals forces.
Observation of this Pd(II) complex under a polarizing
microscope reveals typical textures of a columnar mesophase
from 165 to 234 °C. Differential scanning calorimetry (DSC)
analyses and X-ray diffraction patterns recorded for PdCl2(3)2
at different temperatures were in full agreement with these
observations. Indeed, the X-ray pattern is characterized in the
small angle region by two sharp reflections in the ratio 1+A 3,
corresponding to a two-dimensional hexagonal lattice. The
The synthesis of 3 is depicted in Scheme 1. Treatment of
benzaldimine 17 with 4-iodotoluene under Siegrist conditions
gave iodostilbene E-2. Subsequent Heck coupling of E-2 with
4-vinylpyridine with Pd(OAc)2 as catalyst in Et3N–toluene in
presence of tri-o-tolylphosphine (POT) gave 3 in 80% yield. All
of the spectroscopic studies and elemental analyses were
consistent with the proposed molecular structures.† In partic-
ular, coupling constants of ca. 16.5 Hz for the two AB systems
corresponding to the two sets of vinylic protons in the 1H-NMR
spectrum confirmed the E stereochemistry of both double bonds
in 3. The electronic absorption and emission of styrylstilbazole
Fig. 1 Absorption and (inset) fluorescence (lexc = 313 nm, isosb. point)
spectra of CH2Cl2 solutions of ligand 3 containing 0, 0.5, 1, 2, 4, 6, 10, 100
equiv. of TFA (the emission spectrum recorded after addition of 100 eq. of
TFA is multiplied by a factor of 50).
Scheme 1 Reagents and conditions: i, 4-iodotoluene, t-BuOK, DMF,
80 °C, 1 h (50%); ii, 4-vinylpyridine, Pd(OAc)2, POT, Et3N–toluene, 90 °C,
12 h (80%).
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Chem. Commun., 2001, 1278–1279
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103563k

The treatment of 3 with TFA (1 equiv.) under similar
conditions also leads to an orange crystalline solid and the
development of typical columnar texture was observed from 70
to 143 °C for the resulting compound (Fig. 2). In addition to the
electrostatic interactions among 3 and TFA, it seems that
fluorophilic interactions allow the dimerization of the resulting
3 TFA species and, thus are able to direct their self-organization
into a columnar liquid crystalline phase. It is worth noting that
salts prepared from 3 and inorganic acids such as HCl do not
exhibit any mesomorphic properties. The latter observation
shows that protonation of 3 alone is not sufficient to produce
liquid crystalline derivatives. Thus the fluorophilic interactions
must play an important role.
The mesomorphic properties of the complexes obtained from
3 and perfluorosuberic acid or TFA have been studied by DSC
and X-ray diffraction. The hexagonal lattice parameters de-
duced from the X-ray patterns (50 Å at 140 °C for the adduct
with perfluorosuberic acid and 48 Å at 140 °C for the adduct
with TFA) are similar to that found for the complex PdCl2(3)2
(45 Å at 160 °C). This observation strongly suggests an
effective dimerization of 3 through non-covalent interactions as
discussed above.
Treatment of 3 with a palladium salt, perfluorosuberic acid or
TFA afforded discrete assemblies with new properties. Effec-
tively, 3 itself does not show any mesomorphic properties, but
its dimerization through self-assembling by coordination, ionic
or fluorophilic interactions leads to discrete supramolecular
structures with liquid crystalline properties.
We thank B. Heinrich for his help with the X-ray measure-
ments and L. Oswald for technical assistance.
Notes and references
Fig. 2 Top: optical texture observed with a polarizing microscope at 150 °C
for the complex prepared from 3 and perfluorosuberic acid. Bottom: optical
texture observed with a polarizing microscope at 120 °C for the complex
prepared from 3 and TFA.
† Selected spectroscopic data for 3: Yellow crystals (mp 83 °C). UV-Vis
(CH2Cl2): 367 (40000). dH (200 MHz, CDCl3): 8.58 (d, J 6, 2 H), 7.53 (s,
4 H), 7.37 (d, J 6, 2 H), 7.12 (AB, J 16.5, 2 H), 7.02 (AB, J 16.5, 2 H), 6.73
(s, 2 H), 4.03 (t, J 6.5, 4 H), 4.00 (t, J 6.5, 2 H), 1.80–1.70 (m, 6 H),
1.50–1.20 (m, 54 H), 0.90 (t, J 6.5, 9 H). Anal. calc. for C57H89NO3: C
81.86, H 10.73, N 1.67; found: C 81.58, H: 10.75, H 1.66%.
dimerization of 3 via coordination to a metal center is therefore
able to produce a polycatenar calamitic supramolecule with
liquid crystalline properties.
‡
Selected spectroscopic data for PdCl2(3)2: Yellow crystals. UV-Vis
(CH2Cl2): 392 (118000). dH (200 MHz, CDCl3): 8.70 (d, J 7, 4 H), 7.56 (s,
8 H), 7.42 (d, J 7, 4 H), 7.41 (d, J 16.5 Hz, 2 H), 7.08 (d, J 16.5, 2 H), 7.06
(AB, J 16, 4 H), 6.74 (s, 4 H), 4.01 (t, J 6.5 Hz, 8 H), 3.98 (t, J 6.5, 4 H),
1.80–1.70 (m, 12 H), 1.50–1.20 (m, 108 H), 0.90 (t, J 6.5, 18 H). Anal. calc.
for C114H178N2O6PdCl2: C 74.01, H 9.70, N 1.51; found: C 73.61, H: 9.52,
H 1.55%.
It has been shown that H-bonding can be used to assemble
pyridine derivatives with carboxylic acids to produce linear
structures that exhibit liquid crystalline behavior.5 The treat-
ment of styrylstilbazole 3 with various dicarboxylic acids
(oxalic acid, a,w aliphatic dicarboxylic acids of different
lengths and terephthalic acid) was therefore attempted in order
to obtain supramolecular derivatives with an appropriate shape
to produce columnar mesophases. Unfortunately, homogeneous
samples could not be obtained and macrophase separation was
observed in all the cases. The interactions (if any) of 3 with
these dicarboxylic acid derivatives seem to be too weak to allow
the preparation of the desired supramolecular assemblies. This
observation prompted us to use a more acidic derivative able to
protonate styrylstilbazole 3 in order to produce a stable adduct
thanks to the resulting ionic interactions. Slow evaporation of a
CH2Cl2 solution of 3 and perfluorosuberic acid [HO2C-
(CF2)6CO2H; 0.5 equiv.] afforded an orange crystalline solid.
The orange color and the presence of a strong band at 1623
cm21 in the IR spectrum (neat) are in good agreement with the
formation of a carboxylate salt.
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Observation of the resulting complex under a polarizing
microscope reveals an optical texture characteristic of a
columnar mesophase from 124 to 170 °C (Fig. 2). The
dicarboxylic acid and 3 seem therefore able to form a
supramolecular dimer stabilized by electrostatic forces with the
appropriate polycatenar calamitic shape to produce a columnar
liquid crystalline phase.
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Chem. Commun., 2001, 1278–1279
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