Alkali cation induced liquid crystalline properties of an oligophenylenevinylene-benzocrown ether conjugate

Article abstract of DOI:10.1016/S0040-4039(03)00560-4

An oligophenylenevinylene-benzocrown ether conjugate has been prepared and its binding selectivity for alkali metal ions evaluated by electrospray mass spectrometry. Whereas this benzocrown-ether derivative does not exhibit any liquid crystalline properties, the self-assembly by coordination to potassium or cesium leads to supramolecular sandwich complexes with mesomorphic properties.

Full text of DOI:10.1016/S0040-4039(03)00560-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3039–3042
Alkali cation induced liquid crystalline properties of an
oligophenylenevinylene-benzocrown ether conjugate
Manuel Gutie´rrez-Nava,a Matthieu Jaeggy,a He´le`ne Nierengarten,b Patrick Masson,a Daniel Guillon,a
Alain Van Dorsselaerb,* and Jean-Franc¸ois Nierengartena,*
aGroupe des Mate´riaux Organiques, Institut de Physique et Chimie des Mate´riaux de Strasbourg, Universite´ Louis Pasteur et
CNRS, 23 rue du Loess, B.P. 43, 67034 Strasbourg Cedex 2, France
bLaboratoire de Spectrome´trie de Masse Bio-Organique, Universite´ Louis Pasteur et CNRS, 25 rue Becquerel,
67087 Strasbourg Cedex 2, France
Received 30 January 2003; revised 24 February 2003; accepted 27 February 2003
Abstract—An oligophenylenevinylene-benzocrown ether conjugate has been prepared and its binding selectivity for alkali metal
ions evaluated by electrospray mass spectrometry. Whereas this benzocrown-ether derivative does not exhibit any liquid crystalline
properties, the self-assembly by coordination to potassium or cesium leads to supramolecular sandwich complexes with
mesomorphic properties. © 2003 Elsevier Science Ltd. All rights reserved.
Organic semiconducting materials have been extensively
investigated for their electronic properties,1 and the
mesoscopic organization of this class of compounds
appears today as an important issue for their applica-
tions. In this respect, p-conjugated derivatives exhibit-
ing liquid crystalline properties are of particular
interest,2 as they spontaneously form ordered assem-
blies that can be easily oriented. In recent years, the
self-assembly of p-conjugated oligomers based on non-
covalent interactions is an emerging area of
investigations3,4 and the potential of the resulting dis-
crete assemblies has not been fully explored yet for the
preparation of new liquid crystalline materials. As a
part of this research, we have developed the synthesis of
versatile mesogenic oligophenylenevinylene (OPV)
building blocks.4 In particular, we have shown that the
self-assembly of OPV molecular units through H-
bonds4a or coordination to a transition metal4b is an
efficient strategy for the preparation of supramolecules
with liquid crystalline properties. We now report the
functionalization of our OPV building blocks with a
benzocrown ether and show that the complexation of
the resulting ligand with potassium and cesium affords
a new class of alkalinomesogens. In addition, the bind-
ing properties of this new crown ether derivative have
been investigated by electrospray mass spectrometry
(ES-MS). The later technique is particularly interesting
to estimate the binding selectivities as it is very fast and
requires small amounts of analyte.
The preparation of the crown ether derivative 1 is
depicted in Scheme 1. Compound 2 was obtained in five
steps from methyl 3,4,5-trihydroxybenzoate as previ-
ously reported.5 Reduction of aldehyde 2 with LiAlH4
gave 3 in 80% yield. Treatment of 3 with trimethylsilyl
bromide (TMSBr) in CHCl3 yielded bromide 4. It is
worth noting that the choice of the appropriate condi-
tions for the preparation of bromide 4 was the key to
this synthesis. The bromination step could also be
achieved with CBr4/PPh3; however, the separation of
the resulting bromide from the triphenylphosphine
oxide by-product could not be achieved by crystalliza-
tion, neither by column chromatography. Actually,
benzylic bromide 4 was found to be unstable and
quantitative hydrolysis was observed when chromato-
graphic separation on SiO2 or Al2O3 was attempted. In
contrast, under bromination conditions using TMSBr,
the volatile by-products can be eliminated by simple
evaporation and no further purification step was
required. Therefore, compound 4 could be used in the
next step as received. Treatment of bromide 4 with
P(OEt)3 under Arbuzov conditions then gave phospho-
nate 5 in 76% yield. Finally, reaction of 5 with aldehyde
66 in the presence of t-BuOK in THF afforded the
OPV-benzocrown ether conjugate 1 in 82% yield. All
the spectroscopic studies and elemental analysis results
were consistent with the proposed molecular structures.
* Corresponding authors. Fax: 33 388 10 72 46; e-mail: niereng@
ipcms.u-strasbg.fr
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00560-4

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Scheme 1. Reagents and conditions: (i) LiAlH4, THF, 0°C (80%); (ii) TMSBr, CHCl3, 0°C; (iii) P(OEt)3, 150°C (76% from 3); (iv)
5, t-BuOK, THF, 0°C to rt (82%).
In particular, coupling constants of ca. 17 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 1.
of relative peak intensities were obtained. As expected,
the ES mass spectrum reveals the presence of three
peaks corresponding to the [1·Na+], [1·K+], and [1·Cs+]
1:1 host–guest complexes. The relative proportions,
obtained by summing the relative intensities, including
the isotope distribution, of each complex ion observed
in the ES mass spectrum are 8.4% ([1·Na+]), 84.0%
([1·K+]), and 7.6% ([1·Cs+]). As shown earlier,8 the
proportion of the various host–guest complexes
deduced from the ES-MS measurements can be directly
related to the binding selectivity of compound 1 for
Na+, K+, and Cs+. The preferred complexation of K+
with 1 is in good line with previous studies on related
crown ethers8a showing the high capability and reliabil-
ity of the ES-MS method.
The ability of crown ethers to form host–guest com-
plexes with alkali cations has been largely documented.7
The stability of the complexes and the binding selectiv-
ity depend on the size of the crown-ether macrocycle,
the best bound cation being that which fits best to the
cavity. In the present study, the binding properties of
crown ether 1 were investigated by electrospray mass
spectrometry (ESMS). To determine the selective com-
plexation of 1 with K+, Na+ and Cs+, a competitive
experiment was carried out with an equimolar mixture
(10−4 M) of CH3CO2Na, CH3CO2K, CH3CO2Cs, and 1
in CH3OH:CH2Cl2 (1:1). The resulting ES mass spec-
trum recorded at an extraction cone voltage of 60 V is
depicted in Figure 1. At this cone voltage no fragmen-
tation was observed and reproducible spectra in terms
The appeal of ES-MS stems also from its ability to
produce intact molecular ions (or pseudomolecular
ions) from the solution to the gas phase.9 Due to the
numerous successful characterizations by ES-MS of
large noncovalently bound complexes in solution,9 the
technique may be advantageously applied to the char-
acterization of the highly labile 1:2 sandwich complexes
that compound 1 is able to form with alkali metal ions.
For this study, a mixture of CH3CO2Na, CH3CO2K,
CH3CO2Cs and 1 in a 1:1:1:9 molar ratio was prepared
in CH3OH:CH2Cl2 (1:1) and analyzed by ES-MS. The
resulting ES mass spectrum recorded in the positive
mode at an extraction cone voltage of 40 V is depicted
in Figure 2. It gives rise to three peaks dominated by
the peak at m/z=2272.2 corresponding to the [Cs+·(1)2]
sandwich complex (calculated m/z=2272.1). The two
other peaks are identified as the [K+·(1)2] (calculated
m/z=2178.3), and the [Rb+·(1)2] (calculated m/z=
2224.6) sandwich complexes, respectively. The observa-
tion of the Rb+ complex is due to the presence of traces
of rubidium salt into the commercially available alkali
metal salts. In good agreement with previous studies on
18-C-6 crown ethers, no peak corresponding to the
[Na+·(1)2] sandwich complex could be observed.9
Figure 1. ES mass spectrum (Vc=60 V) recorded from an
equimolar mixture (10−4 M) of CH3CO2Na, CH3CO2K,
CH3CO2Cs, and 1 in CH3OH:CH2Cl2 (1:1).

M. Gutie´rrez-Na6a et al. / Tetrahedron Letters 44 (2003) 3039–3042
3041
the cooling transitions was observed during the follow-
ing heating run. DSC analysis of [Cs+·(1)2] gave similar
results. Finally, the X-ray diffraction patterns of both
sandwich complexes were typical of a mesomorphic
phase with a sharp diffraction peak in the small angle
region and a diffuse band in the wide angle region. The
exact structure determination of these mesophases is
still under progress.
In conclusion, a new OPV-benzocrown ether conjugate
was prepared and its binding selectivity for alkali metal
ions evaluated by ESMS. Interestingly, treatment of 1
with a potassium or a cesium salt afforded sandwich
complexes with new properties. Effectively, 1 itself does
not exhibit any liquid crystalline properties, but the
association of two ligands 1 by coordination leads to
supramolecular assemblies with mesomorphic proper-
ties. In addition, it can be added that the OPV-benzo-
crown conjugate 1 is a strongly luminescent compound
and further studies aiming to determine its potential as
a sensor for the detection of cations are under progress.
Figure 2. ES mass spectrum (Vc=40 V) recorded from a
mixture of CH3CO2Na, CH3CO2K, CH3CO2Cs and 1 in a
1:1:1:9 molar ratio in CH3OH:CH2Cl2 (1:1) (ꢀ=non-iden-
tified impurity, ꢁ=Rb+·(1)2 sandwich complex resulting
from the presence of traces of rubidium salt into the commer-
cially available alkali metal salts).
Acknowledgements
The alkali metal ion complexes of crown ether 1 were
prepared by slow evaporation of CH2Cl2/MeOH (1:1)
solutions of 1 and CH3CO2 M+ (M+=Na+, K+ or Cs+)
−
This research was supported by the French Ministry of
Research (ACI Jeunes Chercheurs) and a fellowship
from the CONACyT to M.G.N.; H.N. and A.V.D.
thank the TMR Project EU No. ERBFM-
RXCT1980226 on Nanometer Size Metallic Complexes
for the financial support of the ES-MS work. We
further thank L. Oswald and B. Heinrich for technical
help.
in various proportions. Homogeneous samples were
only obtained when the stoichiometry of the initial
mixture of 1 and CH3CO2 M+ was appropriate. In
−
other words, the complexes were obtained in a pure
form when the molar ratio of 1 and CH3CO2 M+
−
corresponded to the stoichiometry of the host–guest
complex. In the case of sodium, only equimolar mix-
tures afforded homogeneous samples suggesting the
formation of the 1:1 host–guest complex [Na+·1]. In
contrast, the potassium and cesium host–guest species
were obtained as 2:1 sandwich complexes. These obser-
vations are in good agreement with the ESMS studies.
Effectively, 2:1 complexes have been observed for
potassium and cesium, but not for sodium.
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