New platforms integrating ethynyl-grafted modules for organogels and mesomorphic superstructures

Article abstract of DOI:10.1021/ol048503n

(Chemical Equation Presented) A methyldiacylaminophenyl core substituted with gallic derivatives and alkynyl functions has been synthesized from 2,6-diamino-4-iodotoluene. By heating the iodo and ethynyl molecules, typical columnar mesophases are observed

Full text of DOI:10.1021/ol048503n

ORGANIC
LETTERS
2004
Vol. 6, No. 23
4171-4174
New Platforms Integrating
Ethynyl-Grafted Modules for Organogels
and Mesomorphic Superstructures
Franck Camerel, Gilles Ulrich, and Raymond Ziessel*
Laboratoire de Chimie, Mole´culaire, Associe´ au CNRS, UniVersite´ Louis Pasteur,
Ecole de Chimie, Polyme`res, Mate´riaux de Strasbourg (ECPM), 25,
rue Becquerel, 67087 Strasbourg Cedex 02, France
ziessel@chimie.u-strasbg.fr
Received July 29, 2004 (Revised Manuscript Received September 30, 2004)
ABSTRACT
A methyldiacylaminophenyl core substituted with gallic derivatives and alkynyl functions has been synthesized from 2,6-diamino-4-iodotoluene.
By heating the iodo and ethynyl molecules, typical columnar mesophases are observed and the C12 synthons bearing a protected terminal
alkyne group gelified acetone, via the formation of interlocked fibers which are promoted by intermolecular hydrogen bonding.
The design and preparation of highly functionalized platforms
for applications in material sciences, optoelectronics, elec-
trochromism, light-emitting devices, and sensor chemistry
is an important and topical area of research.1 In addition to
the potential applications of mesogenic nanostructures in
display technology2 and charge transport,3 it appears that
liquid crystalline phases in which molecules self-assemble
into columnar superstructures recently led to the fabrication
of efficient photovoltaic devices.4 For this purpose, many
flat molecules, so-called discoid mesogens (e.g., phthalo-
cyanines, porphyrins, perylenes, dibenzopyrene, and hexa-
substituted benzene derivatives), have been engineered to
easily arrange in an axial stack in order to ensure charge
transport along the column.5 However, the shape of the
molecules is not mandatory for the emergence of columnar
mesophases. We recently found that nondiscoid compounds
with tetrahedral cores or polycatenar or phasmidic scaffolds
also provide columnar arrangements.6,7
The implementation of strong intermolecular attractive
interactions provided by arene-arene stacking or hydrogen
bonding strongly stabilizes mesophases but also favors the
emergence of fibrous superstructures such as found in
organo-gels. Hydrogen bonds are commonly responsible for
solvent immobilization by organogelators8 and to a lesser
(1) (a) Bruce, D. W., O’Hare, D., Eds. In Inorganic Materials; Wiley:
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El-ghayoury, A.; Douce, L.; Skoulios, A.; Ziessel, R. Angew. Chem., Int.
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10.1021/ol048503n CCC: $27.50
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extent to water gelation by hydrogelators.9 The main
representatives of the hydrogen-bond-based gelators are the
amide derivatives known to self-assemble into fiberlike
structures forming three-dimensional networks.10,11
Scheme 1a
As a matter of fact, the difficulty to overcome in the
engineering of such molecules able to achieve solvent
gelation and to melt into a mesomorphic state is to find the
balance between the tendency of the molecules to dissolve
or to aggregate, to melt or to microsegregate into a
noncrystalline state. One particular interesting area of
investigation is to import within the molecules a chelating
center able to interact with an incoming cation, metal salt,
or other substrate. The construction of such molecules is
dependent on the availability of key building blocks that
implement the hydrogen-bonding vectors, the flexible and
rigid parts, as well as a function able to be activated under
mild conditions to provide the conjugated ligand. Our own
experience with conjugated molecular systems allowed us
to target molecules featuring easily activable functional
groups such as iodine and capable of reacting in the presence
of low-valent palladium(0).12 Among such pivotal starting
materials, we sought to develop the preparation of a
tetrasubstituted phenyl ring 1 that provides the prerequisite
to construct molecules A integrating two secondary amides
and alkynyl-substituted bipyridine fragments. Recently, one-
dimensional organic nanostructures were engineered in
solution or self-assembled on the gold surface from hexa-
substituted phenyl rings incorporating three amido and three
ether or ethynyl functions.13
a Key: (i) (S)-(+)-citronellylbromide, Na2CO3, CH3CN, reflux,
27%; (ii) H2 (40 Bar), Pd/C, 89%; (iii) KOH, EtOH/H2O, reflux,
90%; (iv) SOCl2, quantitative.
corresponding acids that are converted to the acid chloride
by reaction with neat SOCl2 (Scheme 1). The C8, C12, and
C16 derivatives were produced in a similar way.16
With this viable protocol in hand, we turned our attention
toward the condensation of these derivatives with the diamino
compound 1 (Scheme 2). Utilizing an anhydrous acetone
solution and dry Na2CO3, the target iodo platforms 4a-d
were isolated in good yields.
Scheme 2a
Access to such platforms first requires producing the key
starting materials 1. Reduction of 2,6-dinitro-4-iodotoluene14
by unstabilized HI15 gave the desired compound in 91%
yield. This path allows the production of large quantities of
the novel and chemically stable derivative 1.
a Key: (i) Na2CO3, acetone, 27% for 4a, 86% for 4b, 76% for
4c, and 33% for 4d.
The synthesis of the phenyl rings carrying the paraffin
chains required the alkylation of the methyl ester of gallic
acid with the alkyl bromide as depicted in Scheme 1 for the
(S)-(+)-citronellyl derivative. Saponification provides the
Next, we chose to investigate the coupling of these iodo
molecules with a variety of terminal acetylenes in THF using
diisopropylamine to quench the nascent acid. We first
targeted trimethylsilylacetylene, and we were able to pro-
duced the ethynyl grafted molecules 5a and 5b in good yields
(73-83%) using a Pd-promoted cross-coupling reaction.
Deprotection of the alkynes is straightforward in the presence
of KF/methanol or NaOH/methanol and afforded 6a and 6b
in good yields (Scheme 3).
We then decided to probe the cross-coupling reaction in
a stoichoimetric ratio with 5-ethynyl-2,2′-bipyridine or 5,5′-
diethynyl-2,2′-bipyridine12b with molecule 4b in order to form
molecules bearing donor sites for the chelation of potentially
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61, 6535. (b) Grosshenny, V.; Romero, F. M. Ziessel, R. J. Org. Chem.
1997, 62, 1491. (c) Khatyr, A.; Ziessel R. J. Org. Chem. 2000, 65, 3126.
(13) (a) Nguyen, T.-Q.; Martel, R.; Avouris, P.; Bushey, M. L.; Brus,
L.; Nuckolls, C. J. Am. Chem. Soc. 2004, 126, 5234. (b) Tuleski, G. S.;
Bushey, M. L.; Kosky, J. L.; Ruter, S. J. T.; Nuckolls, C. Angew. Chem.,
Int. Ed. 2004, 43, 1836.
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Org. Lett., Vol. 6, No. 23, 2004
4172

200 and 218 °C, respectively, all of the compounds were
found to be stable up to 260 °C. DSC analyses shows that
compounds exhibit reversible first-order thermal transitions
after heating to the isotropic melt. Their phase behavior is
summarized in Table 1. The liquid-crystalline behavior of
Scheme 3a
Table 1.
a
compd
T (°C), ∆H (J g-1
)
4a
4b
4c
4d
5a
5b
6ab
6bb
7
Cr 172 (18.6) Col dec > 255.0
Cr 95 (35.2) Col1 159 (10.4) Col2 236 (11.3) I
Cr 95 (61.8) Col1 154 (7.7) Col2 215 (8.4) I
Cr < -50 Col 256 (26.2) I
Cr 82 (24.2) Col 203 (14.3) I
Cr 174 (23.1) Col 185 (9.8) I
Cr 94 (41.7) Col1 183 (13.9) Col2 200 (5.7) I
Cr 122 (19.8) Col 218 (13.3) I
Cr 86 (29.3) Col1 177 (19.7) Col2 210 (5.5) I
Cr 94 (12.4) Col dec > 289.5
a Key: (i) KF (for 6a, 62%) or NaOH (for 6b, 80%), MeOH.
luminescent transition metals. In the initial studies, the cross-
coupling in the presence of [Pd0(PPh3)4] at 60 °C failed due
to a very efficient oxidative homocoupling reaction of the
terminal alkyne providing the butadiyne derivatives. After
some experimentation, the most suitable method we found
involves the use of [PdII(PPh3)2Cl2] and CuI as reducing agent
at rt, providing ligand 7 in fair yields (Scheme 4).
8
a Transition temperature (T) and enthalpies ∆H associated to each
transition are given in parentheses, dec ) decomposition temperature.
b Transition temperature and enthalpy values obtained on the first heating
curve.
the functionalized platforms was probed by the observation
of optical textures showing homogeneous, birefringent, and
Scheme 4
During the preparation of 8, it was soon established by
thin-layer chromatography that the monosubstituted deriva-
tives are preferentially formed at the early stage of the
reaction and a steady-state concentration of this compound
is always present. The isolation of this monosubstituted
derivative is not targeted in this context but is auspicious
for the potential engineering of dissymmetric molecules
bearing different polarized vectors.
The thermal and polymorphic behavior of iodo and ethynyl
platforms was investigated by thermogravimetric analyses
(TGA), differential scanning calorimetry (DSC), and polar-
ized optical microscopy (POM). From TGA analyses, except
for 6a and 6b which decompose above the clearing point at
Figure 1. Liquid crystalline compounds viewed by optical
microscopy under crossed-polarizers (symbolized by the cross in
the corner of the picture): (a) compound 4c at 203 °C and (b)
compound 5a at 199 °C.
Org. Lett., Vol. 6, No. 23, 2004
4173

fluid domains. The optical texture observed upon slow
cooling from the isotropic melt clearly show the existence
of columnar phase associated with the appearance of the
pseudo focal-conic-like patterns for 4c or dendritic texture
for 5a (Figure 1).17 The described platforms are mesomorphic
and one can note that compound 4d is liquid crystal from
room temperature to 256 °C. X-ray experiments are currently
under progress in order to determine the exact symmetries
of these columnar phases.
The presence of amide tethers on these ambipolar struc-
tures led us to probe the gelation ability of molecules 5a
and 5b. Compound 5a displayed gelation abilities in acetone
with a minimum gelation concentration (MGC) of 1.4 g‚L-1
(Figure 2a), whereas compound 5b is soluble in acetone in
this range of concentrations (Figure 2b). Turbid gels were
formed in acetone at 25 °C after the hot and homogeneous
fluid solution of 5a was cooled. The sol to gel phase
transitions are thermally reversible.
Figure 3. SEM picture of a freeze-dried gel from compound 5a
in acetone (1.25 g L-1) deposited on silicon wafer showing the
presence of interlocked bundle of fiber.
directional intermolecular interactions. To ascertain whether
H-bonding plays a role in the formation of the observed
fibers, infrared spectroscopy was performed in these gels.
Single CdO and NH stretching vibrations were observed
for compound 5a, respectively, at 1638 and 3413 cm-1 in
the gelated state. These spectroscopic data are an unambigu-
ous signature of hydrogen-bonded amides inducing a hy-
drogen bonded network,18 in line with the aggregation of
the platform into elongated fibers. It is also surmised that
π-π interaction of the phenyl subunits might add additional
stabilization interactions.
In conclusion, we have been able to prepare new seg-
mented compounds bearing a controlled number of intercon-
nected fragments by a rational approach, using transition-
metal-catalyzed coupling reactions. In preliminary experiments
it has been observed that these compounds may be used
successfully as solvent gelators and for the emergence of
columnar nanostructures. The next phase of this work is to
complex the empty coordination sites in a controlled manner
with specific transition metal salts and to study their
properties in the gels or mesophases. It is foreseen that the
triple bond will serve as an efficient conduit for shuttling
information and the amide will stabilize the supramolecular
edifice via hydrogen bonding. Future investigations will be
tailored toward applications in material science.
Figure 2. Gelation test in acetone: (a) 5a (2.4 gL-1) and (b) 5b
(11.0 gL-1). Gel formation is confirmed by observing that the
sample does not flow when the test tube is inverted. Upon cooling
below the temperature of gelation, the complete volume of solvent
is immobilized and can support its own weight without collapsing.
SEM experiments were used to examine the morphology
of the formed gels. Images obtained from a diluted gel of
compound 5a in acetone (1.25 g L-1) freeze-dried onto
silicon wafer reveals the presence of a 3D network of
interlocked bundle of fibers with an average diameter of
∼150 nm extending over several tens of micrometers (Figure
3). It is likely that the formation of a 3D network of
interlocked fiberlike aggregates is responsible for the gelation
of acetone. The width of the fibers is large compared to the
dimension of the molecule. Therefore, the observed fibers
correspond to the bundle of smaller elongated molecular
assemblies.
Acknowledgment. The Centre National de la Recherche
Scientifique and the Universite´ Louis Pasteur supported this
work.
Supporting Information Available: Experimental pro-
cedures and characterization for all compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
The formation of elongated fiberlike aggregates indicates
that the self-assembly of compound 5a is driven by strong
OL048503N
(17) Lee, H.; Kim, D.; Lee, H.-K.; Qiu, W.; Oh, N.-K.; Zin, W.-C.; Kim,
K. Tetrahedron Lett. 2004, 45, 1019.
(18) Nakanishi, K. Infrared Absorption Spectroscopy; Holden-Day: New
York, 1962.
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