Tuning organogels and mesophases will phenanthroline ligands and their copper complexes by inter- to intramolecular hydrogen bonds

Article abstract of DOI:10.1021/ja047091a

A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings each equipped with two long alkoxy chains has been rationally de

Full text of DOI:10.1021/ja047091a

Published on Web 09/11/2004
Tuning Organogels and Mesophases with Phenanthroline
Ligands and Their Copper Complexes by Inter- to
Intramolecular Hydrogen Bonds
Raymond Ziessel,*,† Guillaume Pickaert,† Franck Camerel,† Bertrand Donnio,|
Daniel Guillon,| Miche`le Cesario,‡ and Thierry Prange´§
Contribution from the Laboratoire de Chimie Mole´culaire, UniVersite´ Louis Pasteur,
Ecole Chimie, Polyme`res, Mate´riaux (ECPM), Centre National de la Recherche Scientifique
(CNRS), 25 rue Becquerel, F-67008 Strasbourg Cedex, France, Institut de Physique et Chimie
des Mate´riaux de Strasbourg (IPCMS), Groupe de Mate´riaux Organiques (GMO),
UniVersite´ Louis Pasteur-CNRS (UMR 7504), 23 rue du Loess, BP 43,
F-67034 Strasbourg Cedex 2, France, Institut de Chimie des Substances Naturelles, CNRS,
F-91128 Gif-sur-YVette, France, LURE, Baˆt. 209d, UniVersite´ Paris-Sud, BP 34,
F-91898 Orsay Cedex, France, and LCRB, Faculte´ de Pharmacie, 4 AV de l’ObserVatoire,
F-75006 Paris, France
Received May 18, 2004; E-mail: ziessel@chimie.u-strasbg.fr
Abstract: A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-
diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings
each equipped with two long alkoxy chains has been rationally designed. The presence of amide tethers
and a chelating phenanthroline fragment connected via an ester dipole formed a new class of gelating
reagents and mesomorphic materials. A few of these compounds have the tendency to form macromolecule-
like aggregates through noncovalent interactions in hydrocarbon solvents and were found to exhibit
thermotropic cubic mesophases. In light of the X-ray molecular structure of the methoxy ligand, an infinite
network maintained by intermolecular hydrogen bonds as well as by π-π stacking of the phenyl subunits
was evidenced. FT-IR studies confirm that the common driving force for aggregation in the organogels
and microsegregation in the mesophase is the occurrence of a tight intermolecular H-bonded network that
does not persist in diluted solution. This situation is switched when the ligands are interlocked by a copper-
(I) cation. A strong intramolecular H-bond confirmed by X-ray diffraction of a single crystal for the methoxy
case provides very stable complexes but inhibits the gelation of the solvents. Heating the complexes bearing
long paraffin chains (n ) 12 and 16) in the dried state leads to a self-organization into a columnar liquid-
crystalline phase in which the columns are arranged along a 2D oblique symmetry as deduced from powder
XRD experiments. In this case, the complexes with the appended counteranions self-assemble in a specific
way to form columns. A striking observation is that the intramolecular hydrogen bond persists in the
mesophase as it does in solution without any evidence of an extended network. As far as we are aware,
these ligands and complexes are rare examples in which organogelation and thermotropic mesomorphic
behavior could be observed in parallel with molecules bearing a chelating platform. Due to the synthetic
availability of the 4-methyl-3,5-diacylaminobenzene core and the simplicity by which the chelating platforms
can be graphed, this methodology represents a practical alternative to the production of functionalized
organogelators and mesomorphic materials.
Introduction
science. The tailoring and design of such materials with multiple
functions is a subject of intense research that holds promise for
the rational design of nanostructures with well-defined channels
and physical properties.2 Along these lines, intermolecular
hydrogen bonding has been employed for the engineering of
Intermolecular hydrogen bonding is a very topical area of
study because of its relevance to a variety of naturally occurring
processes, such as recognition and assembly of complementary
nucleobases that stabilize the double helical structure of nucleic
acids, but also in hydrogels of biopolymers for biomedical
applications.1 The ability to manipulate, switch, or frustrate
H-bonded arrays also opens novel perspectives for materials
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† LCM, ECPM, ULP.
‡ Institut de Chimie des Substances Naturelles.
§ Universite´ Paris-Sud & Faculte´ de Pharmacie.
| IPCMS, GMO.
9
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J. AM. CHEM. SOC. 2004, 126, 12403-12413
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A R T I C L E S
Ziessel et al.
organic liquid crystals, (organo)gelators, well-defined nanom-
eter-scale or mesoscopic assemblies, thin films, and polymeric
materials that self-assemble from simple elementary units.3-5
It is well accepted that hierarchical self-organization events
involving hydrogen bonding are of significant importance to
the formation of liquid-crystalline phases but also for the
stabilization of existing mesophases.6,7 However, H-bonded
metallomesogens8 have been scarcely described despite the
realization that they might possess unusual properties.9
Hydrogen bonds are commonly responsible for solvent
immobilization by organogelators and to a lesser extent for water
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and exhibiting a thermotropic mesomorphic behavior.11-13
Molecular recognition and self-assembling processes directed
by complementary hydrogen bonding have received attention
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bonding in artificial molecular systems is most effective in solid
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recent findings demonstrate that molecular recognition via
hydrogen bonding was also effective at the air-water interface,
in bulk water, or in the lyotropic liquid-crystalline state.17,18
In some studies, pyridyl or bipyridyl moieties have been used
as basic building blocks for the construction of mesomorphic
frameworks19,20 and also as hydrogen-bond acceptors.21,22 In
most of these examples, the engineering of liquid-crystalline
ligands is feasible but when complexed with transition metals,
the mesomorphic property is lost due to the introduction of
additional dipoles, geometric demands of the metal, charges,
and bulkiness of the metal center and surrounding ligands.
However, in other cases, nonmesomorphic ligands allowed one
to prepare metallomesogens by wrapping the ligands around
metals.23,24 The shape of the molecule and the balance between
the rigid core and the paraffin chains must sustain microseg-
regation during the heating cycle. In some rare cases, both
ligands and complexes display mesomorphism.25,26 One way
to overcome these difficulties is to engineer a system in which
additional supramolecular binding directors (such as hydrogen
bonding) would stabilize the mesophase in both cases (ligand
and complex). It is surmised that these additional vectors would
stabilize the mesophase within the ligand via intermolecular
hydrogen bonding, whereas in the case of the complex this
hydrogen bonding is switched into intramolecular bonds stabi-
lizing the interlocked molecular structure. Extension of the
concept of directional intermolecular interactions to organogels
would allow the gelation or nongelation of specific solvents.
Such a system is the aim of the present contribution, where we
use derivatized phenanthroline (phen) cores equipped with amide
linkage to generate cationic copper(I) and mesogenic materials.
The switching from inter- to intrahydrogen bonding is high-
lighted by the reversible immobilization of the solvent by the
ligands but not by the complexes.
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12404 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004

Tuning Organogels with Phenanthroline Ligands
A R T I C L E S
liquid-crystalline phenanthroline.29 This is surprising because
the flat and planar structure of phenanthroline should be highly
suitable for assembling such frameworks via π stacking inter-
actions into ordered mesophases. No results involving phenan-
throline gels were disclosed, whereas terpyridine-containing
carboxylic acid hydrogels have been described.30 Furthermore,
the straightforward coordination of metal centers could lead to
the formation of novel metallomesogens displaying useful
optical and/or electrical properties.31
The copper(I) complex was prepared under anaerobic condi-
tions from ligands and [Cu(CH3CN)4]BF4.41 For ligand L1, both
BF4- and ClO4- counteranions were used. The last choice was
motivated for single crystal growing. All organic compounds
were purified by careful chromatography on silica, followed
by double crystallization in adequate solvents. The air-stable
copper complexes required two crystallizations to recover the
analytically pure sample.
1
Characterization. The H NMR spectra of the copper(I)
complexes show major differences as compared to that of the
free ligand (Figure 1). The most spectacular shifts are found
for the phenyl protons Hc (for labeling, see the drawing of the
copper complex in Scheme 1), which are shielded by ca. 0.8
ppm, and for the methylene protons, which are shielded by ca.
1 ppm due to their proximity with the neighboring phenanthro-
line ring of the second ligand. This latter signal changes from
a singlet in the free ligand to an AB quartet (with JAB ) 10 Hz
and ∆ν ) 24 Hz). The prochirality of these methylene protons
is consistent with rigidification of the phenanthroline arms upon
metal coordination. Finally, a significant upfield shift of the
amides protons is observed from 8.23 ppm for ligand L12 to
8.54 ppm for the complex. Similar shifts are found with
complexes of ligands L8 and L16.
We now describe phenanthroline ligands bearing a 1,3-
diacylaminobenzene platform, which offers a suitable route for
the inclusion of transition metals. The presence of a methyl
group bisecting the central phenyl cycle is motivated by the
need to tilt the amide vectors out-of-the-plane to favor inter-
molecular hydrogen bonding in the metal free ligand. From a
general point of view, acylamino platforms have previously been
selected as efficient gelators of various organic solvents.32,33
Some of these gels appeared to have great potential as functional
soft materials.34,35 Recently, the gelation of room-temperature
nematic liquid crystals by self-aggregation of low molecular
weight molecules through hydrogen bonding has been reported,
and this new class of liquid-crystalline materials have great
potential in dynamic materials such as in electrooptic devices
and as systems responsive to fast stimuli.36,37 Furthermore,
hexasubstituted phenyl rings with amide and alkynyl functions
grafted with paraffin chains appear to be very interesting for
the generation of one-dimensional nanostructures and surface-
bonded lock-and-key receptors.38
Upon diluting a sample of [Cu(L1)2]+ from 5 to 0.05 mM,
the single NH signal exhibits a very weak shift (<0.01 ppm),
confirming the existence of intramolecular H-bonds.42,43 Ad-
ditional evidence for the prevalence of intramolecular H-bonds
is provided by variable-temperature 1H NMR studies from -10
to 50 °C in CDCl3 which show progressive upfield shifts of
-1.8 × 10-3 ppm K-1. A similar trend of upfield shift was
observed for the other copper complexes with weaker values
(around -1.0 × 10-3 ppm K-1 with L8, L12, L16). Such effects
were not observed in the metal-free ligands, excluding the
presence of H-bonding at millimolar concentration in CDCl3.
Infrared spectroscopy in solution and in the solid state for
the copper complexes supports the claim that strong intra-
molecular H-bonding is occurring, whereas for the ligands only
intermolecular interactions are effective. For the copper com-
plexes, the νNH and νCO stretching vibrations were found,
respectively, between 3230-3290 cm-1 and 1650-1660 cm-1
in the solid state. Very little changes were found in dilute CH2-
Cl2 solution (5 mmol), highlighting the occurrence of H-bonds
in both states.44 For the four ligands, the νNH and νCO stretching
vibrations were found, respectively, between 3240-3260 cm-1
and 1645-1655 cm-1 in the solid state. In contrast, these νNH
and νCO stretching vibrations were found at 3426 and 1677 cm-1
for L12 and L16 in dilute CHCl3 solutions (ca. 1 mmol). This
set of values is in the expected range for NH and CdO bonds
involved in hydrogen bonds (for the complexes in solution and
in the solid state and for the ligands only in the solid state),
whereas for the ligands in solution the NH and CdO bonds are
not involved in any supramolecular H-bonds as confirmed by
the free NH and CdO stretching vibrations found at higher
wavenumbers.44
Results and Discussion
Synthesis. The synthetic pathway for all compounds is
sketched in Scheme 1. The starting materials 4-methyl-3,5-
diaminobenzoic acid39 and 2-hydroxy-9-methyl-1,10-phenan-
throline40 were prepared according to literature procedure. The
key esterification step required the cross coupling between the
acids (1n) and the hydroxymethyl phenanthroline derivative with
the chloride salt of 1-ethyl-3-[3-(dimethylamino)propyle]car-
bodiimide (EDC‚HCl) in the presence of dimethylamino-
pyridine (DMAP). The use of dicyclocarbodiimide (DCC) was
prohibited due to residual impurities always present in the final
compounds.
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J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004 12405

A R T I C L E S
Scheme 1a
Ziessel et al.
a Reagents and conditions: (i) EtOH reflux, H2SO4 (cat.), 89%; (ii) H2, Pd/C (10 wt %), room temperature, EtOH/CH2Cl2, 99%; (iii) 1-bromoalkane,
K2CO3, CH3CN, reflux, 90% n ) 8, 95% n ) 12, 99% n ) 16; (iv) EtOH, KOH, reflux, HCl (10%), 98% n ) 8, 94% n ) 12, 92% n ) 16; (v) neat SOCl2,
reflux, quantitative; (vi) anhydrous Na2CO3, acetone, reflux, 75% n ) 1, 72% n ) 8, 78% n ) 12, 78% n ) 16; (vii) NaOH (50 equiv), THF/H2O (v/v),
reflux, 80% n ) 1, 74% n ) 8, 72% n ) 12, 78% n ) 16; (viii) m-CPBA, CH2Cl2, room temperature, 85%; (x) Ac2O, reflux, 87%; (xi) K2CO3, EtOH, 50
°C, 92%; (xii) EDC‚HCl (2 equiv), DMAP (1 equiv), CH2Cl2/THF, room temperature, 57% n ) 1, 73% n ) 8, 76% n ) 12, 75% n ) 16; (xiii)
[Cu(CH3CN)4]BF4, CH2Cl2, room temperature, 81% n ) 1, 99% n ) 8, 68% n ) 12, 95% n ) 16.
Single Crystals. Ligand L1. The unique coordination features
of the bis-amide framework were confirmed by a crystal-
lographic structural determination of the parent compound in
which the dialkoxy termini are replaced with methoxy groups.
As expected, ligand L1 is nonplanar; the peripheral trisubstituted
rings are almost perpendicular with the central phenyl ring, while
the phen subunit is tilted by 68° with respect to the central core
(Figure 2a). It is notable that the two amides vectors point in
the opposite direction, which is auspicious for the formation of
a polymeric network. Two remarkable features are evident from
the packing of this compound. First, the central core of the
crystal results from trimerization of the ligand via intermolecular
hydrogen bonds, leading to the supramolecular edifice (Figure
2b). The tight hydrogen bonds connect, in a centrosymmetric
9
12406 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004

Tuning Organogels with Phenanthroline Ligands
A R T I C L E S
Figure 1. Proton NMR spectra of ligand L12 (a) and [Cu(L12)2]BF4 (b)
recorded in CDCl3 at 200 MHz and 298 K. For the sake of clarity, the CH2
and CH3 paraffin protons are not shown. S accounts for residual CHCl3
and CH2Cl2 solvents.
fashion, the amide groups of the sandwich molecule I to the
top molecule I′ (NH‚‚‚O ) 3.012(3) Å) and to the bottom
molecule I′′ (NH‚‚‚O ) 2.889(3) Å). The aromatic cores are
loosely packed in a zigzag manner by weaker interactions
(phenyl centro¨ıd I/I′ ) 4.4 Å and I/I′′ ) 3.7 Å).
Another interesting feature of the molecular structure is the
π-π stacking of the phen fragments which runs along the
diagonal of the â angle of the cell (standard distances lie in the
range 3.5-3.6 Å, Figure S1). This strong stabilizing effect forces
the welded hydrogen-bonded core to swing out of the stacks.
As a consequence, one benzamide unit (Ar4) becomes involved
in its own π-π stacking (d ) 3.51 Å), while the Ar3 ring is
essentially free of such interactions. Obviously, this infinite
network is maintained by intermolecular hydrogen bonding, as
well as by π-π stacking of the phen subunits.
Copper Complex [Cu(L1)2]ClO4. As suggested by the
ORTEP views of Figure 3, two L1 ligands are interlocked around
a single copper(I) atom in a pseudo-tetrahedral arrangement as
usually found with this type of complexes.45 The bite angles
are 83.1(3)° and 82.0(3)°, respectively, for N(1B)-Cu-N(12B)
and N(1A)-Cu-N(12A). For the sake of clarity and to describe
the molecular structure and the crystal packing, we need to apply
for ligand A the following: phenA, æ2A, æ3A, and æ4A will
be, respectively, the phenanthroline subunit, the central aromatic
core, and the two peripheral aromatic rings. For the second
ligand B, we used phenB, æ2B, æ3B, and æ4B.
Figure 2. (a) Molecular structure of L1 showing the atom-labeling scheme.
The hydrogen-bonding schemes involving the amide groups are plotted as
dotted lines. Angles between Ar2 and Ar3 ) 82°; Ar2 and Ar4 ) 84°. (b)
Central trimeric core, setting up the building block of the supramolecular
edifice. A single molecule I of L1 [symmetry (x,y,z)] is surrounded by two
others [symmetry (-x,-y,1 - z)] for I′ and [symmetry (1 - x,-y,1 - z]
for I′′. The hydrogen bonds (dotted lines) connect centrosymmetrically the
amide groups of I to I′ (NH‚‚‚O ) 3.012(3) Å, NHO angle 173(1)°), and
I to I′′ (NH‚‚‚O ) 2.889(3) Å, NHO angle ) 156(1)°). For the sake of
clarity, the dimethoxyphenyl groups have been removed.
of one of those amide functions and the N-atom (Ns) of a solvent
molecule (dNa-Ns ) 3.082 Å). The two other amide functions
interact with the N-atom of the acetonitrile molecule (dNa-Ns
)
2.949 Å) and the perchlorate counteranion (dNa-O ) 2.990 Å).
In contrast with the free ligand, we noticed the lack of
intermolecular hydrogen bonds between two neighboring com-
plexes.
Concerning the two chelating subunits, we can first notice
that phenA and phenB around a single copper are almost
perpendicular with a dihedral angle of 89.96°. These fragments
are almost parallel to the respective central phenyl ring with
tilt angles of 7.95° between phenA and æ2B and of 7.87°
As compared with the free ligand, we can first notice a strong
intramolecular hydrogen bond (dN-O ) 3.015 Å) between two
amide functions of the two interlocked ligands. Furthermore,
additional hydrogen bonds are found between the N-atom (Na)
(45) Juris, A.; Ziessel, R. Inorg. Chim. Acta 1994, 225, 251.
9
J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004 12407

A R T I C L E S
Ziessel et al.
Figure 4. Gelation test in cyclohexane: (a) L16 (5 g L-1), (b) L12 (5 g
L-1), (c) [Cu(L16)2]BF4 (30 g L-1). Gel formation is confirmed if the sample
does not flow when the test tube is inverted (white arrows).
C6 to C12. Transparent gels were formed in cyclohexane and
turbid gels were formed in hexane and dodecane at 25 °C, after
heating a mixture of the ligand and the solvent to form a
homogeneous fluid solution. Upon cooling below the temper-
ature of gelation, the complete volume of solvent is immobilized
and can support its own weight without collapsing as shown in
Figure 4. The organogels exhibited thermally reversible sol-to-
gel phase transitions. The minimum gelation concentrations
(MGC) observed showed that these mesogenic ligands displayed
good gelation abilities comparable with the ones already reported
for other low molecular weight organogelators (see Table
S5).11c,46 Notice that ligand L8, at a concentration of 6 g L-1
,
does not give a gel in cyclohexane due to its lower solubility
and its strong tendency to precipitate under these conditions.
The unfavorable balance between the rigid part and the more
flexible parts of the L8 molecule avoids gelation.
SEM experiments were used to examine the morphology of
these materials. Images obtained from a solution of L12 ligand
in hexane (15.0 g L-1) and L16 ligand in cyclohexane (16.5 g
L-1) dried onto silicon wafer reveals the presence of a 3D
network of interlocked fibers with a diameter of ∼140 nm
extending over several micrometers (Figure 5a). It is likely that
the formation of a 3D network of interlocked fiberlike aggregates
is responsible for the immobilization, that is, the gelation, of
hexane, cyclohexane, and dodecane. The width of the fibers is
large compare to the dimension of the molecule. Therefore, the
observed fibers correspond to a bundle of smaller elongated
molecular assemblies (Figure 5b). The formation of elongated
fiberlike aggregates indicates that the self-assembly of ligand
L12 and L16 is driven by strong 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. A single CdO stretching band was
Figure 3. (a) ORTEP view of the molecular structure of the [Cu(L1)2]BF4
complex; one of the interlocked ligands is colorized in black for the sake
of clarity. (b) π-π packing between two neighboring units.
between phenB and æ2A. There is no intramolecular π-π
stacking, but we can observe a significant intermolecular π-π
interaction at a distance of 3.8 Å between phenA of one complex
and æ3A of an adjacent complex. Furthermore, a second very
strong π-π stacking (at a centro¨ıd-centro¨ıd distance of 3.5
Å) involves two adjacent æ4B fragments belonging to two
different complexes (Figure 3b).
Gelation Properties into Fibrous Aggregates. In light of
the one-dimensional alignment of the amide-based phenanthro-
line ligands in the crystal structure supported by hydrogen-
bonding interactions and π-π interactions and the fact that the
related copper complexes do not exhibit significant inter-
molecular interactions, it was tempting to involve such com-
pounds in the formation of organogels and to observe the
resulting behavior. The presence of the paraffin chains, in
ligands L12 and L16, features additional van der Waals inter-
actions, useful in stabilizing the three-dimensional edifice. The
gelation ability was evaluated in various polar or nonpolar, protic
or nonprotic solvents (see Table S5). Both ligands displayed
gelation abilities in cyclohexane and with linear alkanes from
observed for L12 and L16, respectively, at 1642 and 1645 cm-1
,
in the three gelated solvents. Furthermore, a single NH stretching
vibration at 3218 cm-1 in hexane, 3222 cm-1 in cyclohexane,
and 3208 cm-1 in dodecane was observed for both ligands (as
(46) (a) Yasuda, Y.; Lishi, E.; Inada, H.; Shirota, Y. Chem. Lett. 1996, 575. (b)
Hanabusa, K.; Koto, C.; Kimura, M.; Shirai, H.; Kakehi, A. Chem. Lett.
1997, 429. (c) Kimura, M.; Kobayashi, S.; Kuroda, T.; Hanabusa, K.; Shirai,
H. AdV. Mater. 2004, 16, 335.
9
12408 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004

Tuning Organogels with Phenanthroline Ligands
A R T I C L E S
plays a crucial role in the gel formation but is not mandatory
for the emergence of mesophase (vide infra).
Characterization and Mesomorphic Behavior of the
Ligands and the Corresponding Complexes. The thermotropic
properties of both ligands Ln and complexes [Cu(Ln)2][BF4] (n
) 8, 12, 16) were investigated by a combination of differential
scanning calorimetry (DSC), polarizing optical microscopy
(POM), and small-angle X-ray diffraction studies (XRD).
The Ligands. Their polymorphic behavior was investigated
systematically as a function of increasing and decreasing
temperature on different batches of L12 and L16. It appeared
that the mesomorphic behavior strongly depends on the thermal
history of the compound and upon kinetics that is presumably
in keeping with the dynamics of hydrogen-bonding networks.
L8 was found to be deprived of mesomorphism over the tested
temperature range (mp ) 156 °C).
Observations by POM were rather laborious and confirmed
the complicated thermal behavior of ligands L12 and L16. On
heating L12 above 90 °C, a cubic phase can be identified by
X-ray diffraction through a diffraction pattern exhibiting two
sharp reflections with the reciprocal spacing ratio x1 and x2
corresponding to a 3D cubic phase (at 23.3 and 16.2 Å,
respectively, Figure S2) in the small region, and three broad
halos at 7.5, 4.5, and 3.5 Å in the wide-angle region. Above
112 °C, another phase is growing alongside the cubic phase,
and at 140 °C, the X-ray pattern is in agreement with the
presence of a mixture of an isotropic liquid and small floating
crystallites as proved by the presence of several sharp peaks in
the medium and wide-angle regions. On further cooling and
heating cycles, this high-temperature crystalline phase remains
stable down to room temperature. Thus, the cubic phases seem
to be only transient.
At room temperature, the X-ray pattern of ligand L16 is
characteristic of a disordered lamellar crystalline phase. Above
110 °C, the X-ray pattern exhibits three sharp reflections (at
25.8, 17.6, and 14.9 Å) in the small-angle region and three broad
halos at 7.5, 4.5, and 3.5 Å in the wide-range region (Figure
S3). The three sharp reflections in the ratio 1, x2, and x3
suggest also a cubic phase. At higher temperature, above 115
°C, another mesomorphic phase appears, for which no firm
identification was possible. At 125 °C, the compound becomes
isotropic. On cooling, the cubic phase reappears at 115 °C with
the same structural distances as on heating without showing
the other mesophase (transient). Further heating and cooling
cycles showed only the presence of the cubic phase above 71
°C (Cr 71 Cub 115 I). To confirm the thermal stability of the
compounds, NMR spectroscopy was performed on samples kept
2 h at 125 °C (isotropic melt) and shows no sign of degradation.
Furthermore, thermogravimetric analysis shows no weight loss
up to 250 °C.
Figure 5. (a) SEM picture of a dried gel from ligand L16 in cyclohexane
(16.5 g L-1) deposited on silicon wafer showing the presence of fiber with
an average diameter of ∼140 nm extended over several micrometers. (b)
SEM picture of a freeze gel from a solution of L12 in cyclohexane (4.0 g
L-1) deposited on silicon wafer.
compared to 3426 cm-1 for the ligand in millimolar solution in
CHCl3). These spectroscopic data are an unambiguous signature
of hydrogen-bonded amides inducing a hydrogen-bonded net-
work, in line with the aggregation of the ligands into elongated
fibers. It is also surmized that the π-π interaction of the
phenanthroline and/or phenyl subunits might add additional
stabilization interactions.
The gelation ability of the copper(I) complexes with ligands
L12 and L16 and BF4- as counterion has also been investigated
in cyclohexane, hexane, and dodecane. These complexes are
soluble after heating but did not show any hint of gelation
abilities at 25 °C even at a concentration as high as 40 g L-1
.
With respect to the FT-IR data measured in these solvents and
the previous discussion on these copper(I) complexes, it appears
that because two amide groups are strongly engaged in an
intramolecular hydrogen bond and the remaining amide func-
tions are not suitably oriented to form an intermolecular network
of hydrogen bonds, this would not lead to the gelation of the
solvent. It is worth pointing out that the presence of charge is
not inhibitory for the emergence of fiberlike aggregate.13,47 These
results confirmed that the switching of the hydrogen bonds from
inter to intra when the ligands are interlocked around a metal
The Copper Complexes. Optical microscopy was not ef-
ficient in probing mesophase formation in these compounds,
due to the absence of recognizable defects in the optical textures
and to the high viscosity of the mesophases. However, on the
basis of the molecular structure of the precursory ligands (vide
supra), and the clipping effect induced by the copper cation
and our previous experience with copper metallomesogens,23,24
(47) (a) Nakashima, T.; Kimizuka, N. AdV. Mater. 2002, 14, 1113. (b) Kuroiwa,
K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuba, N. J. Am. Chem. Soc.
2004, 126, 2016.
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A R T I C L E S
Ziessel et al.
Table 1. Phase Transition Data from DSC and Data Collected from Powder-XRD Diffraction Patterns for the Copper Complexes
DSCa
XRDb
mesomorphic
properties
parameters
at Tc
dmeas/(Å)
hk
I
dcalc/(Å)
[Cu(L8)2]BF4
[Cu(L12)2]BF4
Cr 161 (28) I
I 96 (-10) Cr
Cr 75 (67) Colo 172 (34) I
I 135 (-29) Colo
41.0
20
VS
41.0
T ) 150 °C
26.7
24.4
20.1
9.0
11h
11
40
VS
VS
M
br
br
26.7
24.4
20.5
h2
ao ) 83.0 Å
bo ) 27.1 Å
γ ) 98.75°
So ) 2225 Å2
4.5
h1
[Cu(L16)2]BF4
Cr 62 (145) Colo 164 (24) I
42.1
20
VS
42.1
T ) 150 °C
I 132 (-22) Colo
32.0
28.5
21.2
15.75
9.0
11h
11
40
22h
VS
VS
M
W
br
32.0
28.5
21.05
16.0
h2
ao ) 85.5 Å
bo ) 32.7 Å
γ ) 99.95°
So ) 2755 Å2
4.5
br
h1
a Transition temperatures (°C), and enthalpy of transitions (∆H/kJ mol-1); Cr, crystalline phases; I, isotropic liquid; Colo, oblique columnar phase. b hk
are the indexations of the reflections corresponding to the Colo phase, and I is the intensity of the reflections (VS, very strong; S, strong; M, medium; W,
weak; br, broad); dmeas and dcalc are the measured and calculated diffraction spacing values. c ao and bo are the lattice parameters of the Colo phase (ao )
2d
20 ; bo )
sin γ
A
sin γ
2
; A )
), γ is the angle between ao and bo, and So is the lattice area (So ) aobo sin γ).
1
1
1
+
-
2
2
2
d
d11h
2d20
x
11
mesomorphic behavior was anticipated for the fully interlocked
complex. During the first heating, several endothermic peaks
could be observed on the DSC traces, suggesting at least a
polymorphic behavior (Table 1). However, on subsequent heat-
cool cycles, the DSC traces appeared simpler. From the large
enthalpy value measured at high temperature, corresponding to
the transition to the isotropic state (POM), again, the octyloxy
derivative did not appear to be mesomorphic. In contrast, the
dodecyloxy and hexadecyloxy derivatives likely exhibited
mesomorphism, as much smaller enthalpies were measured at
the clearing transitions (Table 1).
To identify the mesophases, variable-temperature powder-
XRD experiments were carried out from the crystalline state
up to the isotropic liquid. The transition temperatures detected
by DSC and optical microscopy were in good agreement with
those derived from XRD. The crystalline phases displayed a
series of sharp peaks over the entire range of 2θ. The
transformation to the mesophase was accompanied by the
disappearance of a number of these sharp reflections and
significant changes in intensity in others. The X-ray diffraction
patterns supported the presence of liquid-crystalline mesophases
for the two long-chain complexes (Figure 6) and confirmed the
absence of any mesophase for the octyloxy complex.
The X-ray patterns for the two complexes are similar (Figure
6) and divided into three different regions as follows:
(i) At wide angles, a diffuse and broad-scattering halo was
observed centered around 4.5-4.6 Å (h1), corresponding to the
liquidlike order (short-range correlations) of the molten chains
bound to the complexes, and thus to the fluidlike nature of the
mesophase.
Figure 6. XRD diffraction patterns of the complex [Cu(L12)2]BF4 recorded
at 423 K.
(iii) Finally, in the small-angle region, a series of several
Bragg peaks were present. Three sharp and intense reflections
of more or less equal relative intensity were observed which
could be indexed with the fundamental (20), (11), and (11h)
reflections corresponding to a 2D oblique lattice.48,49 One or
two more higher order reflections of smaller intensity could also
be observed and were indexed as (40) for n ) 12, and (40) and
(22h) for n ) 16. The sharpness and intensity of the small-angle
reflections also indicate the long-range correlation of the 2D
arrangement of the columns (Colo phase).
Therefore, the effect of the complexation of a metallic center
with this organic platform results in a dramatic change of the
mesomorphic properties. First, the mesophase stability has been
greatly enhanced upon the coordination to copper(I), particularly
due to the huge increase of the clearing temperature, the melting
temperature remaining almost the same for the ligands and
(ii) At slightly lower angles, another weak, less diffuse halo,
was seen, associated to a distance of ca. 9.0 Å (h2) which was
attributed to some liquidlike correlations between neighboring
molecules. This broad scattering suggests a loose periodicity
of the molecular species inside the columns, although this was
not correlated over too great a distance.
(48) Levelut, A. M. J. Chim. Phys. 1983, 80, 149.
(49) Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.; Wilson,
C.; Blake, A. J.; Schro¨der, M.; Donnio, B. Chem.-Eur. J. 2003, 9, 2484.
9
12410 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004

Tuning Organogels with Phenanthroline Ligands
A R T I C L E S
gregates.50 As expected, no weakening or shift of the CdC and
COO bands could be seen during the heating process. The
splitting of the CdO stretch and N-H bend peaks, and the
quasi-disappearance of the original vibrations, during the heating
process suggest that the formation of the mesomorphic state
from the solid involves severe internal reorganization and the
occurrence of at least two different types of hydrogen bonds
involving the amide functions. It is likely that the presence of
two hydrogen bonds in the mesophase is associated with a
different packing of the phenanthroline cores upon microseg-
regation. In contrast, for the copper(I) complexes, the single
intramolecular H-bond persists in the solid, mesomorphic, and
isotropic states with no major perturbations of the energy and
intensity of the νCO, νNH, and δNH stretching and deformation
vibrations (Figure 7b). It is surmised that the two remaining
NH-CO junctions are possibly involved in hydrogen bonds with
the counteranions. Such additional intracomplex hydrogen bonds
are present in the X-ray structure of the [Cu(L1)2]ClO4 complex
and would explain the fact that no free NH-CO vibrations were
observed in the FT-IR spectra. Likewise, the switching of one
hydrogen bond provides thermally stable complexes but avoids
the emergence of an extended hydrogen-bonded network that
is a prerequisite for gelation of solvents but is not damageable
for the formation of mesophases. Switching the ester linkage
into an amide linkage could add additional stabilization inter-
actions.
Packing Study of the Copper(I) Complexes and Structure
of the Colo Phase. To get some insight on the molecular packing
within the columns and to have a better understanding of the
2D arrangement of the columns, a geometrical approach
previously used49 and based on the segregation of the chemically
different constituting parts of the molecules (hard parts and
aliphatic chains) has been applied. From such a point of view,
columnar structures are mainly characterized by two geometrical
parameters: the columnar cross section, Scol, and the stacking
periodicity along the columnar axis, h,51 which are mutually
Figure 7. FT-IR spectra (from 1400 to 1750 cm-1) recorded for compound
L12 (a) and its [Cu(L12)2]BF4 complex (b) versus temperature in the range
40-160 °C for L12 and 33-180 °C for the copper complex. The
measurements were carried out with polished KBr disks starting from the
crystalline phase.
linked analytically through the relation hScol ) NVm ) Vcell
,
where N is the number of molecules within a columnar stratum
and Vm is the molecular volume. Note that h is obtained directly
from the X-ray patterns (vide supra) and corresponds to the
stratum thickness, and the molecular volume, Vm, and the cross-
sectional areas of the columns are derived from crystallographic
and XRD data. Moreover, on the basis of the amphiphilic
character of such materials,49,52 it is also possible to express
the molecular volume as the sum of elementary volumes using
corresponding complexes. On average, the mesophase stability
is increased by nearly 50 °C. The second important consequence
was the thermodynamic stability of the Colo phase for the metal
complexes when compared to the corresponding ligands.
Infrared in the Mesophases. Evidence for the persistence
of hydrogen bonding in the mesophase was obtained by variable-
temperature FT-IR spectra for the ligands and complexes
dispersed onto KBr pellets. For both ligands, a weakening of
νN-H stretching vibrations with shifts, from 3228 cm-1 in the
crystalline phase to 3290 cm-1 in the isotropic phase, was
observed. In the crystalline phase, the CdO stretching vibration
appears as a single band at 1645 cm-1, and, after melting in
the mesomorphic phase, a new band appears at 1655 cm-1 with
a concomitant and progressive decrease of the original band.
Both bands are characteristic of H-bonded amides. Furthermore,
in the isotropic phase (over 120 °C), this band is split and shifts
to peaks at 1657 cm-1 (H-bonded amide) and 1673 cm-1 (free
amide, Figure 7a). A similar behavior is found for the NH
bending vibration. These latter values are in agreement with
previous studies on mesogenic hydrogen-bonded amide ag-
the additivity rule of the partial volumes according to Vm
)
VAr + VCH, where VAr is the volume of the aromatic, polar part,
and VCH is the volume of the paraffin chains.48 Knowing these
volumes, and thus the associated volume fraction of the column
occupied solely by the hard molecular parts, fAr ()VAr/Vmol),
the cross section of the columnar inner core, SAr, can also be
calculated according to SAr ) fAr‚Scol. These general equations
will be used in the following discussion.
(50) (a) Kato, T.; Kutsuna, T.; Hanabusa, K.; Ukon, M. AdV. Mater. 1998, 10,
606. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 3rd ed.; Wiley: New York, 1976.
(b) Hanabusa, K.; Kato, C.; Kimura, M.; Shirai, H.; Kakehi, A. Chem.
Lett. 1997, 429.
(51) Guillon, D. Struct. Bonding 1999, 95, 41.
(52) (a) Skoulios, A.; Guillon, D. Mol. Cryst. Liq. Cryst. 1988, 165, 317. (b)
Tschierske, C. J. Mater. Chem. 1998, 8, 1485. (c) Tschierske, C. J. Mater.
Chem. 2001, 11, 2647. (d) Tschierske, C. Annu. Rep. Prog. Chem., Sect. C
2001, 97, 191.
9
J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004 12411

A R T I C L E S
Ziessel et al.
Figure 8. (a) CPK model of two molecules of [Cu(L12)2]BF4 complex (arbitrary number of carbons) forming an elliptical plateau and (b) schematic
-
representation of this plateau within the columnar mesophase with an oblique symmetry and a tilt of ca. 40°. The adjacent BF4 anion within the disk is
located arbitrary to fill the empty space, thus improving molecular packing within the mesophase.
Table 2. Crystallographic Data of the Mesomorphic Complexesa
that the volume of one complex corresponds to exactly the
S
col (Å2)
Vcell
)
Scol
×
h2 (Å3)
V
m (Å3)
N
volume of a columnar stratum having a thickness of 4.5 Å (h1).
The thickness of the complex is, however, larger, in the range
of 9-10 Å. It is therefore necessary to take into account the
doubling of the periodicity, and thus to consider a columnar
slice twice as thick of 9 Å, which in fact corresponds to the
position of the second broad scattering observed in the XRD
patterns (i.e., h2 ) 9 Å). To propose a suitable organization of
the complexes within the column, calculation of SAr, the cross
section of the inner columnar core, seems to indicate that the
two complexes lie roughly side-by-side within the columnar
slice, with the two anions located in the voids created by such
molecular disposition. This value is in the range with the
dimensions of the dimeric arrangement as illustrated by the CPK
model shown in Figure 8. Stricto sensu if the dimer adopts such
a conformation, the area described by the ellipse should be
reduced either by the tilt of the elliptical plane with respect to
the columnar axis or by the shortening of the longer axis of the
ellipse to compensate the calculated surface area of the inner
columnar core. A value of ca. 400-420 Å2 (fAr ) 35.9% for n
) 12, and 30.9% for n ) 16) was actually found for the surface
area of the inner core, smaller than the 525 Å2 obtained from
the CPK model (S ) πab/4, where a ) 29, b ) 23), implying
a tilt of ca. 40° of the assembly with respect to the columnar
axis. To keep the elliptical shape, the tilt must occur around
the long molecular axis of the ellipse to satisfy the limits
imposed by the oblique lattice to fix the columns as schemati-
cally depicted in Figure 8.
[Cu(L12)2]BF4
[Cu(L16)2]BF4
1112.5
1377.5
10 020
12 400
5070
5890
2.0
2.1
a Scol ) So/2. Vcell is the volume of one columnar stratum h2-thick.
VCH (T)
M
2
Molecular volume, Vm
)
; M, molecular weight; VCH (T)
2
0.6022
VCH (T0)
2
) 26.5616 + 0.02023T; T in °C, T0 ) 22 °C; N, number of molecules per
Vcell
.
Finally, let us recall that, in columnar phases, the lattice plane
is defined as the plane that is perpendicular to the columnar
axis, and the 2D symmetry results from the shape (circular or
elliptical), disposition, and mutual orientations (for ellipses only)
of the columnar cross sections. In the Colo case, the columnar
cross section is not circular as in the Colh phase, but adopts an
elliptical shape, and the particular disposition of the elliptical
columnar cross sections leads to an oblique lattice (Figure 8),
with two columns per cell.49 Note that the oblique lattice
determined in this work does not differ much from a rectangular
cell, the angle γ between the lattice parameters ao and bo being
ca. 100°, thus corresponding to a slight distortion of the
rectangular cell.
In the case of the complexes, Vm values were estimated
considering a density of 1 in the mesophase, and were then
corrected by a temperature factor because the volume increases
with temperature (Table 2).53-55 Such a value for the density is
acceptable and is commonly used in liquid crystals. It was found
(53) The molecular volume of the ligand L1 was obtained experimentally by
Conclusion
crystallography (Vm ) 842.5 Å3). VCH was calculated as follows (all
elementary volumes in Å3 and T in °C): Vch ) 4 × 12 × VCH + 4∆VCH
,
2
where VCH ) 26.5616 + 0.02023T is the volume of one methylene grou3p
A central 4-methyl-3,5-diacylaminobenzene core equipped
with an ester group connected to a phenanthroline subunit and
two secondary amide functions each carrying a phenyl ring with
two paraffin chains has been prepared by a rational approach
and subsequently easily and quantitatively transformed into deep
red and stable copper(I) complexes. The amide functions are
clearly involved in a supramolecular hydrogen-bonded network,
whereas one hydrogen bond is frustrated into a tight intra-
and ∆V 2 ) 27.14 + 0.01713T + 0.0004181T2 represents the volume
CH2
difference between the terminal methyl group and a methylene group. VAr
) 625 Å3 at 22 °C and 660 Å3 at 90-95 °C.
(54) Molecular volume Vm ) (M/0.6022)(VCH (T)/VCH (T0)); M, molecular
2
weight; VCH (T) ) 26.5616 + 0.02023T; T in °C, T20 ) 22 °C.
2
(55) The molecular volume of the complex [Cu(L1)2]ClO4 obtained experimen-
tally by crystallography was Vm ) 2095.5 Å3. VAr ) 1660 Å3 at 22 °C and
1820 Å3 at 150 °C. Using the same method as in ref 53 would give 4900
and 5850 Å3 for the volume of [Cu(L12)2]ClO4 and [Cu(L16)2]ClO4,
respectively, at 150 °C, in good agreement.
9
12412 J. AM. CHEM. SOC. VOL. 126, NO. 39, 2004

Tuning Organogels with Phenanthroline Ligands
A R T I C L E S
molecular bond during the interlocking of two such ligands
around a copper center. A remarkable feature of the free ligands
(n ) 12 and 16) is the gelation of hydrocarbon solvents. SEM
measurements reveal that 3D dimensional networks are formed
of interlocked fibers with a diameter of ∼140 nm extending
over several micrometers in length. This supramolecular orga-
nization is likely facilitated by a combination of three types of
molecular interactions: hydrogen bonding, van der Waals
interactions within the paraffin chains, and π-π stacking
interactions between the phenanthroline and/or the phenyl rings,
ensuring an optimal immobilization of the solvents. Because
of the emergence of a very strong intramolecular hydrogen bond,
the copper complexes are not organogelators. Liquid-crystalline
mesomorphism was found for the free ligands (transient for n
) 12 and enantiotropic for n ) 16), and heating the related
copper complexes also affords mesomorphic materials packed
in a columnar mesophase with an oblique 2D symmetry. Even
if the starting molecular shape of the complex does not follow
the strict criteria of structural anisometry, LC mesomorphism
can be provided here through the interplay of specific inter-
actions that favor the desired self-organization of the molecules.
A careful study by FT-IR of the free ligands revealed the
presence of strong H-bonds in the crystalline state, organogel,
and liquid-crystalline phase but not in dilute solutions. However,
in the copper(I) complexes, two ligands are sealed via a strong
intramolecular hydrogen bond between the CdO bond of one
ligand and the NH bond of the second ligand which persists
even in polar solutions.
phic matter, the use of a potential hydrogen-bond director allows
for the production of mesomorphic materials with both ligands
and complexes. This work demonstrates guidelines for program-
ming transition metal complexes into self-assembled nanostruc-
tures. The idea leading to the generation of these materials can
be used to design novel functional complexes ordered in soft
condensed matter with potential applications in nanoelectronics,
light-emitting layers, and photovoltaics. Research along this line
is currently in progress in our laboratory.
Acknowledgment. We acknowledge the University Louis
Pasteur (ULP) for support, the Engineer School of Chemistry
(ECPM) for partial financial support, and the CNRS for
continuous support. Dr. L. Douce (ULP) is also acknowledged
for initiating this work in the laboratory, and Dr. Claude
Estournes and Cedric Leuvrey (IPCMS) for measuring the SEM
of the dried gels.
Supporting Information Available: Complete experimental
section including the preparation and characterization of the
ligands and complexes. Summary of crystal data, intensity
measurements, and structure refinement for L1 (Table S1) and
for [Cu(L1)2]ClO4 (Table S3), selected hydrogen bonds for
ligand L1 (Table S2), and the [Cu(L1)2]ClO4 complex (Table
S4). Schematic representation of the interdigitated phen subunits
is given in Figure S1. Gelation tests and minimum gel
concentration (g L-1) (Table S5). XRD diffraction patterns of
L12 recorded at 373 K and L16 recorded at 383 K are given in,
respectively, Figures S2 and S3. CIF files for the X-ray structure
determinations are also available. This material is available free
of charge via the Internet at http://pubs.acs.org.
The present system describes unique examples of organo-
gelators as well as a thermotropic phenanthroline-based met-
allomesogen. It is worth pointing out that, contrary to the
previous cases where oligopyridines are involved in mesomor-
JA047091A
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