Combination of ionic self-assembly and hydrogen bonding as a tool for the synthesis of liquid-crystalline materials and organogelators from a simple building block

Article abstract of DOI:10.1039/b303552b

In this communication we report on the facile combination of hydrogen bonding and the ionic self-assembly (ISA) process to produce organized materials and fiber-containing organogel superstructures from functionalised oligoelectrolytic building blocks.

Full text of DOI:10.1039/b303552b

Combination of ionic self-assembly and hydrogen bonding as a tool for
the synthesis of liquid-crystalline materials and organogelators from a
simple building block†
Franck Camerel and Charl F. J. Faul*
Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam-Golm,
Germany. E-mail: Charl.Faul@mpikg-golm.mpg.de; Fax: +49 331 567 9502; Tel: +49 331 567 9545
Received (in Cambridge, UK) 2nd April 2003, Accepted 28th May 2003
First published as an Advance Article on the web 19th June 2003
In this communication we report on the facile combination
of hydrogen bonding and the ionic self-assembly (ISA)
process to produce organized materials and fiber-containing
organogel superstructures from functionalised oligoelec-
trolytic building blocks.
99%) in dry dichloromethane in the presence of triethylamine.5
The precursor trisamide was obtained as a pure yellow
precipitate (51–56%). Methylation of the terminal amine
functions was performed in a mixture of methyl iodide and
acetonitrile in a 1 : 1 volume ratio to give molecule 1 in high
yield with only traces of terminal amine groups. It should be
noted that the NH group on the central amide is not affected by
the use of a large excess of CH3I. The synthesis of molecule 1
was confirmed by NMR, IR and elemental analysis.†
Charged molecule 1 (with iodide counterions) was com-
plexed with three equivalents of commercially available single-
tailed sodium n-hexadecyl sulfate (C16SNa, Lancaster, 99%)
and the double-tailed dihexadecyl phosphate (C16DP) (Aldrich,
99%, neutralized with sodium hydroxide). All solutions were
prepared at 0.2 wt% concentration. Complexes were isolated
after precipitation. Complexation in a 3 : 1 ratio was confirmed
by means of elemental analyses. It was found that the 1–C16S
complex is soluble in dimethylformamide (DMF) and dimethyl
sulfoxide (DMSO) but not in chloroform. On the contrary, the
1–C16DP complex was found to be soluble in chloroform but
not in DMF or DMSO. The complexes were dissolved in the
respective solvents and cast into films.
Investigation of the films by polarized optical microscopy
shows strong birefringence, indicative of anisotropic and
ordered materials. Schlieren-type textures were detected. From
thermogravimetric analyses (TGA) the degradation temperature
of the complexes was found to be around 200 °C, whereas the
thermal stability of 1 (with iodide counter-ions) was determined
to be around 150 °C. Differential scanning calorimetric analyses
(DSC) show that the films obtained exhibit reversible thermal
transitions. In the case of the 1–C16S complex cast from
DMSO, a broad transition was found at 223.3 °C on the second
heating curve and is attributed to structural rearrangements of
the side chains. 1–C16DP films cast from chloroform displayed
two reversible thermal transitions on the second heating curve at
29.6 and 59.6 °C.
To elucidate the morphology of these materials, a range of
temperature-dependent wide-angle (WAXS) and small-angle
X-ray scattering (SAXS) measurements was performed.6
WAXS experiments show that 1–C16S (at room temperature)
and 1–C16DP films (above 59.6 °C) are non-crystalline
materials with a broad reflection at approximately 20° in 2q (d-
spacing ~ 0.45 nm, indicative of a liquid-like arrangement of
alkyl tails).
Organised materials, specifically mesogenic materials/mole-
cules, are accessible via covalent as well as a range of
noncovalent strategies, including coordination chemistry and
H-bonding between nonmesogenic molecules.1
Recently a facile route for the production of mesogenic
building blocks (or tectons) by complexation of oligoelec-
trolytes with charged surfactants was introduced.2 This process,
so-called ionic self-assembly (ISA), uses electrostatic inter-
actions between the charged species as primary interaction to
construct the mesogenic molecule. It was shown2b,d that liquid-
crystalline properties could be easily tuned by variation of the
alkyl volume fraction of the surfactants while leaving the
central charged building block unchanged.
So far the ionic self-assembly route has only made use of
hydrophobic and p–p interactions as secondary motifs to
promote self-organization of charged oligoelectrolytic species.
The aim of this study is to show that hydrogen bonding can also
be combined with ionic self-assembly as a secondary motif to
promote self-organization and to produce ordered and liquid-
crystalline materials
Our attention focused on C3-symmetrical disks. This group of
molecules was selected for the following reasons: they bear
multiple sites for intermolecular hydrogen bonding formation
(amide groups), exhibit liquid-crystalline phase behavior and
have been shown to act as organogelators when covalently
functionalized with long alkyl chains.3,4 For the purpose of
applying this interesting versatile tecton within the ISA process,
a new charged trisamide molecule 1 was synthesized (Scheme
1).
The precursor trisamide molecule bearing three terminal
amine functions was synthesized by derivation of the procedure
described by E.W. Meijer and co-workers.4 1,3,5-benzene-
tricarbonyl trichloride (Aldrich, 98%) was reacted with N,N-
dimethyl-1,4-phenylenediamine monohydrochloride or N,N-
dimethyl-1,4-phenylenediamine dihydrochloride (Aldrich,
The SAXS pattern of the 1–C16S complex obtained at room
temperature clearly shows that the film is organized, with a set
of peaks which can be indexed to a hexagonal array with a
lattice parameter a = 4.08 nm (Fig. 1a).
The 1–C16DP complex also forms highly ordered liquid-
crystalline materials, but at temperatures above 59.6 °C. Below
this temperature one sharp reflection is observed at 0.40 nm
(WAXS) and is attributed to a partial crystallization of the alkyl
chains. Temperature-dependent SAXS measurements confirm
the formation of a highly ordered columnar phase which can be
indexed to a monoclinic 2D lattice (a = 4.24 nm, b = 3.61 nm,
g = 94°) (Fig. 1b). A reflection at 2.94 nm (s = 0.34 nm21), as
well as the second order reflection at 1.53 nm21 is also observed
on all the SAXS patterns. This could not be included in the
Scheme 1
† Electronic supplementary information (ESI) available: IR, NMR, DSC
and TGA data of the organic core and complexes. See http://www.rsc.org/
suppdata/cc/b3/b303552b/
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CHEM. COMMUN., 2003, 1958–1959
This journal is © The Royal Society of Chemistry 2003

NMR investigations were performed on these materials.
Infrared spectra of films of the complexes were recorded. This
showed that H-bonding does indeed take place during the
formation of the organised superstructures, but does not involve
all of the CNO groups. This is indicated by the splitting of the
single CNO band (1664 cm21 in CCl4 for 1–C16DP) into two
bands: 1606 and 1672 cm21 for 1–C16S complex; 1608 and
1672 cm21 for 1–C16DP complex.4 The band with lower
frequency arises from CNO…H–N hydrogen bonds, whereas
that with the higher frequency is due to the non-hydrogen
bonded CNO group.8
1
Temperature-dependent H NMR spectroscopy9 in d7-DMF
was used in this study to investigate the extent of H-bonding and
its involvement in the gelation behaviour. At 100 °C, the NMR
spectra displayed the characteristic signals of the hydrogen
atoms of the NH groups of the amide at 12.46 ppm. The
observation of the resolved resonances of both the amide
protons as well as the aromatic protons (8.85 (3H), 8.36 (6H),
8.05 (6H) ppm) indicate that the core part of the complex can
freely rotate.
However, upon cooling to room temperature, these reso-
nances are completely dampened by motional broadening. The
effect is so severe that the signals are hardly detectable and only
signals from the surfactants were observed. These results show
that the core of the complex, i.e. the triamide molecule, is
involved in the gelation process upon cooling and that the NH
group (hydrogen bond donor) is taking part in the aggrega-
tion.
A very important fact to mention here is that no p–p stacking
could be detected by WAXS experiments (performed on both
concentrated organogels and films). This, in conjunction with
the NMR data presented, clearly shows that the aggregation of
the complex into elongated fibers in solution is induced by a
directional extended network of hydrogen bonds.
Here we presented results from the combination of hydrogen
bonding and ISA to produce columnar thermotropic liquid-
crystalline materials and fiber-containing organogel super-
structures. The flexibility, tunability and ease of implementa-
tion of this approach (using one core, varying the surfactant to
tune the properties, minimal covalent synthesis) highlights the
potential for use in industrial organogelator applications such
hydrometallurgy, cosmetics, food processing and lubrication.
We thank the Max Planck Society (financial support), Dr.
Rémi Soula (help in the synthesis of the organic core), Ingrid
Zenke (X-ray measurements) and Rona Pitschke (TEM meas-
urements).
Fig. 1 SAXS pattern of: a) 1–C16S cast from DMSO at RT and b) 1–C16DP
cast from chloroform at 100 °C.
indexation of the proposed 2D columnar phase structure. The
observed columnar phases are to be expected, since the presence
of amide groups on such C3-symmetrical disks generally leads
to the formation of columnar phases via hydrogen bonding.3
Since the related materials are known organogelators, we
investigated the gelling properties of these complexes. As
stated, the 1–C16DP complex is not soluble in DMF and DMSO
at room temperature. Heating leads to the dissolution of the
complex. Clear solutions are obtained at 100 °C and, after
cooling to room temperature, formation of gels is observed. It
should be noted that pure C16DP does not form a gel in DMF.
1–C16S did not form a gel in the explored solvents (1-butanol,
toluene, hexane, DMF, DMSO, octanol, hexadecane, cyclohex-
ane, water, 1,2-dimethoxyethane).
TEM was used to provide further evidence of the organogel
nature of these materials. Micrographs obtained from a solution
of 1–C16DP (0.5 mg mL21) dried onto a carbon-film covered
grid reveals the presence of a 3D network of twisted fibers with
diameter of 10–40 nm extending over several micrometres (see
Fig. 2). The twisted nature of the fibers can be attributed to the
drying process, as has been observed for other organogels.7 CD
measurements confirmed that no preferred twist orientation
existed.
Notes and references
1 (a) A. M. Giroud-Godquin, Coord. Chem. Rev., 1998, 178–180, 1485 and
references therein; (b) C. M. Paleos and D. Tsiourvas, Angew. Chem., Int.
Ed. Engl., 1995, 34, 1696.
2 (a) C. F. J. Faul and M. Antonietti, Chem. Eur. J., 2002, 8(12), 2764; (b)
Y. Guan, M. Antonietti and C. F. J. Faul, Langmuir, 2002, 18(15), 5939;
(c) C. F. J. Faul and M. Antonietti, Adv. Mater., 2003, 15(9), 673; (d) F.
Camerel, M. Antonietti and C. F. J. Faul, Chem. Eur. J., 2003, 9(10),
2160.
In order to ascertain whether H-bonding does play any role in
1
the formation of the observed superstructures, both IR and H
3 (a) Y. Yasuda, E. Iishi, H. Inada and Y. Shirota, Chem. Lett., 1996, 575;
(b) K. Hanabusa, C. Koto, M. Kimura, Hirofusa and A. Kakehi, Chem.
Lett., 1997, 429.
4 J. J. van Gorp, J. A. J. M. Vekemans and E. W. Meijer, J. Am.Chem. Soc.,
2002, 124(49), 14759.
5 In order to neutralize the mono- or dihydrochloride salts of the starting
diamine, extra equivalents of triethylamine need to be added to the
reaction mixture.
6 X-Ray measurements were performed after heating to 130 °C and 180 °C
respectively, to ensure the removal of residual solvent in the films (as
determined by TGA analyses). This was done to ensure that solvent-
swollen (lyotropic) phases were not investigated, but true (solvent-free)
thermotropic LC phases.
7 P. Terech and R. H. Wade, J. Colloid Interface Sci., 1988, 125(2), 542.
8 Y. Katsumoto, T. Tanaka, H. Sato and Y. Ozaki, J. Phys. Chem. A, 2002,
106, 3429.
Fig. 2 TEM picture of 1–C16DP (dried from DMF) revealing the presence
of a 3D network of twisted fibers (scale bar 500 nm).
9 P. Terech, A. Coutin and A. M. Giroud-Godquin, J. Phys. Chem. B, 1997,
101, 6810.
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