Soft Matter
Paper
interactions that can induce molecular self-assembly processes measurements were carried out on a Bruker Alpha-E spec-
in both conventional organic solvents and in the solid state.24–26 trometer, equipped with an attenuated total reectance (ATR)
A variety of 1,3,5-benzenetricarboxamides, substituted with unit. Electrospray ionization mass spectroscopy (ESI-MS)
amino acids and amino acid methyl esters, were synthesized measurements were performed on a JMS-T100LP AccuTOF LC-
and investigated for the gelation of ionic liquids.
plus spectrometer (JEOL, Tokyo, Japan). Matrix-assisted laser
desorption and ionization time-of-ight mass spectroscopy
(MALDI-TOF/MS) measurements were carried out using an
UltraeXtreme TM mass spectrometer (Bruker, Germany).
To assess the gelation of different solvents, the gelators were
rst dissolved in the required solvent at a given concentration in
a glass tube (inner diameter: 10 mm) by heating. The resulting
solutions were then slowly cooled to room temperature.
Formation of the gels (gelation) was conrmed by inverting the
glass tube containing the solution.
Field emission-scanning electron microscopy (FE-SEM)
measurements were carried out on a scanning electron micro-
scope (JSM-7500F, JEOL, Japan), operating at an accelerating
voltage of 7 kV. For sample analysis, the ionogels were con-
verted to xerogels, prepared using the procedure reported by
Hanabusa et al.4 Briey, the ionogel underwent a solvent
exchange from ionic liquid to water by immersing the ionogel in
fresh solutions of water for 3 days. The gel, containing water
only, was then freeze-dried under vacuum to produce the
xerogel. The organogel was also converted to the xerogel by
freeze-drying under vacuum. The xerogel was then mounted on
an aluminum stub and coated with osmium by vapor
deposition.
Experimental
Materials
1,3,5-Benzenetricarbonyl trichloride, 1,3,5-tris(bromomethyl)
benzene, methyl ester hydrochlorides of amino acids (glycine
(Gly), L-alanine (L-Ala), L-valine (L-Val), L-leucine (L-Leu),
L-methionine (L-Met), and L-phenylalanine (L-Phe)), and 1,3,5-
triethylbenzene were purchased from Tokyo Chemical Industry
(Tokyo, Japan). All solvents used in the syntheses were
purchased from Wako Pure Chemical Industries (Osaka, Japan).
Ionic liquids [EtMeIm][TFSA], [BuMeIm][TFSA], [HeMeIm]
[TFSA], and [BuPy][TFSA] were synthesized from [EtMeIm][Cl],
[BuMeIm][Cl], [HeMeIm][Cl], [BuPy][Br], and [Li][TFSA] as
reported previously.14 [EtMeIm][CF3SO3], [BuMeIm][CF3SO3],
[BuMeIm][BF4], [EtMeIm][PF6], and [BuMeIm][PF6] were
purchased from Merck. [TMPA][TFSA] and [EtMeIm][BF4] were
purchased from Kanto Chemical Co., Inc. (Tokyo, Japan).
1-Ethyl-3-methylimidazolium
hydroxide
solution
was
purchased from Sigma-Aldrich and [EtMeIm][Phe] was synthe-
sized as previously reported.27 Full names of the ionic liquids
are given in the ESI.†
Confocal laser scanning microscopy (CLSM) analyses were
performed on an Olympus FV1000-D microscope at room
temperature. Prior to analysis, the ionogel was stained with
rhodamine B (30 mM).
Synthesis
Gelators 1–6. A solution of amino acid methyl ester hydro-
chloride (16.5 mmol) in dichloromethane (200 mL) was cooled
Rheological measurements were carried out using
a
ꢀ
to 0 C, and triethylamine (4.5 mL) was added. 1,3,5-Benzene-
rheometer (Anton Paar Physica MCR301, Germany) with a cone
plate (diameter ¼ 2.5 cm), at a strain of 0.1% and a gap of 1.0
mm. For sample measurements, the gelator-containing ionic
liquid solution (1 wt%) was loaded on a sample plate whose
temperature was set at 90 ꢀC. The sample plate was then cooled
tricarbonyl trichloride (5.5 mmol) in dichloromethane (15 mL)
was added dropwise to the solution, and stirred for 18 h at room
temperature. Aer the reaction, the solution was ltered and
the dichloromethane solution was successively washed with
HCl (10 mM) and deionized water twice. The organic phase was
collected and dried over anhydrous Mg2SO4. The solvent was
evaporated and dried under vacuum. The gelators were
obtained as a dry white powder. The yields of gelators 1–6 were
ꢀ
to 30 C, and the measurement was started.
The gel–sol transition temperature, Tgel, was measured by
differential scanning calorimetry (PerkinElmer 8500, Perkin
Elmer), using aluminum pans with a heating rate of 1 ꢀC minꢁ1
.
1
50, 91, 91, 97, 82, and 84%. H NMR assignments, elemental
The conductivity of the ionogels was measured at 25 ꢀC using
a Hioki Chemical Impedance Meter (3532-80). The isotropic hot
ionic liquid solution, containing the gelator, was injected into a
U-tube, followed by gelation. The cell constant was determined
by measuring the conductivity of a KCl solution (7.44 g Lꢁ1).
analyses, FTIR, and MALDI-TOF/MS are provided in the ESI.†
Compound A. 1,3,5-Tris(bromomethyl)benzene (5 mmol),
L-Leu methyl ester hydrochloride (16.5 mmol), and K2CO3
(30 mmol) were dissolved in dimethyl formamide (30 mL), and
reuxed for 24 h at 60 ꢀC under a N2 atmosphere. Aer the
reaction, the solution was ltered and evaporated. The residue
was dissolved in dichloromethane and successively washed
with HCl (10 mM) and deionized water twice. The organic phase
was collected and dried over anhydrous Mg2SO4. The solvent
was evaporated and dried under vacuum. Compound A was
obtained as a yellow solution. The yield was 58%.
Results and discussion
Gelation of ionic liquids and other solvents
The herein developed gelators with simple molecular structures
are shown in Chart 1. All gelators comprised 1,3,5-benzene-
tricarboxylic acid and amino acid methyl ester groups. The
methyl esters, as derived from the six amino acids (Gly, L-Ala,
L-Val, L-Leu, L-Met, and L-Phe), were used for the conjugation of
Instrumentation
1H NMR spectra were obtained on a 500 MHz Bruker Avance-500 1,3,5-benzenetricarbonyl trichloride. Intermolecular hydrogen
spectrometer. Fourier transform infrared spectroscopy (FTIR) bonding between the amide groups and p–p interactions are
966 | Soft Matter, 2014, 10, 965–971
This journal is © The Royal Society of Chemistry 2014
Ishioka, Yumi
