Amino acid based low-molecular-weight ionogels as efficient dye-adsorbing agents and templates for the synthesis of TiO2 nanoparticles

Article abstract of DOI:10.1002/chem.200901917

The gelation of ionic liquids is attracting significant attention because of its large spectrum of applications across different disciplines. These 'green solvents' have been the solution to a number of common problems due to their eco-friendly features. To expand their applications, the gelation of ionic liquids has been achieved by using amino acid-based low-molecularweight compounds. Variation of individual segments in the molecular skeleton of the gelators, which comprise the amino acid and the protecting groups at the N and C termini, led to an understanding of the structure-property correlation of the ionogelation process. An aromatic ring containing amino acid-based molecules protected with a phenyl or cyclohexyl group at the N terminus were efficient in the gelation of ionic liquids. In the case of aliphatic amino acids, gelation was more prominent with a phenyl group as the N-terminal protecting agent. The probable factors responsible for this supramolecular association of the gelators in ionic liquids have been studied with the help of field-emission SEM, ¹H NMR, FTIR, and luminescence studies. It is the hydrophilic-lipophilic balance that needs to be optimized for a molecule to induce gelation of the green solvents. Interestingly, to maximize the benefits from using these green solvents, these ionogels have been employed as templates for the synthesis of uniform-sized TiO₂ nanoparticles (25-30 nm). Furthermore, as a complement to their applications, ionogels serve as efficient adsorbents of both cationic and anionic dyes and were distinctly better relative to their organogel counterparts.

Full text of DOI:10.1002/chem.200901917

FULL PAPER
DOI: 10.1002/chem.200901917
Amino Acid Based Low-Molecular-Weight Ionogels as Efficient Dye-
Adsorbing Agents and Templates for the Synthesis of TiO2 Nanoparticles
Sounak Dutta, Dibyendu Das, Antara Dasgupta, and Prasanta Kumar Das*[a]
Abstract: The gelation of ionic liquids
is attracting significant attention be-
cause of its large spectrum of applica-
tions across different disciplines. These
ꢁgreen solventsꢂ have been the solution
to a number of common problems due
to their eco-friendly features. To
expand their applications, the gelation
of ionic liquids has been achieved by
using amino acid-based low-molecular-
weight compounds. Variation of indi-
vidual segments in the molecular skele-
ton of the gelators, which comprise the
amino acid and the protecting groups
at the N and C termini, led to an un-
derstanding of the structure–property
correlation of the ionogelation process.
An aromatic ring containing amino
acid-based molecules protected with a
phenyl or cyclohexyl group at the
N terminus were efficient in the gela-
tion of ionic liquids. In the case of ali-
phatic amino acids, gelation was more
prominent with a phenyl group as the
N-terminal protecting agent. The prob-
able factors responsible for this supra-
molecular association of the gelators in
ionic liquids have been studied with
the help of field-emission SEM,
1H NMR, FTIR, and luminescence
studies. It is the hydrophilic–lipophilic
balance that needs to be optimized for
a molecule to induce gelation of the
green solvents. Interestingly, to maxi-
mize the benefits from using these
green solvents, these ionogels have
been employed as templates for the
synthesis of uniform-sized TiO2 nano-
particles (25–30 nm). Furthermore, as a
complement to their applications, iono-
gels serve as efficient adsorbents of
both cationic and anionic dyes and
were distinctly better relative to their
organogel counterparts.
Keywords: gels · green chemistry ·
hydrogen bonds · ionic liquids ·
nanoparticles
Introduction
Nꢀouze et al. showed the confinement of ILs by using an in-
organic matrix as in silica-derived networks.[1 h] Similar con-
finements were described by Carlin and Fuller by using
polymeric organic materials.[1i]
“Ionogels”, as they have been termed on the basis of their
imbibing ability of ionic liquids (ILs), have not been subject-
ed to extensive study and are a rather emerging area of in-
terest among scientists.[1] ILs have been a revelation as an
alternative to toxic and hazardous organic solvents[2] and are
known as “green solvents” owing to characteristics, such as
low vapor pressure, nonvolatility, high thermal stability, non-
flammable character, and eco-friendly nature. Investigations
that utilize ILs are progressively improving from synthetic
chemistry to materials science.[2,3] To add to the properties
and applications of ILs, efforts have been initiated to immo-
bilize and confine them by using external materials.[1]
Until now, small molecules and polymers were extensively
designed to gelate water and/or other organic solvents.[4]
However, the gelation of ILs with low-molecular-weight ge-
lators (LMWGs) has not grown as rapidly, and there are
only a few examples.[1a–d] The gelation of ILs with polymeric
or nonpolymeric compounds has been termed the ꢁquasi-
solidification of ionic liquidsꢂ.[1a] The self-assembly of low-
molecular-weight compounds in ILs would obviously add a
new dimension to the properties and applications of these
green solvents. To this end, Kimizuka and Nakashima suc-
ceeded in the formation of ꢁglycolipid bilayer membranesꢂ in
ILs.[1b] In a few cases, ionogels have been exploited in the
development of technological devices, such as dye-sensitized
solar cells, and in the synthesis of anisotropic gold nanopar-
ticles.[1d,5] However, attempts toward the rational design of
LMWGs for ILs are scarce. This is probably attributed to
the poor understanding of the self-assembly mechanism
during ionogelation, which involves noncovalent interactions
[a] S. Dutta, D. Das, A. Dasgupta, Dr. P. K. Das
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700032 (India)
Fax : (+91)33-24732805
E-mail: bcpkd@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901917.
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such as electrostatic, van der Waals, and hydrogen bonding.
Similar to other gelation processes, an optimized balance be-
tween hydrophilicity and lipophilicity is expected to play a
crucial role in ionogelation.[4d,e,6]
Over the last few years, we have attempted to establish a
rationale behind the synthesis of hydro- and organogelators
that comprise simple amino acids/dipeptides as basic build-
ing blocks through a possible structure–property relation-
ship.[4d,e,6,7] The amino acid/dipeptide moiety at the head of
an amphiphilic molecule with a long hydrophobic chain at
the C terminus and a free amine group at the N terminus is
an excellent precursor for the preparation of both organo-
and hydrogelators.[4d,e,6c] Quaternization of this amine with
methyl iodide followed by ion exchange with chloride ions
yielded an excellent hydrogelator, whereas coupling of the
same amine with a long-chain acid resulted in an efficient
organogelator (Scheme 1).[4d,e,6c] Interestingly, tuning the
counterion of the quaternized amphiphile led to an efficient
conversion of a nongelator into a gelator.[8] At this point, it
would be fascinating if the same precursor amine
(Scheme 1) could be converted into an ionogelator by judi-
cious modification in its structure.
Scheme 2. Structure of the compounds.
ILs generally comprise an organic cation and inorganic
anion, a combination that enhances the potential of this
type of solvent because one can always modulate either of
the two ions to extract a desirable property from the
system.[9] Keeping in mind that ILs are more hydrophilic
than organic solvents, we introduced a phenyl ring at the
N terminus of the amine to retain a critical balance. This
fine-tuning of the l-tryptophan-based amine led to a mole-
cule with limited ionogelation ability (Scheme 2, Table 1)
and further motivated us to introduce a more hydrophobic
moiety, that is, a cyclohexyl group at the N terminus of the
amine group. This change resulted in the development of an
efficient ionogelator (Scheme 2) capable of entrapping a
wide range of ILs (Scheme 3). The gelation results, obtained
by varying the amino acids and the protecting groups at the
N and C termini, helped us to establish a logical structure–
property correlation. These ionogels showed remarkable
dye-adsorption efficiencies from water, which were signifi-
cantly better than those of the analogous organogels. The
synthesis of inorganic metal oxide nanoparticles has been a
focus of research owing to their catalytic efficiency and
unique optoelectronic properties.[10] Ionogel 1c (Scheme 2)
was used as a template to synthesize uniform- and small-
sized TiO2 nanoparticles.
Results and Discussion
Gelation is a process in which the immobilization of solvents
has led to the formation of soft materials with diverse appli-
cations that range from advanced materials to biomedi-
cines.[4,11–13] It is now quite well established that supramolec-
ular gels formed by the self-as-
sembly of LMWGs either in
water or organic solvents is
due to an optimized balance of
several noncovalent interac-
tions.[4,6,11] However, under-
standing the structure–proper-
ty correlations of small-mole-
cule ionogelators and their
self-assembly mechanism is
still in its infancy.
Recently, we designed and
developed
very
efficient
hydro- and organogelators
with minimum gelation con-
centrations (MGC) of 0.3 and
0.15% (w/v), respectively, by
using a simple amino acid as
the basic scaffold.[4d,e,6c] Both
Scheme 1. Synthesis of the hydrogelator, organogelator, and ionogelator from same precursor amine.
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The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
Table 1. Gelation properties of 1–8 in ionic liquids.[a]
We introduced a long hydro-
phobic chain (C16) at the N ter-
minus (Scheme 2) as previous-
ly reported for organogela-
tors.[4d] Protecting groups both
at the C and N termini were
connected through an amide
linkage to the central scaffold.
The l-tryptophan-based com-
pound with two long hydro-
phobic chains at the N and
C termini (1a) was insoluble in
most of the ILs and did not ex-
hibit any gelation ability. In-
stead this compound gelated
the organic solvent toluene
(MGC=1.8% w/v). Consider-
ing that ILs are, on the whole,
more hydrophilic than organic
solvents, we decided to im-
prove the hydrophilicity by in-
Compound
BMIMBr
(10% aq)
BMIMCl
(10% aq)
BMIMBF4
BMIMPF6
BPyBr
ACHTUNGTRNE(NUNG 10% aq)
BPyBF4
BMPyBF4
G
U
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6
ins
sol
ins
sol
2.1
ins
2.3
2.5
ins
2.5
4.2
sol
7.5
8.5
sol
sol
sol
3.0
sol
sol
ins
ins
sol
ins
sol
1.8
ins
2.2
2.6
ins
3.3
3.6
sol
8.2
9.6
sol
sol
ins
3.5
sol
sol
ins
sol
1.7
ins
3.5 (og)
4.0 (tg)
ins
4.0 (og)
4.9
sol
sol
sol
sol
sol
ins
3.2
sol
ins
sol
1.7
ins
4.3 (og)
5.1 (tg)
ins
4.3 (og)
4.7 (og)
sol
7.2
7.1
sol
sol
ins
3.7
sol
sol
4.5 (og)
1.5 (tg)
ins
1.3
1.5
sol
3.1
3.9
sol
8.0
9.5 (tg)
sol
sol
ins
2.6
sol
sol
1.3
ins
3.8 (tg)
2.2
ins
4.6
3.6
sol
7.2
10.0
sol
ins
ins
3.1
sol
sol
2.0 (og)
ins
3.1 (og)
3.5 (og)
sol
sol
ins
sol
ins
ins
sol
sol
ins
3.5 (og)
sol
ins
7
8
ins
[a] tg=Translucent gel, og=opaque gel, sol=soluble, ins=insoluble, concentration in % w/v.
corporating a phenyl ring instead of a long hydrophobic
chain at the N terminus (Scheme 2). The resultant molecule
1b could only gelate BMIMBF4 at a high MGC value
(4.5% w/v). To expand the scope of gelation in different ILs
and to judiciously alter the balance between hydrophilicity
and hydrophobicity, we introduced a nonplanar cyclohexyl
moiety at the N terminus instead of the phenyl ring. The
other reason for introducing the cyclohexane ring is because
it sometimes facilitates intermolecular hydrogen bonding by
positioning itself perpendicular to the amide group.[4h,11a,14]
The resultant molecule 1c exhibited a marked improvement
(i.e., threefold) in the gelation of BMIMBF4 (MGC=
1.5% w/v) relative to 1b and also showed either a compara-
ble or better gelation ability in other ILs, such as BMIMBr,
BMIMCl, and BPyBr with 10% water (Table 1). Interesting-
ly, 1c also showed gelation ability in different organic sol-
vents (toluene, hexane, CCl4, and ethyl acetate) with a
MGC of 1.0–2.5% w/v (see Table S1 in the Supporting In-
formation).
Scheme 3. Structure of the ionic liquids.
As a first step, structural variation was carried out at the
central scaffold by changing the l-amino acid moieties with
nonpolar aliphatic/aromatic residues to obtain a structure–
property correlation for the ionogelators. For each amino
acid residue, the protecting group at the N terminus was
varied with n-hexadecyl, phenyl, and cyclohexyl moieties as
carried out for the l-tryptophan-based molecules (1a–c;
Scheme 2). In the case of all the other amino acid groups
used in the present study (l-phenylalanine, l-isoleucine, l-
valine, and l-alanine) as the central unit, no ionogelation
was observed for the compounds with long-chain protection
(i.e., n-hexadecyl) at the N terminus (2a–5a). However,
when l-tryptophan was replaced by l-phenylalanine, (i.e.,
from 1b to 2b), gelation ability was observed in all the ILs
(Table 1). Also, the phenyl- and cyclohexyl-substituted com-
pounds (2b and 2c, respectively) exhibited almost compara-
ble gelation efficiencies in varying ILs (Table 1); however,
the gelators were synthesized in single steps by two com-
pletely different types of modification of a nongelating pre-
cursor amine of an amino acid (e.g., l-tryptophan) with a
long hydrophobic chain at the C terminus (Scheme 1).
Herein, our prime objective was to develop efficient ionoge-
lators in a single step from the same scaffold so that this
amine can be considered to be a general precursor for the
preparation of any variety of gelators. Initially, we decided
to synthesize ionogelators from the precursor amine of l-
tryptophan (Scheme 2) and tested the gelation efficiency in
different ILs comprising the 1-butyl-3-methylimidazolium
(BMIM) moiety and in 1-butylpyridinium (BPy) salts
(Scheme 3). Because BMIMBr, BMIMCl, and BPyBr are
solids at room temperature, we used 10% water with these
ILs (IL/H2O=9:1 w/w) so that the mixture would be a free-
flowing liquid.
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P. K. Das et al.
2c always had higher MGC values relative to 1c, except in
BMIMBF4, in which both 2b and 2c showed similar gelation
efficiencies. Interestingly, 2b not only gelated all the ILs but
also showed more than a threefold improvement in gelation
efficiency in BMIMBF4 relative to 1b (Table 1). The superi-
or ionogelation efficiencies of 1c and 2b relative to those of
2c and 1b, respectively, might be due to the differences in
the hydrophilic–lipophilic balance (HLB) of the gelators.
This contrasting gelation behavior could be explained by the
difference in the hydropathy index of the two aromatic
amino acid residues.[4e] The hydropathy index represents the
relative hydrophilic and hydrophobic properties of the side
chain of the amino acid residues.[15] Hydrophobic amino
acids have larger hydropathy indices. Despite the presence
of aromatic rings in l-phenylalanine and l-tryptophan, these
amino acids have contrasting HLBs, as l-phenylalanine has
a positive hydropathy number of 2.8, whereas l-tryptophan
has a negative value of À0.9.[15] Thus, l-phenylalanine is
more hydrophobic, whereas an extended indole ring con-
with the concurrent steady decrease in the size of the side-
chain substitution from 3 (l-isoleucine) to 5 (l-alanine) irre-
spective of the nature of the protecting group at the N ter-
minus (Table 1).
So far we had tested the gelation efficiency of amino acid-
based compounds with a long hydrophobic chain at the
C terminus and different protecting groups at the N termi-
nus. We were curious to further investigate whether the ion-
ogelating ability would be affected if the position of the pro-
tecting groups was swapped between the termini. To find
out the importance of the specificity of the position of the
long chain and the phenyl/cyclohexyl group in ionogelation,
protecting groups between the C and N termini of 1c and
3c were exchanged to prepare
6 and 7, respectively
(Scheme 2). Compounds 1c and 3c were chosen to reverse
the termini as they were the most efficient ionogelators that
contained amino acid residues substituted with aromatic and
aliphatic side chains, respectively. The l-tryptophan-based
compound 6 with a cyclohexyl-protected C terminus showed
moderate gelation efficiency in all the ILs with a MGC of
2.6–3.7% w/v (Table 1). But the gelation ability decreased
almost by 1.5–2.5-fold relative to 1c. In the case of the l-iso-
leucine analogue 7, there was no gelation at all of any IL.
Thus, swapping the protecting groups between the termini
did not improve the ionogelation efficiency. Even incorpora-
tion of two cyclohexyl groups at both C and N termini of l-
tryptophan (i.e., 8) did not succeed in gelating the ILs. So,
the presence of the long chain at the C terminus of the
amino acid is almost indispensable for displaying gelation
ability. On the whole, we found that aromatic-ring-contain-
ing amino acid-based molecules protected with a phenyl or
cyclohexyl group at the N terminus were efficient in ionoge-
lation. Hence, an optimum HLB is needed for a specific
molecule to induce gelation of the green solvents. Similarly,
in the case of aliphatic amino acids, such as l-isoleucine and
l-valine, the gelation was more prominent with a phenyl
group as the N-terminal protecting agent.
In the case of ILs that are solid at room temperature, that
is, BMIMBr, BMIMCl, and BPyBr, we tested the gelation of
the molecules in the presence of 10% water. Thus, this free-
flowing medium is a mixture of water and hydrophilic IL.
To expand the scope of gelation in only IL-based systems,
we decided to replace the water with a hydrophilic IL that is
liquid at room temperature. We tested the gelation efficien-
cies of 1c and 3c in a mixture the hydrophilic ILs BMIMBr
(solid at room temperature) and BMIMBF4 in an equal
ratio (1:1 w/w). Encouragingly, in this hydrophilic IL mix-
ture both 1c and 3c showed comparable or better gelation
efficiencies (MGC=1.2 and 3.2% w/v for 1c and 3c, respec-
tively; see Table S1 in the Supporting Information) than
with only BMIMBF4 and BMIMBr in 10% water (Table 1).
À
taining a hydrophilic NH group imparts additional hydro-
philicity to l-tryptophan. Aromatic rings are also an impor-
tant factor in all forms of gelation because they impart addi-
tional p–p stacking interactions that facilitate the formation
of self-assembly. Thus, it can be inferred that the presence
of an extended aromatic residue of l-tryptophan and a cy-
clohexyl moiety maintained the appropriate HLB for 1c,
thus leading to efficient ionogelation. At this point, it is
worth mentioning that the linear chain analogue of the cy-
clohexyl moiety, that is, a n-hexyl-protected N terminus of 1
(1d), did not exhibit any ionogelation. On the other hand,
the presence of two phenyl rings in 2b suitably retained the
HLB for ionogelation contrary to 1b, with one indole
moiety and a phenyl ring.
The replacement of aromatic amino acids by aliphatic res-
idues clarified the importance of aromatic rings in ionogela-
tion. In the case of l-isoleucine (3) and l-valine (4), the
phenyl ring at the N terminus (i.e., 3b and 4b) induced su-
perior ionogelation efficiency relative to the analogous non-
aromatic cyclohexyl-substituted gelators 3c and 4c. For ex-
ample, in the case of BMIMCl and water (9:1 w/w), the
MGC value of 3b was 2.5% w/v, which increased to MGC=
4.2% w/v on replacement of the phenyl ring by a cyclohexyl
group in 3c. A similar trend was observed for 4b and 4c
(Table 1). It seems in the case of nonpolar aliphatic amino
acids, substitution of aliphatic cyclohexyl moiety at the
N terminus decreases the ionogelation ability because of its
overall hydrophobic nature. To be precise, for an aliphatic
amino acid, the presence of a phenyl group at the N termi-
nus facilitates the self-assembling process, thus leading to
improvement in ionogelation. Intriguingly, 3c also showed
gelation ability in different organic solvents with a MGC of
1.6–2.9% w/v (see Table S1 in the Supporting Information).
In accordance with the observed trend, none of the l-ala-
nine-based compounds 5a–c were ionogelators, possibly be-
cause they could not retain the requisite HLB necessary for
gelation. Interestingly, the ionogelation efficiency for the ali-
phatic amino acid-based compounds markedly decreased
Gel-to-sol transition: The gelators were dissolved in the de-
sired IL either by heating or irradiating the gelator/IL mix-
ture in a microwave oven at 320 W for 5–10 s. The mixture
was left to stand for 30 min, left to cool at room tempera-
ture, and checked to see whether they were “stable to inver-
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The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
sion” (Figure 1). All the ionogels were thermoreversible in
nature, thus they melted upon heating and turned into a gel
on cooling, which indicates the self-assembled aggregation
of the small molecules in the ILs. The system is at thermo-
ample because it is the best ionogelator in the present study.
A prominent endothermic peak was visible for ionogel 1c in
BMIMBr (Figure 3) at MGC=1.3% w/v. Very interestingly,
the temperature (72.88C) at which this peak is present is
Figure 1. Gelation of 1c in BPyBF4 before (left) and after the formation
of the ionogel (right).
dynamic equilibrium and there is no kinetic control of the
process. The temperature at which this gel-to-sol transition
takes place is known as the gel-melting temperature Tgel,
which increases with an increase in the concentration of the
gelator molecules (Figure 2).[7,16] Interestingly, these ionogels
Figure 3. DSC thermograph of a) 1c in BMIMBr (ionogel) and b) only
the ionic liquid.
comparable with Tgel =748C, as found in the gel-to-sol tran-
sition study. The visually observed gel-to-sol transition at
Tgel corroborates to the temperature found experimentally
from the DSC study. In a control experiment with aqueous
BMIMBr solution (10% water) without the gelator, no peak
was observed in the same temperature range. The heat flow
throughout the scan was uniform, thus indicating no phase
transition in the absence of a gelator.
Microscopy studies: Field-emission scanning electron mi-
croscopy (FESEM) was utilized to obtain primary informa-
tion as to the cause of this self-aggregation at a supramolec-
ular level. The extent of supramolecular aggregation in the
IL-based systems was concurrent with the gelation efficien-
cies of the molecules. Fibrous networks were visible in the
microscopy images of the ionogels of 1c and 3c (taken as
representatives) in BMIMBr (Figure 4a,b). In the case of
1c, thicker fibrils of approximately 300 nm were observed
relative to the fibers of approximately 100 nm for 3c. The
extended aromatic system of the l-tryptophan moiety and
the hydrogen-bonding ability of 1c possibly facilitated the
supramolecular association of the fibrils into thicker fibers.
On the other hand, the ionogel of 3c in BMIMBr showed
thinner fibers of approximately 100 nm. The absence of an
aromatic system might be the cause of this loosely packed
supramolecular structure, which was consistent with its com-
paratively poor ionogelation ability.
Figure 2. Variation of the gel-to sol transition temperature Tgel with the
gelator concentrations of representative ionogels.
are distinctly high-melting gels relative to the reported hy-
drogels and organogels. A striking difference was observed
in the case of 1c, in which Tgel =748C at MGC=1.3% w/v
in BMIMBr (with 10% water), whereas the same compound
in toluene gave Tgel =488C at MGC=1.0% w/v. Even at a
MGC value of 3.5% w/v (a 3.5-fold increase), Tgel =70 and
1038C in toluene and BMIMBr, respectively (Figure 2). Sim-
ilarly, for other ionogels, the Tgel value is relatively higher,
and in accordance with the gelation efficiency Tgel is always
higher for 1c than for 3c.
Bright field microscopy images of 1c and 3c in aqueous
BMIMBr solution (10% water) also showed fibrous inter-
twined networks (Figure 4c,d), which were concurrent with
the FESEM images. The pattern of supramolecular aggrega-
tion was similar for both molecules in bright field microsco-
Differential scanning calorimetry (DSC): This technique is
important in determining the phase change of a system.[17]
One can follow the temperature at which a specific change
in the system takes place. We take 1c as a representative ex-
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P. K. Das et al.
state.[4d,6c,7a,19] The molecules
participating in gelation are
merely observable on the
NMR timescale due to the
large correlation time of the
assemblies, which results in a
very short transversal relaxa-
tion time and produces
a
broad signal.[18h] At the onset
of this aggregation mechanism
in IL-based systems, there is
an obvious participation of the
amide group in hydrogen
bonding. The amide proton in-
teracts with the neighboring
carbonyl group through C=
À
O····H N hydrogen bonding,
which deshields the proton and
allows the downfield shift to
be observed.[6c,18e–g] In this con-
text, we must say that the
other protons of the gelator
molecule, such as the chiral
proton that appears at d=
Figure 4. SEM images of a) 1c in BMIMBr (10% water) and b) 3c in BMIMBr (10% water), and bright-field
microscopy images of c) 1c in aqueous BMIMBr and d) 3c in aqueous BMIMBr.
py as well as FESEM images. The self-aggregation of small
molecules in ILs thus proceeds through a probable fibrous
supramolecular structure.
1
1H NMR experiments: H NMR spectroscopic analysis is a
powerful tool to investigate the involvement of hydrogen
bonding during the process of self-assembled gelation. The
protons involved directly in gelation suffer notable shifts
when investigated in an immobilized state.[4c,18] In this con-
text, we monitored the change in the chemical shift of the
amide proton that plays key role in the hydrogen-bonding
interactions. Obviously, because of its superior ionogelation
efficiency among the others, 1c was chosen for this study. In
deuterated dimethyl sulfoxide ([D6]DMSO), that is, in the
sol state of 1c, the amide proton at the N terminus of the l-
tryptophan moiety appeared at d=5.56 ppm (Figure 5).
With an increase in the BMIMBr content in the medium, a
downfield shift of the amide proton was observed. Even at
concentration of 10% IL (90% [D6]DMSO), the amide
proton was notably deshielded (d=5.83 ppm). A consequent
increase in IL shifted the proton signal further downfield. In
a composition of IL and [D6]DMSO of 40:60 (v/v), the
chemical shift was shifted as much as up to d=6.25 ppm.
Also with an increase in the amount of IL, the nature of the
peak became broader and the splitting disappeared. As the
amount of IL was increased, a gelation process is induced
that obviously involves the molecule concerned. The amide
proton, or the whole molecule that was in solution in
[D6]DMSO, is drawn into a supramolecular association in
the presence of the IL. Thus, the spinning nuclei cannot pro-
duce individual sharp and strong signals as in the nongelated
Figure 5. Changes in the amide proton of 1c with increasing concentra-
tion of ionic liquid; 1c in IL/[D6]DMSO at ratios (v/v) of a) 0:100,
b) 10:90, c) 20:80, d) 30:70, e) 40:60.
4.44 ppm in [D6]DMSO, did not show any significant shifts
with an increase in IL concentration. Also the cyclohexyl
proton, which appears at d=3.00 ppm, remains more or less
constant with varying concentrations of IL. This finding
gives us the idea that only those protons (amide) involved in
hydrogen bonding in the self-assemblies undergo sufficient
chemical shifts.
FTIR spectroscopic analysis: We performed FTIR spectro-
scopic experiments with gelator 1c in a nongelated solution
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The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
state in CHCl3 and in a gel state in ILs BMIMBF4 and
BMIMBr (in the presence of 10% D2O) at their corre-
sponding MGCs (Figure 6). The FTIR spectrum of the com-
pound in CHCl3 showed transmission bands at n˜ =1645 and
tested the concentration-dependent alteration (below MGC
À
to above MGC) in the C=O stretching frequency of 1c in
aqueous solution of BMIMBr (10% D2O). The concentra-
tions of 1c used were 0.8% w/v (below MGC), 1.3% w/v (at
MGC), and 3% w/v (above MGC), respectively. The corre-
sponding stretching frequencies were n˜ =1629, 1626, and
1625 cmÀ1, respectively (Figure 6C). Thus, by increasing the
concentration up to MGC, the intermolecular hydrogen
bonding distinctly increased as the frequency of the amide I
band decreased. Above MGC, the change in the strength of
the hydrogen bonding is obviously negligible as no addition-
al hydrogen bonding was expected to form beyond
MGC.[4d,7]
Luminescence study: The participation of the gelator mole-
cules through hydrophobic interactions during gelation is
often investigated by luminescence studies that use any pop-
ular hydrophobic probe.[13c,16,20] In the present study, we ex-
amined the participation of the IL in gelation following the
emission spectra of the IL. The luminescence study was per-
formed in BMIMBr solution (10% H2O) by varying the
concentration of gelators 1c and 3c (Figure 7). A fluores-
cence emission was observed at lꢀ460 nm when the imida-
Figure 6. A) FTIR spectra of 1c in CHCl3. B) Ionogel of 1c in a) aqueous
BMIMBr and b) BMIMBF4. C) Shift in the n
ACHTUNGERTN(NUNG C=O) (amide I) absorption
with an increase in the concentration of gelator 1c in aqueous BMIMBr.
1528 cmÀ1, which are the characteristic of n
ACHTNUTRGNE(NUG C=O) (amide I)
À
and d
in the n
G
À
the N H bending band (amide II) was observed in the iono-
gel state relative to the sol state (Figure 6).[4d,11b] The amide
I frequency appeared at n˜ =1634 cmÀ1 for the ionogel in
BMIMBF4 and at n˜ =1626 cmÀ1 in BMIMBr solution (10%
D2O). The variations in the decrease in the carbonyl stretch-
ing frequencies indicate the probable extent to which this
bond is involved in a hydrogen-bonding phenomena with a
neighboring amide moiety. Also in concurrence with report-
À
Figure 7. A) Luminescence spectra of a) aqueous BMIMBr and the addi-
tion of 1c at concentrations of b) 0.05, c) 0.5, d) 1.0, e) 1.5, f) 2.0, and
g) 3.0% w/v. B) Luminescence spectra of a) aqueous BMIMBr and the
addition of 3c at concentrations of b) 0. 5, c) 1.0, d) 2.0, e) 3.0, f) 4.0, and
g) 5.0% w/v.
ed studies, the dACHTUNGTRENNUNG(N H) (amide II) frequency increased (n˜ =
~1570–1575 cmÀ1) for 1c in the gel of both ILs. We also
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1499

P. K. Das et al.
zolium salt is excited at l=320 nm.[21] With a gradual in-
crease in the concentration of the gelator molecule, the
emission intensity steadily decreased. At and above the
MGC of the gelator, there was almost negligible change in
the emission intensity. Without the gelator molecule, the IL
molecule remains free, and hence the fluorescence emission
was at a maximum intensity. However, with an increase in
the concentration of the gelator, the IL becomes immobi-
lized within the self-assembly of the gelator and bound in
the supramolecular networks; as a result, the intensity of the
fluorescence emission steadily
lowed by stirring at room temperature for 48 h. Washing
and calcinations of the white material resulted in the forma-
tion of a fine white powder. The powder obtained was char-
acterized by using FESEM in which uniform-shaped parti-
cles of the dimension of 25–30 nm were observed (Fig-
ure 8a). For atomic force microscopy (AFM), the powder
was taken in HPLC-grade ethanol and sonicated for 30 min.
From this turbid solution, a drop was cast on a silicon wafer
and investigated with AFM. TiO2 nanoparticles of a similar
dimension were seen from the AFM image (Figure 8b).
decreased.
The
excitation
wavelength was maintained at
l=320 nm to minimize the ex-
citation of the aromatic moiety
of the gelator 1c. As the trend
of fluorescence emission was
similar for both aromatic and
aliphatic side-chain-substituted
amino acids (i.e., 1c and 3c,
respectively), it can be safely
concluded that the observed
emission is due to the IL. In
this context, it was necessary
to confirm whether the ob-
served signals were due to
fluorescence
emission
or
Raman bands. We carried out
an excitation of the pure IL
(BMIMBr, 10% H2O) from
l=280 to 400 nm with inter-
vals of Dl=10 nm and record-
ed the emission peaks simulta-
neously. Similar experiments
were done with the ionogel
(1c in BMIMBr (2.0% w/v)).
In both cases, we found an in-
consistent shift in the emission
wavelength with highest inten-
Figure 8. a) FESEM image of TiO2 prepared in an ionogel; b) AFM image of TiO2 prepared in an ionogel
(inset: cross-sectional analysis along the black line in (b)). FESEM image of TiO2 prepared in c) an ionic
liquid medium and d) with 1c in BMIMBr below MGC.
sity (lemmax), which is known to happen in the case of ILs
due to the presence of several energetically different associ-
ated forms of the imidazolium ions in the IL.[21a] However,
the possibility of the signal being a Raman band was ruled
out as the extent of the shift in the excitation and emission
wavelengths was not comparable (no linearity was observed
in the plot of lex versus lem; see Figure S1 in the Supporting
Information). Also, the broad nature of the observed bands
indicates the absence of a Raman band, which is generally
found to be a sharp peak.[21c]
However, we were curious to know whether the nanoparti-
cles formed could have different sizes and shapes if a static
condition of the experiment was maintained instead of stir-
ring. We repeated the experiment under nonstirring condi-
tions while keeping all the other experimental conditions
identical. Similar-sized nanoparticles (25–30 nm) were ob-
served from the FESEM image (see Figure S2 in the Sup-
porting Information). Thus, TiO2 nanoparticles of compara-
ble size and shape were obtained under both stirring and
nonstirring conditions. We believe that diffusion of the parti-
cles in the gel template is facilitated under stirring, whereas
normal spontaneous diffusion takes place without stirring,
thus resulting in similar uniformity of the nanoparticles.
However, in the absence of the gelator or at a concentration
(0.5% w/v) below the MGC, aggregation of the TiO2 parti-
cles was observed (Figure 8c,d). Hence, the supramolecular
gel network of the ionogel at MGC is an important factor in
the formation of uniform- and small-sized TiO2 nanoparti-
Synthesis of TiO2 nanoparticles: The IL-assisted preparation
of TiO2 on the nanoscale level has been studied recent-
ly.[22,23] However, an ionogel as a template has never been
utilized in the synthesis of TiO2 nanoparticles. Herein, we
synthesized TiO2 nanoparticles by adding titanium tetraiso-
propoxide to the ionogel of 1c in aqueous (10% H2O)
BMIMBr solution and adding an equal amount water fol-
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The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
cles. The XRD study of the nanoparitcle powder showed
peaks (Figure 9) that correspond to standard anatase TiO2
without the formation of any other crystalline product, and
the broad peak also indicates the formation of small-sized
TiO2 particles.[24]
Figure 9. XRD spectra of TiO2 obtained from an ionogel template (using
1c in aqueous BMIMBr).
Dye-adsorption study: Environmental hazards have looked
ominous and it is a growing problem that demands serious
attention. Dyes are colored substances that are used hugely
in versatile fields, such as the fabric and tanning industries.
Removal of these toxic dyes from wastewater has been a
matter of concern. Activated charcoal, clay, porcelain, and
substances with a high porosity or large surface area have
been used for dye removal from wastewater.[25] To this end,
we previously used xerogels of organogels for efficient dye
removal because they comprise both hydrophilic and hydro-
phobic domains, which could efficiently adsorb the dye.[4d,e]
Having used ILs as solvents for gelation, we used this iono-
gel as a dye-adsorbing agent. This time, water-immiscible
ionogels of the hydrophobic IL BMIMPF6 was used.
BMIMPF6 (100 mL) containing gelator 1c (2.0%, w/v; just
after heating in the microwave oven so that it forms a gel)
was added to a stock solution (0.01 mm) of crystal violet
(cationic) and naphthol blue black (anionic) in water. The
neat ionogel descended through the aqueous solution of the
dye. A similar method was applied for the organogels
(2.0% w/v) of 1c in toluene. This experiment was carried
out to compare the efficiency of the hydrophobic organo-
and ionogels. Almost 65% of the cationic dye was adsorbed
in the ionogel of 1c within 5 min, whereas this value for the
anionic dye was 45% (Figure 10). More than 90% of the
cationic and anionic dyes were removed within 8 and 20 h,
respectively, by using the ionogel of 1c. On the other hand,
the organogels removed dyes from the aqueous dye solution
at a slower rate because 50% adsorption of dye took almost
6 h for crystal violet and 20 h for naphthol blue black (see
Figure S3 in the Supporting Information). Both the cationic
(Figure 11) and anionic dyes are adsorbed by the ionogels to
a more or less similar extent, which is notably better than
Figure 10. UV/Vis spectroscopic study of the time-dependant removal of
crystal violet and naphthol blue black by using an ionogel (1c in
BMIMPF6).
Figure 11. Solution of 0.01 mm of crystal violet before and after treatment
of the ionogel (1c in BMIMPF6).
the analogous organogels. Although BMIMPF6 is hydropho-
bic like the organic solvent used (i.e., toluene), the inherent
ionic character within the ionogel presumably facilitates the
adsorption of the cationic/anionic dyes. A parallel set of
control experiments (in the absence of a gelator) was car-
ried out to compare the dye adsorption efficiencies of the
neat IL (see Figure S3 in the Supporting Information). The
rate of the adsorption of the dyes with the hydrophobic IL
was even slower. For crystal violet, 90% of the dye removal
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1501

P. K. Das et al.
protected amide was obtained through column chromatography on silica
gel (60–120 mesh) and ethyl acetate/hexane as the eluent. The product
was deprotected with trifluoroacetic acid (TFA; 2 equiv) in dry dichloro-
methane. After stirring for 2 h, the solvents were removed on a rotary
evaporator, and the mixture was dissolved in ethyl acetate. The ethyl ace-
tate portion was thoroughly washed with aqueous 10% sodium carbonate
solution followed by brine to neutrality. The organic portion was dried
over anhydrous sodium sulfate and concentrated to obtain the corre-
sponding amine. The amine was coupled with the desired acid (palmitic
acid/benzoic acid/cyclohexane carboxylic acid) by using DCC (1 equiv) in
dry dichloromethane. The purified product was obtained by column chro-
matography on silica gel (60–120 mesh) and ethyl acetate/toluene as the
eluent to yield the purified product.
took as long as 30 h, whereas in the presence of a gelator
(ionogel) 90% of the dye was removed within 8 h. For naph-
thol blue black, even after 30 h only about 35% of the dye
was removed by the neat IL. However, in the presence of
the gelator, 90% of the dye was removed within 12 h.
Hence, the supramolecular network of the ionogel in the
presence of a gelator might have facilitated the adsorption
process. Also, dye adsorption by solid or quasisolid materi-
als is an advantageous process because the residual clean
water can be removed easily.
1a: [a]2D0 =À18.28 (c=3.5 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.14 (s, 1H), 7.72–7.65 (d, 1H), 7.34–7.26 (d, 1H), 7.22–7.05
(m, 3H), 5.65 (br, 1H), 5.19 (br, 1H), 4.39–4.37 (m, 1H), 3.28–3.27 (m,
2H), 3.17–3.05 (m, 2H), 2.17–2.15 (m, 2H), 1.90–1.67 (m, 4H), 1.55–1.08
(br, 50H), 0.85–0.92 ppm (m, 6H); ESI-MS: m/z: calcd for: 688.7579 [M+
+Na]; found: 665.5859 [M+]; elemental analysis calcd (%) for
C43H75N3O2: C 77.54, H 11.35, N 6.31; found: C 77.98, H 11.18, N 6.86.
1b: [a]2D0 =À11.28 (c=3.2 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.10–8.08 (br, 1H), 7.31–6.97 (m, 10H), 6.45–6.40 (d, 1H),
6.12–6.05 (d, 1H), 4.63–4.56 (m, 1H), 3.14–3.08 (m, 2H), 2.88–2.84 (t,
2H), 1.58–1.49 (m, 2H), 1.29–1.15 (br, 26H), 0.93–0.85 ppm (t, 3H); ESI-
MS: m/z: calcd for: 554.2534 [M++Na]; found: 531.3825 [M+]; elemental
analysis calcd (%) for C34H49N3O2: C 76.79, H 9.29, N 7.90; found: C
77.05, H 9.71, N 7.55.
1c: [a]2D0 =À15.08 (c=2.0 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.08 (s, 1H), 7.77–7.75 (d, 1H), 7.38–7.35 (d, 1H), 7.26–7.12
(m, 3H), 6.31–6.29 (d, 1H), 5.51 (br, 1H), 4.70–4.66 (m, 1H), 3.22–3.11
(d, 2H), 3.07–3.05 (t, 2H), 2.12–2.05 (m, 1H), 1.46–1.08 (br, 38H), 0.88–
0.86 ppm (t, 3H); ESI-MS: m/z: calcd for: 560.1645 [M++Na]; found:
537.4294 [M+]; elemental analysis calcd (%) for C34H55N3O2: C 75.93, H
10.31, N 7.81; found: C 75.48, H 10.53, N 8.07.
1d: [a]2D0 =À13.28 (c=2.8 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.05 (s, 1H), 7.45–7.35 (d, 1H), 7.26–7.22 (d, 1H), 7.05–7.01
(m, 3H), 5.56 (br, 1H), 5.23 (br, 1H), 4.54–4.48 (m, 1H), 3.19–3.16 (m,
2H), 3.03–2.95 (m, 2H), 2.23–2.20 (m, 2H), 1.88–1.77 (m, 4H), 1.43–1.15
(br, 30H), 0.88–0.96 ppm (m, 6H); ESI-MS: m/z: calcd for: 548.2578 [M+
+Na]; found: 525.4294 [M+]; elemental analysis calcd (%) for
C33H55N3O2: C 75.38, H 10.54, N 7.99; found: C 75.57, H 10.15, N 8.26.
Conclusions
Herein, the efficient gelation of ILs has been achieved by
using low-molecular-weight amino acid-based compounds.
Importantly, these ionogelators were prepared from the
same precursor scaffold from which hydrogelators and orga-
nogelators were obtained. A possible structure–property
correlation has been established between the molecule and
its ionogelation ability by the systematic variation of the
amino acids and protecting groups. In concurrence with the
gelation behavior of small molecules in water and organic
solvent, a critical HLB is necessary for the formation of the
supramolecular association in ILs that would lead to the de-
velopment of ionogels. The importance of aromaticity was
established because in many cases the presence of a phenyl
ring either in the amino acid or the protecting group is cru-
cial for the gelation of ILs. The ionogel template was found
to have an application in the synthesis of uniform-shaped
TiO2 nanoparticles (25–30 nm). Additionally, these supra-
molecular aggregates in ILs were exploited in the adsorption
of both cationic and anionic dyes from aqueous solution,
which exhibited markedly higher dye-adsorption efficiencies
relative to those of the analogous organogels. With the
growing significance of ILs and the matrices of immobilized
ILs, the present work has provided an opportunity to under-
stand the gelation of ILs with an important structural corre-
lation and to exploit them in the synthesis of TiO2 nanopar-
ticles and wastewater treatment.
2a: [a]2D0 =À9.08 (c=1.0 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=7.26–7.21 (m, 5H), 6.75–6.72 (d, 1H), 6.02 (br, 1H), 4.63–4.61
(m, 1H), 3.13–3.01 (m, 2H), 2.97–2.93 (t, 2H), 2.36–2.32 (t, 2H), 2.16–
2.03 (m, 2H), 1.63–1.61 (m, 2H), 1.55–1.11 (br, 50H), 0.88 ppm (m, 6H);
ESI-MS: m/z: calcd for: 649.6364 [M++Na]; found: 626.5750 [M+]; ele-
mental analysis calcd (%) for C41H74N2O2: C 78.53, H 11.90, N 4.47;
found: C 78.48, H 11.35, N 4.62.
2b: [a]2D0 =À11.28 (c=3.4 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=7.76–7.26 (m, 10H), 6.97–6.94 (d, 1H), 5.69 (br, 1H), 4.80–4.73
(m, 1H), 3.30–3.19 (m, 2H), 3.16–3.03 (m, 2H), 1.41–1.36 (m, 2H), 1.34–
1.25 (br, 26H), 0.90–0.85 ppm (t, 3H); ESI-MS: m/z: calcd for: 515.3167
[M++Na]; found: 492.3716 [M+]; elemental analysis calcd (%) for
C32H48N2O2: C 78.00, H 9.82, N 5.69; found: C 77.62, H 9.68, N 5.58.
2c: [a]2D0 =À10.38 (c=4.1 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=7.7–7.44 (m, 5H), 6.42–6.39 (d, 1H), 5.80 (s, 1H), 4.62–4.55
(m, 1H), 3.22–3.11 (m, 2H), 3.09–2.98 (m, 2H), 2.37–2.31 (m, 1H), 1.56–
1.58 (m, 2H), 1.53–1.15 (br, 36 H), 0.88–0.86 ppm (t, 3H); ESI-MS: m/z:
calcd for: 521.4594 [M++Na]; found: 498.4185 [M+]; elemental analysis
calcd (%) for C32H54N2O2: C 77.06, H 10.91, N 5.62; found: C 77.33, H
10.58, N 5.85.
3a: [a]2D0 =À9.58 (c=2.1 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.23 (s, 1H), 5.10–5.08 (d, 1H), 4.14–4.09 (m, 1H), 3.44–3.40
(br, 2H), 3.26–3.19 (m, 3H), 1.47–1.43 (br, 4H), 1.35–1.11 (br, 55H),
0.89–0.84 ppm (t, 9H); ESI-MS: m/z: calcd for: 615.5254 [M++Na];
found: 592.5907 [M+]; elemental analysis calcd (%) for C38H76N2O2: C
76.96, H 12.92, N 4.72; found: C 76.56, H 12.69, N 4.55.
Experimental Section
General: All the amino acids, n-hexadecyl amine, dicyclohexylcarbodi-
imide (DCC), 4-N,N-(dimethyl)aminopyridine (DMAP), 1-hydroxyben-
zotriazole (HOBT), benzoic acid, cyclohexane carboxylic acid, cyclohexyl
amine, n-hexadecanoic acid, and all the solvents were purchased from
SRL (India). All the deuterated solvents for the NMR and FTIR spectro-
scopic experiments and titanium tetraisopropoxide were obtained from
the Aldrich Chemical Co. TLC analysis was performed on Merck pre-
1
coated silica gel 60-F254 plates. The H NMR spectra were recorded on an
AVANCE 300 MHz (Bruker) spectrometer. The mass-spectrometric data
were acquired by electron spray ionization (ESI) on a Q-TOF-micro
quadruple mass spectrometer (Micromass).
Synthesis: The small molecules were prepared by following a standard
protocol as done previously.[4d] Briefly, a tert-butyloxycarbonyl (Boc)-pro-
tected l-amino acid was coupled with n-alkylamine or cyclohexyl amine
(for 6–8) by using DCC (1 equiv) in dry dichloromethane. The pure Boc-
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The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
3b: [a]2D0 =À15.78 (c=3.1 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=7.81–7.4 (m, 5H), 6.98–6.95 (d, 1H), 6.29–6.27 (br, 1H), 4.47–
4.41 (m, 1H), 3.48–3.28 (m, 2H), 3.26–3.15 (m, 1H), 1.52–1.47 (m, 2H),
1.25–1.10 (br, 28 H), 0.99–0.85 ppm (m, 9H); ESI-MS: m/z: calcd for:
481.2701 [M++Na]; found: 458.3872 [M+]; elemental analysis calcd (%)
for C29H50N2O2: C 75.93, H 10.99, N 6.11; found: C 75.68, H 11.05, N
5.87.
3c: [a]2D0 =À7.28 (c=3.3 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.10–6.08 (d, 1H), 5.97 (m, 1H), 4.32–4.15 (m, 1H), 3.23 (br,
2H), 2.12–1.82 (m, 2H), 1.82–1.77 (m, 2H), 1.25–1.07 (br, 38H), 0.89–
0.91 ppm (m, 9H); ESI-MS: m/z: calcd for: 487.3672 [M++Na]; found:
464.4342 [M+]; elemental analysis calcd (%) for C29H56N2O2: C 74.94, H
12.14, N 6.03; found: C 74.55, H 11.84, N 6.18.
4a: [a]2D0 =À18.48 (c=4.7 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.27 (s, 1H), 5.28–5.25 (m, 1H), 3.83–3.73 (m, 1H), 3.28–3.19
(m, 2H), 2.75 (m, 1H) 2.35–2.20 (m, 2H), 2.08–2.04 (m, 2H), 1.64–1.58
(m, 2H), 1.49–1.11 (br, 50H), 0.95–0.85 ppm (m, 12H); ESI-MS: m/z:
calcd for: 601.5382 [M++Na]; found: 578.5750 [M+]; elemental analysis
calcd (%) for C37H74N2O2: C 76.75, H 12.88, N 4.84; found: C 76.55, H
13.01, N 5.08.
4b: [a]2D0 =À10.08 (c=6.0 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=7.58–7.43 (m, 5H), 6.31 (br, 1H), 5.45 (br, 1H), 4.63–4.51 (m,
1H), 3.20–3.15 (m, 2H), 2.96–2.89 (m, 1H), 1.57–1.45 (br, 2H), 1.42–1.19
(br, 26H), 0.97–0.85 ppm (m, 9H); ESI-MS: m/z: calcd for: 467.1724 [M+
+Na]; 444.3716 [M+]; elemental analysis calcd (%) for C28H48N2O2: C
75.63, H 10.88, N 6.30; found: C 75.32, H 11.11, N 6.44.
4c: [a]2D0 =À10.18 (c=4.2 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.20 (br, 1H), 5.27 (br, 1H), 3.86–3.84 (m, 1H), 3.26–3.22 (t,
2H), 2.36–2.29 (m, 1H), 2.06–1.91 (m, 1H), 1.74–1.65 (m, 2H), 1.47–1.25
(br, 36H), 0.95–0.88 ppm (m, 9H); ESI-MS: m/z: calcd for: 473.2529 [M+
+Na]; found: 450.4185 [M+]; elemental analysis calcd (%) for
C28H54N2O2: C 74.61, H 12.08, N 6.21; found: C 75.01, H 11.88, N 6.07.
5a: [a]2D0 =À14.08 (c=5.2 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.25 (s, 1H), 5.56 (br, 1H), 3.44–3.26 (m, 1H), 3.24–3.19 (m,
2H), 1.65–1.58 (m, 2H), 1.46–1.44 (m, 4H), 1.30–1.06 (m, 53H), 0.99–
0.87 ppm (m, 6H); ESI-MS: m/z: calcd for: 573.2523 [M++Na]; found:
550.5437 [M+]; elemental analysis calcd (%) for C35H70N2O2: C 76.30, H
12.81, N 5.08; found: C 76.12, H 12.56, N 5.23.
5b: [a]2D0 =À10.68 (c=5.5 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.10–7.44 (m, 5H), 6.15–6.12 (br, 1H), 4.98 (br, 1H), 4.12–4.07
(m, 1H), 3.27–3.20 (m, 2H), 1.48–1.44 (m, 2H), 1.35–1.25 (m, 29H),
0.90–0.85 ppm (t, 3H); ESI-MS: m/z: calcd for: 439.4571 [M++Na];
found: 416.3403 [M+]; elemental analysis calcd (%) for C26H44N2O2: C
74.95, H 10.64, N 6.72; found: C 75.18, H 11.03, N 6.93.
5c: [a]2D0 =À15.28 (c=3.5 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.35 (br, 1H), 5.42 (d, 1H), 3.93–3.89 (m, 1H), 3.24–3.19 (m,
2H), 2.30–2.25 (m, 1H), 1.94–1.89 (m, 2H), 1.76–1.62 (m, 7H), 1.47–1.23
(br, 32H), 0.93–0.84 ppm (t, 3H); ESI-MS: m/z: calcd for: 445.1452 [M+
+Na]; found: 422.3872 [M+]; elemental analysis calcd (%) for
C26H50N2O2: C 73.88, H 11.92, N 6.63; found: C 74.06, H 11.72, N 6.98.
3.15–3.08 (t, 2H), 2.04–1.95 (m, 1H), 1.50–1.37 (m, 8H), 1.25–0.88 ppm
(m, 12H); ESI-MS: calcd for: 418.1471 [M++Na]; found: 395.2573 [M+];
elemental analysis calcd (%) for C24H33N3O2: C 72.88, H 8.41, N 10.62;
found: C 72.52, H 8.59, N 10.71.
Synthesis of ILs: The ILs were prepared by following a standard proto-
col.[2f,26] Briefly, BMIMBr, BMIMCl, and BPyBr were obtained through a
microwave-assisted synthesis, after which they were washed with dry di-
ethyl ether and ethyl acetate. The corresponding tetrafluoroborate ana-
logues were obtained by a simple stirring reaction with ammonium tetra-
fluoroborate followed by extraction into dry dichloromethane, whereas
BMIMPF6 was synthesized by stirring the bromide salts with hexafluoro-
phosphoric acid at low temperature followed by extraction into dry di-
chloromethane. All the ILs were charcoalized with distilled methanol.
The solvent was removed in the rotary evaporator and finally the ILs
were dried in a vacuum oven for 72 h at 608C. The purified ILs were
characterized by NMR spectroscopic, mass-spectrometric, and elemental
analysis.
Preparation of the ionogels and organogels: All the molecules were dis-
solved in ILs or the corresponding IL-based systems were placed in a
glass vial (i.d.: 10 mm) and irradiated in a microwave oven (320 W) for
5–10 s depending on the dissolution of the material. The solutions were
cooled (undisturbed) to room temperature. After 30 min, the formation
of ionogels was verified by checking the stability of the gel under gravita-
tion by inversion of the glass vial. Organogels were prepared by placing
the molecules in organic solvents and slowly heating until the solid com-
pletely dissolved. The solutions were cooled to room temperature and
formation of gel was verified after 1 h to be stable to inversion by invert-
ing the glass vial.
Determination of the gel-to-sol transition temperature (Tgel): The gel-to-
sol transition temperature was determined by keeping a glass vial con-
taining an ionogel (i.d.: 10 mm) in an oil bath and slowly raising the tem-
perature at a rate of 28CminÀ1. The vial containing the ionogel was in-
verted in between to check the “stable-to-inversion” property of the gel.
The temperature (Æ0.58C) at which the gel melts and showed gravita-
tional flow was designated as Tgel.
DSC: DSC was carried out on a Perkin–Elmer Diamond DSC. Gel 1c
(1.3% w/v; 35 mg) in BMIMBr (10% H2O, w/v) was placed in a large-
volume capsule (LVC) and sealed. The sample LVC pan was placed in
the DSC apparatus together with an empty LVC pan as a reference. The
pans were cooled to 208C and aged for 30 min at this temperature. Heat-
ing scans were recorded from 20 to 908C at a scan rate of 18CminÀ1
.
Microscopic study: Field emission scanning electron microscopy
(FESEM) images were obtained on a JEOL-6700F microscope. The iono-
gel at MGC was washed with water five times to ensure complete remov-
al of the ionic liquid.[1a] A very small amount of the semisolid gel was
mounted on a piece of cover slip and SEM images were taken.
FTIR measurements: FTIR measurements of the gelators in CHCl3 solu-
tion and ionogels in BMIMBF4, BMIMBr with 10% D2O were carried
out in a Perkin–Elmer Spectrum 100 FTIR spectrometer with KBr pellets
and in a 1-mm CaF2 cell, respectively.
NMR measurements: IL-induced concentration-dependent 1H NMR
spectra of 1c were taken on an AVANCE 300 MHz (Bruker) spectrome-
ter. The molecule was initially dissolved in deuterated dimethyl sulfoxide
([D6]DMSO) and the ionic liquid content was gradually increased.
6: [a]2D0 =À16.08 (c=5.6 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.11 (s, 1H), 7.78–7.68 (d, 1H), 7.37–7.35 (d, 1H), 7.23–7.05
(m, 3H), 6.33–6.31 (d, 1H), 5.45 (br, 1H), 4.35 (br, 1H), 3.61–3.48 (m,
1H), 3.09–3.01 (t, 2H), 2.21–2.16 (t, 2H), 1.95–1.91 (m, 2H), 1.58–1.11
(m, 34 H), 0.96–0.85 ppm (t, 3H); ESI-MS: m/z: calcd for: 546.3157 [M+
+Na]; found: 523.4138 [M+]; elemental analysis calcd (%) for
C33H53N3O2: C 75.67, H 10.20, N 8.02; found: C 75.42, H 10.28, N 8.23.
7: [a]2D0 =À11.78 (c=6.2 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=6.42–6.40 (d, 1H), 5.51–5.43 (br, 1H), 4.68–4.66 (m, 1H), 3.32–
3.27 (m, 1H), 3.15–3.02 (m, 1H), 2.20–2.15 (m, 2H), 1.70–1.56 (br, 6H),
1.52–1.24 (br, 35H), 0.96–0.87 ppm (m, 6H); ESI-MS: m/z: calcd for:
473.4743 [M++Na]; found: 450.4185 [M+]; elemental analysis calcd (%)
for C28H54N2O2: C 74.61, H 12.08, N 6.21; found: C 74.47, H 11.87, N
6.09.
XRD: A Seifert XRD3000P diffractometer was used with CuKa radiation
as the source (a=0.15406 nm) and a voltage and current of 40 kV and
30 mA. The sample was put on a glass slide and XRD analysis was per-
formed in a wide-angle region 10–608.
Fluorescence spectroscopy: The emission spectra of IL solutions
(BMIMBr+10% H2O) containing gelator molecule 1c and 3c at differ-
ent concentration were recorded on a Varian Cary Eclipse luminescence
spectrometer. The solutions were excited at l=320 nm. The excitation
and emission slit widths were 10 and 10 nm, respectively. The emission
spectra of the IL (BMIMBr, 10% H2O) and ionogel 1c in IL (BMIMBr,
10% H2O) were recorded at different excitation wavelengths that ranged
from l=280 to 400 nm at intervals of 10 nm. The excitation and emission
slit widths were 5 and 5 nm, respectively.
8: [a]2D0 =À21.28 (c=5.7 in CHCl3); 1H NMR (300 MHz, CDCl3, 258C
TMS): d=8.09 (s, 1H), 7.70–7.68 (d, 1H), 7.37–7.35 (d, 1H), 7.20–7.05
(m, 3H), 6.35 (br, 1H), 5.44 (br, 1H), 4.65 (m, 1H), 3.61–3.48 (m, 1H),
Chem. Eur. J. 2010, 16, 1493 – 1505
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1503

P. K. Das et al.
Synthesis of titanium dioxide nanoparticles: Gelator molecule 1c
(7.5 mg) was dissolved in BMIMBr (500 mL, containing 10% water), irra-
diated in a microwave, and cooled to prepare the ionogel. Titanium tet-
raisopropoxide (25 mL) was added to this ionogel, and water (500 mL)
was added after half an hour. The reaction mixture was stirred at room
temperature for 48 h. The resulting material was filtered, washed several
times with water, and calcined at 2008C for 2 h followed by 5008C for an-
other 2 h to ensure that no organic portion was left. In a parallel set of
experiments, the procedure was repeated under nonstirring conditions.
Simultaneous experiments with the gelator below MGC and without ge-
lators were performed by using similar protocols. The white powder was
used for FESEM and XRD studies. For AFM, the white powder was
taken in HPLC-grade ethanol and sonicated for 30 min. This suspended
solution (10 mL) was put on a silicon wafer, which was dried under
vacuum and used for taking AFM images in a noncontact mode (Veeco,
modelAP0100).
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Chem. Eur. J. 2010, 16, 1493 – 1505

The Applications of Self-Assembled Supramolecular Ionogels
FULL PAPER
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drophilic IL mixture, FESEM images of the TiO2 nanoparticles, and
the UV/Vis spectroscopic study of the time-dependent removal of
naphthol blue black and crystal violet with an organogel (1c in tolu-
ene) are given in the Supporting Information.
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Received: July 10, 2009
Revised: September 16, 2009
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Published online: December 17, 2009
Chem. Eur. J. 2010, 16, 1493 – 1505
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1505