Amino acid appended cholic acid-azobenzene dyad: An effective & smart phase selective gelator for aromatic solvents

Article abstract of DOI:10.1039/c6ra13160c

A series of amino acid appended cholic acid-azobenzene dyads have been synthesized and studied for their gelation behaviour. One of the l-alanine based dyads showed excellent gelation behaviour in a variety of solvents at room temperature with minimum gel concentration in the range 0.8% (w/v)-1.8% (w/v). The morphology of the stable gels indicated the formation of a lamellar or a dense sheet network in different solvents. Variable temperature ¹H-NMR and FT-IR studies revealed an evident role of intermolecular hydrogen bonding in the self-assembly process. The photo-isomerization between the trans and cis forms of the azobenzene unit was established by UV-visible spectroscopy, and a comparison of ¹H NMR and SEM images of the gel and sol forms. In addition, π → π stacking between phenyl groups of azobenzene might have provided an additional driving force for the formation of a dense three-dimensional network capable of phase selective gelation of aromatic solvents from a water/solvent mixture. This selective gelation of aromatic solvents remains unaffected even in the presence of the common salts usually present in water from different sources. The phase selective gelation ability of the dyad was successfully explored towards the removal of toxic fat-soluble rhodamine dye from water and selectively gelatinizing petrol and crude oil from an oil/water mixture at room temperature. Thus, these smart systems possess the potential to be used effectively in water purification and oil-spill remediation.

Full text of DOI:10.1039/c6ra13160c

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Amino Acid Appended Cholic acid-Azobenzene Dyad: An Effective
& Smart Phase selective Gelator for Aromatic solvents
Devesh S. Agarwal,[a] Neelam Gogoi,[b] Devasish Chowdhury,[b] and Rajeev Sakhuja*[a]
Received 00th January 20xx,
Accepted 00th January 20xx
A series of amino acid appended cholic acid-azobenzene dyads have been synthesized and studied for their gelation
behaviour. One of the L-alanine based dyads showed excellent gelation behaviour in a variety of solvents at room
temperature with minimum gel concentration in the range 0.8% (w/v)-1.8% (w/v). The morphology of the stable gels
indicated the formation of a lamellar or a dense sheet network in different solvents. Variable temperature 1H-NMR and FT-
IR studies revealed an evident role of intermolecular hydrogen bonding in the self-assembly process. The photo-
isomerization between trans and cis form of azobenzene unit was established by UV-visible spectroscopy, and a
DOI: 10.1039/x0xx00000x
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1
comparison of H NMR and SEM images of the gel and sol forms. In addiꢀon, π→π stacking between phenyl groups of
azobenzene might have provided an additional driving force for the formation of a dense three-dimensional network
capable of phase selective gelation of aromatic solvents from a water/solvent mixture. This selective gelation of aromatic
solvents remains unaffected even in presence of common salts usually present in water from different sources. The phase
selective gelation ability of the dyad was successfully explored towards the removal of toxic fat-soluble rhodamine dye
from water and selectively gelatinizing petrol and crude oil from oil/water mixture at room temperature. Thus, these
smart systems possess the potential to be used effectively in water purification and oil-spill remediation.
either for separating toxic aromatic dyes/solvents during water
purification procedures8 or oil-spill remediation9 which is one
of the environmental concern in minimizing marine pollution.10
Introduction
Immobilization of organic solvents by low-molecular-weight
gelators (LMWGs) through molecular self-assembly process is a
well-established strategy for preparing stable gels that find
wide-ranging applications as templated materials, drug
delivery agents, cosmetics, sensors, enzyme-immobilization
matrices.1-4 These gelators forms a three-dimensional (3D)
network in organic solvents by self-organization of the
monomeric species to higher-order structures, such as fibrous,
tubular, or helical,5 usually driven by non-covalent
intermolecular interactions such as electrostatic, dipole–
dipole, van der Waals, π–π stacking, and hydrogen bonding.6
Although numerous effort has been devoted to establishing a
structure–property relationship for the development of low-
molecular weight gelators,7 however predicting the gelation
Also, the use of eco-friendly, abundant and cheap chemical
precursors such as amino acids,11 carbohydrates,12 lipids13 for
preparing such PSGs provides an additional advantage. Since
the development of first amino acid based phase selective
gelator by Bhattacharya and Krishnan-Ghosh,14 several other
amino acid based PSGs have been developed.15 Das et al. have
reported few examples of peptide based organogelators and
explored their applications in dye absorption, phase selective
gelation and template for nanoparticle synthesis.8 In addition
to amino acids, bile acids have also been proven as potential
components of supramolecular gels due to their amphiphilic
scaffolds, availability of multiple easily derivatizable functional
groups, low cost, and adjustable self-assembling
characteristics.16
ability of
a targeted compound and understanding the
Interestingly, most of the examples reported for phase
selective gelation of aromatic solvents requires external
stimuli such as heating,8a,14,17 mechanical shaking,18
sonication19 etc. to initiate the gelation process and
heating/distillation20 for recovering the aromatic solvent from
gelatinized material. Such protocols appear to be highly
expensive in the context of large scale water purification or oil-
spill remediation processes. Thus, instant gelation of aromatic
solvents at room temperature and their recovery by simple
filtration are dictating factors for an ideal PSG, which could be
addressed by preparing photo-responsive PSGs that could
reformed gel back to sol state by photo-irradiation.
mechanism of self-assembly process remains a challenging
task. In addition, specific effort has been laid towards
exploring phase selective gelation ability of novel organic
structures. Such phase selective gelators (PSGs) could be used
a.Department of Chemistry, Birla Institute of Technology and Science (BITS),Pilani-
333031, Rajasthan, India.
b.Material Nanochemistry Laboratory, Physical Sciences Division, Institute of
Advanced Study in Science and Technology (IASST), Paschim Boragaon, Garchuk,
Guwahati-781035, Assam, India.
Email: sakhuja.rajeev@gmail.com.
Electronic Supplementary Information (ESI) available: [Original copies of 1H NMR
and 13C NMR of 3a-c
, 4a-c, 6a-f, 7a-f, 9a-f]. See DOI: 10.1039/x0xx00000x
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In the last decade, emphasis has been laid on the development Waals force interactions, and π-conjugated planar aromatic
of photo-responsive gels that have offered promising moieties that promote the π-π are typical of the features
opportunities for designing useful functional materials such as found to facilitate the gelation process. In addition, with an
sensors, actuators, etc.21 The gel-sol transition of such smart aim to prepare smart amino acid appended cholic acid based
materials upon light irradiation could provide a potential gelator, we envisaged the hybridization of photo-responsive
solution for recovering aromatic solvents from the recovered azobenzene system to cholic acid scaffold via an α-amino acid
gel without heating/distillation. Very few examples on the use linker. Thus, (E)-ethyl 4-((4-hydroxyphenyl)diazenyl)benzoate
of photo-responsive materials for phase selective gelation of
(1) required to initiate the synthesis of targeted system was
aromatic solvents have been reported.22 Ajayaghosh reported obtained by the reaction of p-ethylcarboxylate aniline and
the use of N-glucosyl azobenzene as an excellent phase phenol via standard diazotization-coupling process.26
selective gelator for removal of toxic organic solvents from Alkylation at phenolic-OH in
water.23 Very recently, Díaz Díaz reported the application of
1 with long chain alkyl bromides
(
2a-c) using K2CO3 in acetone yielded the corresponding O-
tetrapeptides bearing a side-chain azobenzene moiety for alkylatedphenyl diazenyl benzoate derivatives (3a-c), which on
ultrasound-induced phase selective gelation of organic reduction with LAH/THF at room temperature yielded the
solvents, water purification by removal of toxic dyes and oil- corresponding O-alkylatedphenyl diazenyl methanols (4a-c).
spill remediation.24 However, the development of bile acid Coupling of
4 with Boc protected α-amino acids (5a-b) using
based photo-responsive and efficient room temperature PSGs EDC.HCl/HOBt/DMAP in DCM afforded Boc protected amino
for the separation of aromatic solvents remains unexplored. In acyl O-alkylatedphenyl diazenyl conjugates (6a-f) in good
this context, we herein report the first example of amino acid yields. Deprotection of Boc group using HCl/dioxane in
appended cholic acid-azobenzene dyad as an effective and followed by coupling with cholic acid using
smart phase selective gelator for aromatic solvents at room EDC.HCl/HOBt/DMAP in DCM at room temperature in 6-8 h
temperature. The phase selective gelation ability of one of the gave amino acid appended cholic acid-azobenzene dyads (9a-f
dyads has been explored towards the practical utility in water in excellent yields. All the intermediates 3a-c 4a-c 6a-f 7a-f
purification by removal of toxic fat-soluble dyes and oil-spill were characterized on the basis of H & 13C NMR, IR data and
6
(8)
)
,
,
,
1
remediation.
the final compounds 9a-f were characterized on the basis of
1H, 13C NMR, IR and mass spectroscopic studies.
Gelation
Results and Discussion
The gelation behaviour of 9a-f was examined in various non-
polar and polar solvents using inverted test tube method25 and
the results are summarized in Table 1. In general, all the
compounds gelatinized most of the solvents at room
temperature forming either stable or weak gels.
Synthesis
Although it is difficult to generalize the criteria for an ideal
organogelator,
however,
hydrogen-bonding
enabling
functional groups, long alkyl chains that facilitate the van der
Scheme 1. Synthesis of amino acid appended cholic acid-azobenzene dyads
For instance, compound 9a, an L-Alanine linked dyad
possessing 8-carbon alkyl chain at the phenolic-OH showed
remarkable gelation ability in seven solvents, namely
hexane, cyclohexane, toluene, xylene, benzene, mesitylene
and t-butyl benzene. It is worth mentioning that no external
stimuli such as heating/sonication/mechanical shaking was
required for gelation process and the gel was formed within
3-4 minutes upon gentle swirling of organic solvents with
powdered LMWGs. The minimum gel concentration of 9a
was found to be 0.8% (w/v) in xylene (mixture of all
2 | J. Name., 2012, 00, 1-3
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isomers) and mesitylene, and it ranged from 1 to 1.8%
(w/v) in other solvents. Compound 9e, another L-Alanine
linked dyad possessing 16-carbon alkyl chain at the
phenolic-OH gelates dodecane at MGC of 1% (w/v). The
solvents above 20 oC, thus indicating a crucial role of sterics
in gelation process.
Morphology of self-assembled gels
It is well documented27 that organogelation represents a
physical state formed via non-covalent hydrophilic and
hydrophobic interactions that leads to the variation in the
morphologies of the resultant gels, including fibers,
nanosheets, fibrils, helical ribbons and other 3-D structures.
To get an insight into the microscopic morphology of the
stable gels obtained, Scanning Electron Microscopy (SEM)
of the xerogels were recorded. The SEM image of the gels
of 9a obtained in toluene and xylene showed a dense
lamellar sheet network consisiting of bundle of fibres
entangled with each other (Figure 1a-b). The gel of 9a in
cyclohexane, hexane, benzene, tert-butyl benzene and
mesitylene also showed similar lamellar sheet type
structures consisting of dense fibre network (Figure S1a-e,
Supporting Information file). In addition, the gel of
compound 9e in dodecane showed a beautiful flowery
lamellar dense sheet network (Figure S1f, Supporting
Information file).
remaining compounds (9b-d
,
9f) could form weak gels
C (Table 1).
which were not stable above 20
°
Table 1. Minimum Gelation Concentration of CA-AA-AZA (9a-f) in different solvents
Solvent
Ethyl acetate
Hexane
9a
9b
S
I
9c
S
9d
9e
9f
S
S
S
S
G (1)
PS
S
WG (4)
I
PS
S
DCM
S
S
S
S
S
PS
S
S
I
S
S
DCE
S
S
S
S
S
1,4-Dioxane
THF
S
S
S
S
S
S
S
S
S
S
Cyclohexanol
Methanol
Ethanol
PS
PS
S
PS
S
PS
PS
S
S
S
S
S
S
S
S
Cyclohexane
Heptane
Decane
G (1)
PS
PS
PS
PS
PS
PS
S
I
I
PS
PS
PS
PS
PS
PS
S
I
I
I
I
I
I
I
PS
Dodecane
Tetradecane
Hexadecane
Toluene
I
I
I
G (1)
I
I
I
I
I
a)
b)
I
I
I
G (1.8)
S
WG
(6)
WG
(5.5)
WG
(6.5)
WG
(8.1)
WG
(7.6)
S
S
Benzene
Xylene
G (1.2)
G (0.8)
G (0.8)
G (0.9)
S
S
S
S
S
S
S
S
S
S
S
WG
(4)
WG
(4.3)
WG
(5.5)
WG
(6.2)
S
Mesitylene
tert-bb
DMF
WG
(6)
Figure 1. SEM images of xerogels of 9a in (a) xylene (b) toluene
WG
(7.5)
S
The Transmission Electron Microscopy (TEM) images of the
xerogel of xylene and toluene further supported the
lamellar sheet type packing stacked over each other (Figure
2).
WG
(10)
WG
(11)
DMSO
S
S
WG
S
S
WG
(10.5)
(11.5)
G = gel formed at room temperature (minimum gel concentration (% w/v));
WG = weak gel (unstable above 20°C); PS = partially soluble (a part of it
becomes soluble upon heating but re-precipitation was observed after
cooling to room temperature); S = soluble; I = insoluble (was not soluble at all
even on heating)
DMSO = Dimethyl sulfoxide, DMF = N,N-Dimethylformamide, DCE = 1,2-
dichloroethane, DCM = Dichloromethane, tert-bb = tert-butyl benzene, THF =
Tetrahydrofuran
All the stable gels were found to be thermo-reversible i.e.
all the gels can be melted at higher temperature and
reformed into gel upon cooling. Interestingly, the gels were
transformed to solution state under UV irradiation within
few minutes and got reformed under visible light. The
gelation abilities of 9a remains unchanged after repetitions
of several gel-sol cycles. In addition, gels of 9a were found
to be quite stable at room temperature over a period of 2-3
months without any phase separation thus indicating their
Figure 2. TEM images of xerogels of 9a in (a) xylene (b) toluene
Mesomorphic nature of the gelators
To study the mesomorphic behaviour of the gelator,
Polarising Optical Microscopy (POM) was recorded. For this
purpose, a small amount of gel of 9a at minimum gel
concentration in xylene, and toluene was placed on a
microscopic glass slide and was visualized under the optical
microscope at 10x magnification using visible light. A strong
birefringent texture, characteristic of the mesoscopic
stability. Surprisingly, L-phenylalanine linked dyads (9d-f)
possessing different alkyl chains at the phenolic-OH were
found to be incapable in generating stable gels in most
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Figure 5. X-ray powdered diffraction pattern of xerogel of 9a in xylene
alignment of the chromophores during gelation was
observed (Figure 3a-b).4g
FT-IR Analysis
.
In order to anticipate the effective role of H-bonding in self-
assembly process due to presence of amino acid and cholic
acid moieties, FT-IR spectra of dilute solution of 9a and its
xerogel (in xylene) were recorded and compared (Figure 6).
The dilute solution of compound 9a showed a number of
peaks in the range of 3680-3317 cm-1 (O-H/N-H stretching),
1744 cm-1 (C=O stretching of ester), 1651 cm-1 (C=O
stretching of amide bond) and 1551 cm-1 (N-H bending),
while IR of wet gel showed a prominent band at 3371 cm-1
and no band above it, indicating the possibility of strong
involvement of O-H of cholic acid/N-H of amidic bond in
the hydrogen bonding in the gel state. A blue shift was
observed in the amidic carbonyl from 1651 cm-1 to 1659
cm-1 indicated the role of amide bond in hydrogen bonding.
Thus, it can be concluded that hydrogen bonding plays a
crucial role in self-assembly process of 9a in different
solvents. Not much change in the frequency and intensity
of carbonyl ester was observed upon gelation, indicating its
non-involvement in the H-bonding.
Figure 3. Polarising Optical Microscopy (POM) images of 9a in (a) xylene (b)
toluene under visible light between crossed-polarizers
Fluorescence Optical Microscopic Study
A fibrillar structure of the wet gel of 9a in xylene and
toluene respectively was observed by Fluorescence Optical
Microscopy (Figure 4a-b).
Figure 4. Fluorescence Optical Microscopic images of wet gel 9a in (a) xylene
(b) toluene
X-ray Diffraction Pattern
To gain more insight into molecular packing in the gel state,
XRD analysis were performed on the xerogel of xylene
(Figure 5). The WAXRD displayed well-resolved diffraction
pattern indicating the formation of well-defined
arrangment of the molecules during self-assembly process.
The xerogel showed reflection peaks (2θ) 38.399, 44.623,
64.914, 77.976 corresponding to spacing (
1.43, 1.22 Å respectively. The sharp peak corresponding to
d) spacing 2.34 Å could be attributed to the inter-layer
d) 2.34, 2.02,
Figure 6. (a) FT-IR of the dilute solution of 9a, (b) FT-IR of the wet gel of 9a
(in xylene) at minimum gel concentration
(
Variable temperature-dependent 1H NMR studies
distance between the azobenzene unit, while the (d)
spacing corresponding to 2.02 Å could be the H-bonding
distance between the cholic acid and the amino acid units.
The SEM images of most xerogels of 9a suggested the
existence of dense sheet or lamellar network. The IR
studies also clued the existence of strong intermolecular
Hydrogen bonding in the self-assemble process. To further
understand the role of hydrogen bonding in the self-
assembly process, variable temperature 1H NMR
300
250
200
150
100
50
experiments were conducted at
concentration of 9a in gelation solvent benzene-d6 over a
temperature range of 293 to 333 K. Generally,
a minimum gelation
K
temperature dependent shift of –NH and –OH peaks have
been widely used to understand the role of hydrogen
bonding in self-assembly process.28 As evident from the
temperature dependent NMR of 9a, a consistent shift in the
–NH and –OH proton peaks was observed as the
temperature was increased (Figure 7). The broad multiplet
of –NH proton shifted from 7.4 to 6.11 ppm, while the
three -OH groups of cholic acid also showed a small upfield
0
20
40
60
80
2θ (deg.)
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shift (Figure 7). The dependence of chemical shift on
(aromatic) via π-π interaction. It was observed that the gel
of 9a in xylene turned into sol on exposure to UV light
within few minutes. Subsequently, the regeneration of the
gel was observed upon exposure to visible light at room
temperature within few minutes. This photo-responsive
temperature (Δδ/ΔT) provides a handy tool to study the
role of hydrogen bonding in the self-assembly process in
the gelling solvent.
behaviour (gel
⇄ sol) was found to be reversible and could
be repeated many times. This configurable change in the
azobenzene scaffold on irradiation with UV light was
further studied and confirmed by UV-Visible spectroscopy.
With an increase in the UV exposure time (under hand held
UV lamp at 365 nm) of xerogel of 9a, a gradual decrease in
the intensity of the absorption band at 352 nm (
π-π*
transition) and a concomitant increase in the intensity of a
band at 455 nm (n-π* transition) was observed (Figure 9a).
Subsequently an opposite phenomena was observed when
the same sample was irradiated with visible light (> 460 nm
under a normal bulb) (Figure 9c). These spectral changes
can account for the change in the photo-responsive
behaviour of the gel at the macroscopic state. The sol
formed after the exposure of UV light on the gel, when
analyzed through SEM showed small aggregates, while the
subsequent gel obtained from this sol in visible light
showed dense sheets when analyzed through SEM images.
Thus, it could be concluded that the UV irradiation of gel
disturbed the molecular packing of 9a in xylene and thus
the network of gel converted to sol (Figure 9).
Figure 7. Variable temperature 1H-NMR study of 9a in gelling solvent
benzene (d6) at 293k, 303K, 313K, 323K and 333K showing a change in the
chemical shifts of NH and OH protons.
Earlier reports have suggested that higher the value of
temperature coefficient (Δδ/ΔT), greater is the role of NH in
intermolecular hydrogen bonding.29 In the present case, a
relatively large temperature coefficient value for –NH (32.3
ppb/K, calculated from slope of the graph between change
in chemical shift and temperature) as compared to earlier
0s
30s
1min
1.5min
2min
2.5min
3.5min
5.5min
9.5min
17.5min
reports29 (Δ
δ/ΔT of the order of 15.46 ppb/K) confirmed the
1.0
0.5
0.0
formation of strong intermolecular hydrogen bonding using
–NH bond (Figure 8). Generally, the temperature coefficient
0.10
0.08
0.06
0.04
0.02
0.00
(Δδ/ΔT).
7.6
7.4
7.2
7.0
6.8
6.6
6.4
6.2
6.0
400
500
Wavelength(nm)
300
400
500
b)
Wavelength (nm)
a)
Visible
UV
30s
1 min
1.5 min
2 min
3min
5min
9min
17min
32min
62min
92min
122 min
152min
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.1
290
300
310
320
330
340
Temperature (K)
Figure 8. The temperature coefficient for –NH calculated from slope (32.3
ppb/K). A relatively large temperature coefficient value confirmed the
formation of strong intermolecular hydrogen bonding using –NH bond.
0.0
400
500
Wavelength(nM)
300
400
500
600
d)
Sol-Gel Transformation of CA-AA-AZA (9a) by light stimuli
Wavelength (nm)
c)
Azobenzene is a photo-responsive chromophore unit which
undergoes trans-cis isomerization by exposure to UV light
Figure 9. (a) Absorption spectra of 9a under UV-light irradiation (365 nm) at
different time scale (inverted vial showing gel of xylene (2% w/v) after
exposing to visible light). (b) SEM image of the xylene gel showing a lamellar
network. (c) Absorption spectra of 9a upon exposure to visible light (>460
nm) at different time scale (tilted vial showing sol form after exposure to UV-
light of xylene). (d) SEM image of the sol form showing aggregates.
and vice-versa by exposure to visible light respectively. It
has been well documented30 that trans form of the
azobenzene favours self-assembly process by long range
networking which could be an important parameter for the
selective gelation of solvents in water-solvent mixture. As
earlier stated23 that trans form of azobenzene unit favours
self-assembly process that favours long range entangled
network capable of forming gels with solvent molecules
Comparative 1H NMR studies for cis-trans isomerization
The trans to cis photo-isomerization phenomena further
1
got support by recording the H NMR of a sample of 9a (in
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benzene-d6) after exposing it to UV light at different
intervals of time (Figure 10). It became evident from these
1H-NMR experiments that extra peaks of cis-isomer in the
aromatic region appeared after irradiating the sample with
UV light after 20 min. and 120 min respectively. Cis isomer
protons are consistently more shielded than their trans
isomer counterparts. The relative shifts in the position of cis
protons (a'-d') as compared to trans protons (a-d) are in
studies that heating of the gel breaks the strong
intermolecular hydrogen bonding between N-H bonds of
amide bond linker or O-H bonds of two consecutive
possible parallel stacked cholic acid units, thereby distorting
the self-assembled aggregates temporarily, which regains
their original format upon cooling. However, no change in
the chemical shift of the azobenzene protons or
appearance of no extra peaks in the aromatic region
(corresponding to cis isomer) indicated that the proposed
ꢀ → ꢀ stacking between trans-azobenzene units remain
unaffected upon heating. Thus, based on the above
observations and literature report,23 it could be inferred
that CA-AA-AZA (9a) type of scaffold might have formed by
face to face packing stabilized by strong intermolecular
hydrogen bonding between N-H of amide bonds of the
amino acid linker and three O-H of the cholic acid motif
leading to formation of lamellar sheet networks containing
bundle of fibers entangled into each other which are also
evident from SEM, TEM and XRD analysis. Such dense
packing facilitates the entrapping aromatic solvents easily
and this facilitates the phase selelctive gelation behavior of
gelator (Figure 11).
accordance with earlier reports.31 As cis
-trans photo-
isomerization is a reversible phenomenon and trans form is
more is more stable than the cis-form, thus relatively small
amount of cis-form was found even after exposure for a
longer time (Figure 10).
Figure 10. Comparison of 1H-NMR of 9a (in benzene-
d6) in visible light (>460
nm) and after irradiating UV light (365 nm) for 20 min and 120 min.
Appearance of new peaks (a'-d') after UV irradiation confirms cis-trans
isomerism of the azobenzene unit.
Mechanism of self-assembly process
1
Based on the variable temperature H NMR and IR studies,
it can be concluded that the involvement of N-H of amide
bond and O-H groups of cholic acid in strong intermolecular
hydrogen bonding, might be a crucial parameter in self-
assembly process. In addition, trans geometry of the
azobenzene unit could be another important parameter
that favors the face to face orientation/stacking of
molecules that favors gelation. The driving force in the
remaining scaffold could be face to face packing of
amphibilic cholic acid units that gets stabilized due to inter-
molecular H-bonding between three hydroxyl groups,
which further got support from intermolecular H-bonding
between and amide bonds in the extended amino acid
skeleton, which remain undisturbed upon irradiation by UV
light. Complete inter-conversion of trans to cis form is
Figure 11. Schematic representation of plausible mode of aggregation in the
self-assembly process via non-covalent interactions resulting in the
formation of lamellar sheet networks containing bundle of fibers entangled
into each other
Application of 9a for Phase Selective Gelation
Contrary to other compounds, CA-AA-AZA (9a) showed
selective gelation of aromatic solvents from a solvent-water
mixture. The phase selective gelation of aromatic solvents
could be attributed to the entrapping of aromatic solvent
molecules between long range parallel dense network of
amino acid appended cholic acid azobenzene molecule
stacked one over another via H-bonding and π-π
interaction. As model examples, the phase selective
gelation of xylene, toluene and benzene were carried by
mixing an equiv. volume of these with water (1 mL each)
and subsequently shaking with 9a (0.02 g). After few
minutes, gelation of the aromatic solvents in their
respective vials were observed and dense coloured gels
were obtained. (Figure 12). The resultant gels were stable
enough to be scooped out with a spatula (Figure 8e) or via
filtration to give pure water. The solvent trapped in gel
chemically
a slow phenomena due to large steric
repulsions, however photo-isomerization of few molecules
from trans to cis might be enough to perturb the self-
assembled three-dimensional network and thus physically
the gel got transformed into sol. Moreover, re-gelation of
sol within few minutes under visible light because of back-
isomerization indicates that the remaining scaffold of the
gelator favors aggregation process in trans form. Similar,
visual observations could be inferred by gel to sol reversible
conversion that occurs upon cooling and heating process
respectively. As evident from variable temperature 1H NMR
6 | J. Name., 2012, 00, 1-3
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and petrol gel (right) scooped out from the vial and the vials after crude oil
and petrol removal.
could be recovered by vacuum distillation and the gelator
could be reused.
To explore the real application of phase selective gelation
behaviour of our synthesized dyad, 9a in oil-spill
remediation process, we attempted phase selective
gelation of petrol and crude oil. Addition of 25 mg of 9a to a
mixture of petrol-water (1 mL:2.5 mL) and crude oil-water
(1 mL:2.5 mL) followed by gentle swirling gelatinizes petrol
and crude oil effectively in few minutes (4-5 min.). The
stable gels formed were comfortably scooped out with a
spatula as depicted in the figure 14.
Figure 12. Phase selective gelation of 9a (0.02 g) in equiv. volume of water
with aromatic solvents such as (a) xylene (b) toluene (c) benzene. (d) Tilted
vial after phase selective gelation of xylene showing gel stability in water (e)
xylene gel scooped out with a spatula
The phase selective nature of the gelator was also tested in
presence of various metal salts. To our delight, the phase
selective nature of the gelator was unaffected in presence
of various salts such as saturated solutions of NaCl, CaCl2,
CuSO4, NiCl2, FeCl3, and CdCl2, (Figure 13) which provided us
a handy tool for the use of gelator in real samples such as
sea water, tap water etc.
Water Purification
Removal of toxic dyes from waste water is one of the
important task in environmental remediation, as it is known
that fat-soluble dyes usually gets extracted in organic
solvents. Thus, phase selective nature of organogelators
could be used as a handy tool for water purification. In
order to explore the practical applicability of 9a towards
phase selective gelation of xylene from water-xylene
mixture, a 0.05 mM aqueous solution of rhodamine B dye
was prepared that showed red colour (Figure 15).
Rhodamine B was selected due to its good fat-soluble
properties. Upon addition of xylene (1 mL) to the coloured
aqueous layer (1 mL) contaminated with rhodamine B and
subsequent shaking, the overall solution becomes
colourless indicating the solubilization of dye in xylene
layer. This xylene layer got gelatinized quickly upon
addition of powdered gelator (12 mg) in it, which upon
filtration or simple decanting yielded the pure water. Thus,
the rhodamine B dye got trapped in three-dimensional (3D)
network of gel formed and gets immobilized along with
xylene molecules. The water obtained by filtering the gel
was analyzed by recording its UV-Visible absorption
spectrum and comparing it with the UV-Visible of the water
containing rhodamine B dye. The purification efficiency was
found to be around 99.5% (Figure 15b). Thus, phase
selective behaviour of 9a could be efficiently used in waste
water purification by removal of similar fat- soluble dyes
from it.
Figure 13. Phase selective gelation of 9a (0.02 g) from the mixture of xylene
and water (1 mL each) in the presence of saturated solution of common salts
present in water such as (a) NaCl (b) CuSO4 (c) Cd(NO3)2 (d) Ni(NO3)2 (e) CaCl2
(f) FeCl3
Oil-Spill remediation
Oil-spill has emerged as one of the major issues in the ever
growing environmental problem.32 An oil-spill not only
causes damage to marine ecosystems, but also imparts a
disastrous health impact on humans due to the
consumption of oil polluted sea foods.33 Due to scale-up
challenges and a number of other drawbacks associated
with the various oil-spill cleaning techniques including
bioremediation,34
dispersion,35
absorption,36
and
solidification,37 phase selective gelation of oil from a
biphasic mixture of water and oil, by low-molecular-weight
gelators has provided a potential and effective solution to
this problem.38 To overcome this, phase selective gelation
has emerged as one of the effective remedy.
a)
b)
3.0
(Water before Purification)
(Water after Purification)
2.5
2.0
1.5
1.0
0.5
0.0
c)
400
500
Wavelength (nm)
600
700
Figure 15. a) Removal of toxic rhodamine B dye from water by phase
selective gelation of xylene using 9a and recovery of pure water by simple
filtration. b) UV-visible spectra of water before and after purification
depicting the efficacy of process.
Figure 14. Real-time example of oil-spill remediation by phase selective
gelation of petrol and crude oil using 9a from
a biphasic mixture of
petrol/crude oil and water. (a) vial showing mixture of crude oil (2 mL, left)
and petrol (2 mL right) with water (5 mL) (b) tilted vials showing phase
selective gelation of crude oil (left) and petrol (right) (c) crude oil gel (left)
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Journal Name
2005, 34
Chem. Res., 2007, 40, 644; (d) A. Ajayaghosh, V.K.
,
821; (c) A. Ajayaghosh and V.K. Praveen, Acc.
Praveen and C. Vijayakumar, Chem. Soc. Rev., 2008,
37, 109; (e) J. H. Jung, M. Park and S. Shinkai, Chem.
Soc. Rev., 2010, 39, 4286; (f) S. S. Babu, K. K. Kartha and
Conclusions
In summary, we have designed and synthesized a series of
amino acid appended cholic acid-azobenzene dyads as
photo-responsive low-molecular-weight gelators. Among
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Ranjith, E. Bandini, A. Ajayaghosh and N. Armaroli,
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these, L-alanine based dyad possessing
O-octyl chain
proved to be excellent phase selective gelator of a variety
of solvents such as hexane, cyclohexane, toluene, xylene,
2
benzene, mesitylene and
t-butyl benzene at room
temperature. The minimum gel concentration of 9a was
found to be 0.8% (w/v)-1.8% (w/v) in these solvents. The
SEM images of the stable gels of 9a showed a lamellar or a
dense sheet network in different solvents. The self-
assembly process of such morphologies was explained by
variable temperature 1H-NMR studies, FT-IR that revealed a
strong role of intermolecular hydrogen bonding. In
addition, UV-visible spectroscopy and a comparison of SEM
images of the gel and sol forms gave an insight into the
existence of photo-isomerization between the trans and cis
form of azobenzene unit and π→π stacking between phenyl
groups of trans azobenzene. This regular stacking and long
range networking stabilized by H-bonding make them
suitable for phase selective gelation of aromatic solvents
3
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Shinkai, Chem. Mater., 2000, 12 1523; (d) F. S.
,
,
,
from
a water-solvent mixture and which remains
Schoonbeek, J. H. van Esch, B. Wagewijs, D. B. A. Rep,
M. P. de Haas, T. M. Klapwijk, R. M. Kellog and B. L.
unaffected in presence of common salts usually present in
water from different sources. The phase selective gelation
ability of 9a was successfully explored towards the removal
of toxic fat-soluble rhodamine dye from waste-water and
selectively gelatinizing petrol and crude oil from oil/water
mixture at room temperature. Thus, these smart systems
possess the potential to be used effectively in water
purification and oil-spill remediation.
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Acknowledgements
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Technology (DST), New Delhi for providing research grant
(SB/FT/CS-033/2012) for the work. We also thank BITS
Pilani for providing additional research grant for the work.
We would also like to thank Jitendra Kumar and Rajnish
Singh, for providing POM images and Fluorescence Optical
Microscopic images respectively. We would also like to
thank Dr. Subhashish Gangopadhyay and Sumita choudhary
for the XRD analysis. The authors would also like to thank
Materials Research Centre (MRC), MNIT, Jaipur for TEM
analysis. The author DA thank DST, New Delhi, India for
providing JRF fellowship. The author, NG thanks IASST,
Guwahati for the characterization facility and CSIR, New
Delhi for the fellowship.
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