Synthesis and properties of amphiphilic photoresponsive gelators for aromatic solvents

Article abstract of DOI:10.1021/ol203294v

A sugar-based photoresponsive supergelator, N-glycosylazobenzene that shows selective gelation of aromatic solvents is described. The partial trans - cis isomerization of the azobenzene moiety allows photoinduced chopping of the entangled gel fibers to short fibers, resulting in controlled fiber length and gel - sol transition. The gelator is useful for the selective removal of toxic aromatic solvents from water.

Full text of DOI:10.1021/ol203294v

ORGANIC
LETTERS
2012
Vol. 14, No. 3
748–751
Synthesis and Properties of Amphiphilic
Photoresponsive Gelators for Aromatic
Solvents
Ramanathan Rajaganesh,† Anesh Gopal,‡ Thangamuthu Mohan Das,*,† and
Ayyappanpillai Ajayaghosh*,‡
Department of Organic Chemistry, University of Madras, Guindy Campus,
Chennai-600025, India, and Photosciences and Photonics Group, Chemical Science and
Technology Division, National Institute for Interdisciplinary Science and Technology
(NIIST), CSIR, Trivandrum-695019, India
tmdas_72@yahoo.com; ajayaghosh62@gmail.com
Received December 9, 2011
ABSTRACT
A sugar-based photoresponsive supergelator, N-glycosylazobenzene that shows selective gelation of aromatic solvents is described. The partial
transꢀcis isomerization of the azobenzene moiety allows photoinduced chopping of the entangled gel fibers to short fibers, resulting in controlled
fiber length and gelꢀsol transition. The gelator is useful for the selective removal of toxic aromatic solvents from water.
Organogelators have wide-ranging applications as scaf-
folds for energy transfer, as drug delivery agents, cosmetic
additives, and enzyme-immobilization matrices, and for oil
recovery and tissue engineering.1ꢀ4 Sugar-based gelators
have a significant role in many of the above applications.5
For applications such as drug delivery or preparation of
smart materials, itisnecessary tohave reversiblecontrolon
the gelation and the morphological properties.6 Reversible
light-induced control of molecular aggregation and gela-
tion can be achieved by incorporating photochromic
† University of Madras.
‡ CSIR.
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39, 4286–4302. (f) Babu, S. S.; Kartha, K. K.; Ajayaghosh, A. J. Phys.
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r
10.1021/ol203294v
Published on Web 01/17/2012
2012 American Chemical Society

azobenzene moieties intogel-forming molecules.7,8 Herein,
we report the design, synthesis, and self-assembly of a
signals at around 5.40 (J = 4.5 Hz) and 7.00 (J = 8.1 Hz)
ppm corresponding to Ano-H and Gly-NH, respectively,
which supports the product formation.
Among compounds 3aꢀf, 4aꢀf, with the exception of 3b
and 4b all other molecules are able to form gels from aro-
matic solvents. Gelation ability of a representative com-
pound, 3a(GAZ), ina number of different organic solvents
has been carried out by using the “test tube inversion
method”.12 Stable transparent gels were obtained from
different organic aromatic solvents at low critical gelation
concentration (see Supporting Information for more details).
The lowest concentration was observed to be 0.05% (w/v)
in toluene. Since gelation occurs with less than 0.1% (w/v),
this molecule falls under the category of a photoresponsive
super gelators.13 Thermodynamic parameters for GAZ in
1,2-dichlorobenzene was evaluated using DSC. The higher
enthalpy [ΔH = 287.6 J/g] of the gel reflects the greater
stability of the self-assembly (see Supporting Information
for more details).
Scheme 1. Synthesis of N-Glycosylamines (3aꢀf, 4aꢀf)
photoswitchable sugar hybrid that undergoes phase-
selective removal of aromatic solvent from water.
Synthesis of p-aminoarylazobenzenes (1aꢀf)9 was ac-
complished in two steps: synthesis of diazoaminobenzenes
followed by conversion of these derivatives to p-aminoar-
ylazobenzenes from the corresponding aniline derivatives
(see Supporting Information for more details). N-Glyco-
sylamine based azobenzene derivatives 3aꢀf and 4aꢀf
were synthesized from p-aminoazobenzene derivatives
(1aꢀf) and 4,6-O-butylidene-D-glucopyranose10 (2a) and
4,6-O-ethylidene-D-glucopyranose11 (2b) (Scheme 1). The
reactants were chosen to have a desired functional group,
such as the primary amine in the aromatic moiety and
active hydroxyl group in the 4,6-O-protected-D-glucose
moieties. All of the new N-glycosylamine compounds
(3aꢀf, 4aꢀf) were characterized through spectral tech-
1
niques. H NMR spectra of N-glycosylamines showed
Figure 2. Absorption spectral change of GAZ in toluene (2.5 ꢁ
10ꢀ4 M) with respect to UV irradiation. Inset shows the
reversibility in the photoisomerization by alternate irradiation
of UV and visible light.
It is well-known that the azobenzene chromophore
undergoes trans to cis isomerization by irradiation (Figure 1)
with UV light and the reversal by visible light irradiation.14
The trans form of themoleculewhich favors the long-range
self-assembly leads to an entangled network structure that
can form a gel in aromatic solvents. The selectivity of
(9) Sharma, P.; Kumar, A.; Upadhyay, S.; Sahu, V.; Singh, J. Eur. J.
Med. Chem. 2009, 44, 251.
(10) Mellies, P. L.; Mehltretter, C. L.; Rist, E. C. J. Am. Chem. Soc.
1951, 73, 294.
Figure 1. (a) Photoisomerization of trans-GAZ to cis-GAZ. (b)
Gelꢀsol transition photograph of GAZ in toluene (1 ꢁ 10ꢀ3 M).
(11) Baker, R.; Mac Donald, D. L. J. Am. Chem. Soc. 1960, 82, 294.
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Huang, C. J. Phys. Org. Chem. 2008, 21, 338. (c) Somobrata, A.;
Sudarshan, K.; Hil, J. P.; Gary, R.; Ariga, K. Adv. Mater. 2009, 21,
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Commun. 2011, 7341.
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9897. (b) Uchida, K.; Yamaguchi, S.; Yamada, H.; Akazawa, M.;
Katayama, T.; Ishibashi, Y.; Miyasaka, H. Chem. Commun. 2009,
4420. (c) Zhao, Y. L.; Stoddart, J. F. Langmuir 2009, 25, 8442. (d) Stoll,
R. S.; Hecht, S. Org. Lett. 2009, 11, 4790. (e) Wang, C.; Chen, Q.;
Sun, F.; Zhang, D.; Zhang, G.; Huang, Y.; Zhao, R.; Zhu, D. J. Am.
Chem. Soc. 2010, 132, 3092.
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J. Phys. Chem. A 2004, 108, 950.
Org. Lett., Vol. 14, No. 3, 2012
749

aromatic solvents may be due to the induced πꢀπ stacking
by the aromatic solvent molecules. The self-assembly will
be disturbed by trans to cis photoisomerization due to the
geometrical change that occurs in the structure of azo-
benzene chromophore.15 In order to understand the self-
assembly and photoresponsive behavior, we chose toluene
as the solvent because of its strong gelating ability among
the tested aromatic solvents.
AFM analysis of drop casted GAZ (1 ꢁ 10ꢀ4 M) in
toluene on a freshly cleaved mica surface shows a cross-
linked fibrillar network-like morphology (Figure 4). How-
ever, after UV irradiation (370 ( 20 nm) the self-assembled
fibrillar network disintegrates to form short fibers.
The temperature-dependent absorption spectra of GAZ
in toluene shows (see Supporting Information for more
details) a strong aggregation tendency for the molecules.
The molecule shows a strong πꢀπ* band at 373 nm and a
small nꢀπ* band at 495 nm. Upon UV irradiation (370 (
20 nm) the πꢀπ* band decreases and the nꢀπ* band
increases, which is the characteristic absorption spectral
change for trans to cis isomerization of azobenzene chro-
mophore (Figure 2). The reverse isomerization was ob-
served by visible light (450ꢀ600 nm bandpass) irradiation.
On the basis of the absorption spectral change, the percen-
tage of cis isomer formed at photostationary state is found
to be 31%. Even though the percentage conversion was
less, this is enough to make a significant disturbance to the
self-assembly of gelator molecules.
The rheological properties ofthe gelator before and after
irradiation are shown in Figure 3. The storage modulus
(G0) and loss modulus (G00) of GAZ in toluene gel before
and after irradiation are shown as a function of angular
frequency (ω) at 0.01% strain (γ) amplitude. Before and
after irradiation the gel showed a plateau region when the
angular frequency was varied from 100 to 1 rad sꢀ1
.
Figure 4. AFM images of the GAZ molecules in toluene (1 ꢁ
10ꢀ4 M): (a) before irradiation; (bꢀd) after 1, 5, and 10 min of
UV irradiation, respectivly.
The change in morphology was also investigated by
using TEM analysis (see Supporting Information for more
details). TEM images of GAZ (1 ꢁ 10ꢀ4 M) in toluene on a
carbon coated TEM grid before irradiation showed nano-
fiber networks of ca. 10ꢀ20 nm. After irradiation the
population of the fibers has significantly reduced, indicat-
ing partial dissolution.
The SEM images obtained from toluene gels of mole-
cules before and after UV irradiation are shown in Figure 5.
The SEM analysis clearly reveals that the trans form
of the gelator facilitates the formation of super bundles
of fibers having diameters in the micrometer range. The
relatively larger size of the fibers seen in the SEM images
indicates that the elementary nanofibers obtained at low
concentration as observed in the TEM images have fur-
ther bundled to form large fibers at higher concentrations
required for the gel formation. After UV irradiation the
bundles of fibers disintegrates to give short globular ag-
gregates due to the trans-cis isomerization of the gelator
molecules.
Figure 3. Angular frequency sweep (ω) dependencies of dynamic
storage (G0) and loss modulus (G00) of 0.05% w/v GAZ gel in
toluene.
The G0 value showed a substantial elastic response to the
gels, which are larger than G00 over the entire frequency
range. Rheological parameters of the gel before irradiation
indicate enhanced solid-like character when compared
with the gel after irradiation.
The powder XRD of the toluene xerogel exhibited dif-
fraction peaks corresponding to the d spacing of 3.11, 3.23,
˚
9.30, and 26.95 A. The d spacing ratio of 1.0 : 2.99 and 8.65
(15) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.;
Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994,
116, 6664. (b) Yagai, S.; Karatsu, T.; Kitamura, A. Langmuir 2005, 21,
11048.
was consistence with a lamellar structure. After irradiation
the xerogel, the XRD pattern showed different diffraction
˚
peaks corresponding to 2.97, 4.68, and 27.06 A. The XRD
750
Org. Lett., Vol. 14, No. 3, 2012

data of the toluene xerogels before and after photoirradia-
tion indicate no considerable change in the molecular
arrangement (see Supporting Information for more details).
From this observation we infer that the partial trans-cis
isomerization which occurs randomly at different parts
of the fibers allows chopping into small size. The overall
molecular arrangements in the chopped fibers remain
more or less the same. Thus, we were able to achieve a
vacuum distillation. The recovered gelator has been used
further for phase-selective gelation. The recovery of the
aromatic solvents can also be achieved by UV light irra-
diation. The gel was first separated by simple filtration.
Further gelꢀsol transition could be achieved by photo-
irradiation, and the aromatic solvents could be separated
by filter column using Celite as an adsorbent Figure 6. We
also could remove small amounts of aromatic solvents
present in water using the gelator. The homogenous phase
obtained from use of a small amount of organic solvents
resulted in gelation of the organic phase, which ensures the
removal of even a small amount of aromatic solvents from
the mixture (see Supporting Information for more details).
Figure 6. Phase-selective gelation of aromatic solvents from
two-phase systems: (a) phase-separated mixtures of organic
solvents in the presence of water before the addition of the
gelator and (b) selective gelation of the organic solvents after the
addition of the gelator.
In conclusion, a phase-selective low-molecular weight
photoswitchable sugar hybrid has been synthesized, and
the selective gelation of aromatic solvents in a mixture has
been observed even at micromolar concentrations. The
morphological investigation through AFM, TEM, SEM,
and XRD analyses implies the formation of self-assembled
fibrillar networks that upon irradiation get chopped into
short fibers, effecting the gelꢀsol transition. The gelator is
selective for aromatic solvents that allow the removal of
such solvents from an aqueous layer. Such phase-selective
photoresponsive gelators may be useful for the removal of
small amounts of toxic aromatic solvents from contami-
nated water.
Figure 5. SEM images of the self-assembled fibers formed by
GAZ (1.3 ꢁ 10ꢀ2 M) in toluene: (a) before UV irradiation, which
shows entangled fibers and (b) after UV irradiation, which
shows significant morphology change.
morphological state of nonentangled short fibers that is in
between large bundles of fibers (gel) and isotropic solution.
Such a state is useful for the controlled relase of encapsu-
lated molecules. The partial conversion of cis-trans isomer
(36% at PSS) is responsible for the change of bundled long
fibers to short fibers.
Interestingly, irrespective of the amphiphilic nature, the
GAZ molecule selectively gelates aromatic solvents but does
not gelate water and hence is suitable for phase-selective
removal of aromatic solvents from water. In a typical
procedure, 0.5 mL of a required aromatic solvents and
0.5 mL of water were mixed in a sample tube, to which
0.05 g of the gelator was added and stirred vigorously.
The gelation of the aromatic layer was observed, and the
water layer remained intact in solution state. The organo-
gel formed can be separated by filtration, and the aro-
matic solvent trapped by the gelator was recovered by
Acknowledgment. The authors acknowledge DST and
CSIR, New Delhi for financial support. DST is also
acknowledged for an NMR facility under FIST scheme
to the Department of Organic Chemistry, University of
Madras, Chennai, India. We thank Dr. J. D. Sudha,
NIIST, Trivandarum for rheological studies.
Supporting Information Available. Experimental pro-
cedures and spectral, morphological data for all com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Org. Lett., Vol. 14, No. 3, 2012
751