Novel sugar-based azo dyes as multistimuli responsive supramolecular gelators and chemosensors

Article abstract of DOI:10.1016/j.dyepig.2018.06.042

Some novel sugar-based and multistimuli responsive supramolecular gelators, containing azobenzene moieties were synthesized and characterized through their microscopic, rheological, and spectroscopic measurements. The sol-gel behavior of these low molecul

Full text of DOI:10.1016/j.dyepig.2018.06.042

Accepted Manuscript
Novel sugar-based azo dyes as multistimuli responsive supramolecular gelators and
chemosensors
Zohreh Khayat, Hassan Zali-Boeini
PII:
S0143-7208(18)30794-0
10.1016/j.dyepig.2018.06.042
DYPI 6849
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 April 2018
Revised Date: 3 June 2018
Accepted Date: 24 June 2018
Please cite this article as: Khayat Z, Zali-Boeini H, Novel sugar-based azo dyes as multistimuli
responsive supramolecular gelators and chemosensors, Dyes and Pigments (2018), doi: 10.1016/
j.dyepig.2018.06.042.
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ACCEPTED MANUSCRIPT
Novel Sugar-based Azo dyes as Multistimuli Responsive Supramolecular Gelators and
Chemosensors
Zohreh Khayat, Hassan Zali-Boeini*
Novel sugar-based supergelators comprising azo dyes were synthesized and their thermo-, pH-, and photo-responsive
behavior were investigated. Gelator 2b was successfully utilized as chemosensor for selective detection of Cu2+ ion via
supramolecular sol-gel conversion.

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Novel Sugar-based Azo Dyes as Multistimuli Responsive Supramolecular
Gelators and Chemosensors
Zohreh Khayat,a Hassan Zali-Boeini*a
Department of chemistry, University of Isfahan, 81746-73441, Isfahan, Iran
Corresponding author e-mail address: h.zali@chem.ui.ac.ir
Key words: Azo dye; Supramolecular; Gelator; Multistimuli responsive; Chemosensor
Abstract: Some novel sugar-based and multistimuli responsive supramolecular gelators,
containing azobenzene moieties were synthesized and characterized through their microscopic,
rheological, and spectroscopic measurements. The sol-gel behavior of these low molecular
weight gelators in response to temperature, UV-Vis light irradiation, pH changes, and in the
presence of various metal cations was investigated. Compounds 2b and 2d were found to be as
supergelators (gelator concentration≤ 1%) in a mixture of DMF/H2O (1:1) and exhibited very
good response toward temperature, light, pH, and metal ions. Moreover, compound 2b showed
selective gel-sol response toward Cu2+ among a wide variety of two and three-valent metal ions.
1. Introduction
Supramolecular gels involve low molecular weight gelators (LMWGs) which self-assemble via
non-covalent interactions including hydrogen bonding, π-π stacking, ion-dipole, van der Waals
forces, solvophobic and electrostatic interactions to form a three dimensional micro and/or nano-
scale network structure which entraps the solvent and prevent its fluidity [1].
A fascinating category of LMWGs, are multi-stimuli responsive supramolecular gelators
(MRSGs). The assembling processes of these smart materials are affected by various external
stimuli such as heat, light, ultrasound, redox, and pH changes [2]. In recent years, MSRGs have
attracted substantial attention, because of their potential applications as sensors [3], drug delivery
systems [4], optoelectronic devices [5], phase selective gels [6], shape memory [7] and display
devices [8], molecular motors [9], and other applications [10].
Introduction of suitable responsive units into gelator molecules, leads to development of novel
stimuli-responsive gels. As examples, the introduction of azobenzene [11] or stilbene [12]

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moieties leads to photo-stimuli molecular gelators, capable of forming photoresponsive
supramolecular gels. There are some reports on the azobenzene-derived organogelators
containing cholesterol [13] or aryl ether [14] segments. In order to achieve a right balance
between hydrophilicity and hydrophobicity in a molecular gelator and to produce hydrogelators,
hydrophilic blocks like peptide [15], saccharide [16], or PEG parts [11b] have been introduced
into the structure of gelators.
In this paper, we report novel sugar based, light, pH, and metal ion responsive organogelators
(aqueous/organogelators), incorporating aryl azo moieties as photoresponsive and hydroxyl and
azo substituted aromatic groups as pH and metal ion responsive binding sites. The study
encompasses thermal, spectroscopic, microscopic, and rheometeric characterization of the
resultant gels. The effect of pH changes, UV/VIS light exposure, and addition of various metal
ions has also been investigated.
2. Experimental section
2.1 Materials and Instruments
All chemicals were purchased from commercial suppliers and used without further purification.
The solvents were dried and purified using standard methods, prior to use. All yields refer to
isolated products. Thin layer chromatography (TLC) was applied to monitor progress of the
reactions over Merck 0.2 mm silica gel 60 F254 analytical aluminum plates. 1H NMR (400 MHz)
and 13C NMR (100 MHz) spectra were recorded on a Bruker Avance 400 MHz NMR
spectrometer, referenced to residual solvent protons and signals are reported in ppm (δ). Infrared
spectra were obtained using a Jasco FTIR-6300 spectrometer as KBr pellets. Rheological studies
were done by an Anton Paar, MCR 301 reometer. FE-SEM analysis was performed using a
Tescan Mira3 scanning electron microscope.
2.2 General Procedure for the Synthesis of Diazenyl benzaldehydes 1a-d
A) Preparation of diazonium salt: 4-Aminobenzaldehyde (0.61 g, 5.0 mmol) was added to a
mixture of H2O (10 mL), acetone (5 mL) and HCl (32%, 2 mL) in a small beaker placed in an
ice-salt bath and stirred for 5 min. Then, the temperature was kept below 0°C (-5 to 0°C) and a
solution of NaNO2 (0.50 g, 7.25 mmol) in water (6.5 mL) was added dropwise. After completion

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of the reaction and KI-starch test (To avoid an excess of nitrous acid), the diazonium salt
solution was stirred at same temperature for further 30 min.
B) Coupling reaction with hydroxyaromatics: In a 50 mL round bottomed flask containing a
solution of NaOH (0.60 g, 15 mmol) in water (6.5 mL), the corresponding hydroxyaromatic
(6.25 mmol) was dissolved. The flask was placed in an ice bath to maintain the temperature
below 5°C, and the cold diazonium solution was added dropwise with vigorous stirring. After
completion of the addition, the reaction mixture was stirred for 60 min in an ice bath and a
further 30 min at room temperature. Then, the pH was adjusted to four by addition of a solution
of HCl (10%). The mixture was stirred 5 min more and the precipitate was filtered and washed
with water until the filtrate became neutral. In the case of 1c, the dried crude product was
triturated in a mixture of n-hexane: ethyl acetate (1:1, 15 mL) at 50°C for 15 min to remove the
minor impurities and filtered to obtain the desired dyes as reddish orange powders.
2.3 Spectroscopic data for the compounds 1a-d
(E)-4-((4-hydroxyphenyl)diazenyl)benzaldehyde (1a). Orange solid (95%); 1H NMR (400 MHz,
DMSO- d6) δ 10.09 (s, 1H), 8.09 (d, J= 8.4 Hz, 2H), 7.97 (d, J=8.4 Hz, 2H), 7.87 (d, J=8.4 Hz,
13
2H), 6.97 (d, J=8.4 Hz, 2H); C NMR (100 MHz, DMSO- d6) δ 192.6, 162.4, 155.4, 145.2,
136.7, 130.7, 125.5, 122.6, 116.2
1
(E)-4-((2-hydroxynaphthalen-1-yl)diazenyl)benzaldehyde (1b). Bright red solid (86%); H
NMR (400 MHz, DMSO- d6) δ 16.23 (s, 1H), 10.00 (s, 1H), 8.46 (d, J=8.0 Hz, 1H), 7.98 (d,
J=8.6 Hz, 2H), 7.76 (d, J=8.6 Hz, 2H), 7.71 (d, J=9.6 Hz, 1H), 7.56 (m, 2H), 7.44 (dt, J= 7.5 Hz,
13
1.2 Hz, 1H), 6.74 (d, J=9.6 Hz, 1H) C NMR (100 MHz, DMSO- d6) δ 190.9, 179.5, 147.7,
143.0, 133.5, 133.3, 131.6, 129.6, 129.1, 128.8, 128.5, 127.2, 126.4, 122.4, 117.1.
1
(E)-4-((4-hydroxynaphthalen-1-yl)diazenyl)benzaldehyde (1c). Redish brown solid (65%); H
NMR (400 MHz, DMSO- d6) δ 9.91 (s, 1H), 8.63 (d, J=8.0 Hz, 1H), 8.28 (d, J=9.6 Hz, 1H), 8.10
(d, J=7.6 Hz, 1H), 7.92 (d, J=8.4 Hz, 2H), 7.79 (d, J=8.4 Hz, 2H), 7.66 (t, J=7.2 Hz, 1H), 7.47 (t,
13
J=7.2 Hz, 1H), 6.63 (d, J=9.6 Hz, 1H); C NMR (100 MHz, DMSO- d6) δ 191.3, 178.6, 153.7,
135.3, 134.1, 131.7,131.2,130.3, 128.6, 125.9, 124.7, 123.5, 122.5, 121.2, 117.6.
1
(E)-4-((2,7-dihydroxynaphthalen-1-yl)diazenyl)benzaldehyde (1d). Bright red solid (92%); H
NMR (400 MHz, DMSO- d6) δ 9.93 (s, 1H), 7.95 (d, J=8.0 Hz, 2H), 7.84-7.75 (m, 3H), 7.73 (d,

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J=9.6 Hz, 1H), 7.48 (d, J=8.4 Hz, 1H), 6.86 (dd, J=8.4 Hz, 2.0 Hz, 1H), 6.40 (d, J=9.6 Hz, 1H);
13C NMR (100 MHz, DMSO- d6) δ 191.5, 178.1, 160.9, 148.2, 143.2, 135.1, 133.0, 131.3, 131.2,
130.9, 121.2, 120.5, 117.1, 116.4, 107.7.
2.4 General procedure for the synthesis of benzylidene glucopyranosides 2a-d
Under an argon atmosphere, methyl-α-D-glucopyranoside (1.13 g, 5.8 mmol) was added to a
solution of an azo dye 1 (5 mmol) in DMF (17-30 mL). After dissolution, p-TsOH (0.09 g, 0.5
mmol) was added and the resulting solution was stirred at room temperature for 15 min. Then,
trimethyl orthoformate (1.6 mL, 15 mmol) was added dropwise and the mixture was stirred at
room temperature for further 20 min, followed by 4 h stirring under reduced pressure. Progress
of the reaction was monitored by TLC. Afterwards, the reaction mixture was quenched by adding
saturated aqueous sodium bicarbonate solution. Then water (20 mL) and a few ice cubes were
added and the precipitate (in the case of 2b and 2d, gel-like precipitate) was suction filtered and
washed with water (10 mL). To remove the residual azo dye, the crude product was extracted
several times with a mixture of n-hexane: ethyl acetate (5:1) to achieve the pure compounds as
deep red to reddish orange solids. In the case of compound 2b, the pure product was obtained via
recrystallization from DMF followed by washing with ethyl acetate.
2.5 Spectroscopic data for the compounds 2a-2d
(2R,4aR,6S,7R,8R,8aS)-2-(4-((E)-(4-hydroxyphenyl)diazenyl)phenyl)-6-
methoxyhexahydropyrano[3,2-d][1,3]dioxine-7,8-diol (2a). Orange solid (35%); mp˃250°C; 1H
NMR (400 MHz, DMSO- d6) δ 7.92-7.84(m, 4H), 7.69 (d, J=8.6 Hz, 2H), 7.01 (d, J=8.6 Hz,
2H), 5.73 (s, 1H), 5.30 (d, J=3.4 Hz, 1H), 5.10 (d, J=5.6 Hz, 1H), 4.71 (d, J=3.2 Hz, 1H), 4.26
13
(dd, J=9.6 Hz, 4.4 Hz, 1H), 3.79 (t, J=10.0 Hz, 1H), 3.71-3.62 (m, 2H), 3.51.3.32 (m, 5H); C
NMR (100 MHz, DMSO- d6) δ 161.1, 152.2, 145.2, 139.6, 127.4, 124.9, 121.8, 115.9, 100.5,
100.3, 81.4, 72.4, 69.8, 68.2, 62.3, 54.7; Anal. Calcd. for C20H22N2O7: C, 59.70; H, 5.51; N, 6.96
Found: C, 59.85; H, 5.47; N, 6.88.
(2R,4aR,6S,7R,8R,8aS)-2-(4-((E)-(2-hydroxynaphthalen-1-yl)diazenyl)phenyl)-6-
methoxyhexahydropyrano[3,2-d][1,3]dioxine-7,8-diol (2b). Bright red solid (44%); mp˃250°C;
1H NMR (400 MHz, DMSO- d6) δ 15.84 (s, 1H), 8.53 (d, J=8.2 Hz, 1H), 7.96 (d, J=9.4 Hz, 1H),
7.86 (d, J=8.4 Hz, 2H), 7.78 (d, J=7.8 Hz, 1H), 7.67-7.56 (m, 3H), 7.47 (t, J=7.3 Hz, 1H), 6.90

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(d, J=9.4 Hz, 1H), 5.65 (s, 1H), 5.29 (d, J=3.5 Hz, 1H), 5.09 (d, J=5.3 Hz, 1H), 4.65 (d, J=3.5
Hz, 1H), 4.21 (dd, J=9.7 Hz, 4.6 Hz, 1H), 3.73 (t, J=10.2 Hz, 1H), 3.66-3.57 (m, 3H), 3.42 (t,
J=9.2 Hz, 1H), 3.33 (s,1H), 3.30-3.23 (m, 2H); 13C NMR (100 MHz, DMSO- d6) δ 174.3, 145.0,
140.4, 137.0, 132.7, 129.33, 129.2, 128.9, 127.9, 127.8, 126.0, 124.3, 121.4, 118.3, 100.5, 100.3,
81.4, 72.4, 69.8, 68.2, 62.3, 54.7; Anal. Calcd. for C24H24N2O7: C, 63.71; H, 5.35; N, 6.19
Found: C, 63.89; H, 5.31; N, 6.12.
(2R,4aR,6S,7R,8R,8aS)-2-(4-((E)-(4-hydroxynaphthalen-1-yl)diazenyl)phenyl)-6-
methoxyhexahydropyrano[3,2-d][1,3]dioxine-7,8-diol (2c). Red solid (30%); mp: 207°C
1
(decomposes); H NMR (400 MHz, DMSO- d6) δ 9.07-8.80 (b, 1H), 8.27 (s, 1H), 8.10-7.91 (b,
3H), 7.78 (t, J=7.2 Hz, 1H), 7.74-7.63 (m, 3H), 7.13-6.96 (b, 1H), 5.68 (s, 1H), 5.30 (d, J=4.8
Hz, 1H), 5.10 (d, J=6.4 Hz, 1H), 4.67 (d, J=3.7 Hz, 1H), 4.23 (dd, J=9.8 Hz, 4.7 Hz, 1H), 3.75 (t,
J=10.1 Hz, 1H), 3.70-3.59 (m, 3H), 3.55-3.25 (m, 4H); 13C NMR (100 MHz, DMSO- d6) δ
167.0, 131.6, 128.6, 127.5, 122.6, 111.5, 107.4, 100.5, 100.4, 81.4, 72.4, 69.9, 68.2, 62.4, 54.7;
Anal. Calcd. for C24H24N2O7: C, 63.71; H, 5.35; N, 6.19 Found: C, 63.85; H, 5.29; N, 6.14.
(2R,4aR,6S,7R,8R,8aS)-2-(4-((E)-(2,7-dihydroxynaphthalen-1-yl)diazenyl)phenyl)-6-
methoxyhexahydropyrano[3,2-d][1,3]dioxine-7,8-diol (2d). Red solid (41%); mp: 245°C
1
(decomposes); H NMR (400 MHz, DMSO- d6) δ 7.83(d, J=2.2 Hz, 1H), 7.82-7.75 (m, 3H),
7.62-7.55 (m, 3H), 6.91 (dd, J=8.4 Hz, 2.4 Hz, 1H), 6.60 (d, J=9.2 Hz, 1H), 5.63 (s, 1H), 5.45-
5.01 (b, 2H), 4.64 (d, J=3.6 Hz, 1H), 4.19 (dd, J=9.8 Hz, 4.6 Hz, 1H), 3.72 (t, J=10.2 Hz, 1H),
13
3.65-3.57 (m, 3H), 3.50-3.25 (m, 4H); C NMR (100 MHz, DMSO- d6) δ 171.1, 159.2, 145.1,
140.8, 136.7, 135.1, 130.9, 129.4, 127.9, 121,2, 120.0, 117.9, 116.0, 105.8, 100.5, 100.3, 81.4,
72.4, 69.8, 68.2, 62.3, 54.7; Anal. Calcd. for C24H24N2O8: C, 61.53; H, 5.16; N, 5.98 Found: C,
61.70; H, 5.11; N, 5.91.
2.6 Typical procedure for the gelation tests
In a small vial, 5-10 mg of the gelator was dissolved in 0.5 mL of the organic solvent upon
heating. After cooling down to 40°C, 0.5 mL water was added, slightly shaken, and set aside at
room temperature. After 30 min, the vial inversion test was performed.
2.7 Typical procedure for gel to sol transition temperature (Tgel) measurement

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Gel samples in critical gelation concentrations of each gelator in various solvents were prepared
and placed in a water bath at 25°C for 5 min. The vial was then taken out of the bath and vial
inversion test was carried out to observe weather the sample had kept its gel state or not. When
the sample maintained its gel state, the bath was adjusted to 30°C, the vial was placed in it for
another 5 min, and the same operation was done. The procedure was regularly repeated while the
temperature of the water bath was raised by 5°C, until the gel dropped upon vial inversion.
3. Results and discussion
3.1 Syntheses
A series of 4,6-benzylidene glucopyranosides containing phenyl or naphthylazo moieties linked
to methyl-α-D-glucopyranoside (2a-d), were synthesized through the following steps: the phenyl
or naphthylazo dyes (1a-d)
were synthesized through diazotization reaction of 4-
aminobenzaldehyde followed by azo coupling with the corresponding phenol or naphthol
derivatives. The as-synthesized azo dyes were used as the precursors for the synthesis of
compounds 2a-d via their acetalization reaction with methyl-α-D-glucopyranoside. The structure
of final products and the synthetic pathway has been outlined in scheme 1.

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Scheme 1. Molecular structure and synthetic pathway for preparation of compounds 2a-d; Reagents and conditions: (i) NaNO2,
HCl (aq), -5 to 0 ºC; (ii) ArH, NaOH (aq), 0 ºC; (iii) Me-α-D-glucopyranoside, trimethyl orthoformate, p-TsOH, DMF, r.t.
3.2 Investigation of gelation abilities
The gelation ability of the products 2a-d was investigated in various solvents. A weighed amount
of each sample was mixed with the corresponding solvent and heated to dissolve. After cooling
down to ambient temperature, equal volume of water was added, slightly shaken, and set aside.
In the case of compounds 2b and 2d, instant gelation occurred in DMF and DMSO. These
gelators made stable gels that did not flow down upon the vial inversion test even at low
concentrations. The critical gelation concentration (CGC) of the gelator 2b in 1:1 DMF/H2O was
1% (w/v, 17.8 mM). While, compound 2c formed partial gels in the aforementioned solvents and
conditions, 2a was not a gelator.
Table 1. Gelation test results for the compounds 2a-2da
Solvent
2a
2b
G
2c
2d
G
DMF:H2O (1:1)
S
PG

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DMSO:H2O(1:1)
MeOH:H2O(1:1)
S
S
G
PG
PG
G
PG
PG
[a] S, G and PG stand for solution, gel, and partial gel respectively.
The gelation test results are summarized in Tables 1 and 2. Due to minimum gel concentration
(gelator concentration≤ 1%), the compounds 2b and 2d can be referred to as supergelators.
Table 2. Critical gelation concentration and sol-gel transition
temperature for compounds 2b and 2d a
Compound
Solvent
CGCa (%)
Tgelb (°C)
DMF:H2O
DMSO:H2O
DMF:H2O
DMSO:H2O
0.5
1
70
65
60
55
2b
2d
0.8
1
[a] Critical Gelation Concentration; [b] Gel to Sol Transition Temperature.
The Fig. 1 shows FE-SEM images of xerogel 2b obtained from lyophilizing a gel comprising
1.5% compound 2b in DMF/H2O (1:1). As seen in the figure 1, a dense aggregation of uneven
thin sheets are responsible for the formation of gel state.
Figure 1. FE-SEM images of xerogel 2b obtained from a 1.5% compound 2b in DMF/H2O (1:1).
Scheme 2 graphically illustrates the probable self-assembly mechanism for the compound 2b. As
shown in the scheme 2, the main interactions that lead to aggregates are hydrogen bonding and
face-to-face π-π stacking of aromatic rings. UV light exposure can lead to trans to cis
isomerization, disturbing the hydrogen bonding and stacking of the rings, and to collapse the gel

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state. Moreover, addition of Cu2+ ions can prevent gel formation due to coordination of Cu2+ ions
with O and N atoms (from OH and azo moieties) and disrupting the inter- and intramolecular
hydrogen bondings. As displayed in scheme 3, the pH changes can also affect the self-assembly
process via deprotonation of OH groups in basic medium or protonation of nitrogen atoms of the
azo moieties under acidic conditions.
Scheme 2. Graphical illustration of supramolecular interactions responsible for the gelation of 2b
Scheme 3. Possible equilibrium of azo and hydrazone tautomers of 2b and its anion at different pH conditions
Representative photographs in Fig. 2a and 2b show thermoresponsiveness of the gelators 2b and
2d in a 1:1 mixtures of DMF/H2O. As it is clear from Fig. 2a, when a solution of 2b in a 1:1

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mixtures of DMF/H2O is prepared by heating to a suitable temperature (Table 2), after cooling to
room temperature the mixture is solidified to a red color firm gel (Fig. 2b).
3.3 Investigation of pH responsiveness
The pH responsiveness of the gelators 2b and 2d has been studied. As shown in Fig. 2c, when
2% solutions of gelator 2b in DMF was prepared and equal volumes of H2O, 0.05 M solutions of
NaOH, or HCl were added (total concentration of gelator was 1%), quite different results were
obtained. However, in the case of a 1:1 DMF/H2O solution of compound 2b a firm opaque gel
was produced. After addition of NaOH solution (0.05 M→0.025 M, pH= 12.4) an immediate
color change from red to dark-red (or dark-purple for high concentrations of OH¯ ) was occurred
and formation of gel was mostly inhibited. Surprisingly, once a solution of HCl (0.05 M→0.025,
pH= 1.6) was added to a solution of compound 2b in DMF, the gel formation process was not
prevented but a distinct color change from deep red to rather bright red was detected. It was
revealed that, higher concentrations of NaOH (>0.05 M) and/or HCl (up to 0.2 M, pH= 0.7)
completely inhibit the gel formation of compound 2b accompanied with a drastic color change to
a bright red for addition of HCl. At higher concentrations of HCl (>0.2 M), gelation did not
occur but, rather a precipitate was formed. As TLC test confirmed, it was simply because the
acetal moiety is decomposed to the original dye in strong acidic media. As shown in Fig. 2d,
gelator 2b can be successfully utilize as a pH responsive gel. The same behavior for compound
2d was observed, but it is explicitly unveiled that in this case even lower concentrations of OH¯
and/or H+ (less than 0.05%) can inhibit the formation of gel. In one hand, sensitivity of the
compound 2d towards pH changes was higher than compound 2b and on the other hand, gelator
2b made stronger gels in a mixture of 1:1 DMF/H2O (at 0.5 % concentration) compared with
gelator 2d (table 2). Therefore, both gelators 2b and 2d could be effectively used as a pH
responsive sol-gel colorimetric sensor at pH range of 0.7<pH<1.6 for acidic medium and
pH>12.4 for basic solutions. A reasonable conclusion supported by these observations is that, the
main driving force for aggregation of gelator molecules may be the hydrogen bonding interaction
of phenolic OH (although π-π stacking interactions are also notable) which is disrupted in basic
media.

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Figure 2. (a) Sol of 1% gelator 2b in 1:1 DMF/H2O; (b) gel of 1% gelator 2b in 1: 1 DMF/H2O; (c) from left to right: 2b in
DMF/H2O, DMF/NaOH 0.05 M and DMF/ HCl 0.05 M; (d) 2b in DMF/H2O, DMF/NaOH 0.1 M and DMF/ HCl 0.1 M.
It is more likely, protonation of one of azo nitrogens in acidic medium and afterwards disturbing
the hydrogen bonding interactions (due NH…OH repulsion), is responsible for inhibiting gel
formation, reducing the resonance efficiency, and therefore a clear color change to a vivid red.
3.4 Investigation of the effect of metal ions on gelation of compounds 2b and 2d
In order to investigate the response of the gelators toward metal ions, a 2% solution of gelator 2b
in DMF was prepared and to the vial, equal volumes of 0.1 M aqueous solutions of different M2+
and/or M3+ cations were added and shaken slightly (total concentration for 2b and metal ions
were 2.21×10-2 and 5×10-2 M respectively). The vials were set aside for an hour and after that,
the vial inversion test was done. As shown in figure 3a, among nine cations were examined,
aqueous solution of Cu2+ was the only metal ion that prevented gelation of the DMF solution of
2b alongside with a color change from red to reddish-brown. To confirm the inhibiting effect of
adding Cu2+ ions on the gelation of compound 2b, the FE-SEM image of gelator 2b in the
presence of Cu2+ ion was prepared (Scheme 3b). As it is unveiled in scheme 3b, all thin sheets
that were responsible for the gel formation are deformed and/or collapsed. The IR spectra of
gelator 2b before and after addition of Cu2+ ion showed distinct changes (Scheme 3c), implying
that, there is a strong interaction between gelator 2b and Cu2+ ions leading to total destruction of
gel state. Therefore, gelator 2b can be successfully use for selective detection of Cu2+ ion among
different two and three-valent metal ions in the solution. The test was also carried out for 2d in a
similar manner and almost the same results were obtained. The only difference in this case was

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that, the gel-sol conversion was happened faster than in the case 2b in the presence of Cu2+ ion
(5×10-2 M).
Figure 3. (a) Photograph of selective gel-sol response of a solution of Cu2+ ion among various metal ions (0.1 M) to gelator 2b
(4.42×10-2 M, 2%); The sequence of added metal ions from left to right is as follows: Pb2+, Cu2+, Ni2+, Al3+, Mn2+, Cd2+, Zn2+,
Fe3+, Hg2+, H2O; (b) FE-SEM image of gelator 2b in the presence of Cu2+ ion; (c) The IR spectra of gelator 2b before and after
addition of Cu2+ ion .
3.5 Investigation of photoresponse of compounds 2b and 2d

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Gel to sol phase transition could be triggered by UV light irradiation in a dye/gelator via trans-cis
photoisomerization of a double bond such as azo functional group under certain wavelengths.
Intrigued by the fact that, the synthesized gelators 2b and 2d have an azo functional group, we
decided to investigate their photoresponsiveness under different frequencies of UV light. To
study the effect of UV light, a sample gel containing 1% of gelator 2b in 1:1 DMF/H2O mixture
was prepared and exposed to UV light (254 or 360 nm, Fig. 4). Interestingly, after 30 min of
exposure, the gel collapsed. It was also found that, a sample gel comprising of compounds 2b or
2d could be transform to sol state after prolong (typically 2-4 days) expose to day light. This fact
demonstrate that, trans-cis photoisomerization is even occurred very slowly under daylight
conditions, while gel state could be persist for a long time (over 2 months) if stored in the dark.
Figure 4. Photo-response of gelator 2b when irradiated with UV light (254 nm, 400 W, 30 min).
3.6 Visco-elastic behavior of compound 2d
In order to investigate the visco-elastic behavior of the gels, storage and loss moduli (G' and G'')
measurements on a gel composed 1% compound 2d in DMF/H2O (1:1) were done in strain and
frequency sweep modes. As shown in Fig. 5a, G' and G'' remained approximately constant below
a strain value of 1%. Above this critical strain value, a gradual decline of G' and G'' values was
observed, demonstrating a partial disruption of the gel.
Figure 5b shows the independence of G' and G'' from frequency under 0.5% constant strain. In
addition, higher G' values compared to G'', imply the dominance of elastic character of the
sample.

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Figure 5. Rheological Data of compound 2d 1% gel in 1:1 DMF/H2O (a) Strain sweep under constant frequency of 1 rad/s; (b)
Frequency sweep under constant 0.5% strain.
4. Conclusions
In conclusion, we have synthesized four sugar-based azo dye as multistimuli responsive
supramolecular gelators. Compounds 2b and 2d were supergelators (gelator concentration≤ 1%)
in a mixture of DMF/H2O while compounds 2a and 2c were unsuccessful to form a firm and
uniform gel. Gelators 2b and 2d showed good thermoresponsive and photoresponsive behavior.
Photoresponse of these gelators were rather fast (30-60 min) at UV (254 and 360 nm) and very
slow in daylight (2-5 days). The investigation of pH responsiveness of Gelators 2b and 2d
revealed that, they made a gel at pH range of 1.6<pH<12.4 and at pH higher than 12.4 and/or
lower than 1.6 they transform to sol state. In addition, gelator 2b was effectively applied for
selective detection of Cu2+ ion among a wide variety of two and three-valent metal ions.
Acknowledgements
We are grateful to the University of Isfahan research council for financial support of this work.
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Novel sugar based, light, pH, and metal ion responsive organogelators, incorporating aryl azo
moiety as photoresponsive and hydroxyaromatic groups as pH responsive and metal ion binding
sites have been introduced. It was found that compounds 2b and 2d were supergelators. These
compounds showed good thermoresponsive and photoresponsive behavior. Gelator 2b formed a
gel at pH range of 1.6<pH<12.4 and at pH higher than 12.4 and/or lower than 1.6 it transform to
sol state. In addition, gelator 2b was effectively utilized for selective detection of Cu2+ ion
among diverse two and three-valent metal ions via gel to sol conversion.