Cholesterol appended bis-1,2,3-triazoles as simple supramolecular gelators for the naked eye detection of Ag+, Cu2+ and Hg2+ ions

Article abstract of DOI:10.1039/c5nj02771c

Cholesterol coupled bis-1,2,3-triazoles 1-3 have been synthesized and their gelation abilities are examined. Bis triazoles 1-3 form a gel from CHCl₃-CH₃OH (2: 1, v/v) mixture solvents. In contrast, compounds 4 and 5, which are devoid of triazoles, are unable to form a gel under similar conditions and thus validate the essential role of triazoles in gelation. While the gel phase of 1 detects Cu²⁺ ions by showing phase transformation from gel to sol, gels derived from 2 and 3 undergo disintegration in the presence of Ag⁺, Cu²⁺ and Hg²⁺ ions. Such phase changes corroborate the visual sensing of metal ions. Moreover, the fluorescence titration of 3 discriminates Cu²⁺, Hg²⁺ and Ag⁺ ions by exhibiting differential quenching.

Full text of DOI:10.1039/c5nj02771c

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ARTICLE TYPE
Cholesterol appended bisꢀ1,2,3ꢀtriazoles as simple supramolecular gelators for
naked eye detection of Ag+, Cu2+ and Hg2+ ions
Kumaresh Ghosh*, Atanu Panja and Santanu Panja
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Cholesterol coupled bis 1,2,3ꢀtriazoles 1ꢀ3 have been synthesized and their gelation abilities are examined. Bis triazoles 1ꢀ3 form gel
from CHCl3ꢀ CH3OH (2:1, v/v) mixture solvents. In contrast, compounds 4 and 5 which are devoid of triazoles, are unable to form gel
under similar conditions and thus validate the essential role of triazoles in gelation. While the gel phase of 1 detects Cu2+ ions by
showing phase transformation from gel to sol, gels derived from 2 and 3 undergo disintegration in presence of Ag+, Cu2+ and Hg2+ ions.
Such phase changes corroborate the visual sensing of the metal ions. Moreover, the fluorescence titration of 3 discriminates Cu2+, Hg2+
and Ag+ ions by exhibiting differential quenching.
Introduction
interactions such as the dipoleꢀdipole interaction, van der Waals
forces and hydrogen bonding.11 Perturbation of these weak forces
by external stimuli disrupts the gels to the sols and offers
promising opportunities for designing and constructing new
functional materials.
In continuation of our work on sensing and detection of ionic
analytes by different supramolecular architectures either in
solution12 or in gel phase,13 we report here cholesterol appended
1,2,3ꢀ triazoleꢀbased compounds 1ꢀ3 (Fig. 1) that exhibit
excellent gelation properties in organic solvent. The gel phases of
the gelators act as potential media for detection of metal ions
such as Cu2+, Hg2+ and Ag+ ions. Compounds 4 and 5 (Fig. 1)
without triazole motif was undertaken as reference that did not
show gelation property under identical conditions and thus
emphasized the key role of the 1,2,3ꢀtriazole unit in the designs.
Since the discovery, the copperꢀcatalyzed azideꢀalkyne
cycloaddition (CuAAC) reaction has been noted to be useful for
different purposes, varying from traditional organic synthesis to
advanced materials.1,2 Different advantageous properties such as
high chemical stability, strong dipole moment, heteroaromatic
character and hydrogen bond donating and accepting abilities
have enable the 1,2,3ꢀtriazole motif to interact with molecules
and ions in productive manner.3 The 1,2,3ꢀtriazole motif,
surrogate of amide group, with greater dipole moment (5 D)
provides the polarized CꢀH bond for interaction with anions and
thus its use in devising anionꢀbinding host is noteworthy.4 Due to
the presence of nitrogen atoms in the backbone, the triazole motif
is also excellent in cation binding.5 Thus integration of this
functionality into several structural frameworks has been a topic
of interest in sensing or detection of ionic analytes in
supramolecular chemistry.6
In relation to this aspect, the synthesis of organogelators using
1,2,3ꢀtriazole functionality draws much attention in last few
decades in gel chemistry.7 Proper use of the 1,2,3ꢀtriazole
scaffold in the design has led many supramolecular gelators that
find use in detection of analytes through a change in state of the
gels.8 Sometimes, the use of amide in place of the triazole motif
has been established ineffective in gelation.9 Gels are viscoelastic
material and have been known potential for application in
materials, drug delivery and sensors etc.10 The driving forces
responsible for gel formation are specific or noncovalent
O
N
O
O
N
OR
N
N
O
O
N
N
O
O
N
OR
N
O
O
N
OR
N
N
N
O
O
N
N
N
N
OR
OR
OR
N
N
2
3
1
O
O
O
O
O
O
H
O
O
O
OR
OR
H
OR
OR
H
R =
O
O
O
O
4
O
5
Figure 1. Structure of the compounds 1ꢀ5.
The sensing and detection of Cu2+, Hg2+ and Ag+ ions draw
attention because of their biological relevance. Copper is linked
with
organisms. This metal ion causes environmental pollution and
also serves as catalytic cofactor for variety of
a variety of fundamental physiological processes in
Department of Chemistry, University of Kalyani, Kalyani-741235, India.
Email: ghosh_k2003@yahoo.co.in, Fax: +913325828282; Tel:
+913325828750
a
a
metalloenzymes.14 Exposure to high levels of copper can cause
gastrointestinal disturbance and liver or kidney damage.15
Similarly, mercury and silver ions are also considered to be the
toxic elements in the environment.16 The exposure of mercury
even at low concentration leads to various health problems,
†Electronic Supplementary Information (ESI) available Figures showing
the figures of FTIR, fluorescence and UVꢀvis titration curves, phase
1
changes of gels, Job plots, binding constant curves, H NMR changes
and spectral data. See http://dx.doi.org/10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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DOI: 10.1039/C5NJ02771C
especially neurological disorders. Owing to the broad
employment in industries, such as electronics, photography,
mirrors and pharmacy, a large amount of silver is released to the
environment from industrial wastes and also to surface waters
that causes high toxicity to aquatic organism.17
aggregation in mixture solvent of CHCl3ꢀCH3OH (2:1, v/v). In
contrast, compound 4, devoid of triazole units, exhibited partial
gelation from CHCl3: CH3OH (1:1, v/v) at a concentration that is
three times higher than the mgc (minimum gelation
concentration) of compound 3. Compound 5 remained soluble
under similar conditions. This indicated the key role of triazole
motifs in 3 and 1 with respect to 4 and 5, respectively during
gelation.
Results and discussion
Synthesis
Gels were prepared by dissolving required amount of gelators in
CHCl3:CH3OH (2:1, v/v). Slight warming of the solutions of
compounds 1, 2 and 3 in CHCl3:CH3OH (2:1, v/v) followed by
keeping at room temperature for 5 min led to the formation of
gels.
The syntheses of the compounds 1, 2, 3, 4 and 5 were achieved
according to the Scheme 1. Initially cholesterol was converted to
the chloro ester 6 which on reaction with NaN3 gave the azide
compound 7.13d On the other hand, the different phenolic
substrates such as resorcinol, catechol and 6,7ꢀdihydroxy
coumarin were reacted with the propergyl bromide in presence of
either Cs2CO3 or K2CO3 to generate alkyne functional group
containing compounds 8, 9 and 10, respectively. Then copper
(I)ꢀcatalysed click reaction was pursued on these alkynes using
the azide compound 7. In order to have the compound 4, 6,7ꢀ
dihydroxy coumarin was refluxed with chloro ester 6 in the
presence of Cs2CO3 in dry CH3CN. In the similar way, compound
5 was obtained from the reaction of resorcinol with the chloro
ester 6 in the presence of K2CO3 in dry CH3CN. All the
compounds were fully characterized by usual spectroscopic
methods.
Morphological study and thermal stability of gels
To gain visual insight into the aggregation mode and microscopic
morphology of the gelators, scanning electron microscopy (SEM)
was used. SEM images reveal the aggregated three dimensional
networks with uneven surface (Fig. 2). While the gel state of 1
gives densely packed flakes with very small pores, the xerogel of
2 shows sponge like architecture with more porous surface.
Gelator 3 having larger πꢀsurface than 1 and 2 exhibits rock like
morphology with large number of voids for solvent trapping in its
gel state.
We believe that molecules 1ꢀ3 possibly follow a general packing
shown in Fig. 3 for which a three dimensional network is set up
in solution and solvents are trapped inside. To our opinion, the
triazole which behaves like amide with greater dipole moment
may be involved in intermolecular hydrogen bonding via trapped
methanol (Fig. 3a) or directly without assistance of methanol
(Fig. 3b) to form the self assembly in solution. The possibility of
intermolecular hydrogen bonding of the triazole proton with the
ester carbonyl oxygen to establish a network in the solution
cannot be ruled out. In FTIR, small change in stretching
frequencies of the ester carbonyls in the gel state with respect to
amorphous state supports this proposition (Fig. 1Sꢀ3S). However,
the large hydrophobic surface of cholesterol would stabilize the
network.
(i)
(ii)
96%
89%
O
H
H
H
O
H
H
H
H
Cl
H
H
N3
OH
O
O
6
7
OH
OH
O
O
(vii)
(iv)
85%
(iii)
1
5
62%
90%
8
OH
OH
O
(iii)
(iv)
2
88%
87%
O
9
(vi)
OH
O
O
(v)
(iv)
4
3
68/%
O
O
OH 84%
74%
O
O
10
Scheme 1. (i) Chloroacetyl chloride, pyridine, dry CH2Cl2, rt, 10h; (ii)
CH3CN, NaN3, reflux, 5h; (iii) propergyl bromide, dry CH3CN, K2CO3,
reflux, 4h; (iv) 7, dry DMF, CuSO4, sodium ascorbate, 800C, 8ꢀ12 h; (v)
propergyl bromide, dry CH3CN, Cs2CO3, reflux, 3h; (vi) 6, dry CH3CN,
Cs2CO3, reflux, 14h; (vii) 6, dry CH3CN, K2CO3, reflux, 8h.
Gelation study
Gelation studies were carried out by an inversion tube method,
where the compound dissolved in a suitable solvent which forms
a homogeneous solution was warmed and then cooled to form a
gel which generally was reluctant to flow upon inversion. The
gelation properties of the cholesterol derivatives 1ꢀ 5 were
examined in a wide range of solvents or mixture of solvents
(Table 1S). Due to presence of cholesterol unit having large
hydrophobic surface, all the compounds were either insoluble or
partially soluble in polar protic solvents like MeOH and H2O.
Whereas compounds 1ꢀ3 show partial gelation tendency in DMF,
they exhibit wellꢀorganized molecular packing via self
Figure 2. SEM images of CHCl3:CH3OH (2:1, v/v) gels of 1 (a and a׳), 2
(b and b׳) and 3 (c and c׳) at different scale bar.
2
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In order to determine the thermal stability of the gels, Tgel defined
as the required temperature for the organogel to collapse was
measured by the dropping ball method18 and plotted against the
gelator concentration (Fig. 4). The Tgel increased with increase in
concentration of the gelators. Compound 3 was noticed to be
gelators attracted us to explore the gels in cation sensing.
Compound 1 which exhibited gel in CHCl3:CH3OH (2:1, v/v)
turned into solution in presence of Cu2+ ions. Initially, the gel
started to disintegrate in presence of Cu2+ ions and was
completely ruptured to the sol state within 30 mins. Under similar
conditions, the gel state of 1 remained unaffected in presence of
other tested metal ions such as Pb2+, Ag+, Mg2+, Co2+, Ni2+, Zn2+,
Cd2+, Fe2+ and Hg2+ ions. Figure 5 demonstrates this feature.
These findings suggested the selective interaction of the gelator 1
to Cu2+ ions over the other metal ions tested. To our opinion, the
strong interaction of Cu2+ ion with the triazoles in the pseudo
cavity of 1 disrupts the gel.
(a)
(b)
H3
C
O
H
H3
C
O
O
H
O
O
H
O
H
O
O
H
H
N
N
N
CH3
H
N
N
H
N
N
O
N
N
N
N
N
N
H
N
N
N
N
O
O
N
Aromatic spacer
Binding site
O
O
O
O
O
O
O
CH3
CH3
H
H
CH3
H
H
O
O
O
O
O
O
N
N
N
However, Cu2+ ionꢀinduced broken gel of 1 was recovered
instantly upon addition of 1ꢀdodecanethiol (Fig. 6) and thereby
interpreted the chemical reversibility of the process.
N
N
N
H
N
N
N
N
N
N
N
N
N
O
O
H
H
O
N
N
N
Hydrophobic surface
O
O
O
Figure 3. Suggested modes of interaction of the gelators 1ꢀ3 to form the
networks in solution.
good gelator than 1 and 2 as it has lower minimum gelation
concentration (mgc) value compared to others emphasizing the
stacking interaction between πꢀsurfaces of coumarin motifs. The
gels were thermo reversible and gave distinct phase changes upon
heating and cooling (Fig. 4S). At the Tgs, the gels were
completely transformed into sols and readily became thick on
cooling.
Figure 6. Chemical responsiveness of the gel of 1 [15 mg/ mL in CHCl3:
MeOH (2:1, v/v)] on successive addition of 5 equiv. amounts of Cu2+ (c =
0.2 M) and 1ꢀdodecanethiol (c = 0.2 M, 10 equiv).
A similar study was carried out with the gelator 2 which showed
interaction with Ag+, Cu2+ and Hg2+ ions in the gel state (Fig. 7).
The gel state of 2 showed quick response towards Ag+ ions and
was converted to the sol within 20 mins. In comparison, Hg2+ and
Cu2+ ionsꢀinduced gel breaking took almost 1h. Inspite of having
similar structural feature of 2 with respect to the isomeric
structure 1 the additional preference for Hg2+ and Ag+ is possibly
linked with the selfꢀassemble pattern of the gelators as well as
dimension of the pseudo cavity of 2.
90
80
70
60
50
40
gel of 1
gel of 3
gel of 2
10
15
20
25
30
35
40
w/v (mg/mL)
Figure 4. Variation of gel melting temperature (Tg) with increase in
concentration of gelators.
Cation responsive behaviour of gels
The presence of triazole motifs in the structural backbones of the
Figure 7. Photograph showing the changes in the CHCl3:CH3OH (2:1,
v/v) gels of 2 (15 mg/ mL) after keeping contact with 0.5 mL of different
metal ions [c = 0.2 M in CHCl3:CH3OH (2:1, v/v) as perchlorate salt] for
2h. Similar observation was obtained when the gels were prepared with 5
equiv. amounts of different metal ions (c = 0.2 M) in CHCl3:CH3OH (2:1,
v/v).
Figure 5. Photograph showing the changes in the CHCl3:CH3OH (2:1,
v/v) gels of 1 (15 mg/ mL) when the gels were prepared with 5 equiv.
amounts of different metal ions [c = 0.2 M in CHCl3:CH3OH (2:1, v/v) as
perchlorate salt]. Similar observation was obtained when the gels were
kept in contact with 0.5 mL of different metal solution (c = 0.2 M) in
CHCl3:CH3OH (2:1, v/v).
Like 1, metal ionsꢀinduced broken gels of 2 were quickly
recovered with the addition of 1ꢀdodecanethiol for Cu2+, Hg2+
ions. Under identical conditions, Ag+ ionꢀ induced broken gel was
not recovered upon addition of 1ꢀdodecanethiol (Fig. 5S). But
addition of tetrabutylammonium chloride (TBACl) retrieved
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gelation (Fig. 6S). These experimental findings are thus useful in
distinguishing Ag+ ion form Cu2+ and Hg2+ ions.
with the metal ions (Fig. 14Sꢀ15S). Both 1 and 2 (c = 2.0 x 10ꢀ5
M) in CH3CN: CHCl3 (4:1, v/v) exhibited absorption peak at
~274 nm. In presence of Cu2+ ions the intensity of this peak was
increased considerably in both cases. In case of titrations with
Ag+ and Hg2+ ions, the change in absorbance of 2 was minor.
Other metal ions showed negligible change in the
Cation interaction in solution
In order to realize, the metalꢀbinding characteristics of the
triazoleꢀbased compound
2 in the solution phase using
fluorescence, we tuned the design 2 introducing coumarin as a
fluorescent probe to get a new structure 3. The design was
undertaken in such a way that the binding site was alike to that of
2. In the study, compound 3 also formed gel in CHCl3:CH3OH
(2:1, v/v). The light yellowish gel phase of 3 behaved to the
cations in the similar way as noticed for compound 2 (Fig. 7S).
Here also the gel was transformed into the solution in the
presence of Ag+, Hg2+ and Cu2+ ions. While in presence of Ag+
ion the gel was readily converted to sol within 15 mins, almost 30
mins were taken for Hg2+ and Cu2+ ions to collapse the gels (Fig.
8S). Like the case with gelator 2, Hg2+ and Cu2+ ionsꢀinduced
broken gels of 3 were converted to the gel states in the presence
of 1ꢀdodecanethiol. On the other hand, TBACl on scavenging
Ag+ ions induced further gelation of the sol state (Fig. 8S).
Careful investigation on the Figs. 5, 7 and 7S reveals that the gel
states of 1, 2 and 3 are completely converted to their sol states
(within time span of 0.25hꢀ1h) in presence of 5 equiv. amounts of
the selective metal salts. On using 1 equiv. amount of the metal
salts, the gels were slowly disrupted and completely converted to
the sol states on keeping for a longer time (1.5h – 5h) and thereby
indicated that the sensitivity of the gels are dependent on the
amount of the metal salts used (Fig. 9S).
Table 1: Binding constant values for 1 ꢀ 3 (c = 2.0 x 10ꢀ5 M) with the
metal ions(c = 8.0 x 10ꢀ4 M) in CH3CN: CHCl3 (4:1, v/v).
Metal
ligand
complex
Binding constant values ( Mꢀ1)
From fluorescence titration From UVꢀvis titration
1 ꢀ Cu2+
2 ꢀ Hg2+
2 ꢀ Ag+
2 ꢀ Cu2+
3 ꢀ Hg2+
3 ꢀ Ag+
3 ꢀ Cu2+
ꢀ
(1.14 ± 0.11) x 105
(2.89 ± 0.61) x 104
(8.42 ± 0.78) x 104
(1.84 ± 0.17) x 104
(2.69 ± 0.31) x 104
(3.42 ± 0.86) x 104
(1.11 ± 0.13) x 105
ꢀ
ꢀ
ꢀ
(2.33 ± 0.75) x 104
(2.59 ± 0.72) x 104
(5.00 ± 1.10) x 104
absorption spectra of both 1 and 2. The non linear fitting of the
emission and UVꢀvis titration data wherever applicable (Fig. 16Sꢀ
21S) gave the binding constant values (Table 1).20 As can be seen
from Table 1, compounds 1 and 3 show marginally higher
binding with Cu2+ ion than Ag+ and Hg2+ ions.
We further investigated the interaction of 3 with the said metal
ions in CH3CN: CHCl3 (4:1, v/v) (Fig. 10S). Among the different
metal ions Ag+, Cu2+ and Hg2+ brought about significant
quenching in emission (Fig. 8a). Cu2+ ions quenched the
emission to the greater extent than Ag+ and Hg2+ ions as evident
from the SternꢀVolmer plot (Fig. 8b). The quenching of emission
is here attributed to the activation of photoinduced electron
transfer (PET) process occurring in between the excited state of
coumarin and the binding site. The metal ions interacted with 3 in
1:1 stoichiometric fashion as evident from the Job plots (Fig.
11Sꢀ12S).19
1H NMR study
It is important to be mentioned that compounds 1, 2 and 3
showed small change in chemical shift values in the presence of
the metal ions such as Ag+, Cu2+ and Hg2+. For 1, the triazole ring
proton of type ‘a’ underwent downfield chemical shift to the
small extent. The protons of types ‘c’ and ‘d’ became broad in the
presence of Cu2+ ions (Fig. 9). In case of 2, the change in
chemical shift value of the triazole ring protons in the presence of
metal ions was too small. This was also true with the compound 3
(Fig. 22Sꢀ23S). This indicated that the triazoles in 1, 2 and 3
interacted with the said metal ions very weakly. In this context,
the mode of interaction of the triazoles with the metal ions is
assumed to be in accordance with the observation of Noor et al.5i
Such weak interaction is noted to be enough functional in
breaking the gels.
Cu2+
12
Hg2+
Ag+
9
6
3
Hb
0
2.0x10ꢀ5
[G]
4.0x10ꢀ5
0.0
O
N
Hc
N
O
N
Ha
Hd
Figure 8. (a) Change in fluorescence ratio (λex = 330 nm) of 3 (c = 2.0 x
10ꢀ5 M) at 420 nm upon addition of 5 equiv. amounts of metal ions (c =
8.0 x 10ꢀ4 M) in CH3CN: CHCl3 (4:1, v/v); (b) Stern – Volmer plot for 3
(c = 2.0 x 10ꢀ5 M) with metal ions (c = 8.0 x 10ꢀ4 M) at 420 nm in CH3CN:
CHCl3 (4:1, v/v).
N
N
N
1
OR
OR
O
O
1
Figure 9. Partial H NMR (400 MHz, CDCl3) of (a) compound 1 (c =
6.29 x 10ꢀ3 M) and (b) 1 with Cu2+ (1:1).
In UVꢀvis, small change in absorbance of 3 with ratiometric
behaviour was observed during titration with the metal ions such
as Cu2+, Ag+ and Hg2+. In case of other metal ions, the change in
absorbance of 3 was negligible (Fig. 13S).
Conclusion
Due to absence of fluorophore in 1 and 2, we performed only the
UVꢀvis titrations to understand their solution phase interactions
In conclusion, cholesterol appended 1,2,3ꢀtriazolesꢀbased
compounds 1ꢀ3 have been designed and synthesized. Triazole
4
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arms are disposed around different aromatic spacers. The
compounds 1ꢀ3 form nice gels in CHCl3/CH3OH (4/1, v/v). The
hydrogen bonding role of the triazole units and the hydrophobic
interaction between the cholesterol units allow the gelators to
form gel in CHCl3/CH3OH (4/1, v/v). Model compounds 4 and 5,
devoid of triazoles did not produce gels and thereby suggested the
important role of triazoles in the designs. The gels show excellent
cation responsive behavior. It is to note that inspite of similar
gelation properties of the triazoleꢀbased compounds; the metal
ionꢀresponsive character of the gels varies with the disposition of
the triazole motifs around the aromatic spacer. While the gel of 1
is rapidly transformed into sol state in the presence of Cu2+ ion,
gels of 2 and 3 with identical binding sites exhibit the gel to sol
transformation in the presence of Cu2+, Hg2+ and Ag+ ions.
Interestingly, while Cu2+ and Hg2+ ionsꢀinduced broken gels of
both 2 and 3 were changed into their gel states immediately upon
addition of 1ꢀdodecanethiol, under similar conditions Ag+ ionꢀ
induced broken gel remained unaffected. Addition of TBACl to
the Ag+ ion containing sols of 2 and 3 brought about gelation and
thus enabled us to distinguish this ion effectively from Cu2+ and
Hg2+ ions. Furthermore, compound 3 with coumarin fluorophore
distinguishes Cu2+, Hg2+ and Ag+ ions in solution
fluorometrically by showing PETꢀbased differential quenching of
emission. Further study along this direction is underway in our
laboratory.
atmosphere. The mixture was allowed to stir for 10 h at room
temperature. After completion of reaction, the solvent was
evaporated and the crude was extracted with CHCl3 (3 × 30 mL).
The organic layer was washed several times with water, separated
and dried over Na2SO4. Evaporation of the solvent gave white
solid compound which on recrystallization from petroleum ether
afforded pure product 6 (0.58 g, yield 96%), mp 148 °C. 1H NMR
(400 MHz, CDCl3): δ 5.37 (m, 1H), 4.72 (m, 1H), 4.03 (s, 2H),
2.36 (m, 2H), 2.02–0.85 (m, 38H), 0.67 (s, 3H); FTIR (KBr,
cm−1): 2939, 2907, 2821, 1753, 1620, 1195.
Azidoꢀacetic acid 17ꢀ(1,5ꢀdimethylꢀhexyl)ꢀ10,13ꢀdimethylꢀ
2,3,4,7,8,9,10,11,12,13,14,15,16,17ꢀtetradecahydroꢀ1Hꢀ
cyclopenta[a]phenanthrenꢀ3ꢀyl ester (7)13d
To a stirred solution of compound 6 (0.5 g, 1.08 mmol) in
CH3CN (20 mL), NaN3 was added (0.11 g, 1.6 mmol) and the
reaction mixture was refluxed for 5 h. The progress of reaction
was monitored by TLC. After completion of reaction, solvent was
evaporated off and water was added. The reaction mixture was
extracted with CHCl3. Evaporation of the solvent gave the crude
azide
7 (0.45 g, yield 89%, mp 116 °C), which after
recrystallization from diethyl ether was directly used in the next
step. FTIR (KBr, cm−1): 2107, 1747, 1213.
1, 3ꢀBisꢀpropꢀ2ꢀynyloxyꢀbenzene (8)
A mixture of resorcinol (1 g, 9.09 mmol) and K2CO3 (5.01 g,
33.36 mmol) in dry CH3CN (30 mL) was refluxed for 1h and then
propergyl bromide (4.37 g, 36.36 mmol) was added to it. The
reaction mixture was further refluxed for 4h. After completion of
reaction, organic solvent was evaporated off under reduced
pressure and water was added to it. Then reaction mixture was
extracted with CHCl3, washed with water and dried over
anhydrous Na2SO4. Evaporation of the solvent gave the crude
product which was purified by column chromatography using 8%
ethyl acetate in petroleum ether as eluent to afford the pure
Experimental section
Materials
Cholesterol, resorcinol and catechol were purchased from
Spectrochem. 6,7ꢀDihydroxycoumarin was obtained from Sigmaꢀ
Aldrich. Chloroacetyl chloride, sodium azide, cesium carbonate
were purchased from Spectrochem. Metal perchlorates used in
the study were purchased from SigmaꢀAldrich and were carefully
handled. All solvents used in the synthesis were purified, dried
and distilled as required. Solvents used in NMR experiments
were obtained from Aldrich. Thin layer chromatography was
1
compound 8 (1.52 g, yield 90%). H NMR (400 MHz, CDCl3): δ
7.23ꢀ7.19 (m, 1H), 6.36ꢀ6.61 (m, 3H), 4.67(s, 4H), 2.52 (t, 2H, J
1
performed on Merck precoated silica gel 60ꢀF254 plates. H and
= 2.4 Hz); FTIR (KBr, cm−1): 3291, 2122, 1594, 1490, 1455.
13C NMR spectra were recorded using Bruker 400 MHz
instrument using TMS as internal standard. High resolution mass
data were acquired by the electron spray ionization (ESI)
technique on XEVO GSꢀ2 QTOf Waters mass spectrometer.
FTIR measurements of all the compounds and dried gels
(xerogels) were carried out using a PerkinꢀElmer L120ꢀ00A
spectrometer (νmax in cmꢀ1) using KBr cell and KBr pellets,
respectively. Scanning electron microscopy (SEM) images were
obtained on HitaciꢀSꢀ4800 instrument. Fluorescence and UVꢀVis
studies were performed using Horiba Fluoromax 4C
spectrofluorimeter and Shimadzu UVꢀ2450 spectrophotometer,
respectively.
1, 2ꢀBisꢀpropꢀ2ꢀynyloxyꢀbenzene (9)
Compound 9 was prepared according to the same procedure as
followed for the synthesis of 8. In this case, 1g of catechol was
transformed into the compound 9 in 88% yield (1.49 g). 1H NMR
(400 MHz, CDCl3): δ 7.09 – 7.04 (m, 2H), 7.00 – 6.96 (m, 2H),
4.75 (d, 4H, J = 2.40 Hz), 2.50 (t, 2H, J = 4 Hz); FTIR (KBr,
cm−1): 3289, 2121, 1594, 1501, 1457.
6,7ꢀBisꢀpropꢀ2ꢀynyloxyꢀchromenꢀ2ꢀone (10)
6,7ꢀDihydroxycoumarin (0.1 g, 0.56 mmol) was dissolved in dry
CH3CN (30 mL) containing 2% dry DMF and to this solution
Cs2CO3 (0.55 g, 1.68 mmol) was added under stirring. The
reaction mixture was refluxed for 1h and then propergyl bromide
(0.27 g, 2.25mmol) was added to it. The reaction mixture was
allowed to reflux for another 8h. After completion of the reaction,
the organic solvent was evaporated off and the crude mass was
extracted with CHCl3. Evaporation of the solvent in vacuo gave
the crude product which was purified by column chromatography
using 25% ethyl acetate in petroleum ether to to have the desired
Synthetic procedure
Chloroꢀacetic acid 17ꢀ(1,5ꢀdimethylꢀhexyl)ꢀ10,13ꢀdimethylꢀ
2,3,4,7,8,9,10,11,12,13,14,15,16,17ꢀtetradecahydroꢀ1Hꢀ
cyclopenta[a]phenanthrenꢀ3ꢀyl ester (6)13d
To a stirred solution of cholesterol (0.5 g, 1.29 mmol) in 20 mL
dry CH2Cl2 was added chloroacetyl chloride (0.16 mL, 1.93
mmol) and pyridine (0.05 mL, 0.65 mmol) under nitrogen
This journal is © The Royal Society of Chemistry [year]
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1
product 10 (0.11 g, yield 84%, mp 144 °C). H NMR (400 MHz,
product 3 in 74% yield (0.35 g, mp 262 °C). 1H NMR (400 MHz,
CDCl3): δ 7.84 (s, 1H), 7.79 (s, 1H), 7.60 (d, 1H, J = 8 Hz), 7.11
(s, 1H), 7.00 (s, 1H), 6.28 (d, 1H, J = 8 Hz), 5.37 (br s, 2H), 5.29
(d, 4H, J = 8 Hz), 5.16 (d, 4H, J = 8 Hz), 4.68 (m, 2H), 2.31 (m,
4H), 2.02 – 0.62 (m, 82H, cholesteryl protons); 13C NMR (100
MHz, CDCl3): δ 165.6, 161.1, 152.2, 150.4, 144.9, 143.9, 143.2,
143.1, 138.9, 124.9, 124.8, 123.3, 114.0, 113.3, 112.2, 102.3,
64.1, 63.0, 56.6, 56.1, 51.0, 49.9, 42.3, 39.6, 39.5, 37.8, 36.8,
36.5, 36.1, 35.8, 31.87, 31.80, 29.6, 28.2, 28.0, 27.6, 24.2, 23.8,
22.8, 22.5, 21.0, 19.2, 18.7, 11.8; FTIR (KBr, cm−1): 3409, 2932,
2868, 2851, 1745, 1727, 1615, 1465; HRMS (TOF MS ES+):
calcd 1193.7916 (M+1)+, found 1193.7938 (M+1)+.
CDCl3): δ 7.64 (d, 1H, J = 8 Hz), 7.09 (s, 1H), 7.06 (s, 1H), 6.32
(d, 1H, J = 8 Hz), 4.83 (d, 2H, J = 4 Hz), 4.79 (d, 2H, J = 4 Hz)
2.59ꢀ 2.55 (m, 2H); FTIR (KBr, cm−1): 3265, 2116, 1704, 1696,
1613, 1561.
Compound 1
To a stirred solution of compound 8 (0.1 g, 0.53 mmol) and
compound 7 (0.62 g, 1.32 mmol) in dry DMF (20 mL), sodium
ascorbate (0.02 g, 0.12 mmol) in dry DMF (2 mL) was added and
stirred for
5 minutes at room temperature. Then solid
CuSO4.5H2O (0.01 g, .04 mmol) was added to it and the reaction
mixture was allowed to heat at 80 oC for 9h. After completion of
the reaction, the reaction mixture was extracted with 2% CH3OH
in CHCl3 and dried over anhydrous Na2SO4. Evaporation of the
solvent gave white solid compound. Recrystallization using
methanol followed by washing with diethyl ether afforded pure
product 1 in 85% yields (0.51 g, mp 188 °C). 1H NMR (400
MHz, CDCl3): δ 7.76 (s, 2H), 7.18 (t, 1H, J = 8 Hz), 6.63 – 6.59
(m, 3H), 5.38 (d, 2H, J = 4 Hz), 5.22 (s, 4H), 5.14 (s, 4H), 4.72ꢀ
4.68 (m, 2H), 2.33 (m, 4H), 2.02 – 0.68 (m, 82H, cholesteryl
protons); 13C NMR (100 MHz, CDCl3): δ 165.5, 159.3, 144.5,
138.9, 130.0, 124.1, 123.3, 107.6, 102.1, 76.5, 61.9, 56.6, 56.1,
51.1, 49.9, 42.3, 39.6, 39.5, 37.8, 36.8, 36.5, 36.1, 35.7, 31.88,
31.80, 28.2, 28.0, 27.6, 24.2, 23.8, 22.8, 22.5, 21.0, 19.2, 18.7,
11.8; FTIR (KBr, cm−1): 3450, 2948, 2867, 2851, 1751, 1733,
1593; HRMS (TOF MS ES+): calcd 1147.7915 (M + Na)+,
found 1147.7404 (M + Na)+.
Compound 4
To a stirred solution of 6,7ꢀdihydroxycoumarin (0.2g, 1.12 mmol)
in dry CH3CN containing 2% dry DMF was added Cs2CO3
(1.09g, 3.36 mmol) at room temperature. The reaction mixture
was refluxed for 1h and then compound 6 (1.57 g, 3.36 mmol)
was added to it. Then it was allowed to reflux for 14 h. After
completion of reaction, the organic solvent was evaporated under
reduced pressure and water was added to the crude mass. Then
reaction mixture was extracted with 2% CH3OH in CHCl3.
Evaporation of the solvent gave the crude product which was
purified by column chromatography using CHCl3 as eluent to
0
afford the pure compound 4 in 68 % yield (0.78 g), mp 148 C.
1H NMR (400 MHz, CDCl3): δ 7.58 (d, 1H, J = 8 Hz), 6.99 (s,
1H), 6.74 (s, 1H), 6.30 (d, 1H, J = 8 Hz), 5.37ꢀ5.34 (m, 2H),
4.74(s, 2H), 4.71 (s, 2H), 3.56ꢀ3.51 (m, 2H), 2.36 (m, 4H), 2.02 –
0.67 (m, 82H, cholesteryl protons); 13C NMR (100 MHz, CDCl3):
δ 168.1, 160.9, 151.8, 150.5, 144.7, 142.9, 140.7, 139.1, 123.2,
121.7, 114.3, 114.0, 112.5, 102.1, 75.3, 71.8, 67.6, 66.1, 56.7,
56.6, 56.1, 50.1, 49.9, 42.3, 39.79, 39.70, 39.5, 37.9, 37.2, 36.8,
36.55, 36.50, 36.1, 35.7, 31.9, 31.8, 31.6, 30.9, 29.6, 28.2, 28.0,
27.6, 24.2, 23.8, 22.8, 22.6, 22.5, 21.08, 21.02, 19.3, 19.2, 18.7,
11.8; FTIR (KBr, cm−1):3387, 2933, 1761, 1731, 1614, 1450,
1383; HRMS (TOF MS ES+): calcd 1031.7262 (M + 1)+, found
1031.7262 (M + 1)+.
Compound 2
Compound 2 was achieved according to the same procedure as
followed for the synthesis of 1. Click reaction between
compounds 9 (0.1 g, 0.53 mmol) and 7 (0.62 g, 1.32 mmol)
yielded the crude mass which on purification using 2% CH3OH in
CHCl3 furnished the desired compound 2 in appreciable yield
1
(0.52 g, yield 87%, mp 208 °C). H NMR (400 MHz, CDCl3): δ
7.79 (s, 2H), 7.06 – 7.04 (m, 2H), 6.95 – 6.93 (m, 2H), 5.38 (br s,
2H), 5.27 (s, 4H), 5.19 (s, 4H), 4.69ꢀ4.67 (m, 2H), 2.33ꢀ2.31 (m,
4H), 2.03 – 0.67 (m, 82H, cholesteryl protons); 13C NMR (100
MHz, CDCl3): δ 165.8, 148.4, 144.4, 138.9, 124.7, 123.2, 122.2,
115.3, 76.4, 63.3, 56.6, 56.1, 50.9, 49.9, 42.3, 39.7, 39.5, 37.8,
36.8, 36.5, 36.1, 35.8, 31.8, 31.7, 28.2, 28.0, 27.6, 24.2, 23.9,
22.8, 22.5, 21.0, 19.2, 18.7, 11.8; FTIR (KBr, cm−1): 3421, 2946,
2868, 1748, 1591; HRMS (TOF MS ES+): calcd 1147.7915 (M +
Na)+, found 1147.7524 (M + Na)+.
Compound 5
Compound 5 was prepared according to the same method as
followed for the synthesis of 4. In this case, resorcinol (0.2 g,
1.81 mmol) gave the desired gummy product 5 in 62% yield
1
(1.13 g). H NMR (400 MHz, CDCl3): δ 7.12 (t, 1H, J = 8 Hz),
6.54ꢀ6.43 (m, 3H), 5.37 (s, 2H), 4.73 (m, 2H), 4.56 (s, 4H), 2.35
(d, 4H, J = 8 Hz), 2.02 – 0.67 (m, 82H, cholesteryl protons); 13C
NMR (100 MHz, CDCl3): δ 168.5, 159.0, 139.2, 130.1, 123.0,
107.8, 102.5, 75.3, 65.5, 56.6, 56.1, 49.9, 42.3, 39.7, 39.5, 37.9,
37.2, 36.8, 36.5, 36.1, 35.8, 31.9, 31.8, 28.2, 28.0, 27.6, 24.2,
23.8, 22.8, 22.5, 21.0, 19.3, 18.7, 11.8; FTIR (KBr, cm−1): 3369,
2935, 2867, 1758, 1742, 1604; HRMS (TOF MS ES+): calcd
1001.7000 (M + K) +, found 1001.6927 (M + K)+.
Compound 3
To a mixture of compound 10 (0.1 g, 0.39 mmol) and 7 (0.45 g,
0.97 mmol) in dry DMF (20 mL), was added sodium ascorbate
(0.02 g, 0.12 mmol) in DMF (2 mL) under stirring condition at
room temperature followed by the addition of CuSO4.5H2O (0.01
g, .04 mmol). The reaction mixture was then allowed to heat at 80
oC for 12h. After completion of reaction, solvent was evaporated
off. The crude mass was extracted with 2% CH3OH in CHCl3 (3
x 30 mL). The organic layer was washed with water and dried
over Na2SO4. Evaporation of the solvent under reduce pressure
gave crude mixture which was chromatographed on a silica gel
column using 1% CH3OH in CHCl3 as eluent to get the desired
Gelation test
The respective amounts of 1ꢀ5 were dissolved in desired organic
solvents (1 mL) forming a homogeneous solution, slightly
warmed and then allowed to cool slowly to room temperature to
form a gel. All the gels were tested by an inversion of vial
method.
6
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DOI: 10.1039/C5NJ02771C
Determination of gelꢀsol transition temperature (Tg)
Y. Ju, Bioorg. Med. Chem. Lett., 2010, 20, 4342 and references cited
therein; (d) M. Felici, P. ContrerasꢀCarballada, J. M. M. Smits, R. J. M.
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Smits, R. J. M. Nolte, L. De Cola, R. M. Williams and M. C. Feiters,
Chem.–Eur. J., 2009, 15, 13124; (f) A. Kumar and P. S. Pandey,
Tetrahedron Lett., 2009, 50, 5842; (g) J. Camponovo, J. Ruiz, E.
Cloutet and D. Astruc, Chem.–Eur. J., 2009, 15, 2990; (h) Y. Li, J. C.
Huffman and A. H. Flood, Chem. Commun., 2007, 2692; (i) A. Noor, J.
E. M. Lewis, S. A. Cameron, S. C. Moratti and J. D. Crowley,
Supramol. Chem., 2012, 24, 492.
The gelꢀtoꢀsol transition temperature (Tg) was measured by the
dropping ball method. The Tg was defined as the temperature at
which the gel melted and started to flow. In this test, a small glass
ball was carefully placed on the top of the gel to be tested, which
was present in a test tube. The tube was slowly heated in a
thermostated oil bath until the ball fell to the bottom of the test
tube. The temperature at which the ball reaches the bottom of the
test tube is taken as Tg of that system.
General procedure for fluorescence and UVꢀvis titrations
Stock solutions of the compounds were prepared in CH3CN:
CHCl3 (4:1, v/v) in the concentration of 2.0 x 10ꢀ5 M. Stock
solutions of metal salts were also prepared in the same solvent in
the concentration of 8.0 x 10ꢀ4 M. Solution of each compound (2
mL) was taken in the cuvette and to this solution different metal
solutions were individually added in different amounts. Upon
addition of metal ion, the change in emission of the compound
was recorded. The same stock solutions were used to perform the
UVꢀvis titration experiment in the same way.
6. (a) Y. H. Lau, P. J. Rutledge, M. Watkinson and M. H. Todd, Chem.
Soc. Rev., 2011, 40, 2848; (b) Y. Hua and A. H. Flood, Chem. Soc.
Rev., 2010, 39, 1262; (c) P. Molina, A. Tarraga and M. Alfonso,
Dalton Trans., 2014, 43, 18.
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Serrano and L. Oriol, Langmuir, 2011, 27, 15236; (d) D. D. Diaz, K.
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Acknowledgement
AP and SP thank CSIR, New Delhi, India for fellowship.
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DOI: 10.1039/C5NJ02771C
GRAPHICAL ABSTRACT
Cholesterol appended bis-1,2,3-triazoles as simple supramolecular gelators for
naked eye detection of Ag+, Cu2+ and Hg2+ ions
Kumaresh Ghosh, Atanu Panja and Santanu Panja
Cholesterol coupled bis 1,2,3-triazoles have been designed and synthesized. Their gelation abilities and cation
responsive behaviors are documented.
cholesterol
hydrophobic surface
triazole
metal binder
_____________________________________________________________________________________