Cholesterol linked benzothiazole: A versatile gelator for detection of picric acid and metal ions such as Ag+, Hg2+, Fe3+ and Al3+ under different conditions

Article abstract of DOI:10.1039/c9nj01282f

Cholesterol-appended benzothiazole 1 has been designed and synthesized. Compound 1 exhibits gelation in different solvents. The benzothiazole and cholesteryl motifs play a pivotal role in aggregation in organic solvents involving π-stacking and hydrophobic interactions, respectively. As an application, the nitrobenzene gel with good rheological properties has been noted to be useful in molecular recognition studies of multiple analytes ranging from neutral to ionic in nature. It selectively recognizes picric acid (PA) over a series of nitroaromatics by exhibiting a gel-to-sol phase transition with a distinguishable color change from pale yellow to deep yellow. On the other hand, the nitrobenzene gel shows selective recognition potential for Ag⁺ and Hg²⁺ ions through a gel-to-sol phase transition and the cations are distinguished by F^-. The gelator 1 also reveals a measurable selective interaction with picric acid over a series of nitroaromatics in CH₃CN containing 1% CHCl₃. In CH₃CN containing 1% CHCl₃, compound 1 further shows affinity towards Fe³⁺, Al³⁺ and Hg²⁺ with distinctive features.

Full text of DOI:10.1039/c9nj01282f

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Cholesterol linked benzothiazole: A versatile gelator for
detection of picric acid and metal ions such as Ag+, Hg2+,
Fe3+ and Al3+ under different conditions
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Subhendu Mondal, Rameez Raza and Kumaresh Ghosh*
www.rsc.org/
Abstract: Cholesterol-appended benzothiazole 1 has been designed and synthesized. Compound 1 exhibits gelation in
different solvents. The benzothiazole and cholesteryl motifs plays pivotal role in making aggregation in organic solvents
involving π-stacking and hydrophobic interaction, respectively. As application, the nitrobenzene gel with good rheological
property has been noted to be useful in molecular recognition studies of multiple analytes ranging from neutral to ionic in
nature. It selectively recognizes picric acid (PA) over a series of nitroaromatics by exhibiting gel-to-sol phase transition with
distinguishable color change from pale yellow to deep yellow. On the other hand, the nitrobenzene gel shows the selective
recognition potential for Ag+ and Hg2+ ions through gel-to-sol phase transition and the cations are distinguished by F-. The
gelator 1 also reveals measurable selective interaction with picric acid over a series of nitroaromatics in CH3CN containing
1% CHCl3 . In CH3CN containing 1% CHCl3, compound 1 further, shows affinity towards Fe3+, Al3+ and Hg2+ with distinctive
features.
Introduction
easiest way is considered to be important from environment,
Design and synthesis of low molecular weight gelators that
find applications in detecting toxic ions, chemicals and
hazardous is of paramount interest in the research area of
supramolecular chemistry.1 The effect of toxic and hazardous
social and security issues. Till now, detection of PA either in
solution or vapour phase, involves the use of different
spectroscopic
techniques
(especially
fluorescence),11
nanoparticles/nanofibers12 and polymers13 etc. By contrast,
the detection of PA using supramolecular gel matrix is merely
explored. Gel phase sensing of PA in naked-eye attracts
attention as it does not require any sophisticated instrument.
The literature study reveals that there are few examples of gel
phase detection of PA (Table S2).14
pollutants on the environment is
a major concern in
environment pollution especially in third world countries. It is
thus more important to protect the environment from toxic
and hazardous pollutants.2-3 Toxic materials cover a wide
range of substrates ranging from neutral to charge in nature.
Out of such substrates, different nitroaromatics as well as
Beside picric acid, the detection of metal ions of biological
relevance is also of prime importance. For example, mercury,15
one of the most hazardous pollutants, is released in the
environment through several natural and industrial processes.
The exposure of this ion even at low concentration leads to
various health problems, especially neurological disorders.
Similarly, Ag+ ion16 is biologically relevant mainly due to its
antibacterial activity. It has great applications in organic
synthesis, electronics, photography, and imaging industry.17
Aluminum and iron are the third most abundant elements in
the earth crust. While excess Al3+ may cause damage to certain
human tissues and cells which may lead to Parkinson’s disease
and Alzheimer’s diease,18 deficiency of Fe3+ leads to liver
damage, anaemia, Alzheimer’s disease, Parkinson’s disease
and cancer.19 To sense these metal ions, most of the probes,
found in the literature, work in solution phase; there are very
different
metal
ions
are
mentionable.4-6
Among
nitroaromatics, trinitrotoluene (TNT) and picric acid (PA) are
used as powerful explosives. Thus there is a need for detection
of these species for homeland security and public safty.7-8
Importantly, PA causes several skin diseases, allergy, eye
irritation, liver malfunction which lead to chronic diseases like
cancer.9 Moreover, due to greater water solubility, it
contaminates ground water and causes water and soil
pollutions.10 Therefore, detection and sensing of PA in an
Department of Chemistry, University of Kalyani, Kalyani-741235, India. Email:
ghosh_k2003@yahoo.co.in, kumareshchem18@klyuniv.ac.in
Fax: +913325828282; Tel: +913325828750 Extn. 305.
†Electronic Supplementary Information (ESI) available: Figures showing the
gelation profile change in fluorescence and UV-vis titrations of 1 and 2 with various
metal
ions,
ion
conducting
plots,
spectral
data.
See
http://dx.doi.org/10.1039/b000000x/
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few reports on the probes that detect such ions in the gel nitrobenzene, benzene, toluene and DMF (Table S1, Fig. S1)
phase (Tables S3 and S4).
View Article Online
while in semi-aqueous compositions DiOtI: 1r0e.m10a39in/Ce9dNJ0e1it2h8e2Fr
Recognition and detection of these multiple analytes by using insoluble or gave precipitate. Analysis of the results reveals
a simple design-based probe is challenging and interesting. In that gelation of 1 occurs from DMF with lowest mgc in the
this full account, we wish to report the design and synthesis of series (10 mg/mL). The benzene gel of 1 displays poor thermal
a simple, new cholesterol-linked benzothiazole supramolecular stability in the series inspite of showing maximum mgc. In this
gelator 1 (Fig. 1) that selectively recognizes picric acid (PA) context, the minimum gelation concentration (mgc) and the
from a series of other nitroaromatics and some bio relevant gel melting temperature (Tgel) of the gels in different solvents
metal ions such as Ag+, Hg2+, Fe3+ and Al3+ under different are summarised in Table S1.
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conditions. Gelator 1 forms gel from nitrobenzene, benzene, Structural analysis of compound
1 indicates that the
toluene and DMF etc.
cholesterol as hydrophobic surface is connected to the
benzothiazole moiety via a rigid aromatic spacer (Fig. 1). It is
presumed that π-π stacking between the benzothiazole moiety
and van der Waals interaction between the cholesteryl surface
tune the self-aggregation of molecules.
Metal binding site
O
O
S
N
O
In solution, compound
1 exhibited two closely spaced
1
absorption peaks at 305 nm and 316 nm for the benzothiazole
moiety. In gel state (nitrobenzene), a broad red shifted
absorption at 340 nm was observed (Fig. 2a). Similarly, in
fluorescence, the emission at 365 nm of the sol was shifted to
412 nm in the gel state with increase in intensity (Fig. 2b). Such
spectral appearances were attributed to the aromatic stacking
which encouraged the formation of J-aggregate.22 Due to
aggregation, the emission of 1 in the gel state was enhanced
significantly (Fig. 2b).23
Spacer
Protonation site

-surface
Hydrophobic surface
Fig. 1. Structure of compound 1.
In the presence of PA, the nitrobenzene-gel of 1 is broken to
the sol and validates its visual sensing. In addition, the
nitrobenzene gel of 1 is selectively broken to the sol in the
presence of Ag+ and Hg2+ ions. Alongside, the sensing of Fe3+
and Al3+ ions is further ascribed through ratiometric spectral
change in both emission and absorption spectra of 1 in CH3CN.
Results and discussion
Synthesis
Compound 1 was achieved according to the Scheme 1. Initially,
cholesterol was converted to the chloride 2 according to our
reported procedure.20 Oxidative condensation21 of 2-
aminothiophenol with 4-hydroxybenzaldehyde in EtOH
containing few drops of trifluoroacetic acid (TFA) at 90 °C
afforded compound 3 which on coupling with compound 2 in
the presence of K2CO3 in CH3CN under refluxing condition
introduced the desired compound 1 in appreciable yield. All
the compounds were characterized by usual spectroscopic
techniques.
O
(i)
R
OH
R
Cl
R =
96%
O
2
Fig. 2. Comparison of (a) normalised UV-vis and (b) fluorescence
spectra (λex = 310 nm) of 1 in sol and gel states; (c) X-Ray diffractogram
of the nitrobenzene gel of 1; (d) Probable mode of interaction of 1 in gel
state.
NH2
SH
N
(iii)
(ii)
70%
OH
1
82%
S
Powder X-ray diffraction (XRD) study of the nitrobenzene gel
of 1 was further done to understand the influence of π-
stacking in the aggregation (Fig. 2c). Although the XDR
diffraction did not show any sharp peak, a broad peak at 2 =
23.70 of significant intensity along with a shouldering at 2 =
42.30 was noted. The strong diffraction at 2 = 23.70 (d = 3.7
Å) is likely to be due to the occurence of two types of
interactions within the gel matrix simultaneously viz. the van
der Waals interactions between the cholesterol units and the
π-π stacking interaction of the aromatic units in 1, suggested in
3
Scheme 1. (i) Chloroacetyl chloride, pyridine, dry CHCl3, rt, 10h; (ii) 4-
hydroxybenzaldehyde, trifluoroacetic acid, C2H5OH, reflux 12h; (iii) 2,.
K2CO3, CH3CN, reflux, 10h.
Gelation study
Gel formation of 1 was primarily investigated from various
solvents using the conventional heating-cooling method.
Gelation of 1 was observed in pure organic solvents such as
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Fig. 2d.14e,24 The broad shouldering in the X-ray diffractogram
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DOI: 10.1039/C9NJ01282F
at 2 = 42.30 corresponding to the d-spacing of 2.15 Å
describes the crystalline nature of the gel fibers.
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Morphology of the gels
The surface morphologies of the nitrobenzene and DMF gels of
1 were understood from SEM images. Both the nitrobenzene
and DMF gels revealed the formation of highly cross-linked
fibrous network (Fig. 3). The gels were thermoreversible in
nature and exhibited temperature-induced reversible gel-sol
phase transition.
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Figure 4. Rheology study of the gels of 1; (a and c) amplitude sweep (at
constant frequency of 1Hz) and (b and d) frequency sweep (at
constant 0.05% strain) experiments [gels were prepared at
a
concentration 1.2 wt% of respective mgc and the experiments were
carried out at 25 oC].
Table 1. Summary of rheological properties of the gel of 1.
Solvent
Critical
strain
Crossover
(% strain)
G′av*
(Pa)
G′′av*
(Pa)
tan δ
( G′′av/G′av)
(%)
NB
0.08
0.41
0.6
1.6
34447
59829
8788
2855
0.25
0.04
DMF
* G′av and G′′av values were calculated from frequency sweep data
Fig. 3. SEM images of xerogel of 1 prepared from (a), (b) nitrobenzene
and (c), (d) DMF.
Sensing and detection of Picric Acid (PA)
Gel phase interaction
Rheological study
In order to understand the sensing behavior of the gel towards
nitroaromatics (especially nitrophenols), nitrobenzene gel was
chosen. Importantly, DMF gel was not considered due to its
inertness or nonresponsive behavior towards the
nitroaromatics studied. However, in the experiment, the gel
formation ability of 1 in nitrobenzene at room temperature
was examined in the presence of nitrophenols and phenols
(Fig. 5). It was observed that only picric acid (PA) resisted
gelation. On the other way, when the solution of PA (1 equiv.)
in nitrobenzene was added on the top of the nitrobenzene gel
of 1, the gel was slowly destroyed into yellow colored sol.
Under identical conditions, other nitrophenols in the study did
not disturb the gel. Hence, we believe that PA being strongly
acidic than other phenolic analytes protonates benzothiazole
nitrogen and is involved in developing charge transfer complex
with the protonated benzothiazole for which the gel is
ruptured.14 Under the same conditions, survival of the gel in
the presence of other phenolic analytes of lower acidic
character than PA ruled out the possibility proton transfer
from phenolic oxygen to the benzothiazole nitrogen.
Interestingly, the gel state remained unaffected in the
presence of organic acids like benzoic and acetic acids.
However, protonation of the benzothiazole nitrogen using
strong acid like trifluoroacetic acid (TFA) ruptured the gel
immediately and gave pale yellow colored sol (Fig. 5). Both
To evaluate the mechanical properties of the nitrobenzene
(NB) and DMF gels of 1, rheological studies were carried out
(Fig. 4 and Table 1). In amplitude sweep (at lower strain) and
frequency sweep (over the entire range studied) experiments
of both the gels, the storage modulus (G′) was significantly
higher than the corresponding loss modulus (G′′) indicating
their true gelling behaviour. However, the gels displayed
considerable differences in G’ and G” values, critical strength,
crossover and gel strength.
The DMF-gel of 1 exhibited ~1.7 times higher storage modulus
(G′) than the nitrobenzene gel establishing to be more stiffer
than the later one. Furthermore, the DMF gel showed higher
critical strength (measurement of gel strength) as well as
higher crossover (measurement of maximum strain bearing
capability before complete demolition of the gel) than the
nitrobenzene-gel, suggesting greater strain-bearing capability
of the gel. Again, lower value of tan δ of the DMF-gel, in
comparison to the nitrobenzene gel, demonstrated its higher
mechanical strength. However, both the gels were completely
destroyed (gel-to-sol transition) at very low strain (crossover at
< 10% strain) and thereby suggested their brittle natures. In
short, higher magnitude of G′, critical strain and crossover and
lower tan δ values of DMF gel established DMF as the good
solvent of choice than nitrobenzene for gelation of 1.
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sols, obtained from the treatment of PA and TFA were connection, it is to note that although both TFA and PA
converted to yellow colored gels upon adding triethylamine destroyed the gel of 1, the solutioDnOI:p1h0a.1s0e39/sCt9uNdJy012c8a2nF
(Fig. S2). Thus the gel state of 1 not only recognized PA but differentiate PA from TFA through quenching of fluorescence
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also distinguished acetic/benzoic acid from TFA visually.
of 1 (Fig. S3). Thus this result substantiates that not only the
protonation of benzothiazole as depicted in Fig. 6a is
responsible for introducing the change in emission but also the
charge transfer complex formation between picrate and
benzothiazolium units has the discrete role in quenching of
emission. This is in accordance with the observations noted by
us and others.11,14
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1
To understand the interaction of picric acid with 1, H NMR
was recorded. In presence of picric acid, the aromatic signals,
especially of the benzothiazole motif, exhibited small
downfield chemical shift (Fig. S4). The picric acid signal at
9.137 ppm underwent upfield chemical shift by 0.021 ppm on
complexation. To our belief, this was attributed to the proton
Fig. 5. Photograph showing the phase changes of 1 (20 mg/mL) in
nitrobezene in the presence of 1 equiv. amount of (a) picric acid, (b) o-
nitrophenol, (c) p-nitrophenol, (d) 2,4-dinitrotoluene, (e) m-
dinitrobenzene, (f) p-cresol, (g) o-chlorophenol, , (h) benzoic acid, (i) transfer from picric acid to benzothiazole nitrogen.
acetic acid and (j) trifluroacetic acid after 2h.
The emission titration data of 1 with picric acid were fitted in
Benesi-Hildebrand equation to calculate the strength of
Solution phase interaction
interaction. The linear plot indicated 1:1 complexation (Fig. S5)
To understand the solution phase interaction, UV-vis and
and the association constant25 was evaluated to be 4.59 x 104
fluorescence titrations of 1 were performed with the said
M-1. The detection limit26 of compound 1 for picric acid was
analytes in CH3CN containing 1% CHCl3. Here, nitrobenzene
calculated to be 9.55 x 10-7 M (Fig. S5).
solvent was replaced by CH3CN to avoid its interference in the
spectroscopic study. In UV-vis, compound
1 exhibited
Metal ion sensing
absorptions at 305 nm and 316 nm with equal intensities.
Addition of the nitrophenols except PA did not produce any
change in the UV-vis spectrum. In fluorescence, on excitation
at 310 nm, compound 1 displayed emission at 365 nm. The
intensity of this emission was gradually decreased upon
titration with PA (Fig. 6). Under similar conditions, other
nitrophenols showed moderate or weak change in emission. In
particular, simple phenols and aromatic acids did not produce
any noticeable change in the emission spectrum of 1 (Fig. S3).
Gel phase interaction
Due to presence of benzothiazole unit that has the ability to
bind metal ions,27 we were keen to note the metal ion
interaction features of 1 both in the sol and gel states. In this
regard, the nitrobenzene gel of 1 was treated with several
Fig.7. (A) Photograph showing the phase changes of 1 (20 mg/mL) in
nitrobezene in the presence of 1 equiv. amounts of (a) Hg2+, (b) Al3+
,
(c) Fe3+, (d) Ag+, (e) Cu2+, (f) Fe2+, (g) Zn2+, (h) Co2+, (i) Ni2+, (j) Cd2+
and (k) Cu+ after 2h; (B) Photograph representing the phase change of
the nitrobenzene gel of 1 (20 mg/mL) upon successive addition of 1
equiv. amount of AgClO4, Hg(ClO4)2 and excess TBACl.
Fig. 6. (a) Probable mechanism of interaction of 1 with picric acid; (b)
Change in emission of 1 (c = 2.50 x 10-5 M) upon addition of 12 equiv.
amounts of picric acid (c = 1.0 x 10-3 M) in CH3CN containing 1% CHCl3;
(c) Stern-Volmer plot for 1 (c = 2.50 x 10-5 M) with different analytes (c metal ions such as Al3+, Fe3+, Hg2+, Cu2+, Cu+, Fe2+, Zn2+, Co2+,
= 1.0 x 10-3 M) in CH3CN containing 1% CHCl3.
Ni2+, Cd2+ and Ag+ (used as their perchlorate salts).
Importantly, DMF gel of
transformation or color change in presence of these metal
ions. However, upon individual addition of 1 equiv. amount of
1 did not show any phase
The Stern-Volmer plot as shown in Fig. 6c clearly reveals the
strong quenching of fluorescence of 1 by PA. In this
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these metal ions to the nitrobenzene gel, there was only gel- Al3+ ions to the solution of 1, the emission at 365 nm was
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to-sol phase transformation in the presence of Hg2+ and Ag+ gradually decreased with the formation DoOf Ia: 10n.e10w39e/Cm9iNsJs0io1n282aFt
ions (Fig. 7A). We presume that binding interaction of sulfur of 405 nm. The newly appeared peak at 405 nm was gradually
benzothiazole unit with the thiophilic ions Hg2+ and Ag+ intensified during progression of the titration and gave an
destroys the gel network to give the sol.
isoemissive point at 373 nm. Figure 9 indicates the change in
Comparison of H NMR of 1 in the absence and presence of emission of 1 with Fe3+ and Al3+ ions. In the event, Hg2+ ion
Ag+ and Hg2+ ions showed the downfield chemical shift of the showed a similar spectral change but to the smaller extent as
benzothiazole ring protons and thereby confirmed the compared to that of Al3+ and Fe3+ ions (Fig. S8). Other metal
interaction although it was difficult to predict the binding ions were non-interacting (Fig. S8).
mode (Fig. S6).
1
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To distinguish between Hg2+ and Ag+, tetrabutylammonium
chloride was added to the broken gels. While Ag+-induced
broken gel was retrieved in the presence of
tetrabutylammonium chloride, Hg2+-induced broken gel
remained undisturbed (Fig. 7B).
Solution phase interaction
To understand the binding interaction in solution, UV-vis and
fluorescence titrations were performed with the metal ions in
CH3CN containing 1% CHCl3. In UV-vis, compound 1 showed
strong absorptions at 305 nm and 316 nm. On titration with
Fe3+ ion, the absorption bands at 305 nm and 316 nm were
Fig. 9. Change in emission of 1 (c = 2.50 x 10-5 M) in presence of 3
equiv. amount of (a) Fe3+ and (b) Al3+ (c = 1.0 x 10-3 M) in CH3CN
containing 1% CHCl3.
gradually decreased in intensity with simultaneous growth of a
new peak at 350 nm. This resulted in the formation of a clear
isosbestic point at 325 nm which confirmed the formation of a
new species in solution (Fig. 8a). A similar drastic change in
absorption spectrum of 1 was found in the presence of Al3+ ion
The binding interaction of Fe3+ and Al3+ ions with 1 was
1
realized from H NMR. Significant downfield chemical shift of
benzothiazole protons in 1 occurred in the presence of Fe3+
ions in CDCl3. Spectral shits were small in the presence of Al3+
(Fig. 8b). Under the identical conditions, small change in
ion compared to the case of Fe3+ ion (Fig. 10).
absorption of 1 in the presence of Hg2+ (Fig. 8c) and other
To understand the interference of other metal ions in the
metal ions expressed significantly weak interaction (Fig. S7).
sensing of Fe3+ and Al3+ ion, competitive experiment was
Figure 8d, in this regard, shows the change in relative intensity
of absorbance at 350 nm with the metal ions.
carried out by adding Fe3+ and Al3+ to the solutions of 1 in
CH3CN containing other metal ions (Fig S9). As can be seen
from Fig S9, no metal ion interfered in the binding of Fe3+ and
Al3+ ions.
1
Fig.10. Partial H NMR (400 MHz) of 1 (c = 3.07 x 10−3) in the absence
and presence of 1 equiv. amount of (a) Fe3+ and (b) Al3+ in CDCl3.
Fig.8. UV-vis titration spectra of 1 (c = 2.50 x 10-5 M) upon successive
addition of 3 equiv. amounts of (a) Fe3+, (b) Al3+, (c) Hg2+ (c = 1.0 x 10-3
M) in CH3CN containing 1% CHCl3 and the relative change in
absorbance of 1 (c = 2.50 x 10-5 M) at 350 nm in the presence of 3
equiv. amounts of different metal ions.
To distinguish Fe3+ from Al3+ ion, anion-induced
decomplexation study was performed. In the event, upon
addition of tetrabutylammoniumfluoride (TBAF), the ensemble
of 1-Fe3+ lost its absorption and returned to the initial
spectrum of 1 due to the formation of stable FeF6 complex.
Under identical conditions, no significant spectral change was
-
In fluorescence, compound 1 on excitation at 310 nm gave
sharp emission at 365 nm. Upon gradual addition of Fe3+ and
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followed upon addition of TBAF to the ensemble of 1-Al3+ (Fig. Tetrabutylammonium salts of anions and metal perchlorates used
View Article Online
DOI: 10.1039/C9NJ01282F
11).
in the study were purchased from Sigma-Aldrich and were carefully
handled. All solvents used in the synthesis were dried and distilled
before use. Solvents used in NMR experiments were obtained from
Aldrich. Thin layer chromatography was performed on Merck
1
precoated silica gel 60- F254 plates. H and 13C NMR spectra were
recorded on Bruker 400 MHz instrument. High resolution mass data
were acquired by the electron spray ionization (ESI) technique on
QTOf Micro YA 263 mass spectrometer. FTIR measurements of all
the compounds and dried gels (xerogels) were carried out using a
PerkinElmer L120-00A spectrometer (νmax in cm-1) using KBr cell and
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Fig. 11. Change in absorbance of (a) 1-Fe3+ and (b) 1-Al3+ complexes KBr pellets, respectively. Scanning electron microscopy (SEM)
upon successive addition of TBAF (c = 1 x 10-3 M) in CH3CN containing images were obtained on EVO LS-10 ZEISS instrument. Fluorescence
1% CHCl3.
and UV-Vis studies were performed using ParkinElmer
spectrofluorimeter and Shimadzu UV-2450 spectrophotometer,
The stoichiometry of both the complexes were found to be 1:1 respectively. The X-ray diffraction study was carried out in a Bruker
as determined from the Benesi-Hildebrand plot.25 The D8 Advance X-ray diffractometer operating at 40 kV and 20 mA with
association constants for Fe3+ and Al3+ ions were determined a Cu-target (λ = 1.54 Å).
to be 1.42 x 104 M−1 and 1.04 x 104 M−1, respectively. Analysis
of the results gave the detection limits26 of 4.07 x 10−8 M and Synthesis
1.65 x 10−8 M for Fe3+ and Al3+ ions, respectively (Fig. S10).
Chloro-acetic
acid
17-(1,5-dimethyl-hexyl)-10,13-dimethyl-
2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-
cyclopenta[a]phenanthren-3-yl ester (2):20
Conclusions
To a stirred solution of cholesterol (0.5 g, 1.29 mmol) in 20 mL dry
CHCl3 was added chloroacetyl chloride (0.16 mL, 1.93 mmol) and
pyridine (0.05 mL, 0.65 mmol) under nitrogen atmosphere. The
mixture was allowed to stir for 10 h at room temperature. After
completion of reaction, the reaction mixture was neutralized with
NaHCO3 solution, and then was extracted with CHCl3 (3 × 30 mL).
The organic layer was washed several times with water and
separated and dried over Na2SO4. Evaporation of the solvent gave
white solid compound. Recrystallization from CH3OH afforded pure
product 2 (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, cholesteryl protons), 0.67 (s, 3H); FTIR (KBr, cm−1): 2939,
2907, 2821, 1753, 1620, 1195.
In conclusion, cholesterol appended simple benzothiazole
derivative 1 has been designed and synthesized. Hydrophobic-
hydrophobic interaction between cholesterol units and the π-π
stacking interaction of the benzothiazole moieties led to the
gelation of 1 in various solvents such as nitrobenzene,
benzene, toluene and DMF. Morphology of the nitrobenzene
and DMF gels, as established from SEM study, revealed the
formation of densely packed highly cross-linked fibrous
network. The nitrobenzene gel was impressive in visual sensing
of nitro aromatics and metal ions. The nitrobenzene gel
selectively recognizes picric acid, Ag+ and Hg2+ by exhibiting
gel-to-sol phase transition. Differential gel phase detection of
Ag+ from Hg2+ has been possible with the aid of
tetrabutylammonium chloride. While in fluorescence
compound 1 in CH3CN exhibits selective sensing of PA over
other nitro aromatics through quenching of emission, it
effectively senses Fe3+ and Al3+ ions by showing ratiometric
change in emissions. Further, differential sensing of Fe3+ from
Al3+ has been possible through the addition of
tetrabutylammonium fluoride to the ensembles of 1.Fe3+ and
1.Al3+. Thus, the present report dealing with a structurally
simple and easily synthesizable gelator 1 for effective sensing
of PA and a number of metal ions such as Ag+, Hg2+, Fe3+ and
Al3+ ions under different conditions is praiseworthy and it
deserves attention in material chemistry. Further work along
this direction is underway in the laboratory.
4-Benzothiazol-2-yl-phenol (3):
2-Amonobenzothiol (1 g, 8.0 mmol) and 4-hydroxybenzaldehyde
(0.95 g, 7.78 mmol) were taken in 50 mL of EtOH containing few
drops of trifluoroacetic acid (TFA) and the reaction mixture was
refluxed at 90 0C for 8h. The progress of the reaction was monitored
by TLC. After completion of reaction, the organic solvent was
evaporated under reduced pressure and water was added to the
crude mass. The reaction mixture was extracted with CHCl3.
Evaporation of the solvent gave the product which was purified by
column chromatography using 30% ethyl acetate in petroleum
ether as eluent to afford the pure compound 1 in 70 % yield (1.23
g), mp 212 0C. 1H NMR (CDCl3 containing two drops of d6-DMSO,
400 MHz):  9.45 (s, 1H), 7.93 (d, 1H, J = 8 Hz), 7.87 (d, 2H, J = 8 Hz),
7.80 (d, 1H, J = 8 Hz), 7.39 (t, 1H, J = 8 Hz), 7.27 (t, 1H, J = 8 Hz), 6.89
(d, 2H, J = 8 Hz); 13C NMR (CDCl3 containing two drops of d6-DMSO,
100 MHz): 168.3, 160.2, 154.0, 134.5, 129.1, 126.0, 124.8, 124.5,
122.4, 121.4, 116.0 ; FTIR (KBr)  cm-1: 3443, 2919, 2667, 2587,
1605, 1522, 1482, 1454, 1430, 1381, 1285; HRMS (TOF MS ES+):
calcd. for C13H10NOS [M+H]+: 228.0478 and found 228.0484 (M+H)+.
Experimental
Materials
Cholesterol, chloroacetyl chloride, pyridine, 2-aminothiophenol and
trifluoroacetic acid were purchased from Spectrochem.
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Compound 1:
Benesi-Hildebrand plot was adopted to determine the binding
View Article Online
-1 -1
A mixture of compound 3 (1g, 4.44 mmol) and K2CO3 (1.24 g, 9.0 constant value using the expression: I0/(I-I0) = [εM
mmol) was refluxed in dry CH3CN for 2h and then compound 2 (2.46 +1), where εM and εC represent molar extinction coefficients for
g, 5.32 mmol) was added to it. The reaction mixture was further compound and the complex, respectively, at selected
DOI: 1/0(.ε10M3–9/εCC9)]N(KJ0a 12C8g2F
1
a
6
refluxed for 12h. After completion of reaction, the organic solvent wavelength, I0 denotes the emission intensity of free compound 1
was evaporated under reduced pressure and water was added to at that specific wavelength and Cg is the concentration of the
the crude mass. The reaction mixture was extracted with 2% CH3OH guest/analyte. The measured emission I0/(I-I0) as a function of the
in CHCl3. Evaporation of the solvent gave the product which was inverse of the guest concentration fits a linear relationship,
purified by column chromatography using 20% ethyl acetate in indicating 1:1 stoichiometry of the compound 1- guest complex. The
petroleum ether as eluent to afford the pure compound 4 in 82 % ratio of the intercept to the slope was used to determine the
yield (2.38 g), mp 182 0C. 1H NMR (CDCl3, 400 MHz):  7.98-7.95 (m, binding constant Ka.
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3H), 7.81 (d, 2H, J = 8 Hz), 7.40 (t, 1H, J = 8 Hz), 7.29 (d, 1H, J = 8 Hz),
6.94 (d, 2H, J = 8 Hz), 5.32 (d, 1H, J = 4 Hz), 4.70-4.68 (m, 1H), 4.60 Calculation of detection limit26
(s, 2H), 2.30–0.67 (m, 43H); 13C NMR (CDCl3, 100 MHz): 167.9, Detection limit was calculated by using fluorescence titration data.
167.5, 160.0, 154.1, 139.1, 134.9, 129.1, 127.3, 126.2, 124.9, 123.1, The emission of compound 1 was measured 6 times, and the
122.9, 121.5, 115.0, 75.4, 65.5, 56.6, 56.1, 49.9, 42.3, 39.6, 39.5, standard deviation of blank measurement was achieved. To have
37.9, 36.8, 36.5, 36.1, 35.8, 31.89, 31.81, 28.2, 28.0, 27.6, 24.2, the slope, emission intensities were plotted against concentrations
23.8, 22.8, 22.5, 21.0, 19.3, 18.7, 11.8; FTIR (KBr)  cm-1: 3416, of guests. The detection limits were calculated using the equation:
2933, 2867, 1760, 1606, 1483, 1433, 1206. HRMS (TOF MS ES+): detection limit = 3s/k, where s is the standard deviation of the blank
calcd. for C42H56NO3S [M+H]+: 654.3975 and found 654.3979 measurement, and k is the slope.
(M+H)+.
Conflicts of Interest
There are no conflicts to declare.
Gelation test and SEM imaging
Required amount of compound 1 was dissolved in desired solvents
(nitrobenzene and DMF, shown in Table 1S) (1 mL) forming a
homogeneous solution, slightly warmed and then allowed to cool
slowly to room temperature to form a gel. To study the effect of
nitrophenols and metal ions, required amounts of analytes were
added to the nitrobenzene gel of compound 1. All the gels were
tested by an inversion of vial method. Samples of gels for SEM
imaging were dried under vacuum and then coated with a thin layer
of gold metal.
Acknowledgements
SM thanks UGC, New Delhi, Govt. of India for fellowship. RR
thanks DST, India for a DST Inspire fellowship. We also thank
DST, Govt. of India for providing facilities under DST PURSE
(phase II) program.
Determination of gel melting temperature (Tgel
)
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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 Tgel of
that system.
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DOI: 10.1039/C9NJ01282F
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GRAPHICAL ABSTRACT
Cholesterol linked benzothiazole: A versatile gelator for detection of picric acid and metal
ions such as Ag+, Hg2+, Fe3+ and Al3+ under different conditions
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Subhendu Mondal, Rameez Raza and Kumaresh Ghosh*
Cholesterol appended benzothiazole 1 has been designed, synthesized and studied interaction with nitrophenols and bio relevant metal ions in sol-
gel medium. Compound 1 exhibits gelation in different solvents. As application, the nitrobenzene gel selectively recognizes picric acid (PA) over
a series of nitroaromatics by exhibiting gel-to-sol phase transition with distinguishable color change from pale yellow to deep yellow. On the
other hand, the nitrobenzene gel shows selective recognition potential for Ag+ and Hg2+ ions through gel-to-sol phase transition and the cations are
distinguished by F-. The gelator 1 also reveals measurable selective interaction with picric acid over a series of nitroaromatics in CH3CN
containing 1% CHCl3 . In CH3CN containing 1% CHCl3, compound 1 further, shows affinity towards Fe3+, Al3+ and Hg2+ with distinguishing
features.
Metal binding site
O
O
S
O
N
1
Spacer

-surface
Hydrophobic surface
Protonation site
_____________________________________________________________________________________