Copillar[5]arene-rhodamine conjugate as a selective sensor for Hg2+ ions

Article abstract of DOI:10.1039/c9nj06264e

A new copillar[5]arene coupled rhodamine probe 1 has been designed and synthesized. The molecular probe shows selective sensing of Hg²⁺ ions over a series of metal ions in CH₃CN by exhibiting a color change of the solution as well as

Full text of DOI:10.1039/c9nj06264e

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Copillar[5]arene-rhodamine conjugate as selective sensor
for Hg2+ ion
Received 00th January 20xx,
Accepted 00th January 20xx
Saswati Ghosh Roy, Subhendu Mondal and Kumaresh Ghosh*
DOI: 10.1039/x0xx00000x
Abstract: A new copillar[5]arene coupled rhodamine probe 1 has been designed and synthesized. The molecular probe
shows selective sensing of Hg2+ ions over a series of metal ions in CH3CN by exhibiting a color change of the solution as well
as ‘turn-on’ fluorescence. Compound 1 also shows interaction with Cu2+ by exhibiting a different color and spectral
changes. In the event, Hg2+ ion has been differentiated from Cu2+ by using tetrabutylammonium iodide (TBAI). While in the
presence of TBAI the reddish violet color of 1-Cu2+ complex becomes colorless with the regeneration of original electronic
spectrum of 1, under identical conditions the pink color of 1-Hg2+ complex is not discharged in presence of TBAI. Model
compound 2 with no pillar[5]arene moiety has been synthesized to validate the role of pillararene in the selectivity of
metal ion sensing. Importantly, compound 2 does not show any selectivity in the sensing under identical conditions.
Complexation studies are performed by using UV-vis, fluorescence, FTIR and NMR spectroscopic methods.
www.rsc.org/
Introduction
and colorimetric methods with high sensitivity.8-12 In this
The selective recognition and sensing of metal ion by design-
based synthetic receptors or sensors is an important area of
research in supramolecular chemistry.1 In this context, metal
ions that have harmful effects on living organisms or the
environment are considered as important targets in the study.
Among various metal ions, Hg2+ is considered as a toxic and
soft transition metal ion that has serious effect on both
environment and biological systems. Various natural and
human activities such as volcanic eruption, mining, combustion
of fossil fuel,2,3 and also use of mercury in daily used products
such as paints, batteries, electronic gadgets etc., contaminate
our environment.4 Microorganisms can consume mercury from
the environment and transform it into methyl mercury cation
which is responsible for the dysfunctioning of central nervous
system.5 Weather mercury is consumed by human as inorganic
form (Hg2+) or as methyl mercury through food chain; it
creates serious damage in heart, brain, kidney and immune
systems and causes minamata and coronary heart diseases.6,7
In last few decades, much effort has been given towards the
development of suitable chemosensors of various
architectures for the detection of Hg2+ ion. In most of the
cases, the detections have been carried out by fluorometric
context, various macrocycles including crown ethers13 and
calixarenes14 are known for the detection of Hg2+ ion.
However, chemosensors constructed on pillararenes, a new
macrocyclic host, is not known in detecting Hg2+ ion.
Pillararenes, composed of hydroquinone units linked by
methylene bridges at the para positions, exhibit various
unique properties in host-guest chemistry,15 synthesis of
supramolecular ampiphiles16 and in construction of
supramolecular smart materials.17 Their unique pillar like
structures with the possibility of functionalization of
hydroquinone units are attractive in devising receptor modules
for guests.18 Due to the presence of multiple benzene units,
pillararene possesses certain luminescence property. In this
horizon, there are reports on fluorescent detection of
fluoride19,20 and iron21 using pillararene-based chemosensors.
Importantly, pillararene-based chemosensors for detection of
Hg2+ is rare in the literature.22
Therefore, to develop new colorimetric and fluorometric
sensor for Hg2+ ions, attention was focused on pillararene-
rhodamine conjugate. Rhodamine-labeled chemosensors are
interesting because of their switching from the non
fluorescent, colorless spirolactam form to the fluorescent, pink
colored spiroring opened form in presence of selected metal
ion.23-26 In this account, we wish to report a new receptor
molecule 1 in which pillar[5]arene is connected to the
rhodamine moiety. As model study, compound 2 has been
considered to demonstrate the role of pillar[5]arene in 1.
Receptor 1 shows selective detection of Hg2+ ion over a series
of metal ions examined both by ‘turn-on’ fluorescence and
color change in acetonitrile. In contrast, compound 2, devoid
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: Copies of 1H, 13C-NMR,
mass of the compounds 1-5, Binding constant and detection limit curves, FT-IR,
Comparative Table on Hg2+ sensing. See http://dx.doi.org/10.1039/b000000x/
This journal is © The Royal Society of Chemistry 20xx
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of pillararene, did not show any selectivity in the sensing under compound 1 was present almost exclusively in the spirolactam
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identical conditions.
form under the experimental conditionsD. OHIo: 1w0e.1v0e39r,/Ct9hNeJ0U6V2-6v4iEs
study of 1 (c = 2.5 × 10-5 M) as conducted in CH3CN with
several metal ions such as Hg2+, Fe3+, Al3+, Fe2+, Hg2+, Zn2+, Cu2+,
Ni2+, Co2+, Ag+ and Cd2+ ions (c = 2.5 × 10-5 M) exhibited
interesting observation. Addition of Hg2+ ions to the solution of
1 resulted in strong absorption at 556 nm and the solution
became pink in color (Fig. 2). In comparison, other metal ions
except Cu2+ which induced a moderate absorption at 470 nm,
brought neither any significant color change of the solution of
1 nor showed strong absorption at 556 nm (Fig, 2b). However,
during interaction with the metal ions there was differential
decrease in absorption in the region ~300 nm in the UV-vis
spectrum of 1. This indicated the participation of pillararene
moiety in complexation involving peripheral –OCH3 groups.
H
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1
O
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NH
N
O
O
N
Figure 1. Structures of 1 and 2.
Results and discussion
Synthesis
Compounds 1 and 2 were (Fig. 1) synthesized according to the
Scheme 1. Pillar[5]arene-labelled compound 1 was obtained
from the intermediate compound copillar[5]arene 4,27 which
was synthesized from the reaction of 1,4-dimethoxybenzene
with compound 3. Compound 3 was prepared by refluxing the
mixture of 4-methoxyphenol and ethyl bromoacetate in the
presence of K2CO3 in dry acetonytrile.28 Ester group of
copillar[5]arene 4 was hydrolyzed using LiOH.H2O in THF/H2O
(3:1, v/v) to yield the compound 5. Finally, coupling of
copillar[5]arene acid 5 with rhodamine amine 6 gave the
desired compound 1 in 42% yield.
(b)
1.0
1 + Hg2+
0.8
1 + Cu2+
Ag+,Fe2+,Zn2+
Co2+,Ni2+,Cd2+
Al3+,Fe3+
0.6
0.4
0.2
0.0
1
O
O
O
O
O
O
(i)
(ii)
H
H
H
H
H
H
H
H
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400
500
600
Wavelength (nm)
3 O
O
O
O
O
O
4
4
O
5
4
Figure 2. (a) Image of solution of receptor 1 (c = 2.5 × 10-5 M) in CH3CN after the
addition of 5 eqiuv. amounts of different metal ions, (b) change in absorption
spectra of receptor 1 (c = 2.5 × 10-5 M) at 556 nm upon addition of 5 equiv.
amount of different cations (taken as perchlorate salt) in CH3CN.
O
O
O
O
OH
O
NH2
N
(iii)
O
O
NH2
N
O
N
(iv)
6
N
2
1
Thus the appearence of new absorption at 556 nm along with
a sharp color change of 1 selectively in the presence of Hg2+
ions in the present study is useful and impressive for its naked
eye detection. In this regard, the change in absorbance ratio at
556 nm in Fig. 3 clearly displays the selectivity for Hg2+ ions.
Figure 4 represents the UV-vis titration spectra of 1 with the
Hg2+ ions in CH3CN. In the beginning of titration, the change in
absorbance at 556 nm is small due to low concentration of
Hg2+ ions in the medium. However, this gradually becomes
intense and strong while the concentration of Hg2+ ions in the
medium is increased. This occurred upto the addition of 5
equiv. amounts of Hg2+ ions (Fig. 4). No increase in intensity
was noticed upon further addition of Hg2+ ions (inset of Fig. 4).
Such spectral change is attributed to the different interactional
behaviors of 1 toward Hg2+ ions. While at low concentration of
Hg2+ ions the rhodamine moiety in 1 exists almost in its
spirolactam form due to weak interaction, it is then slowly
N
N
6
Scheme 1. (i) (HCHO)n, BF3.(OC2H5)2, ClCH2CH2Cl; (ii) LiOH.H2O, THF:H2O (3:1 v/v); (iii)
EDC.HCl, HOBt, DIPEA, DCM, 30 0C - rt, 24h; (iv) n-Butyryl chloride, Et3N, rt, 24h.
Model compound 2 was obtained from the reaction of
rhodamine amine 629 with n-butyryl chloride in the presence
of Et3N. Both the compounds 1 and 2 were characterized by 1H
NMR, 13C NMR, FT IR and mass analysis.
Metal ion sensing
Metal ion sensing behavior of compound 1 (c = 2.5 × 10-5 M)
was investigated in CH3CN by UV-Vis, fluorescence, FT IR and
1H NMR spectroscopic methods. The solution of 1 (c = 2.5 × 10-
5
M) in CH3CN was colorless and exhibited absorptions at 276
nm and 294 nm in UV-Vis spectrum. This revealed that
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transformed into the ring opened form (i.e., xanthenoid form
1A shown latter in Fig. 6b) with increase in concentration of
Hg2+ ions in the medium.
However, the appearance of absorbance at 470 nm of
compound 1 in presence of Cu2+ ion is importantly assumed to
be due to complexation of Cu2+ involving the distal amide
nitrogen of ethylenediamine unit and spirolactam oxygen. This
is in accordance with the findings reported in the literature on
a different architecture.30
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DOI: 10.1039/C9NJ06264E
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Figure 5. Change in absorption of 1 (c = 2.5 × 10-5 M) with time after its mixing
with 5 equiv. amounts of Hg2+ ion in CH3CN. Insets represent the change in
absorbance at 556 nm with time.
the solution became pink in color. In comparison, the intensity
of this emission band was insignificant in the presence of all
other metal ions examined (Fig. 6a). Only small change in
Figure 3. Change in absorbance ratio of receptor 1 (c = 2.5 ×10-5 M) at 556 nm emission at 557 nm in the presence of Fe3+, Fe2+ and Zn2+ ions
upon addition of 5 equiv amount of different cations (taken as perchlorate salt) (Fig. 6a) due to formation of negligible extent of xanthenoid
in CH3CN (A0 and A are the absorbances of 1 before and after addition of metal form 1A (Fig. 6b) was observed although there was no color
ion, respectively).
change of the probe solution. Fig. 7 displays the fluorescence
titration profile of 1 with Hg2+ ion. Like UV-vis in Fig. 4, the
initial change in emission at 573 nm is observed to be small
while concentration of Hg2+ ions in the medium is less.
However, on increase in concentration of Hg2+ ions, the
intensity of this emission gradually increases and reaches to a
(a)
800
1 + Hg2+
600
Fe2+, Zn2+, Fe3+
Ag+,Al3+,Cu2+
Co2+,Ni2+,Cd2+
400
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0
1
Figure 4. Absorption titration spectra of compound 1 (2.5 × 10-5 M) with Hg2+ ion
in CH3CN (measured after 40 min in each addition). Inset represents the change
in absorbance of 1 at 556 nm with concentration of Hg2+ ions.
560
600
640
680
Wavelength (nm)
To realize the slow reaction of the probe with Hg2+ to form the
xanthenoid form, a time-dependent change in absorption of
the probe 1 in the presence of 5 equiv. amounts of Hg2+ was
followed. The colorless solution of 1 (c = 2.5 × 10-5 M) was
turned into pink within 5 minutes upon addition of 5 equiv.
amounts of Hg2+ at a time. The pink color of the solution was
then gradually intensified with time. The plot of absorbance at
556 nm with time as shown in Fig. 5 reveals that the increase
in absorbance reaches the plateau within 40 mins.
O
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(b)
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Like UV-vis, fluorescence study of 1 was performed with the
said metal ions in CH3CN. In the absence of metal ions,
compound 1 was non-fluorescent and displayed no emission
1A
1
N
N
above 550 nm when excited at 510 nm. However, upon Figure 6. (a) Change in emission of 1 (c = 2.5 × 10-5 M, λexc = 510 nm) at 573 nm
interaction with only Hg2+ ions, the probe 1 gave a non upon addition of 5 equiv amounts of different metal ions (taken as perchlorate
structured emission at 573 nm with significant intensity and
salt) in CH3CN; (b) Suggested mode of binding of 1 with Hg2+
.
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stage of saturation due to predominance of the ring-opened M-1.31 Sensitivity of 1 towards Hg2+ was determined from
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DOI: 10.1039/C9NJ06264E
xanthenoid form 1A in the equilibrium, shown in Fig. 6b.
fluorescence and the calculated detection limit was observed
to be 2.85 × 10-8 M (Fig. 15).32
To support the binding mode of interaction of compound 1 as
shown in Fig. 6(b), FTIR and 1H NMR studies were performed. A
comparison of FTIR spectrum of compound 1 (Fig. S16a) with
its Hg2+-complex (Fig. S16c) showed the reduction in stretching
frequency of the lactam amide from 1681 cm-1 to 1588 cm-1
(Fig. S16a), which indicated the opening of spirolactam ring. In
comparison, the stretching band at 1681 cm-1 of lactam amide
remained unchanged in the presence of Cu2+ (Fig. S16b) and
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confirmed the intactness of the spiro ring. In H NMR, the
rhodamine ring protons such as He, Hf and Hg became broad
and moved to the downfield directions upon complexation of
Hg2+. Moreover, the protons close to the lactam ring such as
Ha, Hc and Hd exhibited the positional movement during
interaction and thus supported the proposed binding mode,
shown in Fig. S17.
To understand the interference of the metal ions in the
sensing of Hg2+ ion, competitive experiment was carried out by
adding 5 equiv. amounts of Hg2+ to the solution of 1 in CH3CN
containing 5 equiv. amounts of other metal ions (Fig. 9). Color
change of the solution of 1 from colorless to pink was
observed in presence of all other metal ions examined except
Cu2+. In presence of Cu2+ ion, compound 1 failed to detect Hg2+
ion as the reddish violet color of Cu2+ complex obscured the
pink color.
Figure 7. Fluorescence titration spectra of 1 (2.5 × 10-5 M) with Hg2+ ion in CH3CN
(measured after 40 min in each addition). Inset represents the change in
emission at 573 nm with concentration of Hg2+ ions.
Thus both UV-vis and fluorescence studies highlight that
compound 1 is selective to Hg2+ ion and in this selectivity; we
assume a pivotal role of pillararene. Pillararene being closely
spaced to the rhodamine motif presumably does not allow all
the metal ions except Hg2+ to come in contact with the
spirolactam ring to pursue ring opening reaction. Perhaps the
methoxy (-OCH3) groups of pillararene bind the metal ions and
hindered them to interact with rhodamine moiety.21 This was
clarified by considering the model compound 2 which took
part in ring opening reaction with a series of metal ions such as
Fe3+, Hg2+, Al3+ and Cu2+ ions (Fig. 8). Sensing behavior of 2 with
different metal ions was studied by absorption spectroscopy
under identical conditions. Addition of 1 equiv. amount of the
metal ions to the solution of 2 in CH3CN resulted in pink
coloration and introduced new absorption peak at 552 nm
without giving any selectivity.
Compitingmetal ion
Compitingmetal ion + Hg2+
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20
0
1.5
Fe3+
Al3+
Hg2+
1.2
0.9
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Zn2+
Figure 9. Change in fluorescence ratio of 1 (c = 2.5 × 10-5 M) with Hg2+ in absence
and presence of 5 equiv. amounts of other metal ions (taken as perchlorate salt)
in CH3CN (I0 and I are the emissions of 1 before and after addition of metal ion,
respectively).
300
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500
600
Wavelength (nm)
To overcome the interference of Cu2+ ion, a complexation
experiment was carried out. While in the presence of
tetrabutylammonium iodide (TBAI) the reddish violet color of
1-Cu2+ complex became colorless with the regeneration of
original UV-vis spectrum of 1 (Fig. 10a), under similar
conditions the pink color of 1-Hg2+ complex was not discharged
in the presence of TBAI (Fig. 10b). This occurred due to the
formation of stable CuI2 complex. After complexation of Cu2+
ion, addition of Hg2+ led to the enhancement of fluorescence
intensity (Fig. 10a). Therefore, compound 1 behaved as a
selective chemodosimeter for Hg2+.
Figure 8. Change in absorption of compound 2 (c = 2.5 × 10-5 M) upon addition of
1 equiv amount of different metal ions (taken as perchlorate salt) in CH3CN.
Stoichiometric interaction of the probe 1 with Hg2+ ions was
determined from Benesi-Hilderband plot. From UV-Vis
titration experiment, the values of 1/(A-A0) at 556 nm versus
1/[Hg2+] showed good linear relationship with R = 0.996, which
confirmed 1:1 complex formation between 1 and Hg2+ (Fig.
S14). The association constant for 1 with Hg2+ ions as
determined from Benesi-Hilderband equation was 4.67 × 103
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Finally, to test the applicability of the compound in aqueous can be seen from Fig. 11, the rhodamine is closely spaced to
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sensing of Hg2+, the UV-vis study of 1 with the said metal ions the pillararene and there are some intramolecular H-bonds
DOI: 10.1039/C9NJ06264E
was conducted in CH3CN/H2O (3:1, v/v, 10 mM Tris-HCl Buffer, that impart some rigidity to the molecular framework. Such H-
pH = 6.5) (Fig. S18). In the event, there was no change in color bonding association may be the cause to make it reluctant to
and UV-vis spectra as well. However, compound 1 did not work instantaneously with Hg2+ ions.
work even in presence minor amount of water in organic
Conclusion
solvent. We are currently working on this to find out the
reason. At this moment, we believe that the non responsive
behavior of the compound 1 in CH3CN/H2O is associated with
its hydration and the relative alignments of the rhodamine and
pillararene motifs that do not permit metal-induced spiroring
opening.
In summary, rhodamine-appended copillar[5]arene 1 has been
designed and synthesized. Compound 1 successfully recognizes
Hg2+ ions selectively over a series of other different metal ions
studied in CH3CN by exhibiting a change in color from colorless
to pink via spirolactam ring opening and validates the visual
sensing with detection limit of 2.85 × 10-8 M. In fluorescence,
compound 1 shows significant ‘turn-on’ emission in presence
of Hg2+ ions. On the other hand, it exhibits color change from
colorless to reddish violet in presence of Cu2+ following a
different mechanism of interaction. However, the distinction
of Hg2+ from Cu2+ has been possible with the aid of TBAI.
In structure 1, the effect of pillararene in bringing selectivity
has been understood by considering the model compound 2.
Compound 2 with no pillararene unit shows spiroring opening
reaction with a number of metal ions giving no selectivity. Thus
the present example in this report is a new addendum in
pillararene chemistry where its function with rhodamine dye
has been explored to understand the metal ion sensing
behavior. Experimental findings reveal that compound 1 works
well in comparison to the reported rhodamine-based Hg2+
sensors (Table S1). It is further mentionable that although Hg2+
ion sensing using compound 1 is limited in organic medium,
the importance of the present report as that of other reported
systems for Hg2+ detection in purely organic medium34 cannot
be ignored. Further work along this direction is underway in
the laboratory.
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Figure 10. Change in absorbance of 1 (c = 2.5 × 10-5 M) in the presence of (a) Cu2+
and (b) Hg2+ followed by addition of TBAI in CH3CN to distinguish Hg2+ from Cu2+
.
Experimental
In order to understand the structural feature, DFT optimization
of 1 was done using 6-31(G) basis set and b3lyp function.33 As
Material
4-Methoxyphenol, 1,4-dimethoxybenzene, boron trifluoride
etherate (BF3.Et2O), paraformaldehyde (HCHO)n,) were purchased
from Alfa Aesar and used as received. Ethyl bromoacetate, lithium
hydroxide monohydrate (LiOH.H2O), N-(3-Dimethylaminopropyl)-N′-
ethylcarbodiimide hydrochloride (EDC.HCl), Hydroxybenzotriazole
(HOBt) and rhodamine B were purchased from Sigma-Aldrich and
used without further purification. N,N-Diisopropylethylamine
(DIPEA),
triethyl amine (Et3N), dichloromethane (DCM),
dichloroethane (DCE) and acetonitrile (CH3CN) were brought from
Rankame and were dried following the standard literature
processes. Perchlorate salts of Ag2+, Cu2+, Pb2+, Co2+, Ni2+, Cd2+, Zn2+,
Hg2+and Al3+, Fe2+ and Fe3+ were purchased from Sigma-Aldrich.
CDCl3 (99.8% D), DMSO-d6 (99.8% D) were brought from Cambridge
Isotope Laboratories, Inc., USA for NMR study. The solvents such as
pet ether, ethyl acetate, and acetonitrile were purified by standard
procedures.
Instrumentation
1H and 13C NMR spectroscopic studies were done on a Bruker
Avance IIII 400 spectrometer operating at 400 MHz. Mass
Figure 11. DFT optimized geometry of 1 (HF = -4143.7084525 au) in CH3CN
solvent using b3lyp function and 6-31g (d,p) basis set.
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spectrometry (ESI-MS) was conducted on a high resolution Agilent 2036, 1758, 1730, 1611, 1501, 1465, 1400, 1306, 1279, 1212, 1046,
View Article Online
DOI: 10.1039/C9NJ06264E
6535 Q-TOF LC/MS mass spectrometer. The FT-IR spectrum was 929, 878, 854, 799, 773 and 733. ESI-MS: m/z calculated for
recorded on KBr pellets using a Perkin-Elmer Spectrum L120-00A C48H54O12Na ([M+Na]+), 846.35; found 846.45 (40%).
FT-IR Spectrometer. UV-VIS spectra were recorded in Shimadzu UV-
2450 Fluorescence spectroscopic study was conducted using Compound 5
HORIBA FluoroMax-4C.
Compound 4 (0.6g, 0.7435mmol) was dissolved in 20 mL THF:H2O
(3:1, v/v) mixture. Then LiOH.H2O (23.95 mg, 2.9741 mmol) was
added to the reaction vessel and the mixture was stirred at room
temperature for 24 h. The organic layer was removed under
Synthesis
Compound 3
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Potassium carbonate (16.7g, 0.1208 mol) was added to the solution reduced pressure. Water was added to the reaction mixture and
of 4-Methoxyphenol (10g, 0.08055 mol) in 150 mL of dry CH3CN and stirred for 30 min. White precipitate that appeared was collected by
the solution was stirred at room temperature for 30 min. Then, filtration and purified by column chromatography first by ethyl
ethyl bomoacetate (20.2g, 0.1208 mol) was added to this solution acetate: petroleum ether (1/1, v/v) and then with 100 % acetone to
and the solution was refluxed at 80 oC for two days. Reaction give pure compound 5 in 87% yield (0.52 g) as white solid, mp: 190
mixture was cooled at room temperature and salts were removed oC. 1H NMR (400 MHz, CDCl3) δ 6.78-6.57 (m, 10H, ArH), 4.30 (s, 2H,
by filtration. Filtrate was concentrated under reduced pressure and –ArOCH2COO–), 3.78-3.55 (m, 37H, Ar–OCH3 and 10H, Ar–CH2–Ar)
to the residue 150 mL of water was added. Compound was ppm. 13C NMR (100 MHz, CDCl3,) δ 151.08, 150.63, 150.49, 150.22,
extracted with DCM (100 mL x 5). The combined organic layer was 128.65, 128.41, 128.21, 128.00, 127.85, 113.78, 56.19, 55.71, 55.55,
washed with saturated brine solution and dried over anhydrous 39.39, 31.58, 29.77, 29.47 ppm (signal for –COOH is not observed
Na2SO4. Then solvent was removed under reduced pressure. Finally, due to solubility issue). FT-IR (cm-1): 3416, 2931, 1930, 1735, 1633,
the crude product was purified by column chromatography using 1498, 1463, 1400, 1307, 1266, 1211, 1164, 1117, 1047, 881 and
1:9 ethyl acetate/pet ether (v/v) solvent mixture to give compound 772.
1
3 in 88 % yield (15.0 g) as white low melting solid. H NMR (400
MHz, CDCl3) δ 6.89-6.82 (m, 4H, –C6H4–), 4.57 (s, 2H, –OCH2–COO–), Compound 1
4.28 (q, 2H, –OOC–CH2–CH3, J = 8 Hz), 3.77 (s, 3H, –OCH3) and 1.30 To a solution of compound 5 (200.00 mg, 0.224 mmol) in 10 mL dry
(t, 3H, –OOC–CH2–CH3, J = 8 Hz) ppm. 13C NMR (100 MHz, CDCl3) δ DCM containing DIPEA (78 µL, 0.224 mmol) at 0 C, EDC.HCl (85.88
o
169.18, 154.46, 151.97, 115.78, 114.59, 66.18, 61.21, 55.54, 14.13 mg, 0.448 mmol), HOBt (60.53 mg, 0.448 mmol) were added. After
ppm.
stirring for 10 min, compound 6 (139.7 mg, 0.269 mmol) was added.
Reaction mixture was stirred at 0 oC for 1h and then at room
temperature for 24h. After completion of reaction, water was
Compound 4
Compound 3 (1g, 4.775 mmol) and 1,4-dimethoxy benzene (2.63g, added to the reaction mixture and organic layer was separated.
19.02 mmol) were taken in 250 mL double necked round bottom Organic layer was dried over anhydrous Na2SO4 and solvent was
flask equipped with a bar magnet and dissolved in 150 mL dry 1,2- removed under reduced pressure. The resulting residue was
dichloroethane. Paraformaldehyde (2.1g, 13.3mmol) was added to purified by column chromatography to give white solid powder of
the reaction mixture. The flask was placed over ice-water bath and compound 1 (85 mg, yield: 42%), mp: 52 oC. 1H NMR (400 MHz,
purged with dry N2 for 30 min. Then BF3.Et2O (6.73g, 47.552 mmol) CDCl3) δ 7.80-7.79 (d, 1H, ArH), 7.41-7.38 (t, 2H, ArH), 7.04-7.02 (d,
was added dropwise with continuous stirring for 15 min. Ice-water 1H, ArH), 6.76-6.65 (m, 10H, ArH), 6.38-6.35 (t, 3H, ArH), 6.21-6.19
bath was removed and the reaction mixture was left stirring for (t, 3H, ArH), 4.19 (s, 1H, Ar–O–CH2–COO–), 3.95 (s, 1H, –OOC–NH–
another 45 min at 25 oC. Then CH3OH (50 mL) was added and stirred CH2–), 3.80-3.38 (m, 27H, Ar-OCH3 and 10H, Ar-CH2-Ar), 3.35-3.25
for 5 min to quench the reaction. Reaction mixture was filtered and (m, 2H, –OOC–NH–CH2–CH2– and 8H, N(CH2CH3)2), 3.12-3.11 (d, 2H,
precipitate was washed with DCM. Then the combined organic layer –OOC–NH–CH2–CH2–), 1.14-1.11 (t, 12H, N(CH2CH3)2) ppm. 13C NMR
was washed successively with water (100 mL × 2), saturated (100 MHz, CDCl3,) δ 169.32, 169.16, 153.77, 153.25, 151.41, 151.13,
NaHCO3 solution (100 mL × 3) and saturated brine solution (100 150.86, 150.78, 150.73, 148.84, 148.79, 132.58, 130.49, 128.54,
mL×2). Organic layer was dried over anhydrous Na2SO4 and solvent 128.39, 128.19, 128.07, 128.03, 127.97, 127.88, 123.73, 123.00,
was removed under reduced pressure. Finally, the resultant residue 114.92, 114.77, 114.32, 114.13, 114.04, 113.99, 113.88, 113.76,
was purified by column chromatography using CH2Cl2: petroleum 108.23, 104.99, 97.68, 68.01, 65.20, 56.27, 55.83, 55.75, 55.67,
ether (1:1, v/v) as eluant to give compound 4 in 19% yield (0.7 g) as 44.30, 39.51, 39.34, 29.86, 29.73, 29.58, 29.38 and 12.58 ppm. FT-IR
o
1
white solid, mp: 166 C. H NMR (400 MHz, CDCl3) δ 6.84-6.50 (m, (cm-1): 3406, 2929, 2852, 2828, 1681, 1633, 1614, 1542, 1502, 1466,
10H, ArH), 4.43 (s, 2H, Ar–OCH2–CO–), 3.82-3.54 (m, 27H, Ar–OCH3 1427, 1399, 1365, 1327, 1306, 1265, 1212, 1152, 1118, 1046, 926,
& 10H, Ar–CH2–Ar), 2.15 – 2.10 (Ar–OCH2–COO–CH2–CH3) and –1.50 878, 853, 819, 783 and 757. ESI-MS: m/z calculated for
to –1.53 (Ar–OCH2–COO–CH2–CH3) ppm. 13C NMR (100 MHz, CDCl3) C76H84N4O13Na [M+Na]+, 1283.59; found 1283.59 (30% of the main
 169.13, 151.19, 150.74, 150.69, 150.40, 150.37, 150.11, 149.90, peak).
149.54, 129.22. 129.10, 128.93, 128.60, 128.50, 128.38, 128.01,
127.72, 127.18, 127.16, 115.12, 115.04, 114.04, 113.91, 113.77, Compound 2
113.35, 113.31, 113.04, 112.64, 112.38, 64.83, 60.61, 56.40, 55.98, Butyryl chloride (320 µL, 3.093 mmol) was added dropwise to the
55.91, 55.83, 55.80, 55.68, 55.55, 55.45, 55.19, 31.69, 30.28, 28.98, solution of compound 6 (0.5g, 1.031 mmol) in 20 mL dry DCM
28.72, 27.33, 10.98 ppm. FT-IR (cm-1): 3439, 2988, 2933, 2828, containing Et3N (431 µL, 3.093 mmol) in inert atmosphere at 0 – 5
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oC under constant stirring. Then the reaction mixture was stirred at
Conflicts of Interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/C9NJ06264E
room temperature for overnight and filtered. Organic layer was
collected and washed with water followed by saturated brine
solution. Organic layer was dried over anhydrous Na2SO4 and
removed under reduced pressure. The resulting residue was
purified by column chromatography using ethyl acetate : petroleum
ether solvent mixture (2:3, v/v) to give compound 2 as pinkish white
solid (0.3g, yield: 60%), mp: 96 oC. 1H NMR (400 MHz, CDCl3) δ 7.91-
7.89 (q, 1H, ArH ), 7.47-7.44 (q, 2H, ArH), 7.09-7.07 (q, 1H, ArH),
6.83 (s, 1H, –NH–CO–), 6.43-6.37 (m, 4H, ArH), 6.28-6.26 (d, 2H,
ArH), 3.36-3.28 (m, 10H, 8H from –NH(CH2CH3) and 2H from –
NCH2CH2NHCO–), 3.07-3.03 (m, 2H, –NCH2CH2NHCO–), 1.63-1.59 (t,
2H, –NHCOCH2CH2–), 1.27-1.24 (t, 2H, –NHCOCH2CH2–), 1.18-1.15
(m, 12H, –NH(CH2CH3) and 0.90-0.88 (t, 3H, –NHCOCH2CH2CH3)
ppm. 13C NMR (100 MHz, CDCl3) δ 173.26, 169.94, 153.81, 153.26,
148.90, 132.81, 130.38, 128.40, 128.19, 123.89, 122.80, 108.23,
104.66, 97.77, 65.70, 44.34, 40.45, 40.22, 38.58, 19.10, 13.75 and
12.57. FT-IR (cm-1): 3319, 3079, 2967, 2930, 2871, 1676, 1634, 1614,
1546, 1514, 1467, 1448, 1426, 1376, 1356, 1328, 1304, 1264, 1231,
1218, 1151, 1117, 1089, 1077, 1015, 866, 818, 785, 756, 714 and
702. ESI-MS: m/z calculated for C34H43N4O3H [M+H]+, 555.33; found
555.33.
Acknowledgements
Saswati Ghosh Roy gratefully acknowledges the financial
support [NPDF Project: PDF/2015/000982] from The Science
and Engineering Research Board (SERB) of Department of
Science and Technology (DST), New Delhi, India. K.G. also
thanks SERB, New Delhi, India for financial support (File No.
EMR/2016/008005/OC).
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General procedures for fluorescence and UV-vis titrations
Stock solution of compound 1 was prepared in CH3CN in the
concentration range of ~ 10−5 M. An amount of 2 mL of the solution
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perform the UV-vis titration experiment. Solution of the metal ion
was successively added in different amounts to the solution of 1 (2
mL) taken in the cuvette and the absorption spectra were recorded.
Both fluorescence and UV-vis titration experiments were carried
out at 25 °C.
Binding constant determination from fluorescence31
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using the expression: I0/(I-I0) = [εM /(εM – εC)](Ka -1Cg-1 +1), where εM
and εC represent molar extinction coefficients for compound 1 and
the complex, respectively, at a selected wavelength, I0 denotes the
emission intensity of free compound 1 at that specific wavelength
and Cg is the concentration of the metal ion. The measured
emission I0/(I-I0) as a function of the inverse of the metal ion
concentration fits a linear relationship, indicating 1:1 stoichiometry
of the compound 1- metal ion complex. The ratio of the intercept to
the slope was used to determine the binding constant Ka.
Calculation of detection limit32
Detection limit was calculated by using fluorescence titration data.
The emission of compound 1 was measured 6 times, and the
standard deviation of blank measurement was determined. To have
the slope, emission intensities were plotted against concentrations
of metal ions. The detection limits were calculated using the
equation: detection limit = 3s/k, where s is the standard deviation
of the blank measurement, and k is the slope.
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DOI: 10.1039/C9NJ06264E
Graphical Abstract
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__________________________________________________________________________________
Copillar[5]arene-rhodamine conjugate as selective sensor for Hg2+ ion
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Saswati Ghosh Roy, Subhendu Mondal and Kumaresh Ghosh*
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A new copillar[5]arene coupled rhodamine probe 1 has been
designed and synthesized. The molecular probe shows selective
sensing of Hg2+ ions over a series of metal ions in CH3CN by
exhibiting color change of the solution as well as turn on
fluorescence. Compound 1 also shows interaction with Cu2+ by
Hg2+
Fluorescence
“ON”
Fluorescence
“OFF”
Hg2+
exhibiting
a different color and spectral changes. Using
tetrabutylammonium iodide (TBAI) Hg2+ ion has been
differentiated from Cu2+ ions.
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