Selective sensing of Cu (II) by a simple anthracene-based tripodal chemosensor

Article abstract of DOI:10.1080/10610278.2010.544735

A new and an easy-to-make simple tripodal shaped chemosensor 1 has been designed and synthesised for metal ions. In CH₃CN containing 0.04% DMSO, upon excitation at 370nm, chemosensor 1 exhibited structured emission centred at 418nm, which incre

Full text of DOI:10.1080/10610278.2010.544735

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Supramolecular Chemistry
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Selective sensing of Cu (II) by a simple anthracene-
based tripodal chemosensor
Kumaresh Ghosh a & Tanmay Sarkar a
a Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, India
Version of record first published: 15 Apr 2011.
To cite this article: Kumaresh Ghosh & Tanmay Sarkar (2011): Selective sensing of Cu (II) by a simple anthracene-based
tripodal chemosensor, Supramolecular Chemistry, 23:6, 435-440
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Supramolecular Chemistry
Vol. 23, No. 6, June 2011, 435–440
Selective sensing of Cu (II) by a simple anthracene-based tripodal chemosensor
Kumaresh Ghosh* and Tanmay Sarkar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
(Received 2 September 2010; final version received 11 November 2010)
A new and an easy-to-make simple tripodal shaped chemosensor 1 has been designed and synthesised for metal ions. In
CH3CN containing 0.04% DMSO, upon excitation at 370 nm, chemosensor 1 exhibited structured emission centred at
418 nm, which increased to a significant extent upon complexation of Cu (II). The other metal ions except Zn2þ and Hg2þ
examined in this study did not exhibit any marked change in emission of 1 under a similar condition. Although Cu2þ ion
showed strong interaction with 1, Zn2þ and Hg2þ ions exhibited moderate binding.
Keywords: anthracene-based sensor; benzimidazole-based chemosensor; Cu (II), Zn (II) and Hg (II) metal ion recognition;
fluorescence enhancement
The development of fluorescent chemosensors capable of
selective recognition of metal ions is an active field of
research in supramolecular chemistry (1). Among the
heavy metal ions in the human body, copper ion is the third
in abundance after Fe2þ and Zn2þ ions and, thus, draws
significant attention for its recognition (2). Copper plays a
pivotal role in a variety of fundamental physiological
processes in organisms ranging from bacteria to mammals.
This metal ion causes environmental pollution and also
serves as a catalytic cofactor for a variety of metalloen-
zymes (3). However, exposure to high levels of copper,
even for a short period of time, can cause gastrointestinal
disturbance, whereas long-term exposure can lead to liver
or kidney damage (4). Thus, a number of fluorescent
sensors for Cu2þ have been synthesised and reported (2). In
major cases, Cu2þ-induced fluorescence quenching is
observed. But the fluorescent chemosensors that display
fluorescence enhancements upon addition of Cu2þ ion are
very few in numbers (5, 2k). In this context, in continuation
of our previous work (6), we report, here, the synthesis and
the metal binding studies of a simple and easy-to-make
receptor 1 that shows selective sensing of Cu2þ by
exhibiting significant enhancement in emission of CH3CN
containing 0.04% DMSO. Although Cu2þ ion showed
strong interaction with 1, other important cations such as
Zn2þ and Hg2þ ions exhibited moderate binding.
Sensing of Hg2þ and Zn2þ is also equally important
like Cu2þ. Hg2þ is a well-known pollutant. Its exposure to
human body can lead to neurological diseases (7a,b). On
the other hand, zinc ion is an abundant component in the
human body and plays an important role in various
fundamental biological processes, such as gene transcrip-
tion, regulation of metalloenzymes, mammalian reproduc-
tion and neural signal transmission (7c). Therefore,
sensing of such multiple metal ions by a single
chemosensor is attractive. The present example in this
article belongs to this class, although chemosensor 1 is
more effective in sensing Cu2þ
.
Chemosensor 1 was synthesised according to Scheme
1. Our previously reported 4-anthracenyl substituted
morpholin-2-one 5 (6) was subjected to the reaction with
amine 8 which was obtained starting from Boc-protected
glycine after performing a series of reactions as mentioned
in Scheme 1(b). Compound 2 was obtained according to
our previously reported method (6). All the compounds
were characterised by 1H NMR, 13C NMR, FTIR and mass
analysis (supporting data).
The ability of chemosensor 1 towards sensing of
different metal ions was investigated in CH3CN containing
0.04% DMSO using fluorescence and UV–vis spectro-
scopic tools. Additionally, 1H NMR was studied in CDCl3
containing DMSO to understand the nature of interaction
in the binding site.
Figure 1 shows the change in emission of 1 in the
presence of 15 equiv. amounts of the different metal ions in
CH3CN containing 0.04% DMSO on excitation at 370 nm.
It is evident from Figure 1 that the receptor is more
sensitive to Cu2þ ion. Other metal ions except Zn2þ and
Hg2þ merely perturbed the emission of 1. Upon gradual
OH
N
N
N
H
N
N
H
O
1
2
*Corresponding author. Email: ghosh_k2003@yahoo.co.in
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2010.544735
http://www.informaworld.com

436
K. Ghosh and T. Sarkar
a)
OH
(ii)
(iii)
(i)
(iv)
H2N
1
77%
77%
N
87%
OH
90%
CHO
N
N
H
O
OH
3
4
5
O
O
HN
NH
NH2
O
O
b)
O
O
H
H
N
HN
CO2H
NH2
N
HN
(vii)
95%
(v)
(vi)
O
O
65.4%
N
88.5%
N
8
7
6
Scheme 1. Reagents and conditions: (a) – (i) Dry MeOH, reflux, 9 h; (ii) NaBH4, dry MeOH, reflux, 3 h; (iii) ethyl 2-chloroethanoate,
K2CO3, dry Me2CO, reflux, 7 h; (iv) amine 8, THF, stirring, 4 h; (b) – (v) o-phenylenediamine, DCC, DMAP, stirred in CH2Cl2 for 19 h;
(vi) AcOH, heat, 2 h and (vii) 50% TFA in CH2Cl2, stirred for 3 h.
70
60
50
40
30
20
10
0
250
200
150
100
50
Cd(II) Co(II) Cu(II) Fe(II) Mn(II) Hg(II) Ni(II) Pb(II) Zn(II) Mg(II)
Figure 1. Change in fluorescence ratio of
1
0
(c ¼ 4.10 £ 1025 M) upon addition of 15 equiv. amounts of
400
450
500
550
cations at 418 nm.
Wavelength (nm)
Figure 2. Change in emission of 1 (c ¼ 4.10 £ 1025 M) upon
addition of Cu(ClO4)2 solution to the solution of 1
(c ¼ 4.10 £ 1025 M) in CH3CN containing 0.04% DMSO,
the structured emission centred at 418 nm increased to the
significant extent without showing any other change in the
emission spectra. During progression of titration, a red
shift of 3 nm of the emission peak at 418 nm was observed.
Figure 2, in this regard, shows the change in emission
titration spectra with Cu2þ ions. Other metal ions except
Zn2þ and Hg2þ revealed relatively insignificant responses
in this region, manifesting the pronounced OFF–ON type
of Cu2þ selectivity of 1. In this connection, it is well
known that Cu2þ is a paramagnetic ion with an unfilled d-
shell and could strongly quench the emission of the
fluorophore near it via electron or energy transfer. But in
our case, the enhancement of emission of 1 after binding of
Cu2þ ion is quite interesting and mentionable along with
other few related existing systems (5, 2k,n). To our
opinion, such significant change in emission of 1 in the
presence of Cu2þ is attributed to the better coordination of
Cu2þ by aliphatic amine nitrogen, benzimidazole ring
nitrogen and amide, alcohol functionalities in the open
cavity for which either the change in geometry or the
flexibility of the molecule is reduced. The consequence of
gradual addition of Cu(ClO4)2.
which the metal–fluorophore interaction is modulated
(5a,b) and the photo-induced electron transfer (PET) from
the binding sites to the excited state of anthracene is
inhibited. In this context, the role of amide carbonyl as
blocking moiety for PET cannot be ruled out (8a–c). This
is also true for Zn2þ and Hg2þ ions. The difference is in
only lying with the extent of change. We believe that it is
due to the difference in coordinating ability of the binding
site towards Cu2þ, Zn2þ and Hg2þ
.
In the interaction process, the stoichiometry of the Cu-
complex, Zn-complex and Hg-complexes was found to be
1:1, as evident from the break of the titration curve (Figure
3) and also job plots (9) (Figure 4 for Cu2þ and see
supporting data for Zn2þ and Hg2þ). From the titration
data, association constants (Ka) for the formation of 1-
Cu2þ and 1-Hg2þ complexes were estimated by the
nonlinear curve-fitting procedure (10) and found to be
(7.16 ^ 0.473) £ 103 M21 and (5.67 ^ 0.88) £ 102 M21
,
respectively. We were unable to fit the titration data for
Zn2þ by the nonlinear curve-fit method to determine the
binding constant value. In addition, we determined the

Supramolecular Chemistry
437
Cu2+
250
200
150
100
50
Zn2+
Hg2+
Pb2+, Mn2+, Fe2+, Ni2+
Cd2+, Co2+
Mg2+
Figure 5. Fluorescence enhancement factor (Z) of receptor 1
(c ¼ 4.10 £ 1025 M) upon addition of 15 equiv. amounts of
various metal ions.
0
0
1
2
3
4
5
6
[G]/[H]
adding 5 equiv. amounts of Cu2þ in the presence of 5
equiv. amounts of other metal ions, examined in this study.
Figure 6 displays the relative view on the change in
emission of 1 in the presence of Cu2þ when the other metal
ions are absent and present in the receptor solution. Small
increase in emission upon addition of Cu2þ to the solution
of 1 containing other metal ions (Figure 6) demonstrates
the lower selectivity in the binding process. When the
same experiment was done in the absence of Zn2þ and
Hg2þ ions, a better increase in emission of 1 was noted
(supporting data). This underlined the fact that both Zn2þ
and Hg2þ ions interfere in the binding of Cu2þ towards the
binding site of 1.
Furthermore, to investigate the effect of the counter
anion of copper salt in the binding process, change in
emission of 1 in CH3CN containing 0.04% DMSO was
recorded upon gradual addition of Cu(NO3)2 solution
(supporting data). It was noted that the change in emission
of 1 upon addition of Cu(NO3)2 was identical to that of
Cu(ClO4)2. This experimental finding reveals that in the
binding of Cu2þ into the tripodal core of 1, the counter
anion has no effect.
Figure 3. Fluorescence titration curves ([guest]/[host] vs.
change in emission) of 1 (c ¼ 4.10 £ 1025 M) measured at
418 nm in CH3CN containing 0.04% DMSO.
1.2x10–5
8.0x10–6
4.0x10–6
0.0
0.2
0.4
0.6
XH
0.8
1.0
Figure
4. UV–vis
job
plot
for
receptor
1
([H] ¼ [G] ¼ 4.45 £ 1025 M) with Cu2þ at 365 nm.
fluorescence enhancement factor1(relative indicator of
binding strength) of 1 at an emission of 418 nm in the
presence of 15 equiv. amounts of each metal ion in CH3CN
containing 0.04% DMSO to realise the selectivity in the
binding process. Figure 5, in this regard, shows the plot of
fluorescence enhancement factor (Z) in the presence of
different cations. From the plot, it is evident that the
response of the simple sensor 1 towards Cu2þ is significant
and also selective over Zn2þ and Hg2þ ions. In relation to
this, we established earlier that compound 2(6) was
incapable of showing sensing selectivity towards the same
metal ions and explained the necessity of the presence of
proper functionalities around the tripodal nitrogen centre
of 2.
Receptor 1 + 5 equivalent amounts of
Co2+, Cd2+, Fe2+, Ni2+, Pb2+, Mg2+, Hg2+
60
50
40
30
20
10
0
,
Mn2+, Zn2+ + 5 equivalent amounts of Cu2+ ions.
Receptor 1 + 5 equivalent
amounts of Co2+, Cd2+, Fe2+
,
Ni2+, Pb2+, Mg2+, Hg2+, Mn2+, Zn2+ ions
Receptor 1
400
450
Wavelength (nm)
500
Figure 6. Change in emission of receptor
1
(c ¼ 4.10 £ 1025 M) upon addition of Cu2þ in the presence of
However, to comprehend the selectivity in the sensing
of Cu2þ by 1, we recorded emission spectra of 1 upon
other metal ions.

438
K. Ghosh and T. Sarkar
To get insight into the enhancement of emission
the charge transfer occurring in between the benzimidazole
ligand and metal ion. It is noteworthy that this change in
colour is pronounced in the high concentration level of 1
behaviour in the 1-Cu2þ complex, we investigated the
fluorescence decay behaviour of 1 itself and in the
presence of equivalent amount of Cu2þ ion (Figure 7).
Table 1 summarises the exponential fit results. Compound
1 (c ¼ 4.10 £ 1025 M) exhibited a triexponential fluor-
escence decay. Upon addition of equivalent amount of
Cu2þ (c ¼ 8.21 £ 1024 M), while the fast decay com-
ponents were less affected, the time constant (t2) of the
lifetime decay component was greatly affected and
contributed to the total emission with a larger pre-
exponential factor.
(,1023 M) (Figure 8). At a concentration level ,1025
M
of 1, no change in colour was noticeable. However, at the
concentration range ,1023 M of 1, we carried out the
fluorescence titration upon gradual addition of Cu2þ ion
(supporting data). It is mentionable that the Cu2þ-induced
change in emission of 1 was less than the change in CH3CN
containing 0.04% DMSO and thereby demonstrated the
role of polar solvent in the interaction process. DMSO
being more polar than CH3CN reduces the interaction of
Cu2þ at the tripodal core of 1. Similar is the case with Zn2þ
and Hg2þ (supporting data).
Furthermore, we investigated the interaction of 1 with
the metal ions in aqueous CH3CN, and no characteristic
change in emission was observed (supporting data). In
relation to this, we also noticed the interaction of 1 in more
polar non-aqueous solvent such as DMSO. Interestingly,
the light red coloured solution of 1 in DMSO
(c ¼ 2.2 £ 1023 M) was discharged in the presence of
Cu(ClO4)2 and resulted in light green colour of the solution.
Other metal ions did not bring any detectable colour change
in the solution of 1. The appearance of colour upon
complexation of Cu2þ in DMSO is presumably attributed to
The UV–vis study of 1 in the presence of all the metal
ions except Cu2þ, under similar conditions, showed minor
change in absorbance (supporting data). For example,
Figure 9 shows the change in absorbance upon gradual
addition of Cu2þ. During the interaction, a small red shift
(Dl ¼ 5 nm) of the absorption peak for anthracene
1000
Prompt
Receptor 1
Figure 8. Change in colour of 1 (c ¼ 2.2 £ 1023 M) upon
addition of 10 equiv. amounts of different metal ions
(c ¼ 8.9 £ 1024 M) in DMSO: (a) receptor 1 with (b) Ni2þ, (c)
100
Receptor 1 + Cu2+
Mn2þ, (d) Co2þ, (e) Cd2þ, (f) Cu2þ, (g) Mg2þ, (h) Fe2þ, (i) Zn2þ
,
10
(j) Hg2þ and (k) Pb2þ
.
0
11x10–8 13x10–8 14x10–8 16x1010–8
Time (s)
0.5
0.4
0.3
0.2
0.1
Figure 7. Fluorescence decays (at lmax ¼ 418 nm) of receptor 1
upon addition of 1 equiv. of Cu(ClO4)2 ([H] ¼ 4.10 £ 1025 M,
[G] ¼ 8.21 £ 1024 M) in CH3CN containing 0.04% DMSO.
Table 1. Fluorescence decay times (t) and pre-exponential
factors (a) for 1 and 1-Cu2þ in CH3CN containing 0.04% DMSO.
Compound 1
and its complex
with Cu2þ
t1 ns
(a1)
t2 ns
(a2)
t3 ns
(a3)
x 2
0.0
300
350
400
0.916
(11.40)
0.967
4.53
0.035
(42.03)
0.043
Wavelength (nm)
1
(46.58)
6.24
(76.63)
1.00
1 þ Cuþ2
Figure 9. Change in absorbance of 1 (c ¼ 4.10 £ 1025 M) upon
(1:1)
(8.30)
(15.07)
0.997
gradual addition of Cu2þ ion.

Supramolecular Chemistry
occurred. Decrease in absorption with significant red shift
439
of 1 with Cu2þ is presumably due to cation–p interaction
when the metal ion interacts in the tripodal core of 1 in a
1:1 stoichiometric fashion under the guidance of alcohol,
amide and benzimidazole groups. The metal coordination
property of benzimidazole as well as 2-substituted
benzimidazole is well established (11). It is worth
mentioning that the 2-benzimidazole derivatives allow
coordination towards metal ions through a variety of sites,
with groups bearing nitrogen, oxygen and sulphur atoms,
and the coordination takes place through imidazolic
nitrogen (11). This enables us to presume a binding mode
(Figure 10), where the metal ion is anchored at the tripodal
core involving the benzimidazole ring nitrogen, aliphatic
amine, amide and alcoholic groups altogether. This is
clearly understood from the disposition of the binding
moieties in AM1 optimised geometry of 12 (Figure 11).
The Mulliken charge densities on the different centres are
also shown in Figure 11. Closely spaced benzimidazole
and anthracene moieties possibly partially shield the space
in between them so that the metal ion (Cu2þ)–fluorophore
interaction is minimised, and the PET process from the
binding site to the excited state of anthracene is blocked.
We further recorded the 1H NMR of 1 in the presence
of equivalent amount of Cu2þ ion in CDCl3 containing
0.1% d6-DMSO to be acquainted with the involvement of
the binding site for complexation. But the NMR spectrum
upon addition of Cu2þ ions resulted in broadening
particularly for the resonances of the benzimidazolyl
amide and alcoholic moieties of 1. The anthracenyl ring
protons of types ‘b’ and ‘c’ moved to the downfield
direction by 0.02 ppm and other protons of types ‘a’ and
‘d’ were positionally unaffected. The benzimidazole ring
protons of types ‘e’ and ‘f’ indicated a greater downfield
chemical shift (Dd ¼ 0.20 ppm). In addition, the signals
for the methylene protons adjacent to the aliphatic amine
–0.1669
–0.3779
–0.5017
–0.4166
–0.0919
–0.1525
–0.3685
–0.3798
–0.1181
–0.1760
Figure 11. AM1 optimised geometry of 1 in gas phase
(E ¼ 2194.067 au; the closest distance between anthracene and
˚
benzimidazole is 4.89 A).
nitrogen underwent downfield movement by 0.04–
0.06 ppm. This intimated the information that Cu2þ ion
shows the preferential coordination at the aliphatic amine
nitrogen centre involving the aid of benzimidazolyl amide
and the alcoholic groups.
In conclusion, tripodal shaped compound 1, comprising
anthracene as fluorophore, benzimidazolyl amide, aliphatic
amine and alcohol functionalities as binders/chelators,
functions as a new chemosensor for Cu2þ in preference to a
variety of other metal ions studied except Hg2þ and Zn2þ
ions. Although the tripodal cavity of 1 preferentially binds
Cu2þ ion, it shows moderate sensing behaviour towards
Zn2þ and Hg2þ ions. Cu2þ ion binding induced significant
enhancement of emission of 1 in CH3CN containing 0.04%
DMSO, and also the change in colour in pure DMSO upon
complexation of Cu2þ is the insight into this study for
selective sensing of Cu2þ by the new candidate 1. Further
study is underway in our laboratory.
Weak cation - π interaction
e
f
N
NH
a
N
d
b
HN
c
Supporting data
Characterisation data of 1, figures showing the change in
absorption and fluorescence spectra and the job plots of
receptor 1 in the presence of Zn2þ and Hg2þ, selectivity
study in the absence of Zn2þ and Hg2þ, binding constant
curve for 1 with Cu2þ, change in emission and absorption
of 1 upon addition of Cu(NO3)2, change in fluorescence
OH
O
= Metal ions
Figure 10. Suggested mode of binding of 1 for the metal ions.

440
K. Ghosh and T. Sarkar
Kim, J.W.; Yan, S.; Lee, J.Y.; Lee, J.H.; Joo, T.; Kim, J.S.J.
Am. Chem. Soc. 2009, 131, 2008–2012 and references cited
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ratio of 1, change in emission in DMSO and details of the
experiments are available online.
Acknowledgements
We thank DST, Government of India, for providing facilities in
the department under FIST programme. TS thanks CSIR, New
Delhi, India, for a fellowship.
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Notes
(1) Fluorescence enhancement factor (Z) was calculated based
on the equation Z ¼ (F/F0) [(V0 þ V)/V0] where F ¼
observed fluorescence, F0 ¼ fluorescence of sample before
guest addition, V0 ¼ volume before addition of guest,
V ¼ volume after addition of guest.
(2) AM1 calculation was performed using minimal valence
basis as STO 3G in Argus Lab 4.0.1, copyright (c) 1972-
2004 Mark Thompson and Planaria Software LLC, http://
www.Arguslab.com.
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