Dual chemosensing properties of new ferrocene-based receptors towards fluoride and copper(II) ions

Article abstract of DOI:10.1016/j.inoche.2011.06.016

The new ferrocene based receptors N-[4-ferrocenyl-2-methyl-4-oxobut-1-enyl] -N′-phenylthiourea (1), N-[4-ferrocenyl-2-methyl-4-oxobut-1-enyl]- N′-[4-nitrophenyl]thiourea (2) were synthesized and characterized. Fluorescence titrations of receptors 1 and 2 with various transition metal ions showed selective response to Cu²⁺ ions and the emission intensities quenched significantly. Electrochemical titrations with anions revealed that receptors 1 and 2 sensed the F^- anion in high selectivity with a cathodic shift of 100 mV.

Full text of DOI:10.1016/j.inoche.2011.06.016

Inorganic Chemistry Communications 14 (2011) 1596–1601
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Dual chemosensing properties of new ferrocene-based receptors towards fluoride
and copper(II) ions
⁎
Soosai Devaraj , Veeramuthu Stalin Elanchezhian, Muthusamy Kandaswamy
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai-600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The new ferrocene based receptors N-[4-ferrocenyl-2-methyl-4-oxobut-1-enyl]-N′-phenylthiourea (1), N-[4-
ferrocenyl-2-methyl-4-oxobut-1-enyl]-N′-[4-nitrophenyl]thiourea (2) were synthesized and characterized.
Fluorescence titrations of receptors 1 and 2 with various transition metal ions showed selective response to
Cu2+ ions and the emission intensities quenched significantly. Electrochemical titrations with anions revealed
that receptors 1 and 2 sensed the F− anion in high selectivity with a cathodic shift of 100 mV.
© 2011 Elsevier B.V. All rights reserved.
Received 12 May 2011
Accepted 14 June 2011
Available online 21 June 2011
Keywords:
Chemosensor
Ferrocene
Cationic sensor
Anionic sensor
Electrochemical studies
The development of receptors for recognizing cation, anion and
neutral species has attracted much dynamic in molecular recognition
study and supramolecular chemistry [1]. The sensing function is
generally achieved by the coupling of two-well defined parts: a)
selective binding sites and b) signaling subunits e.g. redox shifts,
colour changes and fluorescence quenching or enhancement [2]. The
binding events have been converted into an electrochemical [3] or
fluorescent [4] change or, more directly, a colorimetric change
detectable by the naked eye [5]. The incorporation of luminescent
chromophores into the receptor has recently gained considerable
attention due to their high sensitivity and low detection limit [6–8].
Many fluorescent sensors have been developed on the basis of a
variety of signaling mechanisms such as competitive binding,
photoinduced electron transfer, metal–ligand charge transfer, and
intramolecular charge transfer [9–12]. Commonly, electrochemical
anion sensing has been achieved potentiometrically by ferrocenyl
species or other organometallic derivatives [13,14]. It has been
postulated by Beer et al. that amide spacer units gave good coupling
between the binding and electrochemical readout sites, and the –NH
fragments were excellent hydrogen bond donors for anions [15]. In
addition, recently some papers reported a few compounds, which
display simultaneously both electrochemical and fluorescent signal-
ing for cation sensing [16,17]. Sensing of a fluoride anion, the smallest
anion, has attracted growing attention due to its beneficial effects
(e.g., prevention of dental caries) and detrimental (e.g., fluorosis)
roles [18]. In order to make beneficial use of fluorescent and
electrochemical dual sensing molecules for the detection of the
fluoride and copper (II) ion, here we report N-[4-ferrocenyl-2-
methyl-4-oxobut-1-enyl]-N′-phenylthiourea (1), N-[4-ferrocenyl-2-
methyl-4-oxobut-1-enyl]-N′-[4-nitrophenyl]thiourea (2), which con-
sist of a redox active ferrocene group and nitro phenyl rings as well as
anion binding thiourea groups as a biresponsive fluorescent and
electrochemical chemosensor. The host–guest complexation for
sensing fluoride and copper(II) ions through electrochemical and
optical responses were investigated.
The receptors 1 and 2 were synthesized by Schiff base condensa-
tion (Scheme 1) of ferrocenoylacetone [19] with 1-(2-aminophenyl)-
3-phenylthiourea and 1-(2-amino phenyl)-3-(4-nitrophenyl)thiourea
respectively [20]. The receptors 1 and 2 were characterized by
elemental analysis, IR, 1H and 13C-NMR [Figs. S1–S4], and ESI Mass
[Figs. S5 and S6] spectroscopic methods [21].
The anion binding interactions of receptors 1 and 2 were
investigated using UV–visible, fluorescence and electrochemical
studies. The UV–visible titrations were carried out in CH3CN by
adding aliquots of solutions of different halide anions as its
tetrabutylammonium salts. Fig. 1 displayed absorption spectral
changes of receptor 1 (5×10−5 M in CH3CN) in the presence of F−
(5×10−3 M) ions. As depicted in Fig. 1, three absorption peaks were
observed at 222 or 243, 308 and 364 nm in the absence of F− ions,
might attribute to ICT (intramolecular charge transfer) absorption
bands. A prominent absorption band with λmax 222, 243 and 308 nm,
can ascribed to a high energy ligand-centered π–π* electronic
transition. The band at 364 nm was assigned to n–π* transition. In
addition, another absorption band in the visible region at 480 nm
(ε=200 M−1 cm−1) was assigned to localized excitation with a low
⁎
Corresponding author at: Department of Applied Chemistry, Providence University,
200 Chungchi Road, Sha-Lu, Taichung Hsien 433, Taiwan. Tel.: +886 4 26328001x15218;
fax: +886 4 26327554.
E-mail address: devpushpam@gmail.com (S. Devaraj).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.06.016

S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601
1597
H2N
O
O
HN
N
S
O
HN
Fe
+
HN
S
EtOH
reflux
HN
R
Fe
R
1 = H
2 = NO2
R
R = H, NO2
Scheme 1. Synthesis of receptors 1 and 2.
energy (LE) which was produced by two nearly degenerated
transitions and Fe(II) d–d transition [Fig. 1]. Upon the addition of
F− ions [0 to 2 equiv.], the absorption peak at 480 nm decreased with
a blue shift of 15 nm. A typical titration curve of fluoride ions by
receptor 2 was shown in Fig. S7. The longer wavelength absorption at
increased and the peaks at 310 and 360 nm decreased gradually along
with the increased concentration of F− ion. Exposure to chloride,
bromide and iodide anions did not result in any spectral changes in
the above receptors [22,23]. The binding constants of F− complexa-
tion for the receptors 1 and 2 were estimated and tabulated (see
Table 1, Fig. S9 and Fig. S10 respectively). Receptor 2 showed a higher
binding constant, attributing the presence of nitro group.
λ
max =485 nm decreased with a blue shift of 15 nm in the presence of
0 to 2 equivalents of F− ions. Simultaneously the peak at 222 nm
1.80
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.00
1 equiv
0.66 equiv
0.33 equiv
0 equiv
200.0
250
300
350
400
Wavelength (nm)
1.60
1.4
1.2
1.0
0.8
0.6
0.4
1 equiv
0.9 equiv
0.8 equiv
0.7 equiv
0.6 equiv
0.5 equiv
0.4 equiv
0.3 equiv
0.2 equiv
0.1 equiv
0 equiv
0.2
0.03
400.0
450
500
550
600
650
700
750
800.0
Wavelength (nm)
Fig. 1. Absorption spectra of receptor 1 recorded in CH3CN (5.0×10−5 M) after the addition of 0 to 1 equiv. of TBAF.

1598
S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601
sequential addition of Cu2+ ions from 0 to 2 equivalents to the
receptor 1 [Fig. 2] showed a gradual decrease in absorbance at 481 nm
and intensity enhancement of absorption at 651 nm with colorimetric
change from pale yellow to green [Fig. 3]. As shown in fig. S10, the
intensity of 2 at 640 nm increased, while that of 490 nm decreased
with the colour changes from orange to dark green [Fig. 3]. These new
peaks and intensity of the colours for the receptors 1 and 2 reached
their limiting value after the addition of two equivalents of Cu2+ ions.
The binding constants of copper(II) complexation for the receptors 1
(Fig. S11) and 2 (Fig. S12) were shown in Table 1. No significant
spectral changes are observed for receptors 1 and 2 in the presence of
Table 1
Data obtained from the UV–visible spectra upon titration of receptors 1 and 2 with
anionsa and cations in CH3CN.
Receptor Binding constants
F−
Correlation
coefficient (R)
Cu2+
Cl−, Br−, I−
1
2
(1.2 0.21)b ×103 (1.5 0.12)b ×103 NDc
(24.0 0.12)b ×103 (9.2 0.20)b ×103 NDc
0.9959
0.9940
*ND-not determined.
a
Countercation was tetrabutylammonium salts for anions. All errors are 5%.
The error value was obtained by the result of linear fitting. All errors are 5%.
Changes in the UV–visible spectra were not enough to calculate the binding
b
c
other metal ions viz Mg2+, Ca2+, Cd2+, Ni2+, Co2+, Zn2+ and Mn2+
.
constant.
Fluorescence monitoring of F- and Cu2+ ions were also performed
for the receptors by using 5×10−5 M solution of 1 and 2 in CH3CN.
The emission spectra of 1 and 2 were characterized by the emission
maximum observed at 438 nm and 445 nm respectively upon excited
at 325 nm. Upon increasing the addition amounts of F- ion (0 to 3
equiv.) to the solution of receptor 1, a significant increase in
fluorescence emission at 438 nm was observed [Fig. 4]. The increase
in fluorescence emission induced by F- ion was attributed to 1.F−
complex. Similarly, upon addition of F− ion, the emission band at
445 nm of 2 was also gradually increased [Fig. S13]. Fluorescence
monitoring of other halides such as Cl−, Br− and I− was also carried
The binding ability of 1 and 2 in CH3CN (5×10−5 M) against
cations of environmental relevance, such as Mg2+, Ca2+, Cd2+, Ni2+
,
Co2+, Zn2+, Mn2+ and Cu2+ as its perchlorate salts showed unique
selective response towards Cu2+ ions alone. This observation was
consistent with Irving–William series. Copper(II) has a specific high
thermodynamic affinity for typical N-donor ligands and fast metal-to-
ligand binding kinetics. Figs. 2 and S11 displayed absorption spectral
changes of the receptors 1 and 2 respectively in the presence of Cu2+
ions. The colorimetric sensing ability of 1 and 2 in CH3CN (1×10−3 M)
was monitored by the naked eye and visible spectroscopy. The
2.00
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.00
2 equiv
1.66 equiv
1.33 equiv
1 equiv
0.66 equiv
0.33 equiv
0 equiv
200.0
250
300
350
400
450
Wavelength (nm)
2 equiv
1.8 equiv
1.6 equiv
1.4 equiv
1.2 equiv
1 equiv
0.8 equiv
0.6 equiv
0.4 equiv
0.2 equiv
0 equiv
1.50
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.07
440.0 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800.0
Wavelength (nm)
Fig. 2. Absorption spectra of receptor 1 recorded in CH3CN (5.0×10−5 M) after the addition of 0 to 2 equiv. of Cu2+ ions.

S. Devaraj et al. / Inorganic Chemistry Communications 14 (2011) 1596–1601
1599
On increasing the concentrations of Cu2+ ions [0 to 3 equiv., Fig. 5]
to the receptor 1, the emission band was quenched at 438 nm with the
blue shift of 10 nm. A selective quenching with blue shift (15 nm) was
also observed for the receptor 2 at 455 nm [Fig. S14]. The emission
spectrum of 1 and 2 were unaffected upon the addition of other metal
ions. The ferrocene conjugated system could allow the modulation of
optical or photochemical properties of the molecule via change in the
electronic states of the ferrocene moiety by the involvement of Cu(II)
ion binding with donor atoms. The quenching effect could be
attributed to an electron or energy transfer quenching of the π*
emissive state through low-lying metal-centered unfilled d-orbitals of
paramagnetic Cu(II)[24].
Figs. 6 and S15 show electrochemical changes of 1 and 2 in CH3CN
solution upon addition of F− ions at 100 mV scan rate, using TBAP as a
supporting electrolyte. The cyclic voltammetric (CV) response of 1
showed a reversible oxidation process at E1/2 =645 mV which was
attributed to the Fc/Fc+ couple. On stepwise addition of F− ion (1
equiv.) to a solution of receptor 1 in CH3CN, a decrease in current
intensity of the reduction wave was observed, which might be
attributed to the formation of receptor–anion complex [Fig. 5] [25].
1
1+Cu2+
2
2+Cu2+
Fig. 3. Colour changes of 1 and 2 in CH3CN (5.0×10−5 M) before and after the addition
of 2 equiv. of Cu2+ ions.
out under the same conditions as the F- ion. Beside F− ion; no
significant fluorescence change of receptors 1 and 2 was observed
even in the presence of 10 equiv.
900.0
800
700
600
500
400
300
200
100
2.4
3 equiv
2.66 equiv
2.33 equiv
2 equiv
1.66 equiv
1.33 equiv
1 equiv
0.66 equiv
0.33 equiv
0 equiv
400.0
420
440
460
480
500
520
540
560
Wavelength (nm)
Fig. 4. The changes in the fluorescence emission spectra of receptor 1 (5.0×10−5 M) upon titration with solutions of F− ions (0–3 equiv.) in CH3CN.
310.0
250
200
3 equiv
2.5 equiv
150
2 equiv
1.5 equiv
1 equiv
0.5 equiv
0 equiv
100
50
1.3
400.0 420
440
460
480
500
520
540
560
580
600 620.0
Wavelength (nm)
Fig. 5. The changes in the fluorescence emission spectra of receptor 1 (5.0×10−5 M) upon titration with solutions of Cu2+ ions (0–3 equiv.) in CH3CN.

1600
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the addition of F− ion (0–2 equiv.) at the scan rate of 100 mV.
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Further addition of F− anion had an effect on the position of Eox, which
shifted cathodically from 690 mV to 600 mV. Cathodic shifts were
often seen in 1-F− anion binding because the oxidation process
became easier in the presence of the negatively charged ion as a
consequence of electrostatic stabilization [26]. Receptor 2 showed a
similar response to TBAF, the potential of Eox cathodically shifted from
790 mV to 690 mV with E1/2 =685 mV [Fig. S15]. There was no
significant CV response of receptors 1 and 2 upon addition of other
halide and metal ions.
In this present study, we have developed selective and sensitive
chromo as well as fluorogenic receptors 1 and 2 using ferrocene
derivatives for the determination of F− and Cu2+. These receptors
selectively recognize F− ions even in the presence of other halide ions
and shows higher selectivity towards Cu2+ ions than other metal ions
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Acknowledgements
SD gratefully acknowledges SRF (CSIR) for financial assistance. MK
also acknowledges the Department of Science and Technology
(Government of India) and UGC, Delhi for the project grant.
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Appendix A. Supplementary material
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.06.016.
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[21] To the solution of 1-(2-amino phenyl)-3-phenylthiourea (0.3 g, 1.2 mmol) in
ethanol (25 ml), ferrocenoylacetone (0.33 g, 1.2 mmol) in ethanol (25 ml) was
added under stirring. The resulting mixture was refluxed for 3 h and cooled to
room temperature. The solid product was collected by filtration and washed with
cold ethanol. The solid was recrystallized using hot chloroform. The receptor 1
was obtained as red powder. (a) Receptor 1: Yield : 1.2 g (72%) m.p.: 110 °C.
Analytical data for C27H25N3OSFe Calculated (%) C, 65.46; H, 5.09; N, 8.48 Found
(%) :C, 65.44; H, 5.07; N, 8.45. IR data (KBr, ν/cm-1): 3250 (NH stretching), 1672
(C=O stretching), 1625 (C=N stretching), 1592 (aromatic C-H), 1322 (C=S
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