Pyrazolone as a recognition site: Rhodamine 6G-based fluorescent probe for the selective recognition of Fe3+ in acetonitrile-aqueous solution

Article abstract of DOI:10.1002/bio.2709

Two novel Rhodamine-pyrazolone-based colorimetric off-on fluorescent chemosensors for Fe³⁺ ions were designed and synthesized using pyrazolone as the recognition moiety and Rhodamine 6G as the signalling moiety. The photophysical properties and

Full text of DOI:10.1002/bio.2709

Research article
Received: 10 December 2013,
Revised: 9 April 2014,
Accepted: 2 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2709
Pyrazolone as a recognition site: Rhodamine
6G-based fluorescent probe for the selective
3+
recognition of Fe in acetonitrile–aqueous
solution
Sanjay Parihar,a Vinod P. Borichab and R. N. Jadejaa*
ABSTRACT: Two novel Rhodamine–pyrazolone-based colorimetric off–on fluorescent chemosensors for Fe3+ ions were
designed and synthesized using pyrazolone as the recognition moiety and Rhodamine 6G as the signalling moiety. The
photophysical properties and Fe3+-binding properties of sensors L1 and L2 in acetonitrile–aqueous solution were also inves-
tigated. Both sensors successfully exhibit a remarkably ‘turn-on’ response, toward Fe3+, which was attributed to 1: 2 complex
formation between Fe3+ and L1/L2. The fluorescent and colorimetric response to Fe3+ can be detected by the naked eye, which
provides a facile method for the visual detection of Fe3+. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: Rhodamine; pyrazolone; fluorescence; Job plot; ion recognition
is that the ring-opening process is accompanied by a vivid change
from colourless to pink, thus enabling the metal detection with the
Introduction
The design and synthesis of new chemosensors for selective
naked eye.
detection of heavy and transition metal (HTM) cations are
important subjects in the field of supramolecular chemistry
due to their significance in chemical, biological and envi-
ronmental assays (1–5). Fluorescent sensors are of particular
The selective detection of biologically important metal ions
has become tremendously important because metal ions are
involved in a variety of fundamental biological processes in
organisms. Iron is one of the most important metals in biological
interest because they allow non-destructive and quick
systems and plays a key role at the cellular level in many bio-
chemical processes. Specifically, ferric ions (Fe3+) are widely
detection using a simple fluorescent enhancement (turn-on)
or quenching (turn-off) response (6–9). Fluorescent chemo-
sensors combine two fundamental functional units:
retained in many proteins and enzymes either for structural pur-
a
poses or as part of a catalytic site (17–19). Moreover, the ferric
ion is well-known as a fluorescence quencher due to its para-
magnetic nature, and most reported Fe3+ receptors, such as
analogues of ferrichromes or siderophores, undergo fluorescence
quenching when bound with Fe3+ (20–22), although it is gener-
ally believed that probes with a fluorescence enhancement
signal on interacting with analytes are much more efficient.
Therefore, the development of new fluorescent indicators for
the accurate and specific detection of Fe3+, especially indicators
that exhibit selective Fe3+-amplified emission, remains a chal-
lenge. Recently, a few sensors have been described to exhibit a
turn-on response to Fe3+ ions (23–26).
fluorophore and an ionophore. The ionophore can selectively
bind the substrate, and the fluorophore is attached in the
vicinity of the binding site for signal detection and transduc-
tion. A large number of fluorescent sensors have been
designed to detect different types of heavy toxic metal ions.
In the fluorescent detection of ions, fluorescence enhance-
ment (‘turn-on’) is preferable to fluorescence quenching
(‘turn-off’), because the former lessens the chance of false-
positive data from other fluorescent quenchers existing in
samples (10).
Recently, Rhodamine amide derivatives have been widely
used as fluorescent probes for the detection of various ions
(9,11–15). The Rhodamine framework is an ideal mode for the
construction of fluorescent chemosensors, owing to their
excellent photophysical properties, such as long absorption and
emission wavelengths, large absorption coefficient, and high
fluorescence quantum yield (16). The cation-sensing mechanism
of these probes is based on structural changes between the
spirocyclic and open-cycle forms (11,12). Without cations, the
probes exist in a spirocyclic form, which is less or non-fluorescent.
The addition of metal cations leads to opening of the spirocycle,
resulting in the appearance of pink and orange fluorescence. The
additional advantage of such a Rhodamine-based sensing system
* Correspondence to: R. N. Jadeja, Department of Chemistry, Faculty of
Science, The M. S. University of Baroda, Vadodara 390002, India. E-mail:
rajendra_jadeja@yahoo.com
a
Department of Chemistry, The M. S. University of Baroda, Vadodara 390002,
India
b
Analytical Science Division, Central Salt and Marine Chemicals Research
Institute (Constituents of CSIR, New Delhi), G. B. Marg, Bhavnagar 364002,
India
Luminescence 2014
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S. Parihar et al.
Pyrazolone, as a prominent structural motif, is found in numer-
ous active compounds. Because of its ease of preparation and rich
biological activity, pyrazolone and its metal complexes have
received considerable attention in coordination chemistry and
medicinal chemistry (27–31). 4-Formyl derivatives of pyrazolones
can form a variety of Schiff bases and are reported to be superior
reagents in various biological applications (32–34). As per the liter-
ature, there have been a few attempts to synthesize fluorescent
sensors containing pyrazolone as a recognition moiety for metal
ions (35–38). However, to the best of our knowledge, there is no
example of a fluorescent sensor for the metal ion using a Schiff
base of Rhodamine 6G and 4-formyl derivatives of pyrazolones.
Here, we synthesized two Rhodamine-based fluorescent sensors
using 5-hydroxy-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde
and 5-hydroxy-3-methyl-1-(p-tolyl)-1H-pyrazole-4-carbaldehyde
as the recognition moiety.
1H and 13C NMR spectra were recorded with Avance-III 400
MHz Bruker FT-NMR instrument. Elemental analyses of C, H and
N were determined using a Perkin–Elmer series II 2400 elemental
analyser. Fourier transform infrared (FTIR) spectra were recorded
as the KBr pellet on a Perkin–Elmer FTIR spectrum RX 1 spec-
trometer. Mass spectra were recorded on a Q-TOF Micro™ LC-MS
instrument. The UV/vis spectra were recorded on a CARY 500
Scan UV–Vis–NIR spectrophotometer (Varian). Emission spectra
were recorded using an Edinburgh Instruments model Xe-900.
Synthesis of L1
Ligands L1 and L2 were synthesized from 2 (Scheme 1). Compound
2 (0.184 g, 0.4 mmol) was dissolved in hot methanol (30 mL). Then,
5-hydroxy-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde (0.081 g,
0.4 mmol) was added. The reaction mixture was stirred and heated
to reflux for 4 h, after which it was cooled and filtered. The solid
mass was washed with methanol and dried to afford L1 as an
off-white solid: 0.158 g (60.3%); 1H NMR (CDCl3, 400 MHz), δ
1.3–1.34 (t, J = 7.2 Hz, 6H), 1.88 (s, 6H), 2.19 (s, 3H), 3.19–3.34 (m,
8H), 3.52 (br, 2H, -NH), 6.20 (s, 2H), 6.38 (s, 2H), 7.07–7.12 (m, 2H),
7.35–7.39 (m, 2H), 7.49–7.51 (m, 2H), 7.95–7.99 (m, 2H), 9.33 (s,
1H) ppm; 13C NMR (400 MHz CDCl3): δ 12.64, 14.69, 16.72, 38.33,
40.65, 46.94, 65.28, 96.54, 100.73, 105.22, 118.26, 118.46, 122.94,
123.88, 123.99, 128.14, 128.31, 128.65, 130.75, 132.93, 139.26,
147.65, 147.80, 151.41, 151.73, 153.39, 165.62, 168.63 ppm.
Elemental analyses (%): found: C 73.15, H 6.34, N 13.13; calcd: C
73.10, H 6.29, N 13.12. LC-MS (m/z): 640.51 [M]+.
Experimental
General
All chemicals and solvents involved were of analytical grade.
Dry N,N’-dimethylformamide was purchased from Merk. Solvents
for spectral studies were freshly purified using standard
procedures. The compounds 5-hydroxy-3-methyl-1-phenyl-1H-
pyrazole-4-carbaldehyde (1a) and 5-hydroxy-3-methyl-1-(p-tolyl)-
1H-pyrazole-4-carbaldehyde (1b) were synthesized following a
previously reported procedure (39). Compound 2 was synthesized
following as reported in the literature (40). All metal nitrate salts
were purchased from Sigma-Aldrich. Rhodamine 6G and
ethylenediamine were purchased from Spectrochem. 3-Methyl-1-
toluoyl-5-pyrazolone and 3-methyl-1-phenyl-5-pyrazolone were
obtained from Nutan Dye Chem, Surat.
Synthesis of L2
Compound 2 (0.184 g, 0.4 mmol) was dissolved in hot methanol
(30 mL). Then, 5-hydroxy-3-methyl-1-(p-tolyl)-1H-pyrazole-4-
Scheme 1. Synthesis of compounds L1 and L2.
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Rhodamine–pyrazolone-based fluorescent sensor for Fe3+ ions
carbaldehyde (0.086 g, 0.4 mmol) was added. The reaction mix-
ture was stirred and heated to reflux for 2 h, at the end of which
it was cooled and filtered. The solid mass was washed with
methanol and dried to afford L2 as off-white solid: 0.166 g
(64.8%); 1H NMR (CDCl3, 400 MHz), δ 1.30–1.33 (t, J = 7.2 Hz,
6H), 1.88 (s, 6H), 2.19 (s, 3H), 2.34 (s, 3H), 3.19–3.33 (m, 8H),
3.52 (br, 2H, -NH), 6.20 (s, 2H), 6.37 (s, 2H), 7.07–7.09 (m, 1H),
7.16 (d, J = 8.4 Hz, 2H), 7.49-7.51 (m, 2H), 7.84 (d, J = 8.4 Hz, 2H),
7.95–7.97 (m, 1H), 9.31 (s, 1H) ppm; 13C NMR (400 MHz CDCl3):
δ 12.62, 14.69, 16.71, 20.94, 38.33, 40.67, 46.87, 65.28, 96.54,
100.77, 105.22, 118.25, 118.48, 122.93, 123.99, 128.14, 128.30,
129.18, 130.76, 132.92, 133.36, 136.84, 147.53, 147.65, 151.33,
151.73, 153.89, 165.43, 168.61 ppm. Elemental analyses (%):
found: C 73.42, H 6.53, N 12.85; calcd: C 73.37, H 6.47, N 12.83.
LC-MS (m/z): 654.64 [M]+.
peak at 65.28 ppm in the 13C NMR spectrum also supports the
existence of the spirolactam form. Condensation of compound
2 with the 4-fomyl derivative of pyrazolone (1a and 1b) pro-
duces L1 and L2, respectively.
The fluorescence and UV/vis spectra of both the
fluoroionophores (L1 and L2) were recorded in an acetonitrile–
aqueous (1: 1) medium at room temperature. Compounds L1
and L2 are insoluble in water and generally the metal ions
containing samples are in the aqueous medium. Thus, we chose
an acetonitrile–water mixture for better solubility of the
compounds and their applicability to the aqueous samples.
Compounds L1 and L2 in acetonitrile–aqueous solution were
non-fluorescent, indicating that the spirolactam form is predom-
inant. Earlier reports reveal that certain TM ions bind selectively
with appropriate derivatives of Rhodamine, where metal–ion
binding induces opening of the spirolactam ring and generation
of the xanthene form, with associated changes in the electronic
and fluorescence spectral patterns. Thus, we checked the binding
affinity of L1 and L2 toward all common metal ions, e.g. Na+, K+,
Ag+, Ni2+, Co2+, Cu2+, Ca2+, Zn2+, Cd2+, Hg2+, Pb2+, Fe3+, Cr3+ and
Al3+ (10 eq.) by observing changes in the electronic and fluores-
cence spectral patterns in an acetonitrile-aqueous medium. The
UV/vis absorption spectra and fluorescence emission spectra of
L1 and L2 were recorded individually upon the addition of various
metal ions. The spectral changes for L1 and L2 upon the addition of
various metal ions are shown in Figs. 1 and 2.
Ion-binding study
Stock solutions of compounds L1 and L2 (2 × 10-5M) were pre-
pared by dissolving the compounds in acetonitrile–aqueous
(50: 50) solution. Solutions of the nitrate salts (2 × 10-4 M) of
various cations (Na+, K+, Ag+, Ni2+, Co2+, Cu2+, Ca2+, Zn2+, Cd2+
,
Hg2+, Pb2+, Fe3+, Cr3+ and Al3+) were prepared in acetonitrile–
water solution. Then 2 mL of a stock solution of L1/L2 and
2 mL of the stock solution of each metal salt added to a 5 mL vol-
umetric flask, so that the effective concentration of compounds L1
and L2 was 1 × 10-5 M and that of the metal ions was 1 × 10-4
M
After the addition of metal ions, UV/vis absorption spectra
showed a distinct change and the appearance of a new spectral
band with a maximum at 530 nm for Fe3+ ions. The absorption
band at 530 nm in the case of Fe3+ is due to the formation of
(10 eq.). The luminescence spectra of the resulting solutions and
of the original compounds (1× 10-5 M) were recorded on excita-
tion at the absorption maxima (λmax = 530 nm). The spectra of
the cation-containing solutions were compared with that of the
original solution to ascertain the interactions of the cations with
the ionophore. The UV/vis spectra of all solutions containing metal
ions (10 eq.) were also recorded. Metal ions that exhibited substan-
tial changes in absorption and emission intensities were consid-
ered for the absorption and emission titrations to determine the
association constant. For the emission titration study, the same
stock solutions of compounds L1 and L2 were used and a solution
of the nitrate salts of the selected metal ions (Fe3+, Al3+ and Cr3+) at
the desired concentration (2× 10-5–2 × 10-3 M) were prepared by
proper dilution of the stock solution. Then, 2 mL of each solution
were mixed in a 5 mL volumetric flask to prepare the reaction mix-
ture with 0.1–100 M equivalents of the metal ion with respect to
the concentration of the complexes; the luminescence spectra of
the resulting solutions were recorded.
a delocalized xanthane moiety of Rhodamine by selective Fe3+
-
induced ring opening of spirolactam, which also explains the
change in colour from colourless to pink in the presence of this
Fe3+ ion. Without metal ions, L1 and L2 show almost no fluores-
cence upon excitation at 530 nm, suggesting that L1 and L2 exist
in a ring-closed non-fluorescent spirolactam conformation. The
addition of Fe3+ creates strong fluorescence upon excitation at
530 nm, indicating strong complexation of Fe3+ with L1 and L2.
In other words, free L1 and L2 were non-emissive, but their fluo-
rescence can turn from ‘off’ to ‘on’ when Fe3+ ions are added.
However, L1 and L2 also show weak absorption and emission
intensities with Al3+ and Cr3+ ions. For Fe3+, spectral changes
were also associated with a visually detectable change from
colourless to pink–red; no such change could be detected
visually for any of the other cations studied.
Results and discussion
Stoichiometry and binding mode study
The synthetic route for compounds L1 and L2 is shown in
Scheme 1. L1 and L2 were synthesized by the reaction of inter-
mediates 1a and 1b with 2. All the compounds were character-
ized on the basis of analytical and spectroscopic data. Elemental
analyses and mass spectrometric data are in excellent agree-
ment with the calculated values for the proposed structures of
the compounds. 1a and 1b were synthesized by the condensa-
tion of 3-methyl-1-phenyl-2-pyrazolin-5-one and 3-methyl-1-
toluoyl-5-pyrazolone with DMF and POCl3, respectively (40).
Compound 2 was synthesized by refluxing a mixture of
Rhodamine 6G and ethylenediamine in ethanol. The reaction
was refluxed until the fluorescence of the solution disappeared.
The disappearance of fluorescence indicates the formation of a
spirolactam ring in compound 2. Meanwhile, the characteristic
To elicit the interaction between L1, L2 and Fe3+, the UV/vis and
fluorescence spectral variations in L1 and L2 (2 × 10-5 M) in aceto-
nitrile–aqueous solution were titrated with different concentra-
tions of Fe3+. The UV/vis titration experiments for L1 and L2 in
acetonitrile–aqueous solution are shown in Fig. 3. Upon gradual
addition of Fe3+, the titration experiment shows a sharp increase
in the band at 530 nm. The appearance of the band at 530 nm
might be due to coordination of the imine nitrogen atom of L1
and L2 with the Fe3+ ions. In the fluorescence titration experi-
ment shown in the Fig. 4, upon gradual addition of Fe3+ ions,
the ring-opened Rhodamine emission band at 556 nm is en-
hanced. The Rhodamine emission band at 556 nm attained sat-
uration at an Fe3+ concentration of ~ 6.0 × 10-4 M (60 eq. of Fe3+).
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S. Parihar et al.
Figure 1. (A) The absorption change and (B) the fluorescence intensity change profile of L1 (λexc = 530 nm) to different metal ions (10 eq.) in acetonitrile/water (1: 1v/v).
Figure 2. (A) The absorption change and (B) the fluorescence intensity change profile of L2 (λexc = 530 nm) to different metal ions (10 eq.) in acetonitrile/water (1: 1v/v).
Figure 3. (A) UV/vis titration profile of L1 (2 × 10-5 M) in the presence of Fe3+ ions (0–100 eq.) in acetonitrile/water (1: 1 v/v). (B) UV/vis titration profile of L2 (2 × 10-5 M) in
the presence of Fe3+ ions (0–100 eq.) in acetonitrile/water (1: 1 v/v).
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Rhodamine–pyrazolone-based fluorescent sensor for Fe3+ ions
Figure 4. (A) Fluorescence titration profile of L1 (2× 10-5 M) in the presence of Fe3+ ions (0–60 eq.) in acetonitrile/water (1: 1 v/v) (λexc = 530 nm). (Inset) Linear regression fit
(double-logarithmic plot) of the titration data as a function of the concentration of the metal ion. (B) Fluorescence titration profile of L2 (2 × 10-5 M) in the presence of Fe3+ ions
(0–60 eq.) in acetonitrile/water (1: 1 v/v) (λexc = 530 nm). (Inset) Linear regression fit (double-logarithmic plot) of the titration data as a function of the concentration of the metal ion.
Figure 5. Job plots of (a) L1 with Fe3+ in acetonitrile/water (1:1 v/v) and (b) L2 with Fe3+ in acetonitrile/water (1:1 v/v).
Scheme 2. Proposed binding structure of L1 with metal ion.
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S. Parihar et al.
authors also thank Dr Parimal Paul, Principal Scientist, Analytical
Science Division, CSMCRI, Bhavnagar, India for providing
instrumental facilities and for his fruitful suggestions throughout
this work.
The Job plot with respect to 530 nm showed that the absor-
bance attained a maximum at a molar fraction of~ 1/3, indicating
that a 1: 2 stoichiometry was most likely for the binding of Fe3+
with L1/ L2 (see Fig. 5). It is found in the literature that the majority
of Rhodamine-based fluorescent chemosensors show a 1: 1
binding stoichiometry of Fe3+ and fluorescent chemosensors
(12,23,25,41,42); there are only a few reports of the 1: 2 binding
of Fe3+ and fluorescent chemosensors (43,44). In the fluorescent
chemosensors described here, binding stoichiometry is 1: 2 for
Fe3+ and L1/L2. Two Rhodamine–pyrazolone ligands attach one
metal ion of Fe3+. The Jobs plots for Al3+ and Cr3+ indicate 1: 1
stoichiometry with compounds L1 and L2 (Fig. S1).
From the absorption titration experiment, the association
constant for Fe3+ and L1 was estimated to be 6.08 × 105 M-2 on
the basis of non-linear fitting of the titration curve (45) assuming
1: 2 stoichiometry (Fig. S2). The association constant for Fe3+ and
L2 was estimated to be 4.10 × 106 M-2 (Fig. S3). The association
constants for chemosensors L1 and L2 are higher than the
reported values (43), indicating that L1 and L2 bind more
strongly than the reported chemosensors (43) with Fe3+. Thus,
considering the Job plots and association constants in accor-
dance with 1: 2 stoichiometry, the possible binding mode
between L1 or L2 and Fe3+ is proposed in Scheme 2. The detec-
tion limits of L1 and L2 were also calculated as 6.1 × 10-6 M and
4.2 × 10-6 M, respectively, from the fluorescence titration data.
The higher selectivity of compounds towards Fe3+ ions might
be because the electrode potential of Fe3+ is greater than that
of Al3+ ions (46). The binding stoichiometry of Fe3+ with L1 and
L2 is 1: 2, leading to greater enhancement of the absorption
and emission bands in comparison with Al3+ and Cr3+ , which
show 1: 1 binding stoichiometry.
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In summary, two new pyrazolone–Rhodamine-based signalling
systems were designed and synthesized for the selective recog-
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ognition moiety and Rhodamine 6G was used as a signalling
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Fe3+ in CH3CN/H2O (1: 1, v/v) can be detected even by the naked
eye, which provides a facile method for the visual detection of
Fe3+. Complexation of the Fe3+ ion opens the spirolactam ring
of Rhodamine moieties to produce specific colour change as
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Acknowledgements
R. N. J. thanks UGC, New Delhi for financial assistance in terms of
a major research project [F.37-376/2009 12.01.2010(SR)]. S.P.
thanks UGC-BSR, New Delhi, India for financial assistance. The
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version of this article at the publisher’s web site.
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