Fine tuning of a solvatochromic fluorophore for selective determination of Fe3+: A new type of benzimidazole-based anthracene-coupled receptor

Article abstract of DOI:10.1016/j.tetlet.2011.01.077

We synthesized a novel benzimidazole-based, anthracene-coupled fluorescent receptor capable of recognizing and estimating the concentrations of Fe ³⁺ in semi-aqueous solution by ratiometric estimation. Our sensor can be made highly selective fo

Full text of DOI:10.1016/j.tetlet.2011.01.077

Tetrahedron Letters 52 (2011) 1368–1371
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Tetrahedron Letters
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Fine tuning of a solvatochromic fluorophore for selective determination
of Fe3+: a new type of benzimidazole-based anthracene-coupled receptor
Doo Youn Lee a, Narinder Singh b, Doo Ok Jang a,
⇑
a Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
b Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140 001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized a novel benzimidazole-based, anthracene-coupled fluorescent receptor capable of recog-
nizing and estimating the concentrations of Fe3+ in semi-aqueous solution by ratiometric estimation. Our
sensor can be made highly selective for Fe3+ over other metal ions by changing the solvent composition.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 2 December 2010
Revised 30 December 2010
Accepted 17 January 2011
Available online 20 January 2011
Keywords:
Solvatochromic
Sensor
Fluorescent
Ratiometric
Fe3+
Fluorescence-based techniques are important tools for chemical
and biochemical research because of their non-invasive nature and
high intrinsic sensitivity.1 They rely on several fundamental photo-
physical mechanisms including photo-induced electron transfer
(PET), internal charge transfer (ICT), excimer and exciplex forma-
tions, electronic energy transfer (EET), or Förster resonance energy
transfer (FRET).2 Receptors attached to fluorescent dyes commonly
employ these mechanisms for bioimaging, characterization of bio-
logical micro-structures, and quantitative estimation of analytes.3
The methodology has rapidly progressed to meet new demands,
such as analytes associated with new disease states, the need for
simultaneous estimation of multiple analytes, and molecular logic
gates and switches.4
Solvents are known to influence the physico-chemical nature of
receptor binding subunits as well as the fluorescence output.
Quantitative measurement of the influence of solvents on the rec-
ognition of metal ions would be greatly aided by the investigation
of the solvatochromic behavior of donor–receptor units fabricated
with more than one fluorophore.5 This approach is very helpful for
developing fluorescent sensor materials that exhibit changes in
their fluorescent signals when exposed to different solvents and/
or cations. Under present investigation, we wish to develop selec-
tive sensors for Fe3+ because of the role of Fe3+ as an important
nutrient for human health and in numerous physiological
functions.
We previously reported the development of benzimidazole-
based receptors.6 Here we present the synthesis of a receptor unit
comprised of two fluorophores, benzimidazole, and anthracene.
The solvatochromic behavior of the receptor was fine tuned to be
highly selective for Fe3+. The two fluorophores were chosen in such
a way that both could be excited at the same wavelength, but the
fluorophore output had to be different; the anthracene moiety gen-
erally exhibits ‘on–off’ behavior,7 while the benzimidazole moiety
may undergo both ‘on–off’ behavior and a charge transfer (CT)
band shift.6 This design is important for the improvement of ratio-
metric fluorescent recognition that measures the ratio of fluores-
cence intensity at two wavelengths. The ratiometric approach
eliminates errors associated with receptor concentration, photo-
bleaching, and environmental effects.8
Receptor 1 was synthesized as shown in Scheme 1. Compound 2
was synthesized as described in the literature.9 Compound 4 was
prepared with a 43% yield by the reaction of compound 2 with sal-
icylaldehyde (3) in the presence of KOH.10 Receptor 1 was prepared
by a condensation reaction of dialdehyde 4 with 2-(2-aminophe-
nyl)-1H-benzimidazole (5) in the presence of a catalytic amount
of Zn(ClO4)2, followed by a reduction with NaBH4, with a 46%
yield.11
Receptor 1 displayed a well defined maximum at 414 nm in the
fluorescence spectrum recorded at a concentration of 10 lM in
pure acetonitrile, when excited at 367 nm. There were no other
emission bands at any wavelengths, indicating the absence of duel
channel emission due to two different fluorophores in the pure
host. Sensors used in biological and environmentally important
samples must be capable of monitoring analytes in aqueous media.
⇑
Corresponding author. Tel.: +82 337602261; fax: +82 337602182.
E-mail address: dojang@yonsei.ac.kr (D.O. Jang).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.077

D. Y. Lee et al. / Tetrahedron Letters 52 (2011) 1368–1371
1369
OH
CHO
KOH, propanol
reflux, 8h , 43%
N
O
+
N
Cl
O
Cl
O
2
3
O
4
1. 5, Zn(ClO4)2, MeOH/THF, rt, 17 h
2. NaBH4, THF, reflux, 14 h, 46%
H
N
NH
O
N
N
H
N
NH2
N
NH
O
N
N
H
5
1
Scheme 1.
Receptor 1 was, therefore, investigated in CH3CN/H2O (1:1, v/v) in
a HEPES-buffered solvent mixture. The sensor was excited at
367 nm at a concentration of 10 lM, and showed only one fluores-
cent emission band, as in acetonitrile, but with a slightly longer
wavelength (424 nm).
To investigate the properties of receptor 1 as a sensor for metal
ions, solutions were prepared in pure acetonitrile with a fixed con-
centration of receptor 1, along with a fixed concentration of a un-
ique metal nitrate salt. The binding of Cu2+ and Fe3+ caused a shift
in the fluorescence band (Fig. 1A). This type of shift was expected
because the CT band is affected when metal ions bind very closely
to a fluorophore. The enhancement of fluorescence intensity upon
Fe3+ complexation was most likely due to the cancelation of PET
from nitrogen donors. On the other hand, the shift in fluorescence
was due to ICT processes. Therefore, the shift and accompanying
fluorescence enhancement was the consequence of both PET and
ICT processes.
The different profiles of fluorescence intensity exhibited by the
binding of transition metal ions prompted us to develop a sensor
that was highly selective for a specific metal ion. We, therefore,
studied the effect of metal-binding on fluorescence signal of recep-
tor 1 in different solvent combinations of CH3CN/DMSO and
CH3CN/H2O. Receptor 1 offered high selectivity for Fe3+ in a solvent
combination of CH3CN/DMSO (8:2, v/v) (Fig. 1B), and presented an
entirely different profile for Fe3+ binding in CH3CN/H2O (1:1, v/v;
HEPES) (Fig. 1C). In this system, the emission band for metal com-
plexation was affected by the solvent. The system thus showed sol-
vatochromism, or the ability to change photo-physical properties
due to a change in solvent polarity. The solvatochromism could
be explained in terms of a positive and negative sign, which corre-
sponds to a blue or red shift with the change in solvent polarity.
This property arises due to the polarity difference between the
ground state and the excited state of a fluorophore. Thus, a change
in solvent polarity would lead to differential stabilization of the
ground and excited states, changing the energy gap between these
electronic states. Consequently, variations in the position, inten-
sity, and shape of the spectra can be direct measures of the specific
interactions between the sensor and solvent molecules.
Figure 1. Changes in fluorescence intensity (kex = 367 nm) of receptor 1 (10
upon addition of a particular metal (50 M) nitrate salt in: (A) CH3CN; (B) CH3CN/
DMSO (8:2, v/v); (C) CH3CN/H2O (1:1, v/v; HEPES 100 mM; pH 7.0).
lM)
To learn more about the properties of receptor 1 as a sensor for
l
Fe3+ in CH3CN/DMSO (8:2, v/v) and a sensor for Fe3+ in HEPES-buf-

1370
D. Y. Lee et al. / Tetrahedron Letters 52 (2011) 1368–1371
fered CH3CN/H2O (1:1, v/v), fluorescence titrations were per-
formed. Figure 2 illustrates the emission response of receptor 1
with an increase in concentration of Fe3+ in CH3CN/DMSO (8:2, v/
v). Upon addition of Fe3+ to a 10
lM solution of receptor 1, we ob-
served a monotonic quenching of the fluorescence intensity at
424 nm and a minor enhancement was observed at 500 nm. Recep-
tor 1 exhibited a high sensitivity toward Fe3+, quenching its fluo-
rescence intensity by 75% with 20 equiv of Fe3+. We attempted to
use the data for ratiometric estimation of Fe3+ (inset of Fig. 2) since
Fe3+ binding leads to a change in intensity at two different wave-
lengths. A linear relationship was obtained within a concentration
range of 100–250 lM. The association constant (Ka) of receptor 1
Figure 3. Fluorescence spectra changes (kex = 367 nm) of receptor 1 (10 lM) upon
for Fe3+ was calculated on the basis of a Benesi–Hilderbrand plot
(Fig. S1),12 and was found to be 1.2 Â 103 MÀ1. The stoichiometry
of the complex was determined to be 1:1 by Job’s plot (Fig. S2).13
Next we investigated the changes in the fluorescence spectrum
of receptor 1 with an increase in concentration of Fe3+ in HEPES-
buffered CH3CN/H2O (1:1, v/v) (Fig. 3). The successive addition of
Fe3+ into a solution of receptor 1 caused a gradual quenching in
fluorescence intensity at 424 nm and a prominent enhancement
at 500 nm. Therefore, receptor 1 can be used for selective ratiomet-
addition of Fe3+ (0–10 equiv) as nitrate salt in HEPES-buffered CH3CN/H2O (1:1,
v/v).
ric estimation of Fe3+ along a concentration range of 0–100
lM,
and the ratiometric calibration curve is shown in the inset of Figure
3. The association constant (Ka) of receptor 1 for Fe3+ in HEPES-buf-
fered CH3CN/H2O (1:1, v/v) was calculated on the basis of a Benesi–
Hilderbrand plot (Fig. S3),12 and was found to be 4.1 Â 103 MÀ1
.
The slightly lower binding strength of receptor 1 for Fe3+ in
CH3CN/DMSO (8:2, v/v) than in CH3CN/H2O (1:1, v/v) is quite ex-
pected because metal ions compete with DMSO for the binding site
of receptor 1.14
Figure 4. Fluorescence intensity (kex = 367 nm) of receptor
1 (10 lM) upon
addition of Fe3+ along with interfering metal nitrate salt in DMSO/H2O (8:2, v/v)
at 424 nm.
To judge the effect of other metal ions upon the signal response
induced by Fe3+ complexation with receptor 1, we carried out com-
petitive binding experiments (Fig. 4). Fluorescence intensities were
measured in a series of solutions containing receptor 1, different
amounts of Fe3+, and another metal ion. The fluorescence intensity
was almost identical to that obtained in the absence of any inter-
fering metal ion. The results confirm that metal ions do not signif-
icantly influence the signal response induced by Fe3+ complexation
with receptor 1.
Cu2+ may cause potential interference in the measurement of
Fe3+ by receptor 1 (Fig. 1C). It was thus important to test the ability
of receptor 1 to operate in solutions containing equimolar concen-
trations of Cu2+ and Fe3+. Figure 5 shows plots for solutions con-
taining receptor 1 with Fe3+ and for those containing receptor 1
with equimolar Fe3+ and Cu2+. The plots were in good agreement
between the two sets of data. These data suggest that Fe3+ binding
Figure 5. Plot of relative fluorescence intensity (kex = 367 nm) of receptor
(10
M) in HEPES-buffered CH3CN/H2O (1:1, v/v) along with blue square = [Fe3+],
and red square = [Fe3+] + equimolar [Cu2+], at 424 nm.
1
l
is not affected by the presence of Cu2+
.
The binding cavity of receptor 1 for the complexation of Fe3+
was assigned on the basis of MacroModel calculation (Fig. 6).15
The calculated structure of complex shows that the sp2 nitrogens
of benzimidazole are orientated toward the binding pocket of
Fe3+ coordination sphere. The other two nitrogens, generated
through the reduction of imine linkages, also participate in binding
via making a six-membered ring. The central nitrogen, which is in
the vicinity to the anthracene moiety, is away from the coordina-
tion sphere. Thus, we could not find any change in the fluorescence
spectrum of receptor 1 due to anthracene moiety. In addition, the
calculated structure shows that the steric bulkiness of anthracene
forces the two benzimidazole moieties in proximity to make an
effective pseudocavity for Fe3+ complexation.
In conclusion, we synthesized a sensor capable of recognizing
and estimating the concentrations of Fe3+ in semi-aqueous solution
through changes in its fluorescence spectra at two different wave-
lengths. The sensor could be made very selective for Fe3+ over other
metal ions by making use of the solvatochromic behavior of the
receptor. Our findings provide a method of improving or altering
the selectivity of known receptors by changing the solvents that
can potentially lead to a new class of optical sensors.
Figure 2. Fluorescence spectra changes (kex = 367 nm) of receptor 1 (10 lM) upon
addition of Fe3+ (0–25 equiv) as nitrate salt in CH3CN/DMSO (8:2, v/v).

D. Y. Lee et al. / Tetrahedron Letters 52 (2011) 1368–1371
1371
Figure 6. Energy minimized structure of Fe3+ complex of receptor 1 as obtained by MacroModel calculation (two different views).
Tetrahedron 2009, 65, 4239; (f) Taha, A.; Mahmoud, M. M. New J. Chem. 2002,
26, 953.
Acknowledgment
6. (a) Lee, G. W.; Singh, N.; Jung, H. J.; Jang, D. O. Tetrahedron Lett. 2009, 50, 807;
(b) Singh, N.; Lee, G. W.; Jang, D. O. Tetrahedron 2008, 64, 1482; (c) Lee, G. W.;
Singh, N.; Jang, D. O. Tetrahedron Lett. 2008, 49, 1952; (d) Jung, H. J.; Singh, N.;
Jang, D. O. Tetrahedron Lett. 2008, 49, 2960; (e) Joo, T. Y.; Singh, N.; Jung, H. J.;
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2007, 48, 8846.
This work was supported by the Center for Bioactive Molecular
Hybrids (No. R11-2003-019-00000-0).
Supplementary data
7. (a) Ghosh, K.; Saha, I. Tetrahedron Lett. 2010, 51, 4995; (b) Ghosh, K.; Sarkar, A.
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.01.077.
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–CH2), 6.66–6.68 (m, 2H, Ar), 6.92–6.96 (m, 2H, Ar), 7.35–7.43 (m, 6H, Ar),
7.73–7.75 (m, 2H, Ar), 7.97–7.99 (m, 2H, Ar), 8.41 (s, 1H, Ar), 8.48–8.50 (m, 2H,
Ar), 10.28 (s, 2H, –CHO); 13C NMR (100 MHz, CDCl3): d 52.1, 53.7, 67.5, 112.5,
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–CH2), 4.70 (s, 2H, –CH2), 6.51–6.60 (m, 6H, Ar), 6.75–6.79 (m, 2H, Ar), 7.01–
7.06 (m, 4H, Ar), 7.19–7.25 (m, 6H, Ar), 7.36–7.45 (m, 8H, Ar), 7.69 (s, 2H, Ar),
7.92–7.95 (m, 2H, Ar), 8.35 (s, 1H, Ar), 8.45–8.47 (m, 2H, Ar), 8.93 (br, 2H, –NH),
9.37 (br, 2H, –NH); 13C NMR (100 MHz, CDCl3): d 42.6, 52.0, 53.6, 66.9, 110.9,
111.0, 112.3, 115.3, 120.6, 122.8, 125.1, 126.1, 127.0, 127.4, 128.0, 128.3, 129.2,
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