Highly Fe3+ selective ratiometric fluorescent probe based on imine-linked benzimidazole

Article abstract of DOI:10.1016/j.tetlet.2008.03.002

We synthesized a novel benzimidazole-based fluorescent receptor bearing imine linkages with two sets of sp² nitrogens, and investigated its binding properties toward various metal ions. The receptor exhibited a shift in emission band upon binding with Fe³⁺ ions, and no such significant response was noticed in other metal ions. The receptor shows a property of selective ratiometric fluorescent probe of Fe³⁺ ions without interferences of the background metal ions.

Full text of DOI:10.1016/j.tetlet.2008.03.002

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2960–2964
Highly Fe3+ selective ratiometric fluorescent probe
based on imine-linked benzimidazole
Hee Jung Jung a, Narinder Singh a, Doo Ok Jang a,b,
*
a Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
b Center for Bioactive Molecular Hybrids, Yonsei University, Seoul 120-749, Republic of Korea
Received 19 February 2008; accepted 3 March 2008
Available online 6 March 2008
Abstract
We synthesized a novel benzimidazole-based fluorescent receptor bearing imine linkages with two sets of sp2 nitrogens, and investi-
gated its binding properties toward various metal ions. The receptor exhibited a shift in emission band upon binding with Fe3+ ions, and
no such significant response was noticed in other metal ions. The receptor shows a property of selective ratiometric fluorescent probe of
Fe3+ ions without interferences of the background metal ions.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Ratiometric; Fluorescence; Imine linkage; Benzimidazole; Iron(III)
Development of chemosensors for selective recognition
and detection of important metal ions is an inspiring
research area of modern supramolecular chemistry.1
Chemosensors are fabricated to monitor host–guest inter-
actions through the changes of their physical properties
in response to the presence of a guest. The rationale behind
the design of chemosensor should meet some basic require-
ments such as exhibiting a qualitative response to a parti-
cular analyte, displaying a different type of response to
other analytes, and affording a quantitative determination
for a wide concentration range of analyte.2,3
Among the chemosensors, fluorescent receptors have
been actively investigated because of the high sensitivity
of the system.4 Especially, fluorescent chemosensors that
show the shift of emission bands upon binding with ana-
lytes are particularly attractive since they are capable of
the ratiometric sensing of analytes.5 For precise analyses,
ratiometric chemosensors have offered advantages over
the conventional monitoring of fluorescence intensity at a
single wavelength. A dual emission system can minimize
the measurement errors because of the factors such as
phototransformation, receptor concentrations, and envi-
ronmental effects.6,7 In recent years, a number of fluores-
cent receptors have been reported to have a pronounced
selectivity for a particular metal ion; however, only a few
have reported on fluorescent chemosensors for Fe3+
ions.8–14 Furthermore, to the best of our knowledge, no
ratiometric fluorescent receptor for Fe3+ ions has been
reported while ratiometric fluorescence receptors have been
available for other metal ions.15–24
Iron is one of the most important microelements for
human health, and is known to endow a great deal of phys-
iological functions.25 Many structural units in the form of
iron complex take part in the process of transporting and
exchanging the oxygen, and several enzymes contain ferric
ions as part of a catalytic site.26 As part of our ongoing
studies on benzimidazole-based receptors, here we present
a receptor bearing benzimidazole moiety that can function
as a highly selective ratiometric fluorescent probe for Fe3+
ions.27–31
The design of receptor is based upon the fact that iron
has a strong binding affinity with receptors having sp2
nitrogen of imidazole.32–34 Moreover, imidazole is a ubiq-
uitous ligand present at the active site of many metallopro-
teins.35 The receptor is employed with two sets of sp2
*
Corresponding author. Tel.: +82 337602261; fax: +82 337602182.
E-mail address: dojang@yonsei.ac.kr (D. O. Jang).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.002

H. J. Jung et al. / Tetrahedron Letters 49 (2008) 2960–2964
2961
Host
Na (I)
K (I)
nitrogens: one set from imine-linkages and the other from
benzimidazole.
250
200
150
100
50
Ba (II)
Mg (II)
Ca (II)
Fe (III)
Co (II)
Ni (II)
Cu (II)
Zn (II)
Ag (I)
Hg (II)
Receptor 1 was prepared by the condensation reaction
of isophthalaldehyde with 2-(2-aminophenyl)-benzimid-
azole.36 And receptor 3 was prepared by the condensation
reaction of isophthalaldehyde with 2-(2-aminophenyl)-ben-
zothiazole.37 The corresponding reaction of benzaldehyde
with 2-(2-aminophenyl)-benzimidazole afforded receptor
4.38 Receptor 2 was obtained with the reduction of imine
linkages of receptor 1 with NaBH4 (Scheme 1).39
0
375
400
425
450
475
500
525
550
575
600
The fluorescence spectrum of receptor 1 displayed an
emission band at k = 412 nm, when recorded with a
10 lM concentration of receptor 1 in CH3CN/H2O (95:5,
v/v) upon excitation at k = 288 nm. We evaluated the bind-
ing properties of receptor 1 toward various metal ions
(Fig. 1). Upon addition of a 100 lM solution of Fe3+ ions
to the 10 lM solution of receptor 1, the intensity of emis-
sion band at 412 nm decreased along with the appearance
of a new red-shifted emission band at 475 nm. Other metal
ions (as nitrate salts) including alkali (Na+ and K+), alka-
line earth (Ca2+, Mg2+, and Ba2+) and transition metal
ions (Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Hg2+) revealed
no such shift in the emission band under the same condi-
tions. Cu2+ ions showed some binding affinity with recep-
tor 1, as can be interpreted from the quenching of
fluorescence intensity at k = 412 nm, but Cu2+ binding
did not cause any shift in the emission band. The structural
rigidity of metal complex and metal binding close to the
fluorophore might be the factors that cooperate to induce
the emission band shift. Fluorescence ratiometric response
of receptor 1 toward the surveyed metal ions is displayed in
Figure 2. The results show a highly selective response of
receptor 1 to Fe3+ ions as compared to the other metal
ions.
Wavelength (nm)
Fig. 1. Changes in fluorescent intensity of receptor 1 (10 lM) upon the
addition of a particular metal salt (100 lM) in CH3CN/H2O (95:5, v/v).
Hg (II)
Ag (I)
Zn (II)
Cu (II)
Ni (II)
Co (II)
Fe (III)
Ca (II)
Mg (II)
Ba (II)
K (I)
Na (I)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
I 475 / I
412
Fig. 2. Fluorescence ratiometric response (I475/I412) of receptor 1 (10 lM)
upon the addition of a particular metal salt (100 lM) in CH3CN/H2O
(95:5, v/v).
Our hypothesis that emphasizes two types of sp2 nitro-
gens as a better combination for Fe3+ ion complexation
was testified by the recognition behavior of compounds
2-4 (Fig. S1–3). Both receptors 2 and 3 have a dipodal skel-
eton similar to that of receptor 1, except that receptor 2
lacks imine linkages while receptor 3 has two sulfurs as
potential donor groups. The sulfur donors of receptor 3
can undergo complexation with a metal ion as an alternate
of sp2 nitrogens of benzimidazole. The design of receptor 4
resembles one half part of receptor 1. To evaluate the metal
ion recognition behavior of receptors 2–4, we selected a
10 lM concentration of compounds 2 and 3, and a
20 lM concentration of compound 4, respectively. These
concentration ranges have approximately the same number
of binding sites as that of a 10 lM concentration of 1. The
binding affinity of receptor 2 was very poor with respect to
all metal ions, implying that the reduction of imine linkages
provides a receptor with a structure too much flexible to
bind with any metal ions. The result supports the fact that
the imine linkages of receptor 1 provide not only appropri-
ate binding sites but also some structural rigidity required
to organize the pseudocavity of receptor 1.
N
N
N
N
NH
N
HN
N
HN
NH
HN
NH
1
2
N
N
N
N
N
N
S
S
HN
3
4
Scheme 1.

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H. J. Jung et al. / Tetrahedron Letters 49 (2008) 2960–2964
To confirm our point of structural rigidity of receptor 1
ually. A careful analysis of the changes in fluorescence
intensities of receptor 1 with continuous addition of Fe3+
ions revealed that the addition of first two equivalent of
Fe3+ exhibited a small enhancement in the fluorescence
intensity at k = 475 nm. However, upon addition of the
next several equivalents of Fe3+, a marked fluorescence
intensity enhancement was observed at this wavelength.
Thus, receptor 1 can be used for selective recognition of
Fe3+ ions in quite a wide range of iron concentrations,
and it can detect as little as 2.83 lM concentration of
Fe3+ ions.41 On the basis of Benesi–Hildebrand plot,42
the association constant Ka of receptor 1 for Fe3+ ions
was calculated to be (2.9 0.2) Â 103 MÀ1. The stoichiom-
etry of the complex formed was found to be 1:1 as deter-
mined by Job’s plot.43
To evaluate the effect of other background metal ions
upon the signal response induced with iron complexation
of receptor 1, the cation interference experiments were car-
ried out (Fig. 5). The experiments were performed to mea-
sure the fluorescence intensity in a series of solutions
containing receptor 1, different amounts of Fe3+ ions,
and another metal ion. The fluorescence intensity was
compared to receptor 2, the MacroModel calculations were
performed (Fig. 3).40 The calculated structures clearly sup-
port our hypothesis underlining pre-organization in the
pseudocavity of receptor 1, while such type of pre-organi-
zation of binding sites is not observed in the structure of
receptor 2.
Receptor 3 has high binding affinity toward a wide range
of metal ions, exhibiting metal ion complexation by using
alternative binding sites with a variety of metal ions. Sim-
ilarly, receptor 4 was found to be non-selective for any
metal ions. The metal ion recognition investigations with
receptors 2–4 disclosed a fact that proper selections of
binding sites as well as the steric features of the pseudocav-
ity of receptor 1 are mandatory for effective complexation
of Fe3+ ions.
Figure 4 showed the changes in fluorescence spectra pat-
tern of 1 upon titration of Fe3+ ions. The titration results
show that with the continuous addition of Fe3+ ions to
the 10 lM solution of receptor 1, the intensity of the emis-
sion band decreased at k = 412 nm, and the intensity of a
newly emerged band started increasing at k = 475 nm grad-
Fig. 3. Energy minimized structure of (A) receptor 1 and (B) receptor 2 as obtained by MacroModel calculation.
250
200
150
100
50
0
375
400
425
450
475
500
525
550
575
600
Wavelength (nm)
Fig. 4. Changes in fluorescence spectrum of receptor 1 (10 lM) upon the addition of Fe(NO3)3 (0–196 lM) in CH3CN/H2O (95:5, v/v).

H. J. Jung et al. / Tetrahedron Letters 49 (2008) 2960–2964
2963
Supplementary data
120
100
80
60
40
20
0
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2008.03.002.
Fe(III) Only
Fe(III) + Co(II)
Fe(III) + Ni(II)
Fe(III) + Ag(I)
Fe(III) + Zn(II)
Fe(III) + Cu(II)
References and notes
1. Desvergne, J. P.; Czarnik, A. W. Chemosensors of Ion and Molecule
Recognition; Kluwer: Dordrecht, 1997.
2. Eggins, B. R. Chemical Sensors and Biosensors; John Wiley: New
York, 2002.
0
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20
3. Hancock, R. D.; Melton, D. L.; Harrington, J. M.; McDonald, F. C.;
Gephart, R. T.; Boone, L. L.; Jones, S. B.; Dean, N. E.; Whitehead, J.
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M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. Chem. Rev. 1997, 97,
1515.
Equiv of Fe (III)
Fig. 5. Estimation of Fe3+ ions in the presence of other metal ions in
CH3CN/H2O (95:5, v/v) at k = 475 nm.
5. Lakowicz, J. R. Topics in Fluorescence Spectroscopy; Plenum Press:
New York, 1994.
6. Grynkiewicz, G.; Poenie, M.; Tsien, R. Y. J. Biol. Chem. 1985, 260,
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K. Org. Lett. 2007, 9, 1129.
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J. Am. Chem. Soc. 2007, 129, 1500.
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almost identical to that obtained in the absence of any
interfering metal ion. The result confirmed that no metal
ions show much disturbance with the signal response
induced by iron complexation with receptor 1.
To study the plausible coordination environment of Fe3+
in the iron complex of receptor 1, the iron complex of recep-
tor 1 was synthesized and characterized independently.44
The Fe3+ ion are assumed to have a octahedral geometry
in the complex, in which receptor 1 acts as a tetradentate
ligand and other two coordination sites are occupied by
nitrates. The formula of complex, Fe(receptor 1)(NO3)2, is
supported by FAB mass spectrum having m/z = 697.1237,
which corresponds to M+H+ of the proposed structure of
the complex (the theoretical calculated value is m/z =
697.1241). Nitrate exhibits variable coordination modes
in different metal complexes such as a monodentate (g1-
ONO2) ligand and a bidentate ligand (g2-O2NO). The
high-frequency band at 1522 cmÀ1 is assigned to ma(NO2)
and that near 1384 cmÀ1 to ms(NO2). The smaller separation
of the high-frequency bands is characteristic of monoden-
tate nitrate coordination. The bidentate nitrate coordina-
tion is expected to have three IR-active stretching modes:
m(N@O) for the uncoordinated oxygen and two (symmetric
and asymmetric) modes for the coordinated NO2 fragment.
The absence of three IR-active stretching modes implies
only monodentate coordination of the nitrate.45,46
In conclusion, we synthesized a novel benzimidazole-
based fluorescent receptor bearing imine linkages with
two sets of sp2 nitrogens, investigating its binding proper-
ties toward various metal ions. The receptor shows a selec-
tive recognition behavior for Fe3+ ions along with a shift in
emission band. Receptor 1 exhibits ratiometric fluorescent
probe along a wide concentration range of Fe3+ with no
interferences of background metal ions.
24. Woodroofe, C. C.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125, 11458.
25. Lynch, S. R. Nutr. Rev. 1997, 55, 102.
26. Al-Karadaghi, S.; Hansson, M.; Nikonov, S.; Jo¨nsson, B.; Heder-
stedt, L. Structure 1997, 11, 1501.
27. Singh, N.; Jang, D. O. Org. Lett. 2007, 9, 1991.
28. Joo, T. Y.; Singh, N.; Lee, G. W.; Jang, D. O. Tetrahedron Lett. 2007,
48, 8846.
29. Moon, K. S.; Singh, N.; Lee, G. W.; Jang, D. O. Tetrahedron 2007,
63, 9106.
30. Kim, H. S.; Moon, K. S.; Jang, D. O. Supramol. Chem. 2006, 18, 97.
31. Kang, J.; Kim, H. S.; Jang, D. O. Tetrahedron Lett. 2005, 46, 6079.
32. Chauvin, A. S.; Frapart, Y. M.; Vaissermann, J.; Donnadieu, B.;
Tuchagues, J. P.; Chottard, J. C.; Li, Y. Inorg. Chem. 2003, 42, 1895.
Acknowledgment
This work was supported by the Center for Bioactive
Molecular Hybrids at Yonsei University.

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H. J. Jung et al. / Tetrahedron Letters 49 (2008) 2960–2964
33. Ohta, H.; Sunatsuki, Y.; Kojima, M.; Ikuta, Y.; Goto, Y.; Matsu-
moto, N.; Iijima, S.; Akashi, H.; Kaizaki, S.; Dahan, F.; Tuchagues,
J.-P. Inorg. Chem. 2004, 43, 4154.
(t, 2H, Ar, J = 8.0 Hz), 8.05–8.08 (m, 3H, Ar), 8.22 (d, 2H, Ar,
J = 8.0 Hz), 8.59 (d, 2H, Ar, J = 8.0 Hz), 8.76 (d, 2H, Ar,
J = 8.0 Hz), 9.09 (s, 1H, Ar), 9.14 (s, 2H, CH@N); 13C NMR
(DMSO-d6, 100 MHz) d 119.4, 121.6, 122.6, 124.9, 126.2, 126.7, 128.2,
130.5, 132.0, 133.1, 136.5, 136.8, 149.1, 151.6, 162.0, 163.3. Anal.
Calcd for C34H22N4S2: C, 74.15; H, 4.03; N, 10.17. Found: C, 74.16;
H, 4.02; N, 10.10.
´
34. Laurent Benisvy, L.; Halut, S.; Donnadieu, B.; Tuchagues, J.-P.;
Chottard, J.-C.; Li, Y. Inorg. Chem. 2006, 45, 2403.
35. Frazee, R. W.; Orville, A. M.; Dolbeare, K. B.; Yu, H.; Ohlendorf, D.
H.; Lipscomb, J. D. Biochemistry 1998, 37, 2131.
36. Synthesis of receptor 1: A solution of isophthalaldehyde (100 mg,
0.75 mmol), 2-(2-aminophenyl)-benzimidazole (390 mg, 1.87 mmol)
and catalytic amount of zinc perchlorate in MeOH was stirred at
room temperature for 12 h. Upon completion of reaction, the solid
separated out. The solid material was washed with CH2Cl2 (50 mL)
affording a light yellow colored solid (372 mg, 96%); mp 216–218 °C;
1H NMR (DMSO-d6, 400 MHz) d 6.80 (d, 2H, Ar, J = 8.4 Hz), 6.83
(t, 2H, Ar, J = 4.4 Hz), 6.96–7.00 (m, 4H, Ar), 7.14–7.21 (m, 4H, Ar),
7.24–7.29 (m, 3H, Ar), 7.37 (d, 1H, Ar, J = 11.6 Hz), 7.51 (s, 1H,
CH@N), 7.56 (s, 1H, CH@N), 7.62 (t, 2H, Ar, J = 6.4 Hz), 7.93 (t,
2H, Ar, J = 8.8 Hz); 13C NMR (DMSO-d6, 100 MHz) d 102.6, 103.9,
106.8, 110.4, 110.8, 114.2, 114.3, 116.9, 118.8, 119.9, 123.8, 124.9,
133.2, 133.3, 135.2, 136.1. Anal. Calcd for C34H24N6: C, 79.05; H,
4.68; N, 16.27. Found: C, 79.33; H, 4.30; N, 16.23.
37. Synthesis of receptor 2: NaBH4 (36 mg, 0.95 mmol) was added to a
solution of receptor 1 (100 mg, 0.2 mmol) in 30 mL of MeOH/THF
(8:2, v/v) solvent mixture. The mixture was stirred for 3 h at room
temperature. Upon completion of reaction, the solvent was evapo-
rated, and water was poured into the content of the reaction mixture.
The organic material was extracted with dichloromethane
(3 Â 50 mL). The organic layer was dried over anhydrous MgSO4.
After filtration and evaporation, receptor 2 was obtained as a yellow
solid material (89.7 mg, 89%); mp 241–243 °C; 1H NMR (CD3CN,
400 MHz) d 2.46 (s, 4H, CH2), 5.97 (br, 2H, NH), 6.66–6.76 (m, 6H,
Ar), 6.85 (t, 2H, Ar, J = 6.8 Hz), 6.98 (t, 2H, Ar, J = 6.8 Hz), 7.16–
7.28 (m, 6H, Ar), 7.62–7.67 (m, 2H, Ar), 8.02 (t, 2H, Ar, J = 6.0 Hz);
13C NMR (DMSO-d6, 400 MHz) d 68.3, 110.9, 112.1, 115.1, 118.6,
119.1, 122.4, 122.6, 125.1, 127.2, 132.1, 133.2, 141.5, 143.4, 144.3,
147.1. Anal. Calcd for C34H28N6: C, 78.44; H, 5.42; N, 16.14. Found:
C, 78.43; H, 5.26; N, 16.34.
39. Synthesis of receptor 4: A solution of benzaldehyde (100 mg,
0.94 mmol), 2-(2-aminophenyl)-benzimidazole (296 mg, 1.4 mmol)
and catalytic amount of zinc perchlorate in MeOH was stirred at
room temperature for 12 h. The progress of the reaction was
monitored by TLC. Upon completion of the reaction, half of the
solvent was evaporated and diethyl ether was added to it dropwise till
solid material separated out. The solid material was filtered and
washed with diethylether (50 mL) affording a yellow colored solid
(186 mg, 67%); mp 228–230 °C; 1H NMR (DMSO-d6, 400 MHz) d
6.83 (t, 1H, Ar, J = 7.4 Hz), 7.05–7.09 (m, 2H, Ar), 7.12–7.18 (m, 2H,
Ar), 7.22–7.26 (m, 3H, Ar), 7.30–7.31 (m, 3H, Ar), 7.58 (s, 1H,
CH@N), 7.63 (d, 1H, Ar, J = 7.3 Hz), 7.94 (d, 1H, Ar, J = 7.3 Hz);
13C NMR (DMSO-d6, 100 MHz) d 110.6, 114.81, 118.2, 118.4, 122.2,
122.3, 124.7, 126.0, 128.8, 131.8, 132.7, 140.2, 143.2, 146.8. Anal.
Calcd for C20H15N3: C, 80.78; H, 5.08; N, 14.13. Found: C, 80.49; H,
4.70; N, 13.88.
40. Mohamed, F.; Richards, N. G. T.; Liskamp, W. C. H.; Lipton, M.;
Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput.
Chem. 1990, 11, 440.
41. Shortreed, M.; Kopelman, R.; Kuhn, M.; Hoyland, B. Anal. Chem.
1996, 68, 1414.
42. Benesi, H.; Hildebrand, H. J. Am. Chem. Soc. 1949, 71, 2703.
43. Job, P. Ann. Chim. 1928, 9, 113.
44. Synthesis of Fe(receptor 1)(NO3)2: A solution of receptor 1 (260 mg,
0.50 mmol) and iron nitrate (305 mg, 0.75 mmol) was taken in
acetonitrile. The reaction mixture was stirred with reflux for 2 h.
Upon completion of the reaction the solid material was separated
out. The solid material was filtered and washed with acetonitrile
(20 mL) affording a brown-yellow colored solid (302 mg, 86%);
mp above 300 °C; IR 1522 cmÀ1 ma(NO2), 1384 cmÀ1 ms(NO2);
HRMS (FAB) calcd for C34H25FeN8O6 (M+H+): 697.1241, found
697.1237.
45. Gulyan, G. M.; Kurtikyan, T. S.; Ford, P. C. Inorg. Chem. 2008, 47,
787.
46. Wyllie, G. R. A.; Munro, O. Q.; Schulz, C. E.; Scheidt, W. R.
Polyhedron 2007, 26, 4664.
38. Synthesis of receptor 3: This compound was synthesized by the same
method as adopted for the synthesis of receptor 1 except that 2-(2-
aminophenyl)-benzothiazole (422 mg, 1.86 mmol) was used instead of
2-(2-aminophenyl)-benzimidazole. The final product is a lemon yellow
colored solid (335 mg, 81%); mp 228–230 °C; 1H NMR (DMSO-d6,
400 MHz) d 7.47 (t, 2H, Ar, J = 5.2 Hz), 7.62–7.67 (m, 6H, Ar), 7.82