Benzimidazole-based ratiometric fluorescent receptor exhibiting molecular logic gate for Cu2+ and Fe3+

Article abstract of DOI:10.1016/j.tetlet.2009.07.083

We synthesized a novel fluorescent receptor based upon a benzimidazole moiety in a dipodal framework. The receptor exhibited a dual fluorescence emission which is quenched upon addition of Cu²⁺ or Fe³⁺. Interestingly, the receptor of

Full text of DOI:10.1016/j.tetlet.2009.07.083

Tetrahedron Letters 50 (2009) 5555–5558
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Tetrahedron Letters
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Benzimidazole-based ratiometric fluorescent receptor exhibiting
molecular logic gate for Cu2+ and Fe3+
*
Hee Jung Jung, Narinder Singh, Doo Youn Lee, Doo Ok Jang
Department of Chemistry, Yonsei University, Wonju 220-710, Republic of Korea
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 fluorescent receptor based upon a benzimidazole moiety in a dipodal framework.
The receptor exhibited a dual fluorescence emission which is quenched upon addition of Cu2+ or Fe3+
.
.
Received 3 July 2009
Accepted 17 July 2009
Available online 21 July 2009
Interestingly, the receptor offers a ratiometric property and an ‘OR’ logic gate property to Cu2+ and Fe3+
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Benzimidazole,
Logic gate
Fluorescent
Cu(II)
Fe(III)
Supramolecular chemistry has attracted considerable attention
in the area of molecular electronics1 and probes in biological sys-
tems.2 Fluorescent techniques are preferred because they are quite
sensitive and well suited to meet the need for in vivo applications
such as mapping the spatial and temporal distribution of the bio-
logical analytes.3 Nowadays, fluorescent probes are used for con-
structing molecular devices such as wires,4 switches,5 and
diodes.6 Extensive studies have been conducted on molecular logic
gates such as ‘AND’, ‘NOT’, ‘OR’, and their combinational logic cir-
cuits that generally produce light signals in response to a variety
of inputs.7 A typical ‘OR’ logic gate has two input ports and one
output port. A positive input signal on either one of the input ports
or both input ports activates the output signal of the gate.8 On the
basis of this principle, several anticancer pro-drugs with a molecu-
lar logic gate have been designed for selective activation in malig-
nant tissues by a specific enzyme which generates the input
signal.9 The gate activation leads to signal translation for bond
cleavages that release the active drug molecule to the target.
Recent interest has been focused on the development of a new
chemical system involving multiple fluorescent output modes.10
Therefore, providing more functional materials with excellent
properties is an exigent challenge. As part of our ongoing studies
on the development of benzimidazole-based chemosensors,11 we
herein evaluate a new dipodal receptor developed for ratiometric
fluorescence determination and molecular logic gate applications
for Cu2+ and Fe3+—the most abundant transition metal ions in bio-
logical systems that are found to function in proximity to several
biological processes.12
Receptor 2 was prepared through a series of steps as shown in
Scheme 1. For the synthesis of compound 1, 2-aminobenzimidaz-
ole was treated with
a,
a0-dibromo-m-xylene in acetone in the
presence of KOH at room temperature. Compound 2 was synthe-
sized by a condensation reaction of compound 1 with salicylalde-
hyde in MeOH in the presence of a catalytic amount of Zn(ClO4)2,
followed by reduction with NaBH4.13 The final product was charac-
terized by using the spectroscopic methods, which were fully
interpreted and found to be in accord with the formula of com-
pound 2.
Upon excitation at 285 nm, receptor 2 in a HEPES buffered
(10 mM, pH 7.0) aqueous acetonitrile solution (CH3CN/H2O, 8:2,
v/v) exhibited
a dual fluorescence emission that peaked at
320 nm and 420 nm. The receptors with a dual emission are better
fluorescence sensors than those which offer the measurement of
fluorescence changes on a single wavelength because the dual
emission minimizes the measurement errors expected due to such
factors as phototransformation, receptor concentrations, and envi-
ronmental effects.14
To obtain a quantitative insight into the binding affinity of
receptor 2 with metal ions, the fluorescence intensity changes
were measured upon adding various metal ions (Figs. 1 and 2).
Upon addition of Cu2+ or Fe3+, the fluorescence spectrum under-
went dramatic changes. The fluorescence emissions both at
320 nm (originated from local excited state) and at 420 nm (attrib-
uted to charge transfer state) were quenched, while a new band
appeared at 490 nm. The charge transfer band assignment for
* Corresponding author. Tel.: +82 337602261; fax: +82 337602182.
E-mail address: dojang@yonsei.ac.kr (D.O. Jang).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.083

5556
H. J. Jung et al. / Tetrahedron Letters 50 (2009) 5555–5558
N
OH
N
N
CHO
N
HN
OH
OH
NH2
Br
Br
NaBH4
KOH
acetone, rt
+
Zn(ClO4)2,
MeOH, rt
NH2
MeOH, rt
H
HN
N
N
N
N
NH2
N
N
2
1
Scheme 1. Synthesis of receptor 2.
PF6
N
N
N
NH2
NH2
PF6
N
3
Scheme 2. Structure of receptor 3.
Figure 1. Changes in fluorescence intensity of receptor 2 (10
lM) upon addition of
a particular metal salt (40 M) in a HEPES (10 mM, pH 7.0) buffered CH3CN/H2O
l
(8:2, v/v) solution (kex = 285 nm).
complex formation takes place, two fluorophores come close and
consequently undergo stacking. This point is supported by the
structures calculated with molecular modeling. Finally, the band
at 320 nm is originated from quenching local excited state with
metal binding, causing reduction in fluorescence intensity by en-
ergy/electron transfer mechanisms. Under the same conditions,
the effect of other metal ions such as alkali, alkaline earth, and
transition metal ions on receptor 2 was investigated. The data
clearly demonstrated that only Cu2+ and Fe3+ modulate the fluores-
cence intensity at three wavelengths, and no such significant ef-
fects are observed with other metal ions. The changes in the fate
of fluorescence intensity at three wavelengths can offer advantages
in studying ratiometric fluorescence recognition and molecular lo-
gic functions.
To study the relative effect of different equivalents of metal ions
on the fluorescence intensity of receptor 2 at these three wave-
lengths, titrations were performed on Cu2+ and Fe3+ ions (Figs. 3
and 4). These titrations were performed by increasing the amounts
Figure 2. Fluorescence response of receptor 2 (10 lM) at 320 nm upon addition of a
particular metal salt (40 lM) in a HEPES (10 mM, pH 7.0) buffered CH3CN/H2O (8:2,
v/v) solution (kex = 285 nm).
420 nm is based upon our previous reported research work, where
we found the shift in fluorescence band upon anion binding.11f This
reported pure receptor 3 showed a band at 443 nm, but upon anion
binding this band was shifted, thus the original band was due to
charge transfer transition (Scheme 2). Receptor 2 has the close
structural similarity with receptor 3. This is a sufficient reason to
believe that due to charge transfer the pure receptor 2 shows emis-
sion at 420 nm. One can expect that metal binding can quench the
intensity of band at 420 nm, and a new band must appear. This
expectation can be due to the fact that metal binding may lead
to CT band shifting on accounts of change in energy gap. But in
present case, the band at 420 nm is sandwiched by two other
bands, which limit to see this new band development. The new
band that appeared at 490 nm is attributed to the stacking
between two benzimidazole units (stacking in fluorophores). In
free receptor, the two pods are oriented freely. However, when a
Figure 3. Changes in fluorescence spectrum of receptor 2 (10
Fe(NO3)2 (0–122 M) in a HEPES (10 mM, pH 7.0) buffered CH3CN/H2O (8:2, v/v)
solution (kex = 285 nm).
lM) upon addition of
l

H. J. Jung et al. / Tetrahedron Letters 50 (2009) 5555–5558
5557
Figure 4. Changes in fluorescence spectrum of receptor 2 (10
lM) upon addition of
Cu(NO3)2 (0–60 M) in a HEPES (10 mM, pH 7.0) buffered CH3CN/H2O (8:2, v/v)
l
solution (kex = 285 nm).
of metal ions in the 10 lM solution of receptor 2. After each addi-
tion of metal ions, the solutions were shaken with a mechanical
stirrer to make the solution homogenous and attain equilibrium
before measuring the fluorescence intensity. The titrations showed
three common features upon continuous addition of metal ions: (i)
The fluorescence intensity was quenched at 320 nm in a regular or-
der. (ii) The relative extent of intensity quenching was very fast at
420 nm upon additions of first few equivalents of metal ions, but
after that the changes at this wavelength were relatively small.
Thus, the successive additions of metal ions do not follow the reg-
ular trend of quenching at this wavelength. (iii) Small but regular
fluorescence enhancement was observed at 490 nm.
Figure 5. Plots of ratiometric fluorescence intensity (I490/I320) of receptor 2 against
metal ion concentration for: (A) Cu2+ (2–40 M) and (B) Fe3+ (8–122
l lM) in a HEPES
In the cases of Cu2+ and Fe3+, upon metal binding, the receptor
showed regular fluorescence quenching at 320 nm and a fluores-
cence enhancement at 490 nm. Thus, the receptor can be used
for ratiometric sensing of Cu2+ and Fe3+. The fluorescence ratiomet-
ric response of receptor 2 to a selected metal ion is displayed in
Figure 5. The figure shows that Cu2+ can be analyzed ratiometrical-
(10 mM, pH 7.0) buffered CH3CN/H2O (8:2, v/v) solution (kex = 285 nm).
ly in the concentration range of 2–40
a ratiometric response to Fe3+ in the concentration range of
8–122 M.
lM, while the receptor shows
l
From the viewpoint of logic gate, receptor 2 satisfies the Bool-
ean algebra ‘OR’ gate if the two inputs are defined to be Cu2+ and
Fe3+, and if the output is defined to be the intensity of fluorescence
emission. Receptor 2 is selective in its response (quenching) to
Cu2+ and Fe3+ when either of these two metal ions is present. No
changes were observed in the fluorescence intensity of receptor 2
(no quenching) upon addition of any other investigated metal ions.
When both Cu2+ and Fe3+ were present together in a solution, the
fluorescence intensity was low (quenching). Fluorescence emission
spectra showing ‘OR’ logic gate with receptor 2 are displayed in
Figure 6, and the truth table is summarized in Table 1.
Figure 6. Fluorescence emission spectra showing ‘OR’ logic gate with receptor 2 in
a HEPES (10 mM, pH 7.0) buffered CH3CN/H2O (8:2, v/v) solution (kex = 285 nm)
under four experimental conditions: (a) receptor 2 only, (b) receptor 2 with Cu2+, (c)
receptor 2 with Fe3+, and (d) receptor 2 with both Cu2+ and Fe3+
.
The Job plots revealed that Cu2+ and Fe3+ ions form a 1:1 com-
plex with receptor 2 (Figure S1 and S2).15 Binding constants of
receptor 2 with metal ions were calculated by using the Benesi–
Hildebrand plots and were found to be (3.9 0.1) ꢀ 103 Mꢁ1 for
Cu2+ and (5.1 0.2) ꢀ 103 Mꢁ1 for Fe3+, respectively (Figure S3
and S4).16
The quest for the structure of complexes formed between
receptor 2 and metal ions (Cu2+ or Fe3+) was determined by energy
minimization studies with the MacroModel v 9.0 using MM-2*
force field (Fig. 7).17 The structures calculated with the MacroMod-
el revealed that a metal ion binds in the cavity of receptor 2
through both nitrogen and –OH donor sites. A similar type of bind-
ing modes for Cu2+ and Fe3+ authenticates that the receptor sites
are equally available for both metal ions, and thus, the receptor
shows the ‘OR’ logic gate behavior. All our attempts failed to obtain
a single crystal of the complexes suitable for determination of the
Table 1
Truth table for the operation of receptor 2 with Cu2+ and Fe3+
IN1 (Cu2+
)
IN2 (Fe3+
)
OUTPUT (Flu320nm)
0
1
0
1
0
0
1
1
0 (High)
1 (Low)
1 (Low)
1 (Low)
crystal structure. On the other hand, the paramagnetic nature of
Cu2+ and Fe3+ limited us to use the NMR spectroscopy for the struc-
ture determination of these complexes.
We synthesized a novel fluorescent receptor on the basis of
a benzimidazole moiety in a dipodal framework. The binding

5558
H. J. Jung et al. / Tetrahedron Letters 50 (2009) 5555–5558
Figure 7. Energy minimized structure of receptor 2 and its complexes with Cu2+ or Fe3+ as obtained by the MacroModel calculation.
9. (a) Amir, R. J.; Popkov, M.; Lerner, R. A.; Barbas, C. F., III; Shabat, D. Angew.
investigations were performed in aqueous acetonitrile, and the
receptor exhibited a dual fluorescence emission which is quenched
upon addition of Cu2+ or Fe3+. The receptor acted as a ratiometric
fluorescent probe over a wide concentration range of Cu2+ and
Fe3+. The receptor offered an interesting property of molecular
‘OR’ logic gate.
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W. Curr. Med. Chem. 2001, 8, 1093.
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Acknowledgment
This work was supported by the Center for Bioactive Molecular
Hybrids.
Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.07.083.
13. Synthesis of compound 1:
A solution of 2-aminobenzimidazole (100 mg,
a0-dibromo-m-xylene (100 mg, 0.38 mmol), and KOH (21 mg,
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