Fluorescence turn-on sensor for cyanide based on a cobalt(II)- coumarinylsalen complex

Article abstract of DOI:10.1021/ol902852g

(Chemical Equetion Presentation) A Co(II)-salen based fluorescent sensor (1·Co) that can selectively recognize cyanide anions in 1:2 binding stoichiometry over other anions has been developed. 1·Co displayed fluorescence enhancement upon the addition of cyanide owing to the interruption of photoinduced electron transfer from the coumarin fluorophore to the cobalt(II) ion. A general regression method was developed to calculate the binding constants in the 1:2 binding system, through which the 1:2 binding between 1·Co and cyanide anions was estimated to be In the range of micromolar dissociation constants.

Full text of DOI:10.1021/ol902852g

ORGANIC
LETTERS
2010
Vol. 12, No. 4
764-767
Fluorescence Turn-On Sensor for
Cyanide Based on a Cobalt(II)-
Coumarinylsalen Complex
Jae Han Lee,† A Reum Jeong,† Ik-Soo Shin,† Hae-Jo Kim,*,‡ and Jong-In Hong*,†
Department of Chemistry, College of Natural Sciences, Seoul National UniVersity,
Seoul 151-747, Korea, and Department of Chemistry, Kyonggi UniVersity,
Suwon 443-760, Korea
jihong@snu.ac.kr; haejkim@kgu.ac.kr
Received December 10, 2009
ABSTRACT
A Co(II)-salen based fluorescent sensor (1·Co) that can selectively recognize cyanide anions in 1:2 binding stoichiometry over other anions
has been developed. 1·Co displayed fluorescence enhancement upon the addition of cyanide owing to the interruption of photoinduced electron
transfer from the coumarin fluorophore to the cobalt(II) ion. A general regression method was developed to calculate the binding constants
in the 1:2 binding system, through which the 1:2 binding between 1·Co and cyanide anions was estimated to be in the range of micromolar
dissociation constants.
Optical sensors for anions have gained considerable attention
due to their applicability to the detection of toxic anions,
monitoring of enzyme reactions related to anions, and
bioimaging of anions.1 Cyanide anions are known to inhibit
the process of cellular respiration in mammals by interacting
strongly with a heme unit in the active site of cytochrome
a3.2 The high level of intracellular calcium ion concentration
is caused by the uptake of cyanide anions and triggers a
cascade of enzymatic reactions to increase the level of
reactive oxygen species, which finally inhibits the antioxidant
defense system.3 The widespread use of cyanide anions in
industrial settings (1.5 million tons per year) and potential
threats for terrorism4 also increase the significance to sense
cyanide anions by a simple and fast luminescence method.
Most cyanide sensors operate on the basis of a 1:1 binding
† Seoul National University.
‡ Kyonggi University.
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10.1021/ol902852g  2010 American Chemical Society
Published on Web 01/21/2010

mode whether they are transition metal complexes5 or
nonmetal probes.6 The 1:2 binding system, however, does
closely resemble nature’s binding or recognition systems like
antibody-antigen7 and protein-metal ion binding.8 We
thought that 1:2 binding between a metal complex and a
cyanide anion would provide a useful sensing mechanism.
Herein, we report a Co(II)-salen based fluorescent sensor
(1·Co) bearing coumarin derivatives on the 1,2-positions of
the ethylenediamine part of the salen ligand and cobalt(II)
ion as a fluorescence quencher. The cobalt(II)-salen complex
exhibited selective and tight binding toward cyanide anions
on the basis of a 1:2 host-guest binding mode.
In addition, we developed a general method to determine
the binding constants for a 1:2 binding system using a C
program. Generally, a 1:2 binding system usually involves
difficulties in quantifying binding properties such as binding
constant and stoichiometry compared with a 1:1 binding
system. We expected that the calculation method developed
for the current 1:2 binding system will establish a general
solution for quantification of the binding properties in
multiple binding systems.
Compound 1 was prepared by the treatment of 2,2′-
dihydroxyphenylene ethylenediamine with 2 equiv of 7-di-
ethylaminocoumarin-3-carboxaldehyde through a diaza-Cope
rearrangement reaction.9 The metal complex, 1·Co, was
obtained by mixing 1 with cobalt(II) acetate in the presence
of triethylamine (TEA), followed by recrystallization of the
complex in methanol and dichloromethane (Scheme 1).10
Figure 1. Complexation of the cyanide ion with 1·Co.
for [1·Co·(CN-)2]. The effect of anions on the emission
spectra of 1·Co was examined by adding various anions to
the cobalt-salen complex in acetonitrile. 1·Co exhibited a
significant fluorescence enhancement upon the addition of
cyanide, whereas other anions showed no detectable fluo-
rescence changes (Figure 2). Compared to cyanide, fluoride
Scheme 1. Synthetic Strategy of 1·Co
Figure 2. Relative fluorescence emission of 1·Co (5 µM, CH3CN)
in the presence of various anions (10 µM). IO ) emission of 1·Co.
ICN ) fluorescence emission intensity of 1·Co in the presence of 2
equiv of CN-. All the anions were used as tetrabutylammonium
salts.
showed a little fluorescence increase even though fluoride
is a much more basic anion.11 The nucleophilic azide anion
also did not show any significant fluorescence change. These
The effect of the metal coordination on the emission
spectra was measured by fluorescence spectroscopy: the
cobalt-salen complex (1·Co) showed weak fluorescence
emission compared with ligand 1 because there seems to exist
an intramolecular photoinduced electron transfer (PeT) from
the coumarin moiety to the cobalt cation of 1·Co. We
expected that the addition of cyanide anions would cause
an alteration in HOMO and LUMO energy levels and lead
to a significant change in the fluorescence intensity of 1·Co
by blocking the PeT pathway (Figure 1).
As expected, we found that the fluorescence of the 1·Co
complex was significantly enhanced upon the addition of
cyanide anions. 1:2 Complexation between 1·Co and the
cyanide anion was confirmed by high-resolution mass
spectral analysis: observed m/z 803.2341, calculated 803.2341
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Org. Lett., Vol. 12, No. 4, 2010
765

results suggest that 1·Co has a high selectivity for cyanide
over other anions.
moiety enough to block the donor-excited PeT and
enhance the fluorescence of 1·Co. However, weak or
noncoordinating anions do not complex with the cobalt
ion and thus cannot block the donor-excited PeT pathway
in 1·Co. Therefore, the selectivity of 1·Co plausibly comes
from the stronger coordination ability of the cyanide anion
over other anions.
As expected, a Job’s plot for binding between 1·Co and
cyanide has shown a 1:2 binding stoichiometry (Support-
ing Information).13 To calculate the binding constants for
the 1:2 binding system from the spectral data without
assumptions,14 we have introduced a simple regression
program using C language, which is generally applicable
to any “one-to-two” binding system (Supporting Informa-
tion).
Using the program, K1 and K2 for cyanide binding to
1·Co were determined to be K1 g 107 M-1 and K2 ) 4.0
× 105 M-1 by fluorescence titration in acetonitrile.15 It is
remarkable that the simple cobalt(II)-salen complex
shows such a tight binding toward cyanide anions with
micromolar dissociation constants. The calculation pro-
gram shows satisfactory results. The fluorescence intensity
constants for HG and HG2 were calculated to be 166 and
235, respectively, which are very similar to the observed
intensities (Figure 4).
To address the origin of the fluorescence enhancement
of 1·Co by the coordination of cyanide anions, the HOMO
and LUMO energy levels of the cobalt-salen complex in
the absence and in the prescence of cyanide anions were
measured through cyclic voltammetric and UV-vis spec-
troscopic measurements by using the method of Nagano
and co-workers.12 The energy diagrams indicate that a
donor-excited PeT could occur in the absence of cyanide
anions, and therefore an electron transfer takes place from
the excited coumarin fluorophore to the LUMO of the
cobalt-salen complex. The PeT is, however, prohibited
in the presence of cyanide anions by the raised LUMO
level of the cobalt-salen complex, which can enhance
the fluorescence of 1·Co upon the addition of cyanide
(Figure 3). The fluorescence enhancement possibly comes
Figure 3. Energy diagrams (in eV) of coumarin and Co(II)-salen
without and with cyanide anion, which was measured from
cyclovoltametry and UV-vis spectroscopy.
from the strong coordination ability of the cyanide anion
to 1·Co.5 This strong coordination of cyanide will alter
the energy levels in the cobalt(II)-salen complex, espe-
cially raising the LUMO level of the cobalt(II)-salen
Figure 4. Fluorescence titration curve by the addition of cyanide
(0-21.9 µM) to 1·Co (5 µM) in acetonitrile. Calculated (line) and
observed data (dot).
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In summary, we have developed a Co(II)-salen com-
plex (1·Co) which exhibited selective and tight binding
to a cyanide anion. 1·Co displayed a significant fluores-
cence enhancement upon the addition of cyanide owing
to the interruption of photoinduced electron transfer from
the coumarin fluorophore to the cobalt(II)-salen moiety.
We also developed a general regression method to estimate
(9) (a) Kim, H.-J.; Kim, W.; Lough, A. J.; Kim, B. M.; Chin, J. J. Am.
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(10) See the Supporting Information.
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New York, 1987; pp 24-28.
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(15) Using the program, K1 was calculated over 107 M-1 in acetonitrile,
which is above the limit of detection. Detailed programming for the
calculation of K1 and K2 is described in the Supporting Information.
766
Org. Lett., Vol. 12, No. 4, 2010

the binding constants in the 1:2 binding system and found
the dissociation constants of 1·Co to be in the range of
micromolar concentrations for cyanide anions.
Supporting Information Available: Synthetic procedure
and selected spectral data for compounds 1 and 1·Co,
fluorescence spectra, Job’s plots, electrochemical data, and
a regression program for the determination of binding
constants. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the KRF
Grant (Grant No. 2008-314-C00206) and the NRF grant
funded by the MEST (Grant No. 2009-0080734). ARJ and
ISS thank the Ministry of Education for the BK 21 fellow-
ship.
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