A novel colorimetric and fluorescent anion chemosensor based on the flavone quasi-crown ether-metal complex

Article abstract of DOI:10.1021/ol035818e

The metal-ligand complex 1 ([Mg (L)]²⁺) (or 2 ([Ca (L)] ²⁺)) was demonstrated to selectively bind HSO₄^- (or H₂PO₄^-) over other anions by using UV-vis absorption and fluorescence

Full text of DOI:10.1021/ol035818e

ORGANIC
LETTERS
2
004
Vol. 6, No. 7
071-1074
A Novel Colorimetric and Fluorescent
Anion Chemosensor Based on the
Flavone Quasi-crown Ether−Metal
Complex
1
†
†,‡
†
†
,†
Li-Li Zhou, Hao Sun, Hua-Ping Li, Hui Wang, Xiao-Hong Zhang,*
,†
†,§
Shi-Kang Wu,* and Shuit-Tong Lee
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100101, China, Institute of Functional Material Chemistry, Faculty of
Chemistry, Northeast Normal UniVersity, Changchun, 130024, China, and Center of
Super-Diamond and AdVanced Films, City UniVersity of Hong Kong,
Hong Kong SAR, China
zhangxh8@yahoo.com; skwu@95777.com
Received September 20, 2003 (Revised Manuscript Received January 5, 2004)
ABSTRACT
The metal−ligand complex 1 ([Mg (L)] 2+) (or 2 ([Ca (L)] 2+)) was demonstrated to selectively bind HSO
-
2 4
(or H PO
-) over other anions by
4
using UV−vis absorption and fluorescence spectroscopy. The studied complex exhibits the remarkable color change and fluorescence quenching
-
-
upon introducing HSO
4
2 4
(or H PO ) anion in acetonitrile. Both the mechanism and structure of the secondary complex of complex 1 with
anion were proposed on the basis of theoretical computation.
Anions play numerous indispensable roles in biological
to design new chemosensors for detection of anions.1c,4 The
metal-ligand coordinative interaction displays its superi-
orities to hydrogen bonding and electrostatic interaction in
1
system. For instance, phosphate and sulfate binding proteins
2
are important biomolecular species in transport systems. The
5
importance of anions in biological systems has attracted
a polar environment. Therefore, the metal-ligand complexes
3
much attention on anion recognition. Efforts have been taken
(
3) (a) Beer, P. D. Acc. Chem. Res. 1998, 31, 71-80. (b) Martinez-
†
Chinese Academy of Sciences.
Northeast Normal University.
City University of Hong Kong.
M a´ n˜ ez, R.; Sancen o´ n, F. Chem. ReV. 2003, 103, 4419-4476.
(4) (a) Snowden, T. S.; Anslyn, E. V. Curr. Opin. Chem. Biol. 1999, 3,
740-746. (b) Antonisse, M. M. G.; Reinhoudt, D. N. Chem. Commun. 1998,
443-448. (c) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40,
486-516.
‡
§
(
1) (a) Beer, P. D.; Hayes, E. J. Coord. Chem. ReV. 2002, 240, 1-23.
(
b) Gale, P. A.; Twyman, L. J.; Handlin, C. I.; Sessler, J. L. Chem. Commun.
1
999, 1851-1852. (c) Berger, M.; Schmidtchen, F. P. Chem. ReV. 1997,
(5) (a) Niikura, K.; Bisson, A. P.; Anslyn, E. V. J. Chem. Soc., Perkin.
Trans. 2 1999, 1111-1114. (b) Fabbrizzi, L.; Marcotte, N.; Stomeo, F.;
Taglietti, A. Angew. Chem., Int. Ed. 2002, 41, 3811-3814. (c) Han, M. S.;
Kim, D. H. Angew. Chem., Int. Ed. 2002, 41, 3809-3811. (d) Fabbrizzi,
L.; Faravelli, I.; Francese, G.; Licchelli, M.; Perotti, A.; Taglietti, A. Chem.
Commun. 1998, 971-972.
9
7, 1609-1646.
(2) (a) Luecke, H.; Quiocho, F. A. Nature 1990, 347, 402-406. (b) He,
J. J.; Quiocho, F. A. Science 1991, 251, 1479-1481. (c) Anslyn, E. V.;
Smith, J.; Kneeland, D. M.; Ariga, K.; Chu, F. Supramol. Chem. 1993, 1,
2
01-209.
1
0.1021/ol035818e CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/12/2004

Figure 1. Structure of the ligand L.
Figure 2. Absorption spectra of complex 1 in acetonitrile (3.0 ×
have been reported for effective and selective recognition
-
5
-
-
6
10 M) after the addition of 4 equiv of anions (no anion, F , Cl ,
of anions. However, most work was focused on the
-
-
-
-
-
-
-
3 3 4 2 4
Br , I , AcO , NO , HCO , HSO , H PO ). The photograph
5
,6
transition-metal ions, such as Cu (II), Zn (II), and Hg (II).
-5
shows the color changes of complex 1 in acetonitrile (3.0 × 10
In this paper, we demonstrate that the alkaline earth metal
ions can serve as coordinative centers as well for anion
recognition.
M) after the addition of 4 equiv of anion (from left to right: no
-
-
-
-
-
-
-
-
3 3 4
anion, F , Cl , Br , I , AcO , NO , HCO , HSO , and
-
2 4
H PO ).
Chromogenic sensors have attracted much attention due
to their capability to detect analytes by the naked eye without
resorting to any expensive instruments. For example, Soto
efficiency of intramolecular charge transfer, resulting in the
red shift of maximal absorbance and the fluorescence
7
and co-workers used a Hg(II)-ligand complex to monitor
2+
quenching. Moreover, the titration of ligand L with Mg
6
b
nitrate concentrations via color change in acetonitrile.
2+
or Ca indicated the formation of stable 1:1 ligand-to-metal
Fluorescence is also popular for its in-real-time detection
9
complex.
1
b,5c,8
and sensitivity.
However, such colorimetric and fluo-
2+
In this continuous work, complex 1 ([Mg (L)] ) or 2 ([Ca
1
c,8
rescent sensors for anionic substrates are relatively few.
2+
(
L)] ) is used as a chemosensor for the detection of different
Our study shows that alkaline earth metal-ligand complexes
anions. Our results show that these two complexes exhibit
quite similar behavior in the presence of studied anions. In
function as unique colorimetric and fluorescent chemosen-
-
sors, with high sensitivity and selectivity to HSO
4
(or
the following sections, we present our studies on complex 1
-
H
2
PO
4
).
2+
(
[Mg (L)] ) in acetonitrile at room temperature.
We previously reported that ligand L (Figure 1), charac-
Figure 2 displays the UV-vis absorption spectra and the
teristic of a hard base, prefers to bind alkali metal and
alkaline earth metal cations, rather than transition-metal ions
photograph of the complex 1 upon addition of 4 equiv of
diverse anions (4 equiv are enough for complete color
conversion). As shown in Figure 2, the spectral feature of
complex 1 displays absorption maximum at 402 nm in
2+
2+
2+
2+
3+ 9
such as Cu , Zn , Co , Ni , and Fe . No changes were
observed in the UV-vis absorption and fluorescence spectra
2+
of ligand L in acetonitrile upon addition of 10 equiv of Cu ,
-
-
acetonitrile. Upon addition of HSO
4
2 4
or H PO , the
2
+
2+
2+
2+
2+
3+
Zn , Co , Ni , Pb , Cd , and Fe . The addition of the
absorption maximum shifts even further to 502 nm, affording
a rose-colored solution. However, little response in absorption
spectra, upon addition of other anions, is observed even at
high concentration. These results indicate that complex 1
+
+
+
alkali metal ions, such as Li , Na , and K did not
significantly effect on the electronic transitions of ligand L.
However, the UV-vis absorption peak of ligand L shifted
from 375 to 402 nm upon introducing Mg2 and Ca ,
concomitant with the color change of the solution from green
to greenish yellow, and the fluorescence intensity of lignad
L was quenched. These observed electronic transition
changes might be interpreted with the enhancement of the
intramolecular charge transfer. As assumed, the oxygen of
carbonyl group of ligand L bound to cations enhances the
+
2+
exhibits high selective complexation with tetrahedron oxoan-
-
ions, especially HSO
4
, whereas it does not bind spherical
-
-
-
-
halides such as F , Cl , Br , and I and planar oxoanions
-
-
-
such as HCO
3
, AcO , and NO
3
. This indicates that
-
complex 1 is selective toward the recognition of HSO
4
or
-
-
H
2
PO
4
3
over other anions including HCO .
Figure 3 shows the UV-vis absorption spectra of complex
-
-
1
with various concentrations of HSO
4
2 4
and H PO ,
(6) (a) Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini, P.; Poggi,
-
-
respectively. The addition of HSO
4
(Figure 3A) or H
2
PO
4
A.; Taglietti, A. Coord. Chem. ReV. 2001, 219-221, 821-837. (b)
Sancen o´ n, F.; Mart ´ı nez-M a´ n˜ aez, R.; Soto, J. Angew. Chem., Int. Ed. 2002,
(Figure 3B) to complex 1, results in substantial decrease of
optical density at 402 nm and gradual increase at 315 and
502 nm. These spectral changes give rise to two clear
isosbestic points, localized at 340 and 470 nm in absorption
spectra. These electronic transition changes imply the forma-
tion of new chromophore which is possibly due to the
formation of the secondary complex between complex 1 and
4
1, 1416-1419.
7) (a) Anzenbacher, J. P.; Try, A. C.; Miyaji, H.; Jursikova, K.; Lynch,
(
V. M.; Marquez, M.; Sessler, J. L. J. Am. Chem. Soc. 2000, 122, 10268-
1
1
3
0269. (b) Miyaji, H.; Sessler, J. L. Angew. Chem., Int. Ed. 2001, 40, 154-
57. (c) Miyaji, H.; Sato, W.; Sessler, J. L. Angew. Chem., Int. Ed. 2000,
9, 1777-1780.
(
8) (a) Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn, E. V. Acc.
Chem. Res. 2001, 34, 963-972.
9) Li, H.-P.; Xie, H.-Z.; Wang, P.-F.; Wu, S.-K. New J. Chem. 2000,
4, 105-108.
(
-
-
2
HSO
4
2 4
(or H PO ).
1072
Org. Lett., Vol. 6, No. 7, 2004

-
5
Figure 3. UV-vis absorption spectra of complex 1 (3 × 10 M)
Figure 5. Variation in absorbance of complex 1 in acetonitrile as
-
with different concentrations of (A) HSO
4
anion (corrected
concentrations: 0, 2, 5, 7, 10, 15, 20, 25, 30, and 35 µM) and (B)
2 4
H PO anion (corrected concentrations: 0, 2, 5, 7, 10, 15, 20, 25,
-
2+
a function of HSO
4
concentration (concentration of [Mg (L)] :
-5
3.0 × 10 M).
-
30, 35, 40, 45, 50, 55, 60, 65, and 70 µM).
The variation of optical densities in UV-vis absorption
spectra has been utilized to determine the stoichiometry and
Fluorescent techniques have been widely used to inves-
11
stability constant of the complex. Figure 5 displays the plot
tigate the receptor-substrate interactions due to their high
-
of optical density at 402 nm versus HSO
4
concentration in
1
0
sensitivity. In this work, the complexation behavior of
complex 1 toward various anions was also assessed by
fluorescence titration in acetonitrile. In contrast to the
absorption method, the anions at low concentration (2 ppm)
can be monitored by the fluorescence method. No change is
acetonitrile. As shown in Figure 5, an approximate plateau
is observed at a 1:1 molar ratio of complex 1 to anion guest,
indicative of the formation of a 1:1 stoichiometrical complex
in solution. On the basis of nonlinear least-squares analysis,
the stability constant (log K value) was calculated to be
-
-
-
-
observed in the fluorescence spectra when F , Cl , Br , I ,
HCO
1
0c
5
.75.
To further verify the affinity to preferred anions arising
-
-
-
3 3
, AcO , and NO anions are added into complex 1
solution. However, the fluorescence of complex 1 is signifi-
from complex 1 instead of ligand L, the control experiment
was performed using ligand L. The UV-vis absorption
spectrum of ligand L does not show any absorbance at 502
-
-
cantly quenched by HSO
4 2 4
or H PO . Figure 4 shows the
-
-
nm, after adding the excess of HSO
4 2 4
(or H PO ) anion in
acetonitrile. This clearly demonstrates that the electronic
transition of 502 nm peak (seen in Figure 3) is not from
ligand L perturbated by added anions, but possible from
electronic perturbation on complex 1. Moreover, the suspects
about the protonation of complex 1 can be ruled out since
no color change is observed upon adding strong acids, such
as HCl in acetonitrile.
To understand the possible secondary complex, we simu-
lated the interaction between complex 1 and anion with
theoretical computation. The general procedures are: the
possible secondary complex was first optimized using G98
program at the HF/6-31G level; and then the single point
energy were computed at the B3LYP/6-31G* level based
on the optimized configuration. The optimized configuration
is shown in Figure 6. As seen in Figure 6, two of the
-
5
Figure 4. Fluorescence spectra of compound 1 (1 × 10 M) with
-
different concentrations of (A) HSO
tions: 0, 2, 5, 7, 10, 15, 20, 25, and 30 µM) and (B) H
corrected concentrations: 0, 2, 5, 7, 10, 15, 20, 25, 30, 35, and 40
4
anion (corrected concentra-
-
2
4
PO anion
(
-
-
4 2 4
conjugated oxygen atoms of HSO or H PO with negative
µM). Inset is the fluorescence spectra of complex 1 with different
charge approach to the positively charged metal center. This
facilitates to configure the formation of hydrogen bonding
-
concentrations of F anion.
(
10) (a) Tong, H.; Zhou, G.; Wang, L.-X.; Jing, X.-B.; Wang, F.-S.;
fluorescence spectra of complex 1 with various concentra-
Zhang, J.-P. Tetrahedron Lett. 2003, 44, 131-134. (b) Kubo, Y.; Tsukahara,
M.; Ishihara, S.; Tokita, S. Chem. Commun. 2000, 653-654. (c) Fabbrizzi,
L.; Licchelli, M.; Rabaioli, G.; Taglietti, A. Coord. Chem. ReV. 2000, 205,
-
-
tions of HSO
4
(Figure 4A) and H
PO
2 4
(Figure 4B),
respectively. The fluorescence intensity is decreased upon
8
5-108.
-
-
the increase of HSO
4
or H PO
2 4
concentrations. These
(11) (a) Fery-Forgues, S.; Birs, M. L.; Guette. J.; Valeur, B. J. Phys.
Chem. 1988, 92, 6233-6237. (b) Valeur, B.; Pouget, J.; Bourson, J.;
Kaschke, M.; Ernsting, N. P. J. Phys. Chem. 1992, 96, 6, 6545-6549. (c)
Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552-4557.
results are consistent with the electronic transition changes
in the absorption titration.
Org. Lett., Vol. 6, No. 7, 2004
1073

addition of anions can be interpreted as the further electron
withdrawing from carbonyl group. It is plausible to deduce
that this further electron withdrawal comes from the center-
metal-assisted hydrogen bonding between the anion and the
carbonyl group of complex 1.
The specific sensitivity and selectivity of complex 1 to
-
-
HSO
4
or H PO
2 4
can be rationalized on the basis of the
structures of preferred anions. The complementary structures
are very important for noncovalent interactions, which leads
to effective and selective recognition. Here, the tetrahedral
-
-
structures of HSO
4 2 4
or H PO provide suitable conjugated
oxygen atoms with negative charge for ligating to the metal
center, which assist the formation of hydrogen bonding
between anion and carbonyl group of complex 1.
-
Figure 6. Model structure depicting the association between HSO
4
and complex 1 ([Mg (L)] 2+). The structure was determined using
the G98W program at the HF/6-31G level.
In conclusion, complex 1 or 2 has been exhibited a
-
-
perspective reagent for anion HSO
4 2 4
or H PO recognition
between the hydroxy group in HSO -
-
in UV-vis absorption and fluorescence spectroscopic studies.
4
or H PO
2 4
and the
-
-
The anion HSO
4 2 4
or H PO recognition can be ascribed to
carbonyl group of ligand L. The calculated results show that
the charge density of oxygen atom in carbonyl group is
increased significantly and that of oxygen atom in ether group
at flavone moiety is slightly enhanced. Combined the single
point energy calculation results with the optimized config-
uration, we can conclude that the center-metal assisted
hydrogen bonding between anion and carbonyl group of
flavone moiety are possibly the major contributor to the
observed electronic transition change in the absorption and
the fluorescence quenching.
the formation of a secondary complex between complex 1
-
-
and HSO
4
or H PO
2 4
anion via center-metal-assisted
hydrogen bonding, which was demonstrated with the simula-
1
tion of theoretical computation and the illustration of H
NMR spectra. To the best of our knowledge, this is the first
case of an alkaline earth metal-ligand complex being used
for anion recognition, which provides a new concept for
developing novel chemsensors for anion recognition.
Acknowledgment. We thank the Major State Research
Development Program of China (Grant No. G2000078100)
and the Chinese Academy of Sciences for financial support.
We thank Jia-Sheng Wu for carefully reading and correcting
our manuscript.
This picture of the secondary complex between complex
1
1
1
and anion is illustrated in H NMR spectroscopy. The H
NMR spectra of complex 1 were conducted on 300 MHz
NMR spectrometer in CD CN. Compared to complex 1, the
3
chemical shifts of protons in flavone moiety shift to the
-
downfield upon adding HSO
4
anion. The degree of these
Supporting Information Available: Experimental pro-
cedures and full characterization for ligand L. This material
is available free of charge via the Internet at http://pubs.acs.org.
shifts varied considerably from 0.52 to 0.06 ppm. Analogous
to the explanation for the vibration frequency changes of
carbonyl in ligand L upon complexation with metallic ions,
9
these downfield chemical shifts of protons in complex 1 upon
OL035818E
1074
Org. Lett., Vol. 6, No. 7, 2004