Discriminating detection between Mg2+ and Ca2+ by fluorescent signal from anthracene aromatic amide moiety

Article abstract of DOI:10.1021/ol701976x

(Chemical Equation Presented) Novel fluorescent reagents 1 and 2 were synthesized. In the absence of metal ions, the fluorescence emissions of these compounds were quite weak, but their intensities were much greater in the presence of alkaline earth metal

Full text of DOI:10.1021/ol701976x

ORGANIC
LETTERS
2+
2007
Vol. 9, No. 22
Discriminating Detection between Mg
and Ca by Fluorescent Signal from
Anthracene Aromatic Amide Moiety
2+
4
419-4422
†
‡
,‡
Jeongsik Kim, Tatsuya Morozumi, and Hiroshi Nakamura*
DiVision of EnVironmental Material Science, Graduate School of EnVironmental
Science, Hokkaido UniVersity, Sapporo, Hokkaido 060-0810, Japan, and Section of
Materials Science, Research Faculty of EnVironmental Earth Science, Hokkaido
UniVersity, Sapporo, Hokkaido 060-0810, Japan
nakamura@ees.hokudai.ac.jp
Received June 29, 2007
ABSTRACT
Novel fluorescent reagents 1 and 2 were synthesized. In the absence of metal ions, the fluorescence emissions of these compounds were
quite weak, but their intensities were much greater in the presence of alkaline earth metal ions. The peak shape and maximum wavelength of
the emission of the complex with Mg2+ differed from those of Ca and other alkaline earth metal ions. The peak wavelength difference was
2+
30 nm.
1
Because of their high sensitivity and selectivity, crown ethers
important target in this field is the design and development
of chemosensors for analytical use. Although Na , K , Mg ,
2
+
+
2+
and polyethers with fluorescent detecting moieties are useful
as chemical sensors for the detection and characterization
of alkali and alkaline earth metal cations. A particularly
2
+
2+
2+
Ca , Fe , and Hg are well-known to play critical roles
in environmental systems, only a few examples of chemo-
2+
2+
sensors for discriminating between Mg and Ca have been
reported. The similar chemical properties of Mg and Ca
makes differential detection difficult.
†
3
2+
2+
Division of Environmental Material Science, Graduate School of
Environmental Science.
‡
Section of Materials Science, Research Faculty of Environmental Earth
Science.
(
A reliable analytical method with use of EDTA is known
1) (a) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 197. (b) de Silva,
2+
2+
to be able to determine the concentration of Mg and Ca
A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy,
C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (c) Valeur,
B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3. (d) McQuade, D. T.; Pullen,
A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537.
ions in solution. But EDTA does not show a selectivity for
the two cations, and forms two stable complexes. To evaluate
2+
2+
the concentration of Ca , Mg must be removed from the
(
2) (a) McSkimming, G.; Tucker, J. H. R.; Bouas-Laurent, H.; Desvergne,
J.-P. Angew. Chem., Int. Ed. 2000, 39, 2167. (b) Nabeshima, T.; Hashiguchi,
A.; Saiki, T.; Akine, S. Angew. Chem., Int. Ed. 2002, 41, 481. (c)
Nabeshima, T.; Yoshihira, Y.; Saiki, T.; Akine, S.; Horn, E. J. Am. Chem.
Soc. 2003, 125, 28. (d) Ames, J. B.; Hendricks, K. B.; Strahl, T.; Huttner,
I. G.; Hamasaki, N.; Thorner, J. Biochemistry 2000, 39, 12149. (e)
Watanabe, S.; Ikishima, S.; Matsuo, T.; Yoshida, K. J. Am. Chem. Soc.
(3) (a) Arunkumar, E.; Chithra, P.; Ajayaghosh, A. J. Am. Chem. Soc.
2004, 126, 6590. (b) Youngblood, W. J.; Gryko, D. T.; Lammi, R. K.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2002, 67, 2111. (c)
Arunkumar, E.; Ajayaghosh, A.; Daub, J. J. Am. Chem. Soc. 2005, 127,
3156. (d) Ajayaghosh, A.; Arunkumar, E.; Daub, J. J. Am. Chem. Soc. 2002,
41, 1766. (e) Ajayaghosh, A. Acc. Chem. Res. 2005, 38, 449. (f) Ajayaghosh,
A.; Arunkumar, E.; Daub, J. Angew. Chem., Int. Ed. 2002, 41, 1766. (g)
Suzuki, Y.; Morozumi, T.; Nakamura, H.; Shimomura, M.; Hayashita, T.;
Bartsch, R. A. J. Phys. Chem. B 1998, 102, 7910.
2
001, 123, 8402. (f) Cha, N. R.; Moon, S. Y.; Chang, S.-K. Tetrahedron
Lett. 2003, 44, 8265. (g) Momotake, A.; Arai, T. Tetrahedron Lett. 2003,
4
4, 7277. (h) Kakizawa, Y.; Akita, T.; Nakamura, H. Chem. Lett. 1993,
1
671. (i) Tahara, R.; Hasebe, K.; Nakamura, H. Chem. Lett. 1995, 753.
1
0.1021/ol701976x CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/27/2007

2
system as Mg(OH) in high pH solution. If this differentiation
is able to be achieved in one step, it will be of great
advantage for a fast and reliable analysis.
In our previous work, linear polyethers with fluorescent
substituents (e.g., N,N′-[oxybis(3-oxapentamethyleneoxy)-
2-phenyl]bis(9-anthracenecarboxamide)) have been synthe-
sized and fluorescence “Off-On” behaviors for alkaline earth
4
metal ions were reported. The twisted intramolecular charge
transfer (TICT) quenching process was effectively controlled
by formation of a complex with alkaline earth metal ions
2+
2+
2+
(
Ca , Sr , or Ba ). These results suggested that a torsion
angle between the carbonyl group of the amide bond and a
plane of the anthracene ring plays an important role on TICT
action. It occurred to us that the difference in ionic radii
2
+
2+
between Mg and Ca can vary the torsion angle in the
complex structure. This variation will be reflected in the
fluorescence spectrum as a feature and a position of
fluorescence maximum. However, the previous work indi-
2+
cated that the emission of complexation with Mg was weak
and gave no remarkable spectral changes compared to those
of other cations. This suggested that the anthracene aromatic
amide moiety was too large to stop a twisted motion of
another side of anthracene for the Mg2 complex in the
excited state. In the present work, compact and effective
+
“
stoppers”, n-propyl and ethyl benzene moieties, were
Figure 2. Fluorescence spectra of 2 and its Mg2 (a) and Ca2+ (b)
complexes.
+
chosen. We also recognized that the best binding site is
constructed with tetraethylene oxide units. On the basis of
these ideas, we synthesized fluorescent reagents 1 and 2.
Herein, we report the results of novel fluorescent chemo-
sensors that can distinguish between Mg2+ and Ca2+ using
a fluorescence emission spectroscopy.
weak in the absence of any metal ions, such as compounds
4
,5
reported previously. The λmax of the weak emission of 2
was 460 nm, which was attributed to the exciplex emission
of anthracene and benzene unit attached to the anthracene.
This observation was supported by the fluorescence spectrum
of free 1. The fluorescence spectra of 2‚Mg2 showed
increasing intensity and vibrational splitting at around 410
nm (Figure 2a). These peaks were attributed to emissions
from the anthracene moiety. This result indicates that no
charge transfer interaction existed between the anthracene
and the carbonyl group. On the other hand, in the case of
the Ca complex, the identical increase of fluorescence
intensity was also observed at around 440 nm. The peak
feature and wavelength at the peak maximum differed from
those of the Mg complex. This difference allowed a
discriminating detection between Mg and Ca by means
of fluorescence spectroscopy.
Chemosensors 1 and 2 (Figure 1) were synthesized in the
4
,5
usual manner (see the Supporting Information). Their
+
2
+
2
+
2
+
2+
Figure 1. Structures of 1 and 2.
The fluorescence behavior of 1 or 2 can be explained as
follows. The ionic size of Mg was fitted for a binding site
structures and purities were respectively confirmed by using
H NMR and elemental analyses (see the Supporting
Information). Fluorescence and UV spectra were measured
2+
1
consistent with CO-NH-Ph-OR. Therefore, the torsion angle
remained perpendicular (Figure 3). On the other hand, the
at 25 °C (RF-5300PC, UV-2400PC; Shimadzu Corp.). The
2+
ionic size of Ca is too large to interact with this site. The
-
5
concentration of these fluorescent reagents was 1 × 10
carbonyl group must be twisted to the anthracene ring at the
binding event to avoid a steric repulsion. This torsion motion
of the carbonyl group caused the charge transfer interaction
for the anthracene ring. Consequently, fluorescence spectra
3
mol/dm in purified acetonitrile. Alkaline earth metal cations
were added to the solution as perchlorate salts.
Figure 2 shows the fluorescence spectra of 2 as a function
2
+
2+
of the Mg and Ca concentrations in acetonitrile. Fluo-
rescence emissions from the anthracene moieties were quite
2+
of 2‚Ca exhibited a structureless and longer wavelength.
This “Off-On” phenomenon was elucidated as the following
pathway: the fluorescence emission from the anthracene
moiety was quenched by the TICT process at the aromatic
amide bond. The quenching mode was inhibited by the
(
4) Morozumi, T.; Anada, T.; Nakamura, H. J. Phys. Chem. B 2001,
05, 2923.
5) Morozumi, T.; Hiraga, H.; Nakamura, H. Chem. Lett. 2003, 32, 146.
1
(
4420
Org. Lett., Vol. 9, No. 22, 2007

emission at λmax of the Mg2 complex. In comparison to the
+
2
+
I
0
max/I value of 1 and 2 for Mg , a terminal ethylbenzene
in 2 played an effective role in stopping the twisted motion
of the anthracene ring compared with that of the n-propyl
group. The complex formation constant (log K) was evalu-
2
+
ated from the titration curve of Abs. vs [M ] by means of
a nonlinear least-squares curve-fitting method (Marquardt’s
method). The complex formation constants (log K) of 1 and
Figure 3. Schematic representation of the molecular configuration
6
of Mg2 and Ca . Oxyethylene and terminal benzene moieties were
+
2+
2
were evaluated from the spectral change that occurred upon
omitted for clarity.
the addition of metal ions, and all complexes formed 1:1
stoichiometry. Those values are listed in Table 1. In the
2
+
alkaline earth cations, Ca showed the best affinity with 1
and 2, as indicated also by other chemosensors.4
coordination of a metal ion with carbonyl and polyethylene
,5
oxide moieties.4,5
Fluorescence spectral data showed a structural change in
The “Off-On” responsiveness was evaluated as the
1
and 2 upon complexation with metal cations. To clarify
intensity enhancement ratio (Imax/I
cation and is listed in Table 1. The order of Imax/I
0
) at λmax for each metal
for 1 was
1
the effect of the addition of metal cations, H NMR
examination was carried out in acetonitrile-d at 30 °C. The
spectra of 2 before (a) and after addition of Ca (b) and
Mg (c) are shown in Figure 4 as a typical result. Peak
assignments were determined by using H-H COSY and
NOESY 2D spectra. Regarding the complexation of 2 with
0
3
2+
2
+
Table 1. Wavelength of Fluorescence Maxima (λmax),
Fluorescence Intensity Ratios (Imax/I
0
) at λmax, and Complex
Formation Constants (log K)
2+
Ca , oxyethylene proton peaks f-h shifted to low magnetic
field (∆δ ) 0.39-0.56 ppm) because of the reduction of
electron density by the coordinated cations in the oxygen
atoms. On the other hand, the proton peaks a-c of 2 showed
no particular shift tendency; moreover, the shift values were
small (∆δ ) -0.27 to 0.27 ppm) because of the weak
interaction of the oxygen atom beside b and c to the metal
ions.
Mg2
+
Ca2+
Sr2+
Ba2+
1
2
λmax
411 nm
16.4
438 nm
15.1
5.67
438 nm
14.9
6.21
438 nm
12.1
4.95
439 nm
10.2
5.35
438 nm
5.8
4.68
440 nm
5.9
4.55
Imax/I0
log K
λmax
Imax/I0
log K
4.89
411 nm
28.8
5.13
Amide proton m showed low magnetic shift changes (∆δ
) 0.83 (Mg ) and 0.66 (Ca )). In the Mg complex, the
Mg2 (16.4) > Ca2+ (15.1) > Sr (12.1) > Ba (5.8). The
+
2+
2+
2+
2+
2+
for 2 was Mg2 (28.8) > Ca (14.9) > Sr
+
2+
2+
order of Imax/I
10.2) > Ba (5.9). The Imax/I
an approximately 2-fold larger value than that of Ca . This
large value is due to the low intensity of free compound
0
chemical shift change was about 0.17 ppm lower than that
2
+
2+
2+
(
0
of Mg for 2 obtained as
of the Ca complex. Another amide proton 1 exhibited
2
+
2+
2+
similar results (∆δ ) 1.52 (Mg ) and 0.89 (Ca ), respec-
tively) to those of m, which suggests that the electronic
Figure 4. 1H NMR spectra of 2‚free (a), 2‚Ca2+ (b), and 2‚Mg2+ (c).
Org. Lett., Vol. 9, No. 22, 2007
4421

interaction between Mg2+ and the carbonyl group in 2 was
2+
2+
2+
stronger than that of Ca , Sr , and Ba .
In contrast to proton m and 1, a high-magnetic chemical
2
+
2+
shift of proton l (∆δ ) -1.41 (Mg ) and -0.93 (Ca ))
1
was induced upon complexation. These observations of H
NMR indicate that Mg bound ligand 1 or 2 more tightly
than Ca2 at the carbonyl group of the aromatic amide. On
the basis of fluorescence and H NMR studies, a plausible
complex structure of 2‚Mg2 in the ground state is illustrated
2+
+
1
+
in Figure 5.
In summary, we have shown that fluorescence spectra of
2
+
the 1 or 2‚Mg complex have different spectral shapes from
Figure 5. Schematic representation of the proposed structural
change of 2 after the addition of Mg in the ground state.
2
+
2+
2+
those of Ca , Sr , and Ba . The fluorescence spectra of
2+
1
4
and 2 showed structured anthracene-like peaks at around
10 nm for the Mg2+ complex, and they showed broad peaks
2+
2+
2+
at around 440 nm for Ca , Sr , and Ba complexes. The
Supporting Information Available: Fluorescence spectra
and characterization data of 1 and 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
present chemosensors 1 and 2 will be useful to discriminate
Mg with use of fluorescence spectroscopy.
2
+
(6) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431.
OL701976X
4422
Org. Lett., Vol. 9, No. 22, 2007