Mechanism for different fluorescence response of a coumarin-amide- dipicolylamine linkage to Zn(II) and Cd(II) in water

Article abstract of DOI:10.1021/jp3111315

A coumarin-amide-dipicolylamine linkage (L) was synthesized and used as a fluorescent receptor for metal cations in water. The receptor dissolved in water with neutral pH shows almost no fluorescence due to the photoinduced electron transfer (PET) from the amide and amine nitrogens to the excited state coumarin moiety. Coordination of Zn²⁺ or Cd²⁺ with L creates strong fluorescence at 437 or 386 nm, respectively, due to the suppression of PET. In contrast, other metal cations scarcely show fluorescence enhancement. IR, NMR, and potentiometric analysis revealed that both Zn²⁺ and Cd ²⁺ are coordinated with two pyridine N, amine N, and amide O; however, the Zn²⁺ center is also coordinated with a hydroxide anion (OH^-). The structure difference for Zn and Cd complexes results in longer- and shorter-wavelength fluorescence. Ab initio calculations revealed that π electrons on the excited state Cd complex are delocalized over the molecules and the Cd complex shows shorter-wavelength emission. In contrast, π electrons of OH^--coordinated Zn complex are localized on the coumarin moiety. This increases the electron density of coumarin moiety and shows longer-wavelength fluorescence.

Full text of DOI:10.1021/jp3111315

Article
pubs.acs.org/JPCA
Mechanism for Different Fluorescence Response of a Coumarin−
Amide−Dipicolylamine Linkage to Zn(II) and Cd(II) in Water
Shigehiro Sumiya, Yasuhiro Shiraishi,* and Takayuki Hirai
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka
University, Toyonaka 560-8531, Japan
*
S Supporting Information
ABSTRACT: A coumarin−amide−dipicolylamine linkage (L) was
synthesized and used as a fluorescent receptor for metal cations in
water. The receptor dissolved in water with neutral pH shows almost
no fluorescence due to the photoinduced electron transfer (PET)
from the amide and amine nitrogens to the excited state coumarin
2
+
2+
moiety. Coordination of Zn or Cd with L creates strong
fluorescence at 437 or 386 nm, respectively, due to the suppression
of PET. In contrast, other metal cations scarcely show fluorescence
enhancement. IR, NMR, and potentiometric analysis revealed that
both Zn2 and Cd are coordinated with two pyridine N, amine N,
+
2+
2
+
and amide O; however, the Zn center is also coordinated with a
hydroxide anion (OH ). The structure difference for Zn and Cd complexes results in longer- and shorter-wavelength
−
fluorescence. Ab initio calculations revealed that π electrons on the excited state Cd complex are delocalized over the molecules
−
and the Cd complex shows shorter-wavelength emission. In contrast, π electrons of OH -coordinated Zn complex are localized
on the coumarin moiety. This increases the electron density of coumarin moiety and shows longer-wavelength fluorescence.
INTRODUCTION
+
Scheme 1. Proposed Coordination Structures of a
Naphthalimide−Amide−DPA Linkage at Neutral pH
■2
2
2
Zn is an essential nutrient for the human body and plays
important roles in many physiological and pathological
1
processes. Its deficiency leads to an acrodermatitis enter-
2
opathica, but an excess dose causes several health problems
such as superficial skin diseases, prostate cancer, diabetes, and
3
2+
brain diseases. In contrast, Cd is highly toxic to the human
body. Its intake causes several diseases such as renal
dysfunction, calcium metabolism disorders, and prostate
4
cancer. The design of fluorescent signaling receptors for
2+
2+
Zn or Cd has therefore attracted a great deal of attention,
because they facilitate rapid monitoring of metal cations by
simple fluorescence measurements. A variety of receptors
showing a turn-on fluorescence response upon binding with
2+
2+
Zn or Cd has been proposed. Most of these receptors,
fluorescence enhancement at 514 and 446 nm, respectively.
The different fluorescence emissions were explained by the
different coordination structures of the Zn and Cd complexes
(Scheme 1). Both Zn and Cd complexes have tetracoordinated
structures involving two pyridine N and amine N. The Zn
complex involves amide N, whereas the Cd complex involves
amide O. These different coordination structures are
considered to affect the different fluorescence emissions.
These coordination structures were, however, identified on
however, show similar fluorescence enhancement upon binding
2+
with these cations at the same wavelength, because Zn and
2+
Cd are in the same group of the Periodic Table and have
5
−16
similar electronic and binding properties.
receptors that show fluorescence enhancement at different
The design of
2
+
2+
wavelengths for Zn and Cd is currently the focus of
1
7−21
attention.
The most efficient and simple ligand system that shows
1
the basis of H NMR, 2D NOISY, and IR analysis in pure
MeCN and DMSO solvents. Accurate coordination structures
fluorescence enhancement at different wavelengths upon
2
+
2+
binding with Zn and Cd is the amide−dipicolylamine
22
(
DPA) linkage proposed by Xu et al. They synthesized a
naphthalimide−amide−DPA linkage (Scheme 1). This mole-
Received: November 10, 2012
Revised: January 21, 2013
Published: January 23, 2013
cule, when dissolved in water at a neutral pH, shows almost no
2
+
2+
fluorescence. Addition of Zn and Cd , however, leads to
©
2013 American Chemical Society
1474
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The Journal of Physical Chemistry A
Article
in water are unclear, and the mechanisms for different
fluorescence wavelengths remain to be clarified.
afforded 1 with 73% yield. Condensation of di-2-picolylamine
25
(DPA) and 1 with N,N-diisopropylethylamine (DIPEA) and
KI gave L as a yellow oil with 65% yield (total yield: 47%). The
The purpose of the present work is to clarify the mechanism
for different fluorescence wavelengths for the amide−DPA
receptor. Coumarin is one of the most popular dyes and has
been studied extensively for application to fluorescent receptors
due to high emission quantum yield and high photo-
1
13
purity of 1 and L was confirmed by H, C NMR and FAB-MS
analysis (Figures S1−S6, Supporting Information).
Fluorescence properties of L in water were studied. Figure 1a
(top) shows the fluorescence spectra (λ = 327 nm) of L (10
ex
2
3,24
stability.
L) that shows Zn - and Cd -selective fluorescence enhance-
ment at different wavelengths. This molecule dissolved in water
We synthesized a coumarin−amide−DPA linkage
μM) measured without cations at different pHs (2−11) at 298
2+ 2+
(
K. L exhibits very weak fluorescence at ca. 394 nm (ϕ <0.17)
F
in the entire pH range. This is because the photoinduced
electron transfer (PET) from the amine and/or amide
nitrogens to the photoexcited coumarin moiety quenches the
fluorescence. This is confirmed by the species of L formed at
the respective pHs, which were calculated from the protonation
constants determined potentiometrically (Table 1). The dotted
lines in Figure 1a (bottom) plot the mole fraction distribution
of the species versus pH. The triangle symbols in this figure
plot the fluorescence intensity monitored at 386 nm. The
protonation/deprotonation states of L are summarized in
Scheme 4. At pH >6, L and LH−1 species with deprotonated
amine and amide N exist predominantly. These species
promote PET from both amine and amide N to the
at a neutral pH shows almost no fluorescence (ϕ = 0.05).
F
Addition of Cd2 creates a purple emission at 386 nm (ϕ =
0
+
F
2
+
.36), whereas addition of Zn shows a blue emission at 437
nm (ϕF = 0.63). On the basis of the IR, NMR, and
potentiometric titration analysis in aqueous solvents, the
2
+
2+
different fluorescence response to Zn and Cd was
successfully explained by the coordination structures, as
summarized in Scheme 2, which are different from those
Scheme 2. Proposed Coordination Structures of a
a
Coumarin−Amide−DPA Linkage at Neutral pH
26
photoexcited coumarin moiety and result in fluorescence
quenching. At pH <6, H L and HL species exist mainly, and the
2
fluorescence intensity becomes relatively strong although at the
very weak level. The amine N of H L and HL are protonated,
2
but the amide N are still deprotonated. This thus promotes
PET from the amide N to the photoexcited coumarin moiety.
These data suggest that, in the entire pH range (2−11), PET
from the amine and/or amide N to the coumarin moiety results
in very weak fluorescence.
Effect of Metal Cations. Figure 2 shows the fluorescence
a
These photographs were taken under photoirradiation of water
spectra (λ = 327 nm) of L (10 μM) measured in water (pH
2+
ex
(HEPES 100 mM, pH 7.3) containing L (10 μM) and 1 equiv of Zn
2+
7.3) with each respective metal cation (1 equiv). L itself shows
or Cd with a hand-held UV lamp (365 nm) at 298 K.
2
+
2+
very weak fluorescence (ϕ = 0.05). Addition of Zn and Cd
F
(
=
1 equiv), however, creates strong fluorescence at 437 nm (ϕF
0.63) and 386 nm (ϕ = 0.36), respectively. This suggests
2
2
reported for the naphthalimide−amide−DPA system. Both
F
2+
2+
that L shows different fluorescence spectra upon interaction
Zn and Cd are coordinated with two pyridine N, amine N,
and amide O. A hydroxide anion (OH ) is coordinated to the
Zn center, whereas Cd complex does not involve OH . The
OH coordination to the Zn center leads to a localization of π
electrons on the coumarin moiety and results in red-shifted
emission.
2+
2+
−
with Zn2 and Cd , as does the naphthalimide−amide−DPA
2
2+
2+
−
linkage. In contrast, addition of other cations (Hg , Cu ,
2+
3+
2+
2+
3+
2+
2+
2+
+
−
Pb , Cr , Mn , Fe , Fe , Mg , Ni , Co , Ag ) to the
solution shows almost no fluorescence enhancement. This
indicates that L shows selective emission enhancement toward
2+
2+
Zn and Cd .
RESULTS AND DISCUSSION
It must be noted that the fluorescence enhancement upon
addition of Zn2+ or Cd terminates within 2 min (Figure S7,
Supporting Information), indicating that L enables rapid
detection of Zn and Cd . As shown in Figure S8 (Supporting
Information), the fluorescence emissions created upon addition
of Zn2+ and Cd are unaffected by the addition of other metal
cations (Ag , Co , Fe , Fe , Hg , Mg , Mn , Ni , Pb ).
In contrast, a small quenching effect on the Zn -induced
fluorescence is observed upon addition of Cr . In addition,
Cu strongly quenches the fluorescence created by both Zn
■
2+
Synthesis and Properties of Receptor. The receptor L
was synthesized via two step reactions (Scheme 3). Reaction of
-amino-4-methylcoumarin (2) with chloroacetyl chloride
2+
2+
7
2+
Scheme 3. Synthesis of the Receptor
+
2+
2+
3+
2+
2+
2+
2+
2+
2
+
3+
2+
2+
2+
2+
and Cd , suggesting that the coordination of Cu with L is
2+
2+
much stronger than those of Zn and Cd . These findings
indicate that L detects Zn2+ or Cd selectively even in the
2+
2+
3+
presence of many other cations except for Cu and Cr . It
must also be noted that addition of Zn2+ together with Cd to
the solution containing L leads to an increase in the longer-
wavelength emission at 437 nm (Figure S9, Supporting
2+
2
+
Information). This indicates that L coordinates with Zn
1
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Figure 1. (Top) pH-dependent change in fluorescence spectra (λex = 327 nm) of L (10 μM) measured in water at 298 K (a) without cations, (b)
with Zn(ClO ) (1 equiv), and (c) with Cd(ClO ) (1 equiv). Both excitation and emission slit widths are 3 nm. The numbers denote the pH of
4
2
4 2
solution. (Bottom) change in fluorescence intensity. Triangle and circle symbols show the intensity monitored at 386 and 437 nm, respectively. The
dotted lines denote the mole fraction distribution of the species. All measurements were performed after stirring the solution for 2 min at the
adjusted pH.
Table 1. Protonation Constants (log K) for L and Stability
2
+
2+
a
Constants for Coordination with Cd or Zn
reaction
log K
+
3
H + LH = H2L
17.50
15.32
9.35
−
1
+
2
H + LH = HL
−
1
+
H + LH = L
−
1
+
Cd2+ + H + LH− = CdL
15.12
8.65
1
Cd2+ + LH−1 = CdLH−1
2+
−
Cd + OH + LH−1 = Cd(OH)LH−1
−1.23
13.98
6.19
2+
+
Zn + H + LH− = ZnL
1
2+
Zn + LH−1 = ZnLH−1
Zn + OH− + LH−1 = Zn(OH)LH−1
2+
Figure 2. Fluorescence spectra (λex = 327 nm) of L (10 μM) measured
−0.87
in water (HEPES 100 mM, pH 7.3) with each respective metal cation
a
The measurements were carried out in an aqueous NaCl (0.15 M)
solution at 298 K.
(1 equiv) at 298 K. Both excitation and emission slit widths are 3 nm.
Measurements were performed after stirring the solution for 2 min.
2
+
more strongly than Cd , as is observed for naphthalimide−
22
amide−DPA linkage.
Stoichiometry for Coordination. Figure 3 shows the
stoichiometry. This is confirmed by the Job’s plot analysis
(Figure S10, Supporting Information). The fluorescence
intensity shows a maximum at the mole fraction 0.5, indicative
of the 1:1 association. The 1:1 coordination is also confirmed
by ESI-MS. An MeCN solution containing L with 1 equiv of
2+
change in fluorescence spectra of L with the amount of Zn or
2+
Cd added. Stepwise addition of respective cation leads to an
increase in the fluorescence intensity. The spectral change
2
+
2+
almost terminates upon addition of 1 equiv of Zn or Cd ,
2
+
2+
2+
2+
implying that L interacts with Zn or Cd in a 1:1
Zn or Cd shows a peak at m/z 577.1 or m/z 627.1, assigned
Scheme 4. Protonation/Deprotonation Sequence of L in Water
1
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1
Figure 4. H NMR chart (400 MHz) of L (10 mM) measured in a
D O/CD CN mixture (1/2 v/v, pH 7.3) in the absence and presence
2
3
2+
2+
of Zn or Cd (1 equiv) at 303 K. The pH of solutions was adjusted
with DCl and NaOD (pH = pD − 0.5).
3
0
Figure 3. Fluorescence titration (λ = 327 nm) of L (10 μM) in water
ex
2+
2+
(
HEPES 100 mM, pH 7.3) with (a) Zn and (b) Cd at 298 K. Both
excitation and emission slit widths are 3 nm.
to [L + Zn2+ + ClO ] or [L + Cd + ClO ] ion (Figures
− +
2+
− +
4
4
S11 and S12, Supporting Information).
The fluorescence titration data obtained with Zn2+ or Cd
2+
2
+
(437 and 386 nm) were plotted by log(I − I )/I vs log [M ]
(Figure S13, Supporting Information). A linear increase in the
0
0
fluorescence intensity is observed at the concentration range
2+
2+
>
50 nM (Zn ) and >60 nM (Cd ), respectively. The US
Environmental Protection Agency (EPA) set the maximum
contaminant levels of Zn and Cd in drinking water at 76 and 45
27
2+
2+
nM, respectively. The detection limits of L to Zn and Cd
Figure 5. IR spectra of 2 or L (10 mM) measured in a D O/CD CN
2
3
are close to these regulation levels.
2
+
2+
mixture (1/2 v/v, pH 7.3) in the absence and presence of Zn or Cd
1 equiv).
1
NMR and IR Analysis for Zn and Cd Complexes. H
NMR analysis was performed to clarify the coordination
structures for Zn and Cd complexes. The solubility of L in
water is only ca. 100 μM, which is insufficient for H NMR and
IR analysis (10 mM). We therefore employed a D O/CD CN
mixture (1/2 v/v) for these analysis. Figure 4 shows the H
NMR spectra of L measured in a D O/CD CN mixture (1/2
v/v, pH 7.3) with and without Zn or Cd (1 equiv). All
(
1
band. Addition of Zn2+ or Cd2+ to the solution creates a new
2
3
1
−1
band at 1655 cm , which is assigned to the amide CO band
shifted by the decrease in electron density via the coordination
2
3
2
+
2+
2+
2+ 33,34
2+
of amide O with Zn or Cd .
This suggests that both Zn
1
1
2+
chemical shifts were identified by H− H COSY analysis
and Cd are coordinated with amide O. It must be noted that,
as shown in Figure S17 (Supporting Information), these
2
+
(
Figures S14−16, Supporting Information). Addition of Zn
2+
−1
or Cd leads to a large downfield shift of the pyridine protons
(
complexes show the CO band at 1655 cm also in acidic or
f
g
h
i
e
H , H , H , H ) and methylene protons (H ) adjacent to the
basic media. This indicates that both Zn and Cd complexes
involve the metal cation−amide O coordination at the entire
pH region.
amine moiety. These shifts are due to the decrease in electron
density of the pyridine and methylene moieties by the cation−
2
8,29
2+
2+
N coordination.
This indicates that both Zn and Cd are
Coordination and Fluorescence Properties of Zn
Complexes. The above NMR and IR results indicate that
both Zn and Cd complexes coordinate with two pyridine N,
amine N, and amide O. However, as shown in Figure 2,
different-wavelength-fluorescence appears upon addition of
coordinated with two pyridine N and amine N, as shown in
2
2
Figure 4, similar to the naphthalimide−amide−DPA linkage.
The cation−amide O coordination in both Zn and Cd
complexes is confirmed by IR analysis in aqueous media
2+
2+
(
3
Figure 5). IR spectrum of 2, a reference compound (Scheme
), measured in a D O/CD CN mixture (1/2 v/v, pH 7.3)
Zn (437 nm) and Cd (386 nm). To clarify this difference,
the coordination structures of Zn and Cd complexes and their
fluorescence behaviors were studied at the entire pH region.
Figure 1b (top) shows the fluorescence spectra of L measured
2
3
−
1
shows an absorption band at 1706 cm assigned to the CO
3
1,32
vibrational stretching for coumarin moiety.
L measured
−
1
2+
without metal cation shows a band at 1698 cm , similar to 2,
indicating that amide CO band overlaps the coumarin CO
with 1 equiv of Zn . At lower pH, shorter-wavelength emission
(350−500 nm) appears, but at pH >6, longer-wavelength
1
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emission (350−550 nm) appears along with a decrease in
shorter-wavelength emission.
respectively. At pH >3, the CdL species forms and shows a
shorter-wavelength emission similar to ZnL. At neutral pH (6−
9), the CdLH−1 species forms and still shows a shorter-
wavelength emission. In contrast, for the Zn complex (Figure
Figure 1b (bottom) shows the change in emission intensity
monitored at 386 nm (triangle) and 437 nm (circle), where the
dotted lines denote the mole fraction distribution of the
species, which were calculated from the protonation and
stability constants determined potentiometrically (Table 1).
Coordination structures of the species at different pH can be
summarized in Scheme 5a. Coordination of L with Zn2+ occurs
−
1b), the OH -coordinated Zn(OH)LH−1 species exists at this
pH range and shows a longer-wavelength emission. This
suggests that, at neutral pH, the structure difference for
CdLH−1 and Zn(OH)LH−1 results in different-wavelength
fluorescence (Figure 2).
−
At pH >9 (Figure 1c), OH is coordinated to the Cd center,
Scheme 5. Proposed Coordination Structure for (a) Zn and
as is the case for Zn complex, and the Cd(OH)LH−1 specie−s
shows a longer-wavelength emission. This suggests that OH
coordination to the metal center creates a longer-wavelength
emission for both Zn and Cd complexes, although mass
(b) Cd Complexes at Different pHs
2
+
spectrometric analysis of the solutions containing L with Zn
or Cd2+ does not show ions assigned to the OH -coordinated
−
species due to the instability of these complexes. At neutral pH,
−
OH coordination to the Zn center occurs, thus showing a
−
longer-wavelength emission. In contrast, OH coordination to
the Cd center does not occur at neutral pH and shows a
shorter-wavelength emission. It is well-known that the
electrostatic interaction between the metal cation and the
ligand molecules is strengthened by the decrease in ionic radius
3
5
2+
of metal cations. The ionic radius of Zn (0.74 Å) is smaller
2
+
36
than that of Cd (0.92 Å). The stronger coordination of
−
2+
−
OH with Zn therefore probably results in OH coordination
to Zn2+ at lower pH.
The above findings indicate that, as shown in Scheme 5, both
Zn and Cd complexes show similar coordination sequence,
−
although OH coordination occurs at different pH. The similar
sequences are supported by nanosecond time-resolved
fluorescence decay measurements at different pH (Figure
S18, Supporting Information). As summarized in Table 2, the
at pH >3 and produces ZnL species, which is coordinated with
two pyridine N and amide O, where the amine N is protonated.
The formation of ZnL species is consistent with the appearance
of shorter-wavelength emission. In contrast, at pH >6, two
species form, such as ZnLH−1 species coordinated with two
pyridine N, amine N, and amide O as a minor species, and
Zn(OH)LH−1 species coordinated with two pyridine N, amine
fluorescence decay time (τ ) of ZnL species (0.9 ns) is similar
F
to that of CdL (0.7 ns). In addition, the decay time of ZnLH−1
(
2.1 ns) is similar to that of CdLH−1 (2.2 ns), and the decay
time of Zn(OH)LH−1 (2.7 ns) is also similar to that of
Cd(OH)LH−1 (2.8 ns). These data clearly support the similar
coordination sequence for Zn and Cd complexes. These
−
−
N, and amide O, and a hydroxide anion (OH ) as a major
findings again suggest that the OH coordination to the metal
species. The distribution of Zn(OH)LH−1 species is consistent
with the intensity of longer-wavelength emission. This suggests
that Zn(OH)LH−1 is the emitting species for the longer-
wavelength emission observed at neutral pH upon Zn2+
addition (Figure 2).
center is the crucial factor for different fluorescence spectra
2
+
2+
upon addition of Zn and Cd at neutral pH (Figure 2).
−
Effect of OH Coordination on Fluorescence. Mecha-
nism for the formation of longer-wavelength emission upon
−
coordination of OH to the metal center must be clarified. Ab
Coordination and Fluorescence Properties of Cd
initio calculation was performed with the Gaussian 03
37
Complexes. Figure 1c shows the data obtained with 1 equiv
program using the time-dependent density functional theory
(TDDFT), where the polarizable continuum model (PCM)
was employed with water as a solvent to capture the solvent
2+
of Cd . As also for the Zn complex (Figure 1b), shorter- and
longer-wavelength emissions appear at lower and higher pH,
Table 2. Absorption and Emission Properties of Zn and Cd Complexes and Their Electronic Excitation Properties Determined
by TDDFT Calculations
a
b
c
species
ZnL
τF/ns
λabs,max/nm
main orbital transition
CIC
E/eV
λ/nm
f
0.9
2.1
2.7
0.7
2.2
2.8
324
323
323
325
325
327
S0 → S1
S0 → S1
S0 → S1
S0 → S1
S0 → S1
S0 → S1
HOMO → LUMO
HOMO → LUMO
HOMO → LUMO
HOMO → LUMO
HOMO → LUMO
HOMO → LUMO
0.65217
0.65950
0.65195
0.65236
0.65699
0.64721
3.6391
3.7404
3.6139
3.5642
3.6731
3.4983
340.70
331.47
343.08
347.86
337.54
354.41
0.0730
0.6031
0.7400
0.1163
0.6767
0.6016
ZnLH−1
Zn(OH)LH−1
CdL
CdLH−1
Cd(OH)LH−1
a
b
Fluorescence decay time determined by nanosecond time-resolved analysis (Figure S18, Supporting Information). Maximum wavelengths for the
c
absorption band of respective species (Figure S19, Supporting Information). CI expansion coefficients for the main orbital transitions.
1
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stabilization effects on the molecules. Scheme 6 shows the
optimized structures of Zn(OH)LH−1 and CdLH−1 complexes.
effect, and the effective shielding distance for pyridine ring is ca.
c
8.8 Å. This suggests that the H protons of Zn(OH)LH and
−1
CdLH−1 complexes lie within the shielding zone of pyridine
1
Scheme 6. Optimized Structures for the Zn(OH)LH− and
CdLH−1 Complexes
ring. As shown in Figure 4, H NMR analysis revealed that the
1
a
c
H protons of these complexes shift upfield as compared to the
free ligand. The upfield shift of H proton is therefore
c
attributable to the ring current effect by the pyridine aromatic
ring. Table 2 summarizes the main orbital transition of the
respective Zn and Cd complexes. The lowest singlet electronic
transitions of all Zn and Cd complexes are mainly contributed
by HOMO → LUMO transition. The calculated transition
energies of all species are similar to λmax of the absorption
spectra (Figure S19, Supporting Information). The above
findings clearly suggest that the calculated results precisely
represent the coordination structures and electronic properties
of Zn and Cd complexes.
The π-electrons of HOMO for all complexes are localized on
the coumarin moiety. In contrast, π-electrons of LUMO for
a
CdL, CdLH , ZnL, and ZnLH species, which do not
−1 −1
Gray, blue, red, yellow, and orange atoms of the molecular framework
−
coordinate with OH , are distributed over the coumarin and
pyridine moieties. This indicates that intramolecular charge
transfer (ICT) from the coumarin to pyridine moieties occurs
by the electronic excitation. In contrast, π-electrons of LUMO
indicate the C, N, O, Cd, and Zn atoms.
c
The H protons of these complexes are located above the
pyridine ring. The distances between H proton and the
c
−
for OH -coordinated Zn(OH)LH−1 and Cd(OH)LH−1 species
pyridine N for the respective complexes are determined to be
are localized on the coumarin moiety. This indicates that ICT
from the coumarin to pyridine moiety does not occur. As
reported, fluorescence wavelengths for coumarin derivatives
38
5.62 and 5.97 Å, respectively. As reported, protons located
39
above aromatic ring is magnetically shielded by the ring current
a
Table 3. Optimized Structures and Interface Plots of the Key Molecular Orbitals for Metal Complexes
a
Gray, blue, red, yellow and orange atoms of the molecular framework indicate the C, N, O, Cd, and Zn atoms. Green and deep red parts on
molecular orbitals refer to the different phases of the molecular wave functions, where the isovalue is 0.02 au.
1
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strongly depend on their electron densities; increased electron
density creates a longer-wavelength fluorescence. The electron
densities of coumarin moieties for the excited-state CdL,
CdLH , ZnL, and ZnLH species are decreased by ICT to
Synthesis of L. 1 (74 mg, 0.29 mmol), DPA (70.1 mg, 0.35
mmol), DIPEA (0.5 mL), and KI (32.3 mg, 0.19 mmol) were
added to MeCN (50 mL), and the mixture was refluxed for 12
−1
−1
h under N . The resultant was concentrated by evaporation,
2
the pyridine moieties. This may result in shorter-wavelength
emission. In contrast, the OH− coordination to the metal
centers suppresses ICT and creates a longer-wavelength
emission.
and the obtained brown oil was purified by silica gel column
chromatography (CH Cl /MeOH = 20/1), affording L as a
2
2
yellow solid (78.3 mg, 65.1%). Mp: 112.3−112.8 °C. λ
max
−1
−1
1
(
water, pH 7.3): 326 nm (ε 15391 M cm ). H NMR (400
MHz, DMSO, TMS): δ (ppm) = 10.97 (s, 1H), 8.58−8.60 (m,
H), 7.83 (d, J = 1.83 Hz, 1H), 7.74−7.79 (m, 3H), 7.58 (dd, J
2.29 Hz, 8.70 Hz, 1H), 7.45 (d, J = 7.79 Hz, 2H), 7.27−7.30
m, 2H), 6.27 (d, J = 1.37 Hz, 1H), 3.95 (s, 4H), 3.52 (s, 2H),
The π-electron distribution on the pyridine moieties of
LUMO for CdL, CdLH , ZnL, and ZnLH species are due to
−1
−1
2
=
the stabilization of π-electrons on the pyridine moieties by the
40
−
cation−pyridine N coordination. The OH coordination to
the metal center weakens the cation−pyridine N coordination
and results in the decrease in electron density of the pyridine
moieties. The average cation−pyridine N distances for the ZnL
and ZnLH−1 species, determined by DFT calculation, are 2.16
and 2.12 Å, respectively; however, Zn(OH)LH−1 has a longer
distance (2.21 Å). In addition, the distance for Cd(OH)LH−1
(
2
13
.41 (d, J = 0.92 Hz, 3H). C NMR (100 MHz, DMSO,
TMS): δ (ppm) = 170.19, 159.91, 158.30, 153.64, 152.97,
1
1
48.90, 141.87, 136.64, 125.92, 123.05, 122.36, 115.11, 115.02,
12.20, 105.42, 59.34, 57.83, 17.89. FAB-MS: m/z calcd for
+
C H N O :,414.17; found, 415.17 [M + H] .
24
22
4
3
Spectroscopic Analysis. Absorption spectra were meas-
ured on an UV−vis photodiode-array spectrophotometer
Shimadzu; Multispec-1500) equipped with a temperature
(
2.44 Å) is also longer than that for CdL and CdLH−1 (2.40
−
and 2.34 Å). These data suggest that the OH coordination to
the metal centers indeed weakens the cation−pyridine N
coordination. This destabilizes the π-electrons on the pyridine
moieties and results in the decrease in electron density.
(
41
controller (S-1700). Steady-state fluorescence spectra were
measured on a JASCO FP-6500 fluorescence spectrophotom-
eter. The fluorescence lifetime was measured on a PTI-3000
apparatus (Photon Technology International) at 298 ± 1 K
CONCLUSION
■
42,43
1
13
using a Xe nanoflash lamp filled with N2.
H and C NMR
We studied the fluorescence behaviors of a coumarin−amide−
dipicolylamine linkage (L) for metal cations in water. The
molecule dissolved in water at neutral pH shows selective
fluorescence enhancement upon addition of Zn and Cd at
37 and 386 nm, respectively, whereas other cations show
spectra were obtained by a JEOL JNM-AL400 spectrometer
using TMS as standard. FAB and ESI-MS analysis were
performed on a JEOL-JMS 700 mass spectrometer. IR spectra
were recorded at room temperature using a FTIR-610
spectrometer (JASCO Corp.) using a liquid sample cell with
2
+
2+
4
almost no enhancement. The different fluorescence wave-
lengths for the Zn and Cd complexes are due to their different
coordination structures. Both Zn and Cd complexes involve the
coordination with two pyridine N, amine N, and amide O, but
a CaF window.
2
Potentiometric Titration. The titrations were performed
on a COMTITE-550 potentiometric automatic titrator
44,45
−
−
(Hiranuma Co, Ltd.) with a glass electrode GE-101.
the Zn center is coordinated also with OH . The OH
Aqueous solution containing L with or without metal cations
coordination to the Zn center leads to a localization of π
electrons on the coumarin moiety. This increases the electron
density on the coumarin moiety and results in longer-
wavelength fluorescence.
was kept under N with an ionic strength of I = 0.15 M (NaCl)
2
at 298 K. Titration was done at 298 ± 1 K using an aqueous
NaOH solution. The program HYPERQUAD was used for
46
determination of protonation and stability constants. The Kw
+
−
−14
(
=[H ]·[OH ]) value used was 10
(298 K). The stability
EXPERIMENTAL METHODS
■
constants for Zn hydroxide (298 K) were log K(Zn(OH)/
Materials. All reagents were supplied from Wako, Sigma-
Aldrich, and Tokyo Kasei and used without further purification.
Zn·OH) = −9.25, log K(Zn(OH) /Zn·2OH) = −17.19, log
2
K(Zn(OH) /Zn·3OH) = −28.40, log K(Zn(OH) /Zn·4OH) =
3
4
2
+
2+
2+
2+
2+
3+
2+
2+
Perchlorate (Hg , Cu , Zn , Cd , Pb , Cr , Mn , Mg ,
−
40.63, log K(Zn (OH)/2Zn·OH) = −8.71, and log
2
+
3+
2+
2+
+
2
Fe , Fe ), nitrate (Ni , Co ), and tetrafluoroborate (Ag )
salts were used as the metal cation source. Water was purified
by the Milli-Q system. DPA was synthesized according to
literature procedure. 7-[(2-Chloroacetyl)amino]-4-methyl-
coumarin (1) and 7-[(2-(di-2-picolylamino)acetyl)amino]-4-
methylcoumarin (L) were synthesized as follows.
K(Zn (OH) /2Zn·6OH) = −57.51, respectively. The stability
2
6
constants for Cd hydroxide (298 K) were log K(Cd(OH)/
Cd·OH) = −10.37, log K(Cd(OH) /Cd·2OH) = −20.64, log
2
5
2
K(Cd(OH) /Cd·3OH) = −33.30, log K(Cd(OH) /Cd·4OH)
3
4
=
−46.78, log K(Cd (OH)/2Cd·OH) = −9.10, and log
2
47
K(Cd (OH) /4Cd·4OH) = −32.28, respectively.
4
4
Synthesis of 1. 7-Amino-4-methylcoumarin (149 mg, 0.85
mmol) and chloroacetyl chloride (140 μL, 1.76 mmol) were
Computational Details. Ab initio calculations were carried
37
out with the Gaussian 03 program. Geometry optimization
was performed with DFT using the B3LYP function. The Zn
and Cd complexes were calculated using the 6-31G* basis set
for all atoms except for Zn and Cd, for which the LANL2DZ
basis set with effective core potential was used. The electronic
excitation energies and oscillator strengths were calculated with
TDDFT at the same level of optimization using PCM with
water as a solvent. Cartesian coordinates for respective
complexes are summarized in the end of Supporting
Information.
refluxed in ethyl acetate (4 mL) for 2 h under N . The resultant
was recovered by filtration, affording 1 as a white solid (156 mg,
2
7
1
3%). Mp: 278.8−279.5 °C. λ (water, pH 7.3): 325 nm (ε
max
4011 M−1 cm ). H NMR (400 MHz, DMSO, TMS): δ
−1 1
(
ppm) = 10.71 (s, 1H), 7.73−7.75 (m, 2H), 7.50 (dd, J = 1.83
Hz, 8.70 Hz, 1H), 6.28 (d, J = 0.92 Hz, 1H), 4.32 (s, 2H), 2.41
1
3
(
(
d, J = 1.37 Hz, 3H). C NMR (100 MHz, DMSO, TMS): δ
ppm) = 165.24, 159.81, 153.51, 152.91, 141.62, 125.97,
1
15.42, 115.23, 112.51, 105.83, 43.48, 17.87. FAB-MS: m/z
+
calcd for C H NO Cl, 251.03; found, 252.15 [M + H] .
12
10
3
1
480
dx.doi.org/10.1021/jp3111315 | J. Phys. Chem. A 2013, 117, 1474−1482

The Journal of Physical Chemistry A
Article
(
13) Park, M. S.; Swamy, K. M. K.; Lee, Y. J.; Lee, H. N.; Jang, Y. J.;
ASSOCIATED CONTENT
■
Moon, Y. H.; Yoon, J. A New Acridine Derivative as a Fluorescent
Chemosensor for Zinc Ions in an 100% Aqueous Solution: A
Comparison of Binding Property with Anthracene Derivative.
Tetrahedron Lett. 2006, 47, 8129−8132.
*
S
Supporting Information
Compound characterization data ( H NMR, 13C NMR, FAB-
1
MS, Figures S1−S6), time dependence for fluorescence
enhancement (Figure S7), effect of other metal cations
fluorescence spectra, Figures S8 and S9), Job’s plot (Figure
(
14) Shiraishi, Y.; Ichimura, C.; Hirai, T. A Quinoline−Polyamine
(
Conjugate as a Fluorescent Chemosensor for Quantitative Detection
S10), ESI-MS charts (Figures S11 and 12), fluorescence
titration data (Figure S13), gCOSY charts (Figures S14−S16),
IR spectra (Figure S17), fluorescence decay data (Figure S18),
absorption spectra (Figure S19), and Cartesian coordinates for
complexes. This material (PDF) is available free of charge via
the Internet at http://pubs.acs.org.
of Zn(II) in Water. Tetrahedron Lett. 2007, 48, 7769−7773.
(15) Xu, H.; Miao, R.; Fang, Z.; Zhong, X. Quantum Dot-Based
“
Turn-On” Fluorescent Probe for Detection of Zinc and Cadmium
Ions in Aqueous Media. Anal. Chim. Acta 2011, 687, 82−88.
(16) Wang, Y.; Hu, X.; Wang, L.; Shang, Z.; Chao, J.; Jin, W. A New
Acridine Derivative as a Highly Selective ‘Off−On’ Fluorescence
Chemosensor for Cd2 in Aqueous Media. Sens. Actuators B 2011, 156,
126−131.
+
AUTHOR INFORMATION
Corresponding Author
E-mail: shiraish@cheng.es.osaka-u.ac.jp.
■
2
+
(
17) Xue, L.; Liu, Q.; Jiang, H. Ratiometric Zn Fluorescent Sensor
2+
and New Approach for Sensing Cd by Ratiometric Displacement.
*
Org. Lett. 2009, 11, 3454−3457.
Notes
(18) Xue, L.; Liu, C.; Jiang, H. Highly Sensitive and Selective
The authors declare no competing financial interest.
Fluorescent Sensor for Distinguishing Cadmium from Zinc Ions in
Aqueous Media. Org. Lett. 2009, 11, 1655−1658.
(19) Yunus, S.; Charles, S.; Dubois, F.; Donckt, E. V. Simultaneous
Determination of Cadmium (II) and Zinc (II) by Molecular
Fluorescence Spectroscopy and Multiple Linear Regression Using an
Anthrylpentaazamacrocycle Chemosensor. J. Fluoresc. 2008, 18, 499−
506.
ACKNOWLEDGMENTS
■
This work was supported by the Grant-in-Aid for scientific
Research (No. 23656504) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT). S.S.
thanks the Japan Society for the Promotion of Science (JSPS)
Research Fellowship for Young Scientists.
(20) Charles, S.; Dubois, F.; Yunus, S.; Donckt, E. V. Determination
by Fluorescence Spectroscopy of Cadmium at the Subnanomolar
Level: Application to Seawater. J. Fluoresc. 2000, 10, 99−105.
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