# Electrochemiluminescence Study of Europium (III)  Complex with Coumarin3-Carboxylic Acid

Authors: Stefan Lis; Krzysztof Staninski; Tomasz  Grzyb
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The europium (III) complex of coumarin-3-carboxylic acid (C3CA) has been prepared and characterized on the basis of elemental analysis, IR, and emission (photoluminescence and electrochemiluminescence) spectroscopy. The synthesised complex having a formula Eu(C3CA)2(NO3)(H2O)2 was photophysically characterized in solution and in the solid state. Electrochemiluminescence, ECL, of the system containing the Eu(III)/C3CA complex was studied using an oxide-covered aluminium electrode. The goal of these studies was to show the possibility of the use of electrochemical excitation of the Eu(III) ion in aqueous solution for emission generation. The generated ECL emission was very weak, and therefore its measurements and spectral analysis were carried out with the use of cut-off filters method. The studies proved a predominate role of the ligand-to-metal energy transfer (LMET) in the generated ECL.


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## Body

## 1. Introduction

Coumarins and their derivatives due to their
biological activities, interesting photophysical and photochemical, and metal
binding properties have been a subject of numerous investigations [1–12]. This group
of compounds is known to have diverse applications in biology and medicine, due
to their anticancer, antibiotic, anticoagulant, and anti-inflammatory [1, 2]
properties. It has been found that the binding of a metal to the coumarin
moiety retains or even enhances its biological activity [2, 3].The coumarin-3-carboxylic acid (HC3CA) has previously been used as a ligand in complexation reactions with d-electron
metal ions [5–7] and series of
lanthanide cations (Dy(III), Er(III), Eu(III), Gd(III), Tb(III), and Sm(III))
[8–12]. The binding
mode of coumarin-3-carboxylic acid in its La (III) complex has been
investigated both experimentally and theoretically [8], the studies indicated
strong ionic metal-ligand bonding in La-C3CA complex and
insignificant donor acceptor interaction. A sensitized emission and an
effective ligand to metal energy transfer in the samarium complex with
7-acetoxy coumarin 3-carboxylic acid [9], and efficient emission with long
lifetimes although with low quantum yield values in the systems of Eu3+ and Tb3+ with crown ethers and iminodiacetic subunits attached to
3-aroylcoumarins in methanol [10] have been observed. The samarium (III)
complex of coumarin-3-carboxylic acid proved to be the most active
antiproliferative agent among the complexes [11]. Erbium (III) and europium (III)
luminescent lanthanide complexes based on a coumarin showing effective energy
transfer between the coumarin ligand and the lanthanide ions were designed and
characterized by Kim et al. [12].Coumarin derivatives have been a subject of
electrochemiluminescence its mechanisms, induced by injection of hot electrons
into aqueous electrolyte solution [13]. The studies showed that coumarins can be
suitable candidates as ECL labels for bioaffinity assays or other analytical
applications [13].In our recent
investigations, we have applied specific electrogenerated luminescence, ECL,
which can be observed in Ln (III) (Ln = Tb, Dy and Eu) complexes with organic
ligands containing aromatic rings forming stable complexes, in studying
mechanisms of energy transfer [14, 15]. The ECL was obtained by producing
highly oxidizing and reducing species as: hydrated electrons, hydroxyl, and
sulfate radicals. These strong redox reactants efficiently excite the complexed Ln (III) ions
by ligand to metal energy transfer.The present work contains results of photoluminescence (PL) and  electrochemiluminescence (ECL)
studies concerning the complex of Eu (III) with the C3CA ligand. The goal of
these studies was to show the possibility of ECL generation with the use of
electroexcitation resulting in the ligand-to-metal energy transfer (LMET) in
aqueous solution with participation of the Eu (III) ion.


## 2. Experimental

### 2.1. Synthesis of Compounds

All chemicals were used of AR grade.
The europium oxide Eu2O3 (Merck 99.99%, KGaA, Germany) was dissolved in a slight excess
of HNO3 or H2SO4. Obtained europium nitrate
was dried and its appropriate amount dissolved in ethanol (spectroscopic grade)
and europium sulphate was dried and dissolved in water (doubly distilled).Synthesis of Eu(III) complex with coumarin-3-carboxylic acid.The complex was synthesized by
reaction of the Eu (III) salt, Eu (NO3)3,
with coumarin-3-carboxylic acid (Merck, Figure 1) in a 1:2 metal to ligand
molar ratio. The complex was prepared by adding ethanol solution of Eu (NO3)3
into ethanol solution of the ligand. The reaction mixture was stirred for 2
hours at room temperature. The precipitate was filtered, washed four times with
ethanol, and dried in a desiccator to constant weight. The obtained Eu/C3CA
complex was very limited soluble in water and ethanol (<10−4 mol×dm3).Figure 1
The structure
of the ligand (coumarin-3-carboxylic acid, C3CA) studied.


### 2.2. Methods

The carbon, hydrogen, and nitrogen content of the compounds were determined by
elemental analysis on an elemental analyser model VARIO ELIII. The IR spectra
(4000–400 cm−1) were obtained by means of an FTIR
Bruker IFS 113v spectrophotometer (resolution 1 cm−1), and the
samples (~2 mg) were prepared in KBr. The water content was determined by
luminescence lifetime measurement of the solid complex and was confirmed by
TGA.The luminescence lifetime measurements were carried out using the detection system
consisting of a nitrogen laser (KB6211) and a tuneable dye laser [16].The fluorescence spectra were
recorded using a Perkin-Elmer MP3 and Aminco Bowman AB2 spectrofluorimeters.ECL measurements were carried out using the experimental setup described
recently [14]. Detection of the emitting light was possible through the use of
a spectrometer Triax 180 (Horiba Jobin YVON GmbH, Germany)
and a photon-counting head Hamamatsu H4730-01. The spectrometer allows for
spectral recording in the range of 200–800 nm with a
step of 0.15 nm. The spectrometer control is executed using a built-in digital
controller. This controller enables one to operate the position of a
diffraction grating and the width of the entrance and exit slits by controlling
of the respective stepping motors. It also allows for sharp tuning of an
emission wavelength and its change during measurements. The detection module of
this system also allows the measurements of ultra weak emission (chemi- and
electrochemiluminescence) spectra with a standard resolution of a moderate
quality spectrofluorimeter. The recording module of this equipment consists of
a photon-counting head
Hamamatsu H4730-01 and a scalar cart attached to a PC. The
ECL spectra, because of their weak emission, were recorded using the method of
cut-off filters [17]. The ECL measurements were made in a
double electrode system: Al/Al2O3 as
the working electrode and a Pt-wire as the counter electrode, in aqueous solution.
The aluminium plate electrode (5 mm × 25 mm × 1 mm)
was covered with a natural oxide film (ca. 2 nm thick) and was made of an
aluminium stripe (99.999%, Merck). The platinum anode was made of a Pt-wire
(1.5 mm diameter). The ECL measurements were recorded with the use of the
earlier described equipment [14].


## 2.1. Synthesis of Compounds

All chemicals were used of AR grade.
The europium oxide Eu2O3 (Merck 99.99%, KGaA, Germany) was dissolved in a slight excess
of HNO3 or H2SO4. Obtained europium nitrate
was dried and its appropriate amount dissolved in ethanol (spectroscopic grade)
and europium sulphate was dried and dissolved in water (doubly distilled).Synthesis of Eu(III) complex with coumarin-3-carboxylic acid.The complex was synthesized by
reaction of the Eu (III) salt, Eu (NO3)3,
with coumarin-3-carboxylic acid (Merck, Figure 1) in a 1:2 metal to ligand
molar ratio. The complex was prepared by adding ethanol solution of Eu (NO3)3
into ethanol solution of the ligand. The reaction mixture was stirred for 2
hours at room temperature. The precipitate was filtered, washed four times with
ethanol, and dried in a desiccator to constant weight. The obtained Eu/C3CA
complex was very limited soluble in water and ethanol (<10−4 mol×dm3).Figure 1
The structure
of the ligand (coumarin-3-carboxylic acid, C3CA) studied.


## 2.2. Methods

The carbon, hydrogen, and nitrogen content of the compounds were determined by
elemental analysis on an elemental analyser model VARIO ELIII. The IR spectra
(4000–400 cm−1) were obtained by means of an FTIR
Bruker IFS 113v spectrophotometer (resolution 1 cm−1), and the
samples (~2 mg) were prepared in KBr. The water content was determined by
luminescence lifetime measurement of the solid complex and was confirmed by
TGA.The luminescence lifetime measurements were carried out using the detection system
consisting of a nitrogen laser (KB6211) and a tuneable dye laser [16].The fluorescence spectra were
recorded using a Perkin-Elmer MP3 and Aminco Bowman AB2 spectrofluorimeters.ECL measurements were carried out using the experimental setup described
recently [14]. Detection of the emitting light was possible through the use of
a spectrometer Triax 180 (Horiba Jobin YVON GmbH, Germany)
and a photon-counting head Hamamatsu H4730-01. The spectrometer allows for
spectral recording in the range of 200–800 nm with a
step of 0.15 nm. The spectrometer control is executed using a built-in digital
controller. This controller enables one to operate the position of a
diffraction grating and the width of the entrance and exit slits by controlling
of the respective stepping motors. It also allows for sharp tuning of an
emission wavelength and its change during measurements. The detection module of
this system also allows the measurements of ultra weak emission (chemi- and
electrochemiluminescence) spectra with a standard resolution of a moderate
quality spectrofluorimeter. The recording module of this equipment consists of
a photon-counting head
Hamamatsu H4730-01 and a scalar cart attached to a PC. The
ECL spectra, because of their weak emission, were recorded using the method of
cut-off filters [17]. The ECL measurements were made in a
double electrode system: Al/Al2O3 as
the working electrode and a Pt-wire as the counter electrode, in aqueous solution.
The aluminium plate electrode (5 mm × 25 mm × 1 mm)
was covered with a natural oxide film (ca. 2 nm thick) and was made of an
aluminium stripe (99.999%, Merck). The platinum anode was made of a Pt-wire
(1.5 mm diameter). The ECL measurements were recorded with the use of the
earlier described equipment [14].


## 3. Results and Discussion

Characterization of the
Eu (III) complex with coumarin-3-carboxylic acid.The elemental analysis of
the Eu/C3CA compound showed the following data: C = 39.07%; H = 2.14%; N =
2.16%, which are in a very good agreement with the calculated values, C = 38, 23%;
H = 2.25%; N = 2.24%, for Eu (C3CA)2(NO3)(H2O)2, EuC20H14NO13. The formation of
this Eu (III) complex was also confirmed by IR spectroscopy (see Figure 2) and
luminescence of lifetime measurement.FTIR spectra of the C3CA ligand and Eu(C3CA)2(NO3)
(H2O)2 complex recorded in the range of 500–1800 cm−1 (a) and 2000
and 4000 cm−1 (b).
(a)(b)


### 3.1. FTIR Spectra Analysis

Detailed analysis of vibrational frequencies, the IR
spectra of the HC3CA ligand and its Eu (III) complex, showed a
very good agreement
with the literature data [8] and gave evidence for bidentate coordination of
C3CA ligand to Eu (III) ions through the carbonylic oxygen and the carboxylic
oxygen. The bands in the 3580–3440 cm−1
range are observed in the Eu/C3CA complex IR
spectrum due to the ν(OH) modes of the coordinated water molecules,
while the broad band at ~3180 cm−1 in the IR spectrum of the ligand
is assigned to the ν(OH) vibrational mode. This band is not
observed in the spectra of the complexes, indicating that the deprotonated
ligand form participates in the complexes.The following bands, observed in the
IR spectra of the Eu/C3CA complex, are assigned to the vibrational modes of the
NO3 group: 1260 cm−1 for ν(NO)bonded; 790 cm−1 and 725 cm−1 for δ(ONO). These bands indicate the presence of the
nitrate group in the Eu/C3CA complex molecule. On the basis of the above
detailed vibrational study, we can conclude that the metal-ligand bonding in Eu(III)
complexes of  coumarin-3-carboxylic acid appeared to be strongly ionic with very
small donor-acceptor character, which is in agreement with the previously
reported data for the Gd (III), Sm (III), and Dy (III) complexes with the C3CA
ligand [11].


### 3.2. Luminescence and  Electrochemiluminescence Studies

The Eu(III) luminescence lifetime measured as
411 ± 6 microseconds (average of 6 measurements) for the solid Eu/C3CA complex
was used to calculate the number of water molecules, bond in the inner sphere
of the Eu(III) ion, from the equation below [18]:(1)nH2O=1.05×τ−1−0.7.The decay rate
(k=1/τ, τ in milliseconds) of the 5D0→7Fj emission is proportional to the number
of aqua ligands, coordinating the Eu(III) ion, due to the vibronic coupling of
the 5D0 excited state with vibrational states of the high frequency OH
oscillators of the aqua ligands. The calculated nH2O based on the value of τ (0.411 millisecond) measured for the Eu/C3CA complex
indicates the presence of two water molecules in the Eu(III) inner coordination
sphere. This hydration number (nH2O≈2) confirms the formula Eu(C3CA)2(NO3)(H2O)2 of the complex-formed Eu(III) with
coumarin-3-carboxylic acid ligand. The
studied complex Eu(C3CA)2(NO3)(H2O)2 is weakly soluble in water and alcohol (<10−4 mol/dm3 in water).The electrochemiluminescence of the
europium(III) ion is the least known among the lanthanide series [13, 19].
Studies being done so far, considering europium chelates with the use of
cathodic-generated ECL in aqueous solution, indicate two possible mechanisms of
excitation of the Eu(III) ions: (1) in the process of energy transfer from
electrochemically exited ligand the Eu(III) ion, or (2) in the electroreduction
process of Eu3+ to Eu2+ following its oxidation with the
use of a strong radical oxidizer generated as a result of decomposition of a
coreactant (e.g., S2O82−). It has been
previously shown that this oxidizing excitation process of Eu(II) occurs mainly
in chemiluminescence systems [20, 21].Luminescence excitation and emission
spectra of the solid Eu(C3CA)2(NO3)(H2O)2 complex are
presented in Figure 3. These photoluminescence studies show that the emission
spectrum of the complex (λex=370 nm) exhibits
typical narrow sharp emission bands corresponding to the characteristic D05→Fj7 transition of Eu(III) ion with the strongest
emission band of the characteristic D05→F27 transition of the Eu(III) ion at 615 nm. This observation confirms the crucial
role of the C3CA ligand in the transfer of the absorbed energy to the central
ion of the complex.Figure 3
Normalized excitation and emission spectra of solid state of Eu(C3CA)2(NO3)(H2O)2 complex
(5×10−5 mol×dm−3), λexc=365 nm.In order to find optimal conditions
for the ECL process, we studied the dependence of pH on the photoluminescence
(PL) intensity of Eu(III) in the solution of Eu/C3CA complex. As shown
in Figure4, the PL intensity of Eu(III) in the complex solution considerably decreases
above the value of pH > 5 with a simultaneous change of the intensity ratio of
the bands D05→F17 and D05→F27. The observed changes, especially in the
range of pH > 6, indicate ligand replacements in the inner coordination sphere
of the Eu(III) ion, due to progressive hydrolysis occurring in the aqueous
solution of the complex.Figure 4
Photoluminescence spectrum of aqueous solution of (5×10−5 mol×dm−3), λexc=355 nm as a function of pH.The ECL studies of systems
containing the Eu(III) ion, both in the complex with C3CA ligand and
uncomplexed (as Eu2(SO4)3), were investigated.The quantum yield of the ECL utraweak emission
is assessed (as∼5×10−8, for Eu2(SO4)3).
The quantum yield of this utraweak
emission is given as the ratio of the number of electric charges introduced
into the system, resulting of ECL process, to the number of photons generated
in the process, in the same geometrical conditions. The excitation mechanism of the coumarin 3-carboxylic acid molecules via the ECL method involves emission
of “hot” electrons from the electrode into the Eu(III) complex. This assists in
the formation of active radicals on the electrode surface in solutions
containing peroxodisulfate S2O82− ions, as a
coreactant, which can be easily decomposed in the following reaction:(2)eaq−+S2O82−→SO4•−+SO42−. Under air-saturated solutions, and due to
oxygen evolution at the counter electrode, oxyradicals and hydrogen peroxide
can be formed, if hydrated electrons are produced at the working electrode
[13].  The ECL spectra were recorded in
aqueous solution containing: only the coreactant K2S2O8 (Figure 5(a)), and Eu2(SO4)3 (as uncomplexed
Eu(III)) plus the coreactant K2S2O8 (see Figure 5(b)).
The ECL spectra and spectral analysis, due to a very weak ECL intensity
observed in the studied system, were complete using the method of cut-off
filters [17].ECL spectra of system containingK2S2O8
(2×10−2 mol×dm−3). (a) and K2S2O8 +
Eu2(SO4)3(5×10−2 mol×dm−3). (b) Experimental conditions: Al/Al2O3
as a working electrode, Pt wire as a counter electrode, applied pulse voltage
−50 V, frequency 40 Hz, pulse charge 30 μC, pH of solution 4.5.
(a)(b)In the case of ECL, spectrum recorded for the
coreactant (K2S2O8) predominates a band with
maximum at ~450 nm, corresponding to radiative relaxation from the P3
excited state to the S1 ground state of the
active F-center in Al2O3 [22]. The ECL spectrum
containing additionally the Eu(III) ions exhibits characteristic emission in
the region around 600 nm. The ECL spectrum characteristic for Eu(III),
generated in the system without an organic ligand, shows that Eu(III) can be
excited by the reduction-oxidation process. The Eu(III) ions are easily reduced
to Eu(II) (E0 for  Eu(III)/Eu(II) = −0.35 V) and then are
oxidized by sulfate and hydroxyl radicals present in solution leading to
Eu(III) excitation(3)eaq−+Eu3+→Eu2+,Eu2++SO4•−→SO42−+(Eu3+)∗,(Eu3+)∗→Eu3++hν(595,615nm). The ECL spectrum of the Eu(C3CA)2(NO3)(H2O)2 complex shows, additionally to the characteristic emission of
Eu(III), also a broad emission band in the range of  370–500 nm,
corresponding to radiative transitions in the ligand molecule (see Figure 6). In
the case of complexes containing
phenyl group(s), the radical excitation of the aromatic ring can be accomplished
on the way of redox reactions. Therefore, the studied system consisting of
Eu(C3CA)2(NO3)(H2O)2 complex and S2O82− the central ion can be potentially excited according to two ways:
(I)
as a result of energy transfer from the
excited state (singlet or triplet)
of the ligand to the Eu(III) ion:(4)Eu(III)-L+SO4•−→Eu(III)-LOX+SO42−,Eu(III)-LOX+eaq−→Eu-L*,Eu-L*→Eu*-L→Eu-L+hν(595,615nm),(II)
on the way of reduction and oxidation of the complexed Eu(III):(5)eaq−+Eu(III)-L→Eu(III)-LRED,Eu(III)-LRED→Eu(II)-L,Eu(II)-L+SO4•−→Eu(III)*-L+SO42−,Eu*-L→Eu-L+hν(595,615nm). The ECL intensity (λ=615 nm) observed in the system containing
the complex studied is over one order of magnitude higher than in that with the
uncomplexed Eu(III) ions. This observation proves the predominate role of the
ligand to metal energy transfer on the total efficiency of the electrochemiluminescence.Figure 6
Photoluminescence (λexc=355 nm) and electrochemiluminescence spectra
of Eu(C3CA)2(NO3)(H2O)2 complex in
aqueous solution. Experimental conditions:
Al/Al2O3 as a working electrode, Pt wire as a
counter electrode, applied pulse voltage
−50 V, frequency 40 Hz, pulse charge 30 μC, pH of solution 4.5, concentration
of the complex 5×10−5 mol×dm−3.


## 3.1. FTIR Spectra Analysis

Detailed analysis of vibrational frequencies, the IR
spectra of the HC3CA ligand and its Eu (III) complex, showed a
very good agreement
with the literature data [8] and gave evidence for bidentate coordination of
C3CA ligand to Eu (III) ions through the carbonylic oxygen and the carboxylic
oxygen. The bands in the 3580–3440 cm−1
range are observed in the Eu/C3CA complex IR
spectrum due to the ν(OH) modes of the coordinated water molecules,
while the broad band at ~3180 cm−1 in the IR spectrum of the ligand
is assigned to the ν(OH) vibrational mode. This band is not
observed in the spectra of the complexes, indicating that the deprotonated
ligand form participates in the complexes.The following bands, observed in the
IR spectra of the Eu/C3CA complex, are assigned to the vibrational modes of the
NO3 group: 1260 cm−1 for ν(NO)bonded; 790 cm−1 and 725 cm−1 for δ(ONO). These bands indicate the presence of the
nitrate group in the Eu/C3CA complex molecule. On the basis of the above
detailed vibrational study, we can conclude that the metal-ligand bonding in Eu(III)
complexes of  coumarin-3-carboxylic acid appeared to be strongly ionic with very
small donor-acceptor character, which is in agreement with the previously
reported data for the Gd (III), Sm (III), and Dy (III) complexes with the C3CA
ligand [11].


## 3.2. Luminescence and  Electrochemiluminescence Studies

The Eu(III) luminescence lifetime measured as
411 ± 6 microseconds (average of 6 measurements) for the solid Eu/C3CA complex
was used to calculate the number of water molecules, bond in the inner sphere
of the Eu(III) ion, from the equation below [18]:(1)nH2O=1.05×τ−1−0.7.The decay rate
(k=1/τ, τ in milliseconds) of the 5D0→7Fj emission is proportional to the number
of aqua ligands, coordinating the Eu(III) ion, due to the vibronic coupling of
the 5D0 excited state with vibrational states of the high frequency OH
oscillators of the aqua ligands. The calculated nH2O based on the value of τ (0.411 millisecond) measured for the Eu/C3CA complex
indicates the presence of two water molecules in the Eu(III) inner coordination
sphere. This hydration number (nH2O≈2) confirms the formula Eu(C3CA)2(NO3)(H2O)2 of the complex-formed Eu(III) with
coumarin-3-carboxylic acid ligand. The
studied complex Eu(C3CA)2(NO3)(H2O)2 is weakly soluble in water and alcohol (<10−4 mol/dm3 in water).The electrochemiluminescence of the
europium(III) ion is the least known among the lanthanide series [13, 19].
Studies being done so far, considering europium chelates with the use of
cathodic-generated ECL in aqueous solution, indicate two possible mechanisms of
excitation of the Eu(III) ions: (1) in the process of energy transfer from
electrochemically exited ligand the Eu(III) ion, or (2) in the electroreduction
process of Eu3+ to Eu2+ following its oxidation with the
use of a strong radical oxidizer generated as a result of decomposition of a
coreactant (e.g., S2O82−). It has been
previously shown that this oxidizing excitation process of Eu(II) occurs mainly
in chemiluminescence systems [20, 21].Luminescence excitation and emission
spectra of the solid Eu(C3CA)2(NO3)(H2O)2 complex are
presented in Figure 3. These photoluminescence studies show that the emission
spectrum of the complex (λex=370 nm) exhibits
typical narrow sharp emission bands corresponding to the characteristic D05→Fj7 transition of Eu(III) ion with the strongest
emission band of the characteristic D05→F27 transition of the Eu(III) ion at 615 nm. This observation confirms the crucial
role of the C3CA ligand in the transfer of the absorbed energy to the central
ion of the complex.Figure 3
Normalized excitation and emission spectra of solid state of Eu(C3CA)2(NO3)(H2O)2 complex
(5×10−5 mol×dm−3), λexc=365 nm.In order to find optimal conditions
for the ECL process, we studied the dependence of pH on the photoluminescence
(PL) intensity of Eu(III) in the solution of Eu/C3CA complex. As shown
in Figure4, the PL intensity of Eu(III) in the complex solution considerably decreases
above the value of pH > 5 with a simultaneous change of the intensity ratio of
the bands D05→F17 and D05→F27. The observed changes, especially in the
range of pH > 6, indicate ligand replacements in the inner coordination sphere
of the Eu(III) ion, due to progressive hydrolysis occurring in the aqueous
solution of the complex.Figure 4
Photoluminescence spectrum of aqueous solution of (5×10−5 mol×dm−3), λexc=355 nm as a function of pH.The ECL studies of systems
containing the Eu(III) ion, both in the complex with C3CA ligand and
uncomplexed (as Eu2(SO4)3), were investigated.The quantum yield of the ECL utraweak emission
is assessed (as∼5×10−8, for Eu2(SO4)3).
The quantum yield of this utraweak
emission is given as the ratio of the number of electric charges introduced
into the system, resulting of ECL process, to the number of photons generated
in the process, in the same geometrical conditions. The excitation mechanism of the coumarin 3-carboxylic acid molecules via the ECL method involves emission
of “hot” electrons from the electrode into the Eu(III) complex. This assists in
the formation of active radicals on the electrode surface in solutions
containing peroxodisulfate S2O82− ions, as a
coreactant, which can be easily decomposed in the following reaction:(2)eaq−+S2O82−→SO4•−+SO42−. Under air-saturated solutions, and due to
oxygen evolution at the counter electrode, oxyradicals and hydrogen peroxide
can be formed, if hydrated electrons are produced at the working electrode
[13].  The ECL spectra were recorded in
aqueous solution containing: only the coreactant K2S2O8 (Figure 5(a)), and Eu2(SO4)3 (as uncomplexed
Eu(III)) plus the coreactant K2S2O8 (see Figure 5(b)).
The ECL spectra and spectral analysis, due to a very weak ECL intensity
observed in the studied system, were complete using the method of cut-off
filters [17].ECL spectra of system containingK2S2O8
(2×10−2 mol×dm−3). (a) and K2S2O8 +
Eu2(SO4)3(5×10−2 mol×dm−3). (b) Experimental conditions: Al/Al2O3
as a working electrode, Pt wire as a counter electrode, applied pulse voltage
−50 V, frequency 40 Hz, pulse charge 30 μC, pH of solution 4.5.
(a)(b)In the case of ECL, spectrum recorded for the
coreactant (K2S2O8) predominates a band with
maximum at ~450 nm, corresponding to radiative relaxation from the P3
excited state to the S1 ground state of the
active F-center in Al2O3 [22]. The ECL spectrum
containing additionally the Eu(III) ions exhibits characteristic emission in
the region around 600 nm. The ECL spectrum characteristic for Eu(III),
generated in the system without an organic ligand, shows that Eu(III) can be
excited by the reduction-oxidation process. The Eu(III) ions are easily reduced
to Eu(II) (E0 for  Eu(III)/Eu(II) = −0.35 V) and then are
oxidized by sulfate and hydroxyl radicals present in solution leading to
Eu(III) excitation(3)eaq−+Eu3+→Eu2+,Eu2++SO4•−→SO42−+(Eu3+)∗,(Eu3+)∗→Eu3++hν(595,615nm). The ECL spectrum of the Eu(C3CA)2(NO3)(H2O)2 complex shows, additionally to the characteristic emission of
Eu(III), also a broad emission band in the range of  370–500 nm,
corresponding to radiative transitions in the ligand molecule (see Figure 6). In
the case of complexes containing
phenyl group(s), the radical excitation of the aromatic ring can be accomplished
on the way of redox reactions. Therefore, the studied system consisting of
Eu(C3CA)2(NO3)(H2O)2 complex and S2O82− the central ion can be potentially excited according to two ways:
(I)
as a result of energy transfer from the
excited state (singlet or triplet)
of the ligand to the Eu(III) ion:(4)Eu(III)-L+SO4•−→Eu(III)-LOX+SO42−,Eu(III)-LOX+eaq−→Eu-L*,Eu-L*→Eu*-L→Eu-L+hν(595,615nm),(II)
on the way of reduction and oxidation of the complexed Eu(III):(5)eaq−+Eu(III)-L→Eu(III)-LRED,Eu(III)-LRED→Eu(II)-L,Eu(II)-L+SO4•−→Eu(III)*-L+SO42−,Eu*-L→Eu-L+hν(595,615nm). The ECL intensity (λ=615 nm) observed in the system containing
the complex studied is over one order of magnitude higher than in that with the
uncomplexed Eu(III) ions. This observation proves the predominate role of the
ligand to metal energy transfer on the total efficiency of the electrochemiluminescence.Figure 6
Photoluminescence (λexc=355 nm) and electrochemiluminescence spectra
of Eu(C3CA)2(NO3)(H2O)2 complex in
aqueous solution. Experimental conditions:
Al/Al2O3 as a working electrode, Pt wire as a
counter electrode, applied pulse voltage
−50 V, frequency 40 Hz, pulse charge 30 μC, pH of solution 4.5, concentration
of the complex 5×10−5 mol×dm−3.


## 4. Conclusions

Eu(III) forms with the ligand of
coumarin-3-carboxylic acid (C3CA) the complex of composition Eu(C3CA)2(NO3)(H2O)2. This complex is one of a few examples in which ECL characteristic for
the Eu(III) ion can be observed [13, 15, 19]. The mechanism of excitation of
the central ion can be completed as a result of energy transfer from the
excited state of the ligand to the Eu(III) ion (LMET), which is predominant,
and on the way of reduction and oxidation reactions of the complexed Eu(III)
ion. The observed ECL in this system is utraweak, due to a very limited solubility of
C3CA, and therefore can be detected with the use of a single photon counting method.
Coumarin derivatives
having important biological activities, of better than C3CA solubility in
aqueous solution should exhibit more intensive ECL. This ECL generated with the
participation of the LMET, can be potentially used in analytical applications
of biologically active agents, for example, in pharmaceutical preparations, as we recently have shown using the
chemically generated emission for the determination of
tetracycline derivatives [23].



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*Source: 131702-2008-10-13.xml*