Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2008, Article ID 131702, 6 pagesdoi:10.1155/2008/131702

Research ArticleElectrochemiluminescence Study of Europium (III)Complex with Coumarin3-Carboxylic Acid

Stefan Lis, Krzysztof Staninski, and Tomasz Grzyb

Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60 780 Poznan, Poland

Correspondence should be addressed to Stefan Lis, blis@amu.edu.pl

Received 4 May 2008; Revised 12 June 2008; Accepted 22 July 2008

Recommended by Mohamed Sabry Abdel-Mottaleb

The europium (III) complex of coumarin-3-carboxylic acid (C3CA) has been prepared and characterized on the basis of elementalanalysis, IR, and emission (photoluminescence and electrochemiluminescence) spectroscopy. The synthesised complex having aformula 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 ofthese studies was to show the possibility of the use of electrochemical excitation of the Eu(III) ion in aqueous solution for emissiongeneration. The generated ECL emission was very weak, and therefore its measurements and spectral analysis were carried outwith the use of cut-off filters method. The studies proved a predominate role of the ligand-to-metal energy transfer (LMET) in thegenerated ECL.

Copyright © 2008 Stefan Lis et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. INTRODUCTION

Coumarins and their derivatives due to their biologicalactivities, interesting photophysical and photochemical, andmetal binding properties have been a subject of numerousinvestigations [1–12]. This group of compounds is knownto have diverse applications in biology and medicine, dueto their anticancer, antibiotic, anticoagulant, and anti-inflammatory [1, 2] properties. It has been found that thebinding of a metal to the coumarin moiety retains or evenenhances its biological activity [2, 3].

The coumarin-3-carboxylic acid (HC3CA) has previ-ously been used as a ligand in complexation reactions withd-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 itsLa (III) complex has been investigated both experimentallyand theoretically [8], the studies indicated strong ionicmetal-ligand bonding in La-C3CA complex and insignificantdonor acceptor interaction. A sensitized emission and aneffective ligand to metal energy transfer in the samariumcomplex with 7-acetoxy coumarin 3-carboxylic acid [9],and efficient emission with long lifetimes although withlow quantum yield values in the systems of Eu3+ and Tb3+

with crown ethers and iminodiacetic subunits attached to3-aroylcoumarins in methanol [10] have been observed.The samarium (III) complex of coumarin-3-carboxylic acidproved to be the most active antiproliferative agent amongthe complexes [11]. Erbium (III) and europium (III) lumi-nescent lanthanide complexes based on a coumarin showingeffective energy transfer between the coumarin ligand andthe lanthanide ions were designed and characterized by Kimet al. [12].

Coumarin derivatives have been a subject of electro-chemiluminescence its mechanisms, induced by injection ofhot electrons into aqueous electrolyte solution [13]. Thestudies showed that coumarins can be suitable candidatesas ECL labels for bioaffinity assays or other analyticalapplications [13].

In our recent investigations, we have applied specificelectrogenerated luminescence, ECL, which can be observedin Ln (III) (Ln = Tb, Dy and Eu) complexes with organicligands containing aromatic rings forming stable complexes,in studying mechanisms of energy transfer [14, 15]. The ECLwas obtained by producing highly oxidizing and reducingspecies as: hydrated electrons, hydroxyl, and sulfate radicals.These strong redox reactants efficiently excite the complexedLn (III) ions by ligand to metal energy transfer.

mailto:blis@amu.edu.pl

2 International Journal of Photoenergy

O

COOH

O

Figure 1: The structure of the ligand (coumarin-3-carboxylic acid,C3CA) studied.

The present work contains results of photoluminescence(PL) and electrochemiluminescence (ECL) studies concern-ing the complex of Eu (III) with the C3CA ligand. The goalof these studies was to show the possibility of ECL generationwith the use of electroexcitation resulting in the ligand-to-metal energy transfer (LMET) in aqueous solution withparticipation of the Eu (III) ion.

2. EXPERIMENTAL

2.1. Synthesis of compounds

All chemicals were used of AR grade. The europium oxideEu2O3 (Merck 99.99%, KGaA, Germany) was dissolved in aslight excess of HNO3 or H2SO4. Obtained europium nitratewas dried and its appropriate amount dissolved in ethanol(spectroscopic grade) and europium sulphate was dried anddissolved in water (doubly distilled).

Synthesis of Eu(III) complex with coumarin-3-carbo-xylic 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 complexwas prepared by adding ethanol solution of Eu (NO3)3 intoethanol solution of the ligand. The reaction mixture wasstirred for 2 hours at room temperature. The precipitatewas filtered, washed four times with ethanol, and driedin a desiccator to constant weight. The obtained Eu/C3CAcomplex was very limited soluble in water and ethanol(

Stefan Lis et al. 3

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ν (cm−1)

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Eu(C3CA)2NO3(H2O)2C3CA

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Eu(C3CA)2NO3(H2O)2C3CA

(b)

Figure 2: FTIR spectra of the C3CA ligand and Eu(C3CA)2(NO3)(H2O)2 complex recorded in the range of 500–1800 cm−1 (a) and2000 and 4000 cm−1 (b).

can conclude that the metal-ligand bonding in Eu(III)complexes of coumarin-3-carboxylic acid appeared to bestrongly ionic with very small donor-acceptor character,which is in agreement with the previously reported data forthe Gd (III), Sm (III), and Dy (III) complexes with the C3CAligand [11].

3.2. Luminescence and electrochemiluminescencestudies

The Eu(III) luminescence lifetime measured as 411 ± 6microseconds (average of 6 measurements) for the solidEu/C3CA complex was used to calculate the number of water

molecules, bond in the inner sphere of the Eu(III) ion, fromthe equation below [18]:

nH2O = 1.05× τ−1 − 0.7. (1)

The decay rate (k = 1/τ, τ in milliseconds) of the5D0→7Fj emission is proportional to the number of aqualigands, coordinating the Eu(III) ion, due to the vibroniccoupling of the 5D0 excited state with vibrational states ofthe high frequency OH oscillators of the aqua ligands. Thecalculated nH2O based on the value of τ (0.411 millisecond)measured for the Eu/C3CA complex indicates the presenceof two water molecules in the Eu(III) inner coordinationsphere. This hydration number (nH2O ≈ 2) confirms theformula Eu(C3CA)2(NO3)(H2O)2 of the complex-formedEu(III) with coumarin-3-carboxylic acid ligand. The studiedcomplex Eu(C3CA)2(NO3)(H2O)2 is weakly soluble in waterand alcohol ( 5 with a simultaneous change of the intensityratio of the bands 5D0→7F1 and 5D0→7F2. The observedchanges, especially in the range of pH > 6, indicate ligandreplacements in the inner coordination sphere of the Eu(III)ion, due to progressive hydrolysis occurring in the aqueoussolution of the complex.

The ECL studies of systems containing the Eu(III) ion,both in the complex with C3CA ligand and uncomplexed (asEu2(SO4)3), were investigated.

The quantum yield of the ECL utraweak emission isassessed (as ∼5 × 10−8, for Eu2(SO4)3). The quantum yieldof this utraweak emission is given as the ratio of the numberof electric charges introduced into the system, resulting ofECL process, to the number of photons generated in the

4 International Journal of Photoenergy

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Figure 3: Normalized excitation and emission spectra of solid stateof Eu(C3CA)2(NO3)(H2O)2 complex (5× 10−5 mol×dm−3), λexc =365 nm.

3.55

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Figure 4: Photoluminescence spectrum of aqueous solution of (5×10−5 mol × dm−3), λexc = 355 nm as a function of pH.

process, in the same geometrical conditions. The excitationmechanism of the coumarin 3-carboxylic acid moleculesvia the ECL method involves emission of “hot” electronsfrom the electrode into the Eu(III) complex. This assistsin the formation of active radicals on the electrode surfacein solutions containing peroxodisulfate S2O8

2− ions, as acoreactant, which can be easily decomposed in the followingreaction:

e−aq + S2O82− −→ SO•−4 + SO2−4 . (2)

Under air-saturated solutions, and due to oxygen evolutionat the counter electrode, oxyradicals and hydrogen peroxidecan be formed, if hydrated electrons are produced at theworking electrode [13]. The ECL spectra were recorded inaqueous solution containing: only the coreactant K2S2O8(Figure 5(a)), and Eu2(SO4)3 (as uncomplexed Eu(III)) plusthe coreactant K2S2O8 (see Figure 5(b)). The ECL spectra

700650600550500450400350

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Eu2(SO4)3 + S2O82−

(b)

Figure 5: ECL spectra of system containing K2S2O8 (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, Ptwire as a counter electrode, applied pulse voltage −50 V, frequency40 Hz, pulse charge 30 μC, pH of solution 4.5.

and spectral analysis, due to a very weak ECL intensityobserved in the studied system, were complete using themethod of cut-off filters [17].

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 3P excitedstate to the 1S ground state of the active F-center inAl2O3 [22]. The ECL spectrum containing additionally theEu(III) ions exhibits characteristic emission in the regionaround 600 nm. The ECL spectrum characteristic for Eu(III),generated in the system without an organic ligand, showsthat Eu(III) can be excited by the reduction-oxidationprocess. The Eu(III) ions are easily reduced to Eu(II) (E0 forEu(III)/Eu(II) = −0.35 V) and then are oxidized by sulfate

Stefan Lis et al. 5

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λ (nm)

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PL/

EC

L[n

orm

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]

Figure 6: Photoluminescence (λexc = 355 nm) and electrochemi-luminescence spectra of Eu(C3CA)2(NO3)(H2O)2 complex inaqueous solution. Experimental conditions: Al/Al2O3 as a workingelectrode, 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.

and hydroxyl radicals present in solution leading to Eu(III)excitation

e−aq + Eu3+ −→ Eu2+,

Eu2+ + SO•−4 −→ SO2−4 +(Eu3+

)∗,

(Eu3+

)∗ −→ Eu3+ + hν (595, 615 nm).(3)

The ECL spectrum of the Eu(C3CA)2(NO3)(H2O)2 complexshows, 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 phenylgroup(s), the radical excitation of the aromatic ring canbe accomplished on the way of redox reactions. Therefore,the studied system consisting of Eu(C3CA)2(NO3)(H2O)2complex and S2O8

2− the central ion can be potentiallyexcited 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:

Eu(III)-L + SO•−4 −→ Eu(III)-LOX + SO2−4 ,Eu(III)-LOX + e−aq −→ Eu-L∗,

Eu-L∗ −→ Eu∗-L −→ Eu-L+hν (595, 615 nm),(4)

(II) on the way of reduction and oxidation of thecomplexed Eu(III):

e−aq + Eu(III)-L −→ Eu(III)-LRED,Eu(III)-LRED −→ Eu(II)-L,

Eu(II)-L + SO•−4 −→ Eu(III)∗-L+SO2−4 ,Eu∗-L −→ Eu-L+hν (595, 615 nm).

(5)

The ECL intensity (λ = 615 nm) observed in the systemcontaining the complex studied is over one order of mag-nitude higher than in that with the uncomplexed Eu(III)ions. This observation proves the predominate role of theligand to metal energy transfer on the total efficiency of theelectrochemiluminescence.

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 ECLcharacteristic for the Eu(III) ion can be observed [13, 15, 19].The mechanism of excitation of the central ion can becompleted as a result of energy transfer from the excitedstate of the ligand to the Eu(III) ion (LMET), which ispredominant, and on the way of reduction and oxidationreactions of the complexed Eu(III) ion. The observed ECLin this system is utraweak, due to a very limited solubilityof C3CA, and therefore can be detected with the use ofa single photon counting method. Coumarin derivativeshaving important biological activities, of better than C3CAsolubility in aqueous solution should exhibit more intensiveECL. This ECL generated with the participation of theLMET, can be potentially used in analytical applications ofbiologically active agents, for example, in pharmaceuticalpreparations, as we recently have shown using the chemicallygenerated emission for the determination of tetracyclinederivatives [23].

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6 International Journal of Photoenergy

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