Energy transfer process in the reaction system NH2OH-NaOH-Cu(II)-Eu(III)/thenoyltrifluoroacetone

Chemiluminescence (CL) in the system containing hyrdroxylamine, sodium hydroxide, copper(II) ions and complex of europium(III) with thenoyltrifluoroacetone has been studied. A weak emission arises from hydroxylamine in the presence of copper(II) ions in a basic aqueous solution. Addition of europium(III) chelate to this system causes a significant increase in the chemiluminescence intensity. The dominant band observed in the emission spectrum occurs at λ ∼ 600 nm, confirming that Eu(III) ions are the only emitters in the system studied. A strong correlation between the CL intensity and the concentrations of europium(III) chelate was observed. It means that europium(III) thenoyltrifluoroacetone complex does not influence the oxidation rate of hydroxylamine and indicates its important role in this reaction as a sensitiser. A possible mechanism of the chemiluminescence of the NH2OH-NaOH-Cu(II)-Eu(III) chelate system is presented.


INTRODUCTION
In our earlier works on the role of lanthanide ions in the processes of chemiluminescence [1,2] it has been proved that excited Eu(III) ions can appear in the system containing hydrogen peroxide.Excited Eu(III) has been obtained in the reaction of Eu(II) oxidation by hydrogen peroxide [3,4], and in the process of energy transfer from the excited reaction products generated in the decomposition reaction of hydrogen peroxide to complexed europium ions [5].In aqueous solution Eu(III) is coordinated with nine water molecules, which provides an efficient pathway for radiationless deactivation of the excited states through vibronic coupling with the OH oscillators [6].The emission intensity of the lanthanide ions can be significantly increased by complexing them with appropriate organic ligands [7][8][9][10].
In this paper a chelate of Eu(III) ions with thenoyltrifluoroacetone (TTA) was used as a sensitizer in the system NH 2 OH-NaOH-Cu(II), characterised by very low emission intensity.

EXPERIMENTAL
Measurements of chemiluminescence were performed using the system enabling of ultraweak radiation by the stationary method as described earlier [1].Spectroscopic measurements were made by means of UV-2401 PC Shimadzu spectrophotometer.
Europium(III) chloride was obtained by dissolving Eu 2 O 3 (99.99%Aldrich) in hydrochloric acid (spectral purity, Fluka).The other reagents used for this study were sodium hydroxide NaOH (pure for analysis, POCh), hydroxylamine hydrochloride (pure for analysis, Fluka), thenoyltrifluoroacetone TTA (pure for analysis, Fluka), copper chloride CuCl 2 (pure for analysis, Fluka).All solutions were prepared with the use of doubly dis-tilled water, except of TTA, which was dissolved in 95% ethanol (pure for analysis).Solutions of Eu/TTA complex were obtained by mixing europium chloride solution with a solution of TTA at the molar ratio 1 : 1, 1 : 2, 1 : 3 and 1 : 4. The ethanol content in > 1% v/v did not affect the CL intensity.
For all solutions studied, the kinetic curves of CL decay were determined and the CL light sums were calculated as the area under these curves (S = t t0 I∆t, where I-CL intensity, t-measurement duration).The spectra of chemiluminescence were recorded using the method of cut-off filters.

RESULTS AND DISCUSSION
The chemiluminescence emitted from the systems containing hydroxylamine and Cu(II) ions as catalyst and Eu/TTA complex in aqueous solution was studied.
The introduction of hydroxylamine into 0.01 mol/l NaOH solution induced a short-lived chemiluminescence of low intensity.The kinetic curve of CL decay in the system NH 2 OH-NaOH is shown in Figure 1.
The low intensity CL emission appeared at the moment of NH 2 OH introduction into NaOH solution, then quickly attenuated to baseline in 2 min.The presence of Cu(II) ions in this system resulted in an increase of CL intensity, having the same duration (curve 2 in Figure 1).
The introduction of thenoyltriflouoreacetone into the NH 2 OH-NaOH-Cu(II) and NH 2 OH-NaOH systems caused a significant decrease in CL intensity and shortening of its duration.The intense and long-lived CL was obtained for solutions containing NaOH, Cu(II), Eu/TTA complex and NH 2 OH (curve 3 in Figure 1).The intensity of CL was dependent on the Eu(III): TTA molar ratios.The highest emission intensity was observed in the case of the ratio Eu(III) : TTA = 1 : 3. The most intensive CL was recorded at the moment of hydroxylamine introduction and then the CL decrease was much slower than that observed in the primary system (without Eu(III) chelate).
In order to identify the emitter in the systems studied, their spectral distributions were recorded by the method of cut-off filters (Figure 2).
The emission spectra of the systems NH 2 OH-NaOH-Cu(II) and NH 2 OH-NaOH were similar and revealed four The emission spectrum of NH 2 OH-NaOH-Cu(II)-Eu(III) chelate with a maximum at λ ∼ 600 nm, was similar to the fluorescence spectrum characteristic of Eu(III) ions, corresponding to the transitions of 5 D 0 → 7 F 1 (λ ∼ 595 nm) and 5 D 0 → 7 F 2 (λ ∼ 615 nm) [2,7,8,16].The width of the spectral region, obtained using the cut-off filters method, is equal to 10-20 nm.An accurate resolution of the two bands in the presented spectrum is not possible.This means that in the system studied the only emitter were europium(III) ions.
The chemiluminescence characteristic of Eu(III) ions obtained in the solutions containing the europium(III) chelate complexes suggests that the complex with TTA plays an essential role in the excitation of Eu(III) ions.In order to determine the effect of the complex concentration on the intensity of CL of the system studied, the intensity of CL was measured in the systems containing NH 2 OH, NaOH and Cu(II) ions, at a constant concentration and europium chelate at different concentrations.
Figure 3 presents the dependence of the light sum CL of the NH 2 OH-NaOH-Cu(II)-Eu(III) chelate system on the concentration of europium(III) complex (M : L molar ratio = 1 : 3).As follows from Figure 3 the increase in the light sum of CL is proportional to the increase in the Eu(TTA) 3 complex concentration.
In order to fully characterise the system NH 2 OH-NaOH-Cu(II)-Eu(III) chelate, the effect of the introduction time of europium(III) chelate into NH 2 OH-NaOH-Cu(II) system on the intensity and duration of CL was also studied (Figure 4).
Curve 1 in Figure 4 presents the CL decay of the system studied, in which europium(III) complex was present at the moment of introduction of NH 2 OH (initiation of reaction).Introduction of the Eu(III) chelate at 1, 3, 5 minutes after the reaction initiation does not influence the CL intensity and duration.That means that europium(III) complex does not influence the rate of oxidation of hydroxylamine.
The absorption spectra of the system NH 2 OH-NaOH-Cu(II)-Eu(III) chelate were also recorded before and after the addition of hydroxylamine (Figure 5).The absorption spectra of individual reagents show that TTA absorbs in the 225-325 nm range.The formation of the chelate between Eu(III) ions and TTA is evidenced by a new absorption band which is red shifted with respect to that of the free ligand (with a maximum at λ ∼ 350 nm) [10,17].As shown in Figure 5, the intensity of this band did not change after the introduction of NH 2 OH which means that the Eu-TTA complex does not undergo destruction under the effect of hydroxylamine.
The oxidation mechanism of hydroxylamine in basic solution and metal-ion-catalysed has been investi- gated [18][19][20][21][22].In basic aqueous solution NH 2 OH exists in the form of NH 2 O − , which can be attacked by the dissolved oxygen [19]: The formation of HO 2 − free radicals also included, the oxidation of metal-ion-catalysed hydroxylamines [20], according to the scheme: The recombination reaction of free radicals generated singlet oxygen [23,24]: Both oxidation mechanisms of hydroxylamine indicate that dissolved oxygen plays an important role in the CL reaction.Therefore, the CL intensity of system NH 2 OH-NaOH-Cu(II)-Eu(III) chelate in dependence of time of argon bubbled through the solution was studied.The intensity of emission decreased with increasing time of argon bubbling.The results indicate that in the reactions above dissolved oxygen is an import factor in the free radical creation.According to our earlier study of the reaction systems in which hydrogen peroxide is decomposed in a basic medium in presence of Eu-TTA complex [5], a strong connection between the CL intensity and the concentration of europium(III) chelate and the presence of four bands, characteristic of singlet oxygen in spectrum of NH 2 OH-NaOH-Cu(II) system, leads to a conclusion that the excitation of Eu(III) ions takes place as a result of the energy transfer from the products of oxidation of hydroxylamine-singlet oxygen to lanthanide ions in its TTA complex.The mechanism of this processes is illustrated in the scheme below:

CONCLUSION
The majority of organic and inorganic systems generating chemiluminescence contains H 2 O 2 or any other oxidising agents.In contrast, the CL system studied here comparises hydroxylamine-a reducing agent that reacts with Cu(II) ions giving a low-intensity chemiluminescence typical for singlet oxygen.A strong increase in the intensity of CL characteristic of Eu(III) ions in the system NH 2 OH-NaOH-Cu(II)-Eu(III) chelate relative to the emission of the system NH 2 OH-NaOH-Cu(II), proves that the Eu(III) thenoyltrifluoroacetone complex is an efficient sensitizer of CL accompanying oxidation of hydroxylamine in a basic medium.

Figure 4 .
Figure 4.The kinetic curves of CL decay in the system NH 2 OH-NaOH-Cu(II)-Eu(III) chelate versus the time of introduction of Eu(III) chelate after the initiation of reaction.Curve 1-Eu(III) complex presence at the moment of initiation of reaction; curve 2 introduction after 60 s, curve 3-after 180 s and curve 4-after 300 s.The concentration of Eu(III) ions was 1 × 10 −3 mol/l, the initial concentration of NH 2 OH = 1 × 10 −3 mol/l, and Cu(II) = 5 × 10 −6 mol/l.