Flash Photolysis Study of Complexes between Salicylic Acid and

Article pubs.acs.org/JPCA

Flash Photolysis Study of Complexes between Salicylic Acid and Lanthanide Ions in Water Philipp-A. Primus and Michael U. Kumke* Physical Chemistry, Department of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Golm, Germany S Supporting Information *

ABSTRACT: In the natural environment humic substances (HS) represent a major factor determining the speciation of metal ions, e.g., in the context of radionuclide migration. Here, due to their intrinsic sensitivity and selectivity, spectroscopic methods are often applied, requiring a fundamental understanding of the photophysical processes present in such HS−metal complexes. Complexes with different metal ions were studied using 2-hydroxybenzoic acid (2HB) as a model compound representing an important part of the chelating substructures in HS. In flash photolysis experiments under direct excitation of 2HB in the absence and the presence of different lanthanide ions, the generation and the decay of the 2HB triplet state, of the phenoxy radical, and of the solvated electron were monitored. Depending on the lanthanide ion different intracomplex processes were observed for these transient species including energy migration to and photoreduction of the lanthanide ion. The complexity of the intracomplex photophysical processes even for small molecules such as 2HB underlines the necessity to step-by-step approach the photochemical reactivity of HS by using suitable model compounds.

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INTRODUCTION With respect to long-term safety assessments of nuclear waste disposals and environmental radionuclide contaminations, it is of utmost importance to fully understand transport and immobilization processes of radionuclides in soil and water with special consideration of the specific geochemistry, e.g., in the vicinity of the repository sites.1 In the context of environmental monitoring, lanthanide ions (Ln(III)) have proven to be highly valuable probes because (i) Ln(III) are considered as natural analogues for actinides without the disadvantage of radioactivity and (ii) many Ln(III) complexes (especially Eu(III) and Tb(III)) show a specific luminescence in aqueous solution.2−4 Since the intrinsic Ln(III) luminescence is very sensitive to alterations in the (first) coordination sphere of the Ln(III) ion,5−8 Ln(III) ions are powerful luminescence probes for the investigation of metal ion complexation reactions. Due to the ubiquitous abundance of humic substances (HS) and due to their ability to form strong complexes with metal ions such as actinides, HS play a key role in environmental processes determining the fate the metals.9−11 The major challenge working with HS is their highly complex and heterogeneous nature. Frequently the complexation of metal ions by HS is monitored using fluorescence spectroscopy.4,12,13 On the © 2012 American Chemical Society

basis of the quenching of the intrinsic HS fluorescence, conditional binding constants were determined for many heavy metal ions.14 However, for an unambiguous interpretation of the fluorescence quenching resulting upon metal ion binding, the nature of the intrinsic HS emission as such needs to be understood in more detail. The heterogeneous nature of HS renders this very difficult. The origin and characteristics of the intrinsic HS fluorescence are accompanied by intramolecular processes such as energy and electron transfer, which in turn may be heavily altered due to metal bindingespecially in case the metal ion itself can participate in such processes.15,16 The knowledge and understanding of the photoinduced processes involved are of fundamental importance for the interpretation of spectroscopic data related to humic substances and their metal complexes. To address the electronic interaction between metal ions and HS, it is a sound experimental approach to reduce the complexity of the system parameters by using simpler model compounds as proxies for HS. Such model compounds can be used to specifically address certain Received: May 10, 2011 Revised: January 4, 2012 Published: January 4, 2012 1176

dx.doi.org/10.1021/jp2043575 | J. Phys. Chem. A 2012, 116, 1176−1182

The Journal of Physical Chemistry A

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the metal ions. Dissolved oxygen was removed by purging the sample solutions with argon for 45 min. Each flash photolysis measurement was performed with a fresh sample, and absorption spectra were recorded before and after the flash photolysis measurements to control for possible photodegradation. Absorption and Fluorescence Spectroscopy. Absorption measurements were carried out using a Lambda 750 absorption spectrometer (Perkin-Elmer, Waltham, MA, USA). All UV/vis spectra were recorded using purified water as reference. The spectral data were collected in the wavelength range of 200 nm < λ < 400 nm. Steady-state fluorescence spectra were collected using a Fluoromax 4 spectrometer (Horiba Jobin Yvon, Kyoto, Japan). For the measurements of the lanthanide luminescence the instrument was operated in the phosphorescence mode. The samples were excited at λex < 300 nm in case the ligand fluorescence and sensitized luminescence were investigated. For direct excitation of the lanthanide luminescence, the samples were excited at λex = 395 nm and at λex = 378 nm for Eu(III) and Tb(III), respectively. Laser Flash Photolysis. Laser flash photolysis measurements were carried out using a LP920 flash photolysis spectrometer (Edinburgh Instruments, Livingston, Great Britain) with a pulsed high-pressure Xe lamp as probe light source; the pulse length was set to 5 ms. The samples were photoexcited by a dye laser (Cobra Stretch, Sirah, Kaarst, Germany) pumped by a Nd:YAG laser (Quanta Ray, Spectra Physics, Santa Clara, CA, USA; λpump = 532 nm) with a repetition rate of 10 Hz and a pulse length of 8 ns. The excitation wavelength was set to λex = 290 nm. In a typical experiment the laser power was set to 50 mW. In the measurements absorption spectra as well as decay kinetics of the transient species were collected. All transient spectra were recorded using an iCCD camera (i-Star, Andor Technology, Belfast, Great Britain) in the wavelength range of 350 nm < λ < 750 nm. For the kinetic measurements a combination of a photomultiplier tube R928 (Hamamatsu, Iwata, Japan) and a digital oscilloscope TDS3012B (Tektronix GmbH, Kö ln, Germany) was used. The decay kinetics were analyzed by fitting the experimental data to

properties of HS and are therefore valuable tools to investigate structure−interaction relationships.17,18 Often aromatic carboxylic acids are considered as proxies for complexing substructures in HS.19−21 Understanding the photophysics and photochemistry of the model compounds in the presence of metal ions is a prerequisite for a sound interpretation of the reactivity of HS and the impact on metal ion speciation.17,22 Interactions between excited states of organic ligands and lanthanide ions have been studied for well over 60 years8,23−25 with special emphasis on intracomplex energy transfer. A major driver in such investigations was the search for novel ligands with optimized optical properties in order to overcome the disadvantage of thee.g., due to parity forbiddancelow extinction coefficients of Ln(III) ions using the so-called antenna effect.26 Triplet and singlet energy transfer from ligands to Ln(III) ions have been discussed for various systems on the basis of absorption and luminescence data. An energy transfer via ligand triplet states is widely accepted as the predominant energy migration pathway, although direct evidence is often missing.27,28 Other researchers report compounds where the participation of ligand-to-metal charge transfer (LMCT) states is crucial for the interpretation of luminescence spectroscopy data.29−35 2-Hydroxybenzoic acid (salicylic acid, 2HB) is frequently used as simple model compound since it combines three patterns generally found in HS: (i) aromaticity, (ii) carboxylic groups, and (iii) phenolic groups. The latter two are major contributors to the metal binding properties of HS.19 The photophysical processes discussed for carboxylic acid− lanthanide complexes are usually singlet and/or triplet energy transfers.36 Those energy-transfer processes are also discussed extensively for humic substances.4 Photoreduction, chargetransfer transitions, and energy back-transfers from lanthanide ions to the organic chromophores are often ignored. In experiments using aromatic carboxylic acids, in which the distance between aromatic moiety and carboxylic group (as metal binding site) was systematically varied, the observed energy-transfer efficiencies decreased with increasing distance between binding site and aromatic moiety.37 Compared to former studies, in which mainly indirect evidence from investigations of the lanthanide luminescence was used to conclude on the energy-transfer pathways, laser flash photolysis provides a possibility to directly study transient species of molecules,38 such as molecules in triplet states or in charge-transfer states, or radical ions, respectively. In the present study laser flash photolysis was used to selectively generate and investigate reactions involving triplet states, radical ions, and solvated electrons of complexes between 2HB and different lanthanide ions. On the basis of the experimental data, direct evidence for the involvement of the ligand triplet state and a complex-specific charge-transfer state in deexcitation processes is presented. The influence of the redox properties of the metal ion on the specific deexcitation pathway is investigated using Eu(III), Tb(III), and La(III).

A(t ) = constant +

∑ Ai(0) exp[ − kit ] i

(1)

with i = 1, 2. Ai(0) represents the signal intensity at t = 0, ki is the decay rate constant, and “constant” accounts for the background signal.

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RESULTS AND DISCUSSION UV/Vis Absorption. Figure 1 shows the UV/vis absorption spectrum of 2HB in the absence and in the presence of Eu(III). At λabs > 310 nm the absorption increased with increasing concentration of Eu(III) (up to a molar ratio of 2HB:Eu(III) of 1:500). In a control experiment using La(III) no such change in the 2HB absorption spectrum was observed (see Figure 1). The observed alteration in the UV/vis spectrum may be attributed to a charge-transfer (CT) transition, which has been described for the 2HB−Eu(III) complex before.40 Compared to CT absorption bands typically seen for d-metals such as Cu(II), the intensity of the CT absorption band in the case of Eu(III) is weak. This can be attributed to the effective shielding of the Eu(III) f-orbitals by s- and p-orbitals, allowing only a weak

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EXPERIMENTAL SECTION Chemicals. Salicylic acid has two acidic protons with a pKa = 3.0 and 13.4, respectively.39 At pH 5.5 the predominant form is the 2HB monoanion. All samples were prepared from stock solutions of sodium salicylate (c2HB = 10−3 M) and metal chlorides (cMe = 10−1 M). The final concentration of 2HB was c2HB = 10−4 M in all samples. The pH was adjusted to 5.5 to avoid the formation of carbonate and hydroxide complexes of 1177



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Figure 2. Relative fluorescence spectra of 2HB (10−4 M, λex = 290 nm) and 2HB with a 500-fold excess of different metal ions (case B scenario; vide infra). Note the intensive Tb(III) emission lines at λem = 488 and 543 nm (in case of Eu(III) the related sensitized luminescence was observed at wavelength λem > 575 nm; see inset).

Figure 1. Absorption spectra of 2HB (10−4 M, black) with a 500-fold excess of Eu(III) (red) and La(III) (green). The peaks correspond to absorption lines of the Eu(III) ion (λ = 286, 298, 317, and 326 nm).

direct interaction with the MOs of 2HB. The fact that this CT band is only seen at a large excess of Eu(III) is a consequence of the low complexation constant of the 2HB−Ln(III) systems (for Eu(III), log(β 1 ) = 2.02 and log(β 2 ) = 3.84). 41 Consequently, to achieve a high degree of complexation, the laser flash photolysis experiments on 2HB were performed in the presence of a large excess of metal ions. On the basis of the conditional binding constants β1 and β2 reported (vide supra) and with use of the mass balance law, a fraction of 96% bound 2HB was calculated for 2HB:Eu(III) at the molar ratio of 1:500. Because of the very small extinction coefficients, no significant contribution from Eu(III) to the overall absorption of the sample was observed (only at the highest Eu(III) concentration weak absorption bands became visible; see Figure 1). Fluorescence of 2HB. In the fluorescence spectra of 2HB a large Stokes shift of the emission maximum relative to the absorption maximum is observed, which is the result of an excited-state proton transfer. Depending on the solvent (and the pH of the solution) this proton transfer can be (i) intramolecular between the hydroxyl and carboxyl group of 2HB (in aprotic solvents) or (ii) intermolecular between the hydroxyl group of 2HB and surrounding solvent molecules (in protic solvents such as water).42−44 Upon metal binding the intrinsic fluorescence of 2HB is quenched. From the data evaluation of steady-state and time-resolved fluorescence measurements Aoyagi et al. concluded that the observed fluorescence quenching by metal ions, e.g., by Eu(III), is of pure static character.45 In the present study samples containing Tb(III) or Eu(III) (cLn = 5 × 10−2 M), the fluorescence intensity was quenched by 50 and 90% of its initial intensity, respectively. On the other hand, La(III) and Ca(II), which were used as reference ions, induced a significantly smaller decrease of the 2HB fluorescence intensity (see Figure 2). The observed quenching of the 2HB fluorescence can be caused either by an energy transfer to the Ln(III) ion from the 2HB singlet state27 or by an enhanced intersystem crossing efficiency in the Ln(III)−2HB complex due to an enhanced spin−orbit coupling induced by a heavy atom effect. The effective fluorescence quenching in the case of Eu(III) has also been attributed to a photoreduction of Eu(III) to Eu(II).46 To further resolve the contributions from different reaction pathways, transient absorption measurements were performed.

Transient Absorption of 2HB. The transient absorption spectrum of 2HB showed three typical bands (see Figure 3):

Figure 3. Transient absorption spectra of 2HB (10−4 M, λex = 290 nm) recorded at different excitation powers, normalized to the TTA signal at λmax = 455 nm.

the triplet−triplet absorption (TTA) with λTTAmax = 455 nm, the absorption of a phenoxy radical species at λmax = 390 nm, and a broad absorption band of the solvated electron around λmax = 720 nm.38 The assignment of the three transient absorption bands was confirmed by measurements in the absence and presence of oxygen (quenching of triplet state by 3 O2) and by variation of the excitation energy. It has been shown that the formation of the phenoxy radical and of the solvated electron is induced by a two-photon absorption process.38 Thus, the formation of these species was found to be dependent on the excitation power (see Figure 3). By adjusting the excitation power the relative yield of the 2HB triplet state and solvated electrons could be addressed (increasing phenoxy radical absorption is not visible due to the weak intensity). Transient Absorption of 2HB in the Presence of Eu(III). The interaction of 2HB and Eu(III) ions was investigated at two different boundary conditions: at low Eu(III) concentrations up to cEu = 10−4 M, where only a very minor fraction of 2HB is bound in a complex (at 2HB:Eu(III) of 1:1 1178



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observedindicating the absence of an energy transfer from 2HB to the Eu(III) ion. The success of collisional quenching over energy-transfer-based quenching may be related to the rate constants for both processes. While collisional quenching is a diffusion-controlled process, the rate constant of energy transfer isamong other parametersdetermined by the overlap integral of the donor emission and the acceptor absorption. Due to the small extinction coefficients of Eu(III), which would act as the acceptor part, the overlap integral becomes small and subsequently also the corresponding rate constant for the energy-transfer step is small.47 Therefore, the diffusioncontrolled quenching is the operating process. Case B. At high Eu(III) concentrations the fraction of 2HB coordinating Eu(III) ions is significantly increased. Consequently, at increasing Eu(III) concentrations, 2HB is predominantly found in complexes with Eu(III) which are now observed in the transient measurements. At high Eu(III) concentrations, no triplet absorption of 2HB (λTTA = 455 nm) can be detected even at short times after the laser flash (δt < 1 μs) because of the abundance of Eu(III), which proved to be an efficient triplet quencher for 2HB already in the nonbound state (vide supra, case A). This quenching is also highly efficient in the 2HB−Eu(III) complexes; however, some electronic energy is transferred to the Eu(III) ion, which is seen from the sensitized luminescence (see Figure 2, inset). With increasing concentrations of 2HB−Eu(III) complexes, a new transient absorption band is building up around λmax = 440 nm. Because of the strong spectral overlap with the absorption of 2HB radicals, this new transient absorption band is only visible at about 50 μs after the laser pulse (see Figure 7, gray spectrum). A time trace of the transient absorption of 2HB−Eu(III) recorded at λ = 455 nm for different concentrations of Eu(III) is shown in Figure 4. A rise time of τr = 9.0 ± 0.4 μs and a decay time of τd = 134 ± 7 μs were obtained from evaluation of the experimental data using eq 1 (Figure 4, green). The transient species of 2HB alone have decay times in the lower microsecond time regime with τTTA = 14.6 ± 0.1 μs for pure 2HB being the longest. That is valid for the radicals, the solvated electron as well as the other excited states of 2HB. Compared to the TTA decay of 2HB in the absence of Eu(III) ions, the decay time of the new absorption signal at λmax = 440 nm is a factor of about 10 slower (τTTA = 14.6 μs versus τd = 134 μs). The observed rise can be explained neither by a transient absorption of Eu(II) that may be formed as an intermediate nor by absorption of photochemical products that may be formed after irradiation with intensive laser light. None of these species absorb light in the spectral region of 430 nm < λ < 460 nm, as the respective absorption spectra showed.48 Furthermore, solutions of Eu(II) quickly react with water under formation of hydrogen and Eu(III),49,50 which has not been tested for in the present study. The complex transient absorption decay kinetics in combination with the long transient absorption decay time and the spectral shift compared to the TTA band of 2HB are strong indicators that this long living transient absorption can be attributed to a new state which is specifically formed in the complexes of 2HB with Eu(III). In Figure 6, the time traces of the transient absorption measured at λ = 455 nm (corresponding to a wavelength at which both the 2HB triplet state and the new Eu(III)−2HB absorption can be monitored) for different Eu(III) concentrations are

only 4% are bound; case A) and at high Eu(III) concentrations (2HB:Eu(III) = 1:500), in which 96% of 2HB is complexed (case B). Case A. Free Eu(III) ions quenched the triplet state of 2HB, which was indicated by the decrease in the triplet absorption intensity and by a reduction of the 2HB triplet decay time from τTTA = 14.6 ± 0.1 μs to τTTA = 1.8 ± 0.1 μs (see Figure 4, black

Figure 4. Transient absorption decay of 2HB (10−4 M, black) with different concentrations of Eu(III) in water at λ = 455 nm (case A, red; case B, green). The sharp negative peaks accrue from collected fluorescence light at 455 nm. The samples were excited at λex = 290 nm.

and red lines, respectively). The latter effect directly points to a dynamic interaction between 2HB and Eu(III). In a Stern− Volmer analysis of the 2HB triplet-state decay time a bimolecular quenching constant kq = (4.9 ± 0.8) × 109 M−1 s−1 was determined, suggesting that the observed triplet quenching under conditions of case A is a diffusion-controlled process (Figure 5). The quenching of the 2HB triplet absorption can

Figure 5. Stern−Volmer analysis of the 2HB (c = 10−4 M) triplet absorption decay time (τTTA) quenched by Eu(III) ions. Each value has been measured three times.

be assigned to a collisional process, e.g., causing an increased intersystem crossing rate to the 2HB singlet ground state due to an external heavy atom effect. This assignment is further supported by the fact that under the experimental conditions of case A no sensitization of the Eu(III) luminescence was 1179



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species of 2HB. In contrast to Eu(III), no distinct alterations in the transient spectra were found. Both ions (La(III), Ca(II)) do not change their redox state, and thus no electronic levels for an energy transfer from the 2HB triplet state are available. Therefore, the TTA as well as the absorption of the solvated electron is seen, and no LMCT related absorption is found (see Figure 7). An intermediate behavior is found for Tb(III). Here, the TTA of 2HB is strongly quenched, while the solvated electron and the phenoxy radical is almost unaffected by the presence of Tb(III). However, no LMCT related transient absorption is found, probably due to the lack of a stable +II oxidation state for terbium.

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CONCLUSION Monitoring the different transient species in the absence and presence of lanthanide ions paved the way to better characterization of the nonradiative intracomplex reaction pathways. Free Eu(III) led to an effective diffusion-controlled quenching of the 2HB triplet state. Due to the lack of sensitized Eu(III) emission, there is no indication for a triplet energy transfer from excited 2HB to Eu(III) as long as 2HB and Eu(III) are not bound in a complex (case A conditions). In Eu(III)−2HB complexes (case B conditions) a new transient species is formed after photoexcitation and can be observed in the transient absorption spectra. Delayed transient absorption measurements revealed the absorption band with a maximum around λ = 440 nm, a rise time of τr = 9.0 ± 0.4 μs, and a decay time of τd = 134 ± 7 μs. This specific absorption band can either be a LMCT absorption that appears after nuclear reorganization or a reactive intermediate of a photoinduced electron-transfer reaction between 2HB and europium. In contrast to Eu(III), complexes with redox inactive (under the experimental conditions) metal ions did not show a comparable absorption band in the transient absorption spectrum of 2HB. Those ions, such as Tb(III), which have adequate electronic states, still act as effective TTA quenchers. In that case only the electronic energy is transferred from 2HB to the metal ion, which subsequently shows an enhanced (sensitized) luminescence. The new absorption at λ = 320 nm observed in the regular UV/vis absorption spectrum may be attributed to a transition in a higher LMCT state. In that case the transient absorption band at λmax = 440 nm could belong to an electronic transition between two LMCT states. The relatively long built-up time of the transient species at λmax = 440 nm may be attributed to the fact that a higher vibronic state is populated from one of the electronic levels of the europium, which requires the exchange of electronic excitation energy to vibrational energy and involves a large reorganization effort, making the process slow. The 5D1-state of europium may be a possible initial state because the decay time of it is in the range of 5−10 μs. In Figure 8 the proposed deactivation pathways after formation of LMCT states for 2HB−Eu(III) complexes are summarized. Due to the formation of an LMCT state, an additional deactivation channel is opened. Recently, we investigated the temperature dependence of the luminescence of 2HB−Eu(III) complexes in the range of 77 K < T < 293 K. At room temperature a luminescence decay time between 90 and 78 μs, depending on the degree of complexation, was determined, which strongly points to an additional ligandspecific quenchingalthough water molecules, which are considered to be responsible for the highly effective quenching due to coupling to OH-vibrations, are removed from the first

Figure 6. Transient absorption decay of 2HB and its Eu(III) complexes in water at λ = 455 nm.

shown. The rise times τr are independent of the Eu(III) concentration which further supports the idea of an intracomplex process, such as a ligand-to-metal charge transfer. The intensity of the proposed transient LMCT absorption scales well with the calculated amount of complex formed in the samples at different Eu(III) concentrations (see Figure 6). Similar results have been found in the presence of Cu(II) ions (see the Supporting Information), whichlike Eu(III)can change its redox state under certain experimental conditions. Nevertheless, this interpretation could be challenged by the possibility of photoinduced electron-transfer reactions between Eu(II) and 2HB radical species, as Eu(II) reacts with mild oxidants on the microsecond time scale in pulse radiolysis experiments.51 Transient Absorption of 2HB in the Presence of Other Lanthanide Ions. In the next step the influence of other redox inactive metal ions on the formation of the different transient species (triplet state, phenoxy radical, solvated electron, formation of LMCT) was monitored. Depending on the metal ion added, specific changes in the transient absorption spectra were observed. In Figure 7 the transient absorption spectra of 2HB

Figure 7. Transient absorption spectra of 2HB (10−4 M, black) and 2HB in the presence of an excess of La(III) (red) and Eu(III) (green and gray). The right axis represents the 2HB spectrum at δt = 50 μs in the presence of Eu(III).

in the presence of Eu(III) and La(III) are compared to that of pure 2HB. The addition of La(III) (and also of Ca(II); see the Supporting Information) had only minor effects on the intensity and on the decay kinetics of any of the transient 1180



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coordination sphere17 (in contrast to these decay times, Eu(III) in water has a luminescence decay time of 110 μs). On the basis of the results presented, the observed luminescence quenching in 2HB−Eu(III) complexes could be attributed to the formation of a LMCT state. Such charge-transfer states have been proposed before as a possible source for luminescence quenching,29 but direct experimental evidence is missing. The spectroscopic data are of particular interest for studies using lanthanide ions as luminescent probes in aquatic systems, such as studies on the HS−lanthanide interactions. LMCT states of the chelating chromophores must be taken further into account when discussing the photophysical properties of complex systems such as complexes with HS and concluding on the speciation of metal ions. Work is in progress to investigate the role of such LMCT states in the deactivation of Eu(III) in complexes with further model ligands featuring diverse electronic properties such as anthranylic acid and tropic acid.

ASSOCIATED CONTENT

* Supporting Information S

Figures showing transient absorption spectra of 2HB in the presence of Ca(II) and Tb(III) ions, transient absorption decays at λ = 455 nm in the presence of different concentrations of Cu(II), and absorption spectra of 2HB in the presence of different concentrations of Eu(III). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 8. Scheme of photophysical processes possible in the salicylate−Eu(III) complex after irradiation with intensive laser light under case B conditions. Straight arrows indicate radiative transitions, dotted arrows indicate nonradiative transitions, and dashed arrows indicate energy- or electron-transfer processes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS Part of the experimental work was funded by the Federal German Ministry of Economics and Technology under the supervision of the Project Management Agency Karlsruhe (Grant Agreement 02E10216). The authors are thankful for the assistance of Mrs. Ursula Eisold (University of Potsdam) in some of the presented measurements. 1181



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Flash Photolysis Study of Complexes between Salicylic Acid and

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