Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+

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A: Kinetics, Dynamics, Photochemistry, and Excited States 22+

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Ultrafast Transient Absorption Spectroscopy of UO and [UOCl] Toni Haubitz, Satoru Tsushima, Robin Steudtner, Björn Drobot, Gerhard Geipel, Thorsten Stumpf, and Michael U. Kumke

J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05567 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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The Journal of Physical Chemistry

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]+ Toni Haubitz1, Satoru Tsushima2,3, Robin Steudtner2, Björn Drobot4, Gerhard Geipel2, Thorsten Stumpf2, and Michael U. Kumke1* 1

University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, D-01328 Dresden, Germany 3 Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, Tokyo, 152–8550, Japan 4 Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstraße 108, D-01307 Dresden, Germany 2

ABSTRACT: For the only water coordinated "free" uranyl(VI) aquo ion in perchlorate solution we identified and assigned several different excited states and showed that the 3∆ state is the luminescent triplet state from Transient Absorption Spectroscopy. With additional data from other spectroscopic methods (TRLFS, UV/Vis) we generated a detailed Jabłoński diagram and determined rate constants for several state transitions, like the inner conversion rate constant from the 3Φ state to the 3∆ state transition to be 0.35 ps1 . In contrast to luminescence measurements it was possible to observe the highly quenched uranyl(VI) ion in highly concentrated chloride solution by TAS and we were able to propose a dynamic quenching mechanism, where chloride complexation is followed by the charge transfer from the excited state uranyl(VI) to chloride. This proposed quenching route is supported by TD-DFT calculations.

Introduction The dioxouranium cation UO22+ (uranyl) is of major interest for the long-term safety assessment of nuclear waste repositories and in the remedation of former uranium mining sites. Moreover, as a naturally occurring element uranium is ubiquitously present in the environment as a “natural radioactive burden”. Because of its high chemical toxicity, the content of uranium in different environmental compartments (such as groundwater) is important to know as they may accumulate in the kidney and other organs1. The bioavailability and –toxicity of uranium are determined by its chemical form (ligands in the coordination sphere). Therefore, a sound speciation of uranium and in particular for uranyl(VI) in the different environments is indispensable, e.g., for risk assessment. Uranyl(VI) speciation analysis using optical spectroscopy, especially that based on luminescence, was proven to have an outstanding selectivity and sensitivity with detection limit of uranyl(VI) in the order of 10-12 M2,3. The emission spectrum is found in the spectral range of 480 nm < λem < 620 nm and the corresponding emission decay times are in the range of a few µs up to several hundred µs depending on the ligands and the solution conditions (e.g., pH, temperature, ionic strength).3 From the change in the spectral intensity distribution and the alteration of the luminescence decay kinetics, uranyl(VI) complexes were successfully identified for many inorganic and organic ligands4–6. Consequently, the fundamental understanding of the uranyl(VI) photophysics in its different complexes is of central interest because this will allow to predict the luminescence properties of uranyl(VI) complexes and may help to improve the experimental design when investigating those. Especially, for complexes with reduced (or no) luminescence the understanding of radiationless deactivation processes,

which are competing with the emission, is indispensable for a successful redesign of the experiments (e.g., improving the time resolution of the experiments)7. Some uranyl(VI) complexes, such as carbonates and halides, show only weak or no luminescence at all in aqueous solution or only at cryogenic temperatures.8 In these cases the luminescence-based speciation analysis (e.g., identification of complexes with respect to the number of coordinated ligands or with respect to the binding constants) is limited or not possible at all. Uranyl(VI) complexes with chloride belong to a group of complexes that are only very weakly luminescent. The quenching effect of halide ions are described in literature as a dynamic quenching process based on the evaluation of intensity-based and decay-time based luminescence data9. Stern-Volmer data analysis of the luminescence decay times showed a linear relationship with chloride concentration. Beyond the phenomenological description of the quenching process a mechanistic interpretation of it is under debate. Theoretical calculations suggest that the quenching is the result of a charge transfer reaction between the ligand(s) and the electronically excited uranyl(VI) ion. Tsushima et al showed an increase in redox potential with increasing halide radius, while fluoride complexes are still luminescent but chloride complexes are not. 10,11 Marcantonatos et al. found that the redox potential of the excited uranyl(VI) to uranyl (V) with 2.61 V is in the range of fluorine12 and is vastly higher than in the ground state (0.062 V).13

Experimental Section A uranyl(VI) stock solution was prepared by dissolving solid UO3 in HClO4. The final uranyl(VI) concentration in each sample was set to be (0.098±0.001) M (checked by ICP) to

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The Journal of Physical Chemistry receive a maximum signal in the transient absorption with low degree of self-quenching. Wavelength [nm] 385

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Figure 1. Comparison of spectroscopic data of the uranyl(VI) aquo ion obtained by different techniques: UV/Vis (blue), TRLFS (green), TAS (orange, red).

As complexation ions and background electrolytes, 1 M hydrochloric acid and perchloric acid were chosen. The pH values were set to 0 and were measured by a double junction pH electrode Profitrode 125 mm from Merck with a 3 M NaClO4 solution as a bridge electrolyte, to avoid chloride contamination by the electrode. For dilution deionized water was used and was generated by a Milli-Q Reference Water Purification System of Merck Millipore to 18.2 MΩ. All measurements were performed at room temperature (T = 295 K) if not stated otherwise. For the transient absorption spectroscopy (TAS) a TitanSapphire-laser system Spitfire Ace PA of Spectra-Physics was used with a fundamental wavelength of 800 nm and a pulse length of approximately 40 fs at a repetition rate of 1 kHz. The fundamental laser beam is split into a pump and a probe beam. The wavelength of the pump beam can be tuned using a HETOPAS (Light Conversion) parametric amplifier. As pump wavelength λpump = 310 nm (bandwidth ∆λ approx. 40 nm), unless stated otherwise, was chosen, as uranyl(VI) complexes studied here can be excited in this range. Both beams were fed into the transient absorption spectrometer (TAS system, Newport). The probe beam was coupled into a CaF2 crystal for white light generation between 350 nm < λprobe < 700 nm. The white light beam is sent over a variable optical delay line, adjustable between 0 m and 2.4 m delay path length, which corresponds to a maximum time delay between λpump and λprobe of ∆t = 8 ns with a maximum time resolution of 26.6 fs. The transient signal (∆OD) was calculated based on the difference between the white light intensity with and without pump beam by the controller software (TAS software Newport, version 2.1). The pump power in the sample was adjusted by neutral density filters. The pump power was measured by a PEPS-39.5 sensor and a 1918-R power meter (Newport) prior and after each measurement. The samples were measured in quartz cuvette of 2 mm optical path length (Typ 21, Starna GmbH) and were rigorously stirred during the measurements (Electronic Stirrer Model 300, Rank Brothers Ltd.).

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TRLFS measurements were conducted with a quadrupled Nd:YAG Laser (Minilite, Continuum) at 266 nm. For detection a iHR550 spectrograph and an iCCD camera (Horiba Jobin Yvon) with a spectral resolution of 0.5 nm was used. UV/Vis measurements were performed on a CARY 5G spectrometer (Varian) with a spectral resolution of 1 nm. In both cases samples were measured in 4 mL quartz cuvettes with 1 cm pathway (Hellma Analytics). For data analysis the PARAFAC algorithm14 implemented in the optimization toolbox of Matlab 2017 (MathWorks)15 was used to deconvolute the spectra and time traces into the single species. To fit the time traces to a kinetic model, a numerical approach has been chosen, in which the corresponding ordinary differential equations of the model has been solved by the ODE45 algorithm in Matlab 2017 (MathWorks). The resulting rate constants k of the rate equations of the transitions have been varied to achieve a least square minimum between the numeric model and the experimental time traces by the fmincon algorithm. Quantum chemical calculations were performed in an aqueous phase using the Gaussian 09 program employing the density functional theory (DFT) by using a conductor−like polarizable continuum model as previously described.16 Structure optimizations were performed at the B3LYP level followed by vibrational frequency analysis at the same level to confirm no imaginary frequency is present. For the calculations of the excited states non-equilibrium time-dependent (TD) DFT calculations were applied in the aqueous phase. The energy−consistent small−core effective core potential (ECP) and the corresponding basis set suggested by Küchle et al.17 were used for uranium. For oxygen and hydrogen, the valence triple-ζ plus polarization basis was used. The spin−orbit effects and basis set superposition error corrections were neglected.

Results and Discussion Uranyl(VI) Aquo Ion Considering the low pH value (pH = 0) and ionic strength of the sample investigated no hydrolysis species are expected based on speciation data available18. This was also supported by the experimental TRLFS data showing a monoexponential luminescence decay with a luminescence decay time τ = (4.52 ± 0.02) µs and the main luminescence peak positions at λem = 487.2 nm, 509.6 nm, and 533.8 nm (Figure 1). Both are in good agreement with literature data for the uranyl(VI) aquo ion at 1 M ionic strength at room temperature.19 The time-resolved TAS spectra (Figure 2) were measured up to a delay time of Δt = 3 ns and showed a strong positive transient absorption (ΔOD) signal at around λ = 575 nm, which implies the population of a transient state. No ground state depletion signal (expected at around λ = 420 nm from the absorption spectra, Figure 1) was observed due to the small extinction coefficient of uranyl(VI). While the TAS spectra were very broad at short ∆t with almost no vibrational pattern, they changed into a better-defined (presence of a vibrational structure) and blue shifted spectrum within the first 5 ps after the pump pulse. The isosbestic point at 614.5 nm (Figure 2 (B)) indicates the involvement of two different electronic states and shows that uranyl relaxes from the initially excited state to a second (lower) electronically excited state. In the latter one, the uranyl “remains” for the observation time accessible with the TAS setup used.

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∆OD x10-3

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Figure 2. A: Complete 2D TAS spectrum of the uranyl(VI) aquo ion. B: Spectra at different delays. C: Time traces at different wavelengths. (1 M HClO4; pH = 0; 3.5 mW pump power; λPump = 310 nm; maximum delay of 3 ns)

Based on the TAS experimental configuration a decay time of >> 3 ns was anticipated and it is tempting to connect this “final” electronic state to the luminescent T1 state with a decay time of τ = 4.5 µs (vide supra, obtained from TRLFS measurements). The results for the ClO4--system are in good agreement with TAS measurements reported for uranyl(VI) in nitrate solution. Here, in nitrate solution the broader and shorter-lived state was assumed to be the T2 state of the uranyl20 and the change of the transient spectra was attributed to the transition from the T2 to the luminescent T1 state. Until now it is unclear whether the 3∆ or the 3Φ electronic state is the lowest electronic state in energy and corresponding to the luminescent state, as theoretical calculations suggest different results depending on the calculation method applied.21 However, the different theoretical calculations showed good agreement for the relative position of the higher electronic states. Based on the energetic order, the transient absorption processes were attributed to be the transitions from the 3∆ to the 3Π and the 3Φ to the 3Γ state. For now, we will only refer to the electronic states involved in the observed transient absorption as T1 to T3 and the T2 to T4 transitions (see Figure 3). The PARAFAC algorithm was applied to deconvolute single absorption spectra for the transition T2T4 and T1T3 and for the corresponding time traces (see Figure 4 (A) and (B)). It was found that the T1T3 absorption was blue-shifted, more structured and narrower than the T2T4 absorption. In the time traces it was also observed that the decay of the T2 coincided with the building kinetics of the T1 states. The kinet-

ics were fitted using a consecutive reaction model according to equation (1). /

(1)

A direct transition from S0 to T2 was assumed in this model as any kinetics like absorption from the S0 to higher states, vibronic relaxation and intersystem crossing (ISC) processes in between absorption and reaching the T2 state were not resolved due to the time resolution of the experiment, as these excitation processes usually happen on the low femtosecond time scale. Transition rates of kP/kISC=(2.15 ± 0.07) ps-1 and kIC=(0.354 ± 0.003) ps-1 were found and are in reasonable agreement with uranyl(VI) in nitrate solution, for which rate constants of 1.18 ps-1 and 0.625 ps-1, respectively, were reported.20 Table 1. Spectroscopic properties of the water coordinated uranyl(VI) ion State

Energy gap ∆E to the ground state [cm-1]

Vibrations [cm-1]

0

886 ± 10

–––

702 ± 10

20517 ± 11

3

Decay rate [ps-1]

T3/ Π

36737 ± 40

633 ± 9

>5

T4/3Γ

>34970 ± 100

759 ± 8

>5



qualitatively agree that the energy gap of the 3∆ to the 3Π should be bigger or at least equal to the energy gap between the 3Φ and the 3Γ state.21 Thus we assumed that the transition higher in energy (from T1 to T3) corresponded to the 3∆ 3Π transition, while the transient absorption lower in energy (T2 to T4) was attributed to the 3Φ 3Γ transition. Consequently, we assigned the T1 to the 3∆ state, while the T2 was attributed to the 3Φ state (see Figure 3). 0.14

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By fitting the absorption and transient absorption data (see Figure 1) with Gaussian peaks, under the condition that the peaks are equidistant due to the regular vibronic structure of

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Figure 3. Proposed Jabłoński diagram of the uranyl(VI) aquo ion based on data obtained from UV/Vis (blue), TRLFS (green), transient absorption (red, orange) and luminescence excitation scan (purple) measurements

the higher states, we were able to calculate the energy gaps between the different electronic states. The absorption band lowest in energy at 20520 cm-1 corresponds to the energy difference between the S0 and T1 state and the energy difference between the vibrational bands is the gap between the vibronic states of the T1 state with an energy spacing of ∆E = 702 cm-1 (Table 1). This band corresponds to the band found in the emission spectrum and was attributed to the 0-0 transition between the ground and luminescent state. The energetic difference of the emission peaks of 886 cm-1 is equal to the energy spacing between the vibronic states of the ground state which corresponds to the respective Raman band of the ground state (see Figure 3)22. The low intensity and highest energy band in the emission spectrum is considered in the literature as a so called “hot-band” (HB), i.e. an emission originating from the first excited vibronic state of the T1 state.23 The low intensity is due to the low occupation ratio of around 3% at room temperature (calculated based on the Boltzmann distribution). Considering the energy of the lowest band of the T1 to T3 transition (16150 cm-1, approx. 620 nm, Figure 1), we calculated the energy gap from the ground state to the T3 state by adding the 0-0 transition of 20520 cm-1 to this signal, which resulted in an energy of 36740 cm-1 (approx. 270 nm). Due to the lack of information on the energy gap between the T2 and the ground state, we can only state that it should be higher than the T1 state, as the system should relax into the energetically lowest electronic state. Based on the energy difference measured in the TAS experiments (T2 to T4 transition was found at 14440 cm-1) we assumed that the T4 state must be higher than 34970 cm-1 in energy. Using the energy differences calculated from the transient absorption we also got the vibronic spacing of the higher states with 630 cm-1 and 760 cm-1 for the T3 and T4, respectively (see Figure 3). While theoretical calculations differ distinctly in the relative energetic order of the 3∆ and the 3Φ state, almost all methods

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Figure 4. PARAFAC deconvoluted single species spectra (A, C) and fitted time traces (B, D) of the uranyl(VI) ion with different background electrolytes: 1 M HClO4 (A, B) and 1 M HCl (C, D). (pH = 0; 3.5 mW pump power; λPump = 310 nm; maximum delay of 3 ns)

For now the energy of the 3Φ state relative to the ground state and of the 3Γ state to the ground state cannot be deduced from our experimental data (therefore, their position shown in Figure 3 represent a rough estimate based on the conclusion of them being energetically higher compared to 3∆ and 3Π). The exact energy difference between 3∆ and 3Φ and thus the energy gap of 3Φ to the ground state might be obtained from IRpump-probe experiments. Based on our current data a transient IR absorption signal due to a 3∆-3Φ -transition can be expected at 500 cm-1 to 1500 cm-1 (estimation based on theoretical calculations21). From further comparison of the TAS data with literature excitation spectra16 we are now able to properly assign transitions to the excitation bands of the uranyl(VI) aquo ion (see Figure 5). For comparison the frequencies of the TAS signals were again combined with the energy of the state they were originating from, i.e. the luminescent state 3∆. The highest energy band (40500 cm-1, 245 nm) was then consequently a superposition of transitions from the 1Σ to the 1Π and 1Γ states, the main band (36000 cm-1, 278 nm) transitions from the 1Σ to the 3Π and 3Γ states and the lowest energy excitation band (31800 cm-1, 315 nm) transitions from the 1Σ to the 1Φ and 1∆ states. On the right hand side of the excitation spectrum one can see the corresponding visible light absorption spectrum, which we assigned to be transitions from the 1Σ to the 3Φ and 3 ∆ states (24600 cm-1, 405 nm). Based on this assignment we can estimate the energy of the singlet states 1Φ and 1∆. Subsequently, we are now in a position to speculate where to expect a TAS signal for the transition of the luminescent 3∆ to the respective singlet states, which is expected at around 11700 cm-1 or 850 nm, currently outside of the TAS white light continuum spectral range. However, these transitions


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might be of low intensity as triplet-singlet transitions are considered “forbidden”. Wavelength [nm] 200 1.2

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of chloride ions, while for the uranyl(VI) aquo ion the population remained constant for the time monitored in the TAS experiments. While luminescence measurements of the 1:1 uranyl(VI) chloride complex are hampered by the strong quenching effect and therefore,

Excitationa νTAS T1+νLum.

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Figure 5. Comparison of excitation data from Drobot with our TAS data relative to the 0-0 transition from the uranyl(VI) aquo ion

Chloride quenching Halide ions are known quenchers of the uranyl(VI) luminescence and although chloride24 and water25 quenching is long known across the literature, the exact process, time scales and mechanisms are still under discussion.26,27 Typical rate constants kq for the uranyl(VI) luminescence quenching by chloride ions have been reported with kq = 1.9 ns-1 M-1 which corresponds to a diffusion controlled reaction rate constant.9 In recent literature an electron transfer from an halide ion to the excited uranyl(VI) was assumed subsequently leading to the reduction of uranium(VI) to uranium(V) to be the source of the observed luminescence quenching.11 In order to further elucidate processes possibly leading to the strong luminescence quenching of the uranyl(VI) ion in the presence of halide ions, TAS measurements were carried out with samples containing a 1 M chloride ion concentration (at pH 0). Even though chloride is known to be a weak complexation agent, it is known from literature28 that in 1 M chloride solutions the 1:1 uranyl chloride complex [UO2Cl]+ is formed with more than 30 % of uranyl complexed. Thus, in the sample solution a mixture of the uranyl(VI) aquo ion and 1:1 uranyl(VI) chloride complexes was present. This was also supported by a slight red shift/broadening observed in the absorption spectrum indicating the presence of more than one uranyl(VI) species in the solution (see Figure 6). The time resolved transient spectra of the chloride solution were distinctly different from the sample containing only the uranyl(VI) aquo ion. In Figure 4 (C) and (D) the single component TAS spectra as well as the respective time traces obtained from the PARAFAC analysis of the experimental data of the chloride solution are shown. The strong quenching effect of the chloride ions on the uranyl(VI) luminescence is complemented by the increased decay rate found for the population of the T1 state. A monoexponential decay of the T1 state population on the ps time scale was observed in the presence

26000

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Wavenumber [cm-1] Figure 6. Comparison of the uranyl(VI) spectra found in 1 M HClO4 and 1 M HCl solution, respectively. Spectra of the HClO4 solution are purely from the free uranyl(VI) aquo ion, while spectra in HCl solution are originating from the free uranyl(VI) aquo • ion and the 1:1 uranyl(VI) chloride complex as well as the Cl2 radical.

a rate constant for the deactivation is difficult to obtain, in TAS measurements the kinetic analysis could be carried out with good precision (see Figure 4 (C) and (D)). From the calculated species distribution, also two different T1-related transient absorption time traces were expected. However, only one time trace, nicely following a first order decay law, was found (see Figure 4 (D)). Presence of only one monoexponential T1-related decay in the TAS for two different uranyl(VI) species indicated a kinetic coupling between both species (and their respective states) yielding the overall observed luminescence quenching. This is further supported by the spectral analysis of the TAS signals. Next to the T1 and T2 state, which are also found the uranyl(VI) aquo ion in perchlorate solution, a new species emerges after 10 ps delay at around 350 nm/28500 cm-1 (see Figure 4 (C)). Its formation was more apparent with increasing pump laser power. The formation time of this new species coincides with the decay of the T1 state and implies a direct connection between both (indicating that the quenching path leads to the formation of this new species). The transient spectra of the uranyl(VI) in chloride solution for the T1 state are similar to that of the uranyl(VI) in perchlorate solution of the T1 state, but shows a slight broadening similar to the linear absorption spectrum (Figure 6). This is most likely due to the spectral overlap of the uranyl(VI) aquo ion and the 1:1 uranyl(VI) chloride complex. The T2 absorption is more red shifted, which indicates a change in the relative position of the T2 and T4-state (a lowering in the T4 or rise of the T2 state or both). Again there seems to be an overlap of the spectra of aquo and chloride species. Due to the monoexponential decay and very similar transient spectra of the uranyl(VI) aquo ion and the 1:1 uranyl(VI) chloride complex, by using PARAFAC it was only possible to distinguish three


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species: i) The T1 state of the uranyl(VI) aquo and chloride species as one component (spectral and kinetic data), ii) the T2 state of the uranyl(VI) aquo and chloride species as another component (spectral and kinetic data), and iii) a third distinctly

Figure 7. Proposed uranyl(VI) quenching mechanism by chloride considering the photophysical processes of the two considered uranyl(VI) species

different kinetics and spectrum (see Figure 4). Burrows assumed from laser flash spectrometry the formation of a dihalide radical as the product of a reduction process.9 The third species resembled spectrally the absorption spectrum of a Cl2•radical.29 The formation of this radical requires therefore a diffusion step from solution to the excited uranyl chloride complex. The uranyl(VI) aquo ion and the 1:1 uranyl chloride complex are coupled in a dynamic equilibrium which consequently includes a diffusion step of chloride association (k+Cl,Diff) and dissociation (k-Cl,Diff). The redox step was assumed to happen in the chloride complex with consecutive diffusion controlled chloride-adding step. As no uranium(V) species nor a dichloride uranium complex [UO2Cl2]0 were directly detected, the determination if as a first step a second chloride ion associates and then the electron is transferred or vice versa or even simultaneous was not possible. Therefore, this step was handled as a dark intermediate species (DS) in the kinetic analysis (see Figure 7). Table 2. Comparison of experimentally obtained rate constants (in ns-1) of the uranyl aquo ion and the 1:1 uranyl chloride complex, error less than 10 %

Rate constants

Without chloride

With chloride

UO22+

UO22+

[UO2Cl]+

2150

1100

4100

kIC ( Φ ∆)

354

404

4500

kDiff

–––

1.1

0.9

k2

–––

–––

36

k3

–––

–––

0.54

kP+ kISC 3

3

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From this model we determined rate constants (Table 2) involved in the different deactivation steps shown in Figure 7 from numeric fitting of the corresponding ordinary differential equations. The diffusion-controlled steps should be dependent on the temperature, viscosity and chloride concentration. The rate constant for the chloride association and dissociation step of 1.1 ns-1 and 0.9 ns-1, the slowest rate in the whole model, was assumed to be the quench step of the free uranyl(VI) molecule, which fitted well with quench rate constants of 1.9 ns-1 9 and 1.7 ns-1 10 for 1 M chloride solution reported in literature. Moreover, this model also implied a thermal-driven equilibrium between the 3∆ and 3Φ state of the uranyl(VI) chloride complex, which means that both are energetically close. The observation of thermal equilibration implied that the 3Φ state is lower than the 3∆ state in the chloride complex, due the rate of 3 Φ formation being bigger than the reverse transition.

Theoretical Investigations In an attempt to theoretically explore the mechanism of dynamic quenching, we performed DFT and time–dependent DFT (TD–DFT) calculations and obtained the structures of the singlet ground state (S0) and the lowest–lying triplet states (T1) of the outer–sphere complex between uranyl(VI) complex and free Cl- ion. Free Cl- ion and uranyl(VI) complex were merely H–bonded in the ground state whereas additional interaction with “yl”–oxygen occurred in the T1 state of the [(UO2(H2O)52+)(Cl–)] adduct (Figure 8 (A)). Asymmetrically elongated U–Oax distances were found in the T1 of [(UO2(H2O)52+)(Cl–)] (1.805 Å and 1.863 Å compared to 1.753 Å for the S0). According to the TD–DFT calculations, excitation of [(UO2(H2O)52+)(Cl–)] occurred through the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transition of the S0 molecular orbitals (MOs). The HOMO and LUMO comprised mainly of Cl 2p atomic orbitals (AOs) and U 5f AOs, respectively. The corresponding vertical de–excitation energy from T1 to S0 was remarkably reduced to 0.724 eV in the adduct [(UO2(H2O)52+)(Cl–)] compared to 2.421 eV in the case of [UO2(H2O)5]2+. Photoemission upon electronic relaxation was thereby excluded for the loosely associated [(UO2(H2O)52+)(Cl–)] complex due to the increase in radiationless deactivation processes. Consequently, the chloride–to– uranyl(VI) charge transfer (LMCT) was identified to be responsible for outer–sphere quenching (dynamic quenching). When similar calculations were performed for the adduct [(UO2Cl(H2O)4+)(Cl–)], during the structural optimization of T1 the coordinating Cl was pushed out to the second shell and interacted with unbound Cl– eventually forming Cl2 (Figure 8 (B)). The final structure has the stoichiometry [(UO2(H2O)4)(Cl2)]0. Closer look into Mulliken charge and spin density of the complex revealed that the two unpaired electrons of the T1 state were distributed to UO2 (1.01) and Cl2 (1.00) units in a roughly half–to–half fashion. The net charge of the Cl2 unit was –0.87. Accordingly, the complex could be regarded as an adduct of [UVO2(H2O)4]+ and Cl2–, the latter being a radical.


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The Journal of Physical Chemistry Figure 8. Structure and major bond distances of the lowest-lying triplet state (T1) of the outer–sphere complex between (A) [UO2(H2O)5]2+ and Cl–, and (B) [UO2Cl(H2O)4]+ and Cl–, as obtained by TD–DFT calculations.

Table 3. Major bond distances (in Å) in the singlet ground state (S0) and the lowest-lying triplet state (T1) of uranyl(VI) aquo and chloro-aquo complexes and emission energy (in eV) from T1 to S0 as obtained by DFT and TD-DFT calculations. Complex [UO2(H2O)5]

Ground state

Sym. 2+

U-Oax

D5h

U-Ow

Lowest triplet U-Cl

U-Oax

U-Ow

U-Cl

emiss. [eV]

1.750

2.462

–––

1.796

2.469

–––

2.421

[UO2Cl(H2O)4]+

C2v

1

A1

1.758

2.505

2.674

3

B2

1.798

2.560

4.488

0.759

[UO2Cl2(H2O)3]0

C2v

1

A1

1.763

2.524

2.731

3

B2

1.794

2.553

3.107

1.385

Since static quenching required inner–sphere uranyl(VI)– chloro complexes, comparable calculations were performed on this system (see Table 3). In the S0 of uranyl(VI) aquo complex, the DFT calculated U–Oax and U–Owater distances were 1.750 Å and 2.462 Å, respectively, which matched EXAFS bond distances of 1.764 Å and 2.403 Å30. The U-Oax bond was elongated about 0.05 Å in the T1 because of electron transfer from “yl”– oxygen to uranium. The calculated symmetric stretching vibrational frequencies corresponding to the same bond is 926.6 cm-1 for S0 and 824.5 cm-1 for T1. These numbers and the trend are in agreement with those obtained by Su et al.23 (911 cm-1 and 805 cm-1 for S0 and T1, respectively) although the bond expansion is somewhat underestimated in theoretical calculations when compared to the experimental result. The U–Oax and U–Owater distances were slightly elongated in S0 of aquo–chloro complexes compared to that of pure aquo complex (0.01 Å and 0.04 Å in [UO2Cl(H2O)4]+ compared to 0.01 Å and 0.05 Å in [UO2Cl2(H2O)3]0). Correspondingly calculated U–Cl distances were 2.67 Å and 2.73 Å, respectively, which excellently matched the EXAFS results obtained by Hennig et al..31 The U-Cl distances were significantly elongated to 4.488 Å ([UO2Cl(H2O)4]+) and 3.107 Å ([UO2Cl2(H2O)3]0) in T1, which implied that chloride ligands got virtually dissociated upon photoexcitation. The deexcitation energies of aquo-chloro complexes were significantly lower (0.759 eV for [UO2Cl(H2O)4]+, 1.385 eV for [UO2Cl2(H2O)3]0) compared to [UO2(H2O)5]2+ (2.421 eV). It is tempting to assume that photoemission may be consequently suppressed in [UO2Cl(H2O)4]+ and [UO2Cl2(H2O)3]0. In the case of cryogenic measurements of uranyl chlorides, where luminescence is visible at similar wavelengths to free uranyl, we assume that there is no complex reorganization and U-Cl bond elongation possible upon excitation due to the frozen state of the sample. This way the T1 state remains nearly unchanged and luminescence is still apparent with only minor changes in wavelength. In the present study, more pronounced and distinct elongation of the U– Cl distance in the T1 state was observed when compared to our previous investigation.11 The difference with the previous study was that here we applied more sophisticated theory to the systems, with the use of TD–DFT, larger basis sets and no symmetry constraint. One may still argue that TD-DFT is not

necessarily the best choice for studying excited states of uranyl(VI). Earlier works21,23 have shown that the excitation energy of uranyl(VI) obtained by TD-DFT is not as accurate as those obtained by wave-function based theory or by CASPT2 calculations. Although the structure and the emission energy obtained by TD-DFT appear to illustrate fairly well the mechanism of luminescence quenching, for accurate reproduction of the uranyl(VI) excited state chemistry, application of more sophisticated theory may be needed.

Conclusion Pump and probe experiments were carried out to investigate the electronic deactivation processes in the uranyl(VI) aquo ion and the 1:1 uranyl(VI) chloride complex. The experimental data were complemented with TD-DFT calculations. The TAS experiments allowed a unique view on the interconnection of the different electronic states in both uranyl(VI) complexes. Due to the outstanding time-resolution of the experiments it was possible to determine the respective rate constants for the deactivation processes involved. In the combination with PARAFAC analysis also the energy splitting and the vibronic structure of the higher electronic states of the uranyl(VI) aquo ion and the 1:1 chloro complex were deduced. A special emphasis in our study was given to the investigation of the observed luminescence quenching in uranyl(VI) chloride complexes, which is a major limitation in the speciation analysis of such. Sarakha et al.32 found in flash photolysis studies for the oxidation of chlorphenols by photoexcited uranyl(VI) ions that due to a photoinduced electron transfer U(V) is formed. But in the absence of oxygen the back electron transfer reaction is dominating and no net reaction was observed, while in the presence of oxygen a photoinduced degradation of three different chlorphenols was observable. The intermediate phenol radicals were observed on a ns-time scale (20 ns – 250 ns). For the uranyl(VI)-chloro complexes direct experimental data showing the difference in the deactivation were missing so far. The intermediate formation of a uranyl(V) has been proposed as the reason for the observed quenching. Intramolecular LMCT occurs within uranyl(VI)– aquo–chloro complexes upon photoexcitation. The photoexcited state can be described as a uranyl(V)–radical pair, i.e. (UO2+)…(Cl•). This idea was initially postulated by Burrows9 for uranyl(VI) bromide system. He also proposed a quenching



mechanism that involves further formation of halide dimer anion radical X2•– that oxidizes uranyl(V) back to uranyl(VI). The second mechanism matches with the TD–DFT calculations performed for the dynamic quenching mechanism and is in excellent agreement with our experimental data. In frozen samples however, the excited [UO2Cl(H2O)4]+ is fixed in its Franck-Condon state and it can deactivate via photon emission. Also diffusion is supressed and no [UO2Cl2(H2O)3]0 can be formed for a charge transfer deactivation mechanism. The knowledge on the involvement of photoredox reactions as a competing deactivation process is important for the speciation analysis, which is often based on luminescence (due to the outstanding sensitivity and selectivity). Therefore, other classes of ligands that may be prone to photooxidation in the presence of uranyl(VI) should be identified. Moreover, the observed luminescence quenching in uranyl(VI) carbonate complexes needs to be investigated. Here, the quenching mechanism will be different because of the different redox properties of this ligand. Future experiments will also involve transient absorption measurements in the NIR spectral region to investigate the proposed 3∆ 1∆ and 3∆ 3Φ transient absorption signals.

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AUTHOR INFORMATION

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Corresponding Author *Michael U. Kumke, University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany, [email protected].

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Author Contributions The manuscript was written through contributions of all authors.

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ACKNOWLEDGMENT TH and MUK are grateful to the Federal Ministry for Economic Affairs and Energy for the financial support (contract number 02E11415F).

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ABBREVIATIONS TAS, Transient Absorption Spectroscopy; PARAFAC, Parallel Factor Analysis; TRLFS, Time-resolved Laser-induced Fluorescence Spectroscopy; IC, Inner Conversion; ISC, Inter System Crossing; LMCT, Ligand to Metal Charge Transfer; TD-DFT, Time-dependent Density Functional Theory; NIR, Near Infrared.

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