J . Chem. SOC., Faraday Trans. I , 1985, 81, 49-60 Photophysics of the Excited Uranyl Ion in Aqueous Solutions Part 4.-Quenching by Metal Ions BY HUGH D. BURROWS,* AUGUSTO C. CARDOSO, SEBASTI~O J. FORMOSINHO* AND MARIA DA GRACA M. MIGUEL Chemistry Department, University of Coimbra, 3000 Coimbra, Portugal Received 5th March, 1984 The quenching of the excited state of the uranyl ion by metal ions in aqueous solutions has been studied under conditions where (UOi+)* decay is biexponential. The effect of metal ions always follows Stern-Volmer behaviour both for real quenching and for the second process, which is suggested to involve reversible crossing via solvent exchange between two energetically close excited states. With Tl+, Ag+, Fez+, Pb2+, Mn2+, Ce3+ and Ni2+ quenching is suggested to occur by electron transfer.Theoretical calculations using a quantum-mechanical tunnelling model support a mechanism involving an inner-sphere exciplex. With Eu3+ preferential quenching of the emitting U* state is suggested to involve enhanced hydrogen-atom abstraction by (UOi+)* following complexing and overlap of europium and uranyl orbitals. Initial fluorescence enhancement by metal ions in the uranyl system is observed in both dynamic and static studies. In cases where there is no complexing in the ground state with UOi+, this initial enhancement is interpreted in terms of an effect of the metal ion on the initial relaxation of the emitting (UOi+)* state. The time-resolved fluorescence spectra of UOi+, together with other data on the photophysics of (UOg+)*, suggest that the UOi' ground state and the lowest excited state, X*, have a slightly bent geometry (ca.176"), whereas the second excited state, U*, is linear. The quenching of excited uranyl ion by metal ions in aqueous solutions has been studied exten~ively.l-~ The mechanism of quenching with several ions involves an electron-transfer process, but energy transfer has also been invoked with E u ~ + , ~ where electron transfer is not energetically feasible. Marcus theory does not seem to be applicable to these electron-transfer proce~ses.~ It is suggested that this rules out an outer-sphere mechanism, and Marcantonatos has revealed that the quenching processes depend on the chemical ionization energy and on the spatial extension of the orbital containing the electron to be tran~ferred.~ In all these experiments, either in the steady-state or by dynamic studies, uranyl decay was considered to be a single exponential. However, (UOi+)* is known to undergo biexponential decay under certain experimental conditions and this has been attributed to a reversible crossing between two energetically close excited The states have a different electronic nature, n3,& and n3,4E, and the reversible crossing between the two states has been attributed to a solvent-exchange process.Since solvent can play a significant role in the mechanisms of quenching of (UOi+)* by metal ions, we have investigated such quenching processes under conditions where the decay of the excited uranyl ion is biexponential. When analysed in terms of a reversible-crossing mechanism, the decays permit the study of the effect of the metal ion on quenching and on solvent-exchange processes.Such studies have also helped elucidate the mechanism of enhancement of uranyl fluorescence (particularly in aged solutions) caused by ions such as Ce3+, Zn2+ and 3 49 FAR 150 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION EXPERIMENTAL Excited uranyl decays were studied using a nanosecond flash-photolysis apparatus with a pulsed N, laser (A,,, = 337 nm) at constant laser intensity. Fluorescence spectra were run on a Spex Fluorolog model 111 instrument and absorption spectra were obtained using a Shimadzu UV-240 spectrophotometer. Solutions were prepared with triply distilled water from uranyl nitrate (analytical grade).All metal ions were taken as their nitrate salts and were of analytical grade. RESULTS FLUORESCENCE QUENCHING Fluorescence decays were obtained for aqueous solutions of uranyl nitrate (0.02 mol dm-3, pH 2.8, T = 20 "C) in the absence and in the presence of varying concentrations of other metal nitrates under conditions where the decays were biexponential. Both components were observed to decay more rapidly with an increase in metal-ion concentration. The decays are analysed according to the following ki scheme : U * r X * The effect of metal ions always follows Stern-Volmer plots, k,+k2[Q], for the rate constants of irreversible decays (true quenching) and for the rates of reversible crossing. Nitrate ion has also some effect on the rates' but its effect is much smaller than that of the metal ions; NO; has the following quenching rate constants (from addition of NaNO,): kro; = 8.0 x lo4 dm3 mol-1 s-l; k,Noi = 4.5 x lo4 dm3 mol-l s-l.The rate k, decreases (1.6 x lo5 dm3 mol-1 s-l) up to [NO;] = 0.3 mol drn-,. Table 1 presents the rate constants for 'quenching' by T1+, Ag+, Fe2+, Pb2+, Mn2+, Ce3+, Ni2+ and Eu3+ together with those for autoquenching by UO;+.' The rate constants were estimated in terms of metal-ion concentrations and not in terms of activities because we have used low ionic strengths. Marcantonatos did not find a great difference between the two kinds of rate^,^ although for the less efficient quenchers the increase in ionic strength (p) may lead to a small increase ( x 2) in the estimated quenching rate constants.To compare the present results with other published data it is useful to estimate the average quenching rate constants. The rates are estimated from Stern-Volmer plots of the biexponential decays with rate constants k, and k, and amplitudes C, and Cz.5 The average quenching rate constant is obtained from the Stern-Volmer plots of the average decay constants kav = (C, k, + C, k,)/(C, + C2). Table 1 also presents these values, which are in good agreement with other published data,1-4 with the exception of Ce3+, which has a rate constant ca. 20 times higher than that obtained by steady-state measurements.2 This discrepancy can be attributed to an enhancement in fluorescence which will be discussed later. Acidity has been found to have virtually no effect on the autoquenching rates of uranyl within a pH range 2.0-4.0.The same seems also to be true for Ag+, which does not undergo hydrolysis within this pH r e g i ~ n . ~ Table 2 confirms also the small increase in the quenching rates with an increase in ionic strength at different pH values. FLUORESCENCE ENHANCEMENT Enhancements in the fluorescence intensities have previously been reported in steady-state measurements with aged solutions of uranyl ion and Ce3+, Zn2+ andH. D. BURROWS, A. c. CARDOSO, s. J . FORMOSINHO AND M. DA G. M. MIGUEL 51 Table 1. Stern-Volmer rate constants" for the quenching of the excited uranyl ion by metal ions at 20 "C and pH 3 ion k"4" T1+ Fez+ Pb2+ Mn2+ Ce3+ Ni2+ Eu3+ Ag+ uo;+ 1600 1700 390 24 2.5 1.6 1.1 1.3 10 1900 1400 3 70 22 2.0 1.8 1.5 4.0 ca.0.19 850 900 130 10 8.5 5.0 0.8 2.6 19 I100 700 150 15 6.0 4.5 0.6 7.3 ca. 0.4 3500 1600 590 14 17 6.0 0.76 35 " Determined as described in ref. (5) with an estimated error of &20% ; units lop6 dm3 mol-1 s-1. Table 2. Rate constants for quenching of (UO;+)* by Ag+ as a function of pH and ionic strength 1 O9 dm3 mol-l s-' P /mol dmP3 PH 2 PH 3 PH 4 - - 0.016 1.1" 1 .oc 0.06 1.6" 1.5b 1 .6b 1 .@ 0.1 2.1" 1.76 - 1 .7c 0.2 - 2.4c 0.355 2.2b - - - - - " M. D. Marcantonatos and M. Deschaux, Chern. Phys. Lett., 1980, 76, 359. Our results by dynamic studies. Our results by steady-state measurements. CO~+.~-* With Ag+ we have been able to observe an enhancement under steady-state conditions, but only at pH 4 and p 0.1 mol dmP3. At a lower pH only the normal quenching effect was observed.Attempts were made to detect complex formation at pH 3 by conductivity measurements. However, within experimental error (3 % ) there is no difference in conductivity between the sum of conductivities of the separated ions and that of the mixed solution, suggesting that the degree of complexing, if any, is not very big. Marcantonatos has also found negative deviations in the Stern-Volmer plots for the quenching of (UOi+)* by relatively high concen- trations of Ag+ lo and Ce3+4 under conditions where there are some changes in the absorption spectra of UOE+. We have also observed under dynamic conditions an increase (up to ca. 30%) in the initial fluorescence intensity of (UOi+)* with the addition of all the metal ions (T1+, Ag+, Fe2+, Mn2+, Ce3+ and Eu3+ were investigated).Since we are monitoring the fluorescence intensity at a particular wavelength (A = 485 nm), the apparent enhance- ment could be caused by a change in the fluorescence spectra accompanying the addition of metal ions. In order to elucidate this problem time-resolved emission spectra of (UOE+)* were recorded. As fig. 1 shows, the fluorescence spectrum of (UOi+)* does not change significantly in overall area, but suffers a red shift from 3-252 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION 460 4 50 500 A/nm Fig. 1. Time-resolved spectra of UOi+ fluorescence: 0, 0 and 0, 300 ns. 470 nm to 480 nm with time. This shift is much faster than the overall excited-state decay and occurs with a lifetime of 200 f 30 ns for [UOi+] = 0.02 mol dm-3.In the presence of metal ions the emission spectra seem to be relaxed within 100 ns (the time resolution of our detection system). DISCUSSION FLUORESCENCE ENHANCEMENT Both static and dynamic studies of uranyl fluorescence reveal an enhancement in initial luminescence intensity in the presence of metal ions. When there are changes in the absorption spectrum of UO;+ on addition of the metal ion, this increase can be attributed to an increase in the extinction coefficients. Such an interaction between UO!+ and the metal ion is favoured by hydrolysis of the uranyl ion, which decreases the latter’s charge, and by an increase in ionic strength, which decreases the repulsion between the ions. This is particularly evident with Ag+, which causes an enhancement in the uranyl fluorescence under steady-state conditions only at pH 4 and high ionic strength ( p 3 0.1 mol dm-3).Under these conditions a new absorption band is observed at 435 nm. Nitrates of Ce3+ and Co2+ are known also to form complexes with uranyl ion under certain conditions,ll and with Fe2+ Marcantonatos4 has shown that the absorption spectra of Fe2+ and UO;+ in aqueous solutions are non-additive. The enhancement of fluorescence under certain experimental conditions can be caused by an increase in light absorption and by an increase in the radiative rate constants of the hydroxo-oxo complexes between UOi+ and the metal ion which supersedes the current quenching effect. The initial enhancement of the fluorescence intensity observed in dynamic studies is interpreted in terms of effect of the metal ions on the shift in the emission spectrum of (UOg+)*.With (UO,2+)* the shift in the spectrum is relatively slow (ca. 200 ns) and the fluorescence intensity at 485 nm increases slightly with time up to ca. 150 ns. However, in the presence of metal ions such an increase is not observed, revealing that in the presence of metal ions the relaxation of the fluorescence spectrum has a rate constant > lOlo dm3 mol-1 s-l.H. D. BURROWS, A. C. CARDOSO, S. J. FORMOSINHO AND M. DA G. M. MIGUEL 53 GEOMETRY OF THE URANYL ION AND POSSIBLE STRUCTURES OF GROUND- AND EXCITED- STATE AQUO-URANYL SPECIES Comparison of the UOi+ absorption spectral data of Bell and Biggersl, and the emission spectra from the U* and X* states5 reveals that whereas the X* state shows a shift or stabilization of 420 cm-l after being populated through light absorption, the U* state exhibits a perceptibly larger shift of 870 cm-l.The difference in these stabilizations is equal to the red shift (450 cm-l) observed in the time-resolved emission spectra in the present study (fig. 1). An attractive interpretation of these observations is that the UOi+ ground state and X* have the same equilibrium geometry, whilst the geometry of the U* state is slightly different. Whilst the geometry of O=U=02+ in the ground state has generally been considered to be linear,13 there has been some controversy as to why this should be so when the isoelectronic species Tho, possesses a bent geometry (8 = 122&20).14 Recent theoretical calculation^^^ support the view that the bare, gas-phase uranyl cation is linear whereas Tho, is strongly bent, since the Sforbitals, which are dominant in UOi+, prefer linear geometries while the 6d orbitals, dominant in Tho,, prefer bent geometries.However, calculations on naked UO;+ species may not be completely appropriate for this species in solution surrounded by ligands in the equatorial position. The uranyl ion seems to be able to accommodate between 3 and 6 ligands in the equatorial plane. Recent magnetic circular dichroism studies on aqueous uranyl perchlorate so1utions15 indicate that the dominant species is [U0,(OH,),]2+. This is supported by X-ray diffraction and n.m.r. studies.lG Having five equatorial ligands might be expected to lead to a slight distortion of the uranyl species from linearity.This has been observed experimentally in both neutron17 and X-ray18 diffraction studies of single crystals of uranyl compounds. We wish to suggest that ground-state UOi+ and X* are both slightly bent, whereas U* is linear under our experimental conditions. The degree of distortion may be expected to be relatively small. For a harmonic U=O oscillator the 450 cm-l energy shift is calculated to correspond to a deviation from linearity of ca. 4"; the displacement of the oscillator, x, is taken as x = (1/2) no'/ 180 where I is the length of the U-0 bond and 8' the angular change from linearity. The suggested 0-U-0 bond angle (176") is intermediate between those observed in crystal structures of uranyl perchlorate heptahydrate (161 ')la and [U0,(OH),(OC(NH,),)4](N0,), (177.7 +0.4").li Support for such changes in geometry come from considerations of the oscillator strengths of the transitions. The lowest absorption bands of UOi+ are considered to be Laporte-forbidden.lS Since there is no change in the geometry of a molecular species during an electronic transition, the degree to which the transition is forbidden will be smaller for a transition where the initial state is slightly bent and larger for a linear species.The calculated oscillator strengths are f(U*)/f(X*) = 3.7 (absorption) andf(U*)/f(X*) = 2.3 (emi~sion),~ in agreement with the suggestion that the ground state and X* are slightly bent, whereas U* is linear. Since the relaxation of the U* state illustrated in fig.1 occurs on a relatively long timescale it can be suggested to result from some chemical change. Since the solvent-exchange process in (UO?j+),*, has more associative character in X* and more dissociative character in U*,i it is possible that this relaxation involves an increase in the primary hydration sphere from 5 to 6, a process which may be54 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION Table 3. Metal-ion effect on the non-radiative processes of the electronic states U*(ni &) and X* ( x i dt) of the uranyl ion ion k,&/kF Ag+ 1.1 1.28 Fe2+ 1.05 0.90 Mn2+ 1.26 1.04 Ce3+ 0.9 1.1 Eu3+ 3.4 6.5 UOi+ ca. 7.0 ca. 6.0 facilitated by the presence of metal ions. Such changes in hydration number have previously been postulated in excited states of Eu~+.~O ELECTRON-TRANSFER MECHANISM To discuss the effect of metal ions on reversible crossing and irreversible rates of decay in (UO,Z+>* it is convenient to compare the relative rate constants for each of the non-radiative processes in different states.Table 3 allows a distinction to be made between two kinds of mechanism. The first is relative to the metal ions where an electron transfer between Mn+ and (UO,Z+)* is energetically possible (TI+, Ag+, Fe2+, Pb2+, Mn2+, Ce3+ and Ni2+) and where the 'quenching' effect is the same in both U* and X*: for these ions ku&/kz and kF/kF are close to 1.0. In the second mechanism, found in Eu3+ and UOg+, the metal ion has a much stronger effect on the non-radiative rates of U* than of X*. For the first group of ions the quenching process is currently interpreted in terms of an electron-transfer pro~ess.~-~ Direct evidence for electron transfer has been obtained in the cases of Mn2+ and Ce3+.2v21 As table 3 shows, the effect of the metal ions on the irreversible rates of decay of (UOi+)* is of the same order of magnitude as the effect on the rates of solvent exchange. This fact suggests that both effects should have a common nature.A possible mechanism that will interpret such a situation is an inner-sphere electron- transfer mechanism where (UOg+)* and the metal ion Mn+ are linked by a water molecule. Electron transfer to the (UOg+)* ion leads to quenching. The transfer of a water molecule to the metal ion leaves (UOg+)* with one less water molecule in the first coordination shell, and the subsequent entrance of another H 2 0 molecule into the hydration shell of (UO;+)* can change the electronic nature of the excited uranyl ion.A kinetic scheme which represefits such a mechanism for the process from the state U* is Uzq + Mtg' kd k- d 7 (U**H,O*M"+),q (U* - H 2 0 * Mn+)aq represents an inner-sphere exciplex that leads to quenching via an electron-transfer process; the metal-ion-induced U* -+ X* transition is a non- quenching process. We have not considered the electronic-state transition within the inner-sphere exciplex : (U* - H,O - Mn+),, f (X* - H,O Mn+)aq. In fact such a transition occurs via a solvent-exchange mechanism and consequently requires the dissociation of the exciplex in order to be induced by the metal ion.H.D . BURROWS, A. C. CARDOSO, S. J. FORMOSINHO AND M. DA G. M. MIGUEL 55 1 I I I 4.0 4-5 5.0 5.5 6.0 (IleV)” Fig. 2. Correlation between the logarithm of the quenching rate constants k,& (0) and k,& (0) and the square root of the ionization potentials of various metal ions. (1) Tl’, (2) Ag+, ( 3 ) Fez+, (4) Pb2+, (5) Mn2+, (6) Ce3+ and (7) Ni2+. According to the proposed mechanism the rate constant for the electron-transfer For the reversible crossing induced by the metal ion Equivalent expressions can be established for the state X*. Marcantmato9 has considered the formation of an intimate pair of solvated ions, P*, prior to the formation of an exciplex. A poor correlation was found between the quenching rates and the equilibrium constant of P*, expressed in terms of the molar refractivity of the metal ions.However, other factors, such as coulombic repulsion, can be relevant, and this is invoked to explain the poor character of the correlations. Owing to the impossibility of assessing with some accuracy the effect of the nature of the ions on intimate pair formation, we will consider its equilibrium constant invariant to the metal ions and equal to 1 dm3 mol-l. The quenching rates k,& and k,& decrease with an increase in the ionization potential, I, of the metal ions in the vapour phase, as reported in ref. (2). Fig. 2 presents correlations between logk: or logk? and Pie. A linear plot is found for metal ions with high ionization energies ( I > 30 eV). For metal ions with I < 30 eV the rate of quenching is diffusion controlled.A similar correlation (fig. 3) is also found for the56 4 a cv -Y en - 7 6 5 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION 1 A 2 h A 4.0 4.5 5.0 5.5 6.0 Fig. 3. Correlation between the logarithm of the rate constants k? (A) and k? (A) and P. (1) T1+, (2) Ag+, (3) Fez+, (4) Pb2+, (5) Mn2+, (6) Ce3+ and (7) Ni2+. (I/eV)x rates of induced U* + X* transitions, kp and k?, but with a different slope. Such correlations can be interpreted if both processes are due to an electron-transfer process which occurs by a tunnelling mechanism. For the tunnelling of an electron of mass m through a rectangular barrier of height I and width Y the transmission factor, T, is22, 23 T = exp [ -47~(2rnI)l/~ r/h] log kQ = log Y - 5 . 2 ~ ( 2 m I ) l / ~ r / h (3) and logkQ is a linear function of PI2.For a three-dimensional cubic barrier (4) where v is a frequency factor. Estimation of values of Y from the experimental slopes adds support to the proposed mechanism for the effect of metal ions on the excited uranyl ion. The value found for the induced solvent-exchange process is 4 A (fig. 3), which is close to the diameter of a water molecule (3 A).24 In contrast, a value of r = 5 A can be estimated for the electron-transfer process from the slope of fig. 2. This is close to the diameter of the water molecule plus the radius of the uranium 5forbital in UOi+ (0.57 A).25 These values suggest that when charge is transferred to the uranium atom, reduction of UOi+ occurs and there is quenching, whilst charge transfer to the bridging water molecule between Mn+ at (UOi+)* leads to a breaking of the (UOg+)* .-.OH, bond such that there is no net oxidation and reduction. The (UOi+)* species, thus deprived of a water molecule in its hydration shell, can capture another water molecule and be transformed into the other state, X* or U*. The similar behaviour of the U* and X* states follows from them having empty nu orbitals of similar energy. 9 l9H. D . BURROWS, A. C. CARDOSO, S. J. FORMOSINHO AND M. DA G. M. MIGUEL 57 The use of gas-phase ionization potentials in correlations of (UOi+)* quenching has been questioned on the grounds that these values are higher than the nu energy in the uranyl ion.26 Covalent interactions or polarization effects may have a significant effect on bringing the energy of the electron to be transferred above the nu energy of (UOi+)*, as can be seen by the facts that hard? cations with relatively low gas- phase ionization energies24 such as Ba2+ ( I = 35.5 eV) and Cs+ ( I = 25.1 eV, k , < 3 x lo5 dm3 mol-1 s-l) have very much lower quenching rates than soft cations such as Ni2+ ( I = 35.16 V) or Cu2+ (36.83 V).However, attempts to correlate quenching rates with either the ionization energies of aquo-metal ions26 or with redox potentials of the metal ions3 have been unsuccessful. Similarly, although Marcantonato~~ has found a correlation between quenching rates of some metal ions and E;, the energy necessary to overcome electrostatic repulsion in a (U02,+)*-Mn+ exciplex, we did not find such a correlation when other ions, such as Fe2+ and Ce3+, are included.Where overall electron-transfer occurs, the use of vapour-phase I values in estimating barrier widths for electron tunnelling from metal ions to (UOg+)* can be justified as the process is very fast, and consequently does not allow reorientation of the water molecules in the hydration shell.8 A correlation between quenching rates and either redox potentials or ionization energies of aquo-ions would be expected if the electron transfer was a non-radiative process involving solvent-ion vibrational or multiphonon 28 However, in the reduction of (UO;+)* to UO; there is a significant increase in the U=O bond length, estimated to be ca. 0.3 A.29330 Such a bond-length change requires participation of the U=O vibration in any non-radiative transitions, and the Franck-Condon factors, and consequently rate constants,31 for such a process are low owing to the large reduced mass of the oscillator and to the large energy barrier.Further insight into the quenching process comes from consideration of the cases of Ag+ and T1+, where the rates are diffusion controlled. Combining eqn (1) and (2) gives and since in these cases kg,+ k& % k-, then Further, for diffusion-controlled processes between ions 8RT o , / R T 300011 exp (w,/RT) - 1 k , = ~ (7) where 11 is the medium viscosity and w, the work for the formation of the collision complex.32 This is given as Z1Z2 Ne2 Er cur = where E is the dielectric constant for water and Y is the charge separation. From the data for Ag+ and T1+ for the states U* at X*, at pH 2.8, we can estimate k, = 2.6 x lo9 dm3 mol-1 s-l, and using this and taking r = 3 x lo-* cm for the electron-transfer process gives 2, 2, = 0.7.Since there is negligible hydrolysis of Ag+ or T1+ under these condition^,^^^ 34 this value indicates that (UOi+) must be significantly hydrolysed at this pH, in agreement with earlier suggestions.6 Activation energies were determined for quenching of (UOi+)* by Ag+ and Mn2+ (table 4). Although the solvent modes do not seem to play an explicit role in the ?- The terms hard and soft mean non-polarizable and polarizable, respectively.3758 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION Table 4. Activation energies for the quenching of (UOi+)* by metal ions (EJkJ mol-l) AgZ 25 27.5 a a Mn+ 57 42 44 59 Eu3+ 29.5 11.5 35.5 11.5 uo;+ 24 24.5 66.5 50 a Non-linear Arrhenius plot.electron-transfer process, a decrease in the H,O-metal and H,O-uranyl ion distances with an increase in energy would decrease the barrier width r for electron transfer and consequently increase the rate of electron tunnelling. An activation energy would be observed and would be highest for the highest barrier height. Consequently it is no surprise that quenching by Mn2+ will have a higher activation energy than that by Ag+. Some support for the tunnelling mechanism comes from the fact that with Ag+ strongly non-linear Arrhenius plots are observed (fig. 4) for ki& and k9. QUENCHING BY Eu3+ Eu3+ quenches the U* (n;&) state of excited uranyl ions more strongly than the X* (n",;) state.Moriyasu et al., have stated that quenching of (UO;+)* by Eu3+ occurs via energy transfer, because luminescence was observed for Eu3+. Although we have observed such luminescence, more than 95% under the present conditions can be accounted for by direct excitation of the Eu3+ ion. Consequently an energy-transfer mechanism does not account for the quenching process in water, although it may be important in more viscous solvents or in glasses. Furthermore, electron transfer is not energetically possible and a heavy-atom quenching effect is expected to be similar for both U* and X* states. Europium ion has a quenching effect similar to the autoquenching of the uranyl This would suggest that quenching involves the overlap of atomic orbitals of Eu3+ and molecular orbitals of UOi+ and consequently that H,O hydration molecules are also labile in Eu3+, since Eu3+ has a very high coordination number and there is a fast equilibrium between Eu(H,O);+ and EU(H,O);+.~O The labile character of the H,O molecules is also evident in changes in the inner coordination sphere that are observed for some lanthanide ions, including Eu3+, upon direct electronic e~citation.,~ In the U* state with an n3,& configuration the half-filledf,, orbital has all the lobes pointing in the equatorial direction and can overlap one of the half-filled or empty 5f,, orbitals of Eu3+, which have energies comparable to those of UO;+.However, in the ~ $ 6 ; configuration of the state X*, the half-filled &(fk ,) orbital makes an angle of 45" with the equatorial plane and the overlap is much weaker.So, as with UOi+, Eu3+ affects the state U* more strongly. In the exciplex [(UOi+)* * Eu3+Jaq the rates of reversible crossing, for the state U*, have enthalpies and entropies of activation of AH+ = 35.5 kJ mol-1 and AS? x 4 J K-l mol-l. When such values are compared to those in (UOi+)* (AH? = 67 kJ mol-l, AS7 = 75 J K-l mol-l) it seems that Eu3+ increases ki by increasing the association character of solvent e~change.~ This effect can be attributed to an increase in the radius and charge of the exciplex. The irreversible decays in (UO;+)* have been attributed to hydrogen abstraction from coordinated H,O molec~les.~~ 7 9 26 On increasing the association character of theH. D. BURROWS, A.C. CARDOSO, S. J. FORMOSINHO AND M. DA G. M. MIGUEL 59 24 23 4 c - 22 21 201 - - - - I 1 1 I I 1 24 23 22 a c e 21 2c 15 3.0 3.2 3.4 3.6 1 0 3 ~ 1 ~ A, ki and A, k,. Fig. 4. Arrhenius plots of the quenching rate constants for Ag+: 0 , k,; 0, k,; coordinated water molecules on the solvation shell of the exciplex, Eu3+ increases the number of H,O molecules available for hydrogen abstraction by (UOt+)* and consequently the frequency factor. The activation energy is similar to that in the excited uranyl ion at pH 3. Since for ((UOi+)* * Eu3+) the frequency factor is very high ( A z 1 x 1013 s-l) it seems that hydrogen abstraction from H,O is controlled by the frequencies of metal-water bonds and OH modes, rather than diffusion of water molecules to the U=O bonds. Other rare-earth ions such as Gd3+ and Yb3+ which have empty orbitals of higher energy than thef orbitals of UOi+, because they have no low-lying ( < 22000 cm-l) excited electronic are much less efficient (2-20 times) quenchers than E u ~ + . ~60 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTION The quenching ability of such ions is probably associated with the orbital overlap between the lowest unoccupied orbitals of (UOt+)* and the highest occupied orbitals of the metal ions.We are grateful to INIC and GTZ for financial support. R. Matsushima, H. Fujimori and S. Sakuraba, J. Chem. Soc., Faraday Trans. I , 1974, 70, 1702. H. D. Burrows, S. J. Formosinho, M. G. Miguel and F. Pinto Coelho, J. Chem. Soc., Faraday Trans. I , 1976, 72, 163. M. Moriyasu, Y. Yokoyama and S.Ikeda, J. Znorg. Nucl. Chem., 1977, 39, 2205. M. D. Marcantonatos, J. Chem. Soc., Faraday Trans. 1, 1979,75, 2252. S. J. Formosinho, M. G. Miguel and H. D. Burrows, J. Chem. SOC., Faraday Trans. I, 1984,80, 1717. M. G. 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