J. Chem. SOC., Faraday Trans. I, 1984, 80, 1735-1744 Photophysics of the Excited Uranyl Ion in Aqueous Solutions Part 2.-Acidity Effects between pH 0.5 and 4.0 BY MARIA DA GRACA M. MIGUEL, SEBASTIAO J. FORMOSINHO* AND ALJGUSTO C. CARDOSO Chemistry Department, University of Coimbra, 3000 Coimbra, Portugal AND HUGH D. BURROWST Chemistry Department, University of Ife, Ile-Ife, Nigeria Receiued 25th July, 1983 Fluorescence quantum yields and decays of excited uranyl ion in aqueous solutions have been studied over the pH range from 0.5 to 4.0. The fluorescence yield decreases between pH 0.5 and 2, is constant up to pH 2.5 and then varies again with a maximum at pH 3.5. The pH dependence of the emission decay is complex but when analysed in terms of a reversible-crossing mechanism reveals that the rate of reversible crossing has a maximum at pH 3 whereas the rate of irreversible decay has a maximum at pH 2 and a minimum at pH 3. All these effects have been interpreted in terms of several aquo, hydroxo-aquo and polynuclear uranyl cations (UO?j+)*, [UO,(OH)+]*, [UO,(OH),]* and [(UO,), (OH);]*, with a pH-dependent distribution curve and with different rates of fluorescence decay.Good agreement between the decay and the stationary-fluorescence data is found within a reversible-crossing kinetic scheme between two uranyl states that are energetically very close, a higher U* state and a lower X* state. Increasing uranyl concentration increases only the rate of decay of state U*, except at pH 1 where some effect is detected in the X* decay.The acidity effect on the autoquenching rate constant is not very pronounced. The laser intensity does not affect the rates of irreversible decay of both states, but affects the rate for the reversible transition between U* and X*. This effect depends on the pH. The photophysics of the uranyl ion in aqueous solutions is dependent on a great number of fact0rs.l One of these factors is the acidity of the solution, which strongly affects the fluorescence quantum yields and lifetime^.^-^ In the first part of this series5 we interpreted the photophysics of (UOi+)* in terms of reversible crossing between two excited electronic states U* and X* : ki u* e x* Here we quantum report a study of the effect of acidity on the decay and the fluorescence yield of the uranyl ion over the pH range from 0.5 to 4.0.The present results extend considerably the acidity range of previous studies and reveal new features that are accounted for by the reversible-crossing kinetic scheme. t Visiting Professor at the University of Coimbra. 17351736 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTIONS 1 2 3 4 5 PH Fig. 1. Relative fluorescence yield (0) of uranyl as a function of pH, under stationary-state conditions; (-) fluorescence yield calculated through eqn (1) from the pH dependence of uranyl decay rates; [UOi+] = 0.02 mol drn-,, [NO;] = 0.55 mol dm-3 and A,, = 337 nm. EXPERIMENTAL Excited uranyl decays were studied using nanosecond flash-photolysis apparatus with a pulsed N, laser. Fluorescence spectra were run on a Spex Fluorolog model 11 1 fluorimeter and absorption spectra were recorded on a Shimadzu UV-240 spectrophotometer.Solutions were prepared with triply distilled water and from uranyl nitrate of the purest grade commercially available; the pH was adjusted with HNO, and NaOH for pH > 3.0. The fluorescence quantum yields were determined at different pH with respect to the quantum yield at pH 3.0, [UOg+] = 0.02 mol drn-,, Aex = 366 nm. Further experimental details have been presented elsewhere. RESULTS AND DISCUSSION FLUORESCENCE QUANTUM YIELDS UNDER STATIONARY CONDITIONS The fluorescence quantum yield of (UOi+)* was studied as a function of pH under stationary conditions for excitations at 337 nm. Fig. 1 shows that & decreases from pH 0.5 to pH 2, increases up to pH 3.5 and decreases again at pH 4.& is independent of the wavelength of excitation up to pH 3, but at pH 3.5 and 4 different yields are obtained for excitations at 337 nm (the laser exciting wavelength for the fluorescence decay) and at 366 nm. At this latter excitation the relative yields increase with increasing pH (pH > 2.5) with relative yields 43,S6(pH 4)/&3pH 3) = 3.6. Since the absorption spectrum of UO;+ changes drastically at pH > 3.5 because of several polynuclear uranyl species obtained by condensation reactions between hydroxocomplexes,6 this wavelength dependence can be attributed to the presence of some strongly fluorescent species that are only excited around 366 nm. The emission- excitation matrix plot (fig. 2) for the uranyl emission at different pH values supports this view.The pH dependence reported in fig. 1 is in agreement with the previous observations of Marcantonato~~? * within the pH region studied (0.5-2.5). Marcantonatos was ableM. DA G. M. MIGUEL, S. J. FORMOSINHO, A. C. CARDOSO AND H. to interpret his observations in terms of a complex mechanism pH-dependent hydrogen abstraction from H20 *UO;+ + H 2 0 f *U02H2+ + HO' and, after several steps, the formation of an emitting exciplex proposed kinetic scheme gave an expression for & D. BURROWS 1737 involving initially (U204H4+)*. The A + B [H+] " = C+D[H+] where A , B, C and D are independent of pH. Although such an expression can be used to fit data up to pH 2.5 it is clear that the mechanism of Marcantonatos is unable to interpret the acidity dependence for pH > 2.5, Moriyasu et aL7 interpret the pH dependence of uranyl decay in terms of a hydrogen-abstraction reaction of (UOi+)* that is slower from H,O+ than from H20.Although such an interpretation could again account for the experimental data at pH -= 2.5 it is unable to explain the pH dependence at higher pH. Our observations can only be fully interpreted after a discussion of the pH dependence of the excited uranyl ion decay rates according to the scheme of reversible cro~sing.~ Fig. 2 reveals that at low pH (ca. 1) uranyl emission has a high-energy shoulder at 452 nm for excitation between 380 and 400 nm. Such an emission has an energy of 22 100 cm-l, which is virtually identical to the third electronic origin (22050 cm-l) found by Bell and Bigger9 in the first absorption band of UOg+. Since significant intensity can be hidden under the most intense emissions at higher wavelengths an accurate quantum yield cannot be provided for such an emission.A rough comparison of the areas reveals that the high-energy emission is ca. 100 times weaker than the normal fluorescence emission, i.e. $F x With the oscillator strength for this transition (3.8 x lo+) reported by Bell and Biggers the non-radiative transition rate for this state is estimated to be 4 x lo7 s-l. FLUORESCENCE DECAY The fluorescence decay of uranyl excited at 337 nm by a short laser pulse was studied between pH 0.5 and 4. Since the fluorescence decay is dependent on the laser intensity, except where otherwise stated the decays were studied at a constant (within 10%) laser intensity.The decays were biexponential and analysis in terms of a reversible- crossing mechanism allowed the determination of the decay rates k,, k,, ki and k,.5 Fig. 3 presents the pH dependence of the different decay rates for [UOi+] = 0.1 mol dm-, at a constant NO; concentration (0.55 mol drn-,). As fig. 3 shows, the rates of the irreversible decays, k, and k,, have an identical pH dependence increasing from pH 0.5 up to pH 2, decreasing to pH 3 and increasing again at pH 4. This dependence contrasts with the pH dependence of the rates of the reversible crossing, ki and k,, both of which have maxima at pH 3.0. Similar variations have been found for lower uranyl concentrations down to 5 x mol dm-3. The most significant differences are above pH 3.The minimum for the rates k , and k , occurs at a higher pH (ca. 3.5) at low concentrations and the increase is not so steep around pH 4. Excited uranyl is known to suffer a significant autoquenching process. This autoquenching process at pH 3 affects only the decays rates of state U*, k , and ki. The same effect is apparent at other pH values (table 1) with the exception of pH 1, whereas k , also suffers significant autoquenching and ki suffers a very small decrease with increasing [UOi+]. Nevertheless, the autoquenching rate constants are not strongly pH dependent.1738 4.49E 05 x U .- 2 U .- O.OOE 00 45 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTIONS I I I I 00 563.00 676 .OO position/nm 1 1.18E .06 ; I - O.OOE 00 450.00 563.00 posit ion/nm Fig.2.(a) and (b). For legend see facing page. 676 .OO The effect of the intensity of the laser was also studied at pH 1 and pH 4. The results at pH 4 resemble those at pH 35 for ki and k,, which decrease with increasing laser intensity, but more strongly at pH 4 than at pH 3. However, k, increases slightly with increasing laser intensity (ca. 1.4 for 3 times the laser intensity), but k , also decreases in a fashion similar to that of ki. At pH 1, however, ki and k , pass through a minimum (fig. 4), but k , and k , are again independent of the laser intensity.M. DA G. M. MIGUEL, S. J. FORMOSINHO, A. C. CARDOSO AND H. D . BURROWS 1739 6.36E .06 O.OOE .OO 450.00 563.00 position/nm 6 76 .OO Fig. 2. Emission-excitation matrix plot of uranyl nitrate aqueous solutions as a function of pH; [UOt+] = 0.02 mol dm-3 at room temperature: (a) pH 1; (b) pH 3; (c) pH 4.HYDROLYSIS OF URANYL Aquo-cations of high charge tend to act as acids in solutiong and UOi+ is no exception. Hydrolysis constants, pKl = 4.1-4.3, have been reported for UO:+.lo In general, the acid dissociation constant for the loss of a second proton from an aquo-cation is 10-100 times smaller than that for the loss of the first p r ~ t o n . ~ Hydroxo-0x0-aquo-cations are formed by uranyl which polymerizes at pH > 3 .06 with equilibrium constants for the dimer of ca. 10+ mol dm-3.67 Consequently at different pH different uranyl ions are present in the aqueous solutions, and these ions can have different rates of decay. The rates of reversible decay of the excited uranyl ion have been attributed to reversible crossing between two electronic states of different R, caused by a solvent- exchange rne~hanism.~ Not much is known about solvent exchange in excited states of metal ions9 nor about the possible influence of coordinated hydroxyl groups.Nevertheless, water exchange at FeOH2+ takes place much more rapidly than at Fe&ll and in U(H20)4,+ the rate of water exchange increases with decreasing acidity.12 A similar situation may also occur for the excited uranyl ion and may explain the increase in ki and k, with increasing pH up to pH 3. To our knowledge nothing is known of solvent exchange in polymer cations, but because of the 0x0, hydroxo and water bridges established between the different metal ions, we expect that solvent exchange rates are lower in such species, since fewer coordinated molecules are avail- able for the exchange.This seems to be the case for uranyl at pH 4. The rates of the irreversible decays, k, and k,, can be attributed to a hydrogen- abstraction reaction from H 2 0 by an excited uranyl ion or to a purely physical radiationless transition where the 0-H vibrations of the coordinated water play a significant role as accepting modes. However, since the 0-H frequencies are not pH1740 5 - 4 - * lv) 3 - 2 In --- 2 - PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTIONS 3 - - I v) In 2 2 - Y' I I I 1 I 0 ' 1 2 3 4 5 PH I I I I I I 0 1 2 3 4 5 PH Fig. 3. pH dependence of the decay rates (a) and [NO;] 0 1 2 3 4 5 PH 't 0 Y 1 2 3 4 5 PH k,, (b) kx, (c) ki and ( d ) k,; [UO;+] = 0.1 mol dm-3 = 0.55 mol dm-3.Table 1. Autoquenching decay rates (s-l) at room temperature as a function of pH PH 1 .o 2.0 3.0 4.0 kll 1.0 x 106 0.96 x lo6 1.3 x lo6 0.97 x lo6 ki Oa 0.95 x lo6 2.6 x lo6 2.2 x 106 kX 1.7 x lo6 0 0 0 kr 0 0 0 0 a Q 2 x 105 S-1.M. DA G. M. MIGUEL, S. J. FORMOSINHO, A. C. CARDOSO AND H. D. BURROWS 1741 l5 t I 1 100 20 0 30 0 Fig. 4. Effect of laser intensity on the rate constants of uranyl decay at pH 1 ; [UOif] = 0.02 mol dm-3: 0, k,; 0, k,; A, ki; A, k,. Ilaser dependent for the different uranyl cations, the pH dependence of k, and k, does not support the latter mechanism for our experimental conditions. Rates of hydrogen abstraction will be dependent on the possibility of the excited uranyl ion abstracting a hydrogen atom from the water molecules loosely coordinated in the equatorial plane, or from water that comes into the hydration shell in an axial direction.The frequency factor and possibly the hydrogen-abstraction rates are higher for the former situation. Since the presence of OH groups can alter the water structure around UOi+, the hydrogen-abstraction rates can also be pH dependent. In order to test the possibility that such qualitative ideas will interpret quantitatively the data of fig. 3, a quantitative assessment of all those factors was established, assuming that all the acid-base equilibria for excited uranyl ion were faster than the fluorescence decay. The following set of equilibria? was considered : *UOi+ + H,O + *UO,(OH)+ + H+ = 2 x lo-, *U02(OH)++HH,0+ *UO,(OH),+H+ = 4 x 2 *UO,(OH),+H,O+*(UO,),(OH)~+H+ = 1 x lop3.Fig. 5 presents the distribution diagram for all these different species as functions of pH. Since NO; is present in solution some NO; complexes are also present but were not considered in the mechanism since the data do not allow a distinction between free uranyl ion and nitrate complexes. Furthermore, with an equilibrium constant t These species have not been established to be present, but are chosen on the basis of simplicity. Products of the first two equilibria should be capable of fast solvent exchange whilst polymer species undergo only slow exchange.1142 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTIONS *uo**+ \ * UOZ (OH), PH Fig. 5. Distribution diagram for excited uranyl ions as a function of pH; [UO~+],,,,, = 0.1 mol dm-3.K* = 0.36 dm3 mol-l for the complex (U0,NO;)*13 we can estimate that at very low pH (< 0.5) (UO:+)* is at most 20% complexed with NO; ([NO;] = 0.55 mol dm-3 and [UOi+] = 0.1 mol dm-3). With those equilibria and the following rates for k, and ki k,/i05 s-1 2.3 5.8 0.1 10 *uo;+ * UO,(OH)+ * UO,(OH), *(UO,),(OH), ki/ lo5 s-l 1.7 5.2 4.5 0.3 good agreement between the calculated and the experimental rates was found (fig. 6). At lower [UOi+],,,,, the fraction of the polynuclear ion decreases and consequently there is a shift to pH ca. 3.5 in the minimum of the k, curve and the increase at pH 4 is not so strong. These facts are also in agreement with the experimental observations. The pK values for the hydrolysis of the excited uranyl ion are ca.2.5 units lower than the pK reported for the ground state. The increase in acid strength upon electronic excitation is a situation common to many other molecules. However, with uranyl, because there is not a great shift in the maximum of the absorption spectra with pH, except for pH > 3.5, the enthalpy contribution for such an increase is notM. DA G. M. MIGUEL, s. J. FORMOSINHO, A. c . CARDOSO AND H. D. BURROWS 1743 4 - - I v1 P 3 - >- a 2 . I 4 . I v1 VI 2 3 . a Y 2 . 0 1 2 3 4 5 0 1 2 3 4 5 PH PH Fig. 6. Calculated (-) and experimental (0) rates (a) k, and (b) ki for excited uranyl decay as a function of pH; [UO~+],,,,, = 0.1 mol dmP3. very high. At most we can estimate it to be 0.5 pK units. The remaining difference can be attributed to an increase in the entropy of the acid-base equilibrium in the excited state of ASAH* -ASAH = 37.5 J K-l mol-l.This value is not unreasonable since the change in entropy for the dissociation of a proton is very high for the aquo-cations of uranium; A S = 138-150 J mol-1 K-I for U4+.lo For UOq+ the situation is very unsatisfactory because the choice of values for the enthalpy is very large (AH = 44-87 kJ mol-l) and consequently A S ranges from 25 to 159 J K-' mol-1 at 298 K. The increase in entropy because of the closer packing of the hydration shell in the excited states seems to be in agreement with the data for solvent exchange in UO;+. For (UO?j+)* we have rates of solvent exchange at room temperature of (2-5) x lo5 s-l. Nuclear magnetic resonance studiesL4 of the kinetics of H,O-exchange process in the equatorial positions of UO,(H,O)q+ allowed an estimation of a rate for all the coordinated water of 3.8 x lo6 s-' at 25 "C [9.5 x lo5 s-'; (U02)(H20)2,t with n = 41, a value that is approximately an order of magnitude higher than the rates of exchange in the excited states.Further support for the hydrolysis of the excited uranyl ion comes from the fluorescence spectra (fig. 2), which are more sensitive to pH than the absorption spectra. Furthermore, the quenching studies of (UOi+)* with Ag+ as a function of ionic strength at pH 2 show a dependence of the quenching rate on the ionic strength. This dependence reveals that in the quenching process there is an intervention of two ions with a + 1 charge.I5 Fig.5 shows that at pH 2 [UO,(OH)+]* is the dominant species, but even the other relevant species, (UOi+)* and UO,(OH),*, are present in equal amounts. At higher pH values (3 and 4) the fluorescence in the presence of Ag+ is perturbed by the formation of polynuclear species between uranium and silver.16 Under stationary conditions the fluorescence spectra of the uranyl ion contains the emission of the excited states U* and X*.5 For a reversible-crossing kinetic scheme the overall fluorescence yield is given by k;' + kj k",(k, + k,) '' = k, + ki - ki k,/(k, + k,)1744 PHOTOPHYSICS OF URANYL ION IN AQUEOUS SOLUTIONS where k , are the radiative rate constants of the two excited states of the uranyl ion (kg = 1.8 x lo2 s-l and kE = 0.80 x lo2 s - ~ ) .~ In order to test the model of reversible crossing the pH dependence of the fluorescence yields can be estimated through eqn (1) using the experimental decay rates. The agreement between the experimental and the calculated bF is good at all pH except at pH 0.5 (fig. 1). However, at [UOi+] = 0.02 mol dm-3 no decay values were experimentally determined at this pH. The values employed in the calculation where extrapolated from the experimental curves. The absolute yield at pH 3 is now lower, &. = 0.7 x than that previously reported (& = 1.1 x [NO;] = 0.04 mol dmP3) since the NO; concentration is higher (0.55 mol dm-3) and NO; increases some of the decay rates of the excited uranyl At a higher uranyl concentration (0.1 mol dm-3) the increase of bF around pH 3.5 is much less than at lower concentrations.In conclusion, we have shown that the effect of acidity on the photophysics of the uranyl ion can be attributed to the different decay rates of the several ionic species caused by the hydrolysis of (UOi+)*. All the data support the previously proposed reversible-crossing mechanism. This work was supported by INIC through the Research Centre QC-1. We thank G.T.Z. for the kind gift of the fluorimeter used in this work and one of the referees for useful suggestions. C. K. Jrargensen and R. Reisfeld, Struct. Bonding (Berlin), 1982, 50, 121. M. D. 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