Quantum yield and electron-transfer reaction of the lowest excited

Quantum yield and electron-transfer reaction of the lowest excited state of uranyl .... nodded to science and women in his second State of the Union s...
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J. Phys. Chem. lS80, 84, 2981-2985

2981

Quantum Yield and Electron-Transfer Reaction of the Lowest Excited State of Uranyl Ion' T. Rosenfeld-Grunwald" and J. Rabanl" Energy Research Center and the Department of Physical Chemistry, The Hebrew University, Jerusalem 9 1000, Israel (Recsived: September 14, 1979; In Final Form: January 16, 1980)

The pulsed photoreduction (using Nz laser photolysis technique) of uranyl ion, U0z2+,both in its ground state and in the lowest excited state (at pH 2 and in 2 M H3P04aqueous solutions) has been studied. We found is quenched by ruthenium tridbipyridine) ions, Ru(bpy)z+,and Ru(bpy)S3+is formed; namely, that *UOZ2+ *UOZ2+is an electron-acceptor species. It is shown here that the rate of decay of *UO?+ in the presence of Ru(bpy)z+ exactly corresponds to the formation rate of Ru(bpy):+, as measured by the bleaching of the R ~ ( b p y )ground-state ~~+ absorption. The excited state of Ru(bpy)$+is found to be quenched by UOz2+,and again R ~ ( b p y ) , ~is+formed, UOzz+being an electron acceptor in its ground state. The quantum yield of the formation of the lowest excited state of U022+,4*UO2z+, multiplied by the yield of electron transfer, betvveen *UO?+ and Ru(bpy):+ is found to be 0.5 at pH 2 and unity in 2 M H3P04solution. In the presence of a negatively charged polyelectrolyte, such as poly(viny1 sulfate), PVS, the value of this product is reduced to an upper limit of 0.1. It is shown that ~ * U O ? +is unaffected by the presence of PVS. The back reaction, U02+ + Ru(bpy)$+ -* UOZ2++ Ru(bpy)z+,is a second-order reaction, k/c (A = 450 nm) = 1.05 X lo5 s-l cm at pH 2 and 1.78 X IO2 s-l cm in 2 M H3P04solution. This reaction is accelerated in the presence of Oz or PV8. The yields and rates of formations and reactions of the photochemical products are discussed.

Introduction The uranyl ion possesses some properties which make it a potential component for a solar energy conversion system. It absorbs light in the shorter-wavelength range of the solar spectrum, producing a relatively long-lived excited state. The excited state has fluorescence peaking at -520 nm, a property which makes it relatively convenient to investigate its reaction. The redox potential of the excited uranyl ion makes it a powerful oxidizing agent. This may be of potential use in the photogeneration of oxygen, which is of great importance for the photocleavage of ~ a t e r . ~ 8 The photochemistry of uranyl ion, UO?+,has been a subject of numerous scientific The photoreduction of UOZ2+was reported to take place by several organic or inorganic reductant^^?^ by abstraction of hydrogen atoms from organic in particular the photoreduction of UO?+ with ethanol13to produce U02+. Another research studied the photoaccelerated U(1V)-U(VI) electron-exchange reaction.14 In order to understand the mechanism of the photorewe made an attempt to charduction reactions of' UOz2+, acterize the excited state of UO?+,*UO?+.The assignment of the electronic excited state of UO?+ is still controver~ial.~?~ 'The lowest excited state has been attributed to either a tri]pletlk17or singlet excited state.ls It has been proposedlg tlhat this frequently djscussed question of whether the first excited state is a siinglet or a triplet has no physical siignificance in the specific case of UO?+,because of the combination of two conditions: the almost identical one-electron energies of at least four of the seven 5f orbitals arid the large spin-orbit separation between 5F7/2 and 2F5/2.20 The lowest excited state has an absorp= 570-590 nm and a mediumtion spectrum with ,A, dependent fluorescent ~ p e c t r u m . ~ Both J ~ ~ the ~ * absor~~ bance and the fluorescence decay with the same rate constant,22which is greatly dependent on the medium;6J2,22 e.g., it is 3 X lo5 s-l in water at pH 2, decreasing 10-50-fold in concentrated acid solutions, depending on the nature of the acid. The reactivity of 'WOZ+ has been extensively studied. We will consider only those systems where the quenching 0022-3654/80/2084-298 1$01.OO/O

mechanism is claimed to be an electron-transfer reaction. The quenching of *UOZ2+by aromatic hydroc a r b o n ~ ~ J and " - ~unsaturated ~ > ~ ~ compoundsz takes place without formation of transient or permanent products. So is the case in the quenching of *UO?+by inorganic ions27 such as halide ions2sand inorganic cation^.^^^^^ Burrow et alamreported a transient absorbance upon flash photolysis in the presence of Mn2+,which was attributed of UOZ2+ to Mn3+. Burrow et reported also that photolysis of an aqueous solution of UO?+ in the presence of I-, CNS-, Br-, or CO2- produced Iz-,CNS,, Br2, and C&- radicals, respectively. It is quite possible that these transient absorbances were the result of the flash photolysis of the ligands t h e m s e l v e ~ . ~ ~ ~ ~ ~ A correlation%between the quenching constants and the oxidation-reduction potentials of the quenchers was taken in many cases as evidence for an electron-transfer quenching reaction of *U022+. The quantum yield of the photoproduction of *UO$+ is not yet unequivocally determined. Steady-state photolysis experiments14indicated that light accelerated the U(IV)-U(V1)exchange. The quantum yield of this process is wavelength and temperature dependent, reaching maximum values of 0.219 f 0.023 at X = 436 nm at 25 "C and 0.879 f 0.082 at 35 "C. In the presence of ethanol the quantum yield of the photoproduction of U02+was found to be near unity for two temperatures: 25 and 45 "C. Since the role of the alcohol in the formation of UOZ+l3 is not well understood,the quantum yield of *UO?+cannot be taken as equal to that of U02+.The quenching of UO?+ by d6 metallocenes was also i n ~ e s t i g a t e d It . ~ was ~ found that the quenchers (those of Fe or Ru) were oxidized. The quantum yield for the oxidation of Fe-metallocene was found to be 0.62; this value is determined by the quantum yield of *UOZ2+and the yield of electron transfer per encounter. The material balance in this system must be known in order to use the data for the evaluation of these quantum yields. The purpose of this work is to demonstrate a direct measurement of a reversible net electron-transfer reaction involving *UOZz+.Our results seem to be the first unequivocal direct observation of the photoreduction of @ 1980 American Chemical Society

2982

The Journal of Physical Chemistty, Vol. 84, No. 22, 1980

*UOZ2+by electron transfer. Experimental Section Reagents. Uranylnitrate and uranyl sulfate were B.D.H. Laboratory reagents and were used without further purification. Tris(2,2'-bipyridine)ruthenium(II) chloride hexahydrate, R ~ ( b p y ) ~was ~ +purchased , from K & K and recrystallized from water. Its €(A = 452 nm) was measured as (1.43 f 0.02) X lo4 M-l cm-l, and the rate of decay of its triplet state (in deaerated aqueous solutions) was found to be (1.58 f 0.02) X lo6 s-l, in agreement with published v ~ l u e s ,indicating ~ ~ , ~ ~ the purity of the sample. The potassium salt of poly(viny1 sulfate) (Sigma) was purified as previously d e ~ c r i b e d .The ~ ~ average molecular weight of the polymer was taken as ~250000.37The pH of the solutions was pH 2, measured with an El-Hama Instruments pH meter, and was adjusted, when necessary, with diluted HN03. Apparatus. The pulsed photolysis technique using a 337.1-nm (10 ns, 0.5 mJ) pulse of an N2 Avco-Everett laser has been previously d e s ~ r i b e d .Transient ~~ spectra and reaction rate constants were calculated from the transient absorbance changes at the appropriate time domains. Permanent absorption spectra were measured on Cary-14 or Beckman DP-G spectrometers, before and after the laser photolysis experiments. No permanent absorbance changes were detected. The deoxygenation was carried out by bubbling argon (Matheson) through the optical cell for 10 min before irradiation. The number of photons in each laser photolysis experiment were measured with either R ~ ( b p y ) , ~or+ Ru(bpy),2+-HC104-Fe3+solutions. The bleaching in optical density at 450 nm, AOD, was measured and from which the photon density was determined. The measurements were based on the fact that the quantum field of this phenomenon is unity.36i39-41The laser intensity density under our experimental conditions was up to 3 X einsteins per liter per pulse and, whenever necessary, was reduced by a factor of -3, so that we worked in the range of linear dependence of the AOD (A = 450 nm) on the laser intensity4z and also avoided excessive bleaching of the Results and Discussion Aqueous solutions (pH 2) of UO$+ in the range of concentrations 5 x 10-3-5 X lom2M, when submitted to the N2 (337.1 nm) pulsed laser excitation, exhibit a transient absorbance with Am, = 570-590 nm and a fluorescence spectra with A, = 515-525 nm. Both phenomena decay in a first-order reaction with a rate constant of k2 = (3.1 f 0.1) x lo5 s-l. This valuez1is unaffected by the concentrations of UO?+ (using the above-mentioned values), by the presence of 02,or upon replacing the NO, by SO:-. The transient absorbance is attributed to *U0$+.4*7J1-14Both the absorbance (at 575 and 585 nm) and the fluorescence (at 515 and 525 nm) were measured (50 ns after the pulse) as a function of the intensity of the exciting pulse.4z The intensity dependence of both phenomena is linear, indicating a monophotonic mechanism of the photoprocess producing *UOz2+,under our conditions. Upon adding R ~ ( b p y ) ~ to~a+ deaerated solution of UO$+, and after laser photolysis, we observed two processes: (a) a shortening of the lifetime of *UOz+, which was indicated by both the faster decay of the absorbance and the fluorescence of *U022+,and (b) the photoproduction of R ~ ( b p y ) characterized ~~+ by the longer-lived bleaching of Ru(bpy)?+, i.e., the decrease in the optical

Rosenfeld-Grunwald and Rabani

density, at 450 nm.35-39 These observations can be explained by reactions 1-3.

2*uo22+ *U022+ UOZz++ hv' UO?+

-

4

*u02z+ *UO?+

UO22+

+ R ~ ( b p y ) 3 ~kq(3) +

(1) (2a) (2b)

U02+ + Ru(bpy)a3+ (3)

Both UO$+ and R ~ ( b p y ) , ~absorb + the N2 laser light (337.1 nm); the t(337.1 nm) of Ru(bpy),2+is 9.5 X lo3 M-l cm-l and that of UO$+ is 23.5 M-l cm-l. The fraction of excitation light absorbed by Ru(bpy)gP+,F , was calculated by using the formula F R = ODR(1- lO-&T)/OD, where ODRis the optical density of R ~ ( b p y ) at ~ ~337.1 + nm and ODT = ODR + OD, (OD, is the optical density of UO?+ at 337.1 nm). We calculated F R at both the beginning and the end of the laser pulse, correcting ODR for the bleaching of R ~ ( b p y ) observed ~~+ at the end of the laser pulse. The difference between the two values of F R was less than lo%, and F R was taken as the average value. Knowing the changes of the concentration of Ru(bpy)t+ during the laser pulse, and correcting for reaction 2, we calculated the quenching rate constant of *UO?+by Ru(bpy),2+,kq(3) = (3.7 f 0.3) X lo9 M-l s-l. In the laser photolysis experiments of deaerated aqueous M Ru(bpy),2+ and 5 X solutions of 5 X 10-"8 X M (and less) UO?+, we found that the typical lifetime of the emission of Ru(bpy)t+was shortened, along with an observed bleaching of the Ru(bpy)t+due to the production of Ru(bpy),3+ according to reaction 4. *Ru(bpy),2++ UO$+

kq(4)

R u ( ~ P Y )+~ UOz+ ~ + (4)

In these experiments the absorbance of UOZ2+was less than 5 % of the total absorbance. We determined k44) = (4.4 f 0.4) X lo* M-l s-l. (The decay of the emission of *Ru(bpy),2+followed at A = 600 nm in the absence of UO?+ was found to be k*Ru = (1.68 f 0.02) X lo6 s-'.) Knowing kR,, kq(4),the number of photons, and the change in Ru(bpy)?+ concentration during the laser pulse, we calculated, according to eq 5, the efficiency of electron AOD(A)mea,d = [NhvlFR~YC~*Ru~etE(A) ( 5 ) transfer in reaction 4, where AOD is the change in optical density (bleaching), [Nhv] is the number of photons in einsteins per liter per pluse, FRis the average value of the fraction of light absorbed by Ru(bpy)?+, 4yc = k&4b k ~ , ] ; f * ~= , 1, €(A) iS the ex[UO?+]/[k (4)[uO?'] tinction coelficient of R u ( b p ~ ) ,~and & is defined as the efficiency of electron transfer between *Ru(bpy)t+and UOzz+.In eq 5 t(U02+)(400 < A < 600 nm) was neglected as we determined its value to be less than 100 cm-l M-l by using pulse radiolysis. Since all the other values can be measured, det was calculated to be d ~ ~ ~ ( *+RUO?+) u = 0.45 f 0.05. (The measurements of AOD were carried out in the range of wavelengths 440-470 nm.) The correction for the contribution of reaction 3 was less than 5% of the experimental AOD. Using a dye laser bxCitation = 437 nm, where the absorbance of UOZ2+is almost zero, we observed the same phenomena and evaluated &(*Ru2+ + UO?12+! = 0.50 f 0.05. Similar calculations were performed for reaction 3. In these experiments the contribution of reaction 4 was important as quite a high concentration of R ~ ( b p y ) 3 (3 ~ +X 5X M) was needed in order to observe a measurable AOD and to calculate kq(3). The calculations were as follows:

The Journal of Physical Chemlstry, Vol. 84, No. 22, 1980 2983

where AOD(,X)(5)is calculated according to eq 5, Fu = OD,O - 10-ODT)/OD~, h c = kq(3)1[Ru(b~~)~+l/[~q(3)' [Ru(bpy)t+] k2], imd q5et is the yield of electron transfer between *UOz+ andl Ru(bpy),2+. Since the quantum yield n of *UO?+, r&~uo~+, is unknown, we could estimate only the 0 a = 0.44 f 0.05. product: &LrOz+& The back-reaction (reaction 7) was3 studied by following w U02+ .t R ~ ( b p y ) , ~--++ UO?' + Ru(bpy)g2+ (7)

+

v

the recovery of the optical density at 450 nm. The reaction was found to be second order with k/c(X = 450 nm) = (1.05 f 0.05) X lo5 s-l cm. In the presence of O2 (aerated aqueous solutions of UO?+ and Ru( bpy),2+)this back-reaction became faster, with Iz/t(X = 460 nm) = (1.52 f 0.07) x IO5 s-l cm. This can be explained by reactions 8 and 9. The for-

-

+0 2 Ru(bpy)lt + 0 2 U02'

+ 02-

(8)

Ru(bpy)S2++ 02

(9)

UO?+

--*

0

I

OOl/ 1

2

I

6

I

,

IO

,

I

I4

,

I8

t , ysec

mation of Ru(bpy):,2+ is expected to be second order if Flgure 1. Photochemical bleaching of the optical densky, = 450 nm, either reaction 8 or 9 is rate determining. It should be in the uranyl-H3P0,-Ru(bpy)l+ system: 2 X lo-* M UO:+, 3.1 X loJ M Ru(bpy):+, 2 M H,PO,, deaerated solutlon; @) experimental, curve emphasized that O2 had no effect on the initial yield of is com uted, taking k,(3) = 2.6 X lo9 M-I s-' and k2(H3P04)= 3 X R ~ ( b p y ) formed ~ ~ + from *UO?+, 10' hl-ps-'. Corrections were made for the change in the [Ru(bpy),2+] Effect of 2 M H8P04. Similar experiments were perduring the laser pulse and for reaction 4. Insert: oscillograms A and aqueous solutions, as it is well esformed in 2 M B from which the data were taken for Flgure 1. Oscillogram B includes tablished that, the lifetime of the excited state in the H3P04 the first 19 hs of A. solution is much longer than its value in water. The N2 these experiments we used solutions containing 2 M pulsed laser photolyses of both aerated and deaerated 2 &Pod, in which the emission of *R~(bpy)~'+ at 600 nm M H3P04sollutions containing 2 X 10-2-4 X M of in the presence of UO2+ was compared to the emission of UO2+ exhibit a very similar transient absorbance and *R~(bpy)~'+ in a control solution (l-cm cell). The absorfluorescence spectra in comparison with pH 2. The decay bance at 337 nm, the excitation wavelength, was identical is again a firnt-order reaction with a rate constant of k2for both the test solutions and the control solutions. This (&PO,) = (3.3 f 0.3) X lo4 s-l. The initial yield of enabled a direct comparison between the emission effi*(U022+-phosphate)measured by the increase of optical ciencies. Thus, the emission intensity of a solution condensity or the fluorescence intensity showed a linear deM R ~ ( b p y ) was ~ ~ +100 (in arbitrary taining 1.27 X pendence on the laser intensity, again indicating that the units), while a solution containing 4.36 X lo4 M Ruphosphate coimplex of *UO?+ undergoes a monophotonic (bpy)t+and 2.46 X M UO+ : which has the same OD photoprocess. = 0.180 at 337 nm showed emission intensity of 8.6. These A deaerated 2 M l13P04solution of Ru(bpy)t+ exhibits results show that no energy transfer took place between the same photoprocess as an aqueous solution of Ru*UO2+and Ru(bpy)t+,since this is precisely the expected ( b ~ y ) ~The ~ + .rate of decay of * R ~ ( b p y )was ~ ~ +found to emission as can be calculated from the reaction rate con= (2 f 0.2) X lo6s-l. Both reactions 3 and be kaRu(H3P04) stants and absorption spectra in this system, taking re4 could be observed upon laser photolysis of deaerated 2 M H3P04 solutions of UO?+ and Ru(bpy)t+. The rate of action 3 as written, Had energy transfer taken place with a yield of 1, the expected emission in the UO?+-Ru(bpy),2+ + found to be k,quenching of *UOZ2+by R ~ ( b p y ) , ~was solutions would have been 2.62-fold higher. This conclu(3)(H3P04) =: (2.2 f 0.2) X lo9 Plvl-l whereas k,(4)(H3P04)is found to be (3.5 f 0.5) X lo* M-l s-l. In this sion was confirmed in another set of experiments, with system we measured qiet(*Ru(bpy)32f+ U022+)= 0.45 f 0.010 M UO2+ and 9.8 X lo+ M Ru(bpy)t+,where the 0.05. The prloduct f$*U02z+$Jet = 1.1 :k 0.1 was calculated M emission was compared to that observed in 3.35 X R ~ ( b p y ) ~which ~ + , again has the same absorbance at 337 from experiments in whiich UOZ2+absorbed most of the nm as the test solution. Here again, the measured ratio exciting light, This means that &UO~Z+(phosphate complex) is unity, and so is the efficiency of electron transfer of the emission intensities (0.074) confirmed that reaction between *UOb?+ (phosphate complex) and R ~ ( b p y ) ~ ~ + .3 proceeds as written, while energy transfer with a yield The rate of decay of'the transient absorbance of *UO?+ of 1 would have resulted with a value 2.58-fold greater. corresponds to the rate of formation of Ru(bpy),3+ as It might be argued that reaction 4 takes place as a measured by the rate of bleaching at 450 nm (Figure 1). geminate reaction after energy transfer between *UO2+ The back-reaction is a second-order reaction with k / = ~ and Ru(bpy)S2t,so that no emission by the product *Ru(1.7 f 0.15) X lo2 s-l cm. This value is smaller by three (bpy)z+is expected under such conditions. However, since orders of magnitude than the value observed at pH 2. we measured k4 = 3.5 X lo* M-l s-l , a value considerably It might be argued that reaction 3 takes place by energy lower than the diffusion controlled limit, one would expect transfer from *UO?+ to R ~ ( b p y ) , ~iFollowed + by reaction that most of the energy transfer pair ion products, if 4 and not by reaction 3 as written. Such an argument formed, diffuse to the bulk of the solution. cannot be ruled out on the basis of the energetics of the Effect of Poly(viny1 sulfate). The formation of Rusystem. However, ateady-state measurements using a (bpy):+ and UOz+is supported by the effect of poly(viny1 Terner spectrofluorinieter showed that no *Ru(bpy):+ was sulfate), PVS, where the rate constants of both photoformed in our system other than by direct excitation. In electron transfer and the back-reaction increased in its

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The Journal of Physical Chemistry, Vol. 84, No. 22, 1980

1

0 i4

c

3

/ - \

PVS

a "F

: l

n 2

w

d

460

500

54C

580

A

620

660

700

nm

Flgure 2. Absorption spectra of *UO:+, 4.2 X IO-' M UO+ : in aqueous no PVS, (0) in the presence of 5.8 X lo-* M PVS in solution: (0) monomer units. The absorbance was recorded 50 ns after the laser pulse.

presence. In aqueous solutions of PVS, 5.62 X M in monomer units and 1.96 X M of the UOZ2+ions, the rate constant of the decay of *U02z+was found to be k2 = (1.2 f 0.1) X lo6 s-l. The reaction rate constant k2 increased with the ratio of equivalents U022+to PVS in its monomer form (will be referred to as a ) , reaching a value of (2 f 0.2) X lo6 s-l at a = 1.4. The increase in the value of k 2 observed upon increasing the percent of the coverage of the negative charges of PVS by UO?+ can be understood by the known experimental data21that *UO?+ is quenched to a degree by ground-state UO?+, lz, = (4.0 f 0.3) X lo6 M-l s-l, and also by the fact that the UO?+ ions form aggregates43in the presence of PVS. UO?+ and *UO?+ are essentially present in a volume determined by the negative field of the polymer. Therefore, their actual concentrations within the effective may be by several orders of magnitude higher than the apparent concentration in the solution, resulting with the enhancement of the reaction between these species. The initial yield of absorbance was unaffected by the presence of PVS up to a = 1.4 (Figure 2). O2had no effect on either the initial yield or the rate constant of the decay of *UO?+. Upon addition of Ru(bpy)?+ to a deaerated solution of PVS and UO?+, we observed the same processes as found in water; Le., the lifetime of *UO?+ was shortened and the optical density at 450 nm decreased, indicating the formation of Ru(bpy),3+. In these experiments, the concentration of Ru(bpy)g2+ was about equal to the concentration of the PVS polymer. The fractions of light absorbed by Ru(bpy),2+and UO?+ were 0.33 and 0.54, respectively. The apparent rate constant of quenching of *UO?+ by Ru(bpy)Qa+was found to be k (3) = (4.4 f 0.4) X 1O1O M-l s-l (correction was made for tke change in the absorption of light by Ru(bpy)?+). This value of k,(3) is 20 times larger than the value obtained in water. We also studied systems where the photons were absorbed only by Ru(bpy)?+. In these experiments the concentrations of UOzz+were in the range 10-4-10-3 M, and those of Ru(bpy)QB+were nearly equal to the polymer form of PVS, namely, in the range 2.4 X 10"-4 X M. The fraction of light absorbed by U0Z2+was small, and the contribution of reaction 3 to the optical density change at 450 nm was negligible. The value of klRu in the presence of PVS was (1.75 f 0.05) X lo6 s-'. Upon adding UO?+ we observed the same phenomena that occurred in water, i.e., quenching of *Ru(bpy)gP+and formation of R ~ ( b p y ) ~observed ~+, by the bleaching of the

Rosenfeld-Grunwald and Rabani

absorbance at 450 nm. The apparent rate constant of the quenching reaction was found to be k (4) = (4 f 1)x 1O1O M-' s-'. Using eq 546and 6, we calcdated 4*U024ep-'*U < 0.1. Since we observed that 4.UOz~+ is unaffected by the presence of PVS (Figure 2), it is that decreases in PVS. The decrease in the net photoelectron transfer quantum yield can be explained by the influence of the strong electrostatic potential field of the polyelectrolyte. This field may have an effect on the potential curve of the charged species of *UO?+, on the stability of the intermediate complex of *UOZz+and Ru(bpy)?+, and on the geminate back-reaction to the ground-state products. The increase of the values of the quenching constants, k,, upon addition of PVS can be explained by considering the effective ~ o l u m e *in~ which * ~ ~ UOZz+and Ru(bpy),2+ ions move. Note that the value of k, for *UO?+ quenching increased by a factor of 20, whereas that for quenching of *Ru(bpy),2+increased by a factor of 100. This means that a concentration effect alone cannot account for the results. It should be emphasized that each *Ru(bpy),2+could react with one of many UOZ2+ions in the same polymer, but whenever the conditions favored the formation of *UOZ2+ there was on the average only one Ru(bpy)t+in the same polymer molecule. This difference and the possible formation of uranyl dimers or trimers43may be the reasons for the different effect of PVS on the kq's. The back reaction 7 was found to be greatly dependent on the presence of PVS. The half-lifetime of the reaction is reduced to T~ = 6.3 f 0.3 pus compared with r1 = 80 f 4 p s observed in the absence of PVS. The difierence is in agreement with the previously reported volume eff e ~ t . ~This ' > ~ effect ~ results under our conditions, with a 12-fold concentration of the positively charged products of reaction 7 due to accumulation of these ions in the polymer. The factor of 12 was calculated on the basis of the ratio between the volumes of the polymer and the solution, considering also the difference in the electrontransfer quantum yield in the presence and absence of PVS. Conclusions In this work we have demonstrated a photoelectrontransfer reaction from the ground state of R ~ ( b p y ) to ~~+ an excited uranyl ion, with the formation of Ru(bpy)gB+ and UOz+. Although phtooxidation by exciting uranyl ions has been previously reported, our results seem to be the first direct demonstration of electron transfer to *UO?+ in a reversible system. In steady-state experimentswe were able to show that both the absorption and emission spectra of the UO?+-Ru(bpy):+ system remained unchanged after 2 X lo4 photoelectron transfer-back reaction cycles (calculated on the basis of the Ru(bpy)Qa+concentration). This demonstrates a relatively high stability-a property which is required for a potential use in solar conversion and storage. Acknowledgment, We are indebted to M. Ottolenghi for continued discussions during all phases of this work, to R, Reisfeld for bringing to our attention important information on the properties of urnayl ions, to C. R. Goldschmidt and D. J. Lougnot for invaluable help in the alignments of the laser and optical systems, and to G. Dolan and J. Ogdan for maintenance of the electronic system.

References and Notes (1) This research was supported by the Israel1 Ministry of Energy and

Infra-Structure. (2) R. Reisfeld and N. LiebliSofer, "Role of the Uranyl Ion in ConVerSbfl of Solar Energy", Abstracts of the Conference of Photochemical and

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J. Phys. Chem. 1980, 84, 2985-2989

(3) (4) (5) (6) (7) (8) (9) (10) (1 1) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)

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(26) A. Ahmed, A. Cox, T. J. Kemp, and Q. Sultana, J. Chem. Soc., M l n Trans. 2, 1867 (1975). (27) R. Matsushima, H. FuJImOri,and S. Sakuraba, J. Chem. Soc., Faraday Trans. 7, 1702 (1974). (28) Y. Yokoyama, M. Moriyasu, and S. Ikeda, J. Inorg. Nucl. Chem., 38, 1329 (1976). (29) M. Moriyasu, Y. Yokoyama, and S. Ikeda, J. Inorg. Nucl. Chem., 39, 2205 (1977). (30) H. D. Burrows, S. J. Formosinko, M. 0. Miguel, and F. P. Coelho, J. Chem. Soc., Faraday Trans. 7 , 163 (1976). (31) H.D. Burrows, and D. P. DeJesus, J. Photochem., 5, 265 (1970). (32) I n the experimental data given In ref 30 and 31 It is obvious that UV light was emitted from the flash lamps. It is well established in the iiterature33that halide ions and COB" give these translent absorbances upon flash phototysls usirq UV light. We couid not repeat these results, working wtth kbntical solutions and using N2laser photdysls technQue (A = 337.1 nm). (33) L. I. Grossweiner and M. S. Matheson, J . Phys. Chem., 61, 1089 (1957). (34) 0.Fraverso, R. Rossi, L. Magon, A. Cinquantine, and T. J. Kemp, J. Chem. Soc., Dalton Tmns., 569 (1978). (35) C. Creutz and N. Sutin, Proc. Natl. Acad. Sci. U . S . A . , 72, 2858 (1975). (36) D. Meisel, J. Rabani, D. Meyerstein, and M. S. Matheson, J. Phys. Chem., 82, 985 (1978). (37) D. Melsel and M. S. Matheson, J. Am. Chem. Soc., 9% 6577 (1977). (38) C. R. Goldschmldt, M. Ottolenghi, and G. Stein, Isr. J. Chem., 8, 29 (1970). (39) C. Lin, W. Bottcher, M. Cou, C. Creutz, and N. Sutin, J. Am. Chem. Soc., 98, 6536 (1976). (40) C. T. Lin and N. Sutin, J. Phys. Chem., 80, 97 (1976). (41) D. Meisel, M. S. Matheson, W. A. Mulac, and J. Rabani, J. Phys. Chem., 81, 1449 (1977). (42) U. Lachish, A. Shafferman, and G. Steln, J. phys. Chem., 64, 4205 (1976). (43) I. A. Taka and H. Morawetz, J. Am. Chem. SOC.,93, 829 (1970). (44) F. Oosawa, "Polyelectrolytes", Marcel Dekker, New York, 1971, pp 18. (45) We found the value of +d*Ru(bpyh* LO )': to be In the presence of PVS 0.7 f 0.07. Further studies are in progress.

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A y and Pulse Radiolysis Study of the Reactions of Hydrido Complexes of Iron(I1) Containing Orgainonitriles in Methanol P. G. Fuochl,*la Q. G. Mulazzanl,la G. Pilloni,'b and G. ZOttilb Istituto di Fotochirnica e Radiazioni d'Alta Energia del C.N.R., Via de'Castagnoli, I, 40128 Bologna, Italy, and Istituto di Polarografia ed Elettrochimica Preparativa del C.N.R., C.P. 1075, 35100 Padova, Italy (Received: April 23, 1980)

In melthanolic solution (deaeratedwith argon, the complexes of general formula t-FeH(L)(DPE)z+,where DPE = 1,2-bis(diphenylphosphino)ethane and L = CH3CN,CHSCHZCN, CHzCHCN, C6H&N, and p-CH3C6H4CN, are reduced to FeH(L)(DPEl2by e;. For the reaction FeH(L)(DPE)z++ e; --* FeH(L)(DPE)2,k = (2.8 f 0.3) X 1O1O M-l s-l for all of the compounds. With FeH(CHzCHCN)(DPE)2+ only, a reaction with the radical .CH20H has also been observed. This reaction,which is supposed to proceed via addition of the radical to the coordinated CH,CHCN, leads to the saturation of the double bond. When L = CH3CN, CH3CHzCN, C6H&N, and p-CH3C6H4CN,FeH(L)(DPE)Z rearranges, losing the ligand L to give the already known iron(1) complex, FeH(DPE)z,which in methanol has a t l / 22 1 s. The rate constants for this process have been determined. The complex FeH(p-ClC6H4CN)(DPE)2+ on reacting with e; undergoes the detachment of C1- from the ligand, while the radical intermediate FeH(C6H4CN)(DPE)2+ reacts rapidly with the solvent to give the complex FeH(C6H5CN)(DPE)2+ as final product.

Introduction Recently attention has been focused on the electrochemical behavior of transition metal hydride^.^-^ The hydrido complexes of general formula t-FeH(L)(DPE)z+ (DPE = 1,2-bis(diphenylphosphino)ethane;L = CH,CN, CHZCHCN, C6H5CN,lVz, C6H5N)have been electrochemically reduced in 1,2-dimethoxyethane (DME) solution containing 0.2 .M TBAP to FeH(DPE),5 a species syn0022-3654/80/2084-2985$0 1.OO/O

thesized by Sacco et al.6 in 1974. As part of our investigation on unusual oxidation states obtained by radiolytic methods,' we have undertaken a study on a series of these organonitrile complexes, where L = CHSCN, CH3CH&N, CHZCHCN, C,H,CN, p CH3C6H4CN,and p-CIC6H4CN,by steady-state y and pulse radiolysis. The general purpose of this work was to study the reactions of e;, the main reducing species pro@ 1980 American Chemical Society