J. Phys. Chem. 1991, 95, 7717-7721 ab initio calculations, may shed light on this matter. We also plan to explore the microwave spectra of the two perhalogenated compounds. This investigation may provide rotational constants for a combined microwave-electron diffraction analysis to further refine the parameters. TFCB, not surprisingly, is a very polar species. Measurements of Stark splittings for six M components of two transitions are given in Table V1. These data gave a dipole moment of 3.43 (2) D. The most intense vibrational satellite for TFCB was also assigned using the conventional spectrometer. It had rotational constants of A = 2838.59 (2) MHz, B = 1958.45 (1) MHz, and C = 1954.29 ( I ) MHz. Rough relative intensity measurements indicated that the vibrational frequency was 95 f 20 cm-I. While the data are not completely conclusive, it appears that these transitions had the same spin statistics as the ground state, in-
7717
dicating that this vibration state is probably symmetric, i.e., species A, or Ala
Acknowledgment. This work was supported by the Summer Undergraduate Research Program of the UM Chemistry Department with a fellowship to S.L.M. A.M.A. was the recipient of a Regents fellowship from the University of Michigan (1987-1991). The microwave spectrometers were purchased and supported with funds from National Science Foundation grants awarded to R.L.K. The work at Oberlin was supported by a grant from the Petroleum Research Fund, administered by the American Chemical Society. We appreciate discussions with Professor Kenneth Hedberg. We are grateful to Hidong Kim for preliminary work on TFCB and to Elizabeth A. Dudley and Daniel s. Schullery for their assistance in preparing the two samples of TFCB used in microwave spectroscopy.
Cage Escape Yields in the Quenching of "Ru(bpy)t+ by Methylviologen. Presence of Triethanolamine as a Sacrificial Electron Donor Mikhael Georgopoulos and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 0221 5 (Received: May 29, 1990; In Final Form: May 20, 1991)
Continuous and pulsed laser flash photolysis techniques have been used to determine the efficiency (7,) with which the redox products are released into the bulk upon the oxidative quenching of *Ru(bpy):+ by MV2+ in the absence and presence of TEOA as a sacrificial electron donor in aqueous solution. The value of 7, is diminished as the ionic strength (p) of the solution increases, although the extent of the effect depends on whether or not TEOA is present: in the absence of TEOA, I, = (3.8 + 4.8p1l2)-l; in the presence of 0.1 M TEOA in alkaline solution, 7, = (3.8 + 9.Op1I2)-I. TEOA is an effective scavenger of R ~ ( b p y ) ~at~concentrations + 20.05 M and in the pH 8.5-1 1.5 range. In less alkaline solution, its protonation diminishes its reducing ability; in more alkaline solution, reactions between its degradation products and MV2+yield additional equivalents of MV". The value of 7, is also a function of [ R ~ ( b p y ) ~with ~ + ]a transition at 20-50 pM between upper and lower plateau values.
Introduction The factors that control the efficiency with which redox products are formed in the bulk solution upon the bimolecular electrontransfer quenching of excited states of metal complexes are becoming increasingly known and quantified.' The interpretation of the value of the efficiency of cage escape (a,) is based on the mechanistic model (reactions 1-3)2 in which the diffusional en*M + Q + [M+/-...Q- /+I (1) [M+/-...Q/+] [M+/-...Q/+I
kt4
+M
k,
+Q
M+/- + Q-/+
(2)
(3)
counter of an excited state (*M) by an electron-transfer quencher (Q) results in the formation of a geminate redox pair within the solvent cage; escape of the redox species out of the cage into bulk solution competes kinetically with back electron-transfer within the cage to reform the original ground-state materials. Inasmuch as is equal to k,/(k, + kb(), the problem of understanding the dynamics of the quenching process and the events within the solvent cage becomes a matter of knowing the parameters that govern the values of the two rate constants that ( I ) Tazuke, S.;Kitamura, N.; Kim, H.-B. In Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Dordrecht, 1987; p 87. (2) Balzani, V.; Scandola, F. In Energy Resources through Photochemistry and Catalysis; GrBtzel, M., Ed.; Academic: New York, 1983; p I .
0022-3654/9 1 /2095-77 17$02.50/0
describe the competing intramolecular processes. Unfortunately, the events within the solvent cage occur in the subnanosecond time frame, making reaction 1 the ratedetermining step for the usual concentrations of Q;a direct measure of kbt and k, in the bimolecular quenching process cannot be achieved. However, back electron transfer in photoexcited covalently bonded donor-acceptor molecules has been determined and can be used as a model for reaction 2.3 It is well-known that kbt depends on the exergonicity of reaction 2; kbt describes a "bell-shaped" curve as a function of ACbto of the back electron-transfer reaction, carrying it through the "normal" and "inverted" Marcus regions! As well, kbt depends on the distance between the donor-acceptor centers, the nature of the connecting moieties, and the specific pathway of electron t r a n ~ f e r . ~Values of k, can be calculated from theoretical diffusional equations$ k, is expected to be dependent on the charges on the species, solution ionic strength, solvent dielectric constant and viscosity, temperature, and the extent of the specific interactions among the various species within the solvent cage. For bimolecular quenching reactions under conditions where k , has a constant value, kb,/k, (= a,-1 - 1) also describes the "bell-shaped" curve as a function of AGblO.' (3) Mataga, N . In Photochemical Energy Conuersion; Norris, J. R., Jr.; Meisel, D., Eds.; Elsevier: New York, 1989; p 32. (4) Ohno, T. Prog. React. Kinet. 1986, 14, 219. (5) Closs, G. L.; Miller, J . R. Science 1988, 240, 440, and references therein. (6) Comprehensive Chemical Kinetics; Bamford, C . H., Tipper, C. F. H., Compton, R . G.,Eds.; Elsevier: Amsterdam, 1985.
0 199 1 American Chemical Society
7718 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 In order to investigate the details of reactions 2 and 3, it is desirable to hold as many parameters constant as possible while varying one in a systematic manner. We have chosen here to examine the oxidative quenching of * R ~ ( b p y ) , ~(bpy + = 2,2'bipyridine) by methylviologen (N,N'-dimethyl-4,4'-bipyridinium dication; MV2+). Inasmuch as the photochemistry and photophysics of the system are extremely well-known, these species form the basis of the now-classical model system for the photoreduction of H 2 0 in solar energy conversion schemes; the system also serves as the archetype against which other systems are compared.8 We reported recently on the quantitative dependence of qcc as a function of ionic strength ( p ) in the absence of any additional solutes other than those necessary to control p;9 a plot of (qcc-I - 1 ) vs p1I2was linear with an intercept of 3.1 and a slope of 4.7.1° We also observed a dependence of qce on [MV2+] at low concentrations and low pa9 However, this model system rarely is devoid of additional solutes. For example, in order for a charge carrier (e.g., MV") to accumulate in solution, a sacrificial electron donor must be employed to reduce the oxidized photosensitizer before the rapid electron-transfer reaction between Ru(bpy),,+ and MV'+ occurs in bulk solution. An early choice" of sacrificial donor in the model system was triethanolamine (N(CH2CH20H),; TEOA; pK, of conjugate acid = 7.8),I2J3which possesses a number of properties that makes it attractive for use as a probe of the dependence of qa on solution medium: (1) being a neutral molecule, it does not contribute to the ionic strength of the solution or to ion-pairing of the species; (2) the oxidation of TEOA generates a radical (+*N(CH2CH20H),; TEOA,,'+) that transforms, via H abstraction from the parent molecule, into a reducjng radical ( H O C H 2 C H N ( C H 2 C H 2 0 H ) 2 and/or H O C H C H 2 N (CH2CH20H),; TEOArd'),I4 which, in turn, reduces MV2+, apparently quantitatively without competitive degradation.lS TEOA is now finding increasing use as a sacrificial reductive quencher of Ru(ll)-diimine complexes, the excited states of which are more easily reduced than is * R ~ ( b p y ) , ~ + . ' ~ ~ ~ In this paper we report on our determination of qccas a function of solution medium parameters in the Ru(bpy),Z+/MV2+/TEOA system by the use of continuous and pulsed-laser flash photolysis techniques. Experimental Section
Methylviologen dichloride (Aldrich, recrystallized from water/methanol), Ru(bpy),C12 (G.F. Smith), and TEOA (Fluka) were used. Distilled water was further purified by passage through a Millipore purification train. The pH of the solutions was controlled with IO4 M phosphate buffer; final pH adjustments were made with HCI or NaOH. The ionic strength was adjusted with Na2S04. Continuous photolyses (b,,,, 450 nm) were performed at -22 O C on Ar-purged and magnetically stirred solutions as described before;*' the absorbance of the solution at 605 nm, corresponding to the absorption maximum of MV'+ (em5 = 1.37 X IO4 M-' (7) Ohno, T.; Yoshimura, A.; Mataga, N.; Tazuke, S.; Kawanishi, Y.; Kitsmura, N. J . Phys. Chem. 1989, 93, 3456. (8) Kalyanasundaram, K. Coord. Chem. Rev. 1982,46, 159. (9) Hoffman, M. Z.J . Phys. Chem. 1988, 92, 3458. (IO) Hoffman, M. Z. J . Phys. Chem. 1991, 95, 2606. (11) Lehn, J.-M.; Sauvage, J.-P. N o w . J . Chim. 1977, I, 449. (12) Kalyanasundaram, K.; Kiwi, J.; Grgtzel, M. Helo. Chim. Acta 1978, 61, 2720. (1 3) A pK, value of 8. I has also been cited." Sutin, N . J . Am. (14) Chan, S.-F.; Chou. M.; Creutz, C.; Matsubara, T.; Chem. SOC.1981, 103, 369. (15) Prasad, D. R.;Hoffman, M. 2.J . Am. Chem. Soc. 1986,108,2568. (16) Maidan, R.;Willner, 1. J . Am. Chem. Soc. 1986, 108, 2568. (17) Willner, 1.; Maidan, R.; Mandler, D.; DUrr, H.; DBrr, G.; Zengerle, K. J . Am. Chem. Soc. 1987, 109,6080. (18) Neshvad, G.;Hoffman, M. Z. J . Phys. Chem. 1989, 93, 2445. (19) Neshvad, G.; Hoffman, M. Z.; Mulauani, Q.G.;Venturi, M.; Ciano, M.; D'Angelantonio, M. J . Phys. Chem. 1989, 93, 6080. (20) Sun, H.; Neshvad, G.;Hoffman, M. Z. Mol. Cryst. Liq. Cryst. 1991, 194, 141. (21) Mandal, K.; Hoffman, M. Z. J . Phys. Chem. 1984.88, 5632.
Georgopoulos and Hoffman cm-1),22was monitored as a function of irradiation time. The intensity of the light incident on the cell was determined with the
ferrioxalate actinometer; application of Beer's law for each solution gave the value of the intensity of absorbed light (Ia). The overall quantum yield of MV'+ formation (@(MV'+)) was evaluated from the initial linear portion (110pM MV'+) of the plot of [MV*+] vs irradiation time; @(MV'+) = (d[MV*+]/dr)/I,. Replicate experiments showed @(MV'+) to be reproducible to within
f5-10%. Luminescence quenching experiments were made on airequilibrated solutions of at least four different concentrations of MV2+ with a Perkin-Elmer MPF-2A spectrofluorometer set at 450 nm for excitation and 605 nm for emission. Laser flash photolysis experiments on Ar-purged solutions were made using a Nd:YAG pulsed laser system with excitation at 532 nm; details of the apparatus have been described before.23
Results and Discussion The general mechanism of the R u ( ~ ~ ~ ) , ~ + / M V ~ + / T E O A photochemical system can be described by reactions 4-1 1. Ru(bpy),'+
hu ---*
*Ru(bpy)3'+
2 R ~ ( b p y ) , ~++ hv' *R~(bpy)+ ~ ~MV2+ + A Ru(bpy),'+ + MV'+ R ~ ( b p y ) , ~++ MV'+ 5 R ~ ( b p y ) , ~++ MV2+ *Ru(bpy)32+
R ~ ( b p y ) , ~++ TEOA
A R ~ ( b p y ) , ~++ TEOA,,"
+ MV" 5 TEOA + MV2+ + TEOA -.k. TEOArd' + TEOA + H+ kd TEOArd* + MV2+ MV'+ + products TEOA,,"
TEOA,,"
-
(4) (5)
(6) (7) (8)
(9) (10) (1 1)
All the rate constants in the mechanism are well-known from previous investigations or from routine measurements made in the course of this study. The value of ko ( = l / r o )is 1.65 X IO6 s-l. In the absence of any additional solute and at low [MV2+], k, has a value of -4 X IO8 M-' s-l; as p increases, k, increases, as expected from the charges on the reacting species.24 It has been reported that, in the presence of S0.2 M TEOA, k = 2.9 X lo8 M-I s-l in 0.0067 phosphate buffer at pH 7 and 38 OCF5 The value of k,, ranges from 2.6 X lo9 to 1.1 X 1OIo M-' s-I as p is increased from a low ambient level to 2.4 M, likewise consistent with the charges on the reacting species.26 As expected, k, is a function of pH: 2.4 X los,6.5 X lo6, 4.7 X 10' M-l s-I at pH 5, 7, and 9, respectively;I2 Brown et reported a value of 2 X IO7 M-' s-I at pH 8.1 and p = 0.5 M. Chan et aI.l4 estimated k,, to be of the order of lo9 M-I s-I for the diffusioncontrolled bimolecular reaction between free radicals of the same charge; they also evaluated k,, to be (3.3 f 0.5) X IO6 M-I s-I for the reaction of TEO&+ and unprotonated TEOA. The value of kd has been determined to be 2.7 X IO9 M-' S-I.'~ From the mechanism, cP(MV*+)can be expressed in terms of the efficiencies of the various competing steps, involving the formation and destruction of MV'+, by eq 12. @(MV'+) =
21.9q9celsc9,r
(12)
(22) Watanabe, T.;Honda, K. J . Phys. Chem. 1982,86, 2617. (23) Malba, V.; Jones, G.,11; Poliakoff, E. Photochem. Photobiol. 1985.
42 .-, AS1 .- ..
(24) Hoffman, M.Z.; Bolletta, F.;Moggi, L.; Hug, G. L. J . Phys. Chem. Ref. Data 1989, 18, 219. (25) Okura, 1.; Kim-Thuan, N . J . Chem. Soc.,Faraday Trans. 1 1981, 77, 19% 54, 197.
(27) Brown, G. M.;Chan, S.-F.;Creutz, C.; Schwarz, H. A.; Sutin, N . J . Am. Chem. Soc. 1979, 101, 7638.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7719
Quenching of *Ru( b ~ y ) , ~by + Methylviologen
TABLE I: Rate Constants for the Quenching of *Ru(bpy),'+ by MV*+a PH 4.1
[TEOAJ, M
7.5 11.7
12.0 10.3 10.3
0.1
p,
M
k,
X
IO-*, M-I s-I
b
5.9
b b b b 3.O'
6.3 7.5 6.7
5.5
10.5
a [Ru(bp~)~~ =+ 50 ] pM; air-saturated solutions; = 0.40 ps. *Ambient ionic strength. Ionic strength controlled with Na2SO4.
0*051
0.00
!
0.5
0.0
1.5
1.0
2.5
2.0
P I
5.0
4
5.5
M
Figure 2. Dependence of O(MV'+) on p for solutions containing 50 pM Ru(bpy)?+, and 20 mM MV*+,and 0.1 M TEOA at pH 10.0. 0.40
,
-log [TEOA]
Figure 1. Dependence of O(MV'+) on [TEOA] for solutions containing 50 pM Ru(bpy)? and 20 mM MV2+at pH 10.0; p = 0.06 M.
Here q. is the efficiency of generation of *Ru(bpy)t+ in reaction
om!
4; q, is the efficiency of reaction 6 in competition with reaction
5; qce is the efficiency of escape of the redox pair generated in reaction 6 out of the solvent cage into bulk solution; q , is the efficiency of scavenging reaction 8 in competition with electron transfer reaction 7 between the redox pair in bulk solution; qtr is the efficiency of transformation reaction 7 in competition with electron-transfer reaction 9. The factor of 2 in eq 12 is due to the formation of the second equivalent of MV'+ in reaction 11. The various efficiencies can be written in terms of the rates of the competing reactions and, thus, the rate constants of the reactions and the concentrations of the reactants. By taking qr as unity,28 eq 12 becomes @(MV'+) = 2 ~ ~ vtr. 9 ~ 7 A computer simulation of the mechanism i? with the values of the rate constants given above shows clearly that qsc and qtr approach unity as [TEOA] approaches 0.1 M. Thus, under these limiting conditions in continuous photolysis, @(MV'+) = 2 9 , ~ ~ . Furthermore, under flash photolysis conditions in the presence of 0.1 M TEOA, reactions 8, 10, and 11 occur in the same time frame as reaction 6, and the latter equation holds as well. Therefore, from a measurement of @(MV'+)in continuous or flash photolysis, qce can be obtained directly by knowing qq (=k,[MV2+]/(kq[MVZ+]+ ko)). In flash experiments, qq is easily obtained from the lifetime of *Ru(bpy)t+ in the absence ( 7 0 = l/ko) and presence (70b = I/kob)of MV2+;7, = 1 - (ko/kob). Quenching of *Ru(bpy),Z+ by W+. Linear Stem-Volmer plots (lo/lvs [MV2+])from steady-state spectrofluorometry were obtained for [MVz+]I20 mM. Values of k, as a function of pH, p, and the presence of TEOA, calculated by using rOair = 0.40 ps for * R ~ ( b p y ) , ~as + determined by time-resolved spectrofluorimetry, are given in Table I: the estimated error in the value of k is AS%. Because, as expected, the presence of uncharged T E d A has virtually no effect on k,, values of that quantity at the different ionic strengths used in this study were obtained by interpolation of the available information. Continuous Photolysis. There have been a few values of 9(MV") reported in the literature for the Ru(bpy)t+/MV2+/ TEOA photochemical system in aqueous solution; they are of the (28) Demas, J. N.;.Taylor, D.G. Inorg. Chem. 1979, 18, 3177. (29) DAngelantonio. M. Personal communicatbn.
I
6
I
I
7
I
I
8
,
I
9
I
I
10
I
,
11
I
I
12
, 4
13
PH Figure 3. Dependence of @(MV+) on pH for solutions containing 50 pM Ru(bpy)?+, and 20 mM MV2+,and 0.1 M TEOA; p = 0.06-0.16 M.
order of 0.194.25.3*34 For comparison, @(MV'+) is 0.33 for q, = 0.75 in a~etonitrile.~' Figure 1 shows the dependence of @(MV'+)on [TEOA] (-log + 20 mM [TEOA]) for solutions containing 50 pM R ~ ( b p y ) , ~and MVZ+at pH 10.0; the ionic strength was ambient and constant at 0.06 M. It is clear that @(MV'+) is constant at 0.20 for [TEOA] = 0.054.5M, indicating that the relevant efficiencies, especially qa, are constant in that concentration range; this conclusion is supported by the computer simulation.29 The decrease in @(MV'+)as [TEOA] is decreased can be attributed to a diminution in values of qa and qtr. In all the further work reported here, the concentration of TEOA was held a t 0.1 M. The value of @(MV'+) decreases as p is increasded. Figure 2 shows the dependence of iP(MV'+) on p for solutions containg 50 pM Ru(bpy),Z+, 20 mM MV2+, and 0.1 M TEOA at pH 10.0. The variation of @(MI''+) of more than a factor of 2 over the wide range of p is significant; because ?q does not change very much (0.884.96) over that range, the variation in @(MV'+) must result from the dependence of qcCon p. The depencence of @(MV'+) on pH was determined for solutions containing 50 pM Ru(bpy)32+,20 mM MV2+, and 0.1 M TEOA at ambient (0.064.16 M) ionic strengths; plateau values are observed (Figure 3) in the pH 8.5-1 1.5 range. As expected, @(MV'+) falls off in less alkaline solution as q, and qtr diminsh due to the ineffectiveness of the protonated form of TEOA as a scavenger for Ru(bpy)3Z+and TEO&/+. Unexpected, however, (30) (31)
Kitamura, N.; Kawanishi, Y.;Tazuke, S.Chem. Leu. 1983. 1185. Kawanishi, Y.; Kitamura. N.; Kim, Y.; Tazuke. S.Sci. Pap. Insr.
Phys. Chem. Res. (Jpn.) 1984, 78, 212. (32) Crutchley, R.J.; Lever, A. B. P.J . Am. Chem. Soc. 1990, 102,7128. (33) Crutchley, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2276. (34) Creutz, C.; Keller. A. D.; Sutin, N.; Zipp, A. P.J . Am. Chem. Soc. 1982. 104, 3618. (35) Kawanishi, Y.; Kitamura. N.; Tazuke, S. Inorg. Chem. 1989, 28, 2968.
Georgopoulos and Hoffman
7720 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
TABLE 11: Values of *(MV'+)in tbe Absence and Presence of TEOA from Flasb Photolysis Experiments@ P~~ M
% @( MV'+) [TEOA] = 0.0 M 0.23 0.053 0.45 0.073 0.52 0.080 0.55 0.07 1 0.62 0.063
0.006c 0.2 0.4 0.7 1.5
[TEOA] = 0.1 M 0.23 0.10 0.45 0.1 1 0.52 0.1 1 0.55 0.099 0.62 0.084
0.006c 0.2 0.00
!
3.5
4.0
4.5
I
0.4
0.7 1.5
5.0
-log [RU(bPY)32+3 Figure 4. Dependence of @(MV'+) on [Ru(bpy)?+] for solutions containing 20 mM MV2+ and 0.1 M TEOA at pH 10.0; p = 0.06 M.
?cs
0.23 0.16 0.15
0.13 0.10 0.22 0.12 0.11 0.090 0.067
@[R~(bpy)= ~ ~50 + ]pM; [MV2+] = 2.0 mM; pH 10.0; Ar-purged solutions. Ionic strength controlled with Na2S04except when ambient. cAmbient ionic strength.
is the sharp rise in a(MV'+) as the pH passes 11.5. At this alkalinity under these concentration conditions, the OH--induced of the solution (PAf)after the completion of reaction 6, but before thermal reduction of MV2+36was not observed in the dark for reaction 7 had occurred to any appreciable extent, is still less than blank experiments on solutions containing 2 and 20 mM MV2+. that before the flash; PAf = (e3 + c, - e2)A[MV'+]I. The yield In contrast, we found that the yield of MV" from the 355-nm of MV" at 605 nm could also be monitored without any interlaser flash photolysis of solutions containing 50 pM R ~ ( b p y ) ~ ~ + , ference from the other species. The values of a(MV'+) from 2 mM MV2+,and 0.1 M TEOA was the same at pH 9.6 and 12.2. reactions 4-6 were exactly the same regardless of the wavelength It should also be noted that @(MV'+)is unchanged up to pH 12.3 monitored. In the absence of TEOA, where reactions 8-1 1 are in continuous photolysis in the presence of 0.1 M EDTA. We inoperative, a(MV'+) = qqq,. conclude that the sharp rise in a(MV'+) beyond pH 11.5 in the Because of the irreversible nature of the system in the presence continuous photolysis experiments in the presence of TEOA is of 0.1 M TEOA, only one laser pulse could be used,multiple pulses due to secondary thermal reactions between MV2+ and the caused the formation of MV'+ to be permanent, resulting in a products of the oxidation of TEOA,&' that yield MV'+. continuous change in the baseline absorbance. Furthermore, We had reported earlier21~37 the interesting result that a(MV'+) because the change in absorbance relative to the initial baseline in the R u ( ~ ~ ~ ) ~ ~ + / M V ~ +photochemical /EDTA system is a at 450 nm is small, the precision from a single laser pulse is rather function of [ R ~ ( b p y ) ~ ~We + ] .attributed the phenomenon to the poor. However, the use of the absorption of MV'+ at 605 nm gave presence and excitation of ion-paired aggregates of the photovery reliable and reproducible (&lo%) results. Here, PAf = sensitizer with EDTA, resulting in an increase in q, with inc,A[MV'+]I, inasmuch as the other species do not absorb at that creasing [ R ~ ( b p y ) ~ ~To + ] .test this functionality for TEOA, we wavelength. determined @(MV'+) for solutions containing 20 mM MV2+ and Since A [ M V * + ] / A [ * R U ( ~ ~ ~=) ~a(MV'+), ~+] that quantity 0.1 M TEOA a t pH 10.0 (Figure 4); in order to maximize the can be easily calculated from the experimental values of AAf/AAo quantum yield and minimize the concentrations of anionic species, in the flash photolysis in the absence and presence of TEOA. The ambient p (0.06 M) was used. Clearly, the dependence of aresults are given in Table 11. (MV") on [Ru(bpy),2+] is not limited to solutions containing Cage Escape Yields. From the values of a(MV'+) obtained EDTA. The dependence of quantum yield on the concentration in continuous and flash photolysis experiments, q, can be calof the photosensitizer, as mediated by species in the solution, may culated for the various solution medium conditions; values of qbe a general phenomenon that has been relatively unexplored. for the flash experiments are given in Table 11. The data show Flpsh Photolysis. Excitation of solutions of Ru(bpy),2+ resulted that the values of q , in the absence and presence of TEOA are in the formation of *Ru(bpy),2+ and the bleaching of the ground the same at low ionic strength, but deviate with increasing p; a t state. The first-order decay of the excited-state luminescence or p = 1.5 M, q, in the presence of 0.1 M TEOA is lower than in the ground-state bleaching in the absence of MV2+yields the value its absence by an extent that is outside of experimental error. of ko. The extent of bleaching a t 450 nm (PAo)as a result of For comparison purposes, it is useful to plot (q,-l - 1) vs p1I2 the laser pulse reflects A[*Ru(bpy),2+] (= -A[Ru(bpy),2+]) and (Figure 5) from the flash (Table 11) and continuous photolysis the difference in the molar absorptivities of the states; c2 - e. = (Figure 2) results; the error bars represent an uncertainty of &IO% -9.8 X IO3 M-' cm-' 38 Extrapolation of the recovery of the in the experimental value of qw The flash photolysis data in the 450-nm absorption to the midpoint of the laser pulse yields a value absence of TEOA (A) can be fitted to a good straight line with of M0(=(e2 - e.)A[*Ru(bpy)t+]l, where I = optical path length). an intercept of 2.8 and slope of 4.8; the data in the presence of In the presence of MV2+, the decay of the excited state and TEOA (0) yields an intercept of 2.8 and a slope of 9.0. The recovery of the ground state occur more rapidly due to reaction continuous photolysis data (a) cluster around that latter line, 6, yielding kob;extrapolation of the 450-nm absorption gives the indicating the validity of the treatment. same value of AAo. Because of the reversible nature of the system, The first point to note is that the presence of 0.1 M TEOA, the results of many flashes (-50) on the same solution could be in the limit of p 0, has no discernible effect on 7., As would averaged, resulting in highly precise data. be expected from the equations that describe the cage escape The result of reaction 6 is the formation of R ~ ( b p y ) ~and ~+ process,4' if the charges on the diffusing species, the encounter MV"; the loss of an equivalent concentration of R ~ ( b p y ) , ~ + distance, bulk viscosity, and bulk dielectric constant are unchanged remains. Because of the values of the molar absorptivities of the by the presence of that solute, k, would be unaffected. It is also species involved at 450 nm (e2, e3, and e, = 1.46 X IO4, 1.5 X clear that, if all the solution medium parameters were the same IO3, and 8.0 X IO2 M-' cm-I, r e s p e c t i ~ e l y )the , ~ ~absorbance ~~ in the absence or presence of TEOA, the slopes of the lines in
-
(36) Novakovic, V.; Hoffman, M. Z . J. Am. Chem. SOC.1987,109,2346. (37) Mandal, K.;Prasad. D.R.; Hoffman, M. 2.Coord. Chem. Reu. 1985, 64. 175. (38) Ohno, T.; Yoshimura, A.; Prasad, D.R.;Hoffman, M. Z. J . f'hys. Chem. 1991, 95, 4723.
(39) Watts, R. J. J. Chem. Educ. 1983, 60, 834. (40) Summers, L. A. The Bipyridinium Herbicides; Academic: London,
-.
1980 D r 1 1 1.
(41) Chiroboli, C.; Indelli, M. T.;Rampi J . f'hys. Chem. 1988, 92, 156.
Scandola, M. A,; Scandola, F.
7721
J . Phys. Chem. 1991, 95, 7721-7726
20
1
O !
0.0
0.1
1.0 p1/2
1.I
2.0
Figure 5. (?=-I - 1) vs pl/z. Conditions: 50 pM R~(bpy),~+, 20 mM MV2+,0.1 M TEOA, pH 10.0, continuous photolysis (0):50 pM Ru(bp~),~+, 2 mM MV2+,0.1 M TEOA, pH 10.0, flash photolysis (0): 50 pM Ru(bpy)32+,2 mM MV2+, pH 10.0 (A),flash photolysis.
Figure 5 as a function of ionic strength would be the same, and all the data would be superimposable. The fact that the slopes are different indicates that the presence of TEOA has an effect on the nature of the species that are within, or that make up the solvent cage, and that the effect is manifested as p is increased. The net result is that k,, which decreases, in general, as p is increased for the escape of like-charged species into bulk solution, has an even lower value in the presence of 0.1 M TEOA than in its absence at those higher ionic strengths. It would appear that the combination of TEOA and an ionic medium is effective in retarding, albeit to a limit extent, the rate of cage escape of the redox pair. The results suggest that even neutral species that would normally be viewed as "innocent" solutes can cause subtle variations in the structure of the solvent cage and the value of qcc. From the data in Figure 4 for 0.1 M TEOA at p = 0.06 M, we see that 7, has a limiting upper value of -0.1 at [Ru(bpy):+]
1 50 pM,and a lower plateau value of -0.06 at I20 pM. Although the magnitude of the effect and the concentration range over which it occurs is somewhat different than was observed with 0.1 M EDTA at pH 11.0, which was attributed to the presence of R~(bpy)~+...EDTA~-...Ru(bpy)~~+ ion-paired aggregated specie^,^^*^' the behavior with TEOA follows the same general pattern. It is difficult to visualize how TEOA could mediate the formation of aggregates of the photosensitizer in the same way that EDTA has been proposed to do. We must conclude that it is CI-, the counterion of both R ~ ( b p y ) ~and ~ +MV2+ present at a concentration of 40 mM in these experiments a t ambient ionic strength, that causes the effect by mediating the ion-paired aggregation of the photosensitizer. As [ R ~ ( b p y ) ~ is~ +increased ] across the 20-50 pM regime, the formation of ion-paired aggregates could be enhanced, thereby increasing the electrostatic repulsion of the geminate pair, and the values of k, amd v,. Conchrsiorw TEOA can be used as an effective sacrificial donor in the Ru(bpy):+/MV2+ model photochemical system at 10.05 M; at lower concentrations, its efficiencies of scavenging Ru(bpy)?+ and its oxidized radical are less than unity. At 0.1 M, TEOA is effective in the pH 8.5-11.5 range; in less alkaline solution, its protonation diminishes its ability to act as a scavenger for the same species. In more alkaline solution, secondary processes between MV2+ and the products of the TEOA degradation reactions yield additional equivalents of M V . For [ R ~ ( b p y ) ~ ~ + ] = 50 pM in the absence of TEOA, the dependence of ,7 on p in the quenching of *Ru(bpy):+ by MV2+ is given by the following expression: ,7 = (3.8 4.8pl/2)-l. In the presence of 0.1 M TEOA at pH 10.0, = (3.8 + 9.0p'/2)-'.
+
Acknowledgment. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy. We thank Dr. M. DAngelantonio (Istituto FRAE-CNR, Bologna, Italy) for carrying out the computer simulation.
Formation of Thermodynamically Stable Dications in the Gas Phase by Thermal Ion-Molecule Reactions: Ta2+ and Zr2+ with Small Alkanes Yasmin A. Ranasinghe, Timothy J. MacMahon,t and Ben S. Freiser* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: August 3, 1990; In Final Form: May 9, 1991)
The reactions of doubly charged tantalum and singly and doubly charged zirconium with small linear alkanes are reported. The rate constants for the reactions of singly and doubly charged Zr and Ta with methane are also reported. Ta" reacts with methane formally by carbene and hydride abstraction along with some charge transfer. Reactions of Ta2+with longer chain alkanes result in complete charge transfer. Zr2+ also reacts with methane formally by carbene and hydride abstraction and undergoes dehydrogenation and demethanation reactions and hydride and methide abstraction in addition to charge transfer with propane and butane. With alkanes larger than butane only charge transfer is observed. Zr+ reacts similarly to other early transition metals to multiply dehydrogenate alkanes and, to a lesser extent, yield C-C cleavage fragments.
Introduction Although the existence of multiply charged ions has been known since the early days of mass spectrometry, their low abundances relative to singly charged ions, as well as interferences from isobaric singly charged ions, have inhibited their thorough investigation. In the past couple of decades, however, there have been significant advances in the instrumentation and methodology for studying multiply charged ions, particularly involving sector instruments, together with a concomitant increase in theoretical treatments of these interesting species.' 'Current address: IBM, Hopewell Junction, NY.
0022-3654/9 1/2095-7721$02.50/0
One of the fascinating aspects of the work is the origins of the stability of multiply charged ions of very small molecules. For example, it is surprising that even triatomic trications such as CS23+have been observed to be stable on the microsecond time scale despite the expected Coulombic repulsion between positive (1) (a) Ast, T. Ado. Muss Spectrom. 1980, A8, 555. (b) Koch, W.; Maquin, F.: Stahl, D.; Schwarz, H. Chimiu 1985, 39, 376. (c) Koch,W.; Heinrich, N.; Schwarz. H.; Maquin, F.; Stahl, D. Int. J . Muss Spcfrom. Ion Processes 1985.67, 305. (d) Cooks, R. G.; Ast, T.; Beynon, J. H. Inr. J. Mass Spectrom. Ion Phys. 1973, 11,490. (e) Guilhaus, M.; Brenton, A. G.; Beynon, J . H.; Rabrenovic. M.; Schleyer, P. v. R. J . Chem. Soc., Chem. Commun. 1985, 210. (0 Drewello, T.; Schwarz, H. Int. J . Muss Specrrom. Ion Processes 1989, 87, 135.
0 1991 American Chemical Society
