J. Phys. Chem. 1987, 91, 1649-1655
1649
Factors Affecting Cage Escape Yields following Electron-Transfer Quenching John Olmsted III* Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92634
and Thomas J. Meyer Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 2751 4 (Received: July 14, 1986; In Final Form: November 3, 1986)
The factors influencing the escape of donor-acceptor charge-transfer pairs from their solvent cage have been examined, using methylviologen (MV") as the acceptor and several different excited metal complexes and aromatic organic molecules as donors. We determine ratios of cage escape yields by measuring the amount of transient absorption due to MV'+ radical cation in the absence and in the presence of an anthracene derivativeas an energy shuttle, under conditionswhere no irreversible photochemistry takes place. For a series of different Ru and Os complexes, the cage escape yield of the [M3+,MV'+] pair varies only slightly, ranging from 0.14 to 0.27. When the donor is an organic triplet excited state (9-methylanthracene or acridine yellow), the cage escape yield is near unity. Perturbation by heavy atoms reduces the yield to 0.3 (9-bromoanthracene in solution containing CH,I). Our results show that the cage escape yield is strongly affected by the rate of triplet-singlet interconversion of the triplet charge pair generated in the quenching event. When no mechanism for triplet-singlet mixing is present (organic donors), back-electron-transfer, which requires a spin change, is slow compared to diffusion out of the cage. When spin-orbit coupling is substantial (heavy-atom perturbation, transition-metal complexes), back-electron-transfer becomes competitive with diffusional cage escape.
Introduction Light-induced charge separation plays a key role in photosynthesis and is also a prominent feature in the operation of photoredox schemes for the chemical storage of solar energy. The most thoroughly studied sensitizer/quencher combination in such schemes utilizes R ~ ( b p y ) , ~(bpy + is 2,2'-bipyridine) as a sensitizer and methylviologen (MV2+; paraquat, 4,4'-bis(methyL2,2'-bipyridinium) dication) as an electron relay.' In such schemes, the excited sensitizer is oxidized by the viologen in a one-electron charge-transfer reaction R ~ ( b p y ) , ~ ++* MV2+
-
Ru(bpy),,+
+ MV"
and the viologen radical cation (MV") transfers an electron to a second, catalytic site for dihydrogen production. In systems that have been operated cyclically thus far, a sacrificial donor such as EDTA or triethanolamine is used to reduce the sensitizer back to Ru(I1). Although the energetics of the R U ( ~ ~ ~ ) , ~ + - system M V ~ +are quite favorable for hydrogen reduction using visible light, the overall quantum efficiency of MV" production sharply restricts the yield of dihydrogen. Even when viologen concentrations are sufficiently high to quench the excited state of the sensitizer with The cause near-unit efficiency, MV" yields are 0.25 or smaller." of these low yields is the rapid rate of back-electron-transfer within the solvent cage to regenerate ground-state starting materials and dissipate the excitation energy as heat. This back-transfer competes kinetically with diffusion of the ion pair out of the solvent cage
-
-
[ R U ( ~ ~ ~ ) , ~ + , M V[R~(bpy),~+,MV'+l ~+] Ru(bpy),,+
+ MV'+
( 1 ) For a review, see: Kalyanasundaram, K. Cmrd. Chem. Rev. 1982, 46, 159. ( 2 ) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helv. Chim.Acta, 1978, 61, 2720. (3) Maestri, M.; Sandrini, D. Nouv. J . Chim. 1981, 5, 637. (4) Chan, S.-F.; Chou, M.; Creutz, C.; Matsuhara, T.; Sutin, N. J . Am. Chem. SOC.1981, 103, 369. ( 5 ) Kalyanasundaram, K.; Neuman-Spallart, M. Chem. Phys. Lett. 1982, 88, 7. (6) Mandal, K.; Hoffman, M. 2. J . Phys. Chem. 1984, 88, 185
0022-3654/87/2091-1649$01.50/0
and the relative rates of these processes are such that the escape yield is small. Not long ago, Johansen, Sasse, and Mau discovered that the intrinsic inefficiency of separation of the [R~(bpy),~+,MV'+l pair could be overcome by including in the reaction scheme an energy-transfer relay in the form of anthracene-9-carboxylate anion (AnCOz-).7$8 At appropriate concentrations, the ruthenium sensitizer is efficiently quenched by AnC02- in a triplet-triplet energy-transfer process which is 100% efficient. The ,AnCO; excited state is, in turn, charge-transfer quenched with unit efficiency by viologen, yielding AnCOZ' and MV", which escape from their cage with 100% efficiency. The anthracene radical is recycled by a sacrificial donor, making the net chemistry identical with that occurring in the absence of the energy-transfer relay. Johansen et al. reported dihydrogen quantum yields as high as 0.85 from this hybrid system.8 Since that first report, Edel, Mamot, and Sauvage have achieved similar yield enhancements using AnCOZ-as an energy relay with a copper complex as the sensitizer: and Sasse's group has reported that A n C 0 2 enhances the dihydrogen production yield by about eightfold when the sensitizer is Eosin-Y, a xanthene-type organic dye.'O The factors determining cage escape yields for the sensitizerMVZ+system are not yet well understood. While an increase in the cage escape yield of MV" has been identified as the reason for the enhancement, the mechanistic basis for this enhanced escape probability has not been definitively identified. It has been noted by Chan et al. that escape yields of even as high as 0.25 are surprising in light of the exothermicity of the back-reaction and the rapidity of the quenching reaction^.^ They speculated that either a Marcus inverted region effect or the nonadiabaticity of the back-reaction might decrease recombination rates. Juris et al. suggested that, since experiments in fluid solution fail to show the predicted slowing of rate with increased exothermicity, nonadiabaticity was the more likely explanation." To our (7) Johansen, 0.;Mau, A. W.-H.; Sasse, W. H. F. Chem. Phys. Lett. 1983, 94, 107. ( 8 ) Johansen, 0.;Mau, A. W.-H.; Sasse, W. H. F. Chem. Phys. Lett. 1983, 94, 113. (9) Edel, A.; Marnot, P. A,; Sauvage, J. P. Nouv. J . Chim. 1984,8, 495. (10) Mau, A. W.-H.; Johansen, 0.;Sasse, W. H. F. Photochem. Photobiol. 1985, 41, 503.
0 1987 American Chemical Society
1650 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
knowledge, these suggestions have not been experimentally tested, and the reasons for low (or high) cage escape yields of MV" remain undetermined. In contrast to this state of uncertainty, an explanation for varing yields of charge-transfer products in porphyrin-benzoquinone systems has been simply but eloquently proposed by Gouterman and Holten12and experimentally supported by Harriman, Porter, and Wi10wska.l~ Singlet-state excitation of porphyrins leads to negligible production of benzoquinone radical anion, while triplet-state excitation can give a separated ion products with quantum yields as high as 0.25. Spin multiplicity is the determinant of this large difference of yields between singlet-state and triplet-state precursors, according to Gouterman and Holten. The ion pair formed when the singlet excited state is quenched is itself a singlet; hence back-electron-transfer to yield ground states is fully spinallowed and rapid. In contrast, quenching of the triplet state gives a triplet ion pair which must undergo a spin change either before or during back-electron-transfer. This spin change is sufficiently forbidden to reduce the back-electron-transfer rate constant until it and the rate constant for diffusion out of the solvent cage are comparable in magnitude. Substantial cage-escape yields in the face of high exothermicities for back-electron-transfer can arise from at least three different factors operating to reduce the rate of the back-transfer: (1) the very high exothermicity of the back-reaction, which places it within the Marcus inverted or excited-state decay region; (2) the formal triplet nature of the charge-transfer pair, which introduces a spin-forbiddenness into the back-reaction; (3) other sources of restricted coupling between the initial and final states nonadiabaticity, such as poor vibrationally induced overlap of the donor-acceptor electronic wave functions. To test the roles of these factors, we have carried out a number of determinations of comparative cage escape yields for various excited donor-viologen acceptor pairs. The results, which are presented here, show that spin forbiddenness can be a major impediment to back-electrontransfer within this cage.
Experimental Section Chemicals. The complexes, and the PF6- salt of methylviologen, were kindly provided by S. F. McClanahan. The other transition-metal complexes had been synthesized, purified, and characterized earlier as reported in the following references: Os complexes, ref 14 and 15, [R~(bpy)~(bpyz)]~+, ref 16. Anthracene compounds were purchased from Aldrich. Anthracene (99.9% pure) was used as received, while the 9-substituted anthracenes were recrystallized from ethanol prior to use. Tetraethylammonium perchlorate (TEAP) was recrystallized twice prior to use. Acetonitrile and methylene chloride were B & J Brand high-purity solvents, and methyl iodide was from MCB. Apparatus. Conventional flash photolyses were carried out using a Xenon Corp. flashlamp (fwhm = 20 p ) driven by Kilovolt Corp. high-voltage power supply. The flashlamp output was filtered through a Coming 3-73 color flter to eliminate wavelengths shorter than 420 nm. Transient absorptions were monitored by using a tungsten lamp, Bausch and Lomb high-intensity monochromator, and photomultiplier tube; the signal was observed with a Tektronix 7623A storage oscilloscope. Fluorescence quenching was measured with a S L M Instruments, Inc. single-photoncounting spectrofluorimeter. Luminescence lifetimes were determined with a Molectron tunable dye laser and a Tektronix 79 12AD programmable digitizer. Data were computer fitted to single-exponential decays. ( 1 1) Juris, A.; Balzani, v.;Belser, P.;yon Zelewsky, A. Helv. Chim. Acta 1981, 64, 2175. (12) Gouterman, M.; Holten, D. Photochem. Phorobiol. 1977, 25, 85. (13) Harriman, A,; Porter, G.; Wilowska, A. J . Chem. Soc., Faraday Trans. 2 1983, 79, 807.
(14) Kober, E. M. Ph.D. Dissertation, University of North Carolina, Chapel Hill, 1982. (15) Casuar. J. v. Ph.D. Dissertation. University of North Carolina. Chapel Hill; 1982. (16) Conrad, D.;Allen, G. H.; Rillema, D. P.;Meyer, T. J. Inorg. Chem. 1983, 22, 1614.
Olmsted and Meyer
Solutions. Most of the results reported here were obtained in solutions containing 8 parts by volume 0.1 M TEAP in acetonitrile acidified with 70% HC104 (approximately 2 drops acid/lO mL of solution) and 5 parts by volume methylene chloride or methyl iodide. Enough sensitizer was dissolved in these composite solvents to give optical densities (1 cm path length) of 0.1-0.5 at A,., Solutions were thoroughly purged with solvent-saturated argon just prior to each measurement and were maintained under an argon blanket during the measurements. Comparison of results obtained after repeated freeze-pump-thaw cycles showed no difference between the two purging techniques. Quencher concentrations were varied by microsyringe injection of appropriate volumes of 20 mM solutions in acetonitrile (viologen) or methylene chloride (anthracenes). Each injection was followed by thorough mixing and argon purging. The choice of solvent was dictated by the following considerations. (1) Solubilities: the anthracene compounds were insufficiently soluble in water or in acetonitrile alone, while methylviologen was insufficiently soluble in solutions that were less than 60% acetonitrile. (2) Reversibility of redox reactions: In alcoholic solvents, the oxidative products were irreversibly degraded by the solvent, leading to persistent buildup of viologen radical cation. This was also a problem in even the most carefully purified acetonitrile unless a small amount of acid was included. In the presence of acid, no net photochemistry occurred. (3) Prevention of side effects: TEAP (0.1 M in acetonitrile, 6 1 mM overall) was included to stabilize the ionic strength of the solutions. Argon purging removed oxygen, which rapidly oxidizes methylviologen radical cation. Sacrificial donors were deliberately excluded to avoid complications arising from their outer-sphere complexation with methylviologen or the cationic complexes.'' Kinetic Analysis. The cage escape results which we present here have been obtained by using the enhancement in viologen radical cation formation that occurs when an anthracene derivative is introduced as an energy shuttle. In an anthracene-free system, events following photonic excitation of a sensitizer, for example S = Ru(bpy),2+, are described by the kinetic scheme shown in Scheme I. SCHEME I S* S* S*
-
+
+Q
S
S + hu
+ heat
-
k,
(1)
k,,
(2)
[S+,Q-]
k,
In the presence of an oxidative quencher such as MV2+, bimolecular quenching (k,) to give a donor-acceptor charge-transfer ion pair [S+,Q-]competes with unimolecular decay of the excited state, S*, by radiative (k,) and nonradiative (k",)pathways. It is assumed, as is appropriate here, that back-electron-transfer to regenerate S* is unimportant and diffusional effects are incorporated into k,. The ion pair both diffuses apart to give solvent-separated, independent ions (k& and undergoes backelectron-transfer within the solvent cage (kbc).Free ions eventually diffuse together and undergo back-reaction (kbd).In the absence of sacrificial donors or adventitious reactive impurities, there is no net chemical change following absorption of a photon. Our particular interest in these studies-is the iield of cage escape, which is dependent on the competing decay processes for the geminate pair: Yce
= k-d/(k-d +
kbc)
(7)
When anthracene (or a substituted anthracene) is added at relatively high concentration, the reactions of Scheme I are re(17) Prasad, D. R.; Hoffman, M. Z. J . Phys. Chem. 1984, 88, 5660.
Cage Escape Yields following Electron-Transfer Quenching
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1651
0
5
10
1/[Mv2+], m \
-'
15
Figure 2. Dependence of MV" production on MV2+ concentration, at constant Ru(bpy)32+sensitizer concentration.
Tal,mM Figure 1. Stern-Volmer quenching of Ru(bpy)?+ luminescence in mixed CH3CN/CH2Cl2(8:s v/v) solvent. Quenchers were 9-bromoanthracene (X), 9-methylanthracene(O), and methylviologen (+).
placed by reactions in which the anthracene acts as an energy relay (Scheme 11). SCHEME I1 S*
+ - + + -- ++
+ An
3An 3An
S
An
Q
heat
+ QAn+ + S
An+
k,, k,,;
3[An+,Q-]
3[An+,Q-] 3[An+,Q-]
3An
An
Q
An+
Q-
-
+Q An + S+ An
(8) (9)
k,'
(10)
kk'
(11)
k4'
(12)
kbd'
(13)
k,,
(14)
The initial, energy-transfer quenching step (ket) transfers the excitation energy from S to An, making 3An the reactive reductant; it can decay unimolecularly by intersystem crossing (k,,:) to the ground state. The 3An-Q quenching dynamics, which exactly parallel the S*-Q dynamics, have rate constants designated with primes ( I ) and a cage escape yield, ycl, defined by yc: = k-l/(k-d
+ kb')
(15)
When anthracene radical cations have escaped the solvent cage, they may capture an electron from the ground state of the original sensitizer (kCJ or may back-react with Q-(kM'). If the electron-capture reaction is exothermic, it will predominate since [SI >> [Q-1. Under these conditions, the long-lived intermediates Qand S+ appear in the presence and absence of anthracene. However, their mode of generation is quite different being based on Scheme I when anthracene is absent and by I1 when anthracene is present in sufficient quantities to quench the excited state. Under appropriate conditions, 3An produces Q- with unit efficiency. There are two steps that determine that efficiency. The first is quenching of 3An with efficiency y4/= k,'[Q]/k,'[Q] + k n i . The second is cage escape with efficiency y,:. Johansen et al. have shown that yce' = 1 for anthracenecarboxylate,' where the geminate pair is neutral AnC0; and cationic M V + . For other anthracene compounds, the charge repulsion between An" and MV" can only enhance cage escape; thus y$ is always near unity. The quenching yield y4/can be made unity at sufficiently high [Q]. For 3An, k,' is so small (- 1 s-I) that loss of 3An is primarily by adventitious quenching by impurities, especially oxygen. Assuming impurity levels at the micromolar level, [Q] must be M for y4/to be unity.
Results Stern-Volmer Quenching. In the presence of viologen (MV2+), R ~ ( b p y ) ~ and ~ + *related MLCT-based chromophores undergo excited-state oxidative quenching via reactions 1-3 of Scheme I, and with the addition of an anthracene compound (An), quenching also occurs via reaction 8 of Scheme 11. In either case, the emission intensity, Z (reaction l), should be reduced in the presence of an added quencher as described by eq 16, the Stern-Volmer relation.ls ZO/I(Q) = 1 + kq[Ql/(kr + knr) = 1 + Ksv[QI (16) Here, Q represents either a viologen or an anthracene compound. Plots of Zo/Z(Q) vs. [Q] for all the systems reported here were linear with an intercept value of 1, as predicted by eq 16. Typical data for R ~ ( b p y ) ~quenched ~+ by methylviologen, 9-methylanthracene, and 9-bromoanthracene are shown in Figure 1. We have also measured the lifetime decrease caused by addition of these quenchers. The lifetime ratios T,/T fall on the same straight line as the intensity ratios Zo/Z, showing that only dynamic quenching is occurring. All of the quenchers that we have studied have Stern-Volmer quenching constants in the range of several mM-', which when combined with excited-state lifetimes for these complexes (ca. 1 ps) yield bimolecular quenching constants in the range of (1-6) X lo9 M-I s-' , a s one would expect for quenching reactions occurring near the diffusion-controlled limit. The quenching reactions generate a photoproduct as well as consuming the emitting sensitizer. The concentration of photoproduct generated by the flash depends on the kinetic parameters as [MV"],
= nyc,kq[MV2+]/(k,[MV2+] + k ,
+ knr) (17)
where n is moles of photons absorbed per liter. When inverted, eq 17 takes the form l/[MV'+], = a b/[MV2+] (18)
+
When we measure the transient absorption of MV" at 610 nm immediately after flash excitation, we find that it varies with concentration of MV2+ in the manner predicted by eq 18. A set of typical data, giving the change in [MV"] as [MV2+]was varied in a solution containing R ~ ( b p y ) ~ is ~ +shown , in Figure 2. Furthermore, when R ~ ( b p y ) ~ is ~ +quenched * by anthracene or 9-methylanthracene in thoroughly degassed solution, we also can observe transient triplet-triplet absorption at 425 nm. In the presence of methylviologen, this transient is replaced by a new transient absorption at 690 nm (9-MeAn) or 720 nm (An) which is associated with the anthracene radical cations. All of these observations are consistent with kinetic Schemes I and 11. Cage Escape Ratios. When a solution of sensitizer containing enough MV2+ (ca. 1 mM) to quench ca 70% of the excited states (18) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings; Menlo Park, CA, 1978; pp 247-5 1.
1652 The Journal of Physical Chemistry, Vol, 91, No. 6, 1987 I
I
I
l a ]
Olmsted and Meyer TABLE I: Cage Escape Yields for Viologen-Transition-Metal Sensitizer-Quencher Pairs
sensitizera Ru(~PY)~~+ Ru(bPY)3'+ Ru(~PY)~~+
f
' ' '
R ~ ( d m p h )e~ ~ + O~(bpy)~(das j2+' O~(phen)~(dppm)~+g Os(phen)2(dmpph2+*
Ibi
t,ms
is flashed, a transient absorption due to MV" is observed at 610 nm. The magnitude of the absorption produced by this transient immediately after the flash measures the concentration of MV" produced by the flash. Addition of an anthracene compound at a concentration sufficient to quench the sensitizer competitively results in a dramatic increase in the transient MV" signal. This effect is illustrated in Figure 3. The enhancement in signal upon addition of an anthracene can be analyzed to give the ratio of cage escape yields for the [S3+,MV'+] and [An'+,MV'+] ion pairs. That analysis proceeds as follows. The optical density change observed at 610 nm immediately after the flash, A(OD)o, is directly proportional to the concentration of M V , which in turn can be related to the concentration of excited sensitizer, S*o, generated by the flash: ~b[MV'+lo= ~ b f ~ ~ ~ ~ [ S * ] o(19)
Here, E and b are extinction coefficient and path length, fq is the fraction of S* quenched by MV2+,and ye is the cage escape yield (the fraction of quenching that leads to separated products). When anthracene is added to the system, a new optical density change, A(OD)o', is observed, which has a two-term dependence: NOD)o' =
CfIYce
+ frIYce'~f,'~~[S*lO
(20)
In this equation,&' is the fraction of S* quenched by both MV2+ and An, yc; is the cage escape yield for the [An'+,MV'+] pair, andf, andfrI are the fractions of quenching of S* due to MV2+ and An, respectively cfl +AI = 1). If eq 20 is divided by eq 19 and rearranged, the result is Y'ce/Yce
= [A(oD)o'fq/A(oD)dq'A~I -
cfi/f~) (21)
All the terms on the right-hand side of eq 21 are experimentally accessible. The optical density changes are computed directly from changes in transmitted light intensity following flash excitation, while each of the fractional terms can be found from the concentrations of the quenchers and the previously measured Stern-Volmer quenching constants: fq
+ Ksv[MV2+])-1
(22)
+ Ksv[MV2+] + Ksv'[An])-'
(23)
= K ~ V [ M V ~ + I / ( ~ S V [ M+VKsv'[Anl) ~+I
(24)
fq' = (1
fr = (1 -fir)
= (1
4.6
0.21
2.7 2.7 3.8
0.18
4.6
0.14
6.1
0.27
TABLE 11: Cage Escape Yields for Methylviologen-Anthracene Charge-Transfer Pairs" shuttle compd
9-methylanthracene 9-methylanthracene 9-bromoanthracene 9-bromoanthracene
Note 4-fold difference in the y scales.
cz
3.4
0.18C 0.27d
"Solvent was 8:s (v/v) 0.1 M TEAP/CH3CN to CH2C12and viologen was MV2+ unless orherwise noted. bRelative to -yce = 1.0 for 9methylanthracene. CQuencherwas benzylviologen. dMethanol solvent containing 0.1 M NH,Cl. 'dmph = 5,6-dimethyl-l,lo-phenanthroline. fdas = bis( 1,2-dimethylarsino)benzene. gdppm = bis(dipheny1phosphin0)methane. * dmpp = dimethylphenylphosphine.
Figure 3. Oscilloscope traces of transient absorption due to MV" (610 nm) after flash excitation at 450 nm of a solution containing R~(bpy),~+ sensitizer and 1 mM MV2+: (a) signal in the absence of 9-methylanthracene; (b) signal in the presence of 1 mM 9-methylanthracene.
A(OD)o
kblk,
7,)
0.23
We will illustrate this analysis for an experiment in which Ru(bpy),*+ was present in the mixed solvent 8 CH3CN:5 CH2C12 (v:v) quenched by MV2+and 9-MeAn. At [MV2+] = 1.03 mM, the optical density (OD) change arising from MV" following the flash is 0.0477. From Ksv = 2.68, we computefq = 0.735 as the fraction of the ruthenium excited state quenched at this concen-
solvent additiveb CH2CI2 CH31 CH2C12 CH31
-ycL 1 .O
kbc'/kd' 0
0.70 0.74
0.43 0.35
0.30
2.33
"Sensitizer was R~(bpy),~+, viologen was MV2+. bSolvent was 8:s (v/v) 0.1 M TEAP/CH3CN to halocarbon.
2
(dr)
(7,)
(nu,'
(aDf
In the reduced product, the unpaired electron residues in an orbital largely a* (bpy) in character, the set of d a orbitals is filled, and there is no orbital basis for introducing spin-orbit coupling to any appreciable degree. As a result, the back-electron-transfer within the cage to first approximation involves the pair of organic-like ions, (bpy-) and M V + , for which, like the [An+,MV'+] pair, triplet singlet interconversion
-
-'
3 [ ( b ~ ~ ) 2 R u " ( b ~ ~ ' - ) + , D + [(bpy)~Ru"(bpy'-)+,D+I l or back-electron-transfer
-'
3 [ ( b ~ ~ ) 2 R u " ( b ~ ~ ' - ) + , D + [l R u ( ~ P Y ) ~ ~ + , D I do not compete effectively with cage escape. The suggested contribution of spin effects is also germane to other recent observations of anthracene-enhanced photochemical reduction of MV2+. Mau et al. report that anthracenecarboxylate (27) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986,90, 3722. (b) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (c) Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. A m . Chem. SOC.1982, 104, 630. (d) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583; Inorg. Chem. 1983, 22, 2444. (28) Harriman, A.; Porter, G.; Richoux, M. C. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1281 and references cited therein. (29) Shioyama, H.; Masuhara, H.; Mataga, N. Chem. Phys. Left. 1982, 88, 161. (30) Prasad, D.R.; Hessler, D.; Hoffman, M. Z . ; Serpone, N. Chem. Phys. Lett. 1985, 121, 61.
J . Phys. Chem. 1987, 91, 1655-1658 anion substantially enhances the production of MV" sensitized by Eosin Y.31 Eosin Y is a heavy-atom-substituted xanthene dye (tetrabromofluorescein),for which the internal heavy-atom effect would be expected to reduce the cage escape yield by increasing triplet or back-electron-transfer rates within the the singlet solvent cage. Similarly, Edel et al. report a very large enhancement of MV'+ when anthracenecarboxylate acts as a shuttle for a copper(1) c ~ m p l e x . Their ~ data indicate that the anthracene shuttle causes an enhancement of greater than 60-fold in MV" production, indicating that the cage escape yield for the [Cu complex, MV"] pair is C0.02. This suggests a rapid rate of back-electron-transfer within the solvent cage, which in turn can
-
(31) Mau, A. W.-H.;Johansen, 0.; Sasse, W. H. F. Photochem. Phofobiol. 1985, 41,
503.
1655
be attributed to paramagnetism and rapid spin interconversion at the copper complex. A similar explanation was also proposed for the very low yields of charge pairs in the CuTPP-benzoquinone system.I3
Acknowledgment. This research was supported by a grant from the National Science Foundation, Grant No. CHE-8503092. J.O. thanks the California State University, Fullerton, for providing a sabbatical leave and the University of North Carolina at Chapel Hill for a visiting appointment for the time during which this work was accomplished. Registry No. MVZt, 4685-14-7; BzV", 49765-27-7; MV*+, 2523955-8; R ~ ( b p y ) ~15158-62-0; ~+, Ru(dmph)32t, 14975-40-7; O~(bpy)~(dasb)2t, 80502-59-6; O~(phen)~(dppm)~+, 75446-24-1; O~(phen)~CH31, 74-88-4; CH2CI2, 75-09-2; 9-methyl( d m p ~ ) ~75441-77-9; ~+, anthracene, 779-02-2; 9-bromoanthracene, 1564-64-3.
Quaternary Diffusion in Aqueous KCI-KH,PO,-H,PO,
Mlxtures
Robert A. Noulty and Derek G. Leaist* Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 587 (Received: July 18, 1986; In Final Form: October 22, 1986)
Stokes diaphragm cells have been used to measure the nine quaternary diffusion coefficients for three compositions of the system water-KC1-KH2P04-H3P04 at 25 O C . The quaternary diffusivities of KH2P04and H3P04 components are significantly larger than the binary diffusivities of the aqueous components. It is shown that pH gradients in KH,P04-H3P04 buffers can drive large coupled flows of KCl which concentrate KC1 within diffusion boundaries. Onsager's reciprocal relations LIZ= LZ1,L13 = L31, and L23 = L32 for isothermal quaternary diffusion are tested, and equations are developed to estimate quaternary transport coefficientsfrom the concentrations and mobilities of the diffusing species. By transformation of transport coefficients for the system water-KC1-KH2P04-H3P04, coefficients for the system water-KC1-KH2P04-HC1 are obtained. Binary diffusion coefficients for water-KH2P04 at 25 OC are also reported.
Introduction Multicomponent diffusion data for four-component systems are sparse. Prior to this work, quaternary diffusion coefficients had been reported only for the nonelectrolyte system acetone-chloroform-methanol-benzene.' This work explores diffusion in the quaternary electrolyte system water-KC1-KH2P04-H3P04. Diffusion data for these solutions will provide information about coupled flow of KCl driven by pH gradients in KH2P04-H3P04 buffers, and thus shed light on proton-coupled transport of salts. Also, diffusion data for H3P04in KCI solutions will provide information about transport of a weak electrolyte in supporting salt solutions. Water-KC1-KH2P04-H3P04 is one of the few quaternary systems for which precise activity data are Consequently, measured diffusion coefficients for this system can be used to evaluate Onsager transport coefficients and to test Onsager's reciprocal relation^^*^ for isothermal quaternary diffusion. To aid interpretation of the results, binary diffusion coefficients are reported for aqueous KH2P04 solutions. Binary diffusion data for aqueous KCl and H3P04 solutions have already been reported.&* G.P.; Cullinan, Jr., H. T. J . Chem. Eng. Data 1973, 18, 213. (2) Pitzer, K. S.; Kim, J. J. J . Solution Chem. 1976, 5, 269. (3) Pitzer, K. S.; Silvester, L. F. J . A m . Chem. SOC.1974, 96, 5701. (4) Onsager, L. Ann. N.Y. Acad. Sci. 1945, 46, 241. (5) DeGroot, S. R.; Mazur, P. Non-Equilibrium Thermodynamics; (1) Rai,
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Experimental Section Quaternary diffusion coefficients were determined by the magnetically stirred porous-diaphragm cell method of stoke^.^,'^ The cells used in this work were constructed from Pyrex with fine-porosity sintered diaphragms (mean pore diameter 5 X 10" m). Upper and lower solution compartments (approximate volume 0.03 dm3) were fitted with greaseless Teflon stopcocks and stirred at 60 rpm with iron-in-glass stirrers. Cells were calibrated at frequent intervals with aqueous KCl solution^.^ Concentrations of KCl solutions from calibration runs were determined by potentiometric titration against silver nitrate. Cell constants were reproducible to 0.5-1 .O%. Stock solutions made up from distilled deionized water and reagent grade KCl, KH,P04, and H3P04 were filtered and used without further purification. Concentrations of KCl, KHzP04, and H3P04 (Cl, C,, and C3, respectively) were determined by titration of duplicate 0.01 dm-3 samples of solution. Titration with silver nitrate gave CI. Excess chloride was added to precipitate any free silver ion; then titration with standardized sodium hydroxide to end points near pH 4.5 and 9.0 gave C2and C3." C 1 , C,, and C3values were reproducible to 0.10, 0.25, and 0.20%, respectively. (8) Leaist, D. G. J . Chem. SOC.,Faraday Trans. I 1984, 80, 3041. (9) Stokes, R. H. J . Am. Chem. SOC.1950, 72, 763, 2243. (10) Robinson, R. A,; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Academic: New York, 1959; (a) Chapter 10; (b) p 31; (c) Appendix 6.1. (1 1) Vogel, A. Vogel's Textbook of Quantitative Inorganic Analysis, 4th ed; Revised by Basset, J., Denney, R. C., Jeffery, G. H., Mendham, J. Longman: New York, 1978; p 308.
0 1987 American Chemical Society
