J. Phys. Chem. 1985,89,665-670 picture of the level structure associated with IVR through the use of a more conventional spectroscopic approach. -~
We are particularly indebted to w* Holtzclaw for assistance with aspects of the experimental work, model development, and data analysis as well as for discussions concerning the many subtleties of the technique and the subject of IVR. Discussions with David B. Moss, Alan E. W. Knight, and Bradley M. Stone have also been most helpful. The National
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Science Foundation and the donors of the Petroleum Research Fund, administered by the American Chemical Society, have provided the funds that made the work possible. The inteiactions with A. E, W. Knight were made possible by the Australian Research Grants Scheme through the United States (NSF)Australia Agreement (ARGS) for Scientific and Technical cooperation. Registry No. 02,7782-44-7; pDFB, 540-36-3.
Catalysis of the Electroreduction of Dioxygen and Hydrogen Peroxide by an Anthracene-Linked Dimeric Cobalt Porphyrin Hsue-Yang Liu; I. Abdalmuhdi,t C. K. Chang,**and Fred C. Anson*+ Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratories,$ California Institute of Technology, Pasadena, California 91 125, and Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: July 26, 1984; In Final Form: October 23, 1984)
A new type of anthracene-bridged dimeric porphyrin substituted with one or two cobalt centers has been tested as a catalyst for the electroreduction of dioxygen. Both the dicobalt and monocobalt derivatives provide four-electron-reductionpathways. Both derivatives also catalyze the reduction of hydrogen peroxide but at lower rates. Kinetic analysis shows that hydrogen peroxide is not an intermediate along the pathway for the four-electron reduction of dioxygen.
The ability of certain cofacial dicobalt porphyrins to catalyze the electroreduction of dioxygen at unusually positive potentials’” has attracted interest in structurally similar molecules that might be more readily synthesized or that are chemically more robust.’ In the present study we examined the catalytic behavior of several derivatives of a new type of dimeric porphyrin in which the two porphyrin rings are linked together by an anthracene molecule that is bonded to a meso position on both porphyrin rings (Figure 1). Diporphyrins 1-111 contain no stereoisomers; the link connecting the two rings is likely to withstand acidic environments for longer periods than the amide-linked dimers previously synthesized.’” The catalytic activities of the four molecules shown in Figure 1 toward the reduction of dioxygen at graphite electrodes were measured with the porphyrins adsorbed on the electrode surface. The cavities of molecules 1-111 in Figure 1 might be somewhat more accessible to dioxygen molecules, and the distance between the two cobalt center might encompass a wider range of values than is true of the more rigid doubly bridged cofacial porphyrins. The present experiments were carried out to assess the sensitivity of catalytic activity to these two structural parameters and to compare their behavior with that of the analogous diporphyrins studied previously.’”
Experimental Section Materials. The synthesis, purification, and characterization of the anthracene-linked dimeric and monomeric porphyrins are described e l ~ e w h e r e . ~Cobalt ,~ insertion was accomplished by refluxing a chloroform solution of the diporphyrin with a methanolic solution of cobalt acetate and sodium acetate. Partial metalation can be achieved by using less than stoichiometric amounts of cobalt acetate. Alternatively, the monocobalt complex I1 can be obtained from acid washing of a cobalt/zinc complex synthesized ~ e p a r a t e l y .All ~ metalloporphyrins have been characterized by high-resolution fast atom bombardment mass spectroscopy. Other chemicals were reagent grade and were used as received. Aqueous solutions were prepared with distilled water
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California Institute of Technolow. *Michigan State University. @ContributionNo. 7067.
0022-3654/85/2089-0665$01.50/0
that had been passed through a purification train (Barnstead Nanopure). Pyrolytic graphite electrodes were obtained and mounted as previously described.’O The mounting exposed the edge planes rather than the basal planes of the graphite. Electrodes were mechanically polished with silicon carbide paper (Sears No. 320) on a low-speed polishing wheel (Buehler Model 44-1502). The most reproducible results were obtained when the graphite disk was polished before each run. The rotating graphite diskplatinum ring electrode was a commercial model (Pine Instrument Co.). The collection efficiency of the electrode measured with the Fe(CN)?-/& couple was 0.18. As previously discussed,” when the ring reaction was oxidation of HzOz,smaller and less reproducible collection efficiencies were observed. We utilized the electrode only in qualitative experiments to detect the presence or absence of HzOz. Procedures. The polished electrodes were coated with the catalysts by transferring 40-pL aliquots of dichloromethane solutions of the porphyrin to the electrode surface and allowing the solvent to evaporate. The dry, coated electrodes were transferred to aqueous supporting electrolytes and utilized immediately. The instrumentation and electrochemical procedures employed have been previously d e ~ c r i b e d . Potentials ~ were measured and are quoted with respect to a saturated calomel reference electrode. (1) Collman, J. P.; Marrocco. M.; Denisevich, P.;Koval, C.; Anson, F. C. J . Electroanal. Chem. Interfacial Electrochem. 1979, 101, 117. (2) Collman, J. P.; Denisevich, P.; Konai, Y.; Koval, C.; Anson, F. C. J . Am. Chem. Soc. 1980, 102, 6027. (3) Durand, Jr., R. R.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. SOC.1983,105, 2710. (4) Collman, J. P.; Anson, F. C.; Barnes, C. E.; Bencosme, C. S.; Geiger, T.; Evitt, E. R.; Kreh, R. P.; Meier, K.; Pettman, R. B. J . Am. Chem. Soc. 1983, 105, 2694. ( 5 ) Collman, J. P.; Anson, F. C.; Bencosme, C. S.; Durand, Jr., R. R.; Kreh, R. P. J. Am. Chem. Soc. 1983, 105, 2699. (6) Liu, H. Y.;Weaver, M. J.; Wang, C. B.; Chang, C. K. J. Electroanal. Chem. Interfacial Electrochem. 1983, 145, 439. (7) Chang, C. K.; Liu, H. Y.; Abdalmuhdi, I. J. Am. Chem. SOC.1984, 106, 2725. (8) Chang, C. K.; Abdalmuhdi, I. J . Org. Chem. 1983, 48, 5388. (9) Chang, C. K.; Abdalmuhdi, I., manuscript in preparation. (10) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (11) Geiger, T.; Anson, F. C. J . Am. Chem. Soc. 1981, 103, 7489.
0 1985 American Chemical Society
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Figure 1. Structures of the porphyrin derivatives examined in this study.
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Figure 2. Cyclic voltammograms for 1.2 X lo-’ mol of complex I adsorbed on graphite electrodes: dashed line, electrode coated with cobalt-free diporphyrin, 111; dotted line, background current at an uncoated electrode. Supporting electrolyte: 1 M CF@OH saturated with argon. Scan rate: 100 mV s-l.
Results Voltammetric Responses of the Porphyrins in the Absence of 02.The solid line in Figure 2 is a cyclic voltammogram obtained with a graphite electrode coated with I (Figure 1) in the absence of dioxygen. Coatings of I1 yield essentially identical responses. The dashed line resulted when the electrode was coated with the cobalt-free diporphyrin, 111, and the dotted curve represents the response of the uncoated graphite electrode. The current peaks obtained with the bare graphite electrode arise from the reduction and oxidation of quinone-like functional groups present on the graphite surfaceI2 which become sharper and somewhat larger in the presence of the adsorbed diporphyrin. The prominent pair of cathodic and anodic peaks centered at ca. 0.3 V in Figure 3 probably arise from a cobalt(III/II) couple, and the less prominent pair of peaks near 0.7 V may represent the second cobalt center in complex I. However, the poorly resolved response at the more positive potential appears only on the first scan with a freshly polished and coated electrode while the response near 0.3 V is (12) Evans, J. F.; Kuwana, T. Anal. Chem. 1977,49, 1632 and references therein. (13) Durand, Jr., R. R.;Anson, F. C . J . Electroanal. Chem. Inferfacial Electrochem. 1982, 134, 273.
E vs. SCE, Volt Figure 3. Cyclic voltammograms for 1.37 mM I (A), I1 (B),or 111 ( C ) in dichloromethane (polished glassy carbon electrode (0.34 cm2)). Supporting electrolyte: 0.1 M tetrabutylammonium perchlorate. Scan rate: 100 mV s-I.
much more persistent. The transitory nature of the response near 0.7 V makes its assignment uncertain. Cyclic voltammograms for a solution of compound I in dichloromethane (Figure 3A) exhibit two peaks in the potential range where the cobalt(II1) centers are expected to be reduced. (Additional peaks appear at more positive potentials that presumably arise from ligand oxidation processes that were not examined in detail). The presence of two peaks in Figure 3A is our principal reason for suggesting that the small peak near 0.7 V in Figure 2 marks the point at which the first cobalt(II1) center in adsorbed compound I is reduced. The area between the solid and dashed curves at the prominent peak near 0.3 V in Figure 2 corresponds to approximately 0.8 electron per molecule of porphyrin initially deposited on the electrode. It therefore seems unlikely that this wave corresponds to the reduction of more than one cobalt center in the molecule. Two better formed and separate peaks are also present in voltammograms of the analogous amide-linked dicobalt diporphyrin adsorbed on graphite electrode^.^ The separation in peak potentials for the two identical cobalt centers in this complex has been attributed to electronic interactions that cause the formal potentials of the two metal centers to differ.3 Complex 11, with only one cobalt center, yields a single prominent pair of voltammetric peaks centered near 0.35 V when adsorbed on the electrode surface. When dissolved in dichloromethane, compound I1 exhibits only poorly formed voltammo-
Catalysis of the Electroreduction of O2 and H 2 0 2
The Journal of Physical Chemistry, Vel. 89, No. 4, 1985 661 A
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Figure 4. Current-potential curves for the reduction of O2at a rotating graphite disk-platinum ring electrode. The polished pyrolytic graphite disk was coated with 1.2 X lo4 mol cm-* of (A) complex I or (B) complex 11. Ring potential: 0.9 V. Rotation rate: 100 rpm. Supporting electrolyte 1 M CF,COOH saturated with 02.The disk potential was scanned at 10 mV s-I. The dashed curves are the disk and ring currents obtained under the same conditions from coatings of a more active amide-linked cofacial porphyrili C0-co-4.~
grams such as that in Figure 3B. Two anodic waves are evident. The more positive wave is believed to correspond to the oxidation of the metal-free porphyrin ring in I1 because it appears at potentials similar to that of the first oxidation wave for the metal-free dimer, 111 (Figure 3C). However, the electrochemical responses exhibited by 111 and its various cobalt, copper, zinc, and mixedmetal derivativesg in dichloromethane are difficult to interpret unambiguously because the waves for the porphyrin rings are not clearly separated from those for the metal centers. No clear pattern could be discerned as to the effect of monometalation on the formal potentials for the oxidation of the metalated and unmetalated porphyrin rings. It appears that linking the two porphyrin rings by an anthracene molecule produces rather complex coupling between the two rings and metal ions present in them. Catalysis of the Reduction of 02.Current-potential responses for the reduction of O2a t a rotating graphite disk-platinum ring electrode are shown in Figure 4. The solid curves in Figure 4A were obtained when the graphite disk was coated with I and the platinum ring was held a t a potential where any H202(formed at the disk and reaching the ring) would be reoxidized to 02.The small maximum in the disk current response was reported in a previous communication' but was not examined in detail. Similar behavior was also observed in early experiments with amidelinked cofacial dicobalt porphyrins's2 and was found to be strongly dependent on the polishing procedures employed to prepare the graphite surfaces before the porphyrins were adsorbed. The same was true in the present experiments. The maximum became less pronounced on successive scans with the same electrode coating as the limiting current at potentials more negative than the current maximum diminished. The magnitude of the current maximum was a function of the rate at which the potential was scanned. It is not present under true steady-state conditions. These features suggest that the current maximum may result from a small fraction of the adsorbed porphyrin catalyst that temporarily exhibits a higher activity toward the reduction of 02.That the polishing procedure used to prepare the graphite surface strongly influences the prominence of the maximum indicates that interactions of the cobalt centers with functional groups present on the graphite surface may alter the activity of the catalyst. The most noteworthy feature of the disk and ring currents in Figure 4A is their demonstration that the O2reduction yields very little H202at potentials near 0.45 V, and even at 0.2 V the ratio of ring to disk currents indicates that relatively little of the O2 is reduced to H202 Thus, the dimeric cobalt porphyrin, I, provides a four-electron-reduction pathway for 02. We were surprised to find that compound 11, with only one cobalt ion present in the dimeric porphyrin ligand, is also capable of catalyzing the four-electron pathway for the reduction of 02. The rotating ring-disk current-potential curves for O2reduction with this catalyst are shown as the solid curves in Figure 4B. The disk current is as large as that obtained with the dicobalt catalyst, and there is very little ring current. The only major differences
Figure 5. Levich (A) and Koutecky-Levich (B) plots of the plateau current for the reduction of 0,at graphite electrodes coated with 1.2 X lo+ mol cm-* of complex I (m) or complex 11(0). Supporting electrolyte: 1 M CF,COOH saturated with air. The dashed lines are the calculated responses for the convection-diffusion limited reduction of 0,by four electrons taking [O,]= 0.24 mM and Do* = 1.8 X lo5 cm2 sd.
between the responses obtained with electrodes coated with I or I1 are the less positive potential at which the O2 reduction commences for catalyst I1 and the slightly more gradual approach to the limiting current plateau. For comparison, the dashed curves in Figure 4 show the responses that result when the most active form of the amide-linked cofacial dicobalt catalyst described previously6 was applied to the same polished graphite disk electrode. The reduction commences at slightly more positive potentials than with I and significantly more positive than with 11, but the limiting disk currents are about the same for all three catalysts and exceed substantially the value corresponding to the two-electron reduction of 02. LevichI4 and Koutecky-Le~ich'~ plots of the plateau currents and electrode rotation rates for the reduction of O2 at electrodes coated with I and I1 are shown in Figure 5. As the electrode rotation rate increases, catalyst 11, with only a single cobalt center, is able to sustain notably larger plateau currents than catalyst I. The nonlinearity of the Levich plots (Figure SA) with increasing curvature at higher rotation rates signals the likely presence of a chemical step that precedes the electron transfer and limits the current to values below the convection-diffusion limit. Previous studies3 have assigned the current-limiting step to the formation of a cobalt(II)-02 adduct that is the reducible species. The Koutecky-Levich plots in Figure SB, while linear, have slopes that differ somewhat from that of the dashed line calculated for the four-electron reduction of 02.The difference in slopes is in the direction expected if O2 were reduced to a mixture of H 2 0 and H202at the catalyst-coated electrodes. This is also consistent with the magnitudes of the anodic ring currents at potentials on the plateaus of the disk current-potential curves in Figure 4. The presence of a mixed reaction pathway complicates the interpretation of the intercept of the Koutecky-Levich plots in Figure 5B. However, the ring-disk curves in Figure 4A indicate that the major pathway must involve four electrons. On this basis it is possible to obtain an approximate value of the rate constant, k , governing the current-limiting chemical reaction from the equation3
where iF is the reciprocal intercept of the Koutecky-Levich plot, F is Faraday's constant, ratis the quantity of a catalyst adsorbed on the electrode, and C, is the concentration of O2in the solution. (14) Levich, V. G. "Physicochemical Hydrodynamics"; Prentice-Hall: Englewood Cliffs , NJ, 1962. (15) Koutecky, J.; Levich, V. G. Z h . Fiz. Khim. 1956, 32, 1565; ref 14, pp 345-57.
668 The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
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Figure 6. pH dependence of plateau currents for the reduction of O2at rotating graphite disk electrodes coated with 1.2 X mol of complex I (m) or complex I1 ( 0 ) . Electrode rotation rate: 100 rpm. Other conditions as in Figure 5.
The values of k obtained from the intercept of the lines in Figure 5B are 2 X lo4 and 5 X lo4 M-' s-' for catalysts I and 11, respectively. These values are somewhat smaller than the value of 3 X lo5 M-' s-' measured previously for the amide-linked cofacial dicobalt p o r ~ h y r i n .Thus, ~ the rate of reaction between O2and complex I may be somewhat slower than it is with the doubly linked analogue despite the greater cavity accessibility suggested by the more open structure of the former complex. However, the uncertainties in the values of rat and the presence of mixed reaction pathways render this conclusion very tentative. pH Dependence of O2Reduction. The course of the catalyzed reduction of O2 is influenced significantly by changes in the pH of the supporting electrolyte. Disk plateau currents at electrodes coated with I or I1 are plotted in Figure 6 as a function of pH. At electrodes coated with I the current decreases somewhat between pH 0 and 2 but then remains essentially constant up to p H 12 before decreasing to about half its initial value at pH 14. The currents obtained up to pH 12 are significantly larger than the two-electron diffusion-convection limited value so that I continues to provide a four-electron pathway for the reduction of O2 at pH values as high as 12. With 11, the disk current is much more sensitive to pH. Only a t pH 0 is the current close to the value expected for the four-electron reduction of 02.As the pH is raised, an increasing fraction of the O2is reduced to H202instead of H20. Catalysis of the Reduction of H202. If the monocobalt diporphyrin, 11, were a catalyst for the reduction of H202to H 2 0 at potentials close to those where it catalyzes the reduction of 02, an explanation for the unexpectedly large limiting current in Figure 4B would be at hand. This possibility would not be inconsistent with the appearance of a small ring current in Figure 4B if I1 catalyzed the reduction of H202a t a much lower rate than it catalyzes the reduction of 02.The slight rise in disk current and corresponding decrease in ring current at potentials less positive than ca. 0.1 V in Figure 4A suggest that I may also function as a catalyst for the electroreduction of H202to H,O.' Since most previous studies have not reported pronounced catalytic activity of cobalt porphyrins toward H202r e d ~ c t i o n , ' we ~ ~ examined '~ this point in some detail. Shown in Figure 7A are current-potential curves for the reduction of H202at a rotated graphite disk electrode before (curve 1) and after it was coated with I (curve 2) or I1 (curve 3). The potential of the electrode was held at values no more positive than 0.45 V between scans to avoid the formation of O2 by oxidation (16) Fujihira, M.;Sunakawa, K.; Osa, T.; Kuwana, T. J. Electroanal. Chem. Interfacial Electrochem. 1978, 88, 299. Bettelheim, A.; Kuwana, T. Anal. Chem. 1979,51, 2251. Forshey, P.;Kuwana, T. Inorg. Chem. 1981, 20, 693; 1983, 22,699. Zagal, J.; Bindra, P.; Yeager, E. J. Electroanal. Chem. Interfacial Electrochem. 1980, 127, 1506.
Figure 7. (A) Current-potential curves for the reduction of 1 mM H202 at rotated graphite disk electrodes: uncoated electrode (1); electrode coated with 1.2 X 10+ mol of complex I (2), complex I1 (3), or complex IV (4). Supporting electrolyte: 1 M CF,COOH saturated with argon. Rotation rate: 100 rpm. Scan rate: 10 mV s-I. (B) Levich plots for H202 reduction as catalyzed by complex I ( 0 )or complex I1 (A). (C) The corresponding Koutecky-Levich plots. The dashed lines were calculated for the diffusion-convection limited reduction by two electrons.
of the H202. It is clear from curves 1,2, and 3 in Figure 7A that both I and I1 are catalysts for the reduction of H202 However, even with the low rotation rates employed, the plateau currents are much smaller than the calculated diffusion-convection limited Levich currents14 for a two-electron-reduction process. This is evident from the Levich and Koutecky-Levich plots shown in Figure 7, B and C, respectively. Thus, a slow chemical step preceding the electron transfer reactions apparently limits the magnitude of the plateau currents for the reduction of H202as well as 02.That the rate of the preceding chemical step is much slower in the case of H202is evident from the large intercepts of the lines in Figure 7C. It is conceivable that I and I1 catalyze the disproportionation of H202so that the electrode reaction proceeding during its reduction involves only the reduction of O2 as in Figure 4. We regard this possibility as unlikely, however, because the potentials where the catalyzed reduction of H202 commences in Figure 7A are significantly less positive than those where O2is reduced (Figure 4). If the totally irreversible reduction of H202occurred by its prior disproportionation to O2and H 2 0 , the reduction would be expected to commence at about the same potential where O2 is reduced. Only a potential dependence of the disproportionation reaction could alter this conclusion, and there is no reason to invoke such a potential dependence at potentials that are removed from those where the cobalt centers in the adsorbed porphyrins exhibit their redox activity. The lack of significant catalysis of the disproportionation of H202by complex I or I1 adsorbed on the graphite disk was also demonstrated by means of the rotating ring-disk electrode in a dioxygen-free solution of H202. The anodic current measured at the platinum ring at 0.9 V was essentially the same when the graphite disk (not connected to the potentiostat) was coated with complex I or I1 as when it was uncoated. This was true with rotation rates as low as 400 rpm. Disproportionation of the H202 at the surface of the disk would have produced a decrease in ring current so the equality of the ring currents in the two experiments indicates that the disproportionation reaction proceeds too slowly
Catalysis of the Electroreduction of O2 and H202
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 669
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Figure 8. pH dependece of plateau currents for the reduction of 1 mM H202 at rotating graphite disk electrodes coated with 1.2 X lo4 mol cm-2 of complex I (m) or complex I1 (0).Supporting electrolytes at each pH as in Figure 3. Rotation rate: 400 rpm.
to be important in these experiments. p H Dependence of H 2 4Reduction. The catalytic activities of I and I1 toward the reduction of H202show notably different pH dependences. Plateau currents at disk electrodes coated with each catalyst and rotated at a low rate are shown in Figure 8. I sustains small disk currents over a wide pH range while with I1 a decline in current begins as early as p H 2. Both complexes reduction above p H 12. The become relatively inert for H202 difference in the p H dependences of the activities of the two catalysts toward the reduction of H202 helps to explain the corresponding differences in the overall rates with which they catalyze the reduction of O2 in the pH range from ca. 2 to 12 (Figure 6 ) : Complex I1 loses its activity for the reduction of H202 in the same pH range where it yields diminished plateau currents for the reduction of O2 (Figure 6B). If the catalyzed reduction of O2 by complex I1 involved the production of H202as an intermediate, one would expect less disk current than if the reaction proceeded directly to H20(as with compound I). H202exhibits a preference to react with transition-metal reductants by inner-sphere pathways.” It seems likely, therefore, that coordination of H202to the cobalt center in the porphyrin catalysts precedes its catalyzed reduction. The alternative, outer-sphere pathway in which the cobalt(I1) porphyrin transfers an electron to an uncoordinated H202molecule is incompatible with the potentials where the catalyzed reduction proceeds: In 1 M CF3COOH I catalyzes H202reduction at potentials significantly more negative than those where the first cobalt(II1) center is reduced to cobalt(I1) and I1 also exhibits catalytic activity at potentials less positive than that where the cobalt(I1) porphyrin is first generated (compare Figures 2 and 7A). The cobalt(I1) center in the porphyrin is essential: the metal-free diporphyrin ligand and its dicopper(I1) derivate are both inert toward the reduction of H202.
Discussion Complexes I and I1 catalyze the electroreduction of H202at a much lower rate then they catalyze the electroreduction of 0,. This is apparent from a comparison of the normalized intercepts of the Koutecky-Levich plots in Figures 5B and 7C. Both the magnitude of the disk current for the reduction of O2 by complex I in Figure 4A and the lack of ring current on the rising part of the reduction wave at the disk electrode require that the 0,be reduced to H,O, not H202. It follows that the mechanism of the four-electron reduction of O2 by catalyst I cannot involve uncoordinated H202as an intermediate. Any H202that was released into the solution would be subsequently reduced at the disk too (17) Davies, G.; Sutin, N.; Watkins, K. 0. J . Am. Chem. SOC.1970, 92, 1892.
Figure 9. Plateau currents for the simultaneous reduction of O2 and H202at a rotating graphite disk electrode coated with 1.2 X lo4 mol cm-2 of I: (A) reduction of H202in the absence of 0,; (B) repeat of (A) after the solutions were saturated with 0,; ( C ) difference between the plateau currents in (B) and that for an 02-saturated solution in the absence of H202. Supporting eletrolyte: 1 M CF,COOH. Electrode rotation rate: 400 rpm. Plateau currents measured at -0.3 V.
slowly to provide the high disk current observed in Figure 4A or to escape detection at the ring electrode. This assertion was supported by experiments where both O2 and H202were reduced simultaneously at electrodes coated with catalyst I (Figure 9). At the same point, e.g., -0.3 V, where the O2 present was reduced primarily to H20at a high rate, H202present in the solution was reduced much more slowly and the simultaneous reduction of O2 had virtually no effect on the rate of the reduction of H202 (compare A and C in Figure 9). The two oxidants seem clearly to undergo catalytic reduction by independent pathways. The catalysis of 0,reduction to H20by complex I could involve the coordination of both cobalt centers to the O2 molecule with the formation of a p-peroxo intermediate as proposed in previous studies of binuclear cobalt porphyrin catalyst^.^ Such an intermedite is not likely to be formed if the source of oxygen is H202 instead of 02,especially in acidic solutions. Accordingly, the catalyzed reduction of H202probably involves its coordination to a single cobalt center. This would be compatible with the comparable activities toward H202exhibited by catalysts I and 11. The smaller limiting currents obtained with I, 11, and IV for the reduction of H202compared to that of O2 would then reflect the lower rate of coordination of H202to the cobalt centers in these catalysts. The extensive four-electron O2 reduction activity exhibited by I1 was surprising because the analogous doubly amide-bridged cofacial porphyrin containing only a single cobalt center had been reported to serve only as a two-electron reduction catalyst.’ However, more recent experiments with greater quantities of more thoroughly purified material have shown that the monocobalt cofacial porphyrin does support a four-electron reduction of 02.18 Its four-electron activity declines within a few minutes, and this was one reason that its capacity to catalyze the four-electron reduction was overlooked in the previous s t ~ d y . ~ The anthracene bridging group in I1 is not the source of the enhanced activity because the monomeric porphyrin, IV (Figure l), shows essentially the same behavior as other monomeric cobalt porphyrins in catalyzing the two-electron reduction of O2 to H202 with the further reduction to H 2 0 proceeding at a much lower rate (Figure 7A, curve 4). The higher activity of I1 toward the four-electron reduction of 0,might arise from the proximity of (18) Ni, C.-L.; Anson, F. C., unpublished experiments.
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the second porphyrin ring that should be protonated in the acidic medium employed. It is conceivable that these protons, juxtaposed to the coordinated 02,could prevent the premature dissociation of, as well as assist in proton transfer to, the partially reduced O2coordinated to the cobalt center in the second porphyrin ring. If such specific proton catalysis proves to be the case, it would suggest new directions for the design of catalysts for multielectron reductions.
Concluding Remarks The anthracene-bridged cobalt diporphyrins examined in this study clearly provide four-electron pathways for the catalytic
reduction of Oz when adsorbed on graphite electrodes. This is true for both the dimetalated and monometalated derivatives. These molecules and their diphenylene-bridged analogues7 represent the first effective macrocyclic metal complex electrmtalysts that do not depend upon the “four-atom separation” demonstrated to be essential in the case of the diamide-bridged catalysts of ColIman et a1.I” Acknowledgment. This work was supported by the National Science Foundation at both Caltech and MSU. Registry No. I, 94250-18-7; 11, 94250-20-1; 111, 87597-38-4; IV, 94250-19-8; 02, 7782-44-7; H202,7722-84-1.
Photoprocesses in Diphenylpolyenes. 3. Efficiency of Singlet Oxygen Generation from Oxygen Quenching of Polyene Singlets and S. K. Chattopadhyay,+C. V. Kumar,* and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: August 6. 1984)
The efficiencies of singlet oxygen (IO2*) photogeneration from the oxygen quenching of the excited states (singlet/triplet) of retinal-related polyenals and diphenylpolyenes have been measured in cyclohexane and methanol by 337.1-nm laser flash photolysis. The IO2*yields are essentially quantitative with all-trans-retinal and its lower and higher homologues as triplet photosensitizers. For all-trans-l,6-diphenyl-1,3,5-hexatriene (DPH) and all-trans-l,8-diphenyl-l,3,5,7-octatetraene (DPO), significant fractions (0.1-0.7) of the both singlet and triplet quenching by oxygen contribute to the formation of IO2*; oxygen-induced intersystem crossing in these polyene systems take place without energy transfer to oxygen. The triplet-mediated IO2* yield obtained by steady-state photolysis of all-trans- 1,Cdiphenyl-1,3-butadiene(DPB) under energy-transfer sensitization by pyrene-1-aldehyde in 02-saturated benzene is less than unity (0.7 f O.l), suggesting possible fractional quenching by oxygen at an orthogonal geometry of DPB triplet (responsible for “nonproduction” of IOz*).
Introduction Quantitative aspects of singlet (lo2*, IAJproduction as a result of the quenching of singlet and triplet excited states by oxygen (302, 3Z;) constitute a subject of ongoing interest and controversy. The knowledge of the quantum efficiency of IO2* generation is important because of its frequent involvement as an oxidizing intermediate in sensitized photooxygenations of various organic and biological systems. Unless the singlet-triplet energy separation or UT,+) is smaller than the excitation energy (-8000 cm-I) of oxygen, the oxygen quenching of electronically excited states, SI or T I , generally occurs by energy transfer giving rise to assisted intersystem crossing (TI SI or So T,) as well as producing l o 2 * in high yield^.^ However, complications and variations are observed in many cases presumably because of charge transfer, spin exchange, or other interaction^.^-^ I n a n earlier paperza from this laboratory, we have presented kinetic data concerning the efficient quenching of the singlets and triplets of diphenylpolyenes by oxygen. However, the fate of the oxygen molecules participating in the quenching processes was not examined. In the work described in this paper, we have examined the efficacy of three all-trans diphenylpolyenes as well as three all-trans-retinal-related polyenals as photosensitizers for production of IOz*. The results are of interest not only because of the roles the polyenes play as light-harvesting agents in photobiological systems but also from the viewpoint that the geometric distortion of the polyenes in the excited states may favor oxygen quenching via spin exchange. It should be noted that the relatively high oxygen-quenching rate constant (8 X lo9 M-’ s-l in b e n ~ e n e ) ~ for stilbene triplet has been associated7 with spin-exchange interaction at the twisted geometry and the efficiency of l o 2 *
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Present address: Chemistry Department, Georgetown University, Washington, DC 20057. *Present address: Chemistry Department, Columbia University, New York, NY 10027.
0022-3654/85/2089-0670$01.50/0
Chart I
n = l 1,4-diphenyl-l,3-butadiene (DPB) n = 2 1,6-diphenyl-l,3,5-hexatriene(DDH) n = 3 1,8-diphenyl-1,3,5,7-octatetraene(DPO)
RAG OR -
R
~
C , , aldehyde retinal C,, aldehyde O
generation as a result of this quenching has been shown4a to be small. (1) The research described herein was supported by the Office of the Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-261s from the Notre Dame Radiation Laboratory. (2) (a) Paper 1: Chattopadhyay, S. K.; Das, P. K.; Hug, G. L. J . Am. Chem. Soc. 1982,104,4507-4514. (b) Paper 2: Chattopadhyay, S. K.; Das, P, K.; Hug, G. L. Ibid. 1983, 105, 6205-6210. (3) (a) Dobrowski, D. C.; Ogilby, R.; Foote, C. S. J . Phys. Chem. 1983, 87,2261-2263. (b) Drews, W.; Schmidt, R.; Brauer, H.-D. Chem. Phys. Lett. 1983, 100, 466-469. (c) Stevens, B.; Marsh, K. L.; Barltrop, J. A. J . Phys. Chem. 1981,85, 3079-3082. (d) Merkel, P. B.; Herkstroeter, W. G. Chem. Phys. Lett. 1978, 53, 350-354. (e) Gurinovich, G. P.; Salokhiddinov, K. I. Chem. Phys. Lett. 1982,85,9-11. ( f ) Darmanyan, A. P. Chem. Phys. Lett. 1982,86, 405-410. (g) Wu, K.C.; Trozzolo, A. M. J . Phys. Chem. 1979, 83, 2823-2826. (h) Stevens, B. J. Phys. Chem. 1981,85, 3555-3557. (4) (a) Garner, A.; Wilkinson, F. In “Singlet Oxygen”; Ranby, B., Rabek, J. F., Eds.; Wiley: New York, 1978; p 48. (b) Garner, A.; Wilkinson, F. Chem. Phys. Left. 1977, 45, 432-435. ( 5 ) Gorman, A. A.; Lovering, G.; Rodgers, M. A. J. J . Am. Chem. SOC. 1978, 100, 4527-4532. (6) Darmanyan, A. P. Chem. Phys. Lett. 1983, 96, 383-389
0 1985 American Chemical Society
