Dissociative Electron Transfer by Flavio Maran and Mark S

Dissociative Electron Transfer by Flavio Maran and Mark S. Workentin

redicting the rate of chemical reac- analysis it was evident that some DET The Rate/Free Energy tions on the basis of reorganiza- reactions are characterized by a much Relationship tion energies and driving force is a weaker driving force (≡ - ∆G°) depen- fundamental challenge in all areas dence than that of other ET processes. Our ability to predict the rate of a Pof chemistry and biochemistry. Only more recently have the kinetics given reaction relies primarily on an Considerable achievements in this regard of a number of reactions that follow a understanding of how the reaction rate have been reached in the area of electron DET mechanism been quantified sys- changes as a function of driving force. transfer (ET) processes, thanks to the tematically to allow for a significant This has been one of the main goals of Marcus theory of ET and related treat- development of our knowledge in the studies carried out with various dissocia- 1 ments. On the other hand, there are sev- field. Instrumental in this regard was tive-type acceptor molecules. A typical eral chemical systems that, upon ET, are the desire to experimentally test a example is provided by the reduction of σ susceptible to undergo the cleavage of a model proposed by Savéant to describe tert-butyl bromide (t-BuBr) by aromatic bond, leading to formation of reactive concerted DETs and that established radical anions in dipolar aprotic amide species. The dissociation of the bond may ways to distinguish between the two solvents. For this system, which served as 5 follow or be concerted to the ET itself, as mechanistic pathways. In its most used a test of the DET theory,9,10 the driving shown in the following Scheme: and practical form, the Savéant model force could be changed significantly leads to a quadratic rate/free energy (~1.9 eV) and thus an overall variation of relationship that is formally identical to the intermolecular rate constant by 13 1 the well-known Marcus equation. The orders of magnitude could be measured, ∆ ≠ intrinsic barrier ( G0 , which is the acti- thanks to the work of Lund,11 Savéant,12 ∆ vation free energy at G° = 0) of con- Grimshaw,13 and their co-workers. The certed DETs, however, is much larger experimental data, however, seem to fit than that of outer-sphere ETs. This is almost equally well to a parabola, as pre- Both cases are described as a dissociative because of the significant reorganiza- dicted by the quadratic DET theory, and ET (DET) process. In the concerted DET, tion energy associated with the progres- to a straight line. Similar results were the reaction proceeds along a reaction sive stretching of the cleaving bond obtained with other alkyl halides.10 coordinate that, for the most common along the reaction coordinate. In partic- There are plausible and intriguing ∆ ≠ λ case of reduction of a neutral com- ular, Savéant showed that G0 = ( + hypotheses on the reasons for observing λ pound, does not involve the transient BDE)/4, where is the sum of the sol- these experimental trends. In particular, formation of the radical anion that is on vent and inner reorganization energies the relatively fast rates measured at low a more energetic potential energy sur- (except for the mode corresponding to driving force can be attributed to the face. Examples of stepwise DET process- the cleaving bond) and BDE is the bond possible role of a rate-enhancing inner- 5 es are rather common in electrochem- dissociation energy. Concerted DETs 10 sphere (SN2-like) component, a more istry. This is particularly true for reduc- are thus intrinsically very slow ET pronounced SN1-like contribution to the tive processes. processes (the BDE term is usually much transition-state structure,14 or a driving- λ Generally speaking, DET are useful in larger than ). In principle they are eas- force dependent donor/acceptor elec- that they provide an elegant and chem- ily distinguishable from the much faster tronic coupling.15 ically clean way to electrogenerate reac- outer-sphere ETs that constitute the first For halides and other acceptors char- tive species such as bases or nucle- step of stepwise DETs and for which the acterized by relatively large bond disso- ophiles. From a practical point of view, preponderant term is the solvent reor- ciation energies (BDEs) the expected both stepwise and concerted DETs yield ganization energy. quadratic rate/free energy relationship the same reactive species, the main dif- After a relatively slow beginning in may be difficult to detect. This is ference being that for the latter reaction the eighties, research on DET mecha- because the curvature of the parabola is these species are directly generated at nisms and their possible consequences ∆ ≠ inversely proportional to G0 , which contact distance from the donor (either and applications is now an active area in turn is proportional to BDE, and con- a solution species or electrode). of research carried out in laboratories certed DETs are typically characterized Concerted DETs occur less frequently spread all over the world, including sev- by large intrinsic barriers. However, and thus are less known. The first con- eral groups who are active within the ∆ ≠ when G0 is not too large, the parabol- tributions in this area were provided in O&BE Division. The goal of this report ic pattern can be detected and differen- studies by Hush and Eberson that con- is to outline the most important fea- tiated from a linear correlation beyond cerned the reduction of halides and the tures of these DET processes. For the experimental error. This is the typical 2,3 oxidation of carboxylates. These con- sake of simplicity only a few representa- situation that was encountered with the cepts were then developed extensively tive examples have been chosen with homogeneous or heterogeneous reduc- 4 by Eberson, who used the Marcus the- the aim to provide sufficient insight tion of peroxides,15-21 compounds hav- ory for outer-sphere ET to describe the into this area. For more comprehensive ing low BDEs (in the range 25-40 rate/free energy relationship (i.e., log k accounts on the matter, the reader kcal/mol ) and thus lower ∆G ≠. As rep- ∆ 6-8 0 versus G°) of these processes. By this should refer to very recent reviews. resentative examples, the rate/free ener-

44 The Electrochemical Society Interface • Winter 2002 oxides are, in practice, kinetically slow oxidants.

DET Mechanisms and Mechanistic Transition

In recent years several groups have collected data and provided insights into the dynamics of DETs. In some studies, the use of various electrochemical and nonelectrochemical (such as pulse radiolysis) kinetic methods has allowed the exploration of a large driving-force range, as shown very nicely for the concerted DET to t- BuBr.10 It has even been shown that the heterogeneous DET rate/driving force relationship, obtained by using the convolution analysis approach,17 is approximately complementary to the homogeneous DET results.15,20 Using the convolution methodology,

FIG. 1. Plot showing the free-energy dependence of the logarithm of the intermolecular DET rate constant the explored driving-force range can for the reaction between aromatic radical anion donors and di-tert-butyl peroxide (, DMF, MeCN) or be extended quite significantly (Fig. 2). dihydroascaridole (DASC) ( DMF, MeCN) at 25°C. The curve has been drawn by using the DET qua- By combining these methodologies, it dratic equation. The data are from Refs. 16 and 21. is possible to evaluate the average slope of the log k versus ∆G° in any explored ∆G° range. Common stepwise DET processes, such as those encountered with aryl halides (which form usually short-living radical anions), are also well characterized in terms of log k versus ∆G° (or donor E° plots).23 What is particularly important is that the average slope of the activation region of these ETs, i.e., the region where the rate-determining step is the actual ET, is much steeper than that of the concerted processes. Because of this characteristic, the two processes can be often quite easily distinguished. Extrapolation of the kinetic results to other free-energy ranges, however, may be sometimes hazardous. This is because competitive or borderline mechanisms may be triggered. For example, the SN2 mechanism is intrin- ∆ ≠ sically slower (i.e., has a larger G0 ) than the concerted DET because of the need of forming a new bond between nucleophile and the electrophilic carbon. This aspect and related issues have been elegantly tackled by FIG. 2. Comparison between homogeneous ( ) and heterogeneous ( ) rate constants for the reduction of Marcus.24 However, because of the pivaloyl peroxide in DMF at 25°C. The dashed line is the second order fit to the data. The data are from Ref. 15. thermodynamic advantage associated with the formation of this bond, the SN2 may be the main mechanism at gy relationships experimentally deter- DET.5 Interestingly, the result of careful low driving forces. At large driving mined for di-tert-butyl peroxide16 and analyses carried out on these and simi- forces (i.e., with particularly strong an endoperoxide,19 using series of aro- lar results by using both the adiabatic7 reductants), the initial outer-sphere ET matic radical anions as the donors, are and nonadiabatic22 DET theories, led to step of stepwise DETs may overwhelm shown in Fig. 1. For these compounds the discovery that the dissociative the intrinsically slower concerted the curvature is indeed verified beyond reduction of peroxides is intrinsically DET.25 Because of the intrinsic-barrier experimental error, in full agreement nonadiabatic.15,18 ET to peroxides is difference, the two ETs respond to with the nuclear factor of the rate con- thus much slower than predicted, changes in the driving force in a quite stant expression proposed for concerted which leads to a consequence that per- different way. For the stepwise mecha-

The Electrochemical Society Interface • Winter 2002 45 nism, log k is quite dependent on ∆G° with the consequence that log k (function, through the Marcus or the DET ∆ ∆ ≠ equation, of both G° and G0 ) may become larger than that for the concerted DET, in spite of a less favorable driving force. In fact, the concerted reaction is thermodynamically favored over the corresponding stepwise process when the E° for the formation of the radical anion is sufficiently negative, the cleaving bond is weak and B- is a good leaving group (the E° of a concerted DET can be expressed as a function of the bond dissociation free energy, BDFE, of the cleaving bond and the E° of the leaving group: E° = E° - BDFE/F). AB/A•,B- B•/B- Some evidence for a concerted-to- stepwise mechanistic transition was first obtained by Vianello and co- workers who studied the C-S bond cleavage induced by homogeneous ET (aromatic electron donors) to triph- enylmethyl phenyl sulfide.26 More convincing data can be obtained by using the heterogeneous approach, which is intrinsically more sensitive than the corresponding homogeneous one. The heterogeneous intrinsic barrier depends essentially on the acceptor molecule, while the homogeneous ∆ ≠ G0 depends also on the electron donor. In addition, in the homogeneous case the number of donors is often limited, but in the heterogeneous reaction the free energy can be varied continuously by simply chang- ing the applied potential E. This important feature is conveniently FIG. 3. Potential dependence of the logarithm of the heterogeneous rate constant (upper curve) and transfer coefficient a (lower curve) for the reduction of tert-butyl para-acetylperbenzoate in DMF. The solid line has exploited when the convolution been calculated from estimated quantities by assuming a stepwise/concerted competition mechanism. The 6,17 approach is employed. In fact, data are from Ref. 29. unlike conventional electrochemical methods, all of the experimental i-E data are used in the kinetic analysis the potentials where the actual reduc- As we have seen, α is obtained in an and, in addition, the kinetic data can tion occurs are much more negative unbiased form from the experimental be analyzed without the need of defin- than the E°. Therefore, values of α sig- rate/free energy plots and thus, being a ing a priori the ET rate law.27 nificantly lower than 0.5 are found. On derivative, is particularly sensitive to In practice, the heterogeneous rate the other hand, if the initial ET leads to changes in the slope of these plots. This constant khet is obtained as a function the formation of an intermediate radi- feature is very important in detecting of E. By this method, the quadratic cal anion AB•-, the usual effect of the the transition between concerted and rate/free energy relationship of perox- cleavage reaction is to cause the stepwise DETs and has been verified by ides was established very nice- voltammetric peak to appear close or studying the reduction of a family of 15,17,20,21,28,29 α 28,29 ly. The final step of the even before E°AB/AB•-; thus, apparent perbenzoates. Figure 3 illustrates convolution analysis is the determina- values close or larger than 0.5 are the experimental detection of the DET tion of the transfer coefficient α, expected. An analogous description can transition in terms of both khet, in which describes how variations of ∆G° be applied to the homogeneous DET which the overlapping of two different affect the activation free energy (α = counterpart. When the standard poten- rate/driving force curves is evident, and ∂∆G≠/∂∆G°). If the double-layer effect tial for the formation of the electron α, which has a wavelike potential is neglected, α can be easily obtained donor (homogeneous ET) or the elec- dependence connecting the two linear from the experimental data as α = - trode potential E is made less negative, variations describing the pure mecha- ∂ ∂ ∆ 2.303(RT/F) log khet/ E, being G°= it may be possible to observe a progres- nisms. These results point to a mecha- F(E - E°). Provided only one DET mech- sive transition from a stepwise to a con- nistic transition that can be described anism takes place, α depends linearly certed mechanism. In the borderline as a simple competition problem where α ∆ ≠ on E, being = 0.5 + F(E -E°)/8 G0 . situation, the reduction may occur two different reaction coordinates and Because of the large activation overpo- through both mechanisms, although transition states are provided to the tential suffered by the concerted DET, with different rates. DET reaction. Therefore, the actual for-

46 The Electrochemical Society Interface • Winter 2002 neous ET are substituent dependent. This trend is similar to that found also with the substituted benzyl bro- mides.36 The substituent effect on the ET rate is not driven by thermodynamics but is ruled by the ET intrinsic barrier. For these processes the substituent effect is attributed to the exis- tence of a stabilizing ion-dipole type interaction between the halide ion and the substituted benzyl radical within the solvent cage. By enhancing the electron-withdrawing character of the aryl substituent and thus the dipole moment of the benzyl radical, the interaction becomes stronger, the transition state becomes more reactant- like, and the activation energy decreas- es. A quadratic rate/free energy relationship has been developed to describe this particular mechanism.35 In conclusion, it seems now reason- ably well established that a reactivity difference of as much as a few orders of magnitude may be caused by specific interactions between the caged frag- FIG. 4. Comparison between the heterogeneous standard rate constants of the reduction of disulfides. The ments. The intrinsic barrier of this data are plotted as a function of E° and were obtained in DMF at 25°C. The data are taken from Refs. 32 type of fast concerted DET may turn () and 33 (). The solid line is meant to underline the experimental trend. out to be similar to that of the slow stepwise DET mechanism proceeding through formation of loose radical anions. Distinguishing between these two types of mechanism is the current challenge. mation (or lack of formation) of AB•- pelling evidence for this borderline from AB must be considered a function mechanism, in which the ET step is Intramolecular DET of the competition between the rates of almost as slow as that of a concerted the stepwise and concerted pathways. DET. Typically, this situation is verified In the electrochemical literature Besides the already discussed “classi- when the ET yields a σ* radical anion, there are many examples of radical cal” DET mechanisms, there are two which then undergoes an endergonic anions decaying by fragmentation of a borderline mechanisms that are worth cleavage. The formation of these loose σ bond.4,6-8 As a rule, the intramolec- mentioning. Let us consider first the radical anions is accompanied by a sig- ular reaction is an activated rearrange- case of a stepwise mechanism. If the ET nificant increase of the bond length, ment of the electrogenerated radical product is a quite delocalized radical which amounts to 0.8 Å for the S-S anion to yield a transition state lead- anion, such as with aromatic com- bond.33 This large inner reorganization ing to the formation of the fragmenta- pounds, a small inner reorganization is is responsible for the low values of k°het tion products. The overall process may required and thus the most important observed for these compounds. Of be accompanied by a significant sol- contribution to the intrinsic barrier course, this depends on the nature of vent reorganization. Usually, the frag- comes from the solvent reorganization. the substituents. Figure 4 shows that for mentation of radical anions is Consequently, the ET is fast and the the least easily reducible compounds described as heterolytic or homolytic, DET reaction occurs in the endergonic (which form σ* radical anions) there is depending on whether the extra elec- region (α > 0.5). However, there are little difference. Only for diaryl disul- tron crosses the cleaving bond or not. chemical systems for which it is reason- fides containing good electron-with- For heterolytic cleavages, particularly π able to expect that in the formation of drawing groups (e.g., NO2) the * char- but not exclusively, the process can the radical anion the breaking bond acter of the SOMO is such to reduce the sometimes be viewed as an intramole- will weaken and stretch to some extent. inner reorganization term quite signifi- cular DET.37 This happens when the This would cause an increase of the cantly. antibonding orbital initially hosting inner reorganization and thus the ET The other borderline DET mecha- the electron (most often a π* orbital) is becomes slow. The voltammetric peak nism originates when a concerted DET weakly coupled to the σ* orbital of the becomes electrochemically irreversible yields fragments that may interact elec- cleaving bond and thus an intramole- and is pushed to more negative poten- trostatically in the solvent cage.34,35 cular π* →σ* ET appears as a quite tials than E°. Thus, α can be lower than This mechanism has been recently realistic description of the mechanism. 0.5, almost as low as with concerted identified, particularly, with ring-sub- Most often, however, the coupling is DETs, even though the mechanism is stituted benzyl chlorides.34 In fact, strong and thus an actual intramolecu- still stepwise. Studies on the reductive although for these compounds the E°of lar DET cannot be easily invoked.38 cleavage of the C-S30 and, particularly, the concerted DET is almost substituent The fact that the fragmentation occurs S-S bond31-33 have provided com- independent, the rates of the homoge- by either mechanism depends on a

The Electrochemical Society Interface • Winter 2002 47 delicate balance between driving force, (a) intrinsic barrier, and electronic coupling. The former issue has been discussed in some detail for example by Evans, Lessard and their co-workers for the substituent dependent heterolytic/homolytic transition observed upon reduction of α-nitrocumenes.39 Other interesting discussion on these matters can be found in the literature.6,7,37,40 In order to have an unequivocal intramolecular DET, either homolytic or heterolytic, the electron exchanging centers must be clearly identified and so must their thermodynamics (E° and BDE). This is accomplished by connecting two electroactive moieties, the donor D and the acceptor A, by means of a spacer, Sp. Most often the latter is an active molecular bridge mediating the intramolecular ET. This type of strategy has led to fascinating results in the area of intramolecular nondissocia- (b) tive ETs, leading to a deep knowledge on how electrons are transferred through bonds and space.41 Research in the area of DETs is still in its infancy, despite the relevance of such reactions in some chemical systems (for example, O-O and S-S bond cleavage in biologically relevant molecules). This problem was tackled by focusing initially on the DET free-energy dependence in well-defined D-Sp-A molecular systems.42,43 With a first series of com- pound, in which a tertiary bromide was A, ring-substituted benzoates were the donors, and cyclohexyl was the spacer,42 it was found that the intramolecular DET rate was more sensitive to variation of ∆G° than found for the corresponding intermolecular reaction (free diffusing D/A couples). This experimental out- come was explained by considering the effect of the ring substituent. In fact, a FIG. 5. Free energy dependence of the logarithm of the intramolecular ( and ) and intermolecular () ET more electron-withdrawing para sub- rate constants for reduction of either a tertiary bromide (graph a) or a perester (graph b) acceptor in DMF at stituent produces a shift of the centroid 25° C. The data are taken from Refs. 42 and 43; The dashed lines are the fits of the intramolecular and of the donor π* orbital, in which the intermolecular data. The substituents in the intramolecular reaction are as follows: (right to left, graph a, ) unpaired electron is initially located, Y = H, 3-OPh, 3-F, 2,3-benzo, 4-SO2Me, and 4-CN; (right to left, graph b, ) Y = H, 3-F, 3,6-difluoro, away from the acceptor. 3,4,5,6-terafluoro, and 3,4,5,6-tetrachloro, or ( ) Y = 3-NO2 and 4-NO2. With a similar series of D-Sp-A compounds, in which a peroxide was A, substituted phthaloyl groups provided sure that the donor and acceptor orbitals rate is a consequence of random distance the D moieties, and cyclohexyl was the are kept essentially unaffected by driving and orientation distributions in the spacer,43 the relative D/A distance force variations. encounter complex. Further research is could be controlled. As expected, the The final proof to these findings is now underway to investigate the dis- intramolecular slope was found to be provided by the effect of introducing tance dependence of the DET rate, par- slightly smaller than the intermolecular strong electron-withdrawing groups on ticularly, by using biologically-relevant 44 one, thanks to a larger solvent reorgani- the donor moiety. In fact, with two molecular bridges. Other nonelectro- ∆ ≠ nitro-phthaloyl derivatives we found, chemical studies (e.g., photoinduced zation energy (and thus G0 ). Figure 5 illustrates the results obtained with both experimentally and theoretically, DET) on similar problems are expected both series of compounds. It was veri- that the effective D/A distance increases to provide complementary information fied that the intramolecular DET rate and thus that the intramolecular rate on this important issue. obeys the same rules already highlight- constant is much smaller than expected. Conclusions ed for the intermolecular and heteroge- The intermolecular rates are perfectly in The DET concept is continuously neous processes. To predict the rate line with the results obtained with the growing and its relevance in many areas equally well, however, one has to make other donors because the intermolecular

48 The Electrochemical Society Interface • Winter 2002 is only now being realized. Several appli- 11. T. Lund and H. Lund, Acta Chem. Scand., 41. G. L. Closs and J. R. Miller, Science, 240, 440 cations in the area of synthetic organic B40, 470 (1986). (1988). chemistry are obviously possible, as these 12. C. P. Andrieux, I. Gallardo, J.-M. Savéant, 42. S. Antonello and F. Maran, J. Am. Chem. and K. B. Su, J. Am. Chem. Soc., 108, 638 Soc., 120, 5713 (1998). reactions may yield powerful nucle- (1986). 43. S. Antonello, M. Crisma, F. Formaggio, A. ophiles or reactive radicals. A variety of 13. J. Grimshaw, J. R. Langan, G. A. Salmon, J. Moretto, F. Taddei, C. Toniolo, and F. such examples may be found in a recent Chem. Soc. Faraday Trans., 90, 75 (1994). Maran, J. Am. Chem. Soc., 124, 11503 article by Lund45 and a chapter by 14. H. K. Jensen and K. Daasbjerg, J. Chem. Soc., (2002). Peters.46 DETs may be employed in the Perkin Trans 2, 1251 (2000). 44. S. Antonello, F. Maran, F. Formaggio, A. context of the hydrogenation versus elec- 15. S. Antonello, F. Formaggio, A. Moretto, C. Moretto, and C. Toniolo, in Organic Toniolo, and F. Maran, J. Am. Chem. Soc., Electrochemistry, The Electrochemical tron transfer/protonation issue, as a test 123, 9577 (2001). Society Proceedings Volume Series, PV of heterogeneous reduction mecha- 16. M. S. Workentin, F. Maran, and D. D. M. 2002-10, Pennington, NJ (2002). nisms.47 DET processes have been also Wayner, J. Am. Chem. Soc., 117, 2120 45. H. Lund, J. Electrochem. Soc., 149, S21 used as probes of the electrocatalytic (1995). (2002). mechanisms in microemulsions.48 The 17. S. Antonello, M. Musumeci, D. D. M. 46. D. G. Peters, in Organic Electrochemistry (4th usefulness of the stepwise-to-concerted Wayner, and F. Maran, J. Am. Chem. Soc., ed), H. Lund and O. Hammerich, Eds.: 119, 9541 (1997). Marcel Dekker, New York (2000). mechanistic transition has been shown 18. R. L. Donkers, F. Maran, D. D. M. Wayner, 47. J. Lessard, J. M. Chapuzet, R.Labrecque, G. to be instrumental to explain the activa- and M. S. Workentin, J. Am. Chem. Soc., Martin, S. Antonello, and F. Maran, in tion mechanism of the so-called thermal 121, 7239 (1999). Reactive Intermediates in Organic and SRN1 reactions.49 19. M. S. Workentin and R. L. Donkers, J. Am. Biological Electrochemistry, D. G. Peters, H. J. The concept of DET can be extended Chem. Soc., 120, 2664 (1998). Schäfer, M. S. Workentin, and J. Yoshida, to the problem of concerted electron and 20. R. L. Donkers, M. S. Workentin, Chem. Eur. Eds.; The Electrochemical Society 50 J., 7, 4012 (2001). Proceedings Volume Series, PV 2001-14, p. proton transfer, as recently proposed. 21. R. L. Donkers and M. S. Workentin, J. Phys. 140, Pennington, NJ (2001). Very recently, applications of the DET Chem. B, 102, 4061 (1998). 48. Y. Shao, M. V. Mirkin, and J. F. Rusling, J. concepts may be found in the area of bio- 22. E. D. German and A. M. Kuznetsov, J. Phys. Phys. Chem. B, 101, 3202 (1997). logically relevant systems.44,51-53 Chem., 98, 6120 (1994). 49. C. Costentin, P. Hapiot, M. Médebielle, and Applications in surface chemistry can also 23. J.-M. Savéant, Adv. Phys. Org. Chem., 26, 1 J.-M. Savéant, J. Am. Chem. Soc., 121, 4451 be imagined38 by taking advantage of the (1990). (1999). 24. R. A. Marcus, J. Phys. Chem. A, 101, 4072 50. M. W. Lehmann and D. H. Evans, J. Phys. fact that, upon electroreduction of suit- (1997). Chem. B, 105, 8877 (2001). able compounds, reactive radicals can be 25. C. P. Andrieux and J.-M. Savéant, J. 51. A. G. Griesbeck, T. Heinrich, M. generated directly near the electrode, as Electroanal. Chem., 205, 43 (1986). Oelgemöller, J. Lex, and A Molis, J. Am. shown for example in the case of the dis- 26. M. G. Severin, G. Farnia, E. Vianello, and M. Chem. Soc., 124, 10972 (2002). sociative reduction of aryl diazonium salts C. Arévalo, J. Electroanal. Chem., 251, 369 52. C. S. Burns, L. Rochelle, and M. D. E. on carbon surfaces.54 It is also worth men- (1988). Forbes, Org. Lett., 3, 2197 (2001). 53. W.-S Li and H. Morrison, Org. Lett., 2, 15 tioning the growing interest in the mech- 27. J. C. Imbeaux and J.-M. Savéant, J. Electroanal. Chem., 44, 169 (1973). (2000). anisms of photoinitiated DET and their 28. S. Antonello and F. Maran, J. Am. Chem. 54. P. Allongue, M. Delamar, B. Desbat, O. relationship with the corresponding ther- Soc., 119, 12595 (1997). Fagebaume, R. Hitmi, J. Pinson, and J.-M 55-57 mally-activated processes. As we and 29. S. Antonello and F. Maran, J. Am. Chem. Savéant, J. Am. Chem. Soc., 119, 201 (1997). Wayner concluded in a recent account on Soc., 121, 9668 (1999). 55. L. Pause, M. Robert, and J.-M Savéant, J. Am. Chem. Soc., 123, 4886 (2001). DETs,6 significant progress has been made 30. M. G. Severin, M. C. Arévalo, F. Maran, and 56. S. Nath, A. K. Singh, D. K. Palit, A. V. Sapre, E. Vianello, J. Phys. Chem., 97, 150 (1993). in this area over the last few years. On the and J. P. Mittal, J. Phys. Chem. A, 105, 7151 31. T. B. Christensen and K. Daasbjerg, Acta. basis of the increasing interest in the area, (2001). Chem. Scand., 51, 307 (1997). we believe that other key challenges will 57. D. C. Magri, R. L. Donkers, and M. S. 32. K. Daasbjerg, H. Jensen, R. Benassi, F. Workentin, J. Photochem. Photobiol. A: be addressed in the coming years, particu- Taddei, S. Antonello, A. Gennaro, and F. Chem., 138, 29 (2001). larly in view of useful applications of the Maran, J. Am. Chem. Soc., 121, 1750 (1999). concepts described above. 33. S. Antonello, R. Benassi, G. Gavioli, F. Taddei, and F. Maran, J. Am. Chem. Soc., References 124, 7529 (2002). 34. A. Cardinale, A. Gennaro, A. A. Isse, and F. About the Authors 1. R. A. Marcus and N. Sutin, Biochim. Biophys. Maran, in New Directions in Organic Acta, 811, 265 (1985). Electrochemistry, A. J. Fry and Y. Matsumura, Flavio Maran is with the 2. N. S. Hush, Elektrochem., 61, 734 (1957). Eds.; The Electrochemical Society Department of Physical Chemistry 3. L. Eberson, Acta Chem. Scand., 17, 2004 Proceedings Volume Series, PV 2000-15, p. at the University of Padova, Padova, (1963). 136, Pennington, NJ (2000). Italy. His main research interests are 4. L. Eberson, in Electron Transfer Reactions in 35. L. Pause, M. Robert, and J.-M. Savéant, J. molecular electrochemistry and Organic Chemistry, Springer-Verlag, New Am. Chem. Soc., 122, 9829 (2000). electron transfer mechanisms. He York (1987). 36. Y. Huang and D. D. M. Wayner, J. Am. 5. J.-M. Savéant, J. Am. Chem. Soc., 109, 6788 Chem. Soc., 116, 2157 (1994). may be reached by e-mail at (1987). 37. J.-M. Savéant, J. Phys. Chem., 98, 3716 [email protected]. 6. F. Maran, D. D. M. Wayner, and M. S. (1994). Workentin, Adv. Phys. Org. Chem., 36, 85 38. P. D. Burrow, G. A. Gallup, I. I. Fabrikant, Mark Workentin is with the (2001). and K. D. Jordan, Austral. J. Phys., 49, 403 Department of Chemistry at the 7. J.-M. Savéant, Adv. Phys. Org. Chem., 35, 117 (1996). University of Western Ontario, (2000). 39. Z.-R. Zheng, D. H. Evans, E. S. Chan-Shing, London, Ontario, Canada. His main 8. L. Eberson, Acta Chem. Scand., 53, 751 E. S., and J. Lessard, J. Am. Chem. Soc., 121, research interests are in the area of (1999). 9429 (1999). 9. J.-M. Savéant, J. Am. Chem. Soc., 114, 10595 mechanistic organic electrochemistry (1992). 40. P. Masak, in Topics in Current Chemistry; J. and photochemistry. He may be 10. H. Lund, K. Daasbjerg, T. Lund, and S. U. Mattay, Ed.; Springer-Verlag: Berlin, Vol. reached at [email protected]. Pedersen, Acc. Chem. Res., 28, 313 (1995). 168, p. 1 (1993).

The Electrochemical Society Interface • Winter 2002 49