Photoinduced electron transfer from excited [tris(2,2′-bipyridine)ruthenium(II)]²⁺ to a series of anthraquinones with small positive or negative Gibbs energy of reaction. Marcus behavior and negative activation enthalpies

Abstract

In the electron transfer (ET) quenching reactions of electronically excited *Ru(bpy)₃²⁺ in acetonitrile an increase of the rate constant k_q is observed in the series of 2-methyl-, 1-chloro-, and 1-nitro-anthraquinone as quenchers. If alkali salts are used as supporting electrolytes the AQ^- radical anions are found to form specific associates with the alkali cations. In the presence of non-associating tetraalkylammonium salts the system follows the predictions of Marcus theory. Numerical methods are developed which allow the determination of the rate constants of the conventional reaction scheme. This analysis shows that the quantum yield of free AQ^- radical anion formation is governed by the interplay of forward, reverse and back ET. Negative activation enthalpies are found for the activation controlled quenching reactions. From the numerical analysis of the system of rate constants it is inferred that this phenomenon is due to the elementary ET step in the reaction sequence. We discuss the pre-equilibrium and elementary reaction models for reactions with negative activation enthalpy and present, to our knowledge, the first example of successful discrimination between them.

References

1. (a) D. Rehm and A. Weller, Ber. Bunsen-Ges. Phys. Chem., 1969, 73, 834; (b) D. Rehm and A. Weller, Isr. J. Chem., 1970, 8, 259.
2. (a) R. A. Marcus, J. Chem. Phys., 1956, 24, 966; (b) R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155; (c) R. A. Marcus, Angew. Chem., 1993, 105, 1161.
3. (a) L. J. Calcaterra, G. L. Closs and J. R. Miller, J. Am. Chem. Soc., 1983, 105, 670; (b) G. L. Closs, L. T. Calcaterra, N. J. Green, K. W. Penfield and J. R. Miller, J. Phys. Chem., 1986, 90, 3673; (c) I. R. Gould, D. Ege, J. E. Moser and S. Farid, J. Am. Chem. Soc., 1990, 112, 4290.
4. (a) I. R. Gould, R. H. Young, L. J. Mueller and S. Farid, J. Am. Chem. Soc., 1994, 116, 8176; (b) I. R. Gould, R. H. Young, L. J. Mueller, A. C. Albrecht and S. Farid, J. Am. Chem. Soc., 1994, 116, 8188; (c) I. R. Gould and S. Farid, Acc. Chem. Res., 1996, 29, 522; (d) M. G. Kuzmin, Pure Appl. Chem., 1993, 65, 1653; (e) M. G. Kuzmin, J. Photochem. Photobiol. A: Chem., 1996, 102, 51; (f) N. A. Sadovskii, M. G. Kuzmin, H. Görner and K. Schaffner, Chem. Phys. Lett., 1998, 282, 456.
5. M. Z. Hoffmann, F. Bolletta, L. Moggiand and G. L. Hug, J. Phys. Chem., Ref. Data, 1989, 18, 219.
6. (a) D. Plancherel, J. G. Vos and A. von Zelewski, J. Photochem., 1987, 36, 267; (b) J. R. Darwent and K. Kalyanasundaram, J. Chem. Soc., Faraday Trans. 2, 1981, 77, 373; (c) A. Vlcek jr. and F. Bolletta, Inorg. Chem. Acta, 1983, 76, L227.
7. (a) N. Kitamura, S. Okano and S. Tazuke, Chem. Phys. Lett., 1982, 90, 13; (b) N. Kitamura, R. Obata, H.-B. Kim and S. Tazuke, J. Phys. Chem., 1987, 91, 2033; (c) N. Kitamura, H.-B. Kim, S. Okano and S. Tazuke, J. Phys. Chem., 1989, 93, 5750; (d) H.-B. Kim, N. Kitamura, Y. Kawanishi and S. Tazuke, J. Phys. Chem., 1989, 93, 5757; (e) N. Kitamura, R. Obata, H.-B. Kim and S. Tazuke, J. Phys. Chem., 1989, 93, 5764.
8. R. Frank and H. Rau, Recl. Trav. Chim. Pays.-Bas, 1995, 114, 556.
9. E. I. Kapinus and H. Rau, J. Phys. Chem., 1998, 102, 5569.
10. (a) T. Ohno, A. Yoshimura and N. Mataga, J. Phys. Chem., 1990, 94, 4871; (b) T. Kakitani, N. Matsuda, A. Yoshimori and N. Mataga, Prog. React. Kinet., 1995, 20, 347; (c) N. Mataga, T. Ashai, Y. Kanda, T. Okada and T. Kakitani, Chem. Phys., 1988, 127, 249.
11. J. R. Bolton, M. D. Archer, in Electron tranfer in inorganic, organic and biological systems, Adv. Chem. Ser., ed. J. R. Bolton, N. M. Mataga and P. G. McLendon, ACS, Washington, DC, 1991, vol. 228, p. 7.
12. (a) M. V. Smoluchowski, Z. Physik. Chem. (Leipzig), 1918, 92, 129; (b) P. Debye, Trans. Electrochem. Soc., 1942, 82, 265; (c) M. Eigen, Z. Physik. Chem., (Wiesbaden), 1954, 1, 176.
13. C. Chiorboli, M. T. Indelli, M. A. Rampi-Scandola and F. Scandola, J. Phys. Chem., 1988, 92, 156.
14. Estimated to be equal to that of methylviologen.^15a.
15. (a) C. R. Bock, J. A. Connor, A. R. Gutierrez, T. J. Meyer, D. G. Whitten, B. P. Sullivan and J. K. Nagle, J. Am. Chem. Soc., 1979, 101, 4815; (b) W. E. Jones and M. A. Fox, J. Phys. Chem., 1994, 98, 5095.
16. As the quenching process is a bimolecular reaction this quantum yield is not a characteristic property of the quencher or donor but it is a function of the reaction conditions, especially of the quencher concentration. With increasing quencher concentration it approaches an upper limit. The quencher concentration in this work is so high that this limit is reached.
17. R. Frank and H. Rau, EPA Newsletter, 1999, 65, 39.
18. (a) A. E. Brodsky, L. L. Gordienko and L. S. Degtiarev, Electrochim. Acta, 1968, 13, 1095; (b) M. E. Peover, J. Chem. Soc., 1962, 4540; (c) A. Aumüller and S. Hünig, Liebigs Ann. Chem., 1986, 165; (d) H. Shalev and D. H. Evans, J. Am. Chem. Soc., 1989, 111, 2667; (e) R. G. Compton, B. A. Coles, M. B. G. Pilkington and D. Bethell, J. Chem. Soc., Faraday Trans., 1990, 86, 663; (f) K. Umemoto, Chem. Lett., 1985, 1415; (g) J. Posdorfer, M. Olbrich-Stock and R. N. Schindler, Z. Phys. Chem., N. F. München, 1991, 171, 33.
19. (a) I. Piljac, M. Tkalcec and B. Grabaric, Anal. Chem., 1975, 47, 1369; (b) V. D. Bezuglyi, L. V. Shkodina and T. A. Alekseeva, Soviet Electrochemistry, 1984, 19, 1282; (c) M. E. Peover and J. D. Davies, J. Electroanal. Chem., 1963, 6, 46; (d) R. L. Blankespoor, D. L. Schutt, M. B. Tubergen and R. L. DeJong, J. Org. Chem., 1987, 52, 2059; (e) S. Lazarov, A. Trifonov and J. Panajotov, Z. Phys. Chem., 1965, 233, 49.
20. C. Chiorboli, F. Scandola and H. Kisch, J. Phys. Chem., 1986, 90, 2211.
21. Reaction spectra are available as supplementary material.
22. (a) R. M. Wightman, J. R. Cockrell, R. W. Murray, J. N. Burnett and S. B. Jones, J. Am. Chem. Soc., 1976, 90, 2526; (b) J. L. Roberts jr., H. Sugimoto, W. C. Barrette jr. and D. T. Sawyer, J. Am. Chem. Soc., 1985, 107, 4556.
23. J. Mayer and R. Kraslukianis, J. Chem. Soc., Faraday Trans., 1991, 87, 2943.
24. (a) W. R. Fawcett and G. Liu, J. Phys. Chem., 1992, 96, 4231; (b) P. Suppan, J. Photochem. Photobiol. A: Chem., 1990, 50, 293.
25. We have eliminated the change of viscosity due to increasing salt concentration by a multiplicative correction η_r=η/η₀= 1 + 0.018 c^1/2_NaClO4+ 0.73 c_NaClO4. In eqn. (1) the diffusion controlled rate constants are dependent on η, but also the activation controlled ones if they represent adiabatic reactions with the longitudinal relaxation times in the pre-exponential factor. (See e.g. ; D. F. Calef and and P. G. Wolynes, J. Chem. Phys., 1983, 78, 430, 3387; E. M. Kosower and and D. Huppert, Annu. Rev. Phys. Chem., 1986, 37, 127; G. Grampp, W. Harre andr and W. Jaenicke, J. Chem. Soc., Faraday Trans., 1, 1987, 83, 161.).
26. (a) W. Roth, T. Bastigkeit and S. Boerner, Liebigs Ann., 1996, 1313; (b) Wei, J. Chem. Eng. Sci., 1996, 51, 2995; (c) M. Mozuirkewich, J. J. Lamb and S. W. Benson, J. Phys. Chem., 1984, 88, 6435; (d) J. J. Lamb, M. Mozuirkewich and S. W. Benson, J. Phys. Chem., 1984, 88, 6441; (e) A. Menon and N. Sathyamurthy, J. Phys. Chem., 1981, 85, 1021; (f) R.-R. Lii, R. A. Gorse, jr., M. C. Sauer and S. Gordon, J. Phys. Chem., 1979, 83, 1803; (g) D. D. Davies, R. E. Huie and J. T. Herron, J. Chem. Phys., 1973, 59, 628; (h) J. Connor, A. VanRodselar, R. W. Fair and O. P. Strausz, J. Am. Chem. Soc., 1971, 93, 560; (i) J. L. Jourdain, G. Poulet and G. LeBras, J. Chem. Phys., 1982, 76, 5827; (j) M. Meot-Ner (Mautner) and F. H. Field, J. Am. Chem. Soc., 1978, 100, 1356; (k) D. K. Sen Sharma and P. Kebarle, J. Am. Chem. Soc., 1982, 104, 19; (l) R. Hiatt and S. W. Benson, J. Am. Chem. Soc., 1972, 94, 6886.
27. V. D. Kiselev and J. G. Miller, J. Am. Chem. Soc., 1975, 97, 4036.
28. (a) O. Hammerich and V. D. Parker, Acta Chem. Scand. Ser. B, 1983, 37, 379; (b) J. B. Olson and T. H. Koch, J. Am. Chem. Soc., 1986, 108, 756; (c) J. Wang, Ch. Doubleday, jr. and N. J. Turro, J. Am. Chem. Soc., 1989, 111, 3692.
29. B. Reitstoen and V. D. Parker, J. Am. Chem. Soc., 1990, 112, 4968.
30. (a) N. Sutin and B. M. Gordon, J. Am. Chem. Soc., 1961, 83, 70; (b) J. N. Braddock and T. J. Meyer, J. Am. Chem. Soc., 1973, 95, 3158; (c) J. L. Cramer and T. J. Meyer, Inorg. Chem., 1974, 13, 1250.
31. E. Baggott and M. J. Pilling, J. Chem. Soc., Faraday Trans 1, 1983, 79, 221.
32. (a) N. J. Turro, G. F. Lehr, J. A. Butcher, R. A. Moss and W. Guo, J. Am. Chem. Soc., 1982, 104, 1754; (b) R. A. Moss, W. Lawrynowicz, N. J. Turro, I. R. Gould and Y. Cha, J. Am. Chem. Soc., 1986, 108, 7028.
33. (a) S. R. L. Fernando, U. S. M. Maharoof, K. D. Deshayes, T. H. Kinstle and M. T. Ogawa, J. Am. Chem. Soc., 1996, 118, 5783; (b) H. A. Garrera, J. J. Cosa and C. M. Previtali, J. Photochem. Photobiol. A: Chem., 1991, 56, 267; (c) R. B. Murphy and W. F. Libby, J. Am. Chem. Soc., 1977, 99, 39; (d) H. Werner, E. O. Fischer, B. Heckl and C. G. Kreiter, J. Organomet. Chem., 1971, 28, 367; (e) T. Shimomura, K. J. Tölle, J. Smid and M. Scwarc, J. Am. Chem. Soc., 1967, 84, 796; (f) A.-M. Albrecht-Gary, C. Dietrich-Buchecker, Z. Saad and J.-P. Sauvage, J. Chem., Soc., Chem. Commun., 1992, 280.
34. (a) M. Mozurkewich and S. W. Benson, J. Phys. Chem., 1984, 88, 6429; (b) K. N. Houk and N. G. Rodan, J. Am. Chem. Soc., 1984, 106, 4293; (c) R. A. Marcus and N. Sutin, Inorg. Chem., 1975, 14, 213; (d) R. A. Marcus, J. Electroanal. Chem., 1977, 82, 9; (e) W. Stiller, R. Schmidt, E. Müller and N. V. Shokierev, Z. Phys. Chem. (Munich), 1992, 162, 119; (f) Y. Chen, A. Rauk and E. Tschuikow-Roux, J. Phys. Chem., 1991, 95, 9900; (g) R. A. Marcus and N. Sutin, Comments Inorg. Chem., 1986, 5, 119.
35. Experimental quantities are usually indicated by the doubledagger symbol. The relationships are ΔG^‡=ΔG*–RT ln (hZ/kT); ΔS^≠=ΔS*+R ln (hZ/kT)–½R; ΔH^≠=ΔH*–½RT.^34c.
36. K. N. Houk, N. G. Rodan and J. Mareda, J. Am. Chem. Soc., 1984, 106, 4291.
37. This is a simplification, Marcus defines an (N– 1)-dimensional hypersurface, the set of configurations which separate the reactant and product configurations to be the transition state (R. A. Marcus, in Investigation of Rates and Mechanisms of Reactions, ed. E. S. Lewis, Techniques of Chemistry, vol. VI, Part 1, ed. A. Weissberger, J. Wiley & Sons, New York, London, Sydney, Toronto, 1974, pp. 13–46.
38. The form of the surfaces is dependent on the specific reaction. Houk and Rodan^34b have arbitrarily used Morse curves, we have also used parabolas for G, H and TS as well as a Gaussian for G and a parabola for TS: It seems that for all forms of the surfaces the situation of negative activation enthalpy can be reached when the reaction parameters are chosen suitably.