5750
J . Phys. Chem. 1989, 93, 5750-5756
tials.I0 These calculations, while possible, are enormously time-consuming. This makes the ideal case extremely important as a base line for comparison with experiment. In keeping with this philosophy, Table I1 presents calculated parameters and rate constants for small-molecule reactions a t 298 K for two representative viscosities, 0.5 and 1.0 cP, assuming that K = I . As required by the modified Stokes-Einstein equation used to calculate them, the translational and rotational diffusion coefficients are inversely proportional to the viscosity. Ne,,, or more exactly Ne,,- 1, is directly proportional to the viscosity. The duration of an encounter, 7,and the average number of repeat spot contacts per first spot contact, R, are also proportional to the viscosity. The rate constants k,,,, and kNoyes are approximately inversely proportional to the viscosity. The values of k,,, presented in Table 11, along with the simple inverse relationship to viscosity and the dependence on reactive spot size shown in Figures 2 and 3, should allow chemists to estimate reasonable values for max(10) Benesi, A. J. J . Chem. Phys. 1986, 85, 374.
imum bimolecular rate constants for reactions between small molecules. In conclusion, the essential aspects of the theory of elementary bimolecular reaction^^,^ have been presented and discussed, and the results have been compared with experiment for several re1. For these, experiment and theory agree within actions with K a factor of 2. Data have been presented which allow reasonable estimates for bimolecular rate constants for reactions between small molecules assuming maximum reactivity, i.e., K = 1. There is no reason to expect the agreement with experiment to be less for other elementary bimolecular reactions with K < I , especially if the reactants mimic the assumptions of the model. Computer Program. The F O R T R A N program used in these calculations is available on request from the author.
-
Acknowledgment. The programmer for these calculations was Ms. Lori Divins, who I thank for her endless patience and useful suggestions. I also thank Professor E. F. Caldin for his extensive help. It was his interest which spurred the continuation and clarification of my earlier work.
Photoinduced Electron-Transfer Reactions of Ruthenium( I I ) Complexes. 1. Reductive Quenching of Excited Ru(bpy),2+ by Aromatic Amines Noboru Kitamura,* Haeng-Boo Kim, Sumio Okano, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Received: October 24, 1988; In Final Form: March 7 , 1989)
The rate constants (k,) and activation parameters for reductive quenching of *Ru(bpy)32+(bpy = 2,2'-bipyridine) by 19 aromatic amines were determined in acetonitrile. The activation enthalpies increased from + I to +6 kcal/mol with decreasing k, while the activation entropies were almost constant around -15 eu for all quenchers except for those of highly exoergic reactions. The mechanism of the reductive quenching is discussed on the basis of k , as well as of the activation parameters. The quenching was shown to depend on the nature of the aromatic amine; the quenching by tertiary amines was always faster than that by primary and secondary amines when k , are compared at the same free energy change. The results are explained by the variation of outer-sphere reorganization energy with the nature of a quencher. For the quenching by tertiary and primary/secondary amines, the radii of the quenchers (rqeX)were estimated to be 3 and 2.4 A, respectively, by analyzing free energy relationships of the activation free energy of the quenching. The difference in rQcxbetween tertiary and primary or secondary amines (0.5-0.6 A) was in good agreement with that between N,N-dimethylamino and anilino groups estimated by the space-filling model (0.7 A). The solvent reorganization around the amino group is more relevant than that around an aromatic ring or a whole molecule to the actual outer-sphere reorganization energy.
1. Introduction In a series of publication^,^-^ we report elaborate studies on photoredox quenching of ruthenium( 11) complexes by various neutral organic electron donors (D; aromatic amines) and acceptors (A; nitroaromatics, quinones, and so forth), with special reference to the analyses of the activation parameters for the
quenching. The ultimate aim is to present a general approach for designing efficient photoredox systems through detailed kinetic mechanistic analyses. Recent interst in photoinduced electron-transfer reactions seems to be directed to the intramolecular systems in which D and A are linked by a variety of spacers (R)6 or are introduced to bio-
(1) Part 1: this paper. (2) Part 2: Kim, H.-B.; Kitamura, N.; Kawanishi, Y . ;Tazuke, S . J . Phys. Chem., companion paper in this issue. (3) Part 3: Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S . J . Phys. Chem., companion paper in this issue. (4) Part 4: Kim, H.-B.; Kitamura, N.; Tazuke, S. Manuscript in preparation. (5) Preliminary reports have been already reported: (a) Kitamura, N.; Okano, S . ; Tazuke, S . Chem. Phys. Left. 1982, 90, 13. (b) Kim, H.-B.; Kitamura, N.; Kawanishi, Y . ;Tazuke, S . J . Am. Chem. SOC.1987, 109, 2506. (c) Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S . J . Phys. Chem. 1987, 91, 2033. (d) Tazuke, S.; Kitamura, N.; Kim, H.-B. In Supramolecular Phofochemisfry; Balzani. V., Ed.; NATO ACI Series, Series C; Reidel: Dordrecht, The Netherlands, 1987; Vol. 214, p 97.
(6) (a) Siemiarczuk, A.; McIntosh, A. R.; Ho, T.-F.; Stillman, M. J.; Roach, K. J.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S . J . Am. Chem. SOC. 1983, 105, 7224. (b) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J . Am. Chem. Soc. 1985,107,5562. (c) Closs, G.L.; Calcaterra, L. T.; Green, N . J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986, 90, 3673. (d) Heiler, D.; McLendon, G.;Rogalskyj, P. J . Am. Chem. SOC.1987, 109, 604. (e) Danielson, E.; Elliott, M.; Merkert, J. W.; Meyer, T. J. J . Am. Chem. SOC.1987, 109, 2519. ( f ) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N . S . J . Am. Chem. SOC.1987, 109, 3258. ( 9 ) Penfield, K. W.; Miller, J . R.; Paddon-Row, M . N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S . J . Am. Chem. SOC.1987, 109, 5061. (h) Gust, D.; Moore, T. A,; Moore, A. L.; Barrett, D.; Harding, L. 0.; Makings, L. R.; Liddell, P. A.; De Schryver, F. C.; Van der Auweraer, M.; Bensasson, R. V.; Rouge, M. J. Am. Chem. SOC.1988,110,321. (i) Schanze, K. S.; Sauer, K. J . Am. Chem. SOC.1988, 110, 1180.
0022-3654/89/2093-S750$01.S0/0
1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5751
Reductive Quenching of Ruthenium(I1) Complexes logical molecules (i.e., protein^).^ Since the electron-transfer distance and the mutual orientation between D and A are fixed in these systems, understanding of the electron-transfer step a t the molecular level is more accessible than in intermolecular systems. The lifetime of the charge-separated state (D+-R-A-) can be also extended by means of successive electron transfer among linked chromophores in particular, in the D-S-AI (-A2) systems, where S is a photosensitizer.6h Nonetheless, the intramolecular charge-separated state has to be converted eventually to isolable products by intermolecular electron transfer. This is the essential part of the conversion and storage of light energy and/or their synthetic application^.^^^ Profound understanding of intermolecular electron-transfer systems ( S A, S D, or S D A) is thus required. Until now, numerous studies on electron-transfer quenching of luminescent sensitizers have been reported aimed at understanding intermolecular electron-transfer processes.lo,ll Nevertheless, almost all studies are limited to the measurement of quenching rate constants ( k , ) at a fixed temperature, and the discussion was sometimes developed on the assumption of k , to be the actual electron-transfer rate constant. Comparison of the experimental data with available theories is thus confined to the relation between k , and the overall free energy change, and the theories represented by MarcusI2 have been scarcely tested on the activation parameters for photoinduced electron transfer. I n the field of thermal electron-transfer reactions, kinetic measurement with temperature variation is a commonly accepted t e ~ h n i q u e . ' ~ , ' As ~ J ~early as 1960, it was shown that the apparent electron-transfer rate constant could be a complex function consisting of an equilibrium constant between reaction intermediates, revealing the temperature dependence to be negative.I5 Such information had not been applied to kinetic analyses of photoinduced electron-transfer reactions until we demonstrated for the first time that oxidative quenching of excited-state tris(2,2'-bipyridine)ruthenium(II) complex, * R ~ ( b p y ) ~showed ~ + , a negative temperature d e p e n d e n ~ e . ~ In " this case, the observed k, is no longer equal to the electron-transfer rate constant. Detailed kinetic analyses over a wide temperature range are thus absolutely necessary. As we have shown for a limited case, observation of both positive and negative temperature dependences for oxidative quenching of * R ~ ( b p y ) enabled ~ ~ + us to obtain the rate constants of forward (ij = 23) and reverse (ij = 32; see Scheme I) electron-transfer p r ~ c e s s e s . ~ - ~ ~ , ~
+ +
+
+
(7) (a) Mayo, S. L.; Ellis, W. R.; Crutchley, R. J.; Gray, H. B. Science 1986, 233, 948. (b) Bechtold, R.; Gardineer, M. B.; Kazmi, A,; van Hemelryck, B.; Isied, S. S. J . Phys. Chem. 1986, 90, 3800. (c) Lieber, C. M.; Karas, J. L.; Gray, H. B. J . Am. Chem. SOC.1987,109, 3778. (d) McLendon, G.; Pardue, K.; Bak, P. J . Am. Chem. SOC.1987, 109, 7540. (e) Elias, H.; Chou, M. H.; Winkler, J . R. J . A m . Chem. SOC.1988, 110, 429. (0 Axup, A. W.; Albin, M.; Mayo, S. L.; Crutchley, R. J.; Gray, H. B. J . Am. Chem. SOC.1988, 110, 435. (9) Karas, J. L.; Leiber, C. M.; Gray, H. B. J . A m . Chem. SOC.1988, 110, 599. (8) Photochemical Conversion and Storage of Solar Energy; Connoly, J. S., Ed.; Academic Press: New York, 1981. (9) (a) Ishitani, 0.; Pac, C.; Sakurai, H. J . Org. Chem. 1983, 48, 2941. (b) Pac, C.; Miyauchi, Y.; Ishitani, 0.;Ihama, M.; Yasuda, M.; Sakurai, H. J . Org. Chem. 1984, 49, 26. (c) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J . Am. Chem. SOC.1987, 109, 305. (d) Ishitani, 0. Ph.D. Thesis, Osaka University, 1987. ( I O ) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834; Isr. J . Chem. 1970, 8, 259. ( 1 1 ) (a) Kavarnos, G. J.; Turro, N . J. Chem. Rev. 1986, 86, 401. (b) Eberson, L. Adv. Phys. Org. Chem. 1982, 18, 79. (12) (a) Marcus, R. A. J . Chem. Phys. 1956,24,966,979;J. Chem. Phys. 1965, 43, 679. (b) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155. (c) Sutin, N. In Tunneling in Biological Systems; Chance, B., et al., Eds.; Academic Press, New York, 1979; p 201. (d) Sutin, N. frog. Inorg. Chem. 1983, 30, 441. (e) Newton, M. D.; Sutin, N. Annu. Rev. Phys. Chem. 1984, 35, 437. (13) (a) Braddock, J. N.; Meyer, T. J. J. Am. Chem. SOC.1973, 95, 3158. (b) Marcus, R. A,; Sutin, N. Inorg. Chem. 1975, 14, 213. (14) (a) Pennington, D. E. In Coordination Chemistry; Martel, A. E., Ed.; ACS Monograph 174; American Chemical Society: Washington, DC, 1978; Chapter 3. (b) Reynolds, W. L.; Lumry, R. W. Mechanisms of Electron Transfer; Ronald Press: New York, 1966. (c) Cannon, R. D. Electron Transfer Reactions; Butterworth: London, 1980. ( 1 5 ) (a) Collinson, E.; Dainton, F. S.; Mile, B.; Tazuke, S.;Smith, D. R. Nafure 1963. /98,26. (b) Tazuke, S. Ph.D. Thesis, University of Leeds, 1962.
Activation/thermodynamic parameters for photoinduced electron-transfer reactions have been rarely reported1618after our preliminary publication on the temperature dependence of the quenching of * R ~ ( b p y ) , ~in+ 1982.sa This situation is understandable because this is extremely laborious work in practice, in particular to obtain experimental results that can be compared with theoretical predictions. In addition to reaction rate and excited-state lifetime measurements at various temperatures, all physical constants and parameters such as electrochemical potentials of reactants (electron donor and acceptor) as well as the viscosity, refractive index, and dielectric constant of the solvent have to be evaluated as a function of temperature. Because of these technical difficulties, the possible choice of reaction constituents and solvents is restricted. Among various photoredox sensitizers, we chose ruthenium(I1) complexes represented by R ~ ( b p y ) ~ ~The + . reasons are as follows: (i) The spectroscopic and electrochemical properties are well studied."23 (ii) The excited state is subjected to both oxidative (for R ~ ( b p y ) 3 ~ +E,l 2 and reductive quenching16J7~'9*20~24-32 (Ru3+/*Ru2+)= -0.87 and E 1 / 2 ( * R ~ 2 + / R ~=++0.80 ) V vs SdE in acetonitrile at 298 K).21333 (iii) The excited state is sufficiently luminescent and the lifetime is relatively long ( R ~ ( b p y ) ~ ~ + , 840-850 ns in acetonitrile at 298 K).2'933 (iv) The excited-state properties including its temperature dependence of the lifetime have been well s t ~ d i e d . ~ "(v) ~ ~Photo/thermal ligand labilization
(16) Baggott, J. E. J . Phys. Chem. 1983, 87, 5223. (17) Garrera, H . A,; Gsponer, H. E.; Garcia, N. A,; Cosa, J . J.; Previtali, C. M. J . Photochem. 1986, 33, 257. (18) For organic systems: (a) Baggott, J. E.; Pilling, M. J. J . Chem. Soc., Faraday Trans. I 1983,79,221. (b) Murakami, 0.;Kikuchi, K.; Kokubun, H . Abstracts, XI1 International Conference in Photochemistry, Tokyo, 1985; p 540. (19) Kalyanasundaram, K. Coord. Chem. Rev. 1983, 46, 159. (20) Juris, A,; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (21) (a) Kitamura, N.; Kawanishi, Y.;Tazuke, S . Chem. Phys. Lett. 1983, 97, 103. (b) Kawanishi, Y.; Kitamura, N.; Tazuke, S. Inorg. Chem., in press. (22) (a) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (b) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1984, 175, 229. (c) Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Left. 1985, 119, 61. (d) Barigelletti, F.; Juris, A,; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1987, 26, 4115. (23) Kitamura, N.; Sato, M.; Kim, H.-B.; Obata, R.; Tazuke, S. Inorg. Chem. 1988, 27, 651. (24) (a) Bock, C. R.; Connor, J. A,; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J . A m . Chem. SOC.1979, 101, 4815. (b) Meyer, T. J. frog. Inorg. Chem. 1983, 30, 389. (25) (a) Shioyama, H.; Masuhara, H.; Mataga, N. Chem. Phys. Left. 1982, 88, 161. (b) Shioyama, H. Ph.D. Thesis, Osaka University, 1985. (26) Ohno, T.; Yoshimura, A.; Mataga, N. J. Phys. Chem. 1986,90,3295. (27) Anderson, C. P.; Salmon, D. J.; Meyer, T.J.; Young, R. C. J . A m . Chem. SOC.1977, 99, 1980. (28) Ballardini, R.; Varani, G.; Indelli, M. T.;Scandola, F.; Balzani, V. J . Am. Chem. SOC.1978, 100, 7219. (29) Milder, K.; Das, P. K. J . Am. Chem. SOC.1982, 104, 7462. (30) (a) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982, 22, 117. (b) Rau, H.; Franck, R.; Greiner, G.J . Phys. Chem. 1986, 90, 2476. (c) Milosavlzevic, B. H.; Thomas, J. K. J. Am. Chem. SOC.1986, 108,2513. (d) Chiorboli, C.; Indelli, M. T.; Scandola, M. A. R.; Scandola, F. J . Phys. Chem. 1988, 92, 156. (e) Olmsted, J., 111; Meyer, T. J. J . Phys. Chem. 1987, 91, 1649. (f) Olmsted, J., 111; McClanahan, S. F.; Danielson, E.; Younathan, J. N.; Meyer, T. J. J . A m . Chem. SOC.1987, 109, 3297. (31) Sandrini, D.; Maestri, M.; Belser, P.; von Zelewsky, A.; Balzani, V. J . Phys. Chem. 1985, 89, 3675. (32) (a) Juris, A,; Gandolfi, M. T.; Maufrin, M. F.; Balzani, V. J . Am. Chem. SOC.1976, 98, 1047. (b) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1976, 98, 6536. (c) Creutz, C.; Sutin, N. J . A m . Chem. SOC.1977, 99, 241. (d) Balzani, V.; Scandola, F.; Orlandi, G.; Sabbatini, N.; Indelli, M. T. J. Am. Chem. SOC.1981, 103,3370. (e) Sandrini, D.; Gandolfi, M. T.; Maestri, M.; Bolletta, F.; Balzani, V. Inorg. Chem. 1984, 23, 3017. (f) Krishnan, C. V.; Brunschwig, B.; Creutz, C.; Sutin, N. J. A m . Chem. SOC.1985, 107, 2005. (9) Chiorboli, C.; Scandola, F.; Kisch, H. J . Phys. Chem. 1986, 90, 221 1 . (h) Ballardini, R.; Gandolfi, M. T.; Balzani, V. Inorg. Chem. 1987, 26, 862; J . Phys. Chem. 1988, 92, 56. (33) E , ~ ( R u ~ + / * R u ' +=) -0.79 to -0.90 V, E,I~(*Ru'+/Ru+)= f0.76 to f 0 . 8 4 V! and ro = 850-1 1 I O ns (in acetonitrile at room temperature) have been also reported. For details, see ref 22.
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989
is negligible.34b (vi) Variations in size, formal charge number, and electronic properties are possible by modulating the liga n d ~ . ' ~The - ~choice ~ of acetonitrile as a standard solvent is due to its low freezing point (-43.8 0C),38its aprotic nature, its relative inertness to electron-transfer photochemistry, and its adequacy in electrochemistry. In a series of publications, we discuss reductive] and oxidativeZ quenching of *Ru(bpy),2+ as well as redox quenching of cis*Ru(phen)2(CN), (phen = 1 ,lo-phenanthroline) by neutral D and A3 in acetonitrile on the basis of the standard (AGi,, AHi,. and ASij) and the activation parameters (AGij*, AHij*,and AS,') for the electron-transfer processes (ij = 23, 32, and/or 30 in Scheme I). Also, the activation parameters obtained in this study and those reported previously for the quenching of * R u L ~ containing ~+ bulky bpy ligands (L = 4,4'-dialkyl-2,2'-bipyridine)39 will be compared with those predicted by Marcus theory in part 4 of this ~ e r i e s . ~
Kitamura et al. TABLE I: Temperature Effects on Emission Lifetime of Ru(bpy);* and Solvent Properties in Acetonitrile
273 278 283 288 293 298 303 308 313 318 323 328 333 338
2. Experimental Section Materials. Ru(bpy),*' used in this study was the same sample reported earlier.5a Acetonitrile was used as a solvent throughout this study unless otherwise noted and was purified according to the literature.40 Reagent-grade aromatic amines were supplied from Tokyo Kasei Co., Ltd., Kanto Chemical Industries Co., Ltd., or Yoneyama Chemical Co., Ltd. All of the amines were purified prior to use as follows (for the numbering of the amines, see Table 11). 1 was purified by repeated recrystallizations from benzene. 2 was treated with activated charcoal in acetone and then recrystallized from a n-hexane-benzene mixture three times. 3, 4, and 7 were recrystallized twice from ethanol. 11 was purified by recrystallizations from n-hexane followed by sublimation in vacuo. 15 was recrystallized from methanol. All other aromatic amines were refluxed over K O H for several hours and then fractionally distilled under reduced pressure. Sample solutions in a quartz cell (10-mm optical path) were deaerated thoroughly by Ar gas purging over 20 min and then sealed off a t the constriction of a Pyrex branch. Apparatus. Emission spectroscopy was made by a Hitachi MPF-4 spectrofluorometer equipped with a Hamamatsu Photonics R928F photomultiplier. Excited-state lifetimes of Ru(bpy)3Z+in acetonitrile at various temperatures were determined by the system consisting of a Nd:YAG laser (Quanta Ray, Model DCR-I; 355 nm, pulse width -6 ns), a monochromator (Union Giken, Model RA-40 1 ), a photomultiplier (Hamamatsu Photonics, Model R928). a storage scope (Iwatsu, Model TS-8123). and a micro-
(34) (a) Van Houten, J.; Watts, R. J . J . Am. Chem. SOC.1976,98, 4853. (b) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J . Am. Chem. SOC. 1983, 105,4803. (c) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (d) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T . J. J . Am. Chem. Soc. 1984, 106, 2613. (e) Juris, A,; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1985, 24, 202. (f) Dressick, W . J.; Cline, J.. 111; Demas, J. N.; DeGraff, B. A. J . Am. Chem. Soc. 1986, 108, 7567. (g) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. H . Inorg. Chem. 1986, 25, 227. (35) (a) Kitamura, N.; Kim, H.-B.; Kawanishi, Y.; Obata, R.; Tazuke, S. J . Phys. Chem. 1986, 90, 1488. (b) Kim, H.-B.: Kitamura, N.; Tazuke, S. Chem. Phys. Lett. 1988. 143, 77. (c) Kim, H.-B.; Kitamura, N.; Tazuke, S. Asbtracts, XI1 IUPAC Symposium on Photochemistry, Bologna, 1988; p 426. (d) Kim, H.-B.; Kitamura, N.; Tazuke, S. J. Phys. Chem., submitted. (36) Hiraga, T.; Kitamura, N.; Kim, H.-B.; Tazuke, S. J . Phys. Chem. 1989, 93, 2940. (37) (a) Milder, S. J.; Gold, J . S.; Kliger, D. S. J . Phys. Chem. 1986, 90, 548. (b) Ferguson, J.; Krausz, E. R.; Maeder, M. J . Phys. Chem. 1985,89, 1852. (c) Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981, 81, 42. (d) Bradley, P. G.; Kress, N.; Hornberger, B. A,; Dallinger, R. F.; Woodruff, W. H.J . Am. Chem. SOC.1981, 103, 7441. (e) Carroll, P. J.; Brus, L. E. J . Am. Chem. SOC.1987, 109, 7613. (f) Yersin, H.; Gallhuber, E. J . Am. Chem. SOC. 1984, 106, 6582. (g) Ferguson, J.; Krausz, E. J. Lumin. 1986, 36, 129. (h) Blakley, R. L.: Myrick. M. L.; DeArmond, M. K. Inorg. Chem. 1988, 27, 589. ( i ) Myrick. M. L.; Blakiey, R. L.; DeArmond, M. K.; Arthur, M. L. J . Am. Chem. SOC.1988, 110, 1325. (j)Krausz, E. J . Phys. Chem., in press. (38) Riddick, J . A.; Bunger, W. B. Organic Soiuents, 3rd ed.; Techniques of Chemistry; Wiley-Interscience: New York, 1970. (39) Kitamura, N . ; Rajagopal, S.; Tazuke, S. J . Phys. Chem. 1987, 91, 3767. (40) Perrin, D.D.; Armargo, A. L. F.: Perrin, D. R. Purificarion of Laborarory Chemicals, 2nd ed.; Pergamon Press: New York, 1980.
(I
I 480 1 380 I 260 1 120 980 840 710 590 480 390 320 260 210 170
0.446 0.42 I 0.399 0.379 0.361 0.345 0.330 0.3 16 0.304 0.292 0.28 1 0.27 I 0.262 0.253
43.67 42.76 41.86 40.95 40.05 39.14 38.23 37.33 36.42 35.52 34.6 I 33.70 32.80 3 1.89
1.495 1.61 1 1.73 1 1.854 1.980 2.110 2.241 2.377 2.515 2.657 2.802 2.951 3.102 3.257
0.70 0.71 0.73 0.74 0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.90 0.93 0.96
-2.09 -2.14 -2.18 -2.23 -2.28 -2.34 -2.39 -2.45 -2.51 -2.58 -2.64 -2.71 -2.79 -2.87
Emission lifetime of Ru(bpy),2+ in the absence of a quencher. b q ,
D,,and k , , a r e the viscosity, the static dielectric constant, and the diffusion rate constant in acetonitrile, respectively. For Ds,see also ref 43 and 44. 4 and k,, were calculated by the equations in the Experimental Section. CElectrostatic work necessary to bring two product ions together to the close-contact distance (eq 4). Redn and oxidn represent reductive and oxidative quenching of *Ru(bpy)32+by neutral D and A. respectively
computer (NEC, Model PC-9801m). Every emission decay determined around 620 nm obeyed a single-exponential function in the temperature range examined (0-65 "C). For the emission quenching experiments, temperature (5-50 "C) was controlled by circulating a water-ethylene glycol mixture with a Thermoelectric Model TE-12K (Sharp Co., Ltd.), a Thermoelite BH-71 (Yamato Scientific Co., Ltd.), or an EYELA COOL EL-1 (Tokyo Rikakikai Co., Ltd.). Cyclic voltammetry in acetonitrile was carried out by a Hokuto Denko HB-301 potentiostat and an HB-104 function generator. Working, counter, and reference electrodes were a platinum disk, a platinum wire, and a SCE, respectively. A 0.1 M tetra-n-butylammonium perchlorate solution was used as a supporting electrolyte. Determination of the Quenching Rate Constant (k,) and the Free Energy Change of the Forward Electron-Transfer Step (AGZ3). A bimolecular quenching rate constant, k,, was determined by a conventional Stern-Volmer plot (emission intensity) in acetonitrile a t a given temperature (5-50 "C). Emission lifetimes of Ru(bpy),2+ in the absence of a quencher ( T ~ were ) determined by separate experiments. T~ shown in Table I were used to calculate k,. The origin of the temperature dependence of T~ has been discussed e l ~ e w h e r e .Since ~ ~ ~ ~k, ~includes the contributions from both diffusional and electron-transfer steps, a correction for the diffusional effect on k, was made by eq 1.z4
In eq 1, k, and k I z are the corrected activation-controlled quenching rate constant and the diffusion rate constant in acetonitrile, respectively. k i z was calculated by the Smoluchowski equation (eq 2).4i rR and rQare the effective radii of *Ru(bpy)32+ 2RT kl2 = 30007(
6 rR
+
Q '
(7.1 A) and a quencher (3.8 A), r e s p e c t i ~ e l y .7~ is ~ the viscosity in acetonitrile. Although the 7 values from -40 to +50 "C have been reported,4z the data were not sufficient to calculate k I 2 in the temperature region studied. An empirical relation of 7-l = (2.62 X 10-2)T ("C) + 2.24(-30 < T < +50 "C), derived from the literature was used for the calculation of k12at various temperatures. 17 and k l z in acetonitrile are included in Table I. (41) von Smoluchowski, M . Z . Phys. Chem., Stoechiom. Verwandts-
chaffs[. 1917, 92, 129.
(42) Janz. G . R.; Tomkins, R. P. Non-aqueous Electroiyfe Handbook; Academic Press: Yew York, 1972.
Reductive Quenching of Ruthenium(I1) Complexes
T h e J o u r n a l of P h y s i c a l C h e m i s t r y ,
Vol. 93, No, 15, 1989 5753
TABLE 11: Rate Constants and Activation Parameters for Reductive Quenching of *Ru(bpy)32+in Acetonitrile at 298 K
no.
quencher (E(D+/D) vs SCE)' N,N,N',N'-tetramethylphenylenediamine (0.12)' p-phenylenediamine (0.18)' N,N,N',N'-tetramethylbenzidine (0.43)e phenothiazine (0.53)' I-naphthylamine (0.63)g 2.4-dimethylaniline (0.70)" p-anisidine (0.71)g N,N-dimethyl-p-toluidine (0.7 l)c N-methylphenothiazine (0.73)' o-anisidine (0.76)' N,N-diethylaniline (0.76)e N-methylaniline (0.77)* p-toluidine (0.78)'' 2,5-dimethylaniline (0.79)" 2,6-dimethylaniline (0.81)' N,N-dimethylaniline (0.81)c diphenylamine (0.83)' m-toluidine (0.84)" o-toluidine (0.85)''
1 2
3 4 5 6
7 8 9 10 11 12 13 14 15 16 17 18 19
AG23,
kcal/mol -13.7 -12.4 -6.8 -4.3 -2.0 -0.4 -0.1 -0.1 +0.3 +I.O
+1.0 +1.2 +is +i.7 +2.2
+2.2 +2.6 +2.9 +3.1
k, (k0Lb M-i s-l 3.6 (1.3) X 1O1O 2.4 (1.1) X 10" 2.0 (1.0) X 10" 7.6 (5.6) X lo9 1.6 (1.6) X lo8 5.9 (5.9) x 107 7.8 (7.5) X lo8 1.2 (1.1) x 109 1.7 (1.6) X lo9 5.4 (5.4) x 107 2.5 (2.5) X lo8 2.4 (2.4) X lo7 2.0 (2.0) x 107 1.2 (1.2) x 107 7.6 (7.6) X lo6 9.9 (9.9) x 107 1.8 (1.8) X lo7 1.3 (1.3) X lo6 1.2 (1.2) X lo6
AH23*,
AG23*,
kcal/mol 0.9
eu -9.4
kcal/mol 3.7 4.1 4.6 6.9 7.5 6.1 5.8 5.6 7.6 6.6 8.1 8.2 8.5
1.7
-8.1
1.3
-11.1
2.9 3.7 2.1 1.4 1.8 3.5 2.4 3.8 3.7 3.9 3.9 2.8 3.7 5.2 6.2
-13.4 -12.9 -13.3 -14.6 -12.8 -13.7 -14.2 -14.5 -15.0 -15.4 -16.3 -14.5 -1 5.1 -14.1 -12.2
rQ: 8,
8.8
7.1 8.2 9.4 9.8
3.9 3.3 4.7 4.6 3.7 3.5 3.7 3.7 4.8 3.8 3.7 3.6 3.3 3.3 3.5 3.6 4.2 3.4 3.4
"Oxidation potential of D in acetonitrile (volt vs SCE). b k , and ko are the activation-controlled and experimentally observed bimolecular quenching rate constants, repspectively. CRadiiof the quenchers estimated by the space-filling model. See also, ref 51. dCalorie per mole degree. FReference 24. fMann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Systems; Dekker: New York, 1970. gReference 27. hReference 42. 'Kowert, B. A,; Marcoux, L.; Bard, A. J. J . Am. Chem. Soc. 1972, 94, 5538. 'Determined in this study in the presence of 0.1 M tetra-n-butylammonium perchlorate (CH3CN at 298). SCHEME I
*Ru(bpy)T
11
t
0. e k12 * R u ( b p y ) $ ..... Q '2 1
65 "C is only 0.26 kcal/mol (Table I) and, thus, wp is almost temperature independent. Determination of t h e A c t i v a t i o n P a r a m e t e r s . We assumed the quenching mechanism in Scheme I. In Scheme I, * R ~ ( b p y ) ~ ~ + + Q,*R~(bpy),~+-.Q,R~(bpy),~+/+-.Q-/+,and R ~ ( b p y ) , ~ + /++ Q-1' are initial reactants before electron transfer, an association complex, a product ion pair after electron transfer, and product ions after charge separation, respectively (in the present case, Q = D).
e '23 Ru(bpyI3+/3+.....a"k32
c
j
I
I
'30
2 ;
kq
The free energy change of the forward electron-transfer step, AG23, was calculated on the basis of the reduction and oxidation + = -1.35 V)2iband a potentials of R ~ ( b p y ) , ~(Eij2(,Ru2+/Ru+) quencher ( E i 2(D+/D)), respectively, and the excited-state energy of R ~ ( b p y ) ~ ' +(Eo,o= 2.1 V):" AG23
= EI/Z(D+/D) - E ~ / ~ ( R u ' + / R u +-) Eo,o
+ w P- W ,
(3)
w p or w, is the electrostatic work necessary to bring two product ions or two initial reactants together to the close-contact distance ( d = rQ r R ) , respectively. Since we use neutral quenchers throughout this study, w, is zero. wp was calculated by eq 4.12c
+
=
(
IOOOD,kBT
Z , and Zbare the charge numbers of the two product ions, and D, is the static dielectric constant of the medium.43 In the present study, the ionic strength of the medium ( h ) was zero unless otherwise stated. Although w depends on temperature due to the temperature effect on D,,49,44 the variation of w pfrom 0 to (43) In most studies, D,= 37.538has been used. Throughout the present study, however, D,(CH3CN) = 39.2 at 298 K was used: Wurflinger, A. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 653. See also ref 44. (44) Temperature dependence of D,(CH,CN) is available from Wurflinger43and Janz and tom kin^.^* D,reported by Wurflinger gave a linear In DJln Tvs In T plot around the temperature regions of interest (260-300 K ) , while those collected by Janz and Tomkins yielded a nonlinear plot. We took the D,data reported by Wurflinger throughout this study. An empirical relation In DJln T = -1.1956 In T + 104837 was used to calculate D,at arbitrary temperature in Table I.
= Ki2k23k30/(k30 + k32)
(5)
>> k32 k30 -3 k c a l / m ~ l . ~ ~ ~ ~Recently, ~~ systems in the R ~ L ~ ~ + - a r o m a amine-N,N-dimethylformamide tic (iii) With decreasing k,, AH23* increases from + I to +6 by means of nanosecond transient-absorption spectrosc~py?~ The kcal/mol, while AS23* remains almost constant around -15 eu product ion yields were nearly unity regardless of AGZ3as well except for those of highly exoergic reactions at AC23 < -5 as of the nature of L (L = bpy, 4,4-dimethyl-2,2'-bipyridine, phen, kcal/mol. Reductive quenching of * R ~ ( b p y ) , ~by + aromatic 2,2'-bipyrazine, 3,3'-bipyridazine, and 2-(2'-pyridyl)pyrimidine). amines proceeds via the enthalpy-controlled reaction path. It is clear that no back electron transfer to the ground- (kb)and/or 4. Discussion excited-state reactants (k32) takes place at all on the nanosecond time scale. Furthermore, when the case I1 mechanism ( k 3 2>> 4 .I . Mechanism of Reductive Quenching. Reductive quenching k3o) prevails for reductive quenching of * R ~ ( b p y ) ~the ~ + tem, of * R ~ ( b p y ) , ~or+ * R u L ~ ~(L+ = diimine ligands) by aromatic amines has been studied by several research g r ~ ~ p ~ perature . ~ dependence ~ ~ of ~k, should ~ ~become ~ negative ~ ~ with~ increasing ~ - ~ AG23 as for oxidative quenching of * R ~ ( b p y ) ~ ~ However, +.~-~~ Bock et al. reported that the reductive quenching proceeded via AH,,' was always positive in the AG23 range between -13 and case I (eq 6; k30 >> k32) on the basis of the following kinetic +3 kcal/mol (Table 11). W e conclude that the reductive analysis.24 quenching of * R ~ ( b p y ) , ~proceeds + via case I, and therefore, the When the transition-state theory is applied to the forward activation parameters determined for k, represent those of the where electron-transfer step (k23), eq 5 is written as in eq forward electron-transfer process, k23: eq 9. k , = K12u23Fexp(-AC2,*/RT) (10) 4.2. Effects of Chemical Structures of Amines on Electron Transfer. The quenching by T A and PSA obeys the case I 023 is the frequency factor of k23 and F = k,O/(k30 k32). AG23* mechanism, and there is no evidence of the mechanistic difference is given by Marcus (eq I X represents the sum of the innerbetween the quenching by PSA and TA. The results in Figure 2, however, indicate that the quenching of * R ~ ( b p y ) ~is ~de+ pendent on the chemical structures of aromatic amines. Namely, the quenching by PSA is always slower than that by T A a t the same AG23 except for the diffusion-controlled reactions a t AG23 (Xi)and outer-sphere (A,) reorganization energies. Combining < -5 kcal/mol and the data for p-anisidine (7).47 The dependence eq I O and 1 1 , we obtained eq 12. In case I (k30>> k32, F = 1) of AH23*on AG23 for TA is also different from that for PSA. As
+
k, = KI2u2,Fexp[ and
-5 kcal/mol as presented in Figure 4. A&* was almost constant around -15 eu irrespective of the nature of a quencher. Recently, we reported k , and activation parameters for the quenching of a series of * R u L ~ complexes ~+ containing bulky bpy ligands (L = 4-isobutyl-4'-methyl-2,2'-bipyridine (BM-bpy) and
similar to the results by Sandrini et aL3I However, A S 2 3 ' were more favorable for the bulkier complexes. The decrease in k, for the bulkier R U L ~ ~is 'not explicable by the nonadiabaticity of k23. We thus ruled out the possibility of nonadiabaticity. Solvent Interactions. Specific interactions of the N-H bond in PSA with solvent molecules are a likely reason. According to Marcus theory, AG23* and AH23* should be correlated with A,, and the slope of a AC23' or AH23* vs A, plot is expected to be while A S 2 3 ' is independent of A0.48 Nonetheless, we could not obtain any distinct evidence of the relationship between AG23' or AH23' and A,, and instead, the activation parameters were roughly related with the Gutmann's solvent donor number (DN).49 Any significant difference in the solvent effects between the quenching by T A and PSA could not be observed. The specific solvent interactions with R ~ ( b p y ) , ~ and/or + * R ~ ( b p y ) ~are ~+ responsible for the solvent effects on the reductive quenching. Outer-Sphere Reorganization Energy. Outer-sphere reorganization energy (A,) is known to be dependent on the size of the reactants ( r R and rQ) and, thus, the electron-transfer distance: d = rR ra. The dielectric continuum model (spherical model) provides the theoretical expression of A, (eq 14),'2.s0and A, can
+
be easily compared with experiments. In eq 14, Do, is the optical dielectric constant of the medium.51 rR has been reported to be 7.1 8, for * R ~ ( b p y ) ? + and , ~ ~ rQ is estimated to be 4.1 or 3.5 (3.6) 8, as the average of T A or PA (PSA), respectively, by the space-filling model (SFM; Table II).s2 rR, yQ, and eq 14 give ,A. to be 18 and 21 (20) kcal/mol for the quenching of *Ru(bpy)?+ by T A and PA (PSA), respectively. It is apparent that the variation of rQ with the nature of aromatic amines leads to the change in A, by 2-3 kcal/mol and, thus, to that in k , as expected from eq 10 and 11. Qualitatively, the difference in k , (AC23')
-
-
(48) On the assumption of AG'(inner) 0 and AS'(outer and inner) 0, AGi.', AHi;, and ASij' can be written as follows: AG..' = AG,,,,' AGO' = - R f In (hZ/kBT) + (A/4)[ 1 + (AG,,/A)l2; AHij' iJ AH,,,,,' + AH,' = -(RT/2) + (A/4)[1 + (AGi /A)]'; and ASi.' = AStlPnl'= R In ( h Z / k B T )(R/2). When IAGijI -5 kcal/mol). Although it is not clearly seen in Figure 4, the observed S 2 3 * slightly decrease with increasing AC23(>-5 kcal/mol). The larger decreases in a S 2 3 with increasing AG2, have been similarly observed in the quenching of *Ru(BM-bpy)?+ or * R ~ ( M p - b p y ) ~by~ D.4*39 + Unfavorable AS23* are compensated by favorable M23*, resulting in apparent fits of the observed AG23* vs AG23 (Figure 5 ) with eq 11. In the case of the oxidative quenching of * R ~ ( b p y ) by ~ ~ A, + the observed AHi]*and ASij* ( i j = 23 and 32, Scheme I ) agree quite well with calculated AH,*(A) and ASij*(X), r e ~ p e c t i v e l y . ~ . ~ As far as i j = 23 and 32 in the oxidative quenching of *Ru(bpy);+ are concerned, the prediction of' Marcus theory is satisfactorily acceptable. For the reductive quenching, on the other hand, disagreement of the theory with AH23*and A S 2 3 ' suggests that the mechanism of electron transfer in the reductive quenching may be different from that involved in the oxidative quenching. Further detailed discussion will be published in a forthcoming paper.4
(53) Inner-sphere reorganization energy was assumed to be negligibly small as compared with the outer-sphere reorganization energy. (54) Taylor, G . N.; Chandross, E. A,; Schiebel, A. H. J . Am. Chem. Soc.
Registry No. 1, 27215-51-6; 2, 106-50-3; 3, 366-29-0; 4, 92-84-2; 5, 134-32-7; 6 , 95-68-1; 7, 104-94-9;8, 99-97-8; 9, 4020-30-8; 10, 90-04-0; 11, 91-66-7; 12, 100-61-8; 13, 106-49-0; 14, 95-78-3; 15, 87-62-7; 16, 121-69-7; 17, 122-39-4; 18, 108-44-1; 19, 95-53-4; Ru(bpy),*+, 1515862-0.
w
+
1974, 96,
2693.
(55) Israelachvili, J. N. Intermolecular and Surface Forces: Academic Press: London, 1985; Chapter 7.
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