J. Phys. Chem. 1991,95,6919-6924
6919
Role of Solvation in the Electrogenerated Chemiluminescence of Hexanuclear Molybdenum Cluster Ion Robert D.Munoell and Daniel G. NOcera*-+ Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received: March 5, 1990; In F h l Form: April 8, 1991)
The solvent dependence of the electrogenerated chemiluminescence (ecl) of the hexanuclear cluster complex MO,jC114*- has been investigated. Overall ecl quantum yields and relative production yields of electronically excited Mo6CI142-ion have been measured for the annihilation of M@lJ by 4-cyano-N-methylpyridine(CMP) and M0&l14* in acetone (Ac), acetonitrile (AcN), propionitrile (PrN), butyronitrile (BUN),dichloromethane (DCM), and 1,Zdichloroethane (DCE). Partitioning of the electrochemical excitation energy between excited- and ground-statereaction channels can be described by electron-transfer models. A striking result to emerge is that the formation yield of Mo6CIIt-*does not correlate explicitly with the solvent's dielectric constant but rather parallels the kinetic diameter of the solvent. This result suggests that electron transfer occurs to Mo&Il4-encapsulated in a solvent shell for polar solvents such as Ac, AcN, RN, and BUN. Conversely, despite a significant difference in the kinetic diameters of the more weakly coordinating solvents DCM and DCE, similar excited-state formation yields imply that the solvent shell about the M0&l14- ion is not retained during electron-transferannihilation. Our observations, which are in accordance with simple ion-dipole models, establish the utility of chemiluminescence as a sensitive measure of specific solvation of reactants participating in bimolecular electron-transfer events.
Introduction The aim of our research into electrogenerated chemiluminescence (ecl) reactions is to define and understand the factors governing highly exergonic electron-transfer processes. In an annihilation of a powerful oxidant e l c c t m h d u m i n ~system, t and reductant to leave one of the products in a luminescent electronic excited state is directly competitive with reaction to produce ground-state products. Electron annihilation of oxidized and reduced reactants to generate the excited state will proceed in the Marcus normal region (Le., increasing rate with an increasingly negative free energy driving force AG),whereas direct reaction to ground state occurs in the inverted region (i.e., decreasing rate with more negative AG),as shown schematically in Figure 1.I4 Inasmuch as electron-transfer parameters such as distance, driving force, and reorganizational energy are manifest in determining the rates of electron transfer, these factors will also bear directly on the partitioning of the electron transfer between the normal and inverted regions and hence the ecl efficiency. We have found ecl from M&Y6* ions in nonaqueous solutions to be particularly useful in the study of highly exergonic electron-transfer reactions. &I originating from the Mo6CII4*excited = 760 nm, &,, = 0.19) is produced upon electron state (&,,,,mx transfer between the electrochemicallygenerated Mo6Cll) and Mo&l14s iom5 Moreover, owin to the significant magnitudes of the Mo6CI14-12-and reduction potentials and the relatively low energy of the Mo6CII4*excited state, ecl from the annihilation of Mo6CI14-and Mo6CI14)-with a variety of electractive donors D-and acceptors A+, respectively, can be observed6according to the following reactions: M06C114- D- M0&114~-* D (1)
f
+ MO6c114~-+ A+
+
+
M0&114~-* A (2) where D ' represents a one-electron-reduced aromatic nitro or pyridinium compound and A+ is a one-electron oxidized aromatic amine compound. An important result to have emerged from our ecl studies is that the partitioning between the excited- and ground-state reaction channels, and hence ecl efficiency, is extremely sensitive to the distance of the electron-transfer annihilation reaction. Thus the influence of factors mediating the distance of the electron transfer reaction will be manifest to the overall ecl efficiency. TO this end, we became interested in using ecl reactions as a probe of solvation properties at a molecular level. The conventional +
'Alfred P. Sloan Fellow and NSF Presidential Young Invatigator.
viewpoint in electron-transfer reactions is that the solvent may be treated as a dielectric continuum. However, it is becoming increasinglyclear that electron-transfer rates can be influenced by specific solventsolute interaction^."'^ Ultrafast kinetics studies have verified theoretical predictions that the formation of charge-separated states of aromatic organic molecules with small activation barriers can be limited by the motion of solvent owing to strong dielectric coupling between the solvent dipoles and the developing charge distribution of intramolecular electron-transfer event^.^'-^^ Specific solutesolvent effects are also important to inner-16J7 and outer-sphere'"" electron-transfer (1) (a) Sidere, P.; M a w , R. A. J. Am. Chem. Soc. 1981,103,748-752. (b) Marcus, R. A. J. Chem. Phys. 1965,13,2654-2657. ( 2 ) Meyer, T.J. Prog. Inorg. Chem. 1983,30, 389-440. (3) (a) Faulkner, L. R.; Glass, R. S.Chemicul and Biological Generalion ofExdred Srares;Adam, W., Gilcnto, G., M.; Academic: New York, 1982. (b) Faulkner, L. R. Int. Reo. Sci.; Phys. Chem., Ser. n o 1975,9,213-263. (4)(a) Bolktta, F.; Bonafcde, S.Pun Appl. Chem. 1986,58,1229-1232. (b) Balzani, V.; Bolletta, F. Comm. Inorg. Chem. 1983,2,211-226. (5) Noccra, D.0 . ; Gray, H. B. J. Am. Chem. Soc. 1984,106,824-825. (6) (a) Museell, R. D.; Noccra, D. G. J, Am. Chem. Soc. 1988, 110, 2764-2772. (b) Mussell, R. D.; Noccra, D. G. Polyhedron l a , 5,47-50. (7) (a) Barbara, P. F.; Jarzcba, W. Ado. Photochem. 1990, 15, 1-68. (b) Barbara, P. F.; Walker, G. C. Rev. Chem. Inrermed. 1988, 10,1-33. (c) Barbara, P. F.; Jarzcba, W. Acc. Chem. Res. 1988, 21, 195-199. (8) (a) Bagchi, B.; Fleming, G. R. J . Phys. Chem. 1990,94,9-20. (b) Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989,213,1674-1681. (9)Simon, J. D.Acc. Chem. Res. 1988, 21, 128-134. (IO) Weaver, M. J.; McManis 111, G. E. Acc. Chem. Res. 1990, 23, 294-300. (1 1) (a) Kahlow, M. A.; Jarzeba, W.; Kang, T. J.; Barbara,P. F. J. Chem. Phys. 1989,90, 151-158. (b) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J . Chem. Phys. 1988,88,2372-2378. (c) Kang, T.J.; Kahlow, M. A,; Giscr, D.; Swallen, S.;Nagarajan, V.; Jarzcba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 680015807. (d) Nagarajan, V.; Brearley, A. M.; Kang, T.-J.; Barbara, P. F. J. Chem. Phys. 1987, 86, 3183-3196. (e) Kahlow, M. A,; Kang, T.J.; Barbara, P. F. J . Phys. Chem. 1987, 91, 6452-6455. (12) (a) Maroncclli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 6221-6239. (b) Castner, E. W.; Maroncclli, M.; Fleming, G. R. 1. Chem. Phys. 1987,86, 1090-1097. (13) (a) Su,S.-G.; Simon, J. D. Chem. Phys. b i t . 1989, 158,423-428. (b) Su,S.-G.; Simon, J. D. J . Chem. Phys. 1988,89,908-919.(b) Simon, J. D.; Su,S.-G. J . Phys. Chem. 1988,92,2395-2397. (c) Su,S.-G.; Simon, J. D. J . Phys. Chem. 1987,91,2693-2696. (d) Su,S.-0.; Simon, J. D. 1. Chem. Phys. 1986,90,6475-6479. (14) Smit, K. J.; Warman, J. M.; de Haas, M.P.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. Lctr. 1988. 152, 177-182. (15) (a) Kosower, E. M.J. Am. Chem.Soc. 1985,107,3114-1118. (b) Kosower, E. M. Ace. Chem. Res. 1982, IS, 259-266. (16) (a) Hupp, J. T.; Weydert, J. Inorg. Chem. 1987,26,2657-2660.(b) Blackbourn, R. L.; Hupp, J. T. 1.Phys. Chem. 1988, 92,2817-2820. (17) Ennix, K. S.;McMahon, P. T.;Curtis, J. C. Inwg. Chem. 1987,26, 2660-2666.
0022-365419112095-6919$02.50/0 Q 1991 American Chemical Society
6920 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
- reaction c o o r d i n a t e Figure 1. Surface diagram for the electron-transfer annihilation of an acceptor (A+) and donor (D-)in the normal region to leave product A in an electronic excited state or in the inverted region to produce ground-statespecies; the reaction channels are characterized by the rate constants k, and k,, respectively.
reactions of transition-metal complexes in solution and at metal-solution interfaces.21 Insight into the nature of specific solutesolvent interactions has been provided by Truong's theoretical analysis22of the electron-transfer rates between R ~ ( b p y ) ~and ~+ bipyridinium ions. The observed dependence of the rate for electron-transfer quenching of R ~ ( b p y ) ~ on ~ +the * free energy driving force is accounted for by a model predicated on a hard sphere, comprised of Ru(bpy)?+ retaining a primary solvent cage, embedded in a dielectric continuum. In this model, most of the solvent reorganizational energy is ascribed to the first solvation layer. The validity of this approach for specific solvation has been demonstrated recently for inner- and outer-sphere electron-transfer reaction^'+'^*^' of transition-metal complexes in mixed-solvent systems in which the observed reaction rates reflect a localized solvent structure about reactants different from that in bulk solution. In regard to ecl chemistry, such specific solvation may be important in governing the annihilation reaction of the electrogenerated reactants because the electron transfer would be channelled through the solvation cavity of the electrogenerated reactants, thereby perturbing the distance and hence overall efficiency of the ecl reaction. We report herein the ecl efficiencies for the reaction of Mo6CllL with 4-cyano-N-methylpyridine(CMP) and Mo6CIl4*in acetone (Ac), acetonitrile (AcN), propionitrile (PrN), butyronitrile (BUN), dichloromethane (DCM), and 1,2-dichloroethane (DCE). Electron-transfer analysis of these yields provides evidence that the efficiency of the ecl reaction can be manipulated with the solvent environment. Our observations strongly suggest that specific solvation is an important factor governing the mechanism of highly exergonic electron transfer in homogeneous solvent systems. Experimental Section Materials. The tetrabutylammonium salt of was prepared and purified as previously described.24 4-Cyano-N(18) (a) Weaver, M.J.; McManis, G. E.; Jarzeba, W.; Barbara, P. F. J . Phys. Chem. 1990, 91, 1715-1719. (b) Nielson, R. M.;McManis, G. E.; Golovin, M. N.; Weaver, M.J. J . Phys. Chem. 1988, 92, 3441-3450. (c) McManis, G. E.; Mishra, A. K.; Weaver, M.J. J . Chem. Phys. 1987, 86, 5550-5556. (19) Stebler, M.; Nielson, R. M.; Siems. W. F.;Hunt, J. P.; Dodgen, H. W.; Wherland, S . Inorg. Chem. 1988, 27, 2893-2897. (20) McGuire, M.;McLendon, G. J . Phys. Chem. 1986,90,2549-2551. (21) (a) McManis, G. E.; Golovin. M.N.; Weaver, M.J. J . Phys. Chem. 1986.90,6563-6570. (b) Gennett, T.; Milner. D.F.; Weaver, M. J. J. Phys. Chem. 1985, 89, 2787-2794. (c) Weaver, M. J.; Gennett, T. Chem. Phys. L f f . 1985, 113. 213-218. (22) Truong, T. B. J . Phys. Chem. 1984,88, 39063913. (23) (a) Blackbourn, R. L.; Hupp, J. T. Inorg. Chem. 1989, 28, 3786-3790. (b) Blackbourn, R. L.; Hupp, J. T. J . Phys. Chem. 1988, 92, 2817-2820. (b) Curtis, J. C.; Blackbourn, R. L.; Ennix, K. S.;Hu, S.; Roberts, J. A.; Hupp, J. T. Inorg. Chem. 1989, 28, 3791-3795.
Mussel1 and Nocera methylpyridinium ( C M P ) iodide was prepared by the addition of methyl iodide to a 1:l acetone/ethanol solution of 4-cyanopyridine (Aldrich). The pyridinium hexafluorophosphatesalt was obtained by a metathesis of the pyridinium iodide with ammonium hexafluorophosphate in aqueous solution and purified by successive crystallizations from acetone/water solutions. Tetrabutylammonium hexafluorophosphate (Southwestern Analytical Chemicals), used as the supporting electrolyte, was dissolved in ethyl acetate, dried over MgS04, and recrystallized from pentane/ethyl acetate solutions. The salts were dried in vacuo for 12 h at 60 OC to ensure that the ethyl acetate was completely removed. The solvents acetone, acetonitrile, dichloromethane, and 1,2dichloroethane were obtained from Burdick and Jackson Laboratories (Distilled-in-Glassgrade), and propionitrile and butyronitrile were received from Aldrich (Gold Label grade). These solvents were subjected to seven freeze-pump-thaw cycles and, with the exception of acetonitrile, were vacuum distilled onto 4-A molecular sieves contained in a 1-L round-bottom flask equipped with a high-vacuum Teflon valve; acetonitrile was stored over 3-A molecular sieves. Owing to the propensity of acetone to condense to mesityl oxide in acidic environments, the acetone was vacuum distilled from the sieve after 24 h. Propionitrile was treated with dilute HCI to remove isonitriles, sequentially dried over MgSO, and CaH2, and fractionally distilled from P20,. The solvent was stored under high-vacuum conditions. Electrochemical and Electrogenerated Chemiluminescence Measurements. Formal reduction potentials of Mo6C1142-were determined by cyclic voltammetry employing a PAR 173 potentiostat, 175 programmer, and a 179 digital coulometer. The current of a Pt button working electrode was recorded on a Houston Instrument X-Y recorder. The cell configuration employed a Pt mesh reference electrode, and a Ag wire provided a stable reference potential which was related to the SCE reference scale by using ferrocene as an internal standard (El12(Fc+Io)= 0.31 V vs SCE).2S Ecl quantum yield experiments were performed in solutions containing 0.1 M supporting electrolyte (NBu4PF6),and for the MO~C~~:-/CMP+system equimolar concentrations of reactants were used. The nature and concentration of the electrolyte were identical for all experiments in order to minimize its role in perturbing the ecl reactivity of the system across a solvent series. Our techniques for the preparation and manipulation of solutions for ecl experiments and our methods employing an integrating sphere for the determination of ecl quantum efficiencies have been described in detail elsewhere.& Ecl quantum yields were calculated from the average of three experimental runs of ten measurements; error limits of our ecl quantum yield measurements were
