pubs.acs.org/IC Published on Web 05/12/2009 r 2009 American Chemical Society 8084 Inorg. Chem. 2009, 48, 8084–8091 DOI: 10.1021/ic900421v Photochromic Ruthenium Sulfoxide Complexes: Evidence for Isomerization Through a Conical Intersection Beth Anne McClure, † Nicholas V. Mockus, † Dennis P. Butcher, Jr., † Daniel A. Lutterman, †,‡ Claudia Turro,* ,†,‡ Jeffrey L. Petersen, †,§ and Jeffrey J. Rack* ,† † Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, ‡ Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, and § C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045 Received March 4, 2009 The complexes [Ru(bpy) 2 (OS)](PF 6 ) and [Ru(bpy) 2 (OSO)](PF 6 ), where bpy is 2,2 0 -bipyridine, OS is 2-methylthio- benzoate, and OSO is 2-methylsulfinylbenzoate, have been studied. The electrochemical and photochemical reactivity of [Ru(bpy) 2 (OSO)] + is consistent with an isomerization of the bound sulfoxide from S-bonded (S-) to O-bonded (O-) following irradiation or electrochemical oxidation. Charge transfer excitation of [Ru(bpy) 2 (OSO)] + in MeOH results in the appearance of two new metal-to-ligand charge transfer (MLCT) maxima at 355 and 496 nm, while the peak at 396 nm diminishes in intensity. The isomerization is reversible at room temperature in alcohol or propy- lene carbonate solution. In the absence of light, solutions of O-[Ru(bpy) 2 (OSO)] + revert to S-[Ru(bpy) 2 (OSO)] + . Kinetic analysis reveals a biexponential decay with rate constants of 5.66(3) Â 10 -4 s -1 and 3.1(1) Â 10 -5 s -1 . Cyclic voltammograms of S-[Ru(bpy) 2 (OSO)] + are consistent with electron-transfer-triggered isomerization of the sulf- oxide. Analysis of these voltammograms reveal E S ° 0 = 0.86 V and E O ° 0 = 0.49 V versus Ag/Ag + for the S- and O-bonded Ru 3+/2+ couples, respectively, in propylene carbonate. We found k SfO = 0.090(15) s -1 in propylene carbonate and k SfO = 0.11(3) s -1 in acetonitrile on Ru III , which is considerably slower than has been reported for other sulfoxide isomerizations on ruthenium polypyridyl complexes following oxidation. The photoisomerization quantum yield (Φ SfO = 0.45, methanol) is quite large, indicating a rapid excited state isomerization rate constant. The kinetic trace at 500 nm is monoexponential with τ = 150 ps, which is assigned to the excited SfO isomerization rate. There is no spectroscopic or kinetic evidence for an O-bonded 3 MLCT excited state in the spectral evolution of S-[Ru(bpy) 2 (OSO)] + to O-[Ru(bpy) 2 (OSO)] + . Thus, isomerization occurs nonadiabatically from an S-bonded (or η 2 -sulfoxide) 3 MLCT excited state to an O-bonded ground state. Density functional theory calculations support the assigned spectroscopy and provide insight into ruthenium ligand bonding. Introduction In 1970, Forster classified photochemical reactions as either adiabatic or nonadiabatic, depending upon whether the chemical reaction occurred on a potential energy surface of identical multiplicity or one of different multiplicity. 1 These concepts have since been expanded to focus not solely on spin, such that adiabatic reactions are those that separate atomic and electronic movement (Born-Oppenheimer ap- proximation), while nonadiabatic reactions are those that involve nuclear dynamics on two separate potential energy curves. 2,3 The latter may indeed be much more common than originally thought. In mapping potential energy surfaces, recent computational studies have revealed conical intersec- tions at which the excited-state molecule may rapidly under- go nonradiative decay to the ground-state potential energy surface. 4-6 Such regions occur at the intersection of two or more orthogonal reaction coordinates in the evolution of an excited state. These pathways provide a facile mechanism for the formation of a photoproduct with little to no activation barrier. This issue is especially important in photochemical isomerizations where substantial changes in atomic connec- tivity and electronic structure occur on a femtosecond or picosecond time scale. Efficient conversion of photonic en- ergy to potential energy in these reactions requires minimal energy pathways for the formation of photoproducts. *To whom correspondence should be addressed. E-mail: rackj@ohio.edu (J.J.R.). (1) Forster, T. Pure Appl. Chem. 1970, 24, 443–449. (2) Domcke, W.; Yarkony, D. R.; Koppel, H. Conical Intersections: Electronic Structure, Dynamics and Spectroscopy; World Scientific: River Edge, NJ, 2004. (3) Worth, G. A.; Cederbaum, L. S. Annu. Rev. Phys. Chem. 2004, 55, 127–158. (4) Dick, B.; Haas, Y.; Zilberg, S. Chem. Phys. 2008, 347, 65–77. (5) Piryatinski, A.; Tretiak, S.; Chernyak, V. Y. Chem. Phys. 2008, 347, 25–38. (6) Quenneville, J.; Martinez, T. J. J. Phys. Chem. A 2003, 107, 829–837. Downloaded by OHIO UNIV on August 31, 2009 | http://pubs.acs.org Publication Date (Web): May 12, 2009 | doi: 10.1021/ic900421v