Photochromic Ruthenium Sulfoxide Complexes: Evidence for Isomerization Through a Conical Intersection

The complexes [Ru(bpy)(2)(OS)](PF6) and [Ru(bpy)(2)(OSO)](PF6), where bpy is 2,2'-bipyridine, OS is 2-methylthiobenzoate, 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 propylene carbonate solution. In the absence of light, solutions of O-[Ru(bpy)(2)(OSO)](+) revert to S-[Ru(bp)(2)(OSO)](+). Kinetic analysis reveals a biexponential decay with rate constants of 5.66(3) x 10(-4) s(-1) and 3.1(1) x 10(-5) s(-1). Cyclic voltammograms of S-[Ru(bpy)(2)(OSO)](+) are consistent with electron-transfer-triggered isomerization of the sulfoxide. Analysis of these voltammograms reveal E-S(ol) = 0.86 V and E-O(ol) = 0.49 V versus Ag/Ag+ for the S- and O-bonded Ru3+/2+ couples, respectively, in propylene carbonate. We found k(S -> O) = 0.090(15) s(-1) in propylene carbonate and k(S -> O)=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 (Phi(S -> O) = 0.45, methanol) is quite large, indicating a rapid excited state isomerization rate constant. The kinetic trace at 500 nm is monoexponential with tau =150 ps, which is assigned to the excited S -> O isomerization rate. There is no spectroscopic or kinetic evidence for an O-bonded (MLCT)-M-3 excited state in the spectral evolution of S-[Ru(bpy)(2)(OSO)](+) to a[Ru(bpy)(2)(QSO)](+). Thus, isomerization occurs nonadiabatically from an S-bonded (or eta(2)-sulfoxide) (MLCT)-M-3 excited state to an O-bonded ground state. Density functional theory calculations support the assigned spectroscopy and provide insight into ruthenium ligand bonding.