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Photochromic [Ru(bpy)2(OSO)](PF6), where bpy is 2,2'-bipyridine and OSO is 2-methylsulfinylbenzoate, was investigated by femtosecond transient absorption spectroscopy. The results show that for both S- and O-bonded isomers, a ³MLCT state is formed on a femtosecond time scale. Also observed is the formation of multiple metal-to-ligand charge-transfer (MLCT) states, representing different conformers, prior to isomerization on the picosecond time scale. These results and others are compiled in an energy diagram depicting these results. [ABSTRACT FROM AUTHOR]

Abstract: Complexes of the type [Ru(bpy)2(OSOBnR)](PF6) where bpy is 2,2′-bipyridine and OSOBnR is a 4-substituted benzylsulfinylbenzoate with R=NO2, F, Cl, H, CH3, CF3 and OCH3, have been prepared and investigated by 1H NMR spectroscopy, cyclic voltammetry and UV–vis spectroscopy. Despite the distance of the R group from ruthenium, the Ru3+/2+ reduction potential and charge transfer absorption maximum vary predictably with the electron withdrawing nature of the group. [ABSTRACT FROM AUTHOR]

The complexes [Ru(bpy)2(OS)](PF6) and [Ru(bpy)2(OSO)](PF6), where bpy is 2,2'-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 propylene 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) 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 ESo' = 0.86 V and EOo' = 0.49 V versus Ag/Ag+ for the S- and O-bonded Ru3+/2+ couples, respectively, in propylene carbonate. We found ks-o = 0.090(15) s-1 in propylene carbonate and ks-o = 0.11(3) s-1 in acetonitrile on RuIII, which is considerably slower than has been reported for other sulfoxide isometizations on ruthenium polypyridyl complexes following oxidation. The photoisomerization quantum yield (Φs-o = 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 S-O isomerization rate. There is no spectroscopic or kinetic evidence for an O-bonded 3MLCT excited state in the spectral evolution of S-[Ru(bpy)2(OSO)]+ to O-[Ru(bpy)2(OSO)]+. Thus, isometization occurs nonadiabatically from an S-bonded (or η2-sulfoxide) 3MLCT excited state to an O-bonded ground state. Density functional theory calculations support the assigned spectroscopy and provide insight into ruthenium ligand bonding. [ABSTRACT FROM AUTHOR]