The complexes [Ru(bpy)(2)(OS)](PF(6)) and [Ru(bpy)(2)(OSO)](PF(6)), 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(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 E(S)(o)' = 0.86 V and E(O)(o)' = 0.49 V versus Ag/Ag(+) for the S- and O-bonded Ru(3+/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 (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 eta(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.

We report on phototriggered Ru-S --> Ru-O and thermal Ru-O --> Ru-S intramolecular linkage isomerizations in cis- and trans-[Ru(bpy)2(dmso)2]2+. The cis complex features only S-bonded sulfoxides (cis-[S,S]), whereas the trans isomer is characterized by S- and O-bonded dmso ligands. Both cis-[S,S] and trans-[S,O] exhibit photochromism at room temperature in dmso solution and ionic liquid (IL). Rates of reaction in IL were monitored by UV-visible spectroscopy and are similar to those reported in dmso solution (k(O-->S) ranges from approximately 10(-3) to 10(-4) s(-1)). Cyclic voltammetric measurements of cis-[S,S] and trans-[S,O] are consistent with an electrochemically triggered linkage isomerism mechanism. While both cis-[S,S] and trans-[S,O] are photochromic at room temperature, neither complex is emissive. However, upon cooling to 77 K, cis-[S,S] exhibits LMCT (ligand-to-metal charge transfer) emission typical of many ruthenium polypyridine complexes. In contrast to cis-[S,S], trans-[S,O] does not show any detectable emission even at 77 K.

Structure analysis of ground state (GS) and two light-induced (SI and SII) metastable linkage NO isomers of [Ru(py)4Cl(NO)](PF6)2.0.5H2O is presented. Illumination of the crystal by a laser with lambda = 473 nm at T = 80 K transfers around 92% of the NO ligands from Ru-N-O into the isomeric configuration Ru-O-N (SI). A subsequent irradiation with lambda = 980 nm generates about 48% of the side-on configuration Ru<(N)(O) (SII). Heating to temperatures above 200 K or irradiation with light in the red spectral range transfers both metastable isomers reversibly back to the GS. Photodifference maps clearly show the N-O configurations for both isomers and they could be used to find a proper starting model for subsequent refinements. Both metastable isomers have slightly but significantly different cell parameters with respect to GS. The main structural changes besides the Ru-O-N and RU<(N)(O) linkage are shortenings of the trans Ru-Cl bonds and the equatorial Ru-N bonds. The experimental results are compared with solid-state calculations based on density functional theory (DFT), which reproduce the observed structures with high accuracy concerning bond lengths and angles. The problem of how the different occupancies of SI and GS could affect refinement results was solved by a simulation procedure using the DFT data as starting values.

The concept of molecular information storage has long served as inspiration for chemists. [1][2][3] Light is often conceived as the trigger to switch between two (or more) molecular ground states. Light energy performs work on the molecule, thus storing photonic energy as potential energy. Maximizing the work performed on a molecule represents a new strategy in the design of molecular-based information systems. The development of bistable molecules with efficient switching mechanisms is tantamount to success in this field. Photochromic compounds are excellent candidates in this regard. These molecular devices convert light energy to potential energy for excited-state bond rupture and bond construction. Phototriggered molecular motions in stilbenes, azobenzenes, dithienylethenes, overcrowded alkenes, and spiro compounds demonstrate the versatility of organic-based structures that feature this reactivity.[4] An important aspect of this approach is that the switching reactions between states must be rapid to maximize the work performed on the molecule.Photochromic complexes based on ruthenium or osmium polypyridyl complexes have further appeal, as their electrochemical signatures provide an independent means of monitoring the color changes associated with these photoactive complexes. Our efforts in this field have focused on ruthenium and osmium sulfoxide complexes that feature intramolecular excited-state S!O and ground-state O!S isomerization reactions. The change in ligation shifts the redox potential E8' (M 3+/2+ ) between 0.3 V and 0.8 V for these two states, depending upon the compound. [5,6] The time constant for excited-state isomerization has been measured to be as rapid as 475 ps, with isomerization quantum yields as great as 0.80. [7,8] Recently, we incorporated the sulfoxide moiety within a chelate, while retaining the photochromic action associated with the sulfoxide. [9,10] We reasoned that limiting the degrees of freedom of the bound sulfoxide would provide an excited-state O!S isomerization pathway, thus maximizing work and limiting heat loss after excitation. Herein, we report a photochromic ruthenium disulfoxide complex that exhibits excited-state S!O and excited-state O!S isomerization on a femtosecond timescale by two different colors of light.The photochromic disulfoxide complex [Ru(bpy) 2 -(OSSO)] 2+ (bpy = 2,2'-bipyridine, OSSO = dimethylbis(methylsulfinylmethyl)silane) is prepared through oxidation of the dithioether parent with a peroxide oxidant, chloroperoxybenzoic acid (m-CPBA). The molecular structures of both ruthenium complexes were confirmed by single crystal X-ray diffractometry, and that of the disulfoxide complex is shown in Figure 1 . [11][12][13] Similar to [Ru(bpy) , the prototypical molecule in this class, the lowest energy transition is assigned as a Ru dp!bpy p* charge-transfer (CT) transition. Previous studies of [Ru(bpy) 3 ] 2+ have demonstrated femtosecond intersystem crossing following excitation to form a thermally

We report the structure, spectroscopy, and electrochemistry of cis-[Os(bpy)(2)(DMSO)(2)](OTf)(2), where bpy is 2,2'-bipyridine, DMSO is dimethyl sulfoxide, and OTf is trifluoromethanesulfonate. Electrochemical measurements are consistent with S-to-O isomerization following the oxidation of Os(2+) (1.8 V vs Ag/AgCl). Visible irradiation of the metal-to-ligand charge-transfer transition (355 nm) of [Os(bpy)(2)(DMSO)(2)](2+) in the solid state and solution yields an emissive S-bonded excited state and S-to-O excited-state isomerization on a subnanosecond time scale. These results and a comparison to the nonphotoactive [Os(bpy)(2)Cl(DMSO)](+) are discussed.