No abstract

Compounds of the type [Ru(tpy)(L2)(dmso)](z+) (tpy is 2,2':6',2' '-terpyridine; L2 can be 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate (ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. When L2 is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically. Isomerization is not observed when L2 is acac, mal, or ox. Isomerization results in a drastic change in the absorption spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419-490 nm), 4775 (421-527 nm), and 4440 cm(-)(1) (429-530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively. Quantum yields of isomerization (phi(S)(-->)(O)) were determined for the bpy (0.024 +/- 1), pic (0.25 +/- 1), and tmen (0.007 +/- 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization mechanism does not involve the ligand field states. This result is surprising given the importance of these states in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism are discussed.

Photochromic materials are of interest because of their potential applications in optical information storage devices. [1][2][3][4] Measurements on photochromic sodium nitroprusside (Na 2 [Fe(CN) 5 (NO)]) indicate that two metastable states are formed by irradiations of the crystalline solid: the first is an isonitrosyl (O-bonded NO); the second is an η 2 -NO (side-on) complex. 4,5 Phototriggered linkage isomerizations also occur in dimethylsulfoxide (dmso) complexes: notably, both photochemical Ru-S f Ru-O and thermal Ru-O f Ru-S reactions are observed in dmso solutions of [Ru(bpy) 2 (dmso) 2 ] 2+ (bpy ) 2,2′-bipyridine); 6 and, as reported here, we find photochromism attributable to Ru-S f Ru-O rearrangement upon visible excitation of [Ru(tpy)(bpy)(dmso)] 2+ (tpy ) 2,2′:6′,2′′-terpyridine) in single crystals and films as well as in solution. 7 Crystal structure analysis 8 of [Ru(tpy)(bpy)(dmso)](SO 3 CF 3 ) 2 gives Ru-S and S-O distances of 2.282(1) and 1.467(3) Å, respectively (vs 1.492(2) Å S-O distance for free dmso). 9 An increase in the SdO bond strength inferred from IR data (ν(SO) ) 1102 cm -1 ; 10 uncomplexed dmso: ν(SO) ) 1055 cm -1 ) 11 is typical of S-bonded Ru-dmso complexes. 12 Despite the short SdO distance and the relatively high stretching frequency, the blue-shifted MLCT band (CH 3 CN: 412 nm, 24 390 cm -1 ; ) 8080 M -1 cm -1 ) relative to that of [Ru(tpy)(bpy)(CH 3 CN)] 2+ (CH 3 CN: 454 nm, 22 030 cm -1 ;) 10 900 M -1 cm -1 ) 13 indicates that Ru(II) is stabilized by dπ f dmso back-bonding.Cyclic voltammograms of [Ru(tpy)(bpy)(dmso)] 2+ reveal two irreversible electron-transfer processes. 10 Analysis based on an ECEC mechanism indicates that the Ru(III/II) reduction potential

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.