Room-Temperature Photochromism in cis- and trans-[Ru(bpy)2(dmso)2]2+

We report on phototriggered Ru-S (right arrow) Ru-O and thermal Ru-O (right arrow) 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 (kO(right arrow)S ranges from ~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.