An excited state dynamics driven reaction: wavelength-dependent photoisomerization quantum yields in [Ru(bpy)(2)(dmso)(2)](2+)

We report the excited-state behavior of a structurally simple bis-sulfoxide complex, cis-S,S-[Ru(bpy)(2)(dmso)(2)](2+), as investigated by femtosecond pump–probe spectroscopy. The results reveal that a single photon prompts phototriggered isomerization of one or both dmso ligands to yield a mixture of cis-S,O-[Ru(bpy)(2)(dmso)(2)](2+) and cis-O,O-[Ru(bpy)(2)(dmso)(2)](2+). The quantum yields of isomerization of each product and relative product distribution are dependent upon the excitation wavelength, with longer wavelengths favoring the double isomerization product, cis-O,O-[Ru(bpy)(2)(dmso)(2)](2+). Transient absorption measurements on cis-O,O-[Ru(bpy)(2)(dmso)(2)](2+) do not reveal an excited-state isomerization pathway to produce either the S,O or S,S isomers. Femtosecond pulse shaping experiments reveal no change in the product distribution. Pump–repump–probe transient absorption spectroscopy of cis-S,S-[Ru(bpy)(2)(dmso)(2)](2+) shows that a pump–repump time delay of 3 ps dramatically alters the S,O : O,O product ratio; pump–repump–probe transient absorption spectroscopy of cis-O,O-[Ru(bpy)(2)(dmso)(2)](2+) with a time delay of 3 ps uncovers an excited-state isomerization pathway to produce the S,O isomer. In conjunction with low-temperature steady-state emission spectroscopy, these results are interpreted in the context of an excited-state bifurcating pathway, in which the isomerization product distribution is determined not by thermodynamics, but rather as a dynamics driven reaction.