Excited State Distortion in Photochromic Ruthenium Sulfoxide Complexes

A series of photochromic ruthenium sulfoxide complexes of the form [Ru(bpy)(2)(OSOR)](+), where bpy is 2,2'-bipyridine and OSOR is 2-(benzylsulfinyl)benzoate (OSOBn), 2-(napthalen-2-yl-methylsulfinyl)-benzoate (OSONap), or 2-(pentafluorophenylmethanesulfinyl)benzoate (OSOBnF(5)), have been synthesized and characterized. In aggregate, the data are consistent with phototriggered isomerization of the sulfoxide from S-bonded to O-bonded. The S-bonded complexes feature (3)MLCT absorption maxima at 388 nm (R = BnF(5)), 396 nm (R = Bn), and 400 nm (R = Nap). Upon charge transfer excitation the S-bonded peak diminishes concomitant with new peaks growing in at similar to 350 and similar to 495 nm. Spectroscopic and electrochemical data suggest that the electronic character of the substituent on the sulfur affects the properties of the S-bonded complexes, but not the 0-bonded complexes. The isomerization is reversible in methanol solutions and, in the absence of light, thermally reverts to the S-bonded isomer with biexponential kinetics. The quantum yields of isomerization (Phi(s-o)) were found to be 0.32, 0.22, and 0.16 for the R = BnF(5), Bn, and Nap complexes, respectively. Kinetic analyses of femtosecond transient absorption data were consistent with a nonadiabatic mechanism in which isomerization occurs from a thermally relaxed (3)MLCT state of S-bonded (or eta(2)-sulfoxide) character directly to the singlet 0-bonded ground state. The time constants of isomerization (tau(s-o)) were found to be 84, 291, and 427 ps for the R = BnF(5), Bn, and Nap complexes, respectively. Analysis of room temperature absorption and 77 K emission spectra reveal significant distortion between the S-bonded ground state ((1)GS(S)) and singlet metal-to-ligand charge transfer state ((1)MLCT(S)) and thermally relaxed (3)MLCT, respectively. The distortion is primarily attributed to low frequency metal-ligand and S=O vibrational modes, which are intrinsically involved in the isomerization pathway.