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 (OSOBnF5), 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 3MLCT absorption maxima at 388 nm (R = BnF5), 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 ∼350 and ∼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 O-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 (Φs→o) were found to be 0.32, 0.22, and 0.16 for the R = BnF5, 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 3MLCT state of S-bonded (or η2-sulfoxide) character directly to the singlet O-bonded ground state. The time constants of isomerization (τs→o) were found to be 84, 291, and 427 ps for the R = BnF5, Bn, and Nap complexes, respectively. Analysis of room temperature absorption and 77 K emission spectra reveal significant distortion between the S-bonded ground state (1GSS) and singlet metal-to-ligand charge transfer state (1MLCTS) and thermally relaxed 3MLCT, respectively. The distortion is primarily attributed to low frequency metal−ligand and SO vibrational modes, which are intrinsically involved in the isomerization pathway.