Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

The complexes [Ru(bpy)(2)(pyESO)](PF6)(2) and [Os(bpy)(2)(pyESO)](PF6)(2), in which bpy is 2,2-bipyridine and pyESO is 2-((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by (HNMR)-H-1, UV-visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X-ray structural analysis were grown for [Ru(bpy)(2)(pyESO)](PF6)(2). Cyclic voltammograms of both complexes provide evidence for S -> O and O -> S isomerization as these voltammograms are described by an ECEC (electrochemical-chemical electrochemical-chemical) mechanism in which isomerization follows Ru2+ oxidation and Ru3+ reduction. The S- and O-bonded Ru3+/2+ couples appear at 1.30 and 0.76V versus Ag/AgCl in propylene carbonate. For [Os(bpy)(2)(pyESO)](PF6)(2), these couples appear at 0.97 and 0.32V versus Ag/AgCl in acetonitrile, respectively. Charge-transfer excitation of [Ru(bpy)(2)(pyESO)](PF6)(2) results in a significant change in the absorption spectrum. The S-bonded isomer of [Ru(bpy)(2)(pyESO)](2+) features a lowest energy absorption maximum at 390nm and the O-bonded isomer absorbs at 480nm. The quantum yield of isomerization in [Ru(bpy)(2)(pyESO)](2+) was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81ps (propylene carbonate) and 47ps (dichloroethane) in [Ru(bpy)(2)(pyESO)](2+). These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.