Picosecond Isomerization in Photochromic Ruthenium−Dimethyl Sulfoxide Complexes

The complexes [Ru(tpy)(bpy)(dmso)](OSO2CF3)2 and trans-[Ru(tpy)(pic)(dmso)](PF6) (tpy is 2,2‘:6‘,2‘ ‘-terpyridine, bpy is 2,2‘-bipyridine, pic is 2-pyridinecarboxylate, and dmso is dimethyl sulfoxide) were investigated by picosecond transient absorption spectroscopy in order to monitor excited-state intramolecular S→O isomerization of the bound dmso ligand. For [Ru(tpy)(bpy)(dmso)]2+, global analysis of the spectra reveals changes that are fit by a biexponential decay with time constants of 2.4 ± 0.2 and 36 ± 0.2 ps. The first time constant is assigned to relaxation of the S-bonded 3MLCT excited state. The second time constant represents both excited-state relaxation to ground state and excited-state isomerization to form O-[Ru(tpy)(bpy)(dmso)]2+*. In conjunction with the S→O isomerization quantum yield (ΦS → O = 0.024), isomerization of [Ru(tpy)(bpy)(dmso)]2+ occurs with a time constant of 1.5 ns. For trans-[Ru(tpy)(pic)(dmso)]+, global analysis of the transient spectra reveals time constants of 3.6 ± 0.2 and 118 ± 2 ps associated with these two processes. In conjunction with the S→O isomerization quantum yield (ΦS → O = 0.25), isomerization of trans-[Ru(tpy)(pic)(dmso)]+ occurs with a time constant of 480 ps. In both cases, the thermally relaxed excited states are assigned as terpyridine-localized 3MLCT states. Electronic state diagrams are compiled employing these data as well as electrochemical, absorption, and emission data to describe the reactivity of these complexes. The data illustrate that rapid bond-breaking and bond-making reactions can occur from 3MLCT excited states formed from visible light irradiation.