Picosecond isomerization in photochromic ruthenium-dimethyl sulfoxide complexes

The complexes [Ru(tpy)(bpy)(dmso)](OSO2CF3)(2) and trans-[Ru(tpy)(pic)(dmso)](PF6) bpy 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 (MLCT)-M-3 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 (Phi(S -> 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)](2+), 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 (Phi(S -> 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 (MLCT)-M-3 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 (MLCT)-M-3 excited states formed from visible light irradiation.