Photoisomerization Mechanism of Ruthenium Sulfoxide Complexes: Role of the Metal-Centered Excited State in the Bond Rupture and Bond Construction Processes

Phototriggered intramolecular isomerization in a series of ruthenium sulfoxide complexes, [Ru(L)(tpy)(DMSO)](n+) (where tpy = 2,2':6',2 '''-terpyridine; DMSO = dimethyl sulfoxide; L = 2,2'-bipyridine (bpy), n = 2; N,N,N',N'-tetramethylethylenediamine (tmen) n = 2; picolinate (pic), n = 1; acetylacetonate (acac), n = 1; oxalate (ox), n = 0; malonate (mal), n = 0), was investigated theoretically. It is observed that the metal-centered ligand field ((MC)-M-3) state plays an important role in the excited state S -> O isomerization of the coordinated DMSO ligand. If the population of (MCS)-M-3 state is thermally accessible and no (MCO)-M-3 can be populated from this state, photoisomerization will be turned off because the (MCS)-M-3 excited state is expected to lead to fast radiationless decay back to the original (1)GS(S) ground state or photodecomposition along the Ru2+-S stretching coordinate. On the contrary, if the population of (MCS)-M-3 (or (MCO)-M-3) state is inaccessible, photoinduced S -> O isomerization can proceed adiabatically on the potential energy surface of the metal-toligand charge transfer excited states ((MLCTS)-M-3 -> (MLCTO)-M-3). It is hoped that these results can provide valuable information for the excited state isomerization in photochromic d(6) transition-metal complexes, which is both experimentally and intellectually challenging as a field of study.