Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(II) polypyridyl complexes

Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine), [Ru(mbpy)(3)](2+) (mbpy = 6-methyl-2,2'-bipyridine) and [Ru(mphen)(3)](2+) (mphen = 2-methyl-1,10'-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the (MLCT)-M-1 band at 400 nm. While for [Ru(bpy)(3)](2+) no intermediate state can be evidenced for the relaxation of the corresponding (MLCT)-M-3 state back to the ground state, for [Ru(mbpy)(3)](2+) and [Ru(mphen)(3)](2+) an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy (3)dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)(3)](2+) the (3)dd state is at a higher energy than the (MLCT)-M-3 state and that there is a substantial barrier between the two minima. For [Ru(mbpy)(3)](2+) and [Ru(mphen)(3)](2+), the (3)dd state is at a lower energy than the (MLCT)-M-3 state.