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

Abstract

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 ¹MLCT band at 400 nm. While for [Ru(bpy)₃]²⁺ no intermediate state can be evidenced for the relaxation of the corresponding ³MLCT 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 ³dd 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 ³dd state is at a higher energy than the ³MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)₃]²⁺ and [Ru(mphen)₃]²⁺, the ³dd state is at a lower energy than the ³MLCT state.