The Early Picosecond Photophysics of Ru(II) Polypyridyl Complexes: A Tale of Two Timescales

The early picosecond time scale excited-state dynamics of the paradigm tris(2,2'-bipyridyl)Ruthenium(II) ([Ru(bpy)3]2+) and related complexes have been examined by picosecond Kerr-gated time-resolved resonance Raman (ps-TR3) spectroscopy. The evolution of the signature Raman bands of the lowest thermally equilibrated excited (THEXI) state under two-color pump/probe conditions show that this state is not fully populated within several hundred femtoseconds as proposed previously but rather only within the first 20 ps following excitation. In addition to an emission observed within the instrument rise time (τ < 3 ps), the early picosecond dynamics are characterized by a rise in the intensity of the Raman marker bands of the THEXI-3MLCT state, a rise time which, within experimental uncertainty, is not influenced by either partial or complete ligand deuteriation or the presence of ligands other than bpy, as in the heteroleptic complexes [Ru(bpy)2(L1)]+ and [Ru(bpy)2(Hdcb)]+ (where H2dcb is 4,4'-dicarboxy-2,2'-bipyridine and L1 is 2,-(5'-phenyl-4'-[1,2,4]triazole-3'-yl)pyridine). Overall, although the results obtained in the present study are consistent with those obtained from examination of this paradigm complex on the femtosecond timescale, regarding initial formation of the vibrationally hot 3MLCT state by ISC from the singlet Franck-Condon state, the observation that the THEXI-3MLCT state reaches thermal equilibration over a much longer time period than previously suggested warrants a re-examination of views concerning the rapidity with which thermal equilibration of transition metal complex excited states takes place.