Excited-state spectroscopic investigations of multinuclear complexes based on [Ru(bpy)3]2+ moieties connected to 2,2′-bipyridine and 2,2′;6′,2′′-terpyridine ligands

Abstract

A number of multinuclear assemblies based on [Ru(bpy)₃]²⁺ photosensitive moieties covalently linked to Fe(II), Co(II) or Zn(II) polypyridyl complexes are investigated regarding their initial and thermally equilibrated excited states. Ground state absorption and vibrational spectroscopic techniques are carried out, along with resonance Raman, transient absorption, and time resolved resonance Raman measurements. These methods are also supplemented by computational modelling. In all systems, the results clearly show that under visible irradiation, the substituted bpy linker ligand is involved in the initial ¹MLCT excitation of the Ru(II) subunit. For the Ru(II)/Fe(II) linked assemblies, absorption due to [Ru(bpy)₃]²⁺ and [Fe(tpy)₂]²⁺ subunits are identified to give rise to differing resonance Raman spectra. Transient absorption spectra of complexes containing two [Ru(bpy)₃]²⁺ and one [Fe(tpy)₂]²⁺ subunits show a strong depletion for the [Fe(tpy)₂]²⁺ absorption peaks, which decay on a much faster timescale than the remainder of the transient features. This is consistent with a single excitation of the multimetallic assembly, followed by fast depletion (<10 ns) of the spectral signal from the bpy ligand bound to the Fe subunit. The results are supported by time resolved resonance Raman measurements where a number of features assigned to the linker are found at early time-scales. Using transient absorption this process can be followed for most complexes.