Energy Redistribution and Localization in the Excited States of Ruthenium(II) Polypyridyl Complexes

Excited-state absorption and resonance Raman spectra of the mixed-ligand complexes Ru(bpy)₂phen²⁺ (1) and Ru(bpy)₂DIP²⁺ (2) (DIP = 4,7-diphenylphenanthroline) are reported. The excited-state spectra of these mixed-ligand complexes show that the excited-state electron is localized on individual ligands but show nonstatistical contributions from states corresponding to each of the ligands present in the coordination sphere. On the basis of the excited-state absorption spectra, the bpy contribution to 1 is twice as much as that of phen, whereas in case of 2 the contribution of DIP is much more than that of bpy. Excitation of the mixed-ligand complexes at different wavelengths gave identical spectra, showing that the initial excited-state population relaxed quickly to an equilibrium mixture, which depends very much on the type of ligand. Intramolecular energy transfer is rapid. Excited-state absorption spectra of mixtures of parent tris chelates, in contrast, which depend upon the wavelength of excitation, show that intermolecular energy transfer is very slow. Excited-state localization and rapid relaxation to an equilibrium distribution is further supported by excited-state resonance Raman spectra of these mixed-ligand complexes. Vibrational transitions corresponding to bpy dominate the spectrum of 1 whereas the vibrational bands of DIP dominate in case of 2. Consistent with these results are the resonance Raman spectra of the excited states of the mixed-ligand complexes Ru(bpy)₂(4,4′-Ph₂-bpy)²⁺ (3) and Ru(bpy)(4,4′-Ph₂-bpy)₂ ²⁺ (4). These spectra clearly indicate preferential localization of excited-state energy on the 4,4′-Ph₂-bpy ligand. Thus, it may be possible to design systematically mixed-ligand complexes that would absorb light at a wavelength of choice but would funnel the energy onto a desired ligand.