Ultrafast energy transfer in binuclear ruthenium-osmium complexes as models for light-harvesting antennas

The binuclear complexes [(bpy)(2)Ru(mu-2,3-dpp)Ru(bpy)(2)](4+), [(bpy)(2)Ru(mu-2,5-dpp)Ru(bpy)(2)](4+), [(bpy)(2)Ru(mu-2,3-dpp)Os(bpy)(2)](4+), [(bpy)(2)Ru(mu-2,5-dpp)Os(biq)(2)](4+), and [(bpy)(2)Os(mu-2,3-dpp)Os(bpy)(2)](4+) (dpp = bis-(2-pyridyl)pyrazine, bpy = 2,2-bipyridine, biq = 2,2-biquinoline) have been studied with femtosecond pump-probe spectroscopy. Excitation energy transfer from the Ru to the Os center in the heterometallic binuclear complexes occurs within 200 fs. This is a time scale comparable to the singlet-triplet conversion and vibrational relaxation of the lowest metal-to-ligand charge transfer (MLCT) state in this type of complexes. Thus, energy transfer probably involves nonthermalized initial states, which may be an explanation for the fast transfer rate. Small spectral changes with time constants of ca. 400-800 fs were observed for all complexes examined, and are attributed to relaxation (vibrational and/or spin) of the MLCT state localized on the lowest energy unit. Energy transfer seems to occur within 200 fs also in the symmetric Ru-II-Ru-II and Os-II-Os-II complexes, although the reaction driving force is zero. The results suggest that very large antennas or photonic wires could be constructed based on these metal complexes, in which energy transfer can occur in several steps over long distances, with only very small losses.