Covalently linked ruthenium(II)-manganese(II) complexes: Distance dependence of quenching and electron transfer

Continuing our development of artificial models for photosystem II in green plants, a series of compounds have been prepared in which a RU(bpy)(3)(2+) photosensitizer is covalently Linked to a manganese(II) electron donor. In addition to a trispicolylamine Ligand, two other manganese Ligands, dipico lylamine and aminodiacetic acid, have been introduced in order to study Lig ands that are appropriate for the construction of manganese dimers with ope n coordination sites for the binding of water. Coordination equilibria of t he manganese ions were monitored by EPR. The interactions between the ruthe nium and manganese moieties were probed by flash photolysis, cyclic voltamm etry and steady-state and time-resolved emission measurements. The quenchin g of the Ru-II excited state by Mn-II was found to be rapid in complexes wi th short Ru-Mn distances. Nevertheless, each Run species could be photo-oxi dized by bimolecular quenching with methylviologen, and the subsequent elec tron transfer from Mn-II to Ru-III could be monitored.