Ruthenium polypyridine complexes. On the route to biomimetic assemblies asmodels for the photosynthetic reaction center

We describe in this Account the preparation of RuL3 complexes and their sig nificance as biomimetic models for the photosynthetic reation center. Their preparation from simple or more complicated bypyridine ligands L and their photophysical data, especially their stability, are reported. Biomimetic m odels involving three concepts of the interaction of RuL3 with acceptors in coordinatively, mechanically, or covalently linked supramolecular assembli es are also presented. The electron transfer (ET) of the noncovalently link ed assemblies of RuL3 complexes carrying polyether chains with one or two a nisyl binding sites (4 or 5) with the cyclic bisviologen was studied. Molec ular modeling and NMR titration clearly show the formation of supramolecula r assemblies. Time-resolved spectroscopy demonstrated that ET and charge se paration in the RuL3 complexes with two binding sites are more efficient. T he more constrained RuL3-bisviologen-catenane (6) possesses two conformatio ns which exhibit different efficiency in ET, creating a charge-separated st ate in the microsecond domain. The covalently linked Ru(bpy)(3)(2+)-viologe n assemblies having one (7, diad) or two bisviologen arms (8, diad) result in efficient ET. Addition of linear polyethers, cyclic polyethers, or crown s generates new triads and tetrads of the pseudorotaxane type. Molecular mo deling and NMR titration clearly indicate the formation of supramolecular a ssemblies. The analysis of time-resolved studies proves fast ET and especia lly long-lived charge-separated states in these pseudorotaxanes. These data , compared with the findings for the photosynthetic reation center, show co nclusive results. The lifetimes of the charge-separated states increase cle arly in the sequence for noncovalently < mechanically < and covalently link ed assemblies.