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

Abstract

We describe in this Account the preparation of RuL(3) complexes and their significance 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 models involving three concepts of the interaction of RuL(3) with acceptors in coordinatively, mechanically, or covalently linked supramolecular assemblies are also presented. The electron transfer (ET) of the noncovalently linked assemblies of RuL(3) complexes carrying polyether chains with one or two anisyl binding sites (4 or 5) with the cyclic bisviologen was studied. Molecular modeling and NMR titration clearly show the formation of supramolecular assemblies. Time-resolved spectroscopy demonstrated that ET and charge separation in the RuL(3) complexes with two binding sites are more efficient. The more constrained RuL(3)-bisviologen-catenane (6) possesses two conformations which exhibit different efficiency in ET, creating a charge-separated state in the microsecond domain. The covalently linked Ru(bpy)(3)(2+)-viologen assemblies having one (7, diad) or two bisviologen arms (8, diad) result in efficient ET. Addition of linear polyethers, cyclic polyethers, or crowns generates new triads and tetrads of the pseudorotaxane type. Molecular modeling and NMR titration clearly indicate the formation of supramolecular assemblies. The analysis of time-resolved studies proves fast ET and especially long-lived charge-separated states in these pseudorotaxanes. These data, compared with the findings for the photosynthetic reation center, show conclusive results. The lifetimes of the charge-separated states increase clearly in the sequence for noncovalently < mechanically < and covalently linked assemblies.