SUPRAMOLECULAR ASSEMBLIES FOR LIGHT-INDUCED ELECTRON-TRANSFER REACTIONS

In this paper, we present a summary of our work on highly photostable supramolecular ruthenium complexes, which may be incorporated into more complex systems for artificial solar energy conversion. We have used supramolecular chemistry and photochemistry to synthesize highly photostable ruthenium bipyridine coronates and a bipyridazine podate complex and to enhance photoelectron-transfer reactions in physical model systems for artificial photosynthesis. The recent progress of covalent and non-covalent sensitizer-relay assemblies for highly efficient photoelectron transfer is described. A detailed mechanistic investigation of the binding behavior of cationic species to crown-ether-modified bipyridine derivatives is presented as an example of supramolecular binding in systems for photoelectron transfer. The host properties of the free ligands and the derived bis-heteroleptic ruthenium complexes are compared using UV-visible, luminescence quenching and proton nuclear magnetic resonance titrations. The combination of these three methods confirms that supramolecular binding of cations and the electron relay methylviologen (MV2+) to the complexes can be observed. The binding constants determined are of the order of (1-6) X 10(4) l mol-1 for the crown-ether ruthenium complexes and 1 X 10(2)-4 X 10(3) 1 mol-1 for the crown-ether ligands. Single-photon-counting (SPC) investigations give strong indications for the coexistence of different binding mechanisms. The kinetic scheme of Yekta et al. has been adapted to interpret the binding mechanism.