A biomimetic approach to artificial photosynthesis: Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors

The paper describes recent advances towards the construction of functional rr mics of the oxygen evolving complex in photosystem II (PSII) that are co upled to photoinduced charge separation. Some key principles of PSII and ar tificial systems for light-induced charge accumulation are discussed. Syste ms are described where biomimetic electron donors - manganese complexes and tyrosine - have been linked to a Ru(II)-polypyridine photosensitiser. Oxid ation of the donors by intramolecular electron transfer from the photo-oxid ised Ru(III) complex has been studied using optical flash photolysis and EP R experiments. A step-wise electron transfer Mn-2(III,III) --> tyrosine --> Ru(III) has been demonstrated, in analogy to the reaction on the donor sid e of PSII Electron transfer from the tyrosine to Ru(III) was coupled to tyr osine deprotonation. This resulted in a large reorganisation energy and thu s a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reactions in PSH is made. Finally , light-induced oxidation of a manganese dimer linked to a Ru(II)-photosens itiser has been observed. Preliminary results suggest the possibility of ph oto-oxidising manganese dimers in several steps, which is an important adva ncement towards water oxidation. (C) 2001 Elsevier Science B.V. All rights reserved.