Towards an artificial model for Photosystem II: a manganese(II,II) dimer covalently linked to ruthenium(II) tris-bipyridine via a tyrosine derivative

In order to model the individual electron transfer steps from the manganese cluster to the photooxidized sensitizer P-680(+) in Photosystem II (PS II) in green plants, the supramolecular complex 4 has been synthesized. In thi s complex, a ruthenium(II) tris-bipyridine type photosensitizer has been li nked to a manganese(II) dimer via a substituted L-tyrosine, which bridges t he manganese ions. The trinuclear complex 4 was characterized by electron p aramagnetic resonance (EPR) and electrospray ionization mass spectrometry ( ESI-MS). The excited state lifetime of the ruthenium tris-bipyridine moiety in 4 was found to be about 110 ns in acetonitrile, Using flash photolysis in the presence of an electron acceptor (methylviologen), it was demonstrat ed that in the supramolecular complex 4 an electron was transferred from th e excited state of the ruthenium tris-bipyridine moiety to methylviologen, forming a methylviologen radical and a ruthenium(III) tris-bipyridine moiet y. Next, the Ru(III) species retrieved the electron from the manganese(II/I I) dimer in an intramolecular electron transfer reaction with a rate consta nt k(ET)>1.0X10(7) s(-1), generating a manganese(II/III) oxidation state an d regenerating the ruthenium(II) photosensitizer. This is the first example of intramolecular electron transfer in a supramolecular complex, in which a manganese dimer is covalently linked to a photosensitizer via a tyrosine unit, in a process which mimics the electron transfer on the donor side of PS II. (C) 2000 Elsevier Science Inc. All rights reserved.