INTRAMOLECULAR ELECTRON-TRANSFER FROM MANGANESE(II) COORDINATIVELY LINKED TO A PHOTOGENERATED RU(III)-POLYPYRIDINE COMPLEX - A KINETIC-ANALYSIS

For further investigations in the field of artificial photosynthesis, a model compound, 1, has been developed to mimic the electron-transfer steps from the manganese cluster to P-680(+) in photosystem II. In th is model compound the photosensitizer ruthenium(II)-trisbipyridyl was linked to a manganese(II) ion through a bridging ligand. Photoexcitati on of 1 in the presence of the electron acceptor methyl viologen (MV2) lead to electron transfer from the Ru moiety to MV2+. Laser flash ph otolysis experiments at different concentrations of 1 were performed i n order to follow the subsequent reduction of the photooxidized Ru(III ) species. A kinetic model, taking different parallel reactions into a ccount, could explain the experimental data. It was shown that the maj or part of the photooxidized Ru(III) created was reduced again by intr amolecular electron transfer from the attached Mn(II), with a rate con stant of 1.8 x 10(5) s(-1). However, Mn(II) was partially dissociated from 1, giving a fraction of Ru(III) without Mn(II) attached. In these complexes electron transfer could occur only after a rate-limiting re association of Mn(II), with a rate constant 2.9 x 10(9) M-1 s(-1). In the analysis of the data, the fraction of dissociated Mn(II) could be determined independently at each concentration of 1, utilizing the fac t that bound Mn(II) quenched the excited state, probably by energy tra nsfer.