PARENT SESSION
Posters P8D Artificial photosynthesis and biological hydrogen production. Abstracts (759-769)

Photoinduced electron transfer in a Mn₂^II,II—Ru^II—(NDI)₂ triad : Formation of a long-lived charge-separated state. Stenbjörn Styring^*,1, Olof Johansson², Nizamuddin Shaikh¹, Magnus Borgström³, Leif Hammarström³, Magnus Anderlund², Licheng Sun², ¹ Molecular Biomimetics, Uppsala, Sweden² Department of Oraganic Chemistry, Stockholm, Sweden³ Department of Physical Chemistry, Uppsala, Sweden

ABSTRACT- Photoinduced electron transfer (ET) in a covalently linked triad comprised of a magnetically coupled Mn₂^II,II [Mn₂^II,II-bpmp], a Ru^II photosensitizer [Tris(2,2′-bipyridine ruthenium(II)] and NDI [naphthalene diimide] acceptor (1) has been synthesized and characterized by EPR spectroscopy and laser flash photolysis. At 140 K (in dry butyronitrile), the laser flash photolysis reveals that Ru(II) is first excited. It then transfers an electron to the NDI acceptor. The formed NDI radical is remarkably stable with a lifetime exceeding 150 ms at 140K. EPR studies under the same conditions reveal a light dependent decrease of the Mn₂^II,II EPR signal and a concomitant increase of the Mn^II,III signal together with the formation of the NDI radical. Quantification of all the species is consistent with flash induced intramolecular one electron transfer from the Mn₂^II,II-bpmp donor to the NDI acceptor to form a charge separated state, which is very stable at 140K. To confirm the charge separated state, we incubated the flashed sample at 140K (30 min) which resulted in the decay of the NDI radical and reappearance of the Mn₂^II,II signal with simultaneous decrease of the Mn₂^II,II signal. The present experiments suggest that the molecular triad 1 is capable to undergo photoinduced intramolecular electron transfer leading to a long lived charge-separated state similar to that observed in the donor side of natural photosystem-II.

KEY WORDS: Ru-Mn, Charge separation, Triad, Manganese dimer