A binuclear ruthenium-manganese complex is synthesised; upon illumination the ruthenium transfers an electron to an external acceptor, then retrieves it by intramolecular electron transfer from the manganese.

One attractive way to harvest solar energy is to use the concepts of natural photosynthesis in an artificial system. In green plant photosynthesis, the solar energy is transformed into usable energy in the form of reduced compounds. The electrons come from water which is oxidized to molecular oxygen, thereby providing the plants with a never ending supply of reducing equivalents. In photosystem II, the photosensitizer is a chlorophyll species, P680, which is coupled to a cluster composed of four manganese ions that catalyses the water oxidation. We have tried to mimic this by the synthesis of a binuclear compound [Ru(bpy)2(Mebpy-Mebpy)MnCl2(H2O)2]Cl2 built on bipyridine ligands containing a Ru(II) moiety (the photosensitizer) and a Mn(II) ion (the donor) linked via a bridging ligand. In the complex, which is structurally defined by NMR, elemental analysis and electrospray mass spectroscopy, we have observed, (1) the Mn is sufficiently close (about 13Å) to interact with the Ru(II) ion, and (2) intramolecular, photochemically induced electron transfer from Mn(II) to the photogenerated Ru(III) moiety after a light flash in the presence of an electron acceptor. We suggest that the synthesis, characterization and observation of intramolecular electron transfer in this novel Ru-Mn compound is an important step towards artificial photosynthesis.

Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, Tyr(Z)(.). This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five-or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.

An improved high yield synthesis of 4'-(4-bromophenyl)-2,2': 6',2 ''-terpyridine from 2-acetylpyridine (1) and 4-bromobenzaldehyde (2) has been developed, using a two-step aldol condensation. In this, the intermediate azachalcone 3 was isolated, then reacted with N-[2-oxo-2-(2-pyridyl)ethyl]pyridinium iodide, prepared from 1, using ammonium acetate both as a base and as a ring closure agent. It could also be shown that one step aldol condensation of 1 and 2 gave low yields of the desired terpyridine due to facile formation of polycondensation products, Two of these, 8 and 10, could be isolated in moderate yields from condensation reactions. The structure and relative configuration of these compounds were determined by a comparison of observed experimental NMR parameters with theoretical values. calculated by molecular mechanics.

As part of a project aimed at developing models for photosystem II (PSII) in green plants, we have prepared a series of model compounds (7, 8, and 13). In these compounds, a photosensitizer, ruthenium(II) tris(bipyridyl) complex (to mimic the function of P-680 in PSII), was covalently linked to a manganese(II) ion through different bridging ligands. The structures of the compounds were characterized by electron paramagnetic resonance measurements and electrospray ionization mass spectrometry. The interaction between the ruthenium and manganese moieties within the complex was probed by steady-state and time-resolved emission measurements. When the binuclear complexes are exposed to flash photolysis in the presence of an electron acceptor such as methylviologen (MV2+), it could be shown that after the initial electron transfer from the excited state of Ru(II) in compound 7, forming Ru(III) and MV+., an intramolecular electron transfer from coordinated Mn(II) to the photogenerated Ru(III) occurred with a first-order rate constant of 1.8 x 10(5) s(-1), regenerating Ru(II). This is believed to be the first supramolecular system where a manganese complex has been used as an electron donor to a photo-oxidized photosensitizer, Possible extensions to develop the manganese donor, and thus to approach the function of reaction center in PSII, are indicated.

In the natural photosynthetic reaction center photosystem II, absorption of a photon leads to photooxidation of the primary electron donor P-680, which subsequently retrieves electrons from a tyrosyl residue, functioning as an interface to the oxygen-evolving manganese complex. In a first step toward mimicking these reactions, we have made a Ru(II)-polypyridine complex with an attached tyrosyl moiety. The photoexcited ruthenium complex played the role of P-680 and was first oxidized by external accepters. Combined transient absorbance and EPR studies provided evidence that the Ru(III) formed was reduced by intramolecular electron transfer from the attached tyrosine, with a rate constant of 5 x 10(4) s(-1). Thus we show that a tyrosine radical could be formed by light-induced electron transfer reactions, and we indicate future directions for developing a closer analogy with the photosystem II reactions.