Ruthenium polypyridyl photosensitizers

Nasr C, Hotchandani S, Kim WY, Schmehl RH, Kamat PV (1997) Photoelectrochemistry of composite semiconductor thin films. Photosensitization of Sn02/CdS coupled nanocrystal-Utes with a ruthenium polypyridyl complex. J Phys Chem B 101 7480-7487... 

With the aim of mimicking, on a basic level, the photoinduced electron-transfer process from WOC to P680+ in the reaction center of PSII, ruthenium polypyridyl complexes were used (182-187) as photosensitizers as shown in Fig. 19. These compounds are particularly suitable since their photophysical and photochemical properties are well known. For example, the reduction potential [Rum(bpy)3]3+/-[Run(bpy)3]2+ (bpy = 2,2 -bipyridine) of 1.26 V vs NHE is sufficiently positive to affect the oxidation of phenols (tyrosine). As traps for the photochemically mobilized electron, viologens or [Co(NH3)5C1]2+ were used. 

In the example discussed above, the heterotriad consists of a photosensitizer and an electron donor. In the following example, a ruthenium polypyridyl sensitizer is combined with an electron acceptor, in this case a rhodium(lll) polypyridyl center [15]. The structure of this dyad is shown in Figure 6.21 above. The absorption characteristics of the dyad are such that only the ruthenium moiety absorbs in the visible part of the spectrum. Irradiation of a solution containing this ruthenium complex with visible light results in selective excitation of the Ru(ll) center and in an emission with a A.max of 620 nm. This emission occurs from the ruthenium-polypyridyl-based triplet MLCT level, the lifetime of which is about 30 ns. This lifetime is very short when compared with the value of 700 ns obtained for the model compound [Ru(dcbpy)2dmbpy)], which does not contain a rhodium center. Detailed solution studies have shown that this rather short lifetime can be explained by fast oxidative quenching by the Rh center as shown in the following equation ... 

Takahashi Y, Arakawa H, Sugihara H, et al. Highly efficient polypyridyl-ruthenium(II) photosensitizers with chelating oxygen donor ligands P-diketonato-bis(dicarboxybipyridine)ruthenium. Inorg Chim Acta 2000 310 169-74. 

Another chromophore-catalyst assembly supported onto an electrode was reported by Meyer and coworkers.A ruthenium polypyridyl dye was covalently linked both to a single site ruthenium OEC and to the Ti02 electrode. Initial transient laser and photocurrent measurements on the nanosecond timescale reveal that excitation of the photosensitizer leads to a rapid electron injection to the Ti02 followed by an intramolecular electron transfer from the ruthenium center of the catalyst to the ruthenium center of the dye, in a sub-ns timescale. However, injection from the chromophore attached to Ti02 remains inefficient (< 10 %), so this assembly needs further improvement to provide a key material for photoanode fabrication. 

Mihara and co-workers have synthesized some interesting donor-acceptor maquettes that could be incorporated into bilayer membranes [105,106]. The first such system (88) consisted of a molecular triad in which a mthenium polypyridyl photosensitizer, an anthraquinone primary oxidant, and a propylviologen terminal acceptor were placed along the lengths of two amphiphilic 21-residue polypeptides that were covalently attached to the ruthenium template. Circular dichroism... 

---