Photochemical polypyridyls

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. 

A. Kirsch-De Mesmaeker, G. Orellano, J. K. Barton, and N. J.Turro, Light-dependent interactions of ruthenium(II) polypyridyl complexes with DNA probed by emission spectroscopy, Photochem. Photobiol. 52,461 (1990). 

Taqui Khan, M. M. Chatteijee, D. Hussain, A. Moiz, M. A. Synthesis and characteristics of mixed ligand Ru(IIl) complexes with EDTA-polypyridyl, and Pt/TiOj/ Ru02 semiconductor particulate system modified by the complexes, J. Photochem. Photobiol. 1993, A76, 97. 

Recent interest in Rh polypyridyl complexes has centered on the behavior of their excited states this is an active and complicated research area, and our understanding is less than complete. Discussion here will proceed as follows (a) the photochemical properties (b) the chemistry of the reduced polypyridyl complexes and (c) the role of Rhm polypyridyls as electron-transfer agents in the quenching of photoproduced excited states of [Ru(bipy)3]3, and the photogeneration of H2 from water. 

Both our photochemical and radiation studies have focused on the chemistry of very reactive species in aqueous solution. Indeed, it is because the photochemical work involved aqueous media that radiation chemistry techniques could be so useful to us. Our pulse radiolysis work has led to a number of highly unusual mechanistic conclusions. In the area of low-oxidation-state chemistry, several of the systems violate standard organometaUic dogma. We investigated the rate of hydride formation in another cobalt(I) system, that derived from the high-spin d polypyridyl-cobalt(I) complexes (28). Remarkably, electron transfer was found to be the rate-determining step for formation of the hydride complex, and contributions from Bronsted acid pathways contribute neghgibly to the rate. Rather, the hydride formation appears to involve H-atom transfer from the protonated bpy radical. The H-atom receptor may be either Co(bpy)2 or Co(bpy) as shown in Scheme II. 

The ability of the MLCT ES in Ru(ll) polypyridyl compounds to undergo facile excited state ET reactions, combined with the stability of the complexes and the tunability of their ES properties, makes this class of compounds attractive candidates for a number of practical applications. Ruthenium(ll) polypyridyl compounds have already found applications in the field of photovoltaics as sensitizers for wide-bandgap semiconductors (such as Ti02) and as photocatalysts for the photochemical splitting of water. Their photophysical and photoredox properties... 

The basic photophysical properties of a dye molecule are maintained upon immobilisation on a semiconductor surface, but the interaction with the semiconductor may open new reactive routes and/or change the rate of particular photochemical processes. An example of the importance of these routes is the fact that some polypyridyl complexes, intrinsically photolabile in solution, become photostable when bound to the semiconductor titanium dioxide, and actually constitute the class of dyes that has enabled some of the most efficient cells constructed to date [4, 5]. 

The Ru(II) polypyridyl complexes are good photochemical probes. The opportunity to combine the photophysical and ESR techniques became possible with... 

Enormous interest shown on the photochemistry of transition metal polypyridyl complexes in fact is linked to these type of applications in the domain of photochemical conversion of solar energy. Practically every metal complex with fully or partially characterized electronically excited state has been examined as a means of generating key oxidants and/or reductants required and some have shown partial success. A number of reviews of these topics are available [63-65] and hence only the basic principles and summary of progress in these areas will be indicated. 

Harriman, A. Photochemistry of Manganese Complexes, Coord. Chem. Revs. 1979,28,147. Jamieson, M. A. Seipone, N. Hoffinan, M. Z. Advances in the Photochemistry and Photophysics of Chromium(ni) Polypyridyl Complexes in Fluid Solution, Coord. Chem. Revs. 1981,39,121. Kirk, A. D. Chromium(ni) Photochemistry and Photophysics, Coord. Chem. Revs. 1981,39,225-293. Kutal, C. Spectroscopic and Photochemical Properties of d Metal Complexes, Coord. Chem. Revs. 1990,99,213. 

The photochemical and photophysical properties of polypyridyl complexes of Ru, Os l and Re and their derivatives in solution are well understood. They absorb light in the visible due to metal-to-ligand charge transfer (MLCT) transitions. 

---