Sensitizers polypyridine complexes

Figure 3. Photocatalytic cycles based on an oxidative (left) and reductive (right) quenching of an excited polypyridine complex M. (The photosensitizer M in these cycles has been called light absorption sensitizer (LAS), since it enables a photochemical reactions between chemical species which do not absorb light [74, 266].)...

Importantly, it was found [80-82, 311] that interfacial electron transfer from MLCT-excited Ru polypyridine complexes to Ti02 is an ultrafast process, completed in 25-150 fs This groundbreaking discovery implies that the search for new sensitizers need not to be limited to complexes with long-lived excited states. Indeed, [Fe(4,4 -(COOH)2-bpy)2(CN)2], whose MLCT excited state lifetime is only ca 330 ps, was found [304] to act as a sensitizer in a Ti02-based solar cell. In fact, even the classical Gratzel cell [36, 77, 78] would not operate as well as it does, were the interfacial electron transfer not ultrafast, since the [Ru(4,4 -(COOH)2-bpy)2-(NCS)2] sensitizer has an inherent excited state lifetime of only 50 ns. 

Much attention has been devoted to the development of optimal photo sensitizers of semiconductor electrodes [36, 43]. Ruthenium(II) polypyridine complexes are especially well suited for this purpose. They are strong light absorbers in the visible spectral region and bpy or tpy ligands can be easily derivatized with anchoring groups. Moreover, localization of the excited electron on the ligand which is attached to the semiconductor surface facilitates the electron injection. 

Figure 8. Chemiluminescence in redox-catalytic cycles. The metal-polypyridine complex M acts as light-emission sensitizer, LES [74, 266], D and A can be either a chemical reductant and oxidant, respectively, or an electrode polarized at appropriate potential.

PHOTOREDOX AND SENSITIZATION PROCESSES INVOLVING TRANSITION METAL POLYPYRIDINE COMPLEXES... 

A number of sensitized photoredox processes based on the photolysis of Re carbonyl polypyridine complexes in the presence of amines have been identified [56-58] ... 

The progress was painstakingly slow but in recent years there has been tremendous progress in the design of highly efficient sensitized photoelectrochemical cells. Principles and performance features of these photovoltaic cells form the topic for discussion in this chapter. Several topics related to materials considered in this chapter are treated in more detail in other chapters, e.g, principles of photoredox reactions (Balzani and Maestri), their applications in polypyridine complexes (Kalyanasundaram) and polynuclear complexes as light-harvesting units (Scandola et al.). 

The Fe(II) complex of 5 shows a remarkable stability towards strong oxidants such as H2O2. Its rutfaenium(n) complex is of particular interest due to its otochemical properties. In general, ruthraium(n) polypyridine complexes like [Ru(bipy)3] are of importance as sensitizers in photochemical processes Severe limitations to that use are due to the quick radiationless decay of their lowest excited state, which determines their photochemical properties, and are due to their sensibility towards photodissociation One way of avoiding photodissociation... 

Some metal complexes with polypyridine ligands similar to Dye 2 have also been investigated as sensitizing dyes. Ferrere and Gregg73) synthesized cis-Fen(dcbpy)2(CN)2 complex (Dye 8), and Bignozzi el al.74i75) compared properties of m-Osn(dcbpy)2(CN)2 (Dye 9) with comparable m-Rull(dcbpy)2(CN)2 (Dye 10). The efficiencies are much lower than that of Dye 2, but they proposed the... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

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