Principles of Photoredox Reactions

The basic principles governing electron transfer reactions of excited states are reviewed in the chapter of Balzani and Maestri in this volume and elsewhere [27]. Given below is a brief outline of the materials necessary for understanding of the discussions that follow in this chapter. Focussing attention on the excited metal complex, quenching reactions in which the complex is oxidized are labelled oxidative electron transfer (equation 5)  

Similarly reaction wherein the polypyridine complex is reduced is referred to as reductive electron transfer (equation 6) 

Occurrence of both these type of photoinduced electron transfer reactions have been shown for all three types of electronically excited states that one encounters in polypyridine complexes. 

Another important excited state quenching pathway to be considered is transfer of excited state energy from the metal complex to appropriate acceptor molecules (equation 7)  

Whether some or all of the above mentioned processes occur depend on the spectroscopic energies of relevant excited states, the redox potentials of the complex in the excited state and of the quencher molecules in the ground state and possible restrictions imposed by spin-selection rules (for energy transfer case). 

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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.). 

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