Sensitizer, light-absorbing, design

As with other quantitative photochemical studies, it is important to design Stern-Volmer experiments carefully the quencher should be chosen to ensure that it interacts only with the excited state that is of interest, the extent of reaction should be small enough to ensure that substrate depletion does not affect the intensity of light absorbed, and the concentration of quencher should not be so small that it is significantly depleted by its sensitized reaction. Stern-Volmer plots may turn out to be non-linear, for example because the quencher interacts with more than one excited state on the reaction pathway, or because two different excited states lead to the same chemical. product, and such results are of value in unravelling the mechanism. 

Based on the dye-sensitized redox reactions, one can visualise two other cycles (designated C1 and C2, following Balzani) for the photodissociations of water. In C1 cycle, a visible light absorbing sensitizer S (a metal complex, for example) is photo-oxidised to a higher oxidation state which, in turn, oxidises water to 02 in the dark. The reducing equivalents are stored as a stable reduced acceptor QH or QH2. 

The requisites for the supramolecular antenna sensitizers are (1) an efficient antenna effect, vectorially translating absorbed energy toward a molecular component and (2) the capability of the molecular component bound to the semiconductor surface to inject electrons into the semiconductor from its excited state. Antenna sensitizers can increase the fraction of light harvested by a sensitized semiconductor surface. Two simple prototypes, following the branched or onedimensional design, are shown schematically in Fig. 2. An a priori evaluation... 

Photooxygenation may take place eii.ter as (a) a direct process, where light is absorbed by A designated as the substrate or as (b) an indirect process or photosensitized process where light is absorbed by a molecule other thin that which reacts and is called a sensitizer. The reaction is said to occur from the triplet state of the excited molecule. 

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