Homogeneous Systems Quantum Yields

The energy of an excited species must go somewhere, so the Stark-Einstein law leads to the conclusion that the sum of the quantum yields for all primary processes, including deactivation, must be unity. Where experimental data are available, this expectation is well substantiated. 

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The more efficient system of genera ting OH radicals in the homogeneous phase is H2O2/uv, where the quantum yield, < >254 0.50 (20,21). There... 

To avoid purification of the reaction product from the (colored) sensitizer or its oxidation and photolysis products, the use of insoluble sensitizers has been proposed, in particular for sensitized oxidations [14]. Whereas sensitizers adsorbed on solid supports, such as ion exchange resins, silica, or alumina [15], show considerable leading rates [16] and must be discarded, potential application might be found for sensitizers that are chemically bound to inert surfaces [14-17]. However, a loss of efficiency of at least 50% has to be taken into account when comparing overall quantum yields with those determined in homogeneous reaction systems [17]. 

Knowledge of the quantum yield of a given event (i.e. the number of defined events which occur per photon absorbed by the system or as the quantity (mol) of reactant consumed or product formed per mol or Finstein of photons absorbed see Kisch, 1989) has been central to homogeneous photochemistry. Photochemists have routinely determined quantum yields of reactant disappearance, product formation, light emission, and of other events occurring in a photochemical process. 

Methods to determine ph have been reported (Augugliaro et al, 1991 Schiavello et al, 1991 Palmisano et al, 1993 Cabrera et al, 1994). Alternatively, the quantum yield can be defined using the initial rate of reaction of the event, /f , and the rate of photons impinging on and absorbed by the reaction system, as is common practice in homogeneous photochemistry. Thus... 

Due to the heterogeneity of the reaction system, kinetic experiments are not as straightforward to conduct and interpret as in homogeneous photochemistry. Especially, quantum yield determinations are ambiguous because it is difficult to correct for the amount of light scattered by the photocatalyst powder. Various attempts have been made to overcome these difficulties [61-63] in order to allow comparison of quantum yields measured by different experimentalists. However, no general agreement has been reached until present. 

In homogeneous systems, some photocatalysts using transition metal complexes show outstanding performance, such as high absorbance in the visible region, high quantum yields, and high product selectivities. However, no metal complex which can use water as the reductant for CO2 reduction has been reported so far. 

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