Actinometry and Quantum Yield Determinations

The quantitative study of photochemical reactions requires knowledge of the concentrations of reactants and products as well as the number and energy of photons absorbed by the sample. The determination of the number of photons absorbed by the sample is known as actinometry. In some cases these measurements are made by irradiating the sample and a standard chemical reference system simultaneously. Alternatively, the measurements may be made with electronic devices.  

A quantum yield (also called quantum efficiency) of some photochemical process is defined as the number of photochemical events of that process that occur per photon of UV-vis radiation absorbed. Consider the very simple photochemical reaction illustrated in equation 12.4  

Similarly, the quantum yield of appearance ( app) of product (B) is given by 

More commonly, calculations are done on a molar basis, so 

Instead of the simple hjq othetical reaction in equation 12.4, organic molecules normally exhibit a number of photochemical and photophysical processes that compete with each other. A minimal complication is illustrated in equation 12.8, in which the photoexcited molecule both fluoresceses and forms product. The rate constant for fluorescence is kf, and the rate constant for reaction is k.  

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The various actinometer systems applicable to all types of photoprocesses have been summarized by Kuhn et al. (1989) for the International Union of Pure and Applied Chemistry Commission on Photochemistry. The fundamental problem is that actinometry is a technically demanding task, and it is then a complicated matter to relate the photon absorption rate to a reaction rate applicable to a particular experimental setting. In the following sections, the procedure will be briefly described for chemical actinometry and quantum yield determination then it will be explained that the rate of a photochemical reaction can be predicted from knowledge of the quantum yield and the incident photon intensity. Finally, treatment of experimental rate data for a photostability study will be covered in detail. 

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