Excited States, Radiative, and Nonradiative Processes

The nature of the excited state decay processes is studied by the technique of laser flash photolysis, a description of which has been given by Bensasson et al. (1983). Briefly, flash photolysis involves irradiating a sample with a short (nanosecond) intense pulse from a laser, then observing by rapid response spectrophotometry the spectral changes that occur on the time scale nanoseconds to milliseconds. 

Several standard tests have been established to aid in the identification of the transient species. Solvated electrons generated by photoionization in a nitrogen-gassed solution have a characteristic broad structureless absorption peak at about 700 nm, depending on the solvent (720 nm in aqueous solution). Oxygen quenches this absorption and also quenches the triplet state, while nitrous oxide gassing can be used to quench the solvated electron only, thereby gaining an indication of any transient absorption that arises from the triplet state. 

Although the quantum yield of fluorescence is readily determined by reference to quinine fluorescence as described by Calvert and Pitts (1966), those of the other processes can only be obtained by difference. Phosphorescence is usually too weak to be observed in solution at room temperature, but can be measured if the drug is held in a glassy matrix at low temperature. The usual procedure is to dissolve the drug in ethanol and immerse in liquid nitrogen. The phosphorescence accessory of the fluorimeter incorporates a mechanical chopper enabling the phosphorescence to be observed free of interference from any fluorescence. Because of the difference in temperature and matrix, it is not possible to compare the phosphorescence yield with that of fluorescence. Nevertheless, phosphorescence is worth measuring because it is an important indicator of the capacity of a molecule to populate its triplet state. 

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