Ultraviolet-visible absorption substance reactions

ID. Optical Methods of Analysis. Optical methods of analysis of reaction systems are very convenient where they can be applied. The optical properties which characterize the system may be the absorption at one or more particular wavelengths (in the ultraviolet, visible infrared, or microwave region), the refractive index of the mixture, the optical rotation of one or more species, the light-scattering properties of large molecules, or the fluorescent emission of one or more of the substances present. 

More detailed consideration of light absorption and consequent chemical changes is left to Chapter 13, but it is appropriate here to summarize briefly the types of compounds that are convenient photochemical radical sources. Many of the substances we have been discussing as thermal radical sources absorb light in the visible or ultraviolet and can be decomposed photochemically. The azoalkanes are particularly versatile they absorb around 350 nm and decompose cleanly to nitrogen and two radicals just as in the thermal reaction. As we have already noted, a preliminary photochemical isomerization to the cis isomer precedes the homolysis, which is actually a thermal decomposition of this unstable form.78 CIDNP observations confirm a stepwise decomposition pathway, and clarify the various reactions of the radicals produced.79... 

Analytical absorption spectroscopy in the ultraviolet and visible regions of the elechomagnetic spectrum has been widely used in pharmaceutical and biomedical analysis for quantitative purposes and, with certain limitations, for the characterisation of drugs, impurities, metabolites, and related substances. By contrast, luminescence methods, and fluorescence spectroscopy in particular, have been less widely exploited, despite the undoubted advantages of greater specificity and sensitivity commonly observed for fluorescent species. However, the wider availability of spectrofluorimeters capable of presenting corrected excitation and emission spectra, coupled with the fact that reliable fluorogenic reactions now permit non-fluorescent species to be examined fluorimetrically, has led to a renaissance of interest in fluorimetric methods in biomedical analysis. 

Even if direct light absorption as above does not occur, polymerization can still be initiated if photosensitizers are present that produce free radicals when they absorb ultraviolet or visible light. The same substances that are used for thermal initiation are often used for photosensitization. For example, azo compounds and peroxides are photosensitizers, and the photoinitiation reaction is the same as is the thermal initiation process, described earlier in this chapter. However, the photoinitiation can take place at much lower temperatures than in the thermal initiation by the same initiators. Moreover, many initiators can be used as photosensitizers even though they do not dissociate thermally at convenient rates or temperatures to be useful as thermal initiators. For example, azoisopropane does not dissociate sufficiently rapidly below 180°C to be useful thermal initiator. However, it photodissociates even at low temperatures when irradiated with near-ultraviolet light ... 

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