Photochemical reactivity general discussion

The preceding discussion of the relationships between excited state electronic structure and photochemical reactivity focused primarily upon coordination compounds containing cP or low-spin cP transition metals. These relationships are generally applicable, however, to complexes of other d transition elements, the lanthanides and the actinides. A brief survey of the photochemical reactions of these latter systems is presented below. 

General discussions of photochemical reactivity can be found in the following articles ... 

One of the most important and difficult questions to answer for any photochemical reaction is which excited state is involved. Since these are the reagents, it is obviously important, if generalizations are to be made, to know which state is responsible for a given reaction. The question is difficult to answer because several different types of excited states, both singlet and triplet, are attainable even with the simplest of carbonyl compounds, and their reactivity may, in some cases, be similar. All of the discussion thus far has implied that the photocycloaddition reaction is characteristic of the n,n state. What is the evidence that this state can be involved and what is the character of this state which makes it reactive ... 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

The repulsive destabilization of sp -bound fluorine results in peculiar reactivity which has no real counterpart in hydrocarbon chemistry. This quite unique behavior of fluorinated olefins and arenes is summarized under the term special fluorine effect (as opposed to the previously discussed general fluorine effect ). The photochemically induced (Scheme 2.36) or fluoride ion-induced (Scheme 2.37) rearrangements observed can be rationalized as the system trying to reduce the number of energetically unfavorable fluorine atoms bound to sp -hybridized carbon. 

Typical chlorinations of alkanes or alkenes with (dichloroiodo)benzene proceed via a radical mechanism and generally require photochemical conditions or the presence of radical initiators in solvents of low polarity, such as chloroform or carbon tetrachloride. However, the alternative ionic pathways are also possible due to the electrophilic properties of the iodine atom in PhICH or electrophilic addition of CI2 generated by the dissociation of the reagent. An alternative synchronous molecular addition mechanism in the reactions of PhICl2 with alkenes has also been discussed and was found to be theoretically feasible [44]. The general reactivity patterns of ArICh were discussed in detail in several earlier reviews [8, 45, 46]. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is very dependent on the structure of the reactant. We have been able to discuss only some of the best-studied reactions. Many other examples can be found in the literature. This variation in reaction path with reactant structure may make photochemical reactions seem somewhat more complex and capricious than ground-state reactions. The real problem is that structure-reactivity relationships in excited states are not so well established as in ground-state chemistry. Continued study of photochemical reactions will no doubt lead to a general understanding of structure-reactivity effects in excited states. 

The general problem of the relationship between fragmentation reactions in a mass spectrometer and photochemical or thermochemical reactivity has fascinated mass spectroscopists for many years. The correlation in both cases is quite general and scores of examples are known. The scope and limits of these correlations have recently been reviewed from the point of view of PMO theory, and we will not discuss them further here. 

Abstract In this chapter we discuss some of the typical materials used in photochemistry. We describe, in general terms, how their suitability for application as absorber, emitter, sensitiser, energy acceptor or quencher, depends on the energy states within the material and the routes of interconversion between these states, and also how suitability as a redox or chemical sensitiser/acceptor/ trap is determined by specific chemical reactivities. We describe the application of photochemical principles to the design of light sources and displays, and describe the photochemical principles and applications of photochromies and molecular switches. A table giving the structures, characteristics, and uses, of a number of compounds widely used in photochemistry is provided at the end of the chapter. 

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