Photochemical reactions overview

We shall begin with a closer look at electronic excitation, some aspects of which were discussed in Section 9-9. Because transfer of electronic energy from one molecule to another is a basic process in photochemistry, we will discuss energy transfer also before giving an overview of representative photochemical reactions. The closely related phenomena of chemiluminescence and bioluminescence then will be described. Finally, there will be a discussion of several important applications of photochemistry. 

In this overview and review of tropospheric photochemistry, we will examine a limited set of important homogeneous and heterogeneous photochemical reactions of relevance in the troposphere (Table 1). An expanded array of photochemical reactions is considered viable in the upper atmosphere (e.g., stratosphere) due to exposure to actinic radiation at wavelengths below 290 nm. A brief summary of a limited subset of this array of possible photochemical reactions will be provided in this review. 

The most fundamental principle of photochemistry, which actually was discovered by exposing inorganic compounds to sunlight 21), states that light must be absorbed by a chemical substance for a photochemical reaction to occur. We will therefore start our discussion with a brief overview on some of the natural chromophores which are known to be involved in photosensory processes or could be useful for the construction of artificial photoreactive systems. 

This highlight offers an overview of CD-containing nanosystems of various complexity with photoresponsive behaviour mediated by structural changes (reversible isomerizations, reversible or irreversible cleavage of covalent bonds) or release of bioactive species. Representative examples of the last decade have been described as to structural features, functions and operating mechanisms. CD-based systems which respond to light with emission of photons only or act as microreactors for photochemical reactions have been left out. 

Table 16.1 gives an overview of photochemical reactions in microstructures that have been reported in the literature. 

We now present an overview of the basic types of organic photochemical reactions. We begin with acid-base reactions, and then turn to reactions of hydrocarbon tt systems, such as olefin isomerizations, cycloaddition reactions, and the di-rr-methane rearrangement. We then study "heteroatom" photochemistry, the photoreactions of carbonyls and nitrogen-containing chromophores. 

Several comprehensive reviews of photochemical SET reactions of amines are available, as are more specialized reviews of the reactions of amines with specific classes of excited states. - These include reviews of the photochemical reactions of amines with arenes and iminium ions, which appeared in the first edition of this Handbook. The present article provides an overview of the photochemical addition reactions of amines with alkenes. Detailed information about the mechanisms and dynamics of these reactions is provided in the original literature and the comprehensive reviews. 

The atmospheric chemistry of nitrogen is quite complex and involves literally hundreds or thousands of chemical reactions. Although the fluxes are much smaller than the biological fluxes, these processes are important for a variety of reasons, including impacts on climate, stratospheric ozone, and photochemical smog. In this section we present an overview of the most important processes. 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

Electron transfer, photochemically induced, 1, 246 Electron-transfer equilibirum method, overview, 1, 817 Electron-transfer reactions Ga, In, T1 complexes, 3, 301 with Ge-Ge bonds, 3, 795 zinc species, 2, 315 Electron-transfer salts... 

---