Photochemistry basic features

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

The first chapter is an introduction of the general and basic aspects of photochemistry on solid surfaces. The second chapter covers the application of photochemical techniques in order to understand the photochemical and photophysical nature on solid surfaces. Chapter 3 covers the special features of adsorbed-state photochemistry as compared with those of gas-phase and solution-phase photochemistry. Chapter 4... 

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

Early transition metal oxygen anion clusters, or polyoxometalates for short, are a large and rapidly growing class of inorganic complexes [1-15]. These compounds attracted us as it was apparent they simultaneously exhibited a unique set of properties we felt could be utilized to address new catalytic transformations, photoredox chemistry, and, ultimately construction of sophisticated single-molecule multifunctional devices capable of several temporally linked functions. These prognostications have already been borne out to a considerable extent. This article reviews the title subject, an enterprise still in its infancy. Presented sequentially in this chapter are the basic properties of polyoxometalates, general features of the photochemistry, the mechanisms elucidated in polyoxometalate photochemistry thus far, and an overview of the photochemistry of two representative complexes in tabular form (Table I). 

Figure 2 summarizes four features of polyoxometalate (Pox) photoredox catalysis (i) the two general classes of reactions, equations (2) and (3) (top), (ii) the classes of substrates, SubH2, that have been photochemically oxidized or otherwise transformed by polyoxometalates in the presence of light (top), (iii) the basic processes that add to equations (2) and (3) in the form of a catalytic cycle (middle), and (iv) definitions of three classes of polyoxometalate complexes based on their reactivity (bottom). Note that equations (4), (5) and (6) in the cycle sum to equation (2) and equations (4), (5), and (7) sum to equation (3). As is apparent in Figure 2 and will be elaborated below, a major feature of the photochemistry of polyoxometalate systems is the rich thermal chemistry that is induced by the photoredox processes. The fact that coupled and subsequent thermal processes can be extensively modulated by altering reaction conditions is a principal reason why polyoxometalate photochemistry is so versatile and promising. 

The parent molecule of aromatic azo compounds is azobenzene, whose UVATIS spectroscopic features and photochemistry are basically determined by the azo group -N = N- in conjugation with two phenyl substituents. The most important applications of azobenzene derivatives (azobenzenes) are their use as dyestuffs. Azo dyes absorb visible light and generally show remarkable light-fastness, which is due to an effective deactivation mechanism of the excited-state molecules that includes photoisomerization. ZoUinger has discussed this aspect from the viewpoint of dyestuff chemistry. 

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