Photochemical and Photo-induced Reactions

The effect of pressure on photochemical and photo-induced reactions has been investigated in detail for many systems, and a number of reviews have appeared on this subject recently [82-84]. This topic is treated in more detail in Chapter 6, and readers are referred to that chapter for more information. 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

The characterization and utilization of photochemical processes are rapidly developing into one of the major areas of activity in modern inorganic and physical chemistry. In the past, the photochemistry of classical metal coordination complexes has received the greatest amount of attention, but recently the photochemistry of organometallic compounds has attracted notice In particular, the photochemistry and photophysics of uranyl compounds have been investigated for more than four decades and a great deal has been learned about the primary photoprocesses and the photo-induced reaction mechanisms displayed by these complexes (.3,4). The popularity of uranyl compounds in photochemical studies is derived from their ready availability and stability, their facile redox chemistry and photosensitivity and their rich excited state chemistry. Since current reviews of uranyl photochemistry are expected to appear in the near future, vide infra, further discussion of this topic here will be limited. 

Other publications consider the photochemical reactions of Hg(SiMe3)2 with fluoro-olefins, the photo-induced reaction of Me SiCl4 (n = 1—3) with S02Cl2, the e.s.r. properties of photochemically produced substituted triarylsilyl radicals,and a comparison of the phosphorescence spectra of SiPh4, GePh4, SnPh4, and... 

TERP can be photochemically induced, as mentioned. Photo-induced reactions do not require heat and thus are applicable to functional groups and materials that decompose at high temperatures. Photochemical stimuli can also be used as external stimuli to switch the reactions on and off and can trigger the reactions locally at specific positions. Thus, photo-induced LRP can expand the scope of monomers and substrates and offer temporal and spatial control in surface-initiated LRP. In this regard, Tsujii, Yamago and coworkers conducted photo-induced... 

Torikai, A., Murata, X, and Fueki, K., Photo-induced reactions of polycarbonate studied by ESR, viscosity and optical absorption measurements, Polym. Photochem., 4, 255,1984. 

In order to achieve photochromism, a photo-induced reaction leading to a new species possessing different physical and chemical properties, in particular the absorption spectra, should occur. In the case of the spiropyrans, spirooxazines and chromenes, the initial photochemical step consists of the spirocarbon C-O bond cleavage followed by the rearrangement of the molecule. Scheme 2.2. 

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

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