Photochemical processes, types

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

Although the work discussed thus far has covered primarily neutral organic radicals, there are many types of cation and anion radicals that are stabilized on the surface. Some of these ion radicals are formed through photochemical processes however, many others are spontaneously generated on a surface. The type of radical ion that is formed depends on the oxidizing or reducing character of particular sites on the surface, as well as on the ionization potential and the electronegativity of the adsorbed molecule. 

Numerous examples of intermolecular and intramolecular photocycloaddition to heterocyclic systems (including the dimerization of individual heterocycles) have now been reported. Two types of cycloaddition can readily be effected photochemically, namely, [n2 + 2] and [ 4 + 4] additions. Although concerted suprafacial additions of this type are allowed photochemical processes, in reality many cycloadditions occur via diradicals, zwitterions or exciplexes. 

For this type of integrated fluoroionophores, the photochemical processes can be accelerated and can lead to very fast and reversible photochemical ion release or ion takeup. One example has been described recently 140 where Ca2+ or Li+ is ejected in some picoseconds. By this way, the application of biologically useful chelators, which have their binding constant altered by an irreversible photoreaction taking at best some milliseconds, 141 can be extended to ultrashort time scales. 

Although more work is needed to clearly correlate the type of solubilization site occupied by different porphyrins with their reactivity in such sites towards atropisomerization, it is clear that different sites exist and that these sites show quite different reactivity in both thermal and photochemical processes. Preliminary studies have shown that related behavior probably occurs in other organized assemblies formed by dispersion of surfactant molecules in... 

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption photodissociation, intramolecular rearrangements, photoisomerization, photodimerization, hydrogen atom abstraction, and photosensitized reactions. 

PAHs photooxidized by the type II singlet oxygen mediated mechanism include acenaphthylene, whose oxidized products and yields are shown in Fig. 10.29B (Barbas et al., 1994), phenanthrene (Barbas et al., 1996), anthracene (Dabestini et al., 1995), and tetracene (Dabestini et al., 1996). An additional photochemical process, the formation of photodimers, is also observed for acenaphthylene, anthracene, and tetracene. 

In 1966, Chapman and co-workers proposed a nitro-nitrite photorearrangement as an efficient primary photochemical process for nitroarenes in which the nitro group is out of the plane of the aromatic rings. This is followed by dissociation into NO and a phenoxy-type radical ultimately quinones and other oxy products are formed (Chapman et al., 1966). 

Since the photochemistry of many compounds that have been used as triplet sensitizers has been well studied, we will not attempt to cover these reactions in detail. Unless the investigator is unaware of them, common photochemical processes such as the Norrish Type II cleavage are not ordinarily a complication and as will be mentioned later, they can actually serve as mechanistic probes. A discussion of the mechanisms of triplet energy transfer1,3,9 is beyond the scope of this review as are other specific reactions which have been recently covered elsewhere. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

In a thermal reaction R—>TS—>P, as shown in Figure 4.4, the transition state TS is reached through thermal activation, so that the general observation is that the rates of thermal reactions increase with temperature. The same is in fact true of many photochemical reactions when they are essentially adiabatic, for the primary photochemical process is then a thermally activated reaction of the excited reactant R. A non-adiabatic reaction such as R - (TS) —> P is in principle temperature independent and can be considered as a type of non-radiative transition from a state R to a state P of lower energy, for example in some reactions of isomerization (see section 4.4.2). 

Stereo-isomerizations are quite common photochemical processes with unsaturated organic molecules (the primary photochemical reaction of vision is of this type). 

CFC Abbreviation for chlorofluorocarbon, a type of organic compound in which some or all of the hydrogen atoms of an alkane have been replaced by fluorine and chlorine atoms. These substances are generally unreactive but they can diffuse into the stratosphere where they break down under the influence of ultraviolet light. The products of this photochemical process then react with ozone (in the ozone layer). Because of this, their use... 

There are two types of photochemical processes which lead to these various transitions and thence to a realisation of the synthetic possibilities of the processes 4 and 3 above. 

Isomerization is an accepted photochemical process and we will not discuss further examples in detail. A few types are cited below. 

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