Photochemical reactivity, factors

Multiphoton processes are also undoubtedly involved in the photodegradation of polymers in intense laser fields, eg, using excimer lasers (13). Moreover, multiphoton excitation during pumping can become a significant loss factor in operation of dye lasers (26,27). The photochemically reactive species may or may not be capable of absorption of the individual photons which cooperate to produce multiphoton excitation, but must be capable of utilising a quantum of energy equal to that of the combined photons. Multiphoton excitation thus may be viewed as an exception to the Bunsen-Roscoe law. 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

Lactams such as (258) can be synthesized from the phthalimides (259) by irradiation. Again the reactions are controlled by single electron transfer processes that are usually encountered in the photochemical reactions of phthalimides. The outcome of the reaction is a conventional proton transfer from the benzylic site within the zwitterionic biradical formed on irradiation. Cyclization within the resultant 1,5-biradical affords the final product. Griesbeck and his coworkers have studied the photochemical reactivity of the phthalimide derivatives (260). These compounds on irradiation under triplet sensitized conditions undergo decarboxylation and cyclization. The reaction involves SET and the key intermediates are shown as (261) and (262). The biradical anion (262) is the species that either cyclizes to afford (263) or abstracts hydrogen to yield (264). The reaction is controlled by a variety of factors that have been reported in some detail. Some photochemical reactions of phthaloylcysteine derivatives have been described. Typical of the processes are the decarboxylations of the derivative... 

Dimitriades, B., Eccleston, B. H., and Hum, R. W., "An Evaluation of the Fuel Factor Through Direct Measurements of Photochemical Reactivity of Emissions," presented at the 62nd Annual Meeting, Air Pollution Control Assoc.,... 

The experimental results revealed no significant difference in the rate of photoisomerization of azobenzene residues in the backbone of polyamides and in low molecular weight analogous azobenzene derivatives when both were studied in dilute solution.28 However, while the photochemical reactivity of the small species was relatively insensitive to the concentration of added polymer, the quantum yield for the photoisomerization of the azobenzene residues in the polymer backbone dropped precipitously with increasing concentration. In a glassy polymer film containing 8% DMSO plasticizer, the quantum yield for the isomerization of the polymer was reduced by a factor of 2500 while it was reduced only by a factor of 5 for the small molecule (Figure 3). 

The photochemical reactivity of polymers is of considerable interest because photodegradable plastics have a number of applications. Additional interest in polymer photoreactivity stems from the need to limit and control weathering in polymer materials. Photodegradation is an important component of polymer weather, and a proper understanding of degradation processes and of the experimental factors that affect degradation is necessary for the accurate estimation of polymer lifetime and for the development of stabilizing systems. 

Unusual chemical and photochemical reactivity (or, in contrast, incredible stability) of encapsulated species and the unexpected products of such reactions within the confined cavities of molecular capsules have been observed in many cases. This can be explained by steric restrictions due to relative rigidity of covalent and coordination capsules, by isolation of these guests from external factors such as solvent effects, and by relatively strong supramolecular host-guest interactions. 

We might also point out that our earlier thoughts on photochemical reactivity and radiationless transitions, with the energy gap between the excited species and the corresponding ground state structure as a dominant factor, is not likely to be apphcable to predicting HT reactivity. We suspect that the latter is controlled by kinetic factors such as entropy probability during relaxation of the Franck-Condon species. The latter topic will be dealt with in detail in a separate, future publication. ... 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

Trends in air pollutant concentrations can be predicted with simple empirical models based on atmospheric and laboratoiy data. Concentrations of nonreactive pollutants from point sources can be predicted vfith accuracy well within a factor of 2 predictions are more likely to be too high than too low, especially predictions of concentration peaks. Concentrations of reactive pollutants, such as ozone and other photochemical oxidants, can be predicted reasonably well with photochemical-diffusion models when detailed emission, air quality, and meteorolc c measurements are available most such predictions of air pollution in Los Angeles, California, have been accurate to within approximately 50% for ozone. Detailed performance analyses are found elsewhere in this chapter. 

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