Photochemical reactions initiation

Poly(ethylene terephthalate), (PET), is a thermoplastic polymer widely used in the production of fibers and films on exposure to near ultraviolet light, PET fibers tend to lose their elasticity and break easily PET films become discolored, brittle and develop crazed surfaces. Such deterioration in properties has been attributed to photochemical reactions initiated by the... 

A photochemical reaction initiated by a-cleavage in a carbonyl compound is called a Type I process.45 Photolysis of terf-butyl-3,4-0-iso-... 

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

Crntzen, PJ. (1974) Photochemical reactions initiated by and influencing ozone in unpolluted tropospheric air, Tellus, 26,47. 

Finally, ring opening of a,/1-epoxyketones can be accomplished using n-Bu3SnH (equation 17)65. These reactions can be initiated either photochemically or via a thermal initiator such as AIBN. For the photochemical reaction, initiation occurs via excitation of the ketone to its triplet state, which abstracts hydrogen from n-Bu3SnH. 

Ever since the classical paper by Franck21 in 1926 on the relationship between spectroscopic phenomena and the photochemical primary process in diatomic molecules and the equally important papers in 1924 by Henri and Teves22 who clearly pointed out that diffuse bands in the spectrum of the sulfur molecule S2, as shown in Fig. 2b, meant that the molecule must dissociate upon absorption, the study of photochemical reactions initiated by absorption by diatomic molecules has been on a sound basis. 

The synthesis of mixed-valence corrpounds with optical IT transitions in the visible and near infrared region provides an interesting way to low-energy spectral sensitization. However, experimental results of photochemical reactions initiated by excitation of IT states are very scarce, see e.g. (21, 22). 

The absorption of light can lead to a photochemical reaction initiated by a molecule other than the one that absorbed the photon. This phenomenon suggests that electronic excitation can be transferred between molecules, resulting in the excitation of one and the deexcitation of the other. For instance, the excitation energy of might be transferred to a second molecule, represented in the ground state by S2... 

Free radicals and atoms appear as intermediates in a wide range of processes. Almost all photochemical reactions initiated by radiation in the visible or ne2ur ultraviolet regions of the spectrum are propagated by radicals or atoms (but see Section 1.4 for the photochemical formation of ions). Similarly, combustion processes are believed to occur predominantly by a radical mechanism, as are a large number of other thermal reactions. 

Resistance to Photochemical Reaction Initiated by U.V. Light (the sun). With the Introduction and acceptance of silicone sealants the stress tests have had to be made more stringent. Filters are removed from the Weather-O-Meter tests so some change can be noted in 6000 hours. Previously, sealants were considered good if performance changes were not significant after 250 hours. In fact, silicone sealants are now used to seal the test chamber of weather-simulating machines.—... 

UV-curable and dual cure nanocomposites are viable elegant solutions for the production of thermosetting resins based nanocomposites, most effective as they are solvent-free, require little energy and low temperatures, evolve in short intervals, and, when components are wisely selected, are environmental friendly [227-231]. But UV reflection, refraction and absorption related to the nanometric fillers must be all considered, as may affect the rate of the photochemical reaction initiation and this may entail longer exposure time and even a modified chemical and supramolecular stmcture. 

Cmtzen, P.J., 1974 Photochemical Reactions Initiated by an Influencing Ozone in Unpolluted Troposphtaic Air , in Tellus, 26 47. 

Gan, H., Kellet, M.A., Leon, J.W., Kloeppner, L.R., Leinhos, U., Gould, I.R., Farid, S., and Whitten, D.G., Novel intermolecular and intramolecular photochemical reactions initiated by excited state single electron transfer processes, /. Photochem. Photobiol. A Chem., 82, 211, 1994. 

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