Photochemical enhancement

Interaction of light with a chemical species can initiate or enhance a chemical reaction. Reactions of this type are known as photochemical reactions. Of the many distinctive features of photochemistry, the following is particularly noteworthy in thermal excitation processes, all three forms of energy, electronic, transational, and rotational, are raised to higher levels. In contrast, photoexcitation raises only the electronic energy level which leads to higher selectivity, as 

Visible light constitutes a small fraction of the total radiation and is often referred to as white light. If U( is the velocity of light (= 2.998 x I0 ms ), 

The frequency u is usually expressed as hertz (Hz) in cycles per second (s ). A frequently used parameter is the wave number (m ) which is the reciprocal of the wavelength X, the distance between the crests of a wave. 

Light may also be regarded as a collection of photons or packets of energy. A photon has no mass but has a specific energy which is proportional to the frequency of the radiation  

The basic feature of a photochemical reaction is interaction between one molecule or atom of a chemical species A and one photon (q ergs) to produce a molecule /I in an excited state. The overall reaction in which n photons of light are absorbed may be written as 

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Kloeckner J, Prasmickaite L, Hogset A et al (2004) Photochemically enhanced gene delivery of EGF receptor-targeted DNA polyplexes. J Drug Target 12 205-213... 

Bonsted A, Wagner E, Prasmickaite L et al (2008) Photochemical enhancement of DNA delivery by EGF receptor targeted polyplexes. Methods Mol Biol 434 171-181... 

Nishiyama N, Amida JWD et al (2006) Photochemical enhancement of transgene expression by polymeric micelles incorporating plasmid DNA and dendrimer-based photosensitizer. J Drug Target 14 413 124... 

Katayama, A., Matsumura, F. (1991) Photochemically enhanced microbial degradation of environmental pollutants. Environ. Sci. Technol. 25(7), 1329-1333. 

Bossmann SH, Oliveros E, Gob S, Siegwart S, Dahlen EP, Payawan L, Straub M, Womer M, Braun AM. New evidence against hydroxyl radicals as reactive intermediates in the thermal and photochemically enhanced Fenton reactions. J Phys Chem 1998 102 5542-5550. 

Reactive Intermediates in the Thermal and Photochemically Enhanced Fenton Reactions, J. Phys. Chem. A, 102 5542—5550. 

However, it has been found recently [2g] that thermal or photochemically enhanced Fenton reactions, in the presence of 2,4-dimethylaniline, yield primarily 2,4-dimethylphenol as an intermediate product, the genesis of which may only be explained by an electron transfer mechanism ... 

Fine patterns of polymeric material are widely used in the electronics industry. They are required in the manufacturing of integrated circuits, printed circuit boards and solder masks for the latter. However, photopolymerization finds only limited application in this area. Photocrosslinking and photochemical enhancement of the rate of dissolution are primarily used. Larger structures can also be made by screen printing. In this technique, the photopolymerizable material is deposited pattern-wise and then irradiated in an overall exposure. 

There is a substantial literature on the thermal and photochemical degradation of PS and it is well established that polymer properties are sensitive to the manner in which a particular sample of PS is prepared. For example, it has been reported that PS prepared by anionic polymerization shows enhanced stability with respect to that prepared by a radical mechanism.2 10 This has often been attributed to the presence of "weak links" in the latter polymers. However, the precise nature of the "weak links" remains the subject of some controversy. T he situation is further confused by all PS prepared by radical mechanisms often being considered as a class without reference to the particular polymerization conditions employed in their preparation. In many cases the polymers are "commercial samples" with details of the method of preparation incomplete or unstated. 

Finally, surface properties of PTFEP were modified photochemically by light-induced grafting of poly(AT,Ar,-dimethylacryl amide) onto the film surface of this material to achieve a remarkable enhancement of its hydrophilicity [513]. 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

It has been shown that a combination of photolytic and biotic reactions can result in enhanced degradation of xenobiotics in municipal treatment systems, for example, of chlorophenols (Miller et al. 1988a) and benzo[a]pyrene (Miller et al. 1988b). Two examples illustrate the success of a combination of microbial and photochemical reactions in accomplishing the degradation of widely different xenobiotics in natural ecosystems. Both of them involved marine bacteria, and it therefore seems plausible to assume that such processes might be especially important in warm-water marine enviromnents. 

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