Photosensitization indirect

The optode transduces the non-optical signal from the environment to the optical one, readable by the photodetector. Various indirect optical sensors and theirs applications are described in literature35. The optode can work as a chemical sensor that detects certain analytes in aqueous solutions or in air on chemical way. It means that changes in the environment cause the changes in the photosensitive material, which is immobilized in the optode matrix. These chemical changes influence the observed light intensity (for example, due to absorption) or one can analyze the intensity or time decay of luminescence. There are numbers of publications devoted to the family of optical chemical sensors36. 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

Photooxygenation may take place eii.ter as (a) a direct process, where light is absorbed by A designated as the substrate or as (b) an indirect process or photosensitized process where light is absorbed by a molecule other thin that which reacts and is called a sensitizer. The reaction is said to occur from the triplet state of the excited molecule. 

Triazine herbicides absorb sunlight weakly at wavelengths >290 nanometers (nm), thus, dissipation of the triazine herbicides in the atmosphere and in surface waters via photodegradation occurs mainly by indirect photolysis or photosensitized reactions. 

Apart from direct photolysis that is highly substrate-dependent, indirect photoreactions sensitized by quinones, furans, aromatic carbonyls (e.g., benzaldehyde derivatives), all important constituents of atmospheric aerosols, can lead to the transformation of many organic compounds [33-35], In such processes the photosensitizer P absorbs radiation and is then able to cause transformation of a substrate S because of energy transfer, electron or atom abstraction. 

Indirect photolysis mechanisms involve the excitation of an additional compound called a photosensitizer (PS), which in its excited state can directly oxidize the pollutant of interest. This type of mechanism was investigated by Faust and Hoigne [82] using fulvic substances as photosensitizers of phenols in natural waters. These latter mechanisms correspond to the indirect photolysis of M. In fact, Faust and Hoigne [82] reported that there are four possible routes of the excited photosensitizing action ... 

In addition to humic substances, nitrites and nitrates usually found in natural water also act as indirect photosensitizers to produce secondary oxidants such as hydroxyl radicals [91]. A simplified scheme of the mechanism is as follows [92] ... 

The above-mentioned theoretical background shows that, irrespective of the chemical nature of the photosensitizer and its binding mode to the semiconductor surface, one should consider two main ways of the semiconductor CB populating direct and indirect. Direct processes include VB -> CB excitations, photosensitization via bulk doping (TTRS-driven processes) and photophysical processes involving the TTRMs term. Indirect processes, in turn, involve excitation of the surface and a subsequent electron transfer reaction (WRV1 + TTet). 

Figure 17.7b shows that active E. coli concentration decreases as the accumulated energy increases to 37.5 kJL-1. Total bacterial inactivation (< 1 CPU ml A1) was reached after 5.5 h of phototreatment. As mentioned above, the decrease on bacterial culturability is due to the direct action of UV light and the indirect action of organic and inorganic photosensitizers present in LLW. 

If indirect photolytic processes are clearly important contributors to the photolytic degradation of florasulam in the aquatic environment, the role of DOM in the reaction cannot be evaluated without additional experiments using DOM or humic substances as the only photosensitizers. 

Earlier, Saltiel provided indirect evidence for a diradical intermediate by noting variations in the photosensitized cisjtrans ratio of 2-alkenes 128>. Originally, sensitized isomerization of olefins was thought to involve only energy transfer... 

An indirect PET methodology known as redox photosensitization has been developed by Pac [45] and Tazuke [64] for achieving higher yields of nucleophile addition product to alkene cation radicals. One recent example of this approach may be mentioned by illustrating anti-Markonikov alcohol addition (e.g. 61-62) to non conjugated olefin 61 using biphenyl as cosensitizer [65]. More examples on this topic can be found in Farid [5] and Mariano s [11] reviews. 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

There is abundant evidence from culture studies that both AOB and NOB are photosensitive. It is a high priority to investigate the photosensitivity of AOA. Even if aU nitrifiers exhibit photoinhibiton in some form, however, the direct and indirect ecological imphcations of this physiology for the rates and distributions of nitrification in the environment are not easily predicted. Dissolved organic matter in seawater, as well as turbidity due to sediments or phytoplankton, might aU provide photoprotection in surface waters, and regulation of nitrification by other factors discussed in this section may be much more important in many environments. 

Osmani AH, May GS, Osmani SA. The extremely conserved pyroA gene of Aspergillus nidulans is required for pyridoxine syn- 52. thesis and is required indirectly for resistance to photosensitizers. 

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