Photostability mechanism

Perun S, Sobolewski AL, Domcke W (2005) Photostability of 9h-adenine mechanisms of the radiationless deactivation of the lowest excited singlet states. Chem Phys 313 107—112... 

Barrau and coworkers have synthesized a series of iron and ruthenium complexes by irradiation of Me2HGe(CH)KGeMe2H and Me2HGe(CH)K SiMe2H (n = 1, 2) in the presence of Fe(CO)5 and Ru3(CO)i293. In each case irradiation causes CO loss, with the formation of the M(CO)4 species (reaction 43). When n = 2 the products are photostable with n = 1 (65) a mixture of products (66-69) are obtained due to secondary photolysis (reaction 44). The mechanism, outlined in Scheme 23, is presented to explain these observations. 

Conde FR, Churio MS, Previtali CM (2000) The photoprotector mechanism of mycosporine-like amino acids. Excited-state properties and photostability of porphyra-334 in aqueous solution. J Photochem Photobiol 56B 139-144... 

The ionisation state of molecules in the solution state appears to be an important variable in photodegradation mechanisms. A recent pubhcation on riboflavin oral liquid preparations shows that the formulation is most photostable at pHs between 5 and 6, where the non-ionised form predominates [78]. The rate of photolysis increase 80-fold at pH 10.0, owing to increased redox potential. Conversely, at pH 3.0, the increased photolysis is associated with the excited singlet state, in addition to the triplet state. 

Photodegradation. UV/Vis exposure can induce protein oxidation, aggregation, and backbone cleavage. For example, oxidation has been observed in the histidine residue of human growth hormone (hGH) exposed to photostability conditions (6.7 x 106 lux hours). The proposed oxidation mechanism and product is shown in Figure 136. 

An Example The Phosphorescence of Dithiosuccinimide Many thio-carbonyls have photostable excited (n > ji ) and (ti —> ti ) states that tend to relax by photophysical rather than photochemical processes.177,178 Recently, the electronic spectra of dithioimides have been under experimental and theoretical investigation.179-181 The spin-forbidden radiative decay of the lowest-lying triplet state of dithiosuccinimide may serve as an example to illustrate the results of the previous sections. Experimentally a lifetime of 0.10 0.01 ms was determined for the Ti state.179 This value has been corrected for solvent effects, but the transition may include radiative as well as nonradiative depletion mechanisms. 

The acid-catalyzed cyclization to benzimidazole AT-oxides may also be conducted photolytically with good results.40 Denitration is not observed here and very clean reaction products are obtained. In a few cases, however, the corresponding benzimidazole and not its A-oxide is the sole product. Since these A-oxides are photostable under the reaction conditions, it appears that a different mechanism operates for the photocyclization. A plausible alternative (101)-(88) may be visualized by analogy to the photolytic rearrangement of o-nitro to o-nitroso compounds.43 The o-nitroso-A-oxide (110) may... 

Here we discuss a new class of polypropylene stabilizers—the polymeric phenolic phosphites. These compounds exhibit unique, broad-spectrum activity which may allow simplification of polypropylene stabilizer systems. The most active species are synergistic with thiodipro-pionate esters, are effective processing stabilizers when used alone or with other compounds, and contribute to photostability. Compounds of this type appear to function as both free radical scavengers and peroxide decomposers, and through a mechanism not yet completely understood, allow significant reductions in the concentration of ultraviolet absorbers required to achieve high levels of photostability. 

The corresponding chloro and bromo compounds as well as the 3-iodo and 4-iodophenyl compounds are photostable. A mechanism is proposed in which the primary excited S, state crosses into an ncr state in the iodobenzene manifold. This is followed by homolysis of the C—I bond. The aryl radical may undergo ring closure or abstract a hydrogen atom from the solvent. The primary ring-closed product is photo-oxidized under the reaction conditions. 

Our study of the photostability issue of nucleic acid bases began with isolated bases, and later extended to their water complexes [15, 32], Our observation has thus revealed not only intrinsic properties of the bases, but also the effect of the environment on the decay mechanism of the excited state. Most of our results have been obtained from 1,3-dimethyl uracil (DMU) due to the ease of vaporization. To generalize our conclusion, however, we have also investigated 1-methyl uracil (MT), thymine (T), and 1,3-dimethyl thymine (DMT). In particular, we believe that 1-methyl uracil is an excellent mimic of uradine because of the similar substitution position on uracil. 

In parallel to those determinations, Domcke, Sobolewski and coworkers [57, 58] found the existence of a new type of CIs between excited ttct and ground states, in which the reaction coordinate for the relaxation of the system is the stretching and dissociation of a N-H bond. The relevance of this mechanism for the photostability of isolated nucleobase monomers is however unclear. 

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