Secondary photochemical reaction

The narrow bandwidth that can be matched with the maximum absorption of the initiator and thus eliminates undesirable secondary photochemical reactions non-linear processes may however become important at high power densities. 

A profound understanding of the reaction mechanism through which H202 is being formed could obviously enable optimization and lead to a maximal efficiency of such photochemical systems. Laser flash photolysis was thus employed to study the primary and secondary photochemical reactions in which lumiflavin and Ru (Il)-tris (2, 2 -bipyridine) are involved. Fig. 2 shows that photoexcited lumiflavin in its triplet state (picture a) is efficiently quenched by semicarbazide (picture c) this leads to flavosem iquinone formation (picture b). Further addition of molecular oxygen allows oxidation of flavosemiquinone radicals (picture d). On the other hand, Fig. 3 shows that photoexcited Ru (Il)-tris (2, 2 -bipyridine) is efficiently quenched by... 

Photochromism of 1-alkylanthraquinones (which is different from 1-methylan-thraquinone) in ethanol at 77 K was complicated by secondary photochemical reactions that generated a new product.26 This product was formed very inefficiently at room temperature owing to the short lifetime of the photoinduced form of this... 

From the preceding discussions, it should be clear that photochemistry within all phases of the atmosphere is a major driver of chemical transformations in relatively short time scales. With increasing knowledge of the ever-widening array of chromophoric compounds emitted and produced in the atmosphere, there is definitely room for much more fundamental research into primary and secondary photochemical reactions of relevance. In particular, the role of humic-like substances in aerosol, cloud and ice phases needs to be studied. 

Reactive species formed by the CDOM absorption of UVR can then undergo indirect, or secondary, photochemical reactions. In fact, the many possible reactions caused by photosensitized transient intermediates probably account for most of the photodegradation of CDOM that we observe. The many different photoprocesses involved in CDOM photochemistry make for a very complex pathway of reactions beginning with the initial absorption of light energy and ending with the final products of these multiple reactions. 

Apart from the inherent efficiency of the reactions leading to the light-induced formation of a ROS as summarized by the relevant apparent quantum yield and action spectrum, the observed rate of production will depend on other factors that affect the photon exposure including water column composition and depth (Chapter 3), time of day (i.e., solar zenith angle), season, latitude (Chapter 2), and physical transport processes (Chapter 4). For more details regarding the fundamental equations used to define the rates of primary and secondary photochemical reactions and their application to aquatic systems, the reader is referred to recent reviews on this topic [41,42]. 

Parenterals often consist of dilute aqueous solutions thus, a large fraction of the incident optical irradiation (UV irradiation and visible light) will penetrate through the drug formulation. The optical irradiation will reach sensitive molecules in the solution, and photons can be absorbed by the drug substance, excipients, or impurities in the formulation. Absorption may be followed by decomposition of the drug substance or the formulation by primary or secondary photochemical reactions (see Chapter 2). 

Some organic molecules are able to absorb optical irradiation and pass on the increased energy to other molecules, which then degrade these processes are called photosensitized or secondary photochemical reactions. The absorbing molecules, which are denoted photosensitizers, are not decomposed in the photochemical reactions. Excipients, solvents, or small amounts of impurities may act as photosensitizers that initiate the photochemical reaction in the formulation. The photosensitizers may be present in concentrations not detectable by conventional analytical methods, such as UV-visible absorption spectroscopy or HPLC with UV detection. 

Any subsequent thermal chemical reactions or physical energy dissipation processes are of a secondary nature. Specific examples for both primary and secondary photochemical reactions will be given in later chapters. 

Secondary Reactions. The reduction of a Cu(II) complex by a photochemically generated reducing agent constitutes a secondary photochemical reaction. Reductants such as 0 organic... 

Quantitative measurement of the photophysics of the dissolved organic chromophores found In natural water Is needed to better understand the rates of energy transfer from their excited states and to provide Information as to the kinetics of excited state reactions they may undergo. Specific areas of Interest In natural water chemistry are the kinetics of quenching Interactions and energy transfer processes to other chemical species the effect of such Interactions on the Initiation of secondary photochemical reactions is particularly Important ( ). 

The photodecomposition is the result of a disproportionation of the carboxylic acid catalyzed by the excited uranyl ion. The reduction product of the uranyl ion is oxidized back to UO2 by the organic acid. The secondary photochemical reactions vary, however, from acid to acid. ... 

The law of photochemical equivalence is restricted to primary photochemical process, i.e., each reacting species excited by the absorption of one radiation get chemicd transformation and formed products produce no further reaction. In such cases, these will be 1 1 relationship between the number of quantas absorbed and the number of reacting molecules. But in practice, most of photochemical reactions undergoes secondary photochemical reactions, i.e., photochemically activated species or product molecule initiates a series of chemical transformations while in some cases, photochemically activated species undergoes deactivation, they lose their energy in the form of heat or radiation. Under such conditions, there will be no more 1 1 relationship between the number of quanta absorbed and the product molecules. The deviation from photochemical equivalence (1 1 relationship) is described by the idea of quantum yield or quantum efficiency (< )). It is defined as... 

Intensity of light at a point is defined as the number of photons passing through the point per second. Therefore intensity is directly proportional to the number of photons. Primary photochemical reactions are Monophotonic photochemical reactions where each absorbed quanta excites one molecule which then reacts. That s why the rate of primary photoreactions is usually directly proportional to the light intensity [Rate c /]. But in case of secondary photochemical reactions [which are initiated by species formed by primary photochemical reactions], the proportionality depends on the ... 

Introduction of aromatic substituents to the carbon-carbon double bond leads to considerable n-conjugation causing the absorption to move to longer wavelength (>254 nm) with a concomitant increase in the extinction coefficient. 1,2-Diaryl- and triarylvinyUialides, including the bromides and chlorides, have absorptions at wavelengths around 300 nm. Therefore, these systems have been used mostly for synthetic applications because the secondary photochemical reactions can be eliminated by the use of a suitable filter. 

However, the mentioned reaction features in evacuated CTA films are displayed simultaneously with direct proportion of the naphthalene consiunption rate and accumulation of the product A, absorbing UV-light. This fact allows an exclusion of participation of twofold excited triplet states of N. In this case, according to the authors point of view, the attention should be paid to secondary photochemical reactions of free radicals formed at interaction with CTA macromolecules. 

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