Primary process photochemical, definition

The interdisciplinary nature of spectroscopy and photochemistry is well illustrated by the following definition of the primary photochemical process given by Noyes et al. (14) ... 

The redefinition of some terms and the creation of new definitions will facilitate the expression of ideas that are currently of greatest interest. As far as the chemical aspect of the problem is concerned, especially with regard to organic photochemistry, the nomenclature used is largely that of ordinary reactions. For that reason, the development here will be concerned mainly with what Noyes, Porter, and Jolley (12) referred to as the "primary photochemical process." In addition, because of the growing importance of energy transfer in the study of photochemical systems and the almost complete lack of a coherent nomenclature, this aspect will be considered in the following treatment. 

In some physical chemistry texts, the primary photochemical process is incorrectly considered to be no more than the absorption of radiation. Such a definition is not acceptable because absorption is not a chemical transformation and, more important, because it does not correspond to current usage in photochemistry. These texts then label such diverse processes as fluorescence, dissociation of an excited molecule, and chain reactions, all as different types of secondary photochemical processes. This is also unacceptable because "secondary" has come to have a specific meaning (as is discussed in Section III.A.3) which does not apply to all of these transformations, and also because some of them are not chemical. Photochemists instead use primary and secondary in the original sense of Bodenstein. 

Thus, even excluding the first definition quoted at the beginning of this section, there are four different meanings attributed to the term "primary photochemical process" in these recent books. 

Just as there are a number of different definitions of "primary photochemical processes," so also does "primary quantum yield" have different meanings in the various photochemical references. It may be defined, for example, as the sum of the quantum yields of all of the events which lead to dissociation or reaction of the excited molecule. In Figure 1, these would be events 24, 15, and 14. An alternate definition is that each one of the individual quantum yields in that sum is itself one of the primary quantum yields. 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

The efficiency of any photophysical or photochemical process is a function of both the properties of the reaction environment and the character of the excited state species. The fundamental quantity which is used to describe the efficiency of any photo process is the quantum yield (0) it is useful in both quantifying the process and in elucidating the reaction mechanism. Quantum yield has the general definition of the number of events occurring divided by the number of photons absorbed. Therefore, for a chemical process 0 is defined as the number of moles of reactant consumed or product formed divided by the number of einsteins (an einstein is equal to 6.02 X 10 photons) absorbed. Since the absorption of light by a molecule is a one-quantum process, then the sum of the quantum yields for all primary processes occurring must be one. Where secondary reactions are involved, however, the overall quantum yield can exceed unity and for chain reactions reach values in the thousands. When values of 0 are known or can be measured for a specific photochemical reaction the rate can be determined from ... 

The nonequivalence of the rates of photosensitized reactions in heterogeneous nanophases of glassy polymers is proved in experiments with naphthalene phosphorescence decay. For example, it is shown [13] that in aerated PMMA films fluorescence of singlet-excited naphthalene molecules N can be decayed by tinuvin P, but this does not affect the rate of naphthalene dissociation. The latter is consumed in the process, the rate of which is not defined by the concentration of particles responsible for fluorescence. Under such conditions, according to definition by the authors [13], primary chemical acts are inevitable. However, in the absence of oxygen tinuvin P slows the photochemical process down in accordance with a decrease of singlet-excited naphthalene molecule concentration. 

By definition, the sum of the primary quantum yields for all photochemical and photophysical processes taken together must add up to unity, i.e.,... 

III. SUGGESTED TERMS AND DEFINITIONS The difficulties in nomenclature described in the previous sections as well as others which arise in the photochemical "language" may be overcome by using the terms, definitions, and symbols given below. The basis of the suggested nomenclature is the clear distinction between (a) "step" and "process", (b) "primary" and... 

The quantitative assessment of photochemical activity is facilitated by introducing the quantum yield. In the practice of photochemistry a variety of quantum yield definitions are in use depending on the type of application. There are quantum yields for fluorescence, primary processes, and final products, among others. In atmospheric reaction models, the primary and secondary reactions usually are written down separately, so that the primary quantum yields become the most important parameters, and only these will be considered here. Referring to Table 2-4, it is evident that an individual quantum yield must be assigned to each of the primary reactions shown. The quantum yield for the formation of the product Pf in the ith primary process is the rate at which this process occurs in a given volume element divided by the rate of photon absorption within the same volume element. 

A substance can be thought to be a catalyst when it accelerates a chemical reaction without being consumed as a reactant that is to say, it appears in the rate expression describing a thermal reaction without appearing in the stoichiometric equation [49], A catalyst is a compound that lowers the free activation enthalpy of the reaction. Then, photocatalysis can be defined as the acceleration of a photoreaction by the presence of a catalyst [38, pp. 362-375], This definition, as pointed out in [29, pp. 1-8], includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called the photosensitizer [13], but it excludes the photoacceleration of a stoichiometric thermal reaction irrespective of whether it occurs in homogeneous solution or at the surface of an illuminated electrode. Otherwise, any photoreaction would be catalytic [29, pp. 1-8]. Depending on the specific photoreaction, the catalyst may accelerate the photoreaction by interaction with the substrate in its ground or excited state and/or with a primary photoproduct. 

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