Primary photochemical/photophysical process

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

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

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Primary (photo)process See primary photochemical process. The term primary (photo)process for photophysical processes is apt to lead to inconsistencies, and its use is therefore discouraged. 

Secondary Processes. Long-lived intermediates such as free radicals formed in reactions 4 and 19, or final products of primary processes, may undergo further photophysical or photochemical processes, depending upon the variety of experimental conditions used. If an extremely high photon intensity is available, secondary photolyses as well as two-photon absorption could become important. If sufficient amounts of the primary photochemical products accumulate in the system, the final product distribution could reflect further reactions of these products. 

Primary Photochemical Process a term originally used (12) to describe the entire set of photochemical and photophysical primary processes. 

This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties. 

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

Primary photochemical processes are processes initiated from an electronically excited state that yield a primary photoproduct that is chemically different from the original reactant. Photochemical processes are always in competition with photophysical processes that eventually restore the reactant in the ground state. Photoreactions leading to new products can be efficient only if they are faster than the competing photophysical processes. Therefore, it is essential to have a feeling for the time scales of the latter (Table 2.1, Figure 2.2). Photophysical processes of molecules in solution usually obey a... 

The primary photochemical processes performed by electronically-excited molecules. They can be divided further into photophysical processes and photochemical reaction processes. The former include luminescent processes and nonradiative deactivation. 

Here we discuss light absorption by various components of the cell and the effects caused by light absorption. Primary photoinduced cellular effects are produced by light absorption to induce transition between two electronic states (electronic or coupled electronic-vibrational [vibronic] transitions). Purely vibrational transitions (such as IR and Raman) are of significance only in stmctural identification and in conformational analysis. We first discuss the absorption by various constituent molecules and biopolymers. Subsequently we discuss the various photochemical and photophysical processes induced by light absorption. Then we discuss the effects produced from light absorption by an exogenous chromophore added to the cell. 

Therefore, as in the case of parent phenyl azide 47 and its simple derivatives, the photochemistry of polynuclear aromatic azide, especially that of naphthyl azides 79 and 80, is now well understood. Specifically, the dynamics of the primary photophysical processes as well as the subsequent photochemical steps have been directly investigated using a variety of modem and conventional experimental techniques and compntational chemistry. It is clear now, that the difference between the photochemistry of phenyl azide (and its simple derivative) and polynuclear aromatic azide is caused mainly by the difference in the thermodynamics of the singlet nitrene rearrangement to azinine type species. 

K. Evans, D. Heller, S, A. Rice, and R. Scheps, Primary photochemical and photophysical processes in chloro- and bromo-acety-lene, J. Chem. Soc. Faraday Trans. II 69 856 (1973). 

The relative efficiencies of the various photophysical and photochemical primary processes are described in terms of quantum yields, . The primary quantum yield, , for the ith process, either photophysical or photochemical, is given by Eq. (I) ... 

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