Photochemical reactions cage effects

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

Shape selectivity and orbital confinement effects are direct results of the physical dimensions of the available space in microscopic vessels and are independent of the chemical composition of nano-vessels. However, the chemical composition in many cases cannot be ignored because in contrast to traditional solution chemistry where reactions occur primarily in a dynamic solvent cage, the majority of reactions in nano-vessels occur in close proximity to a rigid surface of the container (vessel) and can be influenced by the chemical and physical properties of the vessel walls. Consequently, we begin this review with a brief examination of both the shape (structure) and chemical compositions of a unique set of nano-vessels, the zeolites, and then we will move on to examine how the outcome of photochemical reactions can be influenced and controlled in these nanospace environments. 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

The intermediate charge transfer complex formed in the excited state is now termed as an exciplex. Back recombination of photochemically dissociated products in solution, due to Franck-Rabinowitch cage effect, probabily, is the common cause of low efficiencies of the solvent reaction. 

The key observation in the case of 152 is that photolysis in benzene conforms to the expected a-cleavage and decarbonylation reactions to form diphenylmethyl (A") and benzyl radicals (B"), which are free to diffuse apart. The statistical combination of free radicals A" and B gives a 1 2 1 mixture of products 154,155, and 156. In contrast, photochemical excitation in the crystalline phase led to the exclusive formation of 153 by combination of the geminate radical pair A B with a 100% cage effect. 

Structure and mechanism in photochemical reactions. The reactions of geminal radical pairs created in bulk polymers are presented by Chesta and Weiss in Chapter 13. Of the many possible chemical reactions for such pairs, they are organized here by polymer and reaction type, and the authors provide solid rationalizations for the observed product yields in terms of cage versus escape processes. Chapter 14 contains a summary of the editor s own work on acrylic polymer degradation in solution. Forbes and Lebedeva show TREPR spectra and simulations for many main-chain acrylic polymer radicals that cannot be observed by steady-state EPR methods. A discussion of conformational dynamics and solvent effects is also included. 

Organic photochemical reactions conducted in micellar solutions are reviewed from the standpoint of systematizing and correlating published results. Five common effects are found to distinguish and characterize micellar photochemistry relative to conventional solution photochemistry super cage effects, local concentration effects, viscosity effects, polarity effects, and electrostatic effects. These effects can contribute to the occurence of enhanced selectivity and efficiency of photoreactions relative to those in conventional homogeneous solution. 

The Fischer-Ingold effect can be understood without the need for elaborate -but more rigorous - mathematical modelling. Let us consider a compound A, which can be decomposed thermally or photochemically (with or without solvent) into two radicals, X and Y , which will then undergo recombination. If we neglected eventual cage effects and assume that the rate of such recombinations is diffusion controlled (and therefore comparable), then, statistically, of the three possible reactions shown by in Scheme 9.1, one would expect a yield... 

It is also of significance to incorporate complex molecules into microporous crystals to form photochemically or photophysically active centers. Because of the separation by the host framework, the complexes located in the channels or cages of microporous crystals are isolated. If the isolated centers with oxidation or reduction features are loaded in the connected and adjacent cages of a microporous crystal, redox pairs may be formed. Electron transfer may occur on these redox pairs under the excitation of light, and therefore photochemical reactions may proceed effectively. This is important for the utilization of solar energy. In addition, this type of assembly system may also be used to simulate the electron transfer process of oxidation-reduction in biological systems. 

Quite large enhancements in the rate of photochemical reaction have been observed in heterogeneous environments such as those that occur in aqueous micelle solutions or surface semiconductors (Cooper W.J. and Herr. F.L., I 987). The ways that micelles may influence solute chemical reactivity have been sumimarized below. These influences include cage, localization-compartmentalization, micro viscosity, polarity, pre-orientation, counterion and local electric field effects. 

It seems to me that Stein s self-criticism of this model rules out the model, and an alternative explanation to geminate recombination should be sought. A possible explanation is that in the photochemical studies, where geminate recombination was postulated, the lower quantum yields observed at low scavenger concentrations is caused by back reactions with products. In many of these systems, yields were not linear with dose at low scavenger concentrations, and possibly the initial yield was not obtained by the extrapolation used. [Czapski and Ottolenghi have shown recently that in these systems the cage effect is non-existent, and the back reactions can account for the system s behavior (28).]... 

The geometrical packing constraints that are expressed in the cage effect can be refined and used to advantage in the study of photochemical reactions within clusters. The essential idea is that the cluster is used to orient (favorably or otherwise) the ground state reactants. The photon then initiates the reaction. The simplest examples are for one clustering molecule and we will discuss both electronic orbital steering and more ordinary steric effects. 

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