Photochemical endergonic

Figure 1. Energy profile for a general endergonic photochemical reaction Ef, is the minimum energy gap between the lowest vibrational levels of the excited state R and the ground state R of the absorber. Er is the activation energy for the back reaction P R.

When we wish to convert and store solar energy as, for example, in an endergonic photochemical reaction, an additional kinetic requirement is imposed which can be seen by reference to Fig. 1. The conversion of R to P must be an exergonic reaction so that an activation energy E for the back reaction will be established (16). Otherwise, P would have no stability for storage or subsequent reactions leading to chemical storage. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

The photochemical carboxylation of pyruvic acid by this process is endergonic by about AG° = 11.5 kcal mol and represents a true uphill photosynthetic pathway. The carbon dioxide fixation product can then act as the source substrate for subsequent biocatalyzed transformations. For example, photogenerated malic acid can act as the source substrate for aspartic acid (Figure 35). In this case, malic acid is dehydrated by fumarase (Fum) and the intermediate fumaric acid is aminated in the presence of aspartase (Asp) to give aspartic acid. 

Chemical fuel, however, is not the only means by which energy can be supplied to operate molecular-level devices. As recalled in the previous section, nature shows that in green plants the energy needed to sustain the machinery of life is supplied by sunlight. Photochemical energy inputs can indeed cause the occurrence of endergonic chemical reactions, which can make a device work without formation of waste products. Currently there is an increasing interest in... 

On the other hand, it is well known that inputs of light energy can cause the occurrence of endergonic, reversible, and clean reactions. In the last two decades, the outstanding progress made by supramolecular photochemistry has led to the design and construction of photochemically driven molecular devices and machines which work without formation of waste products. 

As mentioned previously the term photocatalysis was used by Plotkinow to describe the acceleration of a chemical reaction due to the influence of light. Since this definition applies for any photochemical reaction there is no inventive power in it. Also a restriction of the acceleration by light only to endergonic reactions, as proposed by Balzani[3], does not improve the situation. 

A C(sp)-C(sp ) single bond of arylalkynes was cleaved by photoirradiation of a platinum-alkyne complex (Scheme 1.28) [37]. The oxidative addition product was reverted to the starting ii -alkyne complex upon heating. This result indicates the photochemical oxidative addition is an endergonic process. 

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