Photodecomposition, energy stability

The possible utilization of the energy stored In the stabilized photoproducts In further chemical reactions Is discussed. Special attention Is given to the photodecomposition of water as a reaction route. 

Since the early 1990s, ruthenium (II) polypyridine complexes have been used as mediators in the interconversion of light and chemical energy. A quite important property for photosensitizers is their stability toward photodecomposition. For [Ru(bpy)3] cation in aqueous solution, the quantum yield of photodecomposition is from 10- to 10-3, depending on the pH and temperature. In the solution containing anions (CF, Br, and NCS ), this quantum yield can be as high as 0.1 for solvents with a low dielectric constant [386]. A way... 

In so far as the rate of formation of radicals reflects their stability or reactivity the findings of Hart and Wyman are instructive. In carbon tetrachloride the rate of decomposition of benzoyl peroxide was twice as fast as that of biscyclopropanoyl peroxide. Ingold and coworkers have found that in the photodecomposition of benzoyl and biscyclopropanoyl peroxides, in carbon tetrachloride at 298 K, the phenyl radicals produced reacted faster (7.8 x 10 M s ) than the cyclopropyl radicals (1.5 X 10 M s ). These results are consistent with C-H bond dissociation energies for benzene (llOkcalmol) and cyclopropane (106kcal mol ) which implies that the cyclopropyl radical should be less reactive than the phenyl radical. In subsequent work they also showed that at ambient temperatures radical reactivities decreased along the series /c = Ph > (Me)2 C=CH > cyclopropyl > Me. Table 4 records the absolute rate constants for the reaction of these radicals with tri-n-butylgermane. 

Fig. 1 Typical correlation between energy positions of band edges and decomposition potentials, controlling thermodynamic stability against photodecomposition, (a) stable, (b) unstable, (c) unstable against anodic decomposition, (d) unstable against cathodic decomposition (from Ref 39).

Fig. IV.7 shows the crystal structure and the electronic structure of a material like tungsten sulfide or selenide and molybdenum sulfide or selenide. These materials have band gaps in the order of 1 to 1.2 eV and therefore should have similar properties for solar energy conversion as silicon. Their stability against photodecomposition is caused by the fact that the highest valence band and the...

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