Photochemical reaction backward

The decomposition of liquid water and the following reactions are the results of a typical chemical effect. In this case, however, overall water splitting does not occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On the other hand, it is impossible to decompose water by photochemical reaction under illumination with a xenon lamp. Although it is possible to decompose water by photocatalytic reaction using a desirable photocatalyst and photoirradiation, it is difficult to decompose in practice because of rapid backward reaction, the formation and accumulation of intermediates onto the surface of photocatalyst,10) and other reasons. 

A characteristic feature of phytochrome photochemistry is the formation of intermediates between Pr and Pfr- The initial photochemical reaction is followed by a series of dark relaxation processes in both directions. Intermediates in the forward reaction seem to be different from those of the backward reaction. There have been four experimental approaches to these intermediates (1) determination of rapid kinetics after flash irradiation (2) low temperature studies (3) dehydration of phytochrome and (4) investigation of spectral changes after continuous irradiation. 

Many photochemical reactions are superimposed on thermal backward reactions or other thermal reaction steps. Using the same formalism these mechanisms can be handled in a uniform approach. 

The photoinduced initiation reaction may have the disadvantage of poor quantum yields, arising from a fast backward ET which annihilates the ion pair before its cage separation. This means a poorly efficient source of radicals. However, if the photochemical ET is the initiation of a very efficient radical chain process, a poor quantum yield in the reaction may turn to an advantage because small extent of production of the intermediates (Ar, ArX- , ArNu ) will disfavour the proposed termination steps of the mechanism. 

Several dimerization rates of alkyl-substituted silyl radicals were measured earlier [8]. However, the dimerization rates of silyl-substituted silicon-centered radicals have not previously been determined. In this study we have measured, using EPR spectroscopy, the rate constants for the recombination of four silyl radicals (lb, 2b, 3b, and 4b), to produce the corresponding disilane dimers of type a (i.e. la, 2a, 3a, and 4a respectively). This dimerization reaction is shown as the backward reaction of Eq. 2 in Scheme 1. Radicals lb, 2b, and 3b were generated photochemically fiom the corresponding disilane dimers of type a (Scheme 1, Eq. 2), while radical 4b was generated photochemically from the corresponding silylmercury compound 4c (Scheme 1, Eq. 1). 

The second category of compounds—those operating by a photon-photon mode—comprises photochromic systems functioning photochemically in both the forward and backward reactions. Fulgide and diarylethene derivatives are representative examples (Scheme 1). 

There is, however, a snag. ET reactions can go backwards as well as forwards without the symmetry restrictiorrs that act on some molectrlar photochemical rearrangements to cottfer long-term metastabihty on the energy-rich products of the forward reaction. The back ET reaction... 

The number of such reactions is large. The differential equations for the changes of the concentrations with time can be given in general. In the following example a consecutive reaction which contains two photoisomerisation steps (forward, backward) is followed by a consecutive thermal step. This reaction can be chosen to demonstrate the consequence of the definition of partial photochemical quantum yields (see Section 2.1.2.2), the linear dependencies, and the influence of the thermal reaction ... 

Distyrylazobenzenes exhibit a photokinetic equilibrium and a superimposed thermal backward reaction. The absorption coefficient of one of the partners in the equilibrium is principally unknown. Nevertheless, using the dependence of the photostationary state on the intensity of the irradiation, the photochemical quantum yields

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A photochemical with a superimposed backward thermal reaction is the most simple type possible. The following reaction scheme represents the mechanism (see Section 2.1.4.3) ... 

The spectral change due to the UV light irradiation is ascribable to the isothermal phase transition triggered by the photochemical isomerization reaction. The trany-isomer in the metastable solid phase with the head-to-tail chromo-phore orientation is converted to the cfT-isomer by the UV light irradiation. The backward reaction from cis to trans can also partly proceed during the UV light irradiation. When the cast film is irradiated below the solid-to-liquid crystal phase transition temperature, the trans-isomer is regenerated by the backward reaction... 

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