Reversible photoreactions

Once a photostationary state (PSS) is reached upon irradiation at the wavelength X the amount of reactant A no longer changes, — dnA/dt = 0, and by inserting Equation 3.19 for d/jpA and d p B we obtain Equation 3.27  

Hence the composition of the photostationary state, nA(oo)/nB(°°), is defined by the ratio of the quantum yields for the forward and backward reactions and that of the absorption coefficients s. For a spectrophotometric determination of that composition, the absorption spectrum of at least one of the compounds A or B must be known. To determine the 

In practice, many largely reversible photoreactions do not remain in the photostationary state under prolonged irradiation due to the occurrence of minor side-reactions. The composition of the PSS should then be validated by an approach from both isomers or, preferably, from an authentic mixture near the PSS. 

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The object of this study is to develop new photoresists for deep-UV lithography, by using the reversible photoreaction of pyrimidine bases (17-19). Applicability of pyrimidine containing polymers to both negative and positive type photoresists is due to this photoreversible reaction in which cyclobutane dimers are either formed or cleaved depending on the exposure wavelength (Scheme 2). 

Novel Ag1 coordination polymers with a diarylethene derivative, cis-dbe, have been synthesized by Munakata et al. [31]. Polymers with a onedimensional infinite chain structure, 33, and another with a two-dimensional sheet structure, 34, were synthesized and their reversible photoreactions with 450 nm and 560 nm light in the solid state were revealed. 

Quantum yields of photoproduction of ana-quinones of acetoxy-substituted anthraquinones with amino substituents in the anthraquinone ring as well as with an acetyl group proved to be the lowest (Table 7.2) As in the case of photochromic alkylanthraquinones, the reverse photoreaction, from ana-quinone to para-quinone for derivatives of acetoxyanthraquinone, proved to be impossible.21 The transition from ana-quinone to para-quinone occurred during freezing out of a sample owing to thermal bleaching. 

The net number density of molecules undergoing a photochro-mic transition depends on the concentration of the photochromic molecules and the fraction that is converted. The maximum achievable conversion (the photostationary state) is determined by the relative absorption cross sections of the two states and the quantum efficiencies for the forward and reverse photoreactions. 

The reverse photoreaction, i.e. the photocyclization of cinnamylmagnesium bromide to the corresponding cyclopropylmagnesium bromide, has also been described ... 

Reversible photoreactions have been reported for the salicylidene aniline derivative 3, which is isomerized at X = 308 nm, as shown in Figure 6.3. This reaction is reversible because of a thermal back reaction. Derivative 3 is highly reactive in LBK films, because only small-volume changes occur, involving a proton transfer and a rotation of the phenyl ring. The thermal back reaction is retarded in the LBK film as is the case with crystals, however, and the isomerized form is much more stable than in solution. 

We now derive several equations for the spectrophotometric quantum yield determination of unidirectional photoreactions A —> B. Reversible photoreactions will be treated in Section 3.9.3. The reader should not be deterred by the complex appearance of some of these equations. They are easy to use and give highly reproducible results, because absorbance measurements are precise. The photoreaction is induced by continuous irradiation with a monochromatic light source that exposes the sample... 

In a reversible photoreaction (Scheme 3.2), such as the photoisomerization of azobenzene (Section 6.4.1), the differential rate for the disappearance of the reactant A is given by Equation 3.26, where dnVyA and drapB are the amounts of light absorbed by A and B, respectively, as defined by Equation 3.19. 

In the case of a reversible photoreaction one can define A to be the trans and B the cis form, and the following reaction scheme applies ... 

Various derivatives of dihydroindolizines with its reversible photoreaction to betaine and the superimposed thermal reaction have been used to test the algorithms. 

As mentioned above, the most informative method to study biochemical reactions would be time-resolved infrared difference spectroscopy. However, because the spectral changes are very small, all techniques require signal averaging over many reaction cycles. This limits application of the techniques to thermally reversible photoreactions. If such systems are in addition stable enough, the photoreaction can be triggered by thousands of flashes. 

As the geomeMcal structural changes are relatively small in the photochromic reactions, most of these reversible photoreactions could also proceed in the crystalline state. Thus, diarylethenes also exhibit crystalline state photochro-2.3 Benzopyran and naphthopyran (chromene) mism. Single crystals of the cocrystals with two or... 

In the reversible photoreactions of most photochromic families, the planarity and the tt-conjugation of the molecules are altered considerably. This is reflected by the dramatic color changes in the photochromic reactions. Thus, they are ideal candidates in the construction of photoswitch-able bridges for electronic communication or conduction between different termini. 

An analogous mechanism must govern also the reverse photoreaction. According to the calculations [58], the photocyclization of l-(3-phenyl) butadiene-1,3 to l,8a-dihydronaphthalene proceeds by the same route. 

Direct application to optical memories based on a spectral change in photochromism seems relatively difficult, since reading light dways induces the reverse photoreaction with an efficiency as long as the light is absorbed by the photoisomer. 

The photochemistry of sahcyhdeneanUmes suffers from competing reversible photoreactions involving transfer of the phenohc proton to the imino nitrogen, along with the formation of the corresponding o-quinoid forms (photochromism) or a zwitterion. ... 

The reversible photoreaction was found to be controlled by the regulation of the UV wavelength. The thymine compound has a UV absorption with a high extinction coefficient at 270 nm. The absorbance... 

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