Back-isomerization

An example the trans- cis photochemical and cis- trans thermal back isomerization of BMPC... 

Table 2. Prccxponcnlial factors (A) and activation energies (E,) for the ground state cis-trans back isomerization of BMPC in solvents with different dielectric constants (e).

Back Isomerization of N-Acetvl-2-methoxvDvrrolIdine. 2. Six mL of the product of reaction In CH2CI2 above was diluted with 2 mL of MeOH and catalyzed with 0.13 g of XN-1010. After 1 h at room temperature 2 decreased from 90.8 to 77.2X and 1 (acetamidobutyraldehyde dimethyl acetal) Increased from less than IX to 4.9X. The other major product was 9 (14.2X). 

Frequency of the crystal after addition of the DNP-Ab to the dinitrospiropyran (31a)-functionalized crystal. (O) Frequency of the crystal after photoisomerization of the (31a)/DNP-Ab monolayer to the protonated dinitromerocyanine (31b), washing offof the DNP-Ab, and back isomerization to (31a) monolayer state. 

Because of the competition between the pH-dependent rate of the reaction leading from the uncolored Cc form to the colored AH+ and A species and the pH-indepen-dent cis- trans back-isomerization, the amount of colored species formed upon light excitation of the Ct solution depends on pH. In other words, the pH plays the role of a tap for the color intensity generated by light excitation. 41 This also means that this system can be viewed as a light-switchable pH indicator. The color-tap effect is larger in the case of 4, 7-dihydroxyflavylium than in that of 7-hydroxyflavylium. 

A polymer of I-a,y-diaminobutanoic acid almost quantitatively substituted with azobenzene units in the side chains [Scheme 5, VIII(n= 2)] was not completely soluble in HFP when the sample was kept in the dark. The initial, slightly turbid solution became clear on irradiation at 360 nm and the consequent photoconversion of the azo moieties from their trans to the cis configuration (for photosolubility effects see Section 13.2.3). The cis polymer was found to adopt an essentially random coil conformation. Exposure to 460 nm light and the consequent back-isomerization of the azo units to about 70/30 trans-cis isomeric composition gave rise to a reversible photoinduced change from random coil to a-helical structure (helix content, about 60%).1411... 

Photochemical E —> Z isomerization produces a sterically strained state in which the methyl substituents are positioned in close proximity. The strain is released when the methyl groups and the naphthyl moieties switch their positions. This second step is followed by back-isomerization from the Z to the E form of the double bond and again steric hindrance provides an energetically unfavorable state which can reach an energetically more favorable state by a second flip of the sub-... 

Also, a fraction of the photon energy, originally stored in the primary (batho) intermediate, may still be available for driving the back-isomerization process. 

A holographic method has been used to investigate the cis-trans isomerization which occurs in the dye methyl red dissolved in poly(methyl methacrylate) and polystyrene SO Back isomerization... 

At room temperature, the isomerization of e.g. 4-dimethylamino-4 -nitroazobenzene cannot be observed reasonably, because the thermal back isomerization of the Z-form is too fast. To determine the spectrum of the Z-form, Gabor and Fischer applied low-temperature and extrapolation techniques. The photoisomerization quantum yields are (l)g, z 0.20 and (t>z- E 0.75 at 163 K. They decrease to (j)E->z 0-06 to 0.09 at 143 K, whereas (1)z >e does not change much. Newer data regarding quantum yields are not available. 

LBK films of 7, measured by electrochemical methods, indicated activation energies similar to those measured for solutions. This was interpreted to be the result of the sterically demanding ds-isomer having a rather unfavorable state." " Furthermore, the kinetics may be used as an indication that, in the LBK film, the back isomerization proceeds via the inversion mechanism (see Chapter 1). The thermal back reaction of salicylidene anilines 3 in LBK films was also found to deviate significantly from a first-order kinetics. ... 

FlG. 12.14 Example of biexponentiai fits (solid lines) to EFISH recovery (markers) during the ds-trans thermal back-isomerization of the PI-1 (left) and PI-2 (right) polymers. The insets show the time constants of these biexponentials. After Ref. 3. 

Apart from these shortcomings, there are several other pitfalls to look out for when searching for molecular switching units. The chemical instability of the ris-isomer of stilbene, for instance, makes its trans-cis isomerization of little practical use, and many switching molecules suffer from incomplete isomerization or thermal back-isomerization to their more thermodynamically stable state. 

In many cases, one of the states of a photoisomerizable system is less thermodynamically stable, and a slow thermal isomerization back to the more stable isomer is observed, causing problems for the construction of long-term memory devices. Back-isomerization can also occur as a consequence of the read function or ambient environmental conditions. For these reasons, it can... 

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