Tautomerization photochromism

Valence shell electron pair repulsion theory, 1, 564 Valence tautomerism photochromic processes and, 1, 387 y-Valerolactone, o -allyl-a -2-(pyrido[2,3-6]-imidazolyl)-synthesis, 5, 637 Validamycin A as fungicide, 1, 194 Valinomycin... 

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

C. Tautomerism and Photochromism by Homolytic Cleavage IN Triarylimidazole Dimers... 

Photochromism in solution and in the solid state can be the result oi intramolecular changes, e.g. tautomerism, ring opening, cis-trans isomerization, free radical formation, stereoisomenc transition, formation of dimers nd such similar reversible reactions. Some examples are given below ... 

Compounds which exhibit photochromism are classified according to the mode of their photochromic reaction, because changes in colour can arise as a result of heterolytic cleavage, homolytic cleavage, isomerization, tautomerization and oxidation. 

Photochromic tautomerism refers to a photochemically induced shift in the equilibrium between tautomers which have significantly different absorption spectra. 

Benzylchromones (141) and 2-benzyl-Af-methylquinolones (142) are photochromic. The role of the benzyl substituent in the tautomeric equilibrium is made apparent in Scheme 15. 

Valence tautomerism is responsible for the photochromic transformation exhibited by the xanthenone (143). This phenomenon is a type of dynamic isomerism in which the primary change is a shift in the position of the valence bonds as illustrated in Scheme 16 (68JOC3469). 

Fulgides such as the diphenyl derivative (144), dithienylethylenes (145), certain 2H-pyrans (146) and 2H-chromenes (147) and their respective thio analogues (148) and (149) also exhibit photochromism by valence tautomerism. 

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

The well-known photochromic tautomerism of 2-(2, 4 -dinitrobenzyl) pyridine (CH, Scheme 5) was investigated by flash photolysis in aqueous... 

Photochromism and thermochromism derived from proton tautomerism. 244... 

Valence tautomerism refers to the interconversion of isomers without any accompanying rearrangement including proton transfer. Heterocyclic examples are essentially electrocyclic reactions (Scheme 15). If one of the isomers is colored and the position of the equilibrium can be changed by heat or light, the heterocycles can have useful thermochromic or photochromic applications. 

Pyran 200 (R1 = R2 = H) exists only as its acyclic valence tautomer but 5-acetyl-2,2,6-trimethylpyran 200 (R Me, R2 = Ac) is in equilibrium with only -35% of the valence tautomer 201 (R =Me, R2 = Ac) at 30 °C. The equilibrium of the bicyclic derivatives 202 with the ketone isomers 203 is influenced by solvent and temperature. In tetrachloroethene only 9% of the open-chain form is present at 30 °C but this rises to 40% at 113 °C <1981T1571>. Valence tautomerism of 2/7-pyran derivatives is the basis of some commercial photochromic materials. Similar properties are shown by 2/7-thiin derivatives such as the photochromic compound 204, which forms the thione 205 upon irradiation. 

Chapter 8 (Perimidinespirocyclohexadienones). This series constitutes a rather new photochromic or thermochromic system based on proton transfer involving valence and prototropic tautomeric intramolecular reactions (Scheme 8). 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

Proton tautomerism in the solid state has drawn attention from the aspect of application as well. Further investigations in related areas, such as photochromism, thermochromism, photochemical hole-burning and hydrogen-bonded dielectrics, may open a horizon of protonic molecular devices. 

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