Photochromic isomerization Photochromism

Photochemistry. The most important photochemical processes that proceed from the excited state are geometrical isomerization and photochromic reactions. 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

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. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

It has been suggested that the elusive zwitterionic state [75], or a novel nucleophilic addition/elimination mechanism at the central carbon of the exocyclic bridge [79], or solvent-solute H-bonding interactions [76, 80] might play a role in modulating cis-trans interconversion. Cis-trans isomerization gives rise also to a remarkable intrinsic photochromism of HBI, as it can be easily and reversibly induced upon light absorption [74—76, 79, 80]. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Figure 3.34. Photochromism of azo dyes under irradiation with polarized light. In an amorphous matrix, trans-cis isomerizations are coupled to rotational diffusion. After many isomerization cycles, the molecules are trapped in an orientation with the transition moment M perpendicular to the polarization direction of the light P.

Volume 103 of our series contains three substantial chapters. M.M. Krayushkin and M.A. Kalik (affiliated with Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow) summarize synthetic work directed to photochromic dihetarylethenes. Work has been especially concentrated on dithienylethenes that have considerable potential derived from photochemically induced isomerizations between the cis-and trans-forms and cyclized derivatives. 

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

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 ... 

The valence isomerization of the 2H-pyran system is the basis for the commercially valuable photochromic properties exhibited by the benzologs. 

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. 

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). 

We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23]... 

In solution the photochromism of coordination compounds can result from photoinduced isomerization (e.g. metal dithizonates186-188), substitution (e.g. iron(II) phthalocyanine in the presence of benzyl isocyanide189) and oxidation—reduction (e.g. tris(diethyldithiocarbamato)-nickel(IV) bromide190). 

Photoinduced unimolecular reactions often have kinetics of the order of ps. One example of isomerization in ps times is shown in Figure 8.9. This is the photochromic reaction of a spiropyran. The photoinduced process takes place... 

Photochemistry. The most important photochemical processes that proceed from the excited state arc geometrical isomerization and photochromic reactions. Pholoisomerization of polymethines is a reversible Irans-cis transfer. The cis-isomer absorbs at longer wavelength with a smaller intensity than the trans-isnmer. 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

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