Photochromic isomerization

The strong temperature dependence of the decoloration rate constant must undoubtedly be related to the rigidity of the polymeric matrix below the glass transition temperature which inhibits photochromic isomerization. It is only around and above Tg that the molecular mobility becomes appreciable, gradually increasing and facilitating isomerization. 

From Chap. 2, it must be concluded that the photochromic isomerization reactions and their resulting conformational changes are greatly affected by the polymeric medium in which these reactions take place. It was shown that... 

Y. Morishima, M. Tsuji, M. Kamachi, and K. Hatada, Photochromic isomerization of azobenzene moieties compartmentalized in hydrophobic microdomains in a microphase structure of amphiphilic polyelectrolytes, Macromolecules 25, 4406-4410 (1992). 

Photochromic isomerization of salicylaldehyde 2-quinolylhydrazone 80 involves both amino-imino tautomerism and E-Z isomerizations. When an ethanolic solution of 80a is irradiated in the UV region between 250 and 400 nm at room temperature, it is readily converted into the colored isomer 80b, which is stable in both protic and non-protic solvents at ambient temperature. This conversion presumably occurs via an intramolecular hydrogen transfer from the phenolic group, which precedes the double bond isomerization. In the dark, the reverse isomerizations take place (75JOC2512). No substituent effect on this tautomeric equilibrium has been observed (75JCS(P1)2036). 

Fulgides (e.g., 53, see Scheme 15) are capable of photochromic reactions. Yokoyama et al. reported the preparation and subsequent photochromic isomerization of a fulgide which was designed to resist enantiotopomerization [103]. A sample of the resolved fulgide was irradiated (405 nm light in toluene solution) and was observed to come to a pss with a ratio of 19 81. Irradiation with visible light (X > 580 nm) led to the complete recovery of the initial conformation. An advance on this system was made by the same authors, who described the process of diastereomeric photochromism, in which a fulgide derivatized with a binaphthyl auxiliary was allowed to thermally equilibrate, and a photocyclization process carried out [104]. As a result of the relative populations of photoreactive... 

The mechanism involved in the photochromic isomerization reaction is relatively rare, which makes these substances all the more interesting to study. 

The classes of reactions which produce structure range from complex redox chains [1-4,9], through moderately non-linear reactions of the type A + B C [7,9], to simple first order isomerizations [7,12]. Examples are the photoreduction of Fe(III) [1,6] and the air oxidation of reduced methylene blue [5] (very complex), protonation of methyborange [9] (second or quasi-first order), and photochromic isomerization [7]. See Fig. 4. 

Figure 1. Photoinduced phase transition of a nematic LC by the photochromic isomerization of the doped Az.

There has been increasing attention focused on photochromic isomerization, mainly because it may find an application for a photochemical-switching system based on organic molecules [47,48]. In this connection, the photoisomerization of azobenzene (Abz) moieties in various macromolecular ma-... 

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. 

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