Photochromism 149 Reduction

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

The McMurry reaction (the reductive dimerization of carbonyl compounds giving olefins after treatment with the low-valent titanium reagents TiCla/Mg and TiCla/Zn in THF at 40°C) (CR891513) is a key step in the synthesis of photochromic cyclopentene derivatives. Later, it has been shown that TiCU, which is easier to handle than TiCls, can be used. The cyclization products are formed in 50-60% yields (98S1092,03EJO155). The advantages of this method are that the reactions can be scaled and inexpensive starting materials can be used. 

The l,3-dioxol-2-one ring in these compounds is relatively stable, thereby allowing acetylation, reduction, and the replacement of the keto group by a thione group giving photochromes 243-245 (02IZV1588, 05IZV1299). 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

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). 

Much less influence on photochromic properties is exerted by the substituent Ri, and the relationship between character of the substituent and the position of the absorption band maximum of the colored species is difficult to discover. However, it was shown [120] that strengthening of the electron-withdrawing properties of the substituent Ri led to a reduction of the time for decoloration in the dark. 

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

V. Malatesta, F. Renzi, M. L. Wis., L. Montanari, M. Milosa, and D. Scotti, Reductive degradation of photochromic spiroo-xazines. Reaction of the merocyanine forms with free radicals, J. Org. Chem., 60, 5446-5448 (1995). 

Photochromic anthrapyridones (IIB1, IIB2) were produced by cyclization of 1-chloracetylamino-4-(or -5)-phenoxyanthraquinones with the subsequent reductive elimination of the pyridine group from the products obtained in pyridine (Scheme 2).38 Photochromic chloro derivatives of pyridines (IIC) were synthesized by the interaction of the resultant photochromic anthrapyridines with phosphorous chlor-oxide.38... 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

As can be seen from Table 9.12, photochromism is also strongly affected by humidity. It is well known that thiazine and its derivatives undergo photoreduction smoothly in the presence of such activated surfaces as silica gel and alumina with water molecules,37 and that methylene blue is also photochemically reduced in acid solution, even with only available water as the reducing agent38 Therefore the water present in polymer films must produce a thionine-water hydration system, which accelerates the rate of the photoreduction of thionine, as well as promoting the contact of reductants by a plasticizer effect. PVA, used as the matrix, can also play the role of reductant, but its extent may be minor as compared with the added reductant. The effect of water is supported by the fact that a less hydrophilic polymer matrix such as poly(methyl methacrylate) does not exhibit photochromism even though the system contains an appreciable amount of reductants. 

Table 9.13 summarizes the effect of the kind of reductant on the photochromic behavior of thionine dye. All reductants afforded transparent films upon irradiation and complete color recovery in the dark. The greatest photosensitivity was obtained with TEA as the reductant. The reversibility decreased in the order, tri->di->mono-ethanolamine. The difference cannot be explained by an electron-donating character such as the basicity of the reductant (P A a value of their ammonium salt tri-mono-ethanolamine). A potential explanation is that TEA(bp, 335.4°C/760mmHg) is so hygroscopic that the moisture absorbed in TEA and TEA itself acts as a plasticizer, providing tough films that afford higher color reversibility, as mentioned earlier. 

Blair et al. have also carried out Langmuir film balance measurements on monolayers of the photochromic polyamides mentioned above, namely the 3,3 - and 4,4 -poly-(azodibenzoyl-trans-2,5-monomer unit was observed and interpreted as resulting from azo trans to cis isomerization. The differences between the pressure-area curves of the meta(3,3 -) and para (4,4 -)polyamides were interpreted on the assumption that the meta compound has a helical structure and that the para compound is linear. 

There are two ways to achieve higher density the extension of the data-recording space in the axial direction, and the reduction of bit size. In this chapter, we describe a method for overcoming the density limit, namely, introduce an additional axial dimension in the recording process. " " The z or longitudinal axis is used in addition to the surface dimension (x—y space) of conventional optical memory. The data are thus written not on the material surface but within the three-dimensional (3D) thick volume. The media that can be used are photochromic materials. ... 

As to the photochromic activity, it should be pointed out that the radical ions originating in the reduction processes do not exhibit photochromism in all cases, reduction leads to colored solutions which are not bleached by UV irradiation.57-60... 

A. Alberti, C. Barberis, M. Campredon, G. Gronchi, and M. Guerra, An EPR electrochemical, and ab initio investigation on the nature of the radical ions formed in the reduction of some photochromic compounds ofthe sphoindolinic series,/. Phys. Chem. 99, 15779-15784 (1995). 

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