Reverse photochromism

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

Functionalised conjugated polymers such as polythiophenes were studied from the point of view of the detection and transduction of chemical and physical information into an optical or electrical signal. Their ionochromism (reversible change of colour in the presence of ions), photochromism (reversible change of colour on exposure to light), affinity chromism (tendency to colour change) and electroluminescence of polythiophene complexes with crown ethers and other solutions are discussed in detail [295]. 

New Photochemical Reactions Leading to Spatial Dissipative Structures. Gimenez, M. Micheau, J.C. (Lab Chim.-Phys. II, Univ. Libre Bruxelles, B-1050 Brussels, Belg.). Naturwissenschaften 1983, 70 (2), 90-2 (Eng.). Spatial dissipative structures were formed during visible or UV light irradn. of shallow layer solns. of photochromic (reversible) or chromogenic (irreversible) compds. The origin of the structures was independent of the mechanisms involved in the chem. reaction. 

The application of nonlinear optical recording techniques for reversible optical data storage based on the excitation of photochromic molecules by two-photon processes also has been described (154). 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

The reverse reaction, the photochemical ring opening of sphopyranes (22b), takes place by absorption ia the short-wave uv region of the spectmm and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is ia the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colorless to a colored solution (87,88). These photochromic reactions can be used for technical appHcations (89). 

The reverse reaction, B returning to A, can be driven either by thermal or photochemical energy, or both. When the reversion is photochemicaHy driven, the process is called optical bleaching. Optical bleaching is a general characteristic and is a factor in almost all photochromic systems, even those normally thought of as being thermally reversible. 

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). 

Photochromism Based on Dissociation Processes. Both heterolytic and homolytic dissociation processes can result in the generation of a photochromic system. An example of an heterolytic process is the reversible formation of triphenylmethyl cation, by photolysis of... 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

The use of an electron-accepting counter ion leads to a photochromic system that is highly reversible under an inert atmosphere. An anion that has been used successfully is tetra-bis[3,5-di(trifluoromethyl)phenyl]borate anion [79250-20-9], C22H22BF 24 ( )-... 

Photochromism Based on Electrocyclic Reactions. The most common general class of photochromic systems involves reversible electrocychc reactions. Within this general class, the most weU-studied compounds are the indolino spiropyrans and indolino spiroxa2ines. 

Nitro-substituted indolino spiroben2opyrans or indolino spironaphthopyrans are photochromic when dissolved in organic solvents or polymer matrices (27). Absorption of uv radiation results in the colorless spiro compound [1498-88-0], C22H2gN202, being transformed into the colored, ring-opened species. This colored species is often called a photomerocyanine because of its stmctural similarity to the merocyanine dyes (see Cyanine dyes). Removal of the ultraviolet light source results in thermal reversion to the spiro compound. 

Another class of photochromic compounds, which operate through an electrocychc mechanism, is the fulgides although, with this class, the colored species is formed through a ring formation rather than ring-opening (29,30). The reversion to the colorless species [59000-86-1], does not occur... 

By changing the substituents on the ethylenic linkage and exchanging phenyl rings for heteroaromatic rings, photochromic systems that are thermally reversible are transformed into systems that are thermally irreversible but photochemically reversible. The transition between the ben2othiophene-derivative... 

Compound A is photochromic that is, it becomes colored on exposure to light. The process is reversible, giving back the starting material in the dark. Suggest a structure for the colored photoisomer. 

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]. 

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 present photoelectrochemical deposition/dissolution method is applicable to reversible control of the particle size. A typical application taking advantage of the method is the multicolor photochromism. Additional applications include surface patterning and photoelectrochemical actuator. The patterning is possible by using a thiol-modified silver... 

Silver nanoparticles can be deposited on Ti02 by UV-irradiation. Deposition of polydisperse silver particles is a key to multicolor photochromism. The nanoparticles with different size have different resonant wavelength. Upon irradiation with a monochromatic visible light, only the resonant particle is excited and photoelectrochemically dissolved, giving rise to a decrease in the extinction at around the excitation wavelength. This spectral change is the essence of the multicolor photochromism. The present photoelectrochemical deposition/dissolution processes can be applied to reversible control of the particle size. 

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

Instead of metal chelation, an intramolecular hydrogen bonding between the oxygen atom of phenolate and a hydrogen atom of a carboxylic acid in the 8-position leads to stabilization of the colored form, such as compound 12.20,21 This spiropyran exhibits reversed photochromism, which means that thermally stable species change from the spiro form to the colored form, and thus the colorless form produced by photoirradiation soon converts to thermally stable colored form. 

In the presence of acid, unsubstituted BIPS assumes a thermally stable protonated colored form, and shows reversed photochromism, in which the phenolate form changes to a phenol.20 22 In this case, the absorption band remarkably shifts to the short wavelength. For example, in the presence of acid the absorption band of 6-nitro-BIPS occurs at 405 nm in acetone. 

Solid films of spiropyrans are important in optical data storage. Thin films of spirobenzopyran (1.0 (tm) have been prepared by vacuum deposition, and its reversible photochromism has been confirmed.39 The J-aggre-... 

The compound exists normally as the trans or ( )-isomer 21a. This molecule is essentially planar both in the solid state and in solution, although in the gas phase there is evidence that it deviates from planarity. When irradiated with UY light, the ( )-isomer undergoes conversion substantially into the cis or (Z)-isomer 21b which may be isolated as a pure compound. In darkness, the (Z)-isomer reverts thermally to the (F)-isomer which is thermodynamically more stable because of reduced steric congestion. Some early disperse dyes, which were relatively simple azobenzene derivatives introduced commercially initially for application to cellulose acetate fibres, were found to be prone to photochromism (formerly referred to as phototropy), a reversible light-induced colour change. C. I. Disperse Red 1 (22) is an example of a dye which has been observed, under certain circumstances, to give rise to this phenomenon. 

Scheme 10.4 Reversible photochromism of spirooxazine 247 involving interconversion with...

Compounds such as o-nitrotoluene which undergo photochemical reactions that are rapidly thermally reversed are called photochromic or phototropic. Nitro compounds comprise a very important group of compounds which exhibit this behavior. Further examples are as follows(112,113) ... 

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