Photochemical crystal phase

Photochemical isomerization of nitrones to oxaziridines can occur in the crystal phase (458). 

Image Amplification by Means of Photochemically Triggered Phase Transition in Liquid Crystal... 

In the cases of the Af-salicylidene (46) and N-benzylidene (47) anilines (15a and 15b), there is less ambiguity arising from thermal motion. For both series of compounds it is believed, on spectroscopic grounds, that the molecules are nonplanar in solution. While there are some crystal phases known in which the molecules retain this conformation (the dihedral angle between the anilino and salicylidenimino planes being of the order of 40°), in others the molecules are definitely planar. In the case of 15a it has been found that there are marked differences in densities and in photochemical and spectroscopic properties between crystals of the two types. 

The physical and chemical properties of photoisomerizable molecular films or photoisomerizable polymers are controlled by light. Photochemical control of the formation of liquid crystal phases, or sol-gel transitions,137 381 of polymers containing photoisomerizable components demonstrates signal regulation of the structure and properties of microscopic and macroscopic phases. Physicochemical properties of photoisomerizable membrane-mimetic assemblies such as liposomes,1241 mono-... 

Structure of polymer liquid crystals involved in photochemically induced phase transition... 

S. Tazuke, S. Kurihara, and T. Ikeda, Amplified image recording in liquid crystal media by means of photochemically triggered phase transition, Chem. Lett. 1987, 911-914. 

The direct or sensitized irradiation (A > 330 nm) of the enone (157) in solution affords the [2 + 2] cycloadduct (158). When the irradiation is carried out using the same wavelength but with the compound in the crystalline phase the main product (75%) obtained in (159) formed by a hydrogen-abstraction path. Some of the [2 + 2]adduct (158, 25%) is also obtained. The change in the reaction from solution to crystal phase is the result of (157) being the preferred conformation in the crystal, a prediction which was verified by an X-ray structure. Scheffer has reviewed the influence of crystal lattice control on the outcome of such photochemical reactions. 

Tsutsumi, O., Kitsunai, T., Kanazawa, A., Shiono, T., and Ikeda, T. Photochemic.il phase transition behavior of polymer azobenzene liquid crystals with electron-donating and -accepting substituents at the 4,4 -positions. Macromolecules 31, 355 (1998). 

This group has a characteristically flat disk shape, which is responsible for the discotic liquid crystal phase. The nickel bis (dithiolene) core has high thermal and photochemical stability. The substituted R groups impart the solubility in nonpolar solvents. Also, if the R groups are sufficiently long, they can form a nematic liquid crystal, as shown in Figure 5 and 6. 

The spectral change due to the UV light irradiation is ascribable to the isothermal phase transition triggered by the photochemical isomerization reaction. The trany-isomer in the metastable solid phase with the head-to-tail chromo-phore orientation is converted to the cfT-isomer by the UV light irradiation. The backward reaction from cis to trans can also partly proceed during the UV light irradiation. When the cast film is irradiated below the solid-to-liquid crystal phase transition temperature, the trans-isomer is regenerated by the backward reaction... 

Other techniques for decompng nitric acid are photochemical flash photolysis. The photochemical decompn of nitric acid is not solely a gas-phase reaction X-rays have caused the evolution of 02 from nitric acid crystals. The use of flash photolysis has shown the nitrate radical to be an intermediate in the decompn (Ref 30)... 

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]. 

Liquid crystals are classified into lyotropic and thermotropic crystals depending on the way in which the mesomorphic phase is generated. Lyotropic liquid-crystalline solvents are formed by addition of controlled amounts of polar solvents to certain amphiphilic compounds. Thermotropic liquid-crystalline solvents, simply obtained by temperature variations, can be further classified into nematic, smectic, and cholesteric solvents depending on the type of molecular order present. Liquid crystals are usually excellent solvents for other organic compounds. Nonmesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix (Michl and Thulstrup, 1986). Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions (Nakano and Hirata, 1982). 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

These observations on the effects of the environment, especially the case with more than one crystallographically independent molecule, may serve as a useful link between the solution and solid-state photochemical behavior [91]. It would be interesting to encounter cases of photoreactive crystals containing more than two crystallographically independent molecules, which would be somewhat akin to, but by no means the same as the solution-phase reaction. 

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