Crystal-phase photochemical reactions

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. 

Several solid-state photochemical reactions have been investigated with polycrystalline samples suspended in solvents. Solvents such as water, where the reactant and the product are likely to be insoluble, are usually chosen and a surfactant is added to maintain the suspension. There are at least two apparent advantages to this method. First of all, photochemical equipment commonly used for fluid samples can be readily adopted to solid-state reactions. Secondly, it is expected that all microcrystals in a powdered sample will be homogeneously exposed to the incident light in a well-stirred reactor. Interestingly, while several examples of solid-to-solid reactions in suspended crystals have been documented, there are some cases where the solvent is incorporated into the phase of the final product. In a report by Nakanishi et al. [134] it was shown that p-formyl cinnamic acid (51, Scheme 33) forms mirror-symmetric dimers. While irradiation of crystals suspended in hexane gave amorphous cyclobutanes in 85% yield, suspension of the crystals in water gave a 100% yield of a crystalline photodimer with one water molecule of crystallization. 

S. Kurihara, T. Ikeda, T. Sasaki, H.-B. Kim, and S. Tazuke, Tune-resolved observation of isothermal phase transition of liquid crystals triggered by photochemical reaction of dopant, J. Chem. Soc., Chem. Commun. 1990, 1751-1752. 

The molecular interpretation is again based on the crystal structure of the starting material (Figure 22). The molecules 13 are almost perpendicular on (100) (front), perpendicular on (010) (top), and flat on (001) (left). There is enough room on both sides to partially rotate for photodimerization in the orientations found and to move upward [010], This does form volcanoes with pillars and leaves craters (Figure 216). Those features must consist of mixed crystals out of 13 with 14, 15, and 16 admixed. From there mixed phases rich in products will be formed upon continuation of the irradiation forming the features in Figure 21c. However, the details of the secondary processes cannot yet be assessed. Local spectroscopy measurements (Sections IX and X) will have to look at the distribution of chemical compositions and nanometric Laue facilities (which are yet to be developed) at the local crystallinity. However, for the initial photochemical reaction it has... 

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, Demachi Y, Kanazawa A, Shiono T, Ikeda T, Nagase Y. 1998a. Photochemical phase transition behavior of polymer liquid crystals induced by photochemical reaction of azobenzenes with strong donor acceptor pairs. J Phys Chem B 102 2869 2874. 

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... 

The shape of rran -azobenzene is rod-like and tends to stabilize the phase structure of liquid crystals (LCs), whereas cw-azobenzene is bent and tends to destabilize the phase structure of LCs. This property can be used to control the phase of LCs photochemically [20-29], For instance, when a mixture of azobenzene and nematic LC in which the concentration of the azobenzene ranges from 0.1 to 5 mol% is irradiated with UV light (—350 nm) to bring about the /rans-cw-photoisomerization of the azobenzene moiety, the nematic (N) to isotropic (I) phase transition temperature (T i) of the mixture is lowered on accumulation of the cis-form, because this form acts as an impurity to the system. When T , falls below the irradiation temperature, N to I phase transition occurs isothermally. This process is reversible and, when the isotropic mixture is irradiated with visible light (—450 nm) to cause the cis-trans back isomerization, T i is again raised and isothermal I to N phase transition is observed. The irradiation temperature is crucial to induce the photochemical phase transition it must be set between T , of the /ran5-azobenzene-nematic host mixture [ T i)trans] and the T , of cis-azobenzene-nematic host mixture [(r i)cw]. When the irradiation temperature is lower than T,)cis, no phase transition can be induced by the photochemical reaction of the azobenzene guest molecule. The photochemical... 

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)... 

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