Molecular crystals photochemical change

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]. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

In addition to the identification of crystal moditications, SSNMR has been used to monitor reactivity and phase changes in different polymorphic forms. For instance, Harris and Thomas (1991) followed the photochemical conversion of formyl-fran -cinnamic acid with SSNMR (see also Section 6.4). Variable temperature techniques have been used to study the interconversion of four polymorphic modifications of sulphanilamide (/ -amino-benzenesulphonamide), including interpretation of at least some of the molecular motions during the course of the transformation (Frydman et al. 1990). A similar combination was augmented with colourimetric techniques to study the coexistence of two phases in the course of a phase transition (Schmidt et al. 1999). Of course, differences between unsolvated and solvated or hydrated crystal moditications may also be readily characterized by the SSNMR technique, as was done with the anhydrous and monohydrate of oxyphenbutazone (Stoltz et al. 1991). Due to the availability of the crystal structures for both modifications the SSNMR results could be interpreted directly in terms of the different atomic environments, especially for the differences in hydrogen bonding in the presence... 

The molecular mobility can be further restricted by using appropriate excipients. It is obvious that the unknowing use of a metastable crystal form can lead to solubility and stability problems. A change in crystal modification can occur during processing or as a result of a photochemical process (Briggner et al., 1994 Ahmed et al., 1996 Nord et al., 1997). The data available on photochemical stability of drugs in the solid state that include information on the effect of formulation principle and excipients are at present rather limited. 

The crystallization and X-ray diffraction analysis of the reaction center from the bacterium Rb. sphaeroides R-26 has revealed the three-dimensional structure of the protein and bound cofactors to atomic resolution (Allen et al., 1986 Allen et al., 1987 Yeates et al., 1987 Yeates et al., 1988 Allen et al., 1988 Komiya et al., 1988 Chang et al., 1991 El-Kabbani et al., 1991). With this information available, it is of interest to ask how the spectroscopic properties of the reaction center correlate with the structural features. Ultimately this correlation will allow an elucidation of the molecular details that control the spectral features and relate to the primary photochemical events carried out by the reaction center. Polarized light absorption is one technique for correlating the spectroscopic features with its molecular structure (Breton, 1985). In order to make the correlation more precise, it is distinctly advantageous to carry out the spectroscopic experiments directly on the crystalline samples used in the X-ray diffraction analyses. In this way the clearest link between the structure of the complex and its photochemical properties will emerge. 

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