Special Topic 6.15 Photochromism

Photochromism has many potential and existing applications that take advantage of a change in colour or other physicochemical properties during the process, for example variable-transmission optical materials such as photochromic eyeglass or ophthalmic lenses that darken in sunlight (using spiropyran and spirooxazine systems in addition to 

Case Study 6.28 Supramolecular chemistry - photoresponsive crown ethers 

2 Azo Compounds, Azirines, Diazirines, Diazo Compounds, Diazonium Salts, Azides,. V-Oxides, Nitrite Esters and Heteroaromatic Compounds Photofragmentation and Photorearrangement 

Case Study 6.29 Mechanistic photochemistry - singlet-triplet interconversion of carbenes 

Experimental details.1151 A solution of373 in acetonitrile (2.5 x 10 3 m) containing methanol (0.05 m) and isoprene (0.10 10 m) was purged with nitrogen and irradiated 

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The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

Photochemical switches are also discussed in Special Topic 6.15 (Scheme 6.161) and Special Topic 6.19 (Scheme 6.207). Here we illustrate the photoswitching process, which can control the geometry of biomolecules.1101 When an azobenzene-derived cross-linker in the DNA-recognition helix of the transcriptional activator MyoD is irradiated at 360 nm, the linker predominantly attains a Z-configuration that significantly stabilizes the helix (Scheme 6.193).1209 Reverse isomerization can proceed either thermally or photochemically at a different wavelength therefore, the process is photochromic (Special Topic 6.15). 

The great demand for miniaturization of components in electrotechnical, medicinal or material applications has led to the development of a highly multidisciplinary scientific and technological field called nanotechnology to produce devices with critical dimensions within the range 1 100 nm. The ultimate solution to miniaturization is logically a functional molecular machine, an assembly of components capable of performing mechanical motions (rotation or linear translation) upon external stimulation, such as photoactivation.1103,1104,1239-1244 This motion should be controllable, efficient and occur periodically within an appropriate time-scale therefore, it involves photochromic behaviour discussed in the Special Topic 6.15. Such devices can also be called photochemical switches (Special Topics 6.18 and 6.15). Here we show two examples of molecular machines a molecular rotary motor and a molecular shuttle. 

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