Special Topic 6.19 Molecular machines

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. 

Case Study 6.31 Biochemistry - photocatalytic oxidation of DNA guanine 

Explain the following concepts and keywords photochromism photoswitching photoinitiator photoaffinity labelling photo-Wolff rearrangement photoactivatable compound Barton reaction photochemical trigger photolabile linker photoamination redox photosensitization. 

Entry Starting material3 Product(s) Mechanism Section 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

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