A molecular switch is a molecule that can be reversibly shifted between two or more stable states.〔''Molecular Machines & Motors'' (Structure and Bonding) J.-P. Sauvage Ed. ISBN 3-540-41382-0〕 The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electric current, microenvironment, or in the presence of a ligand. In some cases, a combination of stimuli is required. The oldest forms of synthetic molecular switches are pH indicators, which display distinct colors as a function of pH. Currently synthetic molecular switches are of interest in the field of nanotechnology for application in molecular computers. Molecular switches are also important to in biology because many biological functions are based on it, for instance allosteric regulation and vision. They are also one of the simplest examples of molecular machines.
== Photochromic molecular switches ==
A widely studied class are photochromic compounds which are able to switch between electronic configurations when irradiated by light of a specific wavelength. Each state has a specific absorption maximum which can then be read out by UV-VIS spectroscopy. Members of this class include azobenzenes, diarylethenes, dithienylethenes, fulgides, stilbenes, spiropyrans and phenoxynaphthacene quinones.
Chiroptical molecular switches are a specific subgroup with photochemical switching taking place between an enantiomeric pairs. In these compounds the readout is by circular dichroism rather than by ordinary spectroscopy. 〔''Circular Dichroism of Dynamic Systems: Switching Molecular and Supramolecular Chirality'' Angela Mammana, Gregory T. Carroll, and Ben L. Feringa; Comprehensive Chiroptical Spectroscopy, Applications in Stereochemical Analysis of Synthetic Compounds, Natural Products, and Biomolecules; John Wiley and Sons; 17 February, 2012) 〕 Hindered alkenes such as the one depicted below change their helicity (see: planar chirality) as response to irradiation with right or left-handed circularly polarized light
Chiroptical molecular switches that show directional motion are considered synthetic molecular motors:〔''Chiroptical Molecular Switches'' Ben L. Feringa, Richard A. van Delden, Nagatoshi Koumura, and Edzard M. Geertsema Chem. Rev.; 2000; 100(5) pp 1789 - 1816; (Review) 〕
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