A
molecular switch is a
molecule that can be
reversibly shifted between two or more stable states. 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.