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A molecular switch is a molecule that can be switched between two or more stable or metastable states with the use of any external (exogenous) or internal (endogenous) stimuli, such as changes in pH, light, temperature, an electric current, a microenvironment, or in the presence of ions, and other ligands. In some cases, a combination of stimuli is required. Molecular switches are reversible. They have been considered for a wide area of possible applications, but the main uses are in photochromic lenses and windows.