Final answer:
Molecular switches are molecules that can switch between different functional states. An example is a photoresponsive chemical switch based on a diarylethylene moiety in pyrimidine nucleosides.
Step-by-step explanation:
Molecular switches are molecules that can be toggled between two or more stable states under the influence of external stimuli, such as light, temperature, or pH. They can undergo reversible changes in their physical or chemical properties, allowing them to switch between different functional states. One example of a molecular switch is a photoresponsive chemical switch based on a diarylethylene moiety in pyrimidine nucleosides, which undergo reversible photochemical electrocyclic cyclization under UV irradiation and revert to the open form under visible light.