photoswitching

Photoswitching refers to the reversible change in a molecule or material’s properties triggered by light. In a photoswitch, exposure to photons induces a structural or electronic transformation that interconverts at least two states, typically labeled A and B, which possess different optical and physical characteristics. The process allows optical control of properties such as color, absorption spectrum, fluorescence, polarity, or binding behavior.

Many photoswitches belong to well-studied families. Azobenzenes undergo trans–cis isomerization around the N=N bond; the two isomers have distinct shapes and absorption profiles. Spiropyrans convert to merocyanines under ultraviolet light and revert upon visible light or thermal relaxation. Diarylethenes switch through a reversible ring-opening and ring-closure reaction, often offering robust fatigue resistance and thermal stability of the forms.

Key concepts in photoswitching include the photostationary state, which describes the composition of isomers under continuous illumination, and quantum yields for the forward and backward steps. Thermal back reactions can return the system to its initial state in the dark, affecting memory and cycling performance. Wavelength selectivity and fatigue resistance are important for practical uses, and there is ongoing interest in designing switches that operate with visible or near-infrared light for biological compatibility.

Applications span data storage, molecular machines, and smart materials such as light-responsive coatings and textiles. In imaging, photoswitchable fluorophores enable fluorescence switching for advanced microscopy. In biology and nanotechnology, photoswitchable ligands and proteins permit optically controlled signaling, assembly, and actuation.