photoswitches

Photoswitches are molecules or systems that reversibly change structure and properties when exposed to light. Irradiation with specific wavelengths drives an interconversion between two (or more) isomers, typically with different shapes, dipole moments, absorption spectra, and reactivities. In well-designed switches one form is metastable or thermally stable over a usable timescale, enabling noninvasive optical control of materials, devices, or biological processes.

Common families include azobenzenes, spiropyrans/spirooxazines, diarylethenes, and fulgides. Azobenzenes switch between a planar trans form and a bent cis form; UV light usually induces trans-to-cis isomerization, with visible light or thermal relaxation returning to trans. Spiropyrans convert to open merocyanine forms with altered absorption. Diarylethenes toggle between open and closed ring structures and are noted for high fatigue resistance. Fulgides operate via reversible ring-opening and closing, often with strong thermal stability.

Important design considerations involve the wavelengths required for switching, the quantum yields, the thermal stability of the isomers, and fatigue resistance under repeated cycling. For biological applications, red- or near-infrared activation is advantageous due to deeper tissue penetration and reduced damage. Researchers also tailor solubility, two-state contrast, and compatibility with polymers, surfaces, or biological environments.

Applications span photopharmacology, data storage, molecular electronics, and stimuli-responsive materials. Ongoing work aims to access visible-light and near-infrared switching, improve fatigue resistance, and create multi-state or orthogonal switches for complex control in devices and living systems.