photoisomerizes

Photoisomerization is a photochemical process in which a molecule changes its geometric or stereochemical arrangement upon absorption of light. The result is one or more isomeric forms that can be interconverted by light. The process is central to many light-responsive systems and can be reversible, with different wavelengths promoting forward and backward isomerization, or irreversible in some cases.

The mechanism typically involves excitation to a higher electronic state, followed by structural rearrangement such as rotation around a bond (cis–trans isomerization) or inversion at a stereocenter. Many reactions proceed via a conical intersection that connects the excited state to alternative ground-state geometries. The efficiency varies widely and is described by a quantum yield. Common photoisomerizable chromophores include azobenzene (trans–cis switching), stilbene, spiropyrans, and diarylethenes; in biological systems, retinal photoisomerization (11-cis to all-trans) initiates vision.

Applications of photoisomerizable molecules include serving as molecular switches and actuators in photoresponsive materials, data storage, optical sensing, and biology. Key practical considerations include fatigue resistance (how well the system withstands many switching cycles), thermal back reaction (spontaneous reversion in the dark), and photostability. The wavelength of light required for isomerization depends on the chromophore and environment; in some cases, two-color operation enables separate forward and backward control. Overall, photoisomerization describes a broad class of light-driven conformational changes exploited across chemistry, materials science, and biology.