photoactive

Photoactive describes materials or molecules that undergo a change when they absorb light. The changes can be chemical, such as a redox reaction; physical, such as a change in electronic state; or structural, such as isomerization. Photoactivity is central to processes in chemistry, biology, and materials science.

Upon absorption of photons, electrons are promoted to excited states, which may transfer energy, participate in charge separation, or initiate chemical reactions. In semiconductors, light can generate electron-hole pairs that drive photocatalytic reactions or generate electricity in solar cells. In photoresponsive molecules, light can trigger bond rearrangements or shape changes.

Applications include photocatalysis for environmental cleanup, solar energy conversion, and photodynamic therapy in medicine. Photoactive polymers and organic molecules are used to create data storage, sensors, and light-driven actuators.

Common examples include titanium dioxide and zinc oxide as inorganic photocatalysts; perovskites and organic photovoltaics as light-harvesting materials; and azobenzene- or spiropyran-based compounds as reversible photoactive components.

In biology, photoactivity is exploited in vision and photosynthesis, where molecules such as rhodopsin and chlorophyll respond to light to trigger signaling or electron transfer.

Limitations include dependence on wavelength, short carrier lifetimes, thermal or photochemical degradation, and, for some materials, toxicity or environmental impact. Advances focus on increasing light absorption into the visible range, improving stability, and reducing recombination losses.