GrotthussDraper

The Grotthuss–Draper law, also known as the Grotthuss–Draper principle, is a foundational concept in photochemistry. It states that only light absorbed by a system can bring about a photochemical change; light that is not absorbed cannot drive a photochemical reaction. Nonabsorbed light may still produce physical effects such as heating. Consequently, the efficiency of a photochemical process depends on the portion of incident light that is actually absorbed by the reacting species.

Origin and scope: The idea traces to Theodor Grotthuss in the early 19th century and was later formulated and popularized in the photochemistry context by Henry Draper. The law is often described as the first principle of photochemistry, summarizing the notion that photochemical change requires absorption of light.

Implications and applications: In practice, the law guides experimental design by encouraging the selection of wavelengths that the reactant absorbs to maximize photochemical output. It also highlights that absorption by other species in the system, such as sensitizers, can influence results, sometimes without the substrate itself absorbing light. The law emphasizes that light that is not absorbed cannot initiate chemical changes, though it may cause heating.

Limitations: The Grotthuss–Draper law is qualitative and does not specify quantum yields. It does not fully account for complex mechanisms such as sensitized photochemistry, energy transfer between species, or multiple-step absorption processes, where photochemical outcomes depend on detailed pathways beyond simple absorption by the target reactant.