Fotokemiline

Fotokemiline, or photochemical in English, describes chemical processes that are initiated by light. A photochemical reaction begins when a molecule absorbs a photon, promoting it from the ground state to an excited electronic state. The subsequent transformation may proceed via different pathways, often enabling bond making or breaking that are symmetry- or energy-forbidden in the ground state. Compared with purely thermal reactions, fotokemiline processes rely on light energy as a driving force.

Key features include the formation of singlet and triplet excited states, possible intersystem crossing, and the involvement of reactive intermediates such as radicals or charge‑transfer states. Mechanisms include bond cleavage, rearrangement, cycloadditions, and energy transfer to other molecules (photosensitization). The outcomes depend on the light wavelength (actinic region), intensity, and the chemical environment, yielding quantum yields that quantify efficiency.

Examples of photochemical reactions are photodimerization of alkenes, cis‑trans isomerization, photooxidation, and deprotections used in organic synthesis. In biology, ultraviolet light can induce DNA photodamage such as thymine dimers. Photosensitized reactions use a sensitizer to extend reactivity to wavelengths that the substrate alone would not absorb.

Fotokemiline has various applications. Photopolymerization enables fast curing of coatings and 3D printing resins. Photoresists in semiconductor lithography rely on light‑driven transformations. Photochemical methods are used in organic synthesis and analytical chemistry, and they underpin solar energy research and artificial photosynthesis.

History and scope: The study of light‑driven chemistry developed in the 19th and 20th centuries with foundational experiments on photolysis and photochemistry. The field integrates photophysics, spectroscopy, and reaction kinetics and continues to evolve with advances in light sources, including LEDs and ultrafast lasers, and in computational modeling.