fotodimerizrile

Fotodimerizrile, or photodimerization, is a chemical reaction in which two molecules combine to form a single dimer upon exposure to light. The process is photochemical, typically initiated by absorption of ultraviolet light, though some substrates can absorb visible light or use photosensitizers to harvest light energy. The reaction can proceed through excited electronic states, with many cases involving a triplet excited state that couples to form a covalent bond, often by a [2+2] cycloaddition between adjacent double bonds to give a cyclobutane-containing dimer. Other pathways involve radical pair formation and subsequent coupling, yielding non-cycloaddition dimers. The exact outcome depends on the substrate, the environment (solution or solid state), and the irradiation wavelength and intensity.

In the solid state, photodimerizrile can exhibit topochemical control, where the relative orientations of the reacting molecules in the crystal determine which dimers form. In solution, molecular diffusion and a lack of strict orientation can reduce selectivity and allow competing photochemical processes such as rearrangements or polymerization. The choice of light wavelength, solvent, concentration, and the use of photosensitizers all influence rate and selectivity.

Common model systems include the photodimerization of anthracene and the [2+2] cycloaddition of simple alkenes; these reactions have been used to illustrate concepts in photochemistry and to develop applications such as UV-curing technologies and photo-patterning. The resulting dimers often have distinctive stereochemical relationships and can serve as synthetic intermediates or as crosslinking points in polymeric materials.

Terminology: in Romanian scientific literature the process is commonly referred to as fotodimerizarea; "fotodimerizrile" may appear as a variant or plural form.