photodimer

A photodimer is a dimer formed when two identical molecules or identical chromophores within a molecule undergo a photochemical reaction upon absorption of light. Photodimerization is a type of photochemical process in which excited-state species couple covalently to create a dimer or a cyclic product. The reaction can be intermolecular, involving two separate molecules, or intramolecular, joining two chromophores within a single molecule.

The most common photodimerization modes are cycloadditions. In many systems, two units bond via a [2+2] cycloaddition to form a cyclobutane ring, a mechanism well known in the formation of cyclobutane pyrimidine dimers in DNA following ultraviolet exposure. Other photodimerizations include higher-order processes such as [4+4] cycloadditions, observed in certain aromatic compounds like anthracene, which can form a dianthracene photodimer in the solid state. The exact pathway depends on the photoreactive moieties, orbital symmetries, and the surrounding environment.

Biological and materials contexts illustrate the range of photodimers. In biology, cyclobutane pyrimidine dimers are a major form of UV-induced DNA damage that can block replication and transcription and are repaired by nucleotide excision repair pathways. In chemistry and materials science, photodimerization is exploited for crosslinking polymers, photoresponsive materials, and molecular switches. Some photodimers exhibit reversibility under different wavelengths of light, enabling reversible locking and unlocking of molecular systems.

Detection and study of photodimers typically involve spectroscopy, crystallography, and quantum yield measurements to characterize the efficiency, mechanism, and structural outcomes of the photodimerization process.