Photocyclization

Photocyclization refers to a class of photochemical reactions in which a molecule undergoes ring closure upon absorption of light. In these processes, excitation of a substrate by UV or visible light triggers intramolecular cyclization to form a new ring, or an intermolecular process connects two parts of a molecule to form a cycle. Intramolecular photocyclizations are common, often proceeding via excited singlet or triplet states that promote bond formation between suitably placed π-systems or functional groups. The reactions can follow pericyclic stereochemical rules: photochemical electrocyclizations invert the symmetry rules relative to thermal ones, so 4n-electron systems may cyclize in a fashion disallowed thermally.

Applications include the construction of polycyclic aromatics and heterocycles from relatively simple precursors, and the preparation of photochromic materials. A notable class involves diarylethenes, which undergo a reversible photocyclization to a closed-ring isomer under UV light, with ring opening and color change upon exposure to visible light or heat, enabling molecular switching. Other substrates include enones and enediyne-like systems that cyclize to indanes, chromanes, or fused rings upon irradiation.

Challenges include competing photoisomerization, low quantum yields, and the need for UV light. To address these limitations, sensitization and photocatalysis allow visible-light driven photocyclizations, and molecular preorganization or tethering of reactive ends can steer regiospecific cyclization and improve efficiency.

In summary, photocyclization is a versatile method in photochemistry for forming rings by light-induced bonding, playing a role in synthesis, materials science, and the study of light-responsive compounds.