macrocycle

A macrocycle is a molecule that contains a large ring, typically with 12 or more atoms in the ring. Macrocycles may include solely carbon atoms or heteroatoms such as nitrogen, oxygen, or sulfur, and they often form cavities capable of binding guest species. This property underpins much of host–guest chemistry and supramolecular design.

Macrocycles range from fully conjugated systems, such as porphyrins and annulenes, to nonconjugated, shape-persistent rings such as crown ethers, calixarenes, cyclodextrins, and related cyclopeptide or cyclophane families. The defining feature is a ring large enough to accommodate other molecules or ions, imparting rigidity or flexibility depending on structure. Their synthesis typically relies on cyclization strategies that favor intramolecular closure, often under high-dilution or using templates to guide ring formation.

The macrocyclic effect refers to the tendency for macrocyclic ligands to bind cations or anions more strongly than acyclic analogs, due in part to preorganization and reduced entropic penalties. Macrocycles are central to applications in metal ion sequestration, catalysis, sensing, drug delivery, imaging, and materials science, as well as in separations.

Characterization relies on spectroscopic methods, mass spectrometry, and, when possible, X-ray crystallography to reveal ring size, conformation, and binding modes. The term encompasses a broad class of large-ring compounds notable for their defined cavities and complexation behavior.