porphyrinoids

Porphyrinoids are a broad class of nitrogen-containing macrocycles characterized by a conjugated system built from four heterocyclic subunits, typically pyrrole rings, linked by methine (CH) bridges to form a large, planar ring. The core skeleton is highly conjugated, often described as an 18 π-electron aromatic system, and many porphyrinoids feature a central cavity capable of coordinating a metal ion. The term encompasses porphyrins as well as numerous related macrocycles that differ by size, connectivity, or saturation, including corroles, phthalocyanines, texaphyrins, and expanded porphyrins such as sapphyrins.

Porphyrins, the best-known family, occur widely in biology (heme and chlorophyll). Other notable members include corroles (contracted porphyrinoids), phthalocyanines (four isoindole units with peripheral aza-linkages), and expanded porphyrinoids like texaphyrins and sapphyrins. Many porphyrinoids support metal coordination (Fe, Mg, Zn, Cu, among others), which underpins biological functions and many applications.

Historically, porphyrinoids are strongly colored and absorb in the visible region, with characteristic Soret bands around 400 nm and Q bands in the 500–700 nm range. Their optical and redox properties can be tuned by peripheral substituents, central metal, or degree of saturation. The framework enables diverse chemistry, including catalysis, photodynamic therapy, sensing, and materials for organic electronics.

Synthetic approaches to porphyrinoids include classic porphyrin-forming condensations (porphyrinogen-to-porphyrin routes) and modern modular methods, sometimes employing metal templating to drive macrocyclization. Corroles and other contracted or expanded variants require specialized routes but share the same core concept of a conjugated macrocycle capable of metal binding and electron transfer.