Phthalocyanine

Phthalocyanine refers to a family of synthetic macrocyclic compounds with a planar, highly conjugated ring. The core is formed by four isoindole units linked by nitrogen atoms to create a closed cavity that can coordinate a metal ion in the center, producing metallophthalocyanines. The metal-free form is known as metal-free phthalocyanine (H2Pc or 2H-Pc), while many derivatives contain a wide range of central metals such as copper, zinc, iron, nickel, or cobalt. The ring system is an 18-π-electron aromatic framework, contributing to their chemical stability and rich colors. Peripheral substituents can be varied to modify solubility, aggregation, and electronic properties.

Synthesis of phthalocyanines generally involves the cyclotetramerization of substituted phthalonitrile precursors in the presence of a metal salt and a base at elevated temperature. After metallation, metal phthalocyanines are obtained; metal-free variants can be prepared by removing or avoiding the metal during synthesis. The chemistry allows extensive diversification through substituents on the periphery or by choosing different central metals, enabling a range of physical and electronic properties.

Applications and properties center on their stability, color, and electronic characteristics. Copper phthalocyanine is the prototypical member and is widely used as a pigment (notably pigment blue 15) in inks, paints, and plastics due to its brightness and colorfastness. Metallophthalocyanines and related derivatives function as dyes, pigments, and functional materials. They find use in organic electronics (organic light-emitting diodes, solar cells), catalysis and electrocatalysis (including oxidation and reduction reactions), gas sensing, and as photosensitizers in photodynamic therapy. Phthalocyanines are often discussed in relation to porphyrins, sharing similar macrocyclic, metal-coordinating architectures.