Phthalocyanine (H2Pc) is a large, aromatic, macrocyclic, organic compound with the formula (C8H4N2)4H2 and is of theoretical or specialized interest in chemical dyes and photoelectricity.

The extensive delocalization of the π-electrons affords the molecule useful properties, lending itself to applications in dyes and pigments.

Metal complexes derived from Pc2−, the conjugate base of H2Pc, are valuable in catalysis, organic solar cells, and photodynamic therapy.

Phthalocyanine and derived metal complexes (MPc) tend to aggregate and, thus, have low solubility in common solvents.

H2Pc and CuPc dissolve easily in sulfuric acid due to the protonation of the nitrogen atoms bridging the pyrrole rings.

Unsubstituted phthalocyanines strongly absorb light between 600 and 700 nm, thus these materials are blue or green.

[13] Modification of the substituents attached to the peripheral rings allows for the tuning of the absorption and emission properties of Pc to yield differently colored dyes and pigments.

There has since been significant research on H2Pc and MPc resulting in a wide range of applications in areas including photovoltaics, photodynamic therapy, nanoparticle construction, and catalysis.

They feature four pyrrole-like subunits linked to form a 16 membered inner ring composed of alternating carbon and nitrogen atoms.

Both porphyrins and phthalocyanines function as planar tetradentate dianionic ligands that bind metals through four inwardly projecting nitrogen centers.