A phthalocyanine is a macrocyclic compound having an alternating nitrogen atom-carbon atom ring structure.

The molecule is able to coordinate hydrogen and metal cations in its center by coordinate bonds with the four isoindole nitrogen atoms. The central atoms can carry additional ligands. Most of the elements have been found to be able to coordinate to the phthalocyanine macrocycle. Therefore, a variety of phthalocyanine complexes exist.

History

The first known appearance of an unknown blue by-product, which we now know was metal-free phthalocyanine, was reported in 1907. In 1927, two researchers from Switzerland accidentally synthesized copper phthalocyanine, copper naphthalocyanine and copper octamethylphthalocyanine when they tried to convert o-dibromobenzene into phthalonitrile. They remarked the enormous stability of these complexes but failed to appreciate their discovery and to fully characterize these blue complexes.
The real discovery also started as an accident, when a blue product was found in a reaction flask where only white product was expected. However, this accident occurred in a dye company, Scottish Dyes, Ltd., Grangemouth, Scotland (later ICI) and the discovery was followed on.

Overview

The structure of a phthalocyanine molecule is closely related to that of the naturally occurring porphyrin systems.
The phthalocyanine macrocycle is also related to some other macrocyclic complexes, as e.g., the subphthalocyanine, superphthalocyanine or hemiporphyrazine.

Synthesis

The figure below (left side) shows that a phthalocyanine macrocycle consists of four identical corners. A synthesis strategy, therefore, starts from molecules which correspond to these corners. Such molecules are derivatives of phthalic acid: e.g., phthalonitrile, o-cyanobenzamide, phthalanhydride, phthalimide or diiminoisoindole. Several of these starting materials are shown in the figure below (right side).

Applications

Copper phthalocyanine dyes are produced by introducing solubilizing groups, such as one or more sulfonic acid functions in CPC structure. These dyes find extensive use in various areas of textile dyeing (Direct dyes for cotton), for spin dyeing and in the paper industry. Direct blue 86 is the sodium salt of CPC-sulfonic acid whereas direct blue 199 is the ammonium salt of the CPC-sulfonic acid.

The organic amine salts of these sulfonic acids used as solvent dyes because of their solubility in organic solvents, e.g. Solvent Blue 38 and Solvent Blue 48.

The dye derived from Cobalt PC and an organic amine is Phthalogen Dye IBN.

1,3 Diiminoisoindolene, the intermediate formed during phthalocyanine manufacture, used in combination with a copper salt affords the dye GK 161.

Phthalocyanine is also commonly used as a dye in the manufacture of high-speed CD-R media.

Phthalocyanine compounds are often used as donor materials in organic solar cells based on small molecules.