Phthalocyanine is an intensely blue-green-colored aromatic macrocyclic compound that is widely used in dyeing and the first phthalocyanine (Pc) was synthesized accidentally in 1907 as an unidentified blue compound when o-cyanobenzamide was heated at high temperature and that substance is currently known to be the metal-free phthalocyanine.
Phthalocyanine (Pc) molecules are macrocyclic compounds, which are composed of an inner porphyrazine ring that connects four isoindole groups giving rise to the characteristic cross-like shape. In the nonsubstituted or metal-free form, the center of the porphyrazine ring consists of two H atoms (2H-Pc), which are referred to as pyrrolic H atoms. The pyrrolic H atoms may be substituted by a metal (M) ion. Many metal elements may form coordination complexes with phthalocyanines, M-Pc, through d bonds between M and pyrrolic N atoms. Depending on the metal ion, the molecular electronic and optical properties can be tailored to some extent.
M-Pc molecules are known for their use as pigments, dyes, nonlinear optical materials, enzyme-like catalysts, liquid crystals, sensitizers in photochemical reactions or photovoltaic cells, and photodynamic reagents in cancer therapy. For some applications, Pc molecules have to be adsorbed on surfaces. The adsorption very often entails changes in the genuine molecular properties, which may lead to new beneficial characteristics or are detrimental to the appealing free-molecule properties.
Phthalocyanine is of course closely related to the highly coloured porphyrins which are classed as polyene chromogens, and contain no donor or acceptor groups. The very important blue pigment copper phthalocyanine is a tetraazatetrabenzoporphin and the basic polyene chromogen of the molecule is as in (119), which has an aza-annulene type ring. The colour of the molecule is relatively little affected by substitution in the benzene rings. Copper phthalocyanine has a four-fold axis of symmetry in common with the porphin system, and has two absorption bands in or close to the visible region, designated Q-bands and B-bands.