Phthalocyanine () is a large, aromatic, macrocyclic, organic compound with the formula and is of theoretical or specialized interest in chemical dyes and photoelectricity. It is composed of four isoindole units linked by a ring of nitrogen atoms. = has a two-dimensional geometry and a ring system consisting of 18 π-electrons. The extensive delocalization of the π-electrons affords the molecule useful properties, lending itself to applications in dyes and pigments. Metal complexes derived from , the conjugate base of , are valuable in catalysis, organic solar cells, and photodynamic therapy. Properties Phthalocyanine and derived metal complexes (MPc) tend to aggregate and, thus, have low solubility in common solvents. Benzene at 40 °C dissolves less than a milligram of or CuPc per litre. and CuPc dissolve easily in sulfuric acid due to the protonation of the nitrogen atoms bridging the pyrrole rings. Many phthalocyanine compounds are, thermally, very stable and do not melt but can be sublimed. CuPc sublimes at above 500 °C under inert gases (nitrogen, ). Substituted phthalocyanine complexes often have much higher solubility. They are less thermally stable and often can not be sublimed. Unsubstituted phthalocyanines strongly absorb light between 600 and 700 nm, thus these materials are blue or green. Substitution can shift the absorption towards longer wavelengths, changing color from pure blue to green to colorless (when the absorption is in the near infrared). There are many derivatives of the parent phthalocyanine, where either carbon atoms of the macrocycle are exchanged for nitrogen atoms or the peripheral hydrogen atoms are substituted by functional groups like halogens, hydroxyl, amine, alkyl, aryl, thiol, alkoxy and nitrosyl groups. These modifications allow for the tuning of the electrochemical properties of the molecule such as absorption and emission wavelengths and conductance. History In 1907, an unidentified blue compound, now known to be phthalocyanine, was reported. In 1927, Swiss researchers serendipitously discovered copper phthalocyanine, copper naphthalocyanine, and copper octamethylphthalocyanine in an attempted conversion of o-dibromobenzene into phthalonitrile. They remarked on the enormous stability of these complexes but did not further characterize them. In the same year, iron phthalocyanine was discovered at Scottish Dyes of Grangemouth, Scotland (later ICI). It was not until 1934 that Sir Patrick Linstead characterized the chemical and structural properties of iron phthalocyanine. Synthesis Phthalocyanine is formed through the cyclotetramerization of various phthalic acid derivatives including phthalonitrile, diiminoisoindole, phthalic anhydride, and phthalimides. Alternatively, heating phthalic anhydride in the presence