OVERVIEW

The phthalocyanines represent without doubt the most important chromophoric system developed during the 20th century. Historically, the most important event was probably their accidental discovery around 1928 by a dye manufacturing company in Scotland. However, there is little doubt that researchers before this had isolated phthalocyanines, but the significance of their observations was not fully recognised. In 1907, Braun and Tcherniac were engaged in a study of the chemistry of o-cyanobenzamide ( 90) and discovered that when this compound was heated a trace amount of a blue substance was obtained. This compound undoubtedly was metal-free phthalocyanine ( 91). In 1927, de Diesbach and co-workers reported that when 1,2-dibromobenzene was treated with copper(I) cyanide in boiling quinoline for eight hours, a blue product was obtained in reasonable yield. This was almost certainly the first preparation of copper phthalocyanine (CuPc, 92). They obtained the molecular formula of the compound from elemental analysis and noted its remarkable stability to alkali, concentrated acids and heat but were unable to propose a structure. In 1928, in the manufacture of phthalimide by Scottish Dyes (later to become part of ICI) from the reaction of phthalic anhydride with ammonia in a glass-lined reactor, the formation of a blue impurity was observed in certain production batches. This contaminant was isolated as a dark blue, insoluble crystalline substance. Ultimately, the compound proved to be iron phthalocyanine (FePc), the source of the iron being the wall of the reactor which became exposed due to a...