ABSTRACT
Substitution of nitrogen for a meso carbon bridge in a porphyrin gives an azaporphyrin (155): replacement of all four meso carbon bridges by nitrogens gives a porphyrazine (156). If we fuse benzo rings to the (3,[T-bonds of a porphyrin we arrive at benzoporphyrins such as tetrabenzoporphyrin (157). Replacement of all four meso carbon bridges of this by nitrogens gives a substance used extensively in everyday life, although we may not realise it (blue / green pigments, toner for the photocopier, laser imprinted surface layer of the compact disc), and it has a special name of its own, phthalocyanine (30). With increasing meso-nitrogen substitution and fusion of benzene rings, the macrocycles give more stable metal complexes (copper(II)
phthalocyanine dissolves in cold concentrated sulphuric acid without change, and can be precipitated on pouring into ice-water), and become less basic. The electronic spectrum changes dramatically so that the Q band now becomes (for phthalocyanines) the strongest band in the spectrum, and appears in the 670-700nm region. The metal-free phthalocyanines have two strong bands in this region (Qx and Qy, Panel 8.1) whereas the metallated phthalocyanines usually have only one. These Q bands are very intense (ca. 100 000-400 000) and sharp, a property which is exploited in laser address systems.
