Applications and Occurrences of Heterocycles in Everyday Life 181Modern synthetic dyes and pigments include the phthalocyanines, which are reminiscent of natural porphins and are com-
monly used as metal complexes, particularly with copper: such metal derivatives have a cation complexed at the centre,
much like the iron atom in heme (see pages 162–163). Many variations, including solubilised (sulfonated) forms, are used
in shades of blue and green, the latter being chlorinated derivatives. The phthalocyanines comprise about one quarter of all
colorants sold, with copper phthalocyanine being the single largest volume colorant. These compounds have a multitude
of applications, including artists’ pigments, inkjet inks, industrial paints, plastics and in the manufacture of CDs.
Reactive dyes, which bind very strongly (and therefore do not ‘wash out’), usually contain halo-heterocycles that react with
nucleophilic centres, such as OH and NH, in the fabric (cotton, wool, nylon), to give covalent bonds by displacement of
the halogen.
Related to the coloured appearance (refl ected light) of dyed fabrics is fl uorescence (the absorption of UV light, followed
by emission of visible light), which has important practical uses. The intensity of the colour means that low concentra-
tions can be easily detected, such as the use of fl uorescein to trace water courses. Fluorescein is also the molecular core
for many fl uorescent metal and biomolecule sensors. Optical brighteners, which emit blue light, are very prominent in
everyday substances such as washing powders – the addition of more blue results in a whiter appearance.
Polymers
Most polymer plastics and fi bres are composed of aliphatic or carbocyclic aromatic compounds, but a few are hetero-
cyclic, one of the prime properties of which being resistance to heat.
Melamine–formaldehyde polymers are widely-used, relatively cheap plastics for housewares and in laminates such
as Formica. At the other end of the scale, polybenzimidazole is used to make expensive high performance fi bres, for
example for fi reproof clothing, and high performance engineering plastics (e.g. Celazole-PBI), which are suitable for
constant use at over 300 °C. The fi bre is also under investigation for use in high temperature membranes.