Applications and Occurrences of Heterocycles in Everyday Life 193Electrical and electronic
There are many possible applications for heterocycles in electronics, for example as organic conductors and semi-
conductors, and in electrochromic devices and Organic Light Emitting Diodes (OLEDs). The use of such organic
materials offers a potential escape from limitations of supply and cost of classical materials, that is, metals. They
also offer a probable major reduction of dimensions and weight. Extensive research is being carried out in this area,
but large-scale applications have been progressing more slowly than expected. The promise (and expectation) is still
there though.
The most useful materials are conducting polymers that consist of a cationic heterocyclic polymer or oligomer, with a
counter anionic component (monomeric or polymeric). (Note: These polymers are not continuous chains, but over-
lapping chains, with electron transfer between chains.)
Poly(pyrrole), made via the oxidative polymerisation of pyrrole, was the fi rst heterocyclic conducting polymer and
has found applications in, for example, electronic shielding, capacitors and sensors. The initial neutral polymer is
non-conducting but on further oxidation is converted partially into cation radicals or dications, with incorporation of
counterions from the reaction medium – a process known as ‘doping’ – giving conducting materials.
The closely related poly(thiophene)s, which can be made in a much wider variety than poly(pyrrole), are currently
the more prominent, and have seen signifi cant commercialization as anti-static fi lms, and probably as components of
OLEDs. They form fl exible fi lms and can be formed by polymerisation of the monomers in situ, and also applied as
inks. Two prominent commercial examples are CleviosTM and PlexcoreTM.
The tetrathiafulvalene (TTF)–TCNQ complex shown below, and a number of its analogues, have been used as the
basis for much research into conducting organic materials, and depend on the crystal structure for alignment of the
components for a charge transfer process to occur. These complexes can have directionally dependent semi-conducting
properties.