Synthesis and Properties of Substituted Poly(thiophenes)
The aim of this work was to synthesize new substituted poly(thiophenes) with certain desired properties.
Soluble and meltable poly(thiophenes) can be prepared if a long alkyl side-chain is attached to each thiophene ring along the polymer backbone. Such polymers show electrical conductivity when they are doped (oxidised). However, the long-term stability of the conductivity of the doped polymer is low when all the thiophene rings have side-chains. The stability of the conductivity is crucial for the application of doped polymers as conducting polymers. We have shown that it is possible to prepare processable substituted poly(thiophenes) which are stable in the doped state. This has been done by "diluting" the number of side-chains along the polymer backbone in a regular way.
The regioregularity of 3-substituted poly(thiophenes) is important for the properties of the polymers. We have modified the normal polymerisation procedure using FeCl3 to yield a large amount of regioregular polymer when phenyl-substituted thiophenes are polymerised. This is the first time regioregular polymerisation with FeCl3 has been achieved.
Poly(thiophenes) can be used as the active layer in polymer light-emitting diodes (PLED's). The properties of the conjugated polymer used in a PLED determine the colour of the electroluminescence emitted from the diodes. We have synthesised differently substituted poly(thiophenes) which show blue, green, red and near-infrared electroluminescence. The colour of the electroluminescence has been tuned by controlling the dihedral angle between the thiophene rings along the polymer backbone. PLED's prepared from blends of our poly(thiophenes) show voltage-controlled colour changes. The application of this phenomenon can ultimately lead to new types of flat colour screens or diodes with multicolour emissions.