We report a systematic approach to the control of the conjugation length along the poly-(thiophene) backbone. The planarity of the main chain can be permanently modified by altering the pattern of substitution and character of the substituents on the poly(thiophene) chain, and the conjugation length is thus modified. We obtain blue, green, orange, red, and near-infrared electroluminescence from four chemically distinct poly(thiophenes). The external quantum efficiencies are in the range of 0.01-0.6%.

We have shown that it is possible to regioselectively polymerize 3-(4-octylphenyl)thiophene with FeCl3. Adding FeCl3 slowly to the monomer leads to a soft and therefore regioselective polymerization. The head-to-tail content was determined by NMR to be 94 ± 2 %. Thin films of the polymer treated with chloroform vapor have an absorption maximum at 602 nm (2.06 eV) with clear vibronic fine structure. Free standing films have a conductivity of 4 S/cm, which is 100 times higher than for earlier prepared poly(3-(4-octylphenyl)thiophene). A mechanism for the regioregular polymerization is also proposed.