Synthesis and properties of regioregular poly(dialkyl [2,2′-bithiophene]-5,5′-diyl-4,4′-dicarboxylates)

Regioregular (HH-TT) polythiophenes with electron withdrawing ester groups attached directly to the ring, namely poly-dihexyl and dioctyl [2,2′-bithiophene]-5,5′-diyl-4,4′-dicarboxylate) (2a and 2b) have been prepared via the Ullmann reaction of 3a and 3b with copper in DMF, Number average molecular weights (GPC/polystyrene standards) of 7,900 and 11,000 respectively were obtained. Cast films of 2a and 2b were yellow in color and showed UV-vis absorption at 377 and 381 nm respectively, about 55–80 nm blue shifted compared with regiorandom poly(alkyl thiophene-3-carboxylates). Quantum mechanical calculations showed that the head-to-head dyads induce regular interruption of the coplanarity of the polymer backbone. Both polymers showed bright fluorescence and electroluminescence with emission of yellow light. External electroluminescence efficiencies were 8.5 and 4.7 × 10−3 %, respectively.