Poly(2-decylthieno[3,4-b]thiophene-4,6-diyl). A New Low Band Gap Conducting Polymer

A new low band gap conducting polymer, poly(2-decylthieno[3,4-b]thiophene-4,6-diyl) (5), predicted to have nearly equi-energetic aromatic and quinonoidal forms, was synthesized by a multistep sequence involving FeCl3 polymerization of 2-decylthieno[3,4-b]thiophene (7) and studied. A film of 5 showed a vis−NIR spectral λmax = 925 nm, a band edge band gap of 0.92 eV, and an FeCl3-doped conductivity of 3.1 × 10-3 S cm-1. A CHCl3 solution showed λmax = 739 nm and a band-edge band gap of 0.98 eV. GPC (polystyrene standards) gave a number-average molecular weight, M̄n, of 52 000 with a polydispersity M̄w/M̄n of 1.7. The band gap of 0.92 eV makes 5 one of the lowest band gap organic polymers known.