New processable low band-gap, conjugated polyheterocycles

Poly(2,3-dihexylthieno[3,4-b]pyrazine) (2b) has been prepared by ferric chloride polymerization of 2,3-dihexylthieno[3,4-b]pyrazine(4) and studied. It is a dark blue-black soluble material which becomes light yellow in the doped state as either a solution or film. Doped films can be cast directly from solution. The band-gap, obtained from the absorption edge, is ca. 0.86–1.02 eV as the film and 1.14 eV as a CHCl3 solution. This value of about 0.95±0.1 eV is lower than that of poly(isothianaphthene) and poly(5-decylisothianaphthene), which we report here. It is consistent with theoretical predictions and is one of the lowest band-gaps reported to date. When doped with nitrosonium tetrafluoroborate the films show conductivities of up to 3.6 × 10−2 S cm−1. Poly(50decylisothianaphthene) {(1b; poly(5-decylbenzo[c]thiophene)} has been prepared by FeCl3 oxidation of 5-decyl-1,3-dihyhrobenzo[c]thiophene (6). It is a soluble dark blue-black material which forms a light yellow transparent solution when doped.