Synthesis and structure of fused alpha-oligothiophenes with up to seven rings.

To combine the stability of alpha-oligothiophenes with the planarity of acenes, fully fused oligothienoacenes were synthesized and their properties compared to the nonfused alpha-oligothiophenes. By employing removable solubilizing groups, our synthetic methodology made it possible to efficiently prepare and purify oligothienoacenes with up to seven fused rings. The key steps involved the halogen dance reaction and Pd-catalyzed coupling of Bu3SnSSnBu3 to introduce sulfur linkages. This approach eliminates alpha-beta anion equilibration, a significant improvement over the traditional method of introducing sulfur linkages via Li-Br exchange. X-ray diffraction data indicate that pentathienoacene and heptathienoacene adopt pi-stacked packing motifs in contrast to the herringbone packing of nonfused oligothiophenes. On the basis of the linear dependence of the longest lambdamax on the reciprocal number of double bonds of thienoacenes with three to seven rings, the band gap of polythienoacene is extrapolated to be 2.21 eV.