Syntheses and structures of [7]helicene and double helicene based on dithieno[2,3-b:2′,3′-d]thiophene

Based on dithieno[2,3-b:2′,3′-d]thiophene, three novel helicenes including helicene (rac-1), double helicene (rac-2), and benzohexathia[7]helicene (rac-3), and one bull's horn-shaped benzohexathienoacene (4) have been synthesized and reported in this paper. In the helicenes, the sulfur atoms of terminal thiophenes are inside the helical structures. X-ray crystallographic analysis exhibited the helical configuration of rac-1 and the bull's horn-shaped configuration of 4. Multiple types of intermolecular interactions, including S⋯S, C⋯C and π⋯π interactions, were observed between the adjacent molecules in the crystals of rac-1 and 4. In addition, a remarkable bathochromic shift was found in the absorption behaviors for rac-3 and 4 compared to rac-1; the integrated absorbance in rac-2 is approximately twice that of rac-1.