Propeller-shaped fused oligothiophenes: A remarkable effect of the topology of sulfur atoms on columnar stacking

Propeller-shaped regioisomers of fused oligothiophenes F9T_endo, F9T_anti, and F9T_exo were successfully synthesized. DFT calculations indicated that their core parts are distorted from planarity due to intramolecular steric repulsions involving large sulfur atoms. In contrast with soft crystalline F9T_anti and F9T_exo, F9T_endo self-assembles into a hexagonal columnar liquid crystal (Col_h LC), displaying a clear X-ray diffraction (XRD) due to its stacked π-conjugated core. In each LC column, well-organized intermolecular S-S contacts are developed triple-helically along the columnar axis with a helical pitch of 4.04 nm. Among LC semiconductors reported to date, Col_h LC F9T _endo displays a top-class charge-carrier mobility (0.18 cm ² V^-1 s^-1) with a distinct ambipolar character featuring well-balanced hole and electron mobilities. A thin film, prepared by mixing F9T_endo with soluble fullerene PCBM, shows a photovoltaic response, when the fullerene content is large enough to compensate a small absorptivity of F9T_endo for visible light.