Helical Nanostructures for Organic Electronics: The Role of Topological Sulfur in Ad-Hoc Synthesized Dithiahelicenes Studied in the Solid and on a Gold Surface
As the first of a series of molecular solenoids, two classes of dithiahelicenes (coil-shaped molecules with sulfur atoms integrated within a helical conjugated system) have been devised and synthesized to be used in molecular electronics. We used a modular assembly of fragments using Pd catalyzed coupling reactions and a final photocyclization step for the syntheses; this strategy gave us straightforward access to helicenes bearing thiophene end rings with either exo or endo topologies. Unequivocal structural characterization was done by X-ray crystallography. In the solid, their crystal architectures show little similarities; both can be considered an ensemble of heterochiral dimers (P/M) that are themselves very different in nature in the light of their main pairing interactions. On a gold surface, the effect of the sulfur atom is to strengthen their binding to the electrodes, as manifested by scanning tunneling microscopy (STM) performed at room temperature. Different coating patterns were observed for each class of molecules, although the most prominent finding is that we could see resolved STM images of a single molecule, with a full display of its inherent chirality under room temperature conditions.
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