The design of artificial molecular machines often takes inspiration from macroscopic machines. However, the parallels between the two systems are often only superficial, because most molecular machines are governed by quantum processes. Previously, rotary molecular motors powered by light and chemical energy have been developed. In electrically driven motors, tunnelling electrons from the tip of a scanning tunnelling microscope have been used to drive the rotation of a simple rotor in a single direction and to move a four-wheeled molecule across a surface. Here, we show that a stand-alone molecular motor adsorbed on a gold surface can be made to rotate in a clockwise or anticlockwise direction by selective inelastic electron tunnelling through different subunits of the motor. Our motor is composed of a tripodal stator for vertical positioning, a five-arm rotor for controlled rotations, and a ruthenium atomic ball bearing connecting the static and rotational parts. The directional rotation arises from sawtooth-like rotational potentials, which are solely determined by the internal molecular structure and are independent of the surface adsorption site.

No abstract

Molecular trefoil knots have been prepared from metal-assembled precursors using the ring closing metathesis (RCM) cyclization methodology. The templating metal is either copper(I) or iron(II) and the coordinating fragments 1,10-phenanthroline or 2,2′:6′,2"-terpyridine, respectively. The RCM approach, newly applied to the field of catenanes and knots, represents a spectacular synthetic improvement in terms of yield and experimental conditions (no high dilution required). The dicopper(I) trefoil knot has been synthesized in a 74% yield. A similar approach also led to the first knot constructed around two iron(II) bis(terpyridine) moieties, demonstrating that iron(II) can also be used as a highly efficient template. Moreover, for both Cu(I) and Fe(II) knots, it has been possible to quantitatively reduce the cyclic olefins formed during the macrocyclization by catalytic (Pd/C) hydrogenation. An X-ray structure of the double helix of iron(II) bis-(1,2-bis(5-(5"-methyl-2,2′:6′,2"-terpyridinyl))ethane) is given which shows that the double-stranded helical precursor is well predisposed for the formation of a molecular knot.