The present work aims to give insight into the effect that metal coordination has on the room-temperature conductance of molecular wires. For that purpose, we have designed a family of rigid, highly conductive ligands functionalized with different terminations (acetylthiols, pyridines, and ethynyl groups), in which the conformational changes induced by metal coordination are negligible. The single-molecule conductance features of this series of molecular wires and their corresponding Cu(I) complexes have been measured in break-junction setups at room temperature. Experimental and theoretical data show that no matter the anchoring group, in all cases metal coordination leads to a shift toward lower energies of the ligand energy levels and a reduction of the HOMO-LUMO gap. However, electron-transport measurements carried out at room temperature revealed a variable metal coordination effect depending on the anchoring group: upon metal coordination, the molecular conductance of thiol and ethynyl derivatives decreased, whereas that of pyridine derivatives increased. These differences reside on the molecular levels implied in the conduction. According to quantum-mechanical calculations based on density functional theory methods, the ligand frontier orbital lying closer to the Fermi energy of the leads differs depending on the anchoring group. Thereby, the effect of metal coordination on molecular conductance observed for each anchoring could be explained in terms of the different energy alignments of the molecular orbitals within the gold Fermi level.

The present work reports the introduction of pyridine bisamine terdentate ligands in the structure of a pirouetting copper rotaxane. Rotaxane 2[PF6] constitutes the first example of the incorporation of imine-based dynamic covalent chemistry in the synthesis of switchable copper-complexed interlocked systems. In this rotaxane, the substitution of the classical terpyridine terdentate unit by a pyridine bisamine moiety has led to a significant stabilization of the pentacoordinated site. That fact has been evidenced by EPR spectroscopy and cyclic voltammetry. Regarding the tetracoordinated site, the congestion around the coordination sphere has been reduced to accelerate the typically slow reorganization of the Cu(II). Ethynyl-3,8-substitution on the axis phenanthroline along with the 2,9-diphenyl-1,10-phenanthroline (dpp) present in the macrocycle afforded a very stable coordination environment for Cu(I), which is at the same time labile upon oxidation. In summary, the incorporation of a pyridine bisamine unit as a terdentate ligand and the optimization of the bidentate ligand of the axle not only has led to a simplification of the synthetic procedures, but it has also given rise to a bistable systems with an enhanced energetic separation between states and an acceleration of the reorganization processes. Thus far, rotaxane 2[PF6] presents the fastest switching cycle reported to date in copper-interlocked dynamic systems.

Fast shuttling motions in solution have been observed by cyclic voltammetry in a Cu(I/II)-based [2] rotaxane. In the reported system, the different coordination preferences of both copper oxidation states are exploited to promote the electrochemically-triggered gliding of the ring from a tetra to a pentacoordinated site and vice versa. The thread of this rotaxane consists of a tridentate 2,2':6',2''-terpyridine chelating unit directly bonded through its 5-position to the 3-position of the bidentate 1,10-phenanthroline unit. This distribution reduces to a minimum the distance between the two coordination sites and lessens the congestion around the tetrahedral environment. These two factors have been demonstrated to highly increase the kinetics of the switching process. In addition, the electrochemical experiments carried out in different solvent mixtures evidenced the influence of the solvent on the shuttling mechanism.