Effect of Metal Complexation on the Conductance of Single-Molecular Wires Measured at Room Temperature

Abstract

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.

Characterizing the Metal–SAM Interface in Tunneling Junctions

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Recent studies of singlemolecule break junctions have been interpreted to indicate that the presence of covalent Au−C σ-bonds-formed using trimethyltin (-SnMe 3 )-terminated n-alkyl groups, 19,20 and SnMe 3 -terminated aromatics [19][20][21] or trimethylsilyl (TMS)-terminated conjugated systems 22 -increases rates of charge transport across these junctions by approximately a factor of 10-100, relative to amine or thiolate anchoring groups. One possible inference from the increase is that the Au−C σ-bond, and the absence of resistive anchoring heteroatoms, increases "conductivity" (although the meaning of this word is not entirely clear for tunneling junctions).…”

Photophysical Properties of Oligo­(phenylene ethynylene) Iridium(III) Complexes Functionalized with Metal‐Anchoring Groups

“…The molecular conductances of some of these li- gands have been investigated already in mechanically controlled break junctions. [36] Therefore, complexes Ir1-Ir4 combine the low resistance of molecular wires 1-4 with the rich photophysics of Ir III complexes. [Ir(ppy) 2 …”

“…[48,49] Although the oxidation is more irre-versible for the acetylthiol-terminated compounds Ir1 and Ir2 than for the pyridine-terminated derivatives Ir3 and Ir4, this may only reflect the different tendency of the acetylthiol derivatives to become adsorbed on the electrodes following the redox process. [36] +0.75 [b] [a] The electrochemical gaps were obtained as the differences between the cathodic peak of the reduction process (E c red ) and the anodic peak of the oxidation process (E a ox ).…”

Electric-field induced bistability in single-molecule conductance measurements for boron coordinated curcuminoid compounds

“…Although the influence of anchoring groups and electrodes on molecular conductance has been extensively studied in the past,1,14–16 the influence on the conductance when incorporating a metal ion is less clear; in some cases, the presence of the metal ion can be seen as an effective gate but it is not a priori known if incorporation would lead to a conductance increase or decrease. Experiments by Liu et al 17 using metallo-porphyrins show a downshift in the conductance values when a metal atom is included in the structure; however, in other studies, coordination to a metal has been shown to improve electron transport 18. Similarly, the role of the molecular dipole moments in single-molecule transport has also not been studied in great detail; it has only been reported in a few cases 19…”