We investigated the effect of anchoring group position on the formation and electric conductance of single molecule junctions for benzenedithiol and benzenediamine by the scanning tunneling microscope break junction technique. The conductances of the single 1,4-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenediamine, and 1,3-benzenediamine molecules were 0.005 (±0.001) G 0 (G 0 = 2e 2/h), 0.004 (±0.001) G 0, 0.01 (±0.003) G 0, and 0.005 (±0.002) G 0, respectively. No 1,2-disubstituted benzene molecules formed junctions. While the 1,4-position provided larger conductance than the 1,3-position for both anchoring groups, the effect of the anchoring position on conductance was clearer for benzenediamine than benzenedithiol. The resulting anchoring position and its stability are discussed in consideration of the formation of the single molecular junction. The relationship between conductance and anchoring group (position) was analyzed based on ab initio transport calculations. The deformation and change of the energy alignment of the “conductive” molecular orbital give clearer insight to the anchoring position effect than to quantum interference.

There is an increasing interest in the electron conductivity of metal-atom wires at the single-molecule level because of their quantized electric properties applicable to conducting wires in ultra-small devices. [1][2][3] In contrast, the single-molecular conductivity of metal-ion wires has never been studied and whether the metal-ion wires are conductive or insulating has been a matter of debate, [4][5][6] because creating metal-ion arrays with a fixed cross section and length between nanogap electrodes is nontrivial. In recent years, several efficient methods have been developed for preparing multinuclear complexes with ordered metal-ion arrays. Among them, [3 n] gold-ion clusters, enclosed in self-assembled cages 1 (Figure 1), are particularly interesting because the stacking number n is uniquely determined by the length of the pillar ligand (4). In expectation of efficient electron transport through metal-ion arrays, single-molecular electron conductivity through [3 n] stacks of gold-ion clusters (2) n (n = 1, 2, or 3) was measured. The cages, 1, which keep the metal ions highly ordered in a [3 n] manner within the cavity, are excellent frameworks for accommodating p-stacked molecules and evaluating their electron-transport properties. [7, 8] We show that the metal-ion arrays are highly conductive, the electron transport being comparable to that through metalatom wires and the absolute conductance value being much larger than that of metal-linked organic wires.We confirmed by ab initio transport calculations that the [3 n] Au I ion clusters would be suitable for high conductance with a negligibly small decay over longer transport distances. We calculated the conductance of the corresponding model arrays, 3·(2 a) n ·3 (n = 1-4) (See the Supporting Information). Figure 2 shows the calculated conductance values, together Figure 1. [3n] Au I ion clusters (2) n enclosed within coordination cages 1 which self-assemble from triazine panel ligands 3, pillar ligand 4, and [(ethylenediamine)M(ONO 2 ) 2 ] (M= Pd or Pt).Figure 2. Computed conductance of [3 n] Au I ion clusters. In the calculation, the stacked arrays 3·(2 a) n ·3 (n = 1-4) sandwiched between Au electrodes were used as simple models for the [3 n] Au I ion clusters. The transport direction is parallel to the Au[111] direction.

SummaryWe investigated the effect of the environment on the electrical conductance of a single benzene-1,4-diamine (BDA) molecule bridging Au electrodes, using the scanning tunneling microscope (STM). The conductance of the single BDA molecule junction decreased upon a change in the environment from tetraglyme, to mesitylene, to water, and finally to N2 gas, while the spread in the conductance value increased. The order of the conductance values of the single BDA molecule junction was explained by the strength of the interaction between the solvent molecules and the Au electrodes. The order of the spread in the conductance values was explained by the diversity in the coverage of the BDA molecule at metal electrodes and atomic and molecular motion of the single-molecule junction.