To investigate interference phenomena and conductance properties in mechanically controlled break junctions (MCBJs), macrocycles 1 and 2 (BMCs: for BenzeneMacroCycles), containing a meta-substituted benzene moiety with solubilizing tert-butyl groups, as well as structures 3 and 4 (TMCs: for ThiopheneMacroCycles), featuring 2,5-connected 3,4-hexyl-thiophene corners, were synthesized. Macrocycles 1 and 2 respectively 3 and 4 differ in the positions of the acetyl-protected sulfur anchoring groups, which impacts both, the individual transport efficiency of their parallel electronic pathways and their overall molecular wire lengths. All macrocycles were synthesized based on a series of Sonogashira cross-coupling reactions. For 3 and 4, a 2-(4-pyridinyl)ethyl protecting group for the sulfur atoms was successful, while for macrocycles 1 and 2 the more common tert-butyl protecting group did the job. To our delight, proof-of-concept charge transport studies conducted in an MCBJ setup demonstrated the expected trends regarding improved conductance intensities for the TMCs compared to the BMCs. Furthermore, the corresponding molecular plateaus from the breaking experiments were in the expected length range of the S−S distances for all compounds. We also found that the overall conductance seems to follow a more complex transport mechanism than just the sum of contributions from both channels.@en

Quantum interference plays an important role in charge transport through single-molecule junctions, even at room temperature. Of special interest is the measurement of the destructive quantum interference dip itself. Such measurements are especially demanding when performed in a continuous mode of operation. Here, we use mechanical modulation experiments at ambient conditions to reconstruct the destructive quantum interference dip of conductance versus displacement. Simultaneous measurements of the Seebeck coefficient show a sinusoidal response across the dip without sign change. Calculations that include electrode distance and energy alignment variations explain both observations quantitatively, emphasizing the crucial role of thermal fluctuations for measurements under ambient conditions. Our results open the way for establishing a closer link between break-junction experiments and theory in explaining single-molecule transport phenomena, especially when describing sharp features in the transmission.

@en

Break-junction techniques provide the possibility to study electric and thermoelectric properties of single-molecule junctions in great detail. These techniques rely on the same principle of controllably breaking metallic contacts in order to create single-molecule junctions, whilst keeping track of the junction's conductance. Here, we compare results from mechanically controllable break junction (MCBJ) and scanning tunneling microscope (STM) methods, while characterizing conductance properties of the same novel mechanosensitive para- and meta-connected naphtalenophane compounds. In addition, thermopower measurements are carried out for both compounds using the STM break junction (STM-BJ) technique. For the conductance experiments, the same data processing using a clustering analysis is performed. We obtain to a large extent similar results for both methods, although values of conductance and stretching lengths for the STM-BJ technique are slightly larger in comparison with the MCBJ. STM-BJ thermopower experiments show similar Seebeck coefficients for both compounds. An increase in the Seebeck coefficient is revealed, whilst the conductance decreases, after which it saturates at around 10 μV K−1. This phenomenon is studied theoretically using a tight binding model. It shows that changes of molecule-electrode electronic couplings combined with shifts of the resonance energies explain the correlated behavior of conductance and Seebeck coefficient.@en

Quantum interference (QI) within a single-molecule junction has become an essential yet powerful concept to integrate for designing molecular electronic devices.1 Recently we have investigated the correlation between substitution pattern, conductance and mechanosensitivity in [2.2]paracyclophane(PCP)-based single-molecule junction via the mechanically controlled break junction technique (MCBJ).2,3 We study the influence on conductance when we introduce meta/para connection to the PCP core and the phenyl ring attached to the anchoring group. We find that (i) meta-phenyl-anchored PCP yields such low conductance levels that molecular features cannot be resolved; (ii) para-phenyl-coupled anchoring generally gives higher conductance levels which can be detected via MCBJ; (iii) pseudo-para-coupled PCP core manifests large mechanosensitivity while (iv) pseudo-meta-coupled PCP core show the absence of mechanosensitivity. These experimental findings are interpreted in terms of QI effects between molecular frontier orbitals by theoretical calculations based on density functional theory and the Landauer formalism.@en

Quantum interference (QI) of electron waves passing through a single-molecule junction provides a powerful means to influence its electrical properties. Here, we investigate the correlation between substitution pattern, conductance, and mechanosensitivity in [2.2]paracyclophane (PCP)-based molecular wires in a mechanically controlled break junction experiment. The effect of the meta versus para connectivity in both the central PCP core and the phenyl ring connecting the terminal anchoring group is studied. We find that the meta-phenyl-anchored PCP yields such low conductance levels that molecular features cannot be resolved; in the case of para-phenyl-coupled anchoring, however, large variations in conductance values for modulations of the electrode separation occur for the pseudo-para-coupled PCP core, while this mechanosensitivity is absent for the pseudo-meta-PCP core. The experimental findings are interpreted in terms of QI effects between molecular frontier orbitals by theoretical calculations based on density functional theory and the Landauer formalism.

@en