Substitution Pattern Controlled Quantum Interference in [2.2]Paracyclophane-Based Single-Molecule Junctions

Abstract

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.

Intense Molar Circular Dichroism in Fully Conjugated All‐Carbon Macrocyclic 1,3‐Butadiyne Linked pseudo‐meta [2.2]Paracyclophanes**

“…[37] Our group recently reported experimental evidence of destructive and constructive quantum interference (DQI and CQI) effects in pseudo-para and pseudo-meta substituted PCPs, respectively, thus corroborating the theoretical prediction. [38] With this finding, we figured that pseudo-meta PCPs might be ideal candidates for incorporation in π-conjugated materials. Unlike para-benzene, they combine optimal conjugation with an angled arrangement of the substituents, allowing construction of new types of topologies without the formation of excessive amounts of strain.…”

Intense Molar Circular Dichroism in Fully Conjugated All-Carbon Macrocyclic 1,3-Butadiyne Linked pseudo-meta [2.2]Paracyclophanes

“…37 Performing mechanically controlled break junction experiments, our group recently reported experimental evidence of this destructive and constructive quantum interference (DQI and CQI) effects in pseudo-para and pseudo-meta substituted PCPs, respectively, corroborating the theoretical prediction. 38 With this finding, we figured that pseudo-meta PCPs might be ideal candidates for incorporation in π-conjugated materials. Unlike para-benzene, they combine optimal conjugation with an angled arrangement of the substituents, allowing construction of new types of topologies without the formation of excessive amounts of strain.…”

Mechanical compression in cofacial porphyrin cyclophane pincers

“…For instance, the overlap of molecular orbitals can be controlled mechanically either via changing the relative position or the angle between porbitals. 10,[12][13][14]18 The underlying changes in transport mechanisms, resulting from quantum interference 10,12,13,18 and the size of tunnel couplings, 14 are revealed by single-molecule measurements.…”