A key goal in molecular electronics has been to find molecules that facilitate efficient charge transport over long distances. Normally, molecular wires become less conductive with increasing length. Here, we report a series of fused porphyrin oligomers for which the conductance increases substantially with length by >10-fold at a bias of 0.7 V. This exceptional behavior can be attributed to the rapid decrease of the HOMO-LUMO gap with the length of fused porphyrins. In contrast, for butadiyne-linked porphyrin oligomers with moderate inter-ring coupling, a normal conductance decrease with length is found for all bias voltages explored (±1 V), although the attenuation factor (β) decreases from ca. 2 nm at low bias to <1 nm at 0.9 V, highlighting that β is not an intrinsic molecular property. Further theoretical analysis using density functional theory underlines the role of intersite coupling and indicates that this large increase in conductance with length at increasing voltages can be generalized to other molecular oligomers.
Quantum interference (QI) plays an imperative role in the operation of molecular devices within the phase-coherent length, and it is vital to harness the patterns of QI, i.e., constructive and destructive interference. However, the size of the single-molecule device is too small compared to most gate electrodes. Those gates act like a backgate to affect the molecular component uniformly. Switching the patterns of QI in the same molecular skeleton remains challenging. Here, we develop the atomically precise gating strategy that manipulates the frontier orbitals of molecular components, achieving the complete switching of QI patterns between destructive to constructive QI and leading to a significant conductance modulation at room temperature. The chemical gating effect is exerted locally on the pyridine nitrogen through the selective interaction to cationic reagents, with which we can also control the switching reversibility as desired. We demonstrate the unique effect of atomically precise gating to modulate the quantum interference at the single-molecule scale, opening an avenue to develop new-conceptual electronic devices.
We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in CClD). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au-porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S-S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au-S and Au-Au bonds, but higher than that of the Au-SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound.
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