Stereoelectronic switching in single-molecule junctions

Abstract

A new intersection between reaction chemistry and electronic circuitry is emerging from the ultraminiaturization of electronic devices. Over decades chemists have developed a nuanced understanding of stereoelectronics to establish how the electronic properties of molecules relate to their conformation; the recent advent of single-molecule break-junction techniques provides the means to alter this conformation with a level of control previously unimagined. Here we unite these ideas by demonstrating the first single-molecule switch that operates through a stereoelectronic effect. We demonstrate this behaviour in permethyloligosilanes with methylthiomethyl electrode linkers. The strong σ conjugation in the oligosilane backbone couples the stereoelectronic properties of the sulfur-methylene σ bonds that terminate the molecule. Theoretical calculations support the existence of three distinct dihedral conformations that differ drastically in their electronic character. We can shift between these three species by simply lengthening or compressing the molecular junction, and, in doing so, we can switch conductance digitally between two states.

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…Like all other s(SiSi)b onding MOs, it has no nodes between Si neighbors at any w. However, it has an ode at each of them,s ince after all, it is the least stable of all s(SiSi)o rbitals. The contrastb etween the two limits is seen in Figure 5, whichs hows the standard representation of the HOMO of Si 12 Me 26 ,a nd much more clearly in the symbolic representation of the HOMO of Si 16 Me 34 at the top of Figure 6. The all-anti HOMO is evenly distributed over the whole oligosilane chain and in that regard is reminiscent of the p HOMO of al ong polyene.…”

“…The contrast is even more easily seen in the symbolic representation of the MOs of Si 16 Me 34 in Figure 6. The segmentation of the orbitals in the all-gauche case is very different from the even delocalization in the all-anti case and makes it understandable why the energy of the HOMO changes so little with chain length in the former whereas it increases rapidly with increasing chain length in the latter.…”

“…We first obtain inspiration from ap lot of Ladder C p i values for all s(SiSi)M Os of Si 16 Me 34 as af unctiono fw (Figure 10 B; the explicit form of all these MOs is shown in Figure 10 A,C). In the first approximation, the plot has the shape of at ilted saddle, with the highest values (the most s delocalization) for the highest few energy MOs (and especially,t he HOMO) in loose helices and the lowest few energy MOs in tight helices.…”

The low-bias conducting mechanism of single-molecule junctions constructed with methylsulfide linker groups and gold electrodes

“…27 Furthermore, single-molecule switches operating through a stereoelectronic effect were first realized in oligosilane molecules connected to gold electrodes through SMe linkers. 25 It is generally accepted that for thiol groups the Au-S covalent bonds are formed at the molecule-electrode interfaces while pyridine, amine, and methylsulfide groups form donor-acceptor bonds with undercoordinated gold atoms on the electrode surface because of the lone pairs in the corresponding N or S atom. Unfortunately, up to now the detailed atomic structures of the a) Author to whom correspondence should be addressed: smhou@pku.edu.cn molecule-electrode contacts are still not clearly known and thus out of control in experiments.…”

Tuning Charge Transport in Aromatic‐Ring Single‐Molecule Junctions via Ionic‐Liquid Gating

“…Stimulated by the initial proposal that a single molecule could be used as the functional unit in the circuit of electronic devices, [1] researchers have been making great efforts to realize various functionalities at the single-molecule level, [2] as well as to probe and understand the intrinsic properties of materials at the atomic/molecular scale. [3] The development of single-molecule transistors, in which a single molecule is sandwiched between two nanogapped electrodes while the energy levels of the resulting molecular bridge are controlled by a third proximal electrode, [4] is particularly attractive because they form a basic element in molecular nanocircuits for future practical applications. [5] Because the inherent characteristics of molecular orbital energy levels is one of the most critical factors influencing charge transport in molecular junctions, this three-terminal device architecture also provides a universal strategy to explore quantum transport and novel physical phenomena of single molecules.…”