Multicenter‐Bond‐Based Quantum Interference in Charge Transport Through Single‐Molecule Carborane Junctions

Abstract

Molecular components are vital to introduce and manipulate quantum interference (QI) in charge transport through molecular electronic devices. Up to now, the functional molecular units that show QI are mostly found in conventional π‐ and σ‐bond‐based systems; it is thus intriguing to study QI in multicenter bonding systems without both π‐ and σ‐conjugations. Now the presence of QI in multicenter‐bond‐based systems is demonstrated for the first time, through the single‐molecule conductance investigation of carborane junctions. We find that all the three connectivities in carborane frameworks show different levels of destructive QI, which leads to highly suppressed single‐molecule conductance in para‐ and meta‐connected carboranes. The investigation of QI into carboranes provides a promising platform to fabricate molecular electronic devices based on multicenter bonds.

Multicenter‐Bond‐Based Quantum Interference in Charge Transport Through Single‐Molecule Carborane Junctions

“…As shown in Figure 2b, o-CB shows ac onductance cloud around 10 À3.0 G 0 with 0.43 nm relative stretching displacement (inset in Figure 2b). In comparison with o-CB,t he 2D conductance-displacement histograms of m-CB and p-CB show longer stretching distances,w ith circa 1.22 nm and circa 1.33 nm junction lengths for m-CB (Figure 2c)a nd p-CB (Figure 2d), respectively.T he stretching distances are consistent with the SÀS distances in the crystal structures (Supporting Information, Figure S19), [17] suggesting that the formation of carborane junctions is via the two À SMe anchors.A lthough the conductances of molecular wires attenuate with the increase of junction lengths, [1a, 18] the longer junction of p-CB still shows higher conductance than m-CB,i ndicating that the connectivity of electrodes into the carborane framework plays av ital role to tune the charge transport through singlemolecule carborane junctions.I ti si ntriguing that the singlemolecule-scale characterization of the conductivity of carboranes is also correlated to the conductivity of carborane-based conducting polymers,inwhich the polymer films constructed from ortho-connected carborane monomer shows higher conductivity than that based on meta-a nd para-connected carborane monomers. In comparison with o-CB,t he 2D conductance-displacement histograms of m-CB and p-CB show longer stretching distances,w ith circa 1.22 nm and circa 1.33 nm junction lengths for m-CB (Figure 2c)a nd p-CB (Figure 2d), respectively.T he stretching distances are consistent with the SÀS distances in the crystal structures (Supporting Information, Figure S19), [17] suggesting that the formation of carborane junctions is via the two À SMe anchors.A lthough the conductances of molecular wires attenuate with the increase of junction lengths, [1a, 18] the longer junction of p-CB still shows higher conductance than m-CB,i ndicating that the connectivity of electrodes into the carborane framework plays av ital role to tune the charge transport through singlemolecule carborane junctions.I ti si ntriguing that the singlemolecule-scale characterization of the conductivity of carboranes is also correlated to the conductivity of carborane-based conducting polymers,inwhich the polymer films constructed from ortho-connected carborane monomer shows higher conductivity than that based on meta-a nd para-connected carborane monomers.…”

“…After calibration by adding 0.50 nm snap-back distance of gold electrodes, [16] the actual junction length for the o-CB is about 0.93 nm. In comparison with o-CB,t he 2D conductance-displacement histograms of m-CB and p-CB show longer stretching distances,w ith circa 1.22 nm and circa 1.33 nm junction lengths for m-CB (Figure 2c)a nd p-CB (Figure 2d), respectively.T he stretching distances are consistent with the SÀS distances in the crystal structures (Supporting Information, Figure S19), [17] suggesting that the formation of carborane junctions is via the two À SMe anchors.A lthough the conductances of molecular wires attenuate with the increase of junction lengths, [1a, 18] the longer junction of p-CB still shows higher conductance than m-CB,i ndicating that the connectivity of electrodes into the carborane framework plays av ital role to tune the charge transport through singlemolecule carborane junctions.I ti si ntriguing that the singlemolecule-scale characterization of the conductivity of carboranes is also correlated to the conductivity of carborane-based conducting polymers,inwhich the polymer films constructed from ortho-connected carborane monomer shows higher conductivity than that based on meta-a nd para-connected carborane monomers. [19] Asimilar trend was also observed in the phosphorescent organic light-emitting diodes in that the meta-a nd para-connected carboranes act as an insulated spacer, while the charge transfer through ortho-connected carborane is much more efficient.…”

A Chemically Soldered Polyoxometalate Single‐Molecule Transistor

“…Stemming from the original studies employing simple aliphatic and conjugated rod-like oligoaryl moieties, [14] it is now common to read reports on molecular wires incorporating fused polyaromatic/heterocyclic systems, [15,16] supramolecular complexes, [17,18] organometallic centres, [19,20] and, more recently,p olynuclear clusters. [21][22][23][24] Thel atter are particularly interesting from at echnology point of view for their electronic behaviour,a st he presence of multiple metallic centres imparts stability to several oxidation states,w ith the cluster accommodating large charge variations.T hese electron-sink phenomena [25,26] (that is,c lusters are able to accept and release electrons reversibly without significant changes in their structure) resulted in great interest in their synthesis and applications,e specially as electroactive materials to be deployed in electronic, sensing,and catalytic devices. [27,28] Polyoxometalates (POMs) are ac lass of cluster compounds noteworthy for their stability and rich electrochemistry.…”

Single-Molecule Junction Origami

“…PSD has been used in the literature to characterize through-space coupling in molecular junctions where charge transport does not follow the chemical bond organization, but travels through eigenchannels opened by non-bonding interactions. [13,14,34,35] When charge transport is purely through-bond, the (the integral of the power spectral density) scales approximately with , while the scaling increases when charge transport has a through-space character, to approach the value found for pure through-space tunnelling of . When is normalized by the average conductance ( / ) it is therefore generally found to be insensitive to the junction conductance for through-bond coupling.…”