Unconventional Single-Molecule Conductance Behavior for a New Heterocyclic Anchoring Group: Pyrazolyl

Abstract

Electrical conductance across a molecular junction is strongly determined by the anchoring group of the molecule. Here we highlight the unusual behavior of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene that exhibits unconventional junction current versus junction-stretching distance curves, which are peak-shaped and feature two conducting states of 2.3 × 10 G and 3.4 × 10 G. A combination of theory and experiments is used to understand the conductance of single-molecule junctions featuring this new anchoring group, i.e., pyrazolyl. These results demonstrate that the pyrazolyl moiety changes its protonation state and contact binding during junction evolution and that it also binds in either end-on or facial geometries with gold contacts. The pyrazolyl moiety holds general interest as a contacting group, because this linkage leads to a strong double anchoring of the molecule to the gold electrode, resulting in enhanced conductance values.

Tuning the thermoelectrical properties of anthracene-based self-assembled monolayers

“…This method of controlling the thermoelectric properties of molecular lms opens the way to new design strategies for functional ultra-thin-lm materials and electronic building blocks for future integrated circuits. In particular, it means that strategies for optimising single-molecule transport properties using room-temperature quantum interference [5][6][7][8][9][10][11][12][13][14][15][16] can be utilised in SAMs, both by engineering QI within the core structure, 17 by varying the anchor group as shown here, and possibly by electrochemical means for achieving active control.…”

“…Molecular electronic devices have the potential to deliver logic gates, sensors, memories and thermoelectric energy harvesters with ultra-low power requirements and sub-10 nm device footprints [1][2][3][4] Single-molecule electronic junctions have been investigated intensively over the past few years, because their roomtemperature electrical conductance is controlled by quantum interference (QI). [5][6][7][8][9][10][11][12][13][14][15][16][17] As highlighted in recent reviews, [18][19][20][21] Seebeck coefficients of single molecules can be controlled by varying the anchor groups, [22][23][24] which bind them to external electrodes; Seebeck coefficients of single molecules with thiol anchor groups are found to be positive, while those with pyridyl anchor groups are measured to be negative, with room-temperature magnitudes, which are typically a few tens of mV K À1 at room temperature.…”

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

“…Additionally, the use of multidentate or multipodal anchor structures to improve the robustness and stability of the molecular junction has also been explored. For instance, using acids [116,118,279] or diacids [37], methyl esters [121], tetrathiofulvalenes [280], a 2-aminopyridine group [127], a pyrazole moiety [81], or the tripodal head group 2,6-bis((methylthio)methyl)pyridine [126].…”

“…Several strategies have been employed for this purpose, including the use of multidentate anchor groups [57,81], and multipodal platforms [82]; (iii) Compromise between the mechanical stability and electronic coupling since a too strong interface coupling may result in the loss of electrical functionalities and also in poor gating effects in three-terminal structures. This compromise could be reached by the insertion of a spacer between the anchoring group and the conjugated skeleton within the molecular structure [13]; (iv) Control of the geometry of molecules to avoid fluctuations, due to different orientations (and then a different distance for the electrons tunneling between the electrodes), which can be achieved by an exhaustive control of the surface coverage and molecular packing density [83].…”

“…Other relevant properties in multipodal platforms have also been found, including attenuation of tunnel currents more effectively than do the corresponding monodentate SAMs, which may be useful in future applications for gate dielectric modification in organic thin-film devices [101]. More examples of both multidentate [74,81,195] and multipodal [196][197][198] platforms have been studied at the single molecular level, though the extension of these investigations to large-area ensembles is a topic of growing interest.…”