Low variability of single-molecule conductance assisted by bulky metal–molecule contacts

Abstract

A detailed study of the trimethylsilylethynyl moiety, –CCSiMe3 (TMSE), as an anchoring group, using a combination of experiment and DFT is presented.

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

“…14,[95][96][97] This is due to the fact that the CuCSiMe 3 group makes effective electrical contact to defect sites on the gold electrode surfaces in a restricted range of geometries, limited in part due to the steric bulk of the ancillary methyl groups. 67 Thus, in even π-rich oligophenylene ethynylene based molecules, the large footprint and limited range of conducting configurations that are available to CuCSiMe 3 contacts gives rise to conductance histograms featuring only a single conductance peak without complications from alternative conductive molecular orientations or contacts within the junction 14,14,67,96 As will be seen in the discussion that follows, the relatively weak nature of the CuCSiMe 3 -gold interaction (estimated at ca. 0.4-0.7 eV) 14,67 permits competitive, if adventitious, interactions between the STM tip and other regions of the molecule.…”

“…14,67 It is only these latter situations that give rise to CuCSiMe 3 -contacted junctions of sufficient conductance to be measured. However, the potential for molecules to bind in a number of distinct configurations within a molecular junction, each offering a distinct contact and hence conductance value, has been recognized.…”

“…The trimethylsilylethynyl-gold binding energy has been estimated to fall between −0.40 and −0.74 eV over a range of gold-surface features. 67 As the phosphine-contacted junction is evolved to simulate the pulling of the junction, the binding energy naturally decreases from −3.46 eV to −0.12 eV (Table S2 †). It is therefore likely that I(s) junctions formed as the STM tip approaches and withdraws from the surface involve at least one PPh 3 -based contact.…”

“…65,66 Other strategies to minimise the range of conductance signatures observed include the use of bulky anchor groups such as trimethylsilylethynyl, which give rise only to junctions of appreciable conductance in a restricted range of configurations, simplifying conductance profiles and allowing more precise assessment of low conductance contacts. 14,67 However, even when well-defined contacts are placed within a molecule, the contact of the electrodes to different parts of the compound, particularly in the case of long, conjugated molecules, can result in complex electrical behavior. Examples include the potentiometric-like response of thiol-terminated oligoenes, where electrical contact can be made at either the thiol or oligoene π-system, 68 and related observations in oligoyne-based 69 and other molecular wires 70,71 in which the π-system of the molecular backbone interacts with the electrodes at shorter electrode separations leading to enhanced conductance (short circuits).…”

Syntheses and molecular structures of trans-bis(alkynyl) tetrakis-triethylphosphite ruthenium complexes

“…More recently, it was shown that conductance histograms generated from trans-[Ru(C≡CC 6 H 4 -4-C≡CSiMe 3 ) 2 (dppe) 2 ] and Me 3 SiC≡CC 6 H 4 -4-C≡CC 6 H 4 -4-C≡CC 6 H 4 -4-C≡CSiMe 3 , by the I(s) method [24] were simplified in comparison with the thiolatecontacted compounds, with trimethylsilylethynyl binding groups giving only a single, narrow conductance peak as the steric bulk of the termini restricts the range of available, conductive contacts. [25] A higher conductance value was again obtained for the organometallic complex (5.10 ± 0.99 10 -5 G 0 ) compared with the organic analogue. [26] Efforts are now being expended on the preparation and measurement of longer, multi-metallic organometallic wires of ruthenium [27][28][29] and other metals [30][31][32] which have clearly demonstrated the potential for organometallic compounds of this trans-bis(alkynyl) type to find application in molecular electronic platforms.…”