Single-Molecule Conductance of Functionalized Oligoynes: Length Dependence and Junction Evolution

Abstract

We report a combined experimental and theoretical investigation of the length dependence and anchor group dependence of the electrical conductance of a series of oligoyne molecular wires in single-molecule junctions with gold contacts. Experimentally, we focus on the synthesis and properties of diaryloligoynes with n = 1, 2, and 4 triple bonds and the anchor dihydrobenzo[b]thiophene (BT). For comparison, we also explored the aurophilic anchor group cyano (CN), amino (NH2), thiol (SH), and 4-pyridyl (PY). Scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques are employed to investigate single-molecule conductance characteristics. The BT moiety is superior as compared to traditional anchoring groups investigated so far. BT-terminated oligoynes display a 100% probability of junction formation and possess conductance values which are the highest of the oligoynes studied and, moreover, are higher than other conjugated molecular wires of similar length. Density functional theory (DFT)-based calculations are reported for oligoynes with n = 1-4 triple bonds. Complete conductance traces and conductance distributions are computed for each family of molecules. The sliding of the anchor groups leads to oscillations in both the electrical conductance and the binding energies of the studied molecular wires. In agreement with experimental results, BT-terminated oligoynes are predicted to have a high electrical conductance. The experimental attenuation constants βH range between 1.7 nm(-1) (CN) and 3.2 nm(-1) (SH) and show the following trend: βH(CN) < βH(NH2) < βH(BT) < βH(PY) ≈ βH(SH). DFT-based calculations yield lower values, which range between 0.4 nm(-1) (CN) and 2.2 nm(-1) (PY).

A quantum circuit rule for interference effects in single-molecule electrical junctions

“…by z exp. ¼ Dz þ Dz corr , where the correction Dz corr ¼ 0.5 ± 0.1 nm accounts for the snap-back of the electrodes upon breaking of the gold-gold atomic contact 39 . For this molecule, the log(G/G 0 ) versus Dz stretching traces possess well-defined plateaus in the range of log(G/G 0 ) around À 4.5, which we assign to the conductance of single-molecule junctions.…”

“…To elucidate the measured conductance trends, we performed a density functional theory (DFT)-based optimization of each molecule using SIESTA 42,43 and carried out electron transport calculations using the approach discussed in our previous papers 30,39 . Unlike the p-p-p molecule, the other molecules possess anchoring nitrogen atoms located at meta or ortho positions within the terminal rings that do not naturally bind to planar electrode surfaces.…”

“…For this reason, we also carried out BJ simulations using the approach discussed in our previous papers 30,39 . The geometrically optimized molecules were inserted between two opposing 35-atom (111) directed pyramids with four different tip separations.…”

“…A value of E F ¼ À 0.65 eV, relative to the bare DFT Fermi energy (E F DFT ) was chosen to account for the expected LUMO level shift, as discussed in ref. 39, where the Fermi energy shift was adjusted to match measured and computed conductance for the pyridine-terminated oligoyne molecules.…”

Solvent Dependence of the Single Molecule Conductance of Oligoyne-Based Molecular Wires

“…19 More direct assessment of the electrical properties of oligoynes can be achieved by wiring them into electrical junctions with metal contacts. 4,20 Here we report conductance measurements of novel molecular wires Me 3 Si-(C≡C) n -SiMe 3 (n = 2, 3, 4, 5; Scheme 1), using STM-based techniques. In particular the influence of the solvent on conductance and -values has been investigated, and complemented by DFT computations of charge transport through the molecular bridge.…”