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

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).