Length Dependent Conduction of Polyynes: Searching for the Limit of the Tunneling Regime

Rigid, conjugated molecules are excellent candidates as molecular wires since they can achieve full extension between electrodes while maintaining conjugation. Molecular design plays a significant role, through minimizing the accessible pi surface to minimize interactions between the bridging wire and the electrode. Polyynes are archetypal molecular wires that feature both a rigid and a thin molecular framework with a cross-section of a single carbon atom. Understanding the behavior of polyynes in molecular junctions is essential for testing models of length versus electron transport. We report construction of molecular junctions using polyynes with a well-defined length up to ca. 5 nm in devices characterized by scanning tunneling microscopy break junctions (STM-BJ). The polyynes, Py**[n] (n = 4, 6, 8, 10, 12, 16), are endcapped with pyridyl groups, and we demonstrate good agreement between the length of the molecular junction and the calculated molecular length, with an average discrepancy of just 0.1 nm. This highlights the power of STM-BJ experiments to accurately determine molecular length. The range of molecular lengths studied, from 1.8 to 4.8 Å, mark this as the most accurate determination of β in polyynes to date (β = 2.2 nm–1). We have applied a model based solely on the single and triple bond lengths to interpret these β-values, which predicts β = 1.9 nm–1, consistent with the experimental value. This model also confirms that electronic coupling in polyynes is unaffected by the rotation about the single bonds. At all molecular lengths, we observe conductance in tunneling regime continues due to the long effective conjugation length of polyynes.