Orientational control of quantum interference in ferrocene single-molecule junctions

Abstract

In single-molecule junctions, quantum interference (QI) effects manifest even at room temperature and can be explained by simple quantum circuit rules (QCR), and a rather intuitive magic ratio (MR), theory. These rules characterise how individual moieties contribute to the overall electrical conductance (G), of a molecule and how the overall G can change when the connectivities between different moieties is varied. Here we examine the electrical conductance of a single-ferrocene junction when the two metal electrodes connect to both upper and lower cyclopentadienyl (CP) rings and compare this with the conductance when both electrodes are contacted to only the upper CP ring. In the case of the former, the angle of rotation θ between the upper and lower rings could be changed by varying the distance between the electrodes. The main aim of our investigation is to determine how QI within the ferrocene core is affected by the length of linker groups, which connect the core to electrodes. We find that when θ = 0, short and long molecules exhibit destructive QI (DQI) features within the HOMO–LUMO gap, whereas as θ is increased, the DQI is alleviated. However, DQI within the HOMO–LUMO gap is alleviated at entirely different rotation angles of θ >20° for the molecule with longer linkers, compared to >60° for the shorter molecule. This shows that interference patterns within the ferrocene core are not simply a property of the core alone, but are a holistic property of the molecule as a whole. We investigated the Seebeck coefficients S of these molecules and found that S of the longer molecules can reach 250 μV K⁻¹, which is significantly higher that the Seebeck coefficients of the shorter molecules.