Single-Molecule and π–π-Stacked Dimer Electron Transport in Carbazole and Folded Bicarbazole Derivatives in Molecular Junctions

The present work provides insight into how the conformations of flexible molecules can impact their single-molecule conductance. Six thiol-substituted carbazole-based molecules are synthesized and characterized. In four, two carbazole groups are joined by a linking group (1,3-propane or meta-xylene) while the remaining two are model monocarbazoles. Using a combination of X-ray photoelectron spectroscopy (XPS), single-molecule conductance measurements, and density functional theory (DFT) calculations, we demonstrate that upon transitioning from a self-assembled monolayer (SAM) to a single-molecule junction, the intermolecular interactions give way to intramolecular interactions. This resulted in the flexible bicarbazole molecular wire switching conductance mechanisms, which occurred primarily via the covalent conjugated aromatic part of the molecule in the SAM to one including conductance via noncovalent π–π interactions in the single-molecule junction.