Single-Molecule Charge Transport in Discrete, π-Stacked Pyridinium Dimers

Abstract

Charge transport through π-conjugated molecules plays an essential role in biochemical redox processes and energy storage applications. In this work, we show that molecular charge transport is greatly enhanced upon dimerization of certain pyridinium molecules in the cavity of a synthetic host (cucurbit[8]uril, CB[8]). Stable, homoternary complexes are formed between pyridinium molecules and CB[8] with high binding affinity, resulting in an offset stacked geometry of two pyridiniums inside the host cavity. The charge transport properties of free and dimerized pyridiniums are characterized using a scanning tunneling microscope-break junction (STM-BJ) technique. Remarkably, our results show that π-stacked methylated pyridinium dimers exhibit a 10-fold increase in molecular conductance compared to isolated, single pyridinium molecules. Control experiments using CB[8] homologues show that the synthetic host primarily serves to facilitate dimer formation and plays a minimal role on molecular conductance. Molecular modeling is used to determine transmission functions for molecular junctions using the non-equilibrium Green’s function formalism, with simulations showing good agreement with experimental results. Density functional theory (DFT) reveals that the closely stacked pyridinium dimer has a reduced energy gap and favorable orbital energy alignment with gold electrodes, thereby resulting in enhanced molecular conductance. Overall, this work demonstrates that supramolecular assembly provides a useful approach to understand intermolecular charge transport in π-stacked molecules.