Reversible Switching of Molecular Conductance in Viologens is Controlled by the Electrochemical Environment

Charge transport in electrochemical energy-storage systems critically relies on supporting electrolytes to maintain ionic strength and solution conductivity. Despite recent progress, it is not fully understood how the solvation environment affects molecular charge transport of redox-active species near electrode interfaces. In this work, we characterize the charge-transport properties of bipyridinium molecules in a series of different supporting electrolyte and counterion environments using a combination of experiments and computational modeling. Interestingly, our results show that molecular charge transport in viologens critically depends on the chemical identity of counterions and the solvation environment. Using an electrochemical scanning tunneling microscope-break junction (ECSTM-BJ) instrument, we observe a large and reversible 10-fold enhancement in molecular conductance upon electrochemical reduction of the viologen redox pair (V^2+/+) to the radical cationic state in the electrolytic solution. Density functional theory (DFT) simulations show that charge transport is enhanced due to molecular conformational changes and planarization resulting from interactions with different counterions, which ultimately leads to enhanced charge transport in the reduced state. Overall, this work highlights the role of the counterion species on electrochemical charge transport in redox-active molecules that underpin the design of new energy-storage systems or programmable molecular electronic devices.