Capturing the Occurrence of Single- and Multi-Component Ion-Pairing in Electroactive Self-Assembled Monolayers via Spectroscopic Snapshots

Surface-bound redox-active ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAM) are typically employed in studies that use electrolytes containing a single anion. On the other hand, here we shed light on anion mixtures and investigate how electrolytes containing an anion mixture influence the structural and electronic properties of the Fc SAM electrode–monolayer–electrolyte interface. Using an approach that combines an electrochemical cell with X-ray and ultraviolet photoelectron spectroscopy (EC-XPS/UPS), we probe Fc SAM emersed from electrolytes containing binary anion mixtures of ClO4–/BF4– and TFSI–/BF4–. Our results show that ClO4–/BF4– mixtures favor the formation of Fc+–ClO4– ion-pairs over the formation of Fc+–BF4– ion-pairs owing to the lower solvation energy (hydrophobicity) of ClO4–. Unexpectedly, however, in TFSI–/BF4– mixtures, there is a wide cyclic voltammogram featuring 3 peaks, which can be spectroscopically delineated to show potential-dependent and stepwise formation of multi-component ion-pairing beginning with Fc+–TFSI– at lower potentials followed by Fc+–BF4– ion-pairs at increasing anodic potentials, attributed to steric constraints of the large TFSI– anion. Our work shows an alternative way to modulate via rational electrolyte design to enable multi-component ion-pairing which has relevance in extending the functionality of molecular devices in applications such as molecular logic gates and micromechanical shapeshifting.