Electronic-State Changes of Ferrocene-Terminated Self-Assembled Monolayers Induced by Molecularly Thin Ionic Liquid Layers: A Combined Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Ultraviolet Photoelectron Spectroscopy Study

Abstract

Ferrocene-terminated self-assembled monolayers (SAMs) have been studied in the last quarter-century to determine the relationship between their electrochemical properties and microscopic structures as a prototypical chemically modified electrode. Although electronic structures are directly related to the electrochemical properties, knowledge of such a relation concerning ferrocene-terminated SAMs is extremely limited. In this study, we deposited a small amount of ionic liquid onto three types of ferrocene-terminated SAMs possessing different ionization energies and performed X-ray and ultraviolet photoelectron spectroscopies to elucidate the changes in the electronic structure resulting from the adsorption of the ionic liquid. For a low-ionization-energy system, spontaneous oxidation of the ferrocene moieties occurred, while their electrochemical activities were retained; for other systems, only minor changes were observed. This result indicates that the interaction between the ionic liquid and ferrocene-terminated SAMs facilitates electron transfer when the energy difference between the highest occupied molecular orbital of the ferrocene moieties and the Fermi level of the electrode becomes small.