Density Functional Theory Investigations of Ferrocene-Terminated Self-Assembled Monolayers: Electronic State Changes Induced by Electric Dipole Field of Coadsorbed Species

Abstract

Ferrocene-terminated self-assembled monolayers (SAMs) have been widely studied in the past quarter century to reveal the electrochemical properties of chemically modified electrodes. It has been well-known that the formal potential of the system strongly depends on the local environment of the ferrocene moiety. Although electronic states of ferroceneterminated SAMs should directly affect the electrochemical properties, knowledge concerning electronic structures with respect to different local environment is very limited. In this study, we performed density functional theory calculations of ferrocene-terminated SAMs with different coadsorbed species to reveal the relationship between the electronic structures and the local environment of ferrocene moieties. Depending on the local electrostatic potential, density of states derived from the highest-occupied molecular orbital (HOMO) and its vicinities (HOMO−1 and HOMO−2) of the ferrocene moiety were found to largely shift with respect to the electrode Fermi level up to 0.75 eV. This result leads to a novel strategy for designing sophisticated chemically modified electrodes where the electronic properties are electrostatically regulated by coadsorbed inert molecules.

Discerning the Redox-Dependent Electronic and Interfacial Structures in Electroactive Self-Assembled Monolayers

“…This peak is predominantly ascribed to the six electrons in the 3d orbitals of Fe, which form the HOMO of Fc, giving (a 1g ) 2 (e 2g ) 4 as the electronic configuration. 12,58 Following oxidation to Fc + , the HOMO is expected to shift to toward higher binding energy with Fc + possessing the electronic configuration (a 1g ) 2 (e 2g ) 3 . 45 It is noteworthy that Yokota et al observed nearly matching shifts in the Fe 2p 3/2 peak from XPS and the HOMO peak from UPS of Fc SAMs with different electron donating groups ascribed to the Fc HOMO being localized with the Fe atom.…”

“…Ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAMs) on Au are an ideal model system owing to their practical significance and versatility with structural modifications. − The corresponding ferrocene/ferrocenium (Fc/Fc + ) redox couple consists of single electron transfer and ion pairing reactions with anions (X – ) as follows: …”

Single-Molecule Observation of Redox Reactions Enabled by Rigid and Isolated Tripodal Molecules

“…Previous electrochemical studies have utilized Fc-terminated alkanethiol self-assembled monolayers (SAMs) where the strong chemical interaction between the sulfur and gold anchors the molecule onto the electrode surface. ,,,,,,,,,,− However, it is known that Fc-terminated alkanethiol SAMs undergo structural fluctuations and changes in SAM orientation during redox reactions, ,− ,, making it difficult to probe the unperturbed electronic state during redox reactions with EC-STM. Furthermore, in order to investigate the unperturbed redox reaction of Fc, it is desirable to strategically place Fc moieties solvated with electrolyte solution, rather than surrounded by other organic molecules.…”

In situ and ex situ approaches for molecular scale understanding of electrochemical interfaces

“…The 1.6 eV peak in the pristine sample has been attributed to the HOMO states of ferrocene spatially localized on Fe 2+ ion. [53][54][55][56] This peak disappears when the electrode is emersed at the higher potential (the Fe 3+ state). This means that as the molecule is oxidized, the ferrocene becomes positively charged and the entire orbitals shift toward the higher binding energy side.…”