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

We explore the redox-dependent electronic and structural changes of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed in aqueous solution. By exploiting X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we can electrochemically control the Fc SAMs and spectroscopically probe the induced changes with the ferrocene/ferrocenium (Fc/Fc⁺) redox center (Fe oxidation state), formation of 1:1 Fc⁺-ClO₄^- ion pairs, molecular orientation, and monolayer thickness. We further find the insignificant involvement of interfacial water in the Fc SAMs irrespective of redox state. Electrolyte dependencies could be identified with 0.1 M NaClO₄ and HClO₄ when probing partially oxidized Fc/Fc⁺ SAMs. Corroborating the occurrence of electrochemically induced oxidation, EC-UPS shows that oxidation to Fc⁺ is accompanied by a shift of the highest occupied molecular orbital toward higher binding energy. The oxidation to Fc⁺ is also met with an increase in work function ascribed to the induced negative interfacial dipole caused by the presence of Fc⁺-ClO₄^- ion pairs along with a contribution from the reorientation of the Fc⁺ SAMs. The reversibility of our observations is confirmed upon conversion from Fc⁺ back to the neutral Fc. The approach shown here is beneficial for a broad range of redox-responsive systems to aid in the elucidation of structure-function relationships.