Structure-Reactivity Relationships Applied to Diazonium-Based Surface Functionalization: Toward Tunable Redox Bicomponent Monolayers

The precise engineering of surface-bound organic layers remains a central challenge in materials chemistry, particularly for the construction of mixed monolayers with tunable composition. While the electroreduction of diazonium salts produces robust, covalently anchored films, its inherent limitations - poor control over surface coverage and the lack of dynamic molecular exchange - frequently result in uncontrolled multilayer growth, complicating compositional tuning. In this study, we demonstrate that rational molecular design, specifically the incorporation of extended alkyl spacers, overcomes these limitations by enabling controlled co-immobilization of functional and diluent species and by promoting confinement of film growth to the monolayer regime. Using TEMPO as a model redox-active motif, we compare mixed layers derived from diazonium precursors with and without C12 spacers. Electrochemical characterization reveals that the presence of extended linkers enhances molecular organization, suppresses overgrowth, and enables predictable tuning of redox unit surface density simply by adjusting the composition of the functionalization solution. These results establish clear design principles for the controlled assembly of multifunctional organic interfaces and provide new insight into structure-reactivity relationships in diazonium-based surface modification.