Sterically Controlled Functionalization of Carbon Surfaces with -C6H4CH2X (X = OSO2Me or N3) Groups for Surface Attachment of Redox-Active Molecules

Glassy carbon electrodes were modified by electrochemical reduction of a diazonium molecule ((Pr3SiOCH2C6H4N2+BF4-)-Pr-i) featuring a triisopropylsilyl-protected benzylic hydroxyl group. This electrochemical process introduced a monolayer of (Pr3SiOCH2C6H4)-Pr-i- groups onto the surface of the electrode. The bulky -SiiPr(3) protecting group not only prevents the uncontrolled growth of structurally ill-defined and electronically blocking polyphenylene multilayers, but also separates the phenyl groups in the monolayer. Thus, the void spaces between these aryl units should allow a better accommodation of sizable molecules. Removal of the -SiiPr(3) protecting groups by (Bu4NF)-Bu-n exposed the reactive benzylic hydroxyl functionalities that can undergo further transformations to anchor functional molecules. As an example, redox-active ferrocene molecules were grafted onto the modified electrode via a sequence of mesylation, azidation, and copper-catalyzed [3 + 2] cycloaddition reactions. The presence of ferrocenyl groups on the surface was confirmed by X-ray photoelectron spectroscopic and electrochemical studies. The resulting ferrocene-modified glassy carbon electrode exhibits cyclic voltammograms typical of surface-bound redox active species and remarkable electrochemical stability in an acidic aqueous environment.