Adsorption of hexylferrocene phosphonic acid on indium-tin oxide electrodes. evidence of strong interchain interactions in ferrocene self-assembled monolayers

Hexylferrocene phosphonic acid was adsorbed on ITO and amino-primed ITO electrodes from ethanol. The obtained monolayers are stable in acetonitrile where CV shows the formation of domains of self-interacting and free ferrocene molecules. Distinct phases are observed on bare ITO whereas a single Frumkin isotherm is obtained on amino-primed ITO. Dilution experiments with alkylphosphonic acids CnH2n+1PO(OH)2 (n = 3, 6 and 12) have shown that only the dodecyl chain may compete with the hexylferrocene chain which remains 50 times more strongly bound to the surface. The free energy of self-interaction of the ferrocene moieties in the layers is ca 40 kJ mol-1, i.e. it is equivalent to that of a 15 carbon atom alkyl chain. Lateral interactions among hexylferrocene and dodecyl chains form isolated domains of the components ruling out a real dilution of ferrocenes to single isolated molecules.