Journal of the American Chemical Society, Vol.119, No.44, 10763-10773, 1997
Redox-active ferrocenyl dendrimers: Thermodynamics and kinetics of adsorption, in-situ electrochemical quartz crystal microbalance study of the redox process and tapping mode AFM imaging
The thermodynamics and kinetics' of adsorption of the redox-active dendrimers diaminobutane-dend-(NHCOFc)(8), (dendrimer-Fc(8)), diaminobutane-dend-(NHCOFc)(64), (dendrimer-Fc(64)), diaminobutane-dend-(NHCOFc)(32), (dendrimer-Fc(32)), and diaminobutane-dend-(NHCOFc)64, (dendrimer-Fc(64)) containing 8, 16, 32, and 64 ferrocenyl moieties on the periphery, respectively, have been studied using electrochemical and electrochemical quartz crystal microbalance (EQCM) techniques. All of these materials adsorb onto a Pt electrode surface. At an applied potential of 0.0 V (vs SSCE), where the ferrocenyl sites are in the reduced form and the dendrimers are neutral, the adsorption thermodynamics are well-characterized by the Langmuir adsorption isotherm. The kinetics of adsorption were activation-controlled and the rate constant decreased with decreasing size of the dendrimer. Potential scanning past +0.60 V, where the ferrocenyl sites are oxidized, gave rise to the electrodeposition of multilayer equivalents of the dendrimers. The additional material gradually desorbed upon rereduction, so only a monolayer equivalent remained on the electrode surface. Impedance analysis of the resonator response suggests that at multilayer equivalent coverages, the adsorbed dendrimers do not behave as rigid films and that incorporation of significant amounts of solvent and/or salt accompany the adsorption of these materials at such high coverages. On the other hand, at monolayer coverages, the adsorbed films appear to exhibit rigid film behavior. Using tapping mode atomic force microscopy we have been able to image dendrimer-Fc(64) adsorbed onto a Pt(111) surface. The images reveal that the apparent size of the dendrimer adsorbed on the surface is significantly larger than estimated values based on calculations, which is ascribed to a flattening of the dendrimer upon adsorption.