Host-guest interaction at a self-assembled monolayer/solution interface: An electrochemical analysis of the inclusion of 11-(ferrocenylcarbonyloxy)undecanethiol by cyclodextrins

A host-guest equilibrium involving the redox guest 11-(ferrocenylcarbonyloxy)undecanethiol (FcSH) in a self-assembled monolayer on gold electrode surfaces and a beta-cyclodextrin (beta-CD) host in solution is investigated. The effect of varying the ratio of FcSH to a diluent (n-decanethiol) in the self-assembly solution on the electrochemical characteristics of the ferrocene/ferricenium redox process and the inclusion properties of FcSH self-assembled monolayers (FcSH-SAMs) is examined using cyclic voltammetry and chronoamperometry. alpha- and gamma-cyclodextrin have a negligible effect on both the thermodynamics and kinetics of the redox reaction. The electrochemical and ion-pairing behavior of FcSH-SAMs in solutions containing beta-CD suggests that an inclusion complex is formed and that the included form of the ferrocenyl group is electroinactive in the potential window investigated. Inclusion results in a decrease in cyclic voltammetric peak currents (surface coverage of electroactive FcSH) and a slight increase in the standard electron transfer rate constant for the ferrocene/ferricenium redox process. The decrease in surface coverage of electroactive FcSH upon addition of beta-CD exhibits a response similar to that expected for a Langmuir adsorption isotherm. Stability constants for the inclusion complexes of beta-CD and the ferrocenyl or ferricenium forms of an FcSH-SAMs, containing a FcSH surface coverage of 5.2 x 10(-11) mol cm(-2), have been evaluated as 3.6 (0.4) x 10(4) and 3.2 (0.4) x 10(4), respectively. The stability constants thus evaluated were shown to be dependent on the surface coverage of FcSH in the FcSH-SAMs.