Host-Guest Interaction at Molecular Interfaces: Cucurbit[7]uril as a Sensitive Probe of Structural Heterogeneity in Ferrocenyl Self-Assembled Monolayers on Gold

Herein, we combine host-guest recognition chemistry and electrochemical analysis to demonstrate that the nanometer-size, supramolecular hosts can be adapted as sensitive probes for structural heterogeneity in organized molecular assemblies on a surface. In particular, we carried out thorough cyclic voltammetric (CV) studies to evaluate the binding of cucurbit[7]uril on mixed ferrocenylundecanethiolate/n-alkanethiolate self-assembled monolayers (SAMs) on gold (FcC11S-/CmS-Au) prepared with different methods (coadsorption vs postassembly exchanges) and with varied diluting n-alkanethiols. On the basis of the distinct CV responses of CB[7]@Fc complex and free Fc on the SAM surfaces, we were able to determine the conversion ratio from Fc to CB[7]@Fc, a direct indication of its overall density and uniformity. We have shown that the FcC11S-/C8S-Au prepared by coadsorption in a binary solution with low mole fraction of FcC11SH (5%) and by exchanging preassembled C8S-Au SAM with FcC11SH for a short time (1 min) has the "ideal" structure with isolated and uniformly distributed Fc groups on the surface. In contrast, with similar Fc surface coverage, the FcC11S-/C8S-Au prepared by exchanging FcC11S-Au with C8SH for a prolonged time (20 h) has clustered and nonuniformly distributed Fc groups at the surface. While consistent with previous observations based on conventional electrochemical or microscopic studies, the present finding expands the capability of host-guest chemistry as a new tool to probe the structures of organized molecular assemblies at the nanometer scale.