Electron-Transfer Rates in Host Guest Assemblies at beta-Cyclodextrin Monolayers

The effect of the distance between a beta-cydodextrin (beta CD) host core and a conductive substrate on the electron-transfer rate of complexed guests as well as of free-diffusing electrochemically active probes has been studied. First we have evaluated a set of short-tethered beta CD adsorbates bearing different anchoring groups in order to get a reliable platform for the study of short-distance electron transfer. An electrochemically active trivalent guest was immobilized on these host monolayers in a selective and reversible manner, providing information about the packing density. Iodine- and nitrile-functionalized beta CD monolayers gave coverages close to maximum packing. Electron transfer in the presence of Fe(CN)(6)(3-/4-) studied by impedance spectroscopy revealed that the electron transfer of the diffusing probe was 3 orders of magnitude faster than when the beta CD cores were separated from the surface by undecyl chains. When an electrochemically active guest was immobilized on the surface, electron-transfer rate measurements by cyclic voltammetry and capacitance spectroscopy showed differences of up to a factor of 8 for different beta CD monolayers. These results suggest that increasing the distance between the beta CD core and the underlying conductive substrate leads to a diminishing of the electron-transfer rate.