Host-guest complexation in self-assembled monolayers: Inclusion of a monolayer-anchored cationic ferrocene-based guest by cyclodextrin hosts

A novel ferrocenyl disulfide of the type omega,omega'-dithiobis[alkyl(dimethyl)(ferrocenylmethyl) ammonium bromide] was synthesized and coassembled with nonanethiol on vapor deposited Au(lll)electrode surfaces. The coassembled ferrocene-based cationic guest monolayer assembly was found to bind with beta- and gamma-cyclodextrins to form stable host-guest complexes. The cyclodextrin hosts are believed to interact with the monolayer assembly in two ways: by inclusion of the ferrocene moiety within the cyclodextrin cavity and by interactions between the counterions (e.g., Br-, PF6-, or ClO4-) and the hydrophilic planes of the cyclodextrin molecules. Interactions between the monolayer and cyclodextrin are manifested primarily by a decrease in the voltammetric oxidation current for the surface-bound ferrocenyl redox centers after exposure to dilute solutions (e.g., 10 mu M) of the cyclodextrin hosts. This decrease in electrochemical activity stems from the restricted access of charge-compensating anions to the ferrocenyl redox center. It is proposed that the unusual stability of the resulting cationic ferrocene-cyclodextrin complex is due to dual occupancy of the cyclodextrin hosts by the surface-bound cationic ferrocene-based guests and their counterions. Supporting evidence for the interaction of cyclodextrin hosts with the ferrocene-based cationic guest monolayer was obtained using infrared reflection-absorption spectroscopy (IRRAS).