Mechanism and release rates of surface confined cyclodextrin guests

Abstract

The dissociation of a cobalt bisdiphenylterpyridine, [Co(biptpy)₂]²⁺, guest at mixed (γ-CD-(py)₂)-alkanethiol layers (where γ-CD-(py)₂ is di-6^A, 6^B- deoxy-6-(4-pyridylmethyl)amino- γ-cyclodextrin) formed on platinum electrodes is reported. Cyclic voltammetry (CV) shows reversible one-electron surface confined waves consistent with the Co^2/3+ couple bound at the interface. The quantity of [Co(biptpy)₂]³⁺ reduced is found to be dependent on the scan rate employed, with greater amounts at higher scan rates. This behavior is in contrast to the CD guest ferrocene, which upon oxidation to the ferrocenium ion shows little charge associated with reduction even at elevated scan rates. Chronocoulometry was conducted to systematically vary the time spent oxidizing [Co(biptpy)₂]²⁺ and to measure the resulting charge associated with the reduction of [Co(biptpy)₂]³⁺. It is determined experimentally that as the pulse width increases, i.e. greater time spent in the oxidizing region, the amount of charge needed to reduce [Co(biptpy)₂]³⁺ decreases dramatically. This decrease, along with the CV data, suggests strongly that the [Co(biptpy)₂]³⁺ dissociates from the cavity. Significantly, this dissociation of the interfacial host–guest complex occurs on a much longer timescale (the order of seconds) compared to the oxidation of [Co(biptpy)₂]²⁺ to [Co(biptpy)₂]³⁺, which has been measured using high speed chronoamperometry to occur with a rate contant, k⁰, of approximately 10³ s⁻¹. The comparison of the timescale for dissociation of the interfacial complex and for electron transfer signifies that the electron transfer step occurs before dissociation, i.e. dissociation via an EC mechanism. The dissociation mechanism of [Co(biptpy)₂]³⁺ is contrasted with that of the ferrocene/ferrocenium couple.