Ion-gated electron transfer in self-assembled monolayer films

The preparation and electrochemical characterization of self-assembled monolayers (SAMs) of azobenzenebutanethiols Id and ferrocenylazobenzenebutanethiols 2d on Au are reported. Adsorption of these molecules onto Au surfaces has been verified by X-ray photoelectron spectroscopy and reflectance infrared spectroscopy. Optical ellipsometry, capacitance measurements, and cyclic voltammetry indicate that azobenzene-terminated adsorbate molecules form densely packed SAMs on Au(111). Reduction of the azobenzene group in Id or 2d in an aprotic medium results in the formation of an azobenzene radical anion. However, SAMs of Id and 2d exhibit almost no electrochemical accessibility for their azobenzene groups, even though a SAM of 2d exhibits complete electrochemical accessibility for its outer layer of ferrocenyl groups. The azobenzene electrochemical inaccessibility is due to the densely packed structures of these SAMs and their ability to prohibit the incorporation of charge-compensating cations upon their reduction. Addition of free volume to a film of 2d by coadsorption with ethanethiol or more efficient use of the existing free volume in a full monolayer by using smaller charge-compensating cations such as H+ or Li+ results in greater azobenzene accessibility. Therefore, electron transfer processes between the electrode surface and the redox-active azobenzene centers within the film can be gated by controlling charge-compensating cation size and concentration and/or film structure. This gating behavior constitutes a supramolecular response in SAMs as it is a collective property of the film and nor a property of the molecules that comprise the film. Reduction of the azobenzene in the SAM in the presence of H+ results in hydrazobenzene formation, which has been verified by Raman spectroelectrochemistry. The potential for the latter reduction is dependent upon pH. A three-case model has been proposed to,describe the ion-gating behavior of a SAM of 2d.