ION-GATED ELECTRON-TRANSFER IN SELF-ASSEMBLED MONOLAYER FILMS

The preparation and electrochemical characterization of self-assembled monolayers (SAMs) of azobenzenebutanethiols Id and ferrocenylazobenze nebutanethiols 2d on Au are reported. Adsorption of these molecules on to Au surfaces has been verified by X-ray photoelectron spectroscopy a nd reflectance infrared spectroscopy. Optical ellipsometry, capacitanc e measurements, and cyclic voltammetry indicate that azobenzene-termin ated adsorbate molecules form densely packed SAMs on Au(111). Reductio n 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 azobenzen e groups, even though a SAM of 2d exhibits complete electrochemical ac cessibility for its outer layer of ferrocenyl groups. The azobenzene e lectrochemical inaccessibility is due to the densely packed structures of these SAMs and their ability to prohibit the incorporation of char ge-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 char ge-compensating cations such as H+ or Li+ results in greater azobenzen e accessibility. Therefore, electron transfer processes between the el ectrode surface and the redox-active azobenzene centers within the fil m can be gated by controlling charge-compensating cation size and conc entration and/or film structure. This gating behavior constitutes a su pramolecular response in SAMs as it is a collective property of the fi lm and nor a property of the molecules that comprise the film. Reducti on of the azobenzene in the SAM in the presence of H+ results in hydra zobenzene formation, which has been verified by Raman spectroelectroch emistry. The potential for the latter reduction is dependent upon pH. A three-case model has been proposed to,describe the ion-gating behavi or of a SAM of 2d.