Photon-Gated Electron Transfer in Two-Component Self-Assembled Monolayers

Abstract

The preparation and characterization of a photoactive, two-component self-assembled monolayer (SAM) consisting of 99:1 cis-p-(C₆H₅)NN(C₆H₄)O(CH₂)₁₁SH) (1) and trans-p,p-Fc(C₆H₄)NN(C₆H₄)(CH₂)₄SH (Fc = C₅H₄FeC₅H₅; 2) are reported. With this SAM, electron transfer between solution ferrocyanide and the Au substrate is forced to occur through mediating ferrocenyl sites in the film, resulting in a diode-like response. However, by photochemically converting large “footprint” cis-1 to the smaller footprint trans-1, the free volume within the film is increased, thereby allowing for direct electron transfer between the solution ferrocyanide and the electrode surface and a normal electrochemical response (i.e., i_pa = i_pc). All states of the film have been characterized by cyclic voltammetry, FTIR spectroscopy, and differential capacitance measurements. All data from these techniques are consistent with the conclusion that cis−trans azobenzene isomerization within the monolayer results in increased film porosity, which reduces the film's ability to block the access of solution redox-active species to the underlying Au electrode surface. This novel two-component structure establishes the concept for a new type of molecule-based electronic device, a “photoswitchable diode” that is capable of amplifying the signal associated with a photodriven event via an electrochemical response. With such a system, a small number of photons can release a relatively large number of electrons from the solution electron reservoir. Significantly, this system shows how one can regulate electron-transfer events involving SAMs through photochemical control over film structure and free volume.