2D Electrochemical Time of Flight and Its Application in the Measurements of the Kinetics of Lateral Electron Hopping in Monolayer Films at the Air/Water Interface

A 2D electrochemical time-of-flight (ETOF) method was developed to measure diffusion constants of lateral mobility of amphiphiles and lateral electron hopping in Langmuir monolayers at the air/water interface. Photolithographically fabricated generator-collector ETOF devices featured two parallel gold microelectrodes (7 mm in length, 40 mu m wide, spaced by a 10-mu m gap). In 2D ETOF measurements, such a device is touching the water surface where the generator and collector electrodes function as a collinear pair of line microelectrodes. Bulk measurements, with a generator-collector device submerged in an electrolyte solution, were carried out to calibrate the devices by relating the transit times to the known D values of Ru(NH3)62+ in a series of solutions of different viscosity adjusted with sucrose. A new method to define and to measure transit times in the step mode ETOF experiments was developed that requires only the pseudo-steady-state values of the collector current. Reliability of the 2D ETOF technique was established by investigating lateral diffusion of an amphiphilic tetradecane TEMPO derivative for which the D values were also measured by 2D voltammetry. Combination of 2D ETOF and 2D voltammetry allows one to independently measure diffusion coefficients and concentrations of redox species. This advantageous feature was then used to reevaluate kinetics of lateral electron hopping in Os (DPP)3(ClO4)2 (DPP, 4,7-diphenyl-1,10-phenanthroline) solid monolayers on the water surface. The true rate constant of electron self-exchange, kex = 1.0 × 109 M-1 s-1, was obtained. The fact that the latter is more than 1 order of magnitude larger than its value obtained in a homogeneous acetonitrile solution suggests that the structure and locale of the Os(DPP)3III+/II+ monolayer system result in a larger electronic coupling and/or smaller reorganization energy.