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

Abstract

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 microm wide, spaced by a 10-microm 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)6(2+) 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 x 10(9) 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)3(III+/II+) monolayer system result in a larger electronic coupling and/or smaller reorganization energy.

Electrochemical time-of-flight responses at double-band generator-collector devices under pulsed conditions

“…In each case, TOF procedure involves three sequential components: (i) creation of entities from a generator source, (ii) their transit through the length of the flight path and (iii) their detection. In electrochemistry, the Electrochemical time-of-flight (ETOF) methodology was introduced initially by Murray and coworkers [1] and has been used in the investigation of diffusing redox species as well as for diffusive electron transport in media of interest such as in polymer films [1][2][3], aqueous electrolyte solutions [4][5][6][7], polyacrylate gels [8] and monolayers at the air/water interface [9]. This involves generally the generation in amperometric mode of the product of an electrochemically reversible couple at a microelectrode thus termed generator.…”

“…ETOF techniques typically use lithographically fabricated microelectrode devices or interdigitated arrays composed at least of one pair of parallel microbands in a side-by-side arrangement. In the literature, the distance d is defined either as the gap distance between generator and collector electrodes [1,3,7,9], either as the distance from the center of the generator electrode to the nearest edge of the collector electrode [2,4,5,8]. Contrary to rotating ring-disk experiments [15], forced flow conditions are not necessary, because lateral diffusion over small distances between collector and generator maintain spontaneously high fluxes between the microelectrodes [11,12].…”

“…Contrary to rotating ring-disk experiments [15], forced flow conditions are not necessary, because lateral diffusion over small distances between collector and generator maintain spontaneously high fluxes between the microelectrodes [11,12]. The shape of the collector response is dependent on the type of the potential-time variation applied to the generator electrode [1,3,5,8,9] as well as of the device geometry [11,12]. In classical ETOF experiments, the potential applied to the generator electrode is held for a sufficiently long duration of time to allow the collector current to reach its steady-state plateau value.…”

“…This complication is over-amplified for redox species of low diffusion coefficients. To circumvent these difficulties, a method has been developed that does not require the measurement of the true steady-state collector currents but is based on the transit times of two species of different diffusion coefficients [9]. Finally, upon using pulsed potential experiments at the generator, the current at the collector passes through a maximum.…”

Synthesis and study of the light absorbing, redox and photophysical properties of Ru(II) and Os(II) complexes of 4,7-diphenyl-1,10-phenanthroline containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine

“…The Ph 2 phen ligand can also provide enhanced interaction of metal complexes with DNA [15][16][17]. Osmium complexes of Ph 2 phen are studied as oxygen sensors [18] as well as being incorporated into monolayers to observe electron transfer dynamics [19][20][21]. The bridging ligand dpp, Fig.…”