Potential-Modulated Total Internal Reflection Fluorescence for Measurement of the Electron Transfer Kinetics of Submonolayers on Optically Transparent Electrodes

Abstract

An electroreflectance method to determine the electron transfer rate constant of a film of redox-active chromophores immobilized on an optically transparent electrode when the surface coverage of the film is very low (<0.1 monolayer) is described herein. The method, potential-modulated total internal reflection fluorescence (PM-TIRF) spectroscopy, is a fluorescence version of potential-modulated attenuated total reflection (PM-ATR) spectroscopy that is applicable when the immobilized chromophores are luminescent. The method was tested using perylene diimide (PDI) molecules functionalized with p-phenylene phosphonic acid (PA) moieties that bind strongly to indium−tin oxide (ITO). Conditions to prepare PDI-phenyl-PA films that exhibit absorbance and fluorescence spectra characteristic of monomeric (i.e., nonaggregated) molecules were identified; the electrochemical surface coverage was approximately 0.03 monolayer. The tilt angle of the long axis of the PDI molecular plane is 58°r elative to the ITO surface normal, 25°greater than the tilt angle of aggregated PDI-phenyl-PA films, which have a surface coverage of approximately one monolayer. The more in-plane orientation of monomeric films is likely due to the absence of cofacial π−π interactions present in aggregated films and possibly a difference in PA-ITO binding modes. The electron transfer rate constant (k s,opt ) of monomeric PDI-phenyl-PA films was determined using PM-TIRF and compared with PM-ATR results obtained for aggregated films. For PDI monomers, k s,opt = 3.8 × 10 3 s −1 , which is about 3.7-fold less than k s,opt for aggregated films. The slower kinetics are attributed to the absence of electron self-exchange between monomeric PDI molecules. Differences in the electroactivity of the binding sites on the ITO electrode surface also may play a role. This is the first demonstration of PM-TIRF for determining electron transfer rate constants at an electrode/organic film interface.

Molecular Orientation of −PO3H2 and −COOH Functionalized Dyes on TiO2, Al2O3, ZrO2, and ITO: A Comparative Study

“…A similar trend was reported for perylenediimide- p -phenylene-PA adsorbed on TFD ITO; at monolayer surface coverage, the tilt angle was 33°, whereas at ca. 3% of monolayer coverage, the angle was 58° . This result was influenced by the extent of perylene diimide (PDI) aggregationthe monolayer film was highly aggregated, while the 3% film was monomeric.…”

Impedance Analysis of Electrochemical Systems

“…Finally, to complete the electrical/electrochemical analysis of the surface, it could be interesting to couple impedance to other measurement techniques, as has already been done with electrohydrodynamic impedance, − pressure impedance spectroscopy as applied to fuel cells, ,, thermoelectrochemical impedance spectroscopy, , intensity-modulated photocurrent spectroscopy, and electrogravimetry impedance spectroscopy. − Indeed, if we consider local electrochemical impedance spectroscopy (LEIS), which is currently limited by the size of the probe and the duration of the measurement, coupling with optical measurements to measure locally the surface reactivity can be an elegant and effective approach, even if it remains limited to only a few groups over the world. − In the same way, the coupling with inductively coupled plasma techniques, recently proposed by Ogle’s group, , also seems promising since it combines chemical and electrical analyses and can provide a complex transfer function that is rich in information.…”

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films

“…However, when the immobilized submonolayer is luminescent, fluorescence detection can enhance the sensitivity. Saavedra and co-workers further extended the PM-ATR concept to demonstrate the first example of potential-modulated total internal reflection fluorescence (PM-TIRF) to determine electron transfer rate constant for a submonolayer film of a redox-active chromophore perylene diimide functionalized with a phosphonic acid tethered to an ITO surface . With only ∼3% of a surface monomer tagged with the redox-active probe, these researchers were able to obtain an electron transfer rate constant to the isolated monomeric redox species on the surface.…”

“…Saavedra and co-workers further extended the PM-ATR concept to demonstrate the first example of potentialmodulated total internal reflection fluorescence (PM-TIRF) to determine electron transfer rate constant for a submonolayer film of a redox-active chromophore perylene diimide functionalized with a phosphonic acid tethered to an ITO surface. 127 With only ∼3% of a surface monomer tagged with the redox-active probe, these researchers were able to obtain an electron transfer rate constant to the isolated monomeric redox species on the surface. This value was almost 4 orders of magnitude lower than that measured for a film comprised of aggregated but otherwise identical redox species using PM-ATR.…”