Chemistry
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Raman and IR Spectroelectrochemical Methods as Tools to Analyze Conjugated Organic Compounds
Chapters
Summary October 12th, 2018
A protocol of step-by-step Raman and IR spectroelectrochemical analysis is presented.
Transcript
Ramen and IR spectroelectrochemistry can be used for advanced characterization of structural changes in electroconductive components acquiring during an electrochemical process and, for instance, the study of reactions mechanism. The main advantage is the possibility of observing the signal arising from the intermediate products of the electrochemical process, or investigating process, in which the products can not be separated. Prior to performing spectroelectrochemical studies, use cyclic voltammetry to determine the potential ranges of the redox processes of interest.
To begin the procedure, rinse an indium tin oxide coated quartz working electrode with deionized water. Sonicate the quartz ito electrode in actone, and isopropyl alcohol, in sequence, for 15 minutes each. While the ito electrode is being sonicated, burn the working area of a platinum wire, or spiral counter-electrode, in the flame of a high-temperature gas torch, just until the wire becomes red.
Allow the wire to cool to room temperature in ambient air. Remove the reference electrode from its storage electrolyte solution, and rinse it three times with the solvent to be used during the measurements. Clean the appropriate spectroelectrochemical vessel with ethanol, isopropyl alcohol, or acetone, and allow it to dry.
Clean the other components of the cell with acetone, and allow them to air dry for at least one minute. Once sonication of the ito electrode is complete, allow it to air dry. Then prepare at least 10 milliliters of a supporting electrolyte solution with a concentration at least 100 times greater than the target anolyte concentration.
If applicable for the experiment, prepare two milliliters of a one millimolar anolyte solution in the electrolyte. Bubble inert gas through the anolyte or electrolyte solution for 5 minutes at a moderate gas flow, so that only small bubbles appear at the solution's surface. Afterwards, proceed to the chosen spectroelectrochemistry procedure.
When ready to begin the IR sturdy, assemble the clean IR spectroelectrochemical cell. Ensure that the electrodes are not in contact with each other. Fill the assembled cell with pure solvent and check it for leaks.
Adjust the assembly as needed to ensure that the cell is leak-free. When finished, remove the solvent. Next, turn on the IR spectrometer, and open the instrument software.
Fill the cell with the anolyte solution, ensuring that the areas of the electrodes that will be irradiated by the incident beam are submerged, or solution was drawn by the capillary forces between the ATR crystal and the attached electrode. Then, load the cell into the instrument. Connect the electrodes to a pontentiostat, being careful not to allow the electrodes or the connectors to touch each other.
Fill in the IR spectrum acquisition parameters, and register a background spectrum of the solution with no potential applied. Then, apply a potential of zero volts to the working electrode. Acquire and save an initial IR spectrum.
Then increase the applied potential by 100 millivolts, wait for 15 seconds, and acquire another IR spectrum. Repeat this process until spectra have been acquired for the entire potential range of interest. To evaluate the reversability of the redox process of interest, return the applied potential on the initial value in 100 millivolt steps, and acquire a spectrum for each step.
Otherwise, return to the initial value in a single step, and acquire only one spectrum. Next, subtract the initial spectrum from every other spectrum to obtain the differential spectra. Then disconnect the cell, and transfer the solution to an electrochemical cell, per CV.Prior to the ramen spectroelectrochemical study, coat a clean wire, or plate electrode, with the anolyte, by electropolymerization or dip casting.
When ready to being the study, turn on the ramen spectrometer, laser, and control software. Assemble the spectroelectrochemical cell, being careful to keep the electrodes separated. Position the anolyte-coated working electrode as close as possible to the cell wall, facing the incoming incident beam, while leaving space for a solution to flow between it and the wall.
Then, add about two milliliters of the electrolyte or anolyte solution to the cell, so that all electrodes are immersed in solution. Place the cell in the spectrometer and connect the electrodes to a potentiostat, being careful to keep the electrodes from touching each other. Focus the spectrometer camera on the film deposited on the working electrode.
Then, close the spectrometer cover. Select the laser type and grading appropriate for the sample. Focus the laser beam on the working electrode surface so that the sharpest possible dot or line appears.
Set the spectral range, time of illumination, number of repetitions, and laser power in the spectrometer software, appropriately for the sample. Use low laser power to avoid destruction of the sample. Acquire an initial ramen spectrum.
Adjust the data collection parameters and repeat the scan as needed until a good initial spectrum has been acquired. Then, apply a starting potential of zero volts to the working electrode. Collect a spectrum and save it with a descriptive file name.
Then, increase the applied potential by 100 millivolts, wait 15 seconds, and collect another spectrum. Continue acquiring and saving spectra in this way, throughout the desired range of applied potential, then acquire another spectrum at the initial potential to evaluate the reversability of the redox process of interest. Afterwards, correct the potential values, using CV, as previously described.
Differential IR spectra, taken during electropolymerization of a triphenylamine-based hydrazone, derivative with reactive vinyl groups, showed increased transmittance at about 16 hundred inverse centimeters, indicating the loss of some of the monomers conjugated double-bonds during electropolymerization. The changes in transmittance between 675 and 900 inverse centimeters indicated the loss of IR signal, from monosubstituted benzene, and a new IR signal from disubstituted benzene. This suggested an electropolymerization mechanism involving a reaction between the vinyl groups, and the monosubstituted benzene rings.
Ramen spectroscopy of a polyaniline film deposited on a gold electrode, electrographed it with anolyne, showed bands characteristic of the leukoemeraldine form at the starting point potential of zero millivolts. When the applied potential increased beyond the first redox couple of polyaniline, bands indicating a transition to the semiquinone polyaniline structure were observed. Increasing the applied potential beyond the second redox couple resulted in increased intensity of the band's characteristic of the deprotenated quinoid ring, and decreased intensity of a band characteristic of the semi-quinone radical.
This indicated that the polyaniline had transitioned to the pernigraniline form. This techniques paved the way for researchers in the field of organic electronics to explore structural changes acquiring during redox processes, estimate the quality of individual layers, investigate system durability during multiple oxidation reduction cycles, or study diffusion in multilayer structures. While attempting this procedure, remember that some molecular vibrations may be active only in IR or ramen spectroscopy, making them complementary to each other.
The best results are obtained when the changes involve groups active in the technique being used. Don't forget that working with organic solvents can be extremely hazardous. Appropriate precautions should always be taken during this procedure.
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