Name: using cyclic voltammetry, uv-vis-nir, and epr spectroelectrochemistry to analyze organic compounds
Uploaded: 2018-10-18T12:30:00
Duration: 11 min 44 s
Description: Watch this Scientific Journal Video about Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds at JoVE.com

The authors gratefully acknowledge the financial support of &#34;Excilight&#34; project &#34;Donor-Acceptor Light Emitting Exciplexes as Materials for Easy-to-tailor Ultra-efficient OLED Lightning&#34; (H2020-MSCA-ITN-2015/674990) financed by Marie Sk&#322;odowska-Curie Actions within the framework programme for research and innovations &#34;Horizon-2020&#34;.

The authors have nothing to disclose.

Electrochemical and spectroelectrochemical techniques have no limitations; one can analyze both solid state and liquid solutions in a broad range of temperature and other conditions with these techniques. The important thing in all of these cases is that compounds/materials are analyzed under the applied potential, replicating real world conditions for working organic electronics devices. The only difference is that in electrochemistry, the formation of charge carriers, is observed.
The methods presented here show the usefulness of the analysis of charged carriers generated in organic compounds that correlate with their applicability in organic electronics. Moreover, the electrochemical and spectroelectrochemical techniques are cheaper and less demanding than that of typical methods used in charge carrier analysis, but there are some critical steps and modifications to the protocol that are needed depending on the obtained results.
During electrochemical characterization, always start with a particular concentration. If a set of the compounds is compared, then all materials need to have the same molar concentration. The best is to start with 1 mM concentration and 50 mV/s scan rate as indicated in the protocol in this study, but it is good to know the concentration of the sample on the observed electrochemical behavior. Always try to measure at least three scans. The first two scans are usually different because the starting conditions (equilibrium) are different. The second and the third scans should be the same. If the second and third scans are the same, then there are probably no side reactions observed in this system (Figure 2a). In an oxidation process, a new peak at a lower potential appears showing that the conductive material was deposited on the WE18,19,24,25,29,30,31,32. If the height of the lower peak increases in successive scans, then probably the conjugated polymer was deposited18,19,24,25,29,30,31,32. If all the currents decrease in successive scans, then the nonconductive product of degradation was deposited on the electrode. If a very small peak is observed before the main oxidation or reduction peak (especially for polymers), then this is probably charge-trapping process19,23,31,34. If a very sharp dedoping peak of oxidation or reduction is observed, then this is probably caused by the decomposition of crystalline structures on an electrode formed through the electrocrystallization process during oxidation35.
Always check the behavior of the test compound before, during, and after redox peaks. It means that at least three CV scans should be registered: with upper (in the case oxidation) or lower vertex potential lower or higher, respectively, then the potential of peak maximum, with upper or lower vertex potential set to exactly on the peak maximum and with vertex potentials higher (oxidation) and lower (reduction) than potential of the peak maximum. The observed process may vary and sometimes two processes may be observed under one peak theoretically. Always compare the collected cyclic voltammograms of the electrolyte (step 2.6), the ferrocene (step 2.9), the compound (step 2.13), and the ferrocene with compound (step 2.19); there are several issues to be taken into account.
Always compare the CV signals of the electrolyte and the test compound, if any signals from the electrolyte is visible on the cyclic voltammogram of the measured compound, then the electrolyte must be changed because its electrochemical window is too low, or the electrolyte is contaminated. If the signal (redox couple) of ferrocene (step 2.9) and ferrocene with compound (step 2.19) are at the same position, then everything is performed properly. If the peaks are shifted between each other, then check the RE and repeat the measurement. If the signal (oxidation, reduction, or redox couple potential) of the test compound with added ferrocene (step 2.19) is at a higher potential than that of the pure compound (step 2.13), then consider the values (oxidation, reduction, or redox couple potential) from the cyclic voltammogram of the pure compound. The shift is caused by the higher amount of ferrocene in the solution. When two oxidation processes are observed, the first process (oxidation or reduction) which is always on the WE may affect the active surface; this may cause an increase in the oxidation potential of the second process (Figure 9).

Worldwide, researchers are continually searching for new organic materials that can be used in organic electronics with desirable performance or stability, which drops due to extended use. In the case of organic devices, it is important to understand the behavior of the charge carrier to fully know the rules driving the device behavior. Analysis of the effect of the molecular structure on the generation of the charge carrier and the dynamics and maintenance of the balance of injected charge carriers, both positive (holes) and negative (electrons), is crucial to improve the efficiency and stability of the organic devices. This ensures the effective recombination of these individual charges and consequently significantly improves the photoluminescence efficiency of the organic light emitting diodes (OLEDs)1,2. For organic photovoltaics (OPVs)3,4 as well as organic field effect transistors (OFETs)5,6, it is necessary to have materials with high charge carrier mobility. In addition to the analysis of charge carriers, several important parameters of organic electroactive materials help in predicting where the material could be used: ionization potential (IP), electron affinity (EA) energy levels, and band-gap between them7,8,9,10.
In this work, we present a method for the efficient measurement of cyclic voltammetry (CV) that can be used in the analysis of all types of electroactive materials. This technique provides information about redox properties, the doping/dedoping mechanism, the stability, the conversion and storage of energy, etc. It also allows for the estimation of electron affinity and ionization energy of the test compounds in a much cheaper and faster way compared to other high vacuum methods. The aforementioned parameters correlate with the energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
The method presented in this article can be used to analyze all types of conjugated compounds such as those with delocalized &#960;-electrons in their structures. Conjugated compounds may be small molecules with large polymeric chains. Small molecules can also be monomers; during the initial reaction (photochemical, electrochemical, or chemical) monomers can form polymers. In OLED application, the energy level values are necessary to enable the use of the correct host for the emitter in a thermally activated delayed fluorescence (TADF) guest-host system or to decide with which compounds the exciplex donor-acceptor layer could be formed and what additional layers (electron transporting layer (ETL), hole transporting layer (HTL), electron blocking layer (EBL), and hole blocking layer (HBL)) will be necessary to synthesize stable efficiently charged balanced OLED devices11,12,13,14,15,16,17. Additional electrochemical measurements allow the investigation of possible side reactions during the process of degradation of the active layer and the formation of low mobile charge carriers (bipolarons)18,19,20,21,22.
Coupling electrochemical and spectroelectrochemical methods allows for easy, accurate, and reliable determination of the degree of oxidation or reduction of conjugated compounds and their degradation potential, which is crucial for stability23,24,25,26,27,28. Ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopy coupled with electrochemistry can characterize the fundamental chromatic properties of all new conjugated compounds, such as the changing of the absorption band during doping18,19,20,21,22,23,24,25,26,27,28,29,30.
In a study related to the doping mechanism, it is important to define the type of charge carriers. In this process, two classes of charged quasiparticles take part, one with uncompensated spin (polarons) and the second being diamagnetic (bipolarons); electron paramagnetic resonance (EPR) spectroscopy provides invaluable assistance, which directly allows one to observe and track changes in populations of paramagnetic polarons29,30,31,32. In small molecules, it is difficult to form bipolarons, but these molecules can be quite conjugated and have bipolaron-inducing properties; it is important to check if and at which potential polarons and bipolarons are formed in the structure. Bipolarons are at least one order lower in mobility than that of polarons; therefore, if bipolarons are formed in working devices, then it could lead to an unbalanced ratio of the charge carriers, which would result in high current and overheating of the OLED device or may well be the centers of degradation33.
The method of measurement proposed in this study is cheap and faster and allows for the determination of the most valuable operative parameters for a large number of electroactive materials without the need for special devices that are based on newly synthesized materials to check its performance. By applying electrochemistry and spectroelectrochemistry, it is possible to select one material that is really promising from hundreds of new materials. In addition, it is possible to obtain detailed information regarding the processes of doping and their effects on the chemical structure of the test conjugated systems using electrochemical and spectroelectrochemical methods, which allows constructing more efficient organic electronics devices.

<table>
	<tbody>
		<tr>
			<td>Potentiostat</td>
			<td>Metrohm</td>
			<td>Autolab PGSTAT100</td>
			<td></td>
		</tr>
		<tr>
			<td>EPR</td>
			<td>JEOL</td>
			<td>JES-FA200</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-Vis detector</td>
			<td>Oceanoptics</td>
			<td>QE6500</td>
			<td></td>
		</tr>
		<tr>
			<td>NIR detector</td>
			<td>Oceanoptics</td>
			<td>NIRQuest</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichloromethane (DCM)</td>
			<td>Sigma-Aldrich</td>
			<td>106048</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetrabutylammonium tetrafluoroborate (Bu4NBF4)</td>
			<td>Sigma-Aldrich</td>
			<td>86896</td>
			<td></td>
		</tr>
		<tr>
			<td>2-propanol, 99.9%</td>
			<td>Sigma-Aldrich</td>
			<td>675431</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone, 99.9%</td>
			<td>Sigma-Aldrich</td>
			<td>439126</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic Bath</td>
			<td>Elma</td>
			<td>S30H</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetrahydrofuran &#62;99.9%</td>
			<td>Sigma-Aldrich</td>
			<td>401757</td>
			<td></td>
		</tr>
		<tr>
			<td>ferrocene &#62;98%</td>
			<td>Sigma-Aldrich</td>
			<td>F408</td>
			<td></td>
		</tr>
		<tr>
			<td>decamethylferrocene &#62;97%</td>
			<td>Sigma-Aldrich</td>
			<td>378542</td>
			<td></td>
		</tr>
	</tbody>
</table>

using cyclic voltammetry, uv-vis-nir, and epr spectroelectrochemistry to analyze organic compounds

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance (EPR) or ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopies, we obtain useful information such as electron affinity, ionization potential, band-gap energies, the type of charge carriers, and degradation information that can be used to synthesize stable organic electronic devices. In this study, we present electrochemical and spectroelectrochemical methods to analyze the processes occurring in active layers of an organic device as well as the generated charge carriers.

The most common application of CV analysis is the estimation of IP and EA. Even though there are a couple ways to obtain CV data, it is strongly recommended to calculate them based on the onset of redox peaks (Figure 3). This approach allows unifying the calculation procedure. Not all of the tested materials undergo reversible oxidation/reduction processes (Figure 3); in such situations, it is not possible to calculate based on average potentials (average from potentials correspond to a maximum of cathodic-reduction and anodic-oxidation peaks). However, it is almost always possible to run the tangent to a peak as shown in Figure 3. With the intersection with the background line and use of Equations 1 and 2, the IP and EA values are estimated as 5.35 eV and &#8722;2.90 eV, respectively. There are also several different scales used to evaluate IP and EA based on CV measurements. The most commonly used scales for organic materials are &#8722;4.8 and &#8722;5.1 eV as equivalent to 0.00 V versus Normal Hydrogen Electrode (NHE). However, all of the scales are only an approximation; remember this when comparing different results. The crucial thing is to state what parameters were considered for calculations. In this case, the value &#8722;5.1 eV has been chosen as it should correspond to the formal potential of the ferrocene redox couple; in the Fermi scale, it is 0.40 V versus Saturated Calomel Electrode (SCE) in acetonitrile, which is in agreement with the previous measurement.
There are many articles published regarding the analysis of CV. Herein, we show when the process is not going to be as it is expected. The analysis is based on thiophene derivative: NtVTh (structure shown in Figure 4), which undergoes degradation upon oxidation (Figure 5).
NtVTh has two oxidation potentials: the first at 0.70 V and the second at 0.84 V (Figure 5). During the first cycle, the reduction peak is not observed, indicating an irreversible process. Electrochemical characteristics of NtVTh show that polymerization does not occur and after the first oxidation potential, some electro-inactive layer of reaction products deposits at the electrode surface, thus hindering the polymerization process. What is visible is the reaction with the radical cation on the vinyl bond, where the molecule is losing its conjugation and form dimers on the electrode.
While it is difficult to extract information about charge carriers from the CV, it is possible to distinguish between polarons and bipolarons when supported by a UV-Vis-NIR spectrometer. The neutral poly(OiPrThEE) was characterized by two wide absorption &#960;|&#960;* transitions bands with peaks maxima at &#955;max1 = 363 nm and &#955;max2 = 488 nm, related to the aromatic form of the undoped polythiophene derivative. During the oxidative doping, new polaronic and bipolaronic bands are generated. The UV-Vis spectrum obtained during the poly(OiPrThEE) oxidation revealed the diminishing the neutral polymer absorption band (300-550 nm) (Figure 6) together with the formation of a new absorption band (550-950 nm) of the radical cations of bithiophene and p-phenylenevinylene with maxima at 692 nm. The isosbestic point of the oxidation process was located at 604 nm. The bipolaronic band appeared between the 950 nm and 1700 nm with a maximum located at 1438 nm.
EPR spectroscopy is the technique that detects materials with an unpaired electron, this includes organic radicals39. There are several parameters that could be extracted from EPR spectra, but one of the most interesting is to estimate where the radicals are localized. Electrons, similar to protons, possess spin. By placing an electron in an external magnetic field, this spin can be split two ways: parallel and antiparallel to the magnetic field, giving two energy levels. This phenomenon is known as the Zeeman effect40,41. In case of organic radicals, the unpaired electron interacts not only with the external magnetic field but also with magnetic nuclei (nuclei which have a nonzero spin; I&#8800;0). A number of degenerate energy levels are equal to 2I + 1, where I is the spin quantum number of the nucleus with which the unpaired electron interacts42. The interaction of the unpaired electron with a larger number of magnetic nuclei leads to further splitting of the energy levels and to hyperfine structure of EPR spectra registration43 (Figure 7).
For molecules where the unpaired electron interacts with an even larger number of nuclei, the individual spectral line could overlap, which results in registration of a single, broad signal44,45,46 (Figure 8). This is typical for conjugated polymers, where the generated radical ion during a redox process is delocalized47,48.
The combination of EPR spectroscopy with electrochemical methods allows the characterization of charge carriers (radical ion) generated during the redox process as well as the determination of the mechanism of these processes49,50. If well-resolved (peaks are separated; not in the form of one broad peak) spectra are registered, as in the case of electrochemical reduction of s-tetrazine derivative (Figure 7), then the analysis of the hyperfine structure of spectra leads to conclusions about the localization of unpaired electron. One way to analyze this kind of spectra is to conduct simulation with special software and to fit simulated spectra with the experimental one51. This is especially helpful when the hyperfine structure is complex due to the interaction of the unpaired electron with large numbers of protons. In case of the s-tetrazine derivative shown in Figure 7, simulation of EPR spectra (red line) indicates the interaction of the unpaired electron with four nitrogen atoms of s-tetrazine.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56656/56656fig1.jpg" /> 
Figure 1: Electrochemical and spectroelectrochemical cells used for measurements. The figure presents the scheme setup of electrochemical/spectroelectrochemical cells using cyclic voltammetry, ultraviolet-visible and near-infrared (UV-Vis-NIR), and electron paramagnetic resonance (EPR) spectroelectrochemical measurements. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56656/56656fig2.jpg" /> 
Figure 2: Cyclic voltammetry (CV) of properly measured compound COPO1 (a) and the CV with an unstable reference potential dibenzothiophene-S,S-dioxide with ferrocene (b)52. The figure shows two cyclic voltammograms. (a) presents correctly registered CV and (b) shows a voltammogram registered using a reference electrode with no stable potential. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56656/56656fig3.jpg" /> 
Figure 3: Cyclic voltammetry (CV) of compound COPO1 in a wide range of potentials. Estimation of the onset potentials for EA and IP calculations of the COPO1 compounds52. EA=&#8722;2.90 eV and IP = 5.35 eV. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56656/56656fig4.jpg" /> 
Figure 4. EPR six spectral line of manganese standard. The paramagnetic signal of manganese used for calibration of the signal shift. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56656/56656fig5.jpg" /> 
Figure 5: Cyclic Voltammetry (CV) of compound NtVTh. Cyclic voltammograms of 15 mM NtVTh in 0.1 M Bu4NBF4/CH3CN and relative to ferrocene standard presenting the degradation process involved on working electrode. Scan rate 0.05 V/s: (a) was taken in the range &#8722;1.4 V to 0.8 V, and (b) in the range &#8722;1.4 V to 0.9 V. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/56656/56656fig6.jpg" /> 
Figure 6: Ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroelectrochemistry of poly(OiPrThEE) derivative. UV-Vis-NIR spectra presenting the evolution of absorption band through the generation of charge carriers on polymer structure. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/56656/56656fig7.jpg" /> 
Figure 7: EPR spectroelectrochemical analysis of tetrazine derivative. (a) Structure of s-tetrazine derivative; (b) EPR spectra registered during electrochemical reduction of s-tetrazine derivative (black line-experimental and red line simulated spectrum). <a href="https://www.jove.com/files/ftp_upload/56656/56656fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/56656/56656fig8.jpg" /> 
Figure 8: EPR spectroelectrochemical polaron signal of the conjugated polymer. Electron paramagnetic resonance (EPR) spectra registered during the first step of oxidation of conjugated polymer (EPR spectra of polaron species). <a href="https://www.jove.com/files/ftp_upload/56656/56656fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/56656/56656fig9.jpg" /> 
Figure 9: Cyclic voltammetry (CV) of ferrocene and decamethylferrocene. Comparison of two electrochemical standards as pure and as mixture showing the shift of the potential. <a href="https://www.jove.com/files/ftp_upload/56656/56656fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds at JoVE.com

Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

In this article, we describe electrochemical, electron paramagnetic resonance, and ultraviolet-visible and near-infrared spectroelectrochemical methods to analyze organic compounds for application in organic electronics.

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance ...

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