Physics and CompSci Research
Analysis of the Conductivity of Thiophene and its Disubstituted Derivatives when Exposed to Various Solvents J. Marin and C. Andrews ABSTRACT A computational experiment was performed in order to analyze the effects of three solvents on the conductivity of disubstituted thiophene derivatives. Thiophenes are often used for their conductivity and transparency, especially as polymer semiconductors. Applications of these polymers include, but are not limited to, solar panel electrodes and touchscreens. Experiments with PEDOT:PSS, proved that doping this polymer in ionic liquids significantly elevated its conductivity. EDOT is one of the monomers included in PEDOT:PSS and provided as basis for this study in which other solvents were tested through computational chemistry calculations. This experiment was conducted through the North Carolina School of Science and Mathematics Computational Chemistry server by utilizing the MOPAC software package with PM3 theory for all calculations. These calculations consisted of a series of geometry optimizations and molecular orbital calculations for thiophene, EDOT, EDST, VDOT, and ProDOT. Experimental data from CCCBDB was compared to computational values obtained on the heat of formation of thiophene in order to find how accurate the data on these molecules would be through MOPAC. Each molecular orbital calculation was ran in three different solvents: water, acetonitrile, and cyclohexane as well as calculations without a solvent for a point of comparison. Conductivity was measured in a qualitative manner based on a standard set by molecular orbitals calculations in no solvent. Several trends were developed based on the data. The polar solvents, water and acetonitrile, raised the band gap by significant quantities, therefore lowering conductivity. The band gap is simply the gap between the HOMO and the LUMO of the molecules in the study. The polar solvents, which were water and acetonitrile, decreased the heat of formation the most when compared to the nonpolar solvent, cyclohexane. The polarity of thiophene caused an increase of interactions with the polar solvents, leading to more change in structure and properties. Since the molecules experienced minor change in cyclohexane, it is believed that thiophene and its derivatives had little interaction with the nonpolar solvent due to their opposite polarity. As observed in this study, polarity was the main factor of influence on conductivity.
Introduction Computational Chemistry provides chemists with the tools to model and predict the structure, properties, and activities of a molecule. Through a variety of methods, computational chemists can perform experiments without having to step foot into a lab. These methods include ab initio, semi-empirical, molecular mechanics, and density functional theory. For this study, semi-empirical methods were utilized due to their faster processing times and ability to calculate the behavior of molecules in a solution. Through this method, core electrons are not taken into account and only calculations of valence electrons are made. In this study, the effects of polar and nonpolar solvents on the molecules EDOT, EDST, VDOT, and ProDOT are analyzed. Each of these molecules are disubstituted thiophene derivatives, meaning that several hydrogen atoms of the base thiophene molecule have been replaced with oxygen or sulfur. Their structures, as seen on Figure 1, are composed of a ring of 4 carbons and 1 sulfur atom. The ring is characterized by double bonds between carbons 2 and 3 as well as carbons 4 and 5. These derivatives are particularly useful as building blocks for π-conjugated
systems such as PEDOT:PSS. Systems such as these are used primarily for their electrochemical properties as conductive polymers[1]. One of the main features of these systems is that they have low energy and are stable, which is caused by delocalized electrons. Doping by oxidation of such conductive polymers removes these delocalized electrons resulting in higher mobility of other electrons in the system.
Figure 1. Structure of a basic thiophene as Seen When Built in WebMO Volume 3 | 2013-2014 | 71
Physics and CompSci Research Thiophene is an aromatic compound. These types of chemical compounds are unsaturated, meaning they have at least one double bond, and are characterized by at least one planar ring structure[2]. The arrangement of the molecules makes them very stable and with low reactivity. One of the uses of thiophenes is in pharmaceuticals. This monomer is very similar to benzene based on several characteristics and properties. In some cases, they may be used interchangeably without much change in activity. The sulfur in thiophene, however, makes it more conductive due to more delocalization of electrons. Changes in thiophene structure such as the ones in the derivatives in this study serve to make the molecule more reactive and conductive, allowing it to have a greater variety of applications. EDOT (3,4-Ethylenedioxythiophene), demonstrated in Figure 2, is the most common monomer found in this study. When used to form the polymer PEDOT, it can be a useful transparent conductor. For example, PEDOT can be found as a component of solar cells. EDOT is a building block known for its high reactivity and low polymerization potential, facilitating the formation of PEDOT[1]. This monomer is also known for its strong donor electron properties and ability to lower band gaps[3]. Often times in material science, EDOT is combined with other molecules to obtain low band gap polymers with desired qualities. Unfortunately, PEDOT has poor solubility, which is a roadblock for the material considering that it is usually applied in the solvent water. This obstacle can be overcome by mixing the ionomer PSS (polystyrene sulfonate) with PEDOT to form PEDOT:PSS, as shown in Figure 3[4].
The mixture of the two ionomers has more conductivity than EDOT and has found itself in many commercial applications. Touchscreens with capacitive sensing utilize a thin film of PEDOT:PSS as a transparent electrode. Other uses involve using PEDOT:PSS coating to increase the efficiency, control, and conductivity of electrodes in biobots[5]. This allows for low power circuits to send short electrical pulses to the antennae of Madagascar Hissing Cockroaches to be able to control their movement. Although PEDOT:PSS has great conductivity, its properties can be enhanced through ionic liquid secondary doping[6]. Ionic liquids are known for their stability and high ionic conductivity. They also have a great affinity for conductive polymers, which is another cause for experimenting with them. When PEDOT:PSS is doped in a mix containing several types of ionic liquids and salts, its conductivity is greatly and permanently increased[7,8]. It is important to keep in mind that PEDOT and PSS must be doped first in a solvent in order to make the polymer and ionic liquids would be present in a secondary doping. After secondary doping, the conductivity of this molecule rivals that of indium tin oxide, another conductor with many commercial uses. In this study, ionic liquid doping served as point of comparison against polar and nonpolar solvents. It is important to keep in mind that doping is a process where a polymer’s conductivity is changed through contact with a dopant, which adds impurities to the polymer[6]. This process creates films of semiconducting and conducting materials after being in contact with a dopant in a liquid solution. Just testing the stated molecules in a solvent may differ slightly from the doping methodology, but differences processes will be ignored for the purposes of this study.
Figure 3. 2D Structure of PEDOT:PSS Chains[7].
Figure 2. 3D Structure of EDOT as Seen When Built in WebMO
72 | 2013-2014 | Volume 3
EDST (3,4-Ethylenedisulfanylthiophene), shown below in Figure 4, is very similar to EDOT structurally. The key difference between the two molecules is the presence of sulfur rather than the sulfur and oxygen combination in EDOT[9]. EDST does have a slightly lower oxidation potential than EDOT. That being said, EDST is slightly less
Physics and CompSci Research likely to gain electrons than EDOT. In terms of the band gap, poly-EDST has a higher band gap than PEDOT. The band gap is defined as the difference between the frontier orbitals of the molecules[10]. It provides insight into the conductivity of the molecule through the conclusion that the lower the band gap is, the greater the conductivity of the molecule. Like EDOT, it can be mixed with other molecules to have more favorable qualities. When EDST is mixed with EDOT, the new mixture, EDOT-EDST, has a lower band gap than PEDOT and poly-EDST. Like mentioned before, this lower band gap shows that EDOTEDST is more conductive than PEDOT and poly-EDST. In terms of functionality, EDOT-EDST is a better conductive polymer than PEDOT, but it is still overshadowed by PEDOT:PSS.
Figure 5. 3D Structure of VDOT as Seen When Built in WebMO ProDOT (3,4-Propylenedioxythiophene), visualized in Figure 6, is primarily used as an auxiliary electrode in solar panels[12]. These electrodes can have multiple roles. In a two electrode system, the auxiliary electrode serves as the cathode or the anode depending on the charge of the working electrode. Three electrode systems have superseded the two electrode system. In these modern systems, the auxiliary electrode has to balance the amount of electricity going to the working electrode. The polymer, PProDOT, can also be used for displays as it is a stable electrochromic polymer[13]. These types of polymers change and contrast colors when charges are applied. PProDOT is able to change between different shades of blue, according to its charge.
Figure 4. 3D Structure of EDST as Seen When Built in WebMO The next disubstituted derivative of thiophene examined in this study is VDOT (3,4-Vinylenedioxythiophene), and it can be seen in Figure 5. VDOT is composed of the same amount of carbon, oxygen, and sulfur as EDOT. However, the ethylene bridge is replaced with a vinylene bridge as noted in the name of the molecule. Because of this replacement, the band gap of VDOT decreases to a point that is lower than the band gap of EDOT just slightly[11]. Because of the lower band gap and the increased stability, VDOT was considered to be the replacement for EDOT, but PEDOT:PSS soon came after the development of VDOT. Since then, VDOT has been greatly overshadowed by the mixture of PEDOT:PSS.
Figure 6. 3D Structure of ProDOT as Seen When Built in WebMO Volume 3 | 2013-2014 | 73
Physics and CompSci Research Each of these molecules are great building blocks for π-conjugated systems. The polymers created from these building blocks are generally better conductors than their monomer counterparts. Conductivity of these polymers is extremely important due to their demanding applications. To increase conductivity, the polymers can be doped using various solvents. When it comes to ionic solvents, research has shown that conductivity is greatly increased. Particularly, 1-butyl-3-methylimidazolium tetrafluoroborate (bmim)BF4 which increased conductivity of PEDOT:PSS significantly[6]. The goal of this paper is to discuss the conductivity of these thiophene derivatives when exposed to polar and nonpolar solvents. The solvents tested in this paper include water, acetonitrile, and cyclohexane. These three solvents were chosen because of their polarity and their similar boiling points. Water and acetonitrile are both polar solvents, while cyclohexane is nonpolar. Similar boiling points were chosen due to a link between the boiling point and conductivity. Cyclohexane and acetonitrile nearly have the same boiling point, while the boiling point of water is slightly higher. Since the goal of this paper is focused on polarity, the boiling point variable was somewhat eliminated through the selection of solvents with similar boiling points. The two polar solvents have some interesting electrochemical properties. Water is very conductive, and its conductivity increases with heat. Acetonitrile dramatically boosts the conductivity of ionic liquids. These ionic liquids are the same ones that are used in the doping process for PEDOT:PSS in order to boost its conductivity. It would be interesting to see the effects of doping PEDOT:PSS with an ionic liquid mixed with acetonitrile[14]. This would establish a platform for future experimentation both computationally and experimentally. On the other hand, the nonpolar solvent, cyclohexane, is not conductive due to its polarity. When testing for conductivity, it is expected that the acetonitrile solvent will increase conductivity of the molecules due to the fact that it increases the conductivity of ionic liquids, which are used in the doping process for PEDOT:PSS. Even though boiling point seems to be a factor of higher conductivity, the greater boiling point of water seems irrelevant due to the fact that distilled water acts almost as an insulator. Like the ionic liquids, it is believed that acetonitrile will remove a few delocalized electrons to increase the conductivity of the molecule. In contrast, not much change is expected from calculations with cyclohexane. Since the charges are relatively neutral in the nonpolar solvent, the delocalized electrons will not be removed from the thiophenes.
Computational Approach The North Carolina School of Science and Mathematics Computational Chemistry server, along with a WebMO interface, was utilized to run all the jobs in this study. Specifically, the MOPAC software package with 74 | 2013-2014 | Volume 3
PM3 theory served to do all the calculations. MOPAC is designed for semi-empirical calculations, which utilize experimental data to simplify mathematical equations. The main focus of this method is placed on valence electrons since these are involved all chemical reactions. Semiempirical methods are known for their speed while also producing accurate results. They are ideal for molecules containing 50-100 atoms. Ab initio methods are another option, but due to slower processing times they could not be used in this study. The times of calculation were also not suitable for the time frame of this study. At the beginning, B3LYP and MP2 were the two main theories used to analyze the molecules presented. It may be possible that other theories do work with DFT method and these monomers, but lack of time and resources did not allow for further investigation. Initially, DFT methods were used for the calculations in this analysis; however, the inconsistencies that were discovered made it clear that MOPAC was the best method in this situation. The DFT method, MP2, failed to perform a geometry optimization for any of the molecules. B3LYP provided data for some of the molecules, but the time taken and the inconsistencies ruled out DFT. MOPAC is well suited to work with molecules in solvents. Because this study is heavily focused on solvents and it provided reasonable heats of formation, MOPAC was chosen over DFT and the Gaussian software package. There were pitfalls of choosing MOPAC over Gaussian. Even though MOPAC is suited for solvents, it was very limited in terms of the number of built in solvents that were available to be analyzed. PM3 was the theory used along with MOPAC. PM3 is an acronym for Parameterization Method 3. MOPAC may use Modified Neglect of Differential Overlap (MNDO), Austin Method 1 (AM1), or PM3 as its theories. The latter was chosen for this study because it is robust and very common. PM3 was the last theory developed out of the three listed above. It contains parameters similar to AM1. PM3, however, uses a different number of Gaussian functions for calculating repulsing forces in the core of atoms. The first step in running the calculations was defining the structure of the molecules in the study, as seen above. These were built in the WebMO workspace. A comprehensive clean-up using mechanics was performed multiple times in order to add hydrogens, set the hybridization, and roughly optimize the structures by idealizing the bond lengths and angles. The main goal of the comprehensive clean-up was to minimize the total strain energy of the roughly optimized structure. With minimized total strain energy, the molecule will be at its most stable state and yield more accurate results. All of the molecules were symmetrized after the comprehensive clean-ups. Through this process, each molecule is matched with its point group symmetry so the end results and mathematics are more accurate. Point group symmetries are another factor that helps classify and predict the properties of a molecule since molecules with the
Physics and CompSci Research same symmetry tend to be alike. These symmetries can be predicted by looking at the structure. If the molecule can be rotated around an axis and result in the original structure then it has symmetry. The axis can be set on one atom, the center of the molecule, or any of the xyz planes. Another type of symmetry comes from the molecule having a point of inversion. Depending on what type of axis is used for symmetry, the point group symmetry is predicted. From the molecules analyzed, VDOT and thiophene were the only ones with a C2V (Figure 7) point group symmetry. Water is another example of a molecule with a C2V point group. These have four symmetry operations. EDOT and EDST had a C2 symmetry, ProDOT was symmetrized with Cs, and for PEDOT:PSS the point group was C1. Point group symmetries of C1 do not have any axis of rotation, reflection planes, or inversion centers. A default tolerance of 0.05 was utilized for all symmetries.
Figure 7. Planes of Symmetry of VDOT Structure as Seen in WebMO. This graphic depicts how VDOT may be rotated around two different planes with axis on the sulfur atom. Geometric optimizations were then performed on each molecule. Through trials involving EDOT, no differences in the data were observed between structures optimized with and without the solvent when molecular orbital calculations were made. This realization greatly reduced computation time due to the large amount of time required to perform a geometry optimization of molecules in a solvent. After the geometry optimizations, the optimized molecules were used to perform molecular orbital calculations. Molecular orbitals are simply formed by the combination of atomic orbitals from the atoms in the molecule. According to the Molecular Orbital Theory, a wide variation of information can be found through the molecular orbitals. The structure, properties, and activity can be predicted. One of the main uses includes describing how the atomic orbitals overlap to make up a molecule. This leads to predicting the reactivity and conductivity of a molecule. The movement and behavior of electrons can be defined by a wave-like motion. An electron may either be in an up phase or a down phase of the wave. Two atoms or mole-
cules that have waves in the same phases are more likely to react when they come in contact. Reactivity and conductivity have a close correlation as explained by the band gap. The overlap of atomic orbitals is called a linear combination of atomic orbitals (LCAO). Initially, LCAO theory was used to optimize the wavefunction. Now, with computational chemistry, it primarily serves the purpose of determining the structure of molecular orbitals through the properties of atomic orbitals. The structure of these molecular orbitals provides insight into how the electrons move across molecules based on how electrons move in the atoms that comprise the molecule. The molecular orbital calculations provide information related to all of the molecular orbitals found in the molecule. Each molecule was ran through the molecular orbital calculation four times using their optimized structures. The first calculation determined the molecular orbitals of each molecule in no solvent. Then they were tested using each solvent: water, acetonitrile, and cyclohexane. The molecular orbitals of interest in this study are the frontier orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In terms of band theory, the HOMO corresponds to the valence band, and the LUMO corresponds to the conduction band. The valence band is to be completely filled with electrons that are bound to individual atoms. On the other hand, the conduction band is a space where electrons can move around due to them being delocalized. The electrons found in the conduction band are responsible for the conductivity of the molecule. The space in between in valence band and the conduction band is known as the band gap. The band gap, in terms of frontier molecular orbital theory, corresponds to the HOMO-LUMO gap. This band gap also provides information to the conductivity of the molecule. In the case of these thiophene derivatives, the lower the gap the greater the conductivity of the molecule1. The lower gap corresponds to greater reactivity, which is good for the transfer of delocalized electrons and, consequently, conductivity. With molecular orbital theory, the HOMO of a molecule interacts with the LUMO of another molecule in a reaction. These frontier orbitals of EDOT are visualized in Figure 8.
Figure 8. The HOMO of EDOT is on the left, while the LUMO is on the right. Volume 3 | 2013-2014 | 75
Physics and CompSci Research The molecular orbital calculations also provided the heat of formation for each molecule in each solvent. This additional data provided further insight into how the solvents would affect each molecule. Heat of formation is simply the change in energy to form one mole of a substance. Unlike the HOMO-LUMO gap, no formula was needed to find the heat of formation. WebMO provided the heat of formation in kcal/mol, which is the standard unit for heat of formation in this paper. To determine the HOMO-LUMO gap, the HOMO energy must be subtracted from the LUMO energy. This gap is measured in electron volts, which is the standard used for the gap in this paper. In order to calculate the accuracy of the calculations through MOPAC, a percent error formula was utilized. An experimental value was obtained from the Computational Chemistry Comparison and Benchmark Database (CCCBDB) for the heat of formation for thiophene. The CCCBDB is a government resource for computational data on a variety of molecules[15]. This site obtains its results from ab initio methods, where all the calculations are made from scratch. One of its purposes is to provide a source where researchers can compare their data and various computational methods. Their main focus is thermochemical properties of molecules. They also allow for users to download geometries to be used in further studies. The experimental value of heat of formation in thiophene is 27.46 kcal/mol. In the percent error formula, the experimental value is subtracted from the computational value from MOPAC. Then the result is divided by the experimental value. The absolute value of the product is multiplied by 100 to find the percent error. This formula is very useful when comparing different methods in computational chemistry and finding any significant differences. MOPAC, as well as DFT with B3LYP/3-21G, were tested with the formula. In order to find the effects of each solvent on the various molecules, the percent error function was once again used. In this case, the experimental value was replaced with the heat of formation or band gap of the molecule without a solvent. Values obtained from calculations with solvents replaced the computational value above. Selecting the solvent for the different trials was facilitated by the WebMO interface. MOPAC calculations with solvents were limited and the main options were water, acetonitrile, and cyclohexane. Gaussian software package allows for a greater variety options due to possible modifications of the input code. The solvent name as well as static and dynamic dielectric constants can be defined in the code.
Results and Discussion As mentioned in the section above, the primary software package utilized in this analysis was MOPAC. Ini76 | 2013-2014 | Volume 3
tially, the DFT method B3LYP was to be utilized due to its common use for thiophene and its disubstituted derivatives. However, Table 1 shows that there were some great inconsistencies in the data using B3LYP.
Table 1. Model chemistries compared to experimental values from the NIST database to identify accuracy of computational chemistry methods. To test the choice of model chemistry, thiophene was optimized using Gaussian B3LYP/3-21G and MOPAC PM3. The computational values were then compared to the values from the Computational Chemistry Comparison and Benchmark Database. After the revelation that B3LYP provided such a large error for the heat of formation, MOPAC PM3 was utilized for all geometry optimizations and molecular orbitals calculations in this study. The difference between the two methods can easily be seen through the percent error calculation. B3LYP/3-21G produced a percent error of 1,257,287.5% while MOPAC PM3 had a percent error of 11.68%. MOPAC was then used to obtain the data for the analysis. The molecular orbital calculations for the molecules Thiophene, EDOT, EDST, VDOT, and ProDOT demonstrated that the polar and nonpolar solvents increased the band gap of each molecule. Polar solvents, however, had a more significant effect on the band gaps and heats of formation. As mentioned above, each molecule was subject to three solvents. The main data collected came from the heat of formation, in kcal/mol, and the HOMO-LUMO Gap, measured in eV. Table 2 shows each molecule in each solvent. Thiophene, the parent molecule in this study, set the standards for comparison for the other molecules in the study. Every solvent lowered the heat of formation, if only slightly. The polar solvents, water and acetonitrile, lowered the heat of formation by about 1.5 kcal/mol. The nonpolar solvent, cyclohexane, lowered the heat of formation by only 0.57 kcal/mol. Thiophene had the highest band gap out of all the molecules tested. With a higher gap, less reactivity is expected from the molecule and therefore less change overall. The addition of the polar solvents to thiophene caused the band gap to be raised to the highest peak in this study. That peak, 9.415 eV, was raised by the polar solvent, water, 0.064 eV from the molecular orbital in no solvent. The other polar solvent, acetonitrile, produced extremely similar results to water. However, the nonpolar solvent, cyclohexane, only raised the gap by 0.023 eV.
Physics and CompSci Research
Table 2. Heat of formation and band gap thiophenes and its derivatives in various solvents. Molecular orbitals calculations were performed on each molecule using the listed solvents. The resulting heats of formation (kcal/mol) and the HOMO-LUMO gap (eV) are listed for each run. For EDOT, the polar solvents lowered the heat of formation by about 6.5 kcal/mol. The nonpolar solvent only lowered the heat of formation by nearly 2 kcal/mol. Opposite results were seen for the HOMO-LUMO gap. The nonpolar solvent raised the HOMO-LUMO gap by 0.033 eV while the polar solvents raised the gap by about 0.1 eV. It was surprising to see in the first disubstituted derivative that the gap was actually raised for each solvent. EDST had significant changes in the heat of formation. Similar to EDOT, the solvents lowered the heat of formation for EDST. The polar solvents lowered the heat of formation the most by about 9.5 kcal/mol. The nonpolar solvent also lowered the heat of formation but only slightly as compared to the polar solvents. The HOMOLUMO gap also followed the trend set by EDOT. The nonpolar solvent raised the gap by 0.101 eV while the polar solvents raised the gap by about 0.29 eV. It should be noted that EDST was the only disubstituted derivative with a positive heat of formation in this study. This observance is somewhat odd due to all of the molecules being derivatives of thiophene; consequently, they should have very similar properties. ProDOT continued the trend of the polar solvents decreasing the heat of formation the most when compared to the nonpolar solvent. Water and acetonitrile lowered the heat of formation by about 6 kcal/mol. The nonpolar cyclohexane performed as expected, and the heat of formation was lowered by about 1.8 kcal/mol. In terms of
the HOMO-LUMO gap, the polar solvents raised it more than the nonpolar solvent. Water and acetonitrile raised the gap by 0.07 eV while cyclohexane raised it by only 0.03 eV. Most of the heats of formation were either in the positive or negative 30 to 40 kcal/mol range. VDOT was the only exception with heats of formation in the negative 10 to 20 kcal/mol range. Again, the trend of the polar solvents decreasing the heat of formation the most was observed. The polar solvents lowered the heat of formation by about 5 kcal/mol, and the nonpolar solvent lowered the heat of formation by only 1.5 kcal/mol. Like the molecules before, the polar solvents raised the band gap the most, but in this case, only slightly more than that of the nonpolar solvent. Water and acetonitrile raised the gap by 0.04 eV while cyclohexane raised the gap by 0.02 eV. Analysis of the changes in the heats of formation has presented a surprising trend. In each molecule, it was observed that the polar solvents decreased the heat of formation the most. It was also observed that the polar solvents raised the HOMO-LUMO gap more than the nonpolar solvent. Another key observation was that the two polar solvents were so similar in results despite them being completely different molecules.
Table 3. Percent difference between the heat of formation/band gap values calculated with various solvents and values without solvents Overall, water had the most significant effect on the thiophenes. There was an average change of 21.98% in the heat of formation of all molecules with this solvent. The band gaps were raised by an average of 1.34%. Acetonitrile had very similar, but slightly less, effects on the heat and band gap. A change of 21.28% was observed in the heat, while 1.27% increase of the gap was present. Similar results were expected from water and acetonitrile due to Volume 3 | 2013-2014 | 77
Physics and CompSci Research their polarity and similar properties. Cyclohexane had the least effect, which is caused by its nonpolar features. The average change seen from this solvent in heat was 7.79% and only 0.54% in the band gap. The percent difference in the band gap is quite insignificant and it’s possible that it was caused by simple and common calculation errors within the software. The largest effects on heat of formation were seen on VDOT where polar solvents changed that heat by approximately 46%; however, this monomer saw the least change in its band gap with only 0.44% change on its polar molecules as seen on Table 3. The most significant increase in band gap is observed in EDST with water as solvent at 3.54% change. EDST happened to have the second highest effect on heat of formation with approximately 23% on its polar solvents. An interesting trend is that EDST and VDOT had the highest heat of formation out of the four components when no solvent was involved. Both of them also had the lowest band gaps. Thiophene base, which had the highest original band gap, was least affected by the solvents in its heat with a maximum 4.92% change. It’s band gap saw minimal change as well with a range of 0.25% to 0.68% increase. The data from the calculations did not support the stated hypothesis that acetonitrile would increase the conductivity of thiophene and its disubstituted derivatives. While the two polar solvents did have the most significant effect on the molecules, conductivity was actually decreased as can be concluded by a raise in band gaps. Cyclohexane caused minimal change in all cases. The nonpolar solvent, however, had the same effect of lowering conductivity as well. Rather than increasing conductivity, each solvent increased resistivity of the molecules. With a combination of ionic liquid doping and polar solvents, researchers can obtain full control of conductivity. For example, polar solvents can be used to decrease conductivity as a safeguard from overloading an electrical system. The original goal of this analysis was to study the effects of ionic liquids on PEDOT:PSS. Primarily, discussing the increase or decrease of conductivity was the goal. After learning that analyzing large polymers would be too computationally expensive, steps were taken to shorten computational run time. PEDOT:PSS was then divided into its two main components: PEDOT and PSS. Again, these two are polymers, and it was not feasible to run calculations on the polymers. The next step was to simplify the molecules even more. Because these two are polymers, they were split into their monomer counterparts, EDOT and SS. Studies were discontinued with SS due to computational run time. With one molecule left, EDOT became the main focus of the paper. EDOT being a disubstituted derivative of thiophene sparked the study of several related disubstituted derivatives of thiophene. It would be interesting to see how the original study of PEDOT:PSS, and other related polymers, would have panned out if computation time was not a factor. The effects directly from ionic 78 | 2013-2014 | Volume 3
liquids could not be tested either due to limitations on the MOPAC software. Water, acetonitrile, and cyclohexane proved to be the easiest solvents to perform experiments. Their properties and the software limitations created good reasons for their use in this study. The completion of this analysis has prompted questions related to future studies. The main item of interest is how oxygen and sulfur affect properties of thiophenes. To analyze these properties, polymers composed of different thiophenes should be examined. Hopefully, this study will provide insight into the interactions of oxygen and sulfurs in the polymer with varying structure monomers.
Conclusions Through the analysis of thiophene and its disubstituted derivatives, it can be concluded that polar solvents decrease the conductivity of these molecules the most. Water and acetonitrile had very similar effects on all of the molecules concerning heat of formation and band gap, the main parameters in this study. In terms of increasing conductivity, cyclohexane turned out to be the best solvent due to its minor effect. The nonpolar solvent decreased conductivity the least out of the three solvents in this analysis. All three of the solvents raised the band gap, which corresponded to a decrease in conductivity. Polarity seems to be the greatest factor that influenced the results. The polar solvents, water and acetonitrile, raised the band gap by nearly the same amounts. Acetonitrile raised the band gap by slightly less , making water the worst solvent for decreasing the resistance of thiophene and its disubstituted derivatives. These results were uniform across all molecules tested in the analysis, which provides the basis for a safe conclusion that polar solvents would be the least ideal for increasing conductivity of thiophene and its disubstituted derivatives. One conclusion that can be made from the heat of formation data is that oxygen plays a significant role in lowering its values. EDST and thiophene were the only two molecules without an oxygen atom in their structures and they had the highest heat of formation. Adding an atom of oxygen will continuously decrease the heat of formation. No effects were seen, however, on the band gaps as EDST and thiophene had either the lowest or highest band gap. Patterns on the amount of change were not seen either in relevance to oxygen. Polar solvent had the most impact on the molecules in this study. This may have been due to the fact that thiophenes are polar as well. With two polar molecules, it is expected that there is more interaction and leading to greater effects either in the positive or negative direction. The nonpolar solvent did not have uneven distribution of charges, which may have caused less reactivity. VDOT had the most change in its heat of formation and least in its band gap throughout all the solvents. The minor change in the band gap might have been due to its double bond. This extra double bond between carbons
Physics and CompSci Research is in charge of increasing the stability of the molecule and allowing less change to take place in its band gap, which deals directly with reactivity. EDST experienced the most change in its band gap throughout all of the solvents. This change could be due to the addition of sulfur, rather than oxygen, in this particular disubstituted derivative. It is possible that the lower electron affinity of sulfur, when compared to oxygen, is responsible for this change. Electrons are less strongly attracted to sulfur, which leads to more reactivity and a less stable molecule. Several trends were identified in this analysis. To summarize, the polar solvents decreased the heat of formation the most, and they raised the band gap the most when compared to the nonpolar solvent. It is possible that thiophene and its disubstituted derivatives, being polar, interacted with the polar solvents due to polar solvents dissolving polar molecules. Because the molecules experienced little change in the nonpolar solvent, cyclohexane, it is believed that thiophene and its derivatives had little interaction with the nonpolar solvent due to their opposite polarity.
Acknowledgement The authors thank Mr. Robert Gotwals for assistance with this work. Appreciation is also extended to the Burroughs Wellcome Fund and the North Carolina Science, Mathematics and Technology Center for their funding support for the North Carolina High School Computational Server.
References [1] Sotzing, Gregory A., and John R. Reynolds. “Poly[trans-bis(3,4- ethylenedioxythiophene)vinylene]: A Low Band-gap Polymer with Rapid Redox Switching Capabilities between Conducting Transmissive and Insulating Absorptive States.” Journal of the Chemical Society, Chemical Communications 6 (1995): 703. [2] Lenz, Annika, Hans Kariis, Anna Pohl, Petter Persson, and Lars Ojamäea. “The Electronic Structure and Reflectivity of PEDOT:PSS from Density Functional Theory.” Chemical Physics (2011): 44-51. Print. [3] Alper Bozkurt, Amit Lal, Low-cost flexible printed circuit technology based microelectrode array for extracellular stimulation of the invertebrate locomotory system, Sensors and Actuators A: Physical, Volume 169, Issue 1, 10 September 2011, Pages 89-97 [4] Döbbelin, Markus, Rebeca Marcilla, Maitane Salsamendi, Cristina Pozo-Gonzalo, Pedro M. Carrasco, Jose A. Pomposo, and David Mecerreyes. “Influence of Ionic Liquids on the Electrical Conductivity and Morphology of PEDOT:PSS Films.” Chemistry of Materials 19.9 (2007): 2147-149. [5] Xia, Yijie, and Jianyong Ouyang. “Salt-Induced Charge Screening and Significant Conductivity Enhancement of
Conducting Poly(3,4- ethylenedioxythiophene):Poly(styr enesulfonate). “ Macromolecules 42.12 (2009): 4141-147. Print. [6] Onorato, Amber, Michael A. Invernale, Ian D. Berghorn, Christopher Pavlik, Gregory A. Sotzing, and Michael B. Smith. “Enhanced Conductivity in Sorbitol-treated PEDOT–PSS. Observation of an in Situ Cyclodehydration Reaction.” Synthetic Metals160.21-22 (2010): 2284289. [7] Turbiez, Mathieu, Pierre Frère, Magali Allain, Nuria Gallego-Planas, and Jean Roncali. “Effect of Structural Factor on the Electropolymerization of Bithiophenic Precursors Containing a 3,4- Ethylenedisulfanylthiophene Unit.” Macromolecules38.16 (2005): 6806-812. Print. [8] Synthesis, Characterization, and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-Containing Polythiophenes and 2,1,3- Benzothiadiazole Jianhui Hou, Hsiang-Yu Chen, Shaoqing Zhang, Gang Li, and Yang Yang Journal of the American Chemical Society 2008 130 (48), 16144-16145 [9] Leriche, Philippe, Philippe Blanchard, Pierre Frère, Eric Levillain, Gilles Mabon, and Jean Roncali. “3,4-Vinylenedioxythiophene (VDOT): A New Building Block for Thiophene-based P- conjugated Systems{.” ChemComm (2005): 275- 77. Print. [10] Lee, Kun-Mu, Chih-Yu Hsu, Po-Yen Chen, Masashi Ikegami, Tsutomu Miyasaka, and Kuo-Chuan Ho. “Highly Porous PProDOT-Et2 film as Counter Electrode for Plastic Dye-sensitized Solar Cells.” Physical Chemistry (2009): 3375- 379. Print. [11] Gaupp, C. L., Welsh, D. M. and Reynolds, J. R. (2002), Poly(ProDOT-Et2): A High-Contrast, HighColoration Efficiency Electrochromic Polymer. Macromol. Rapid Commun., 23: 885– 889. [12] Influence of Solvent on Ion Aggregation and Transport in PY15TFSI Ionic Liquid–Aprotic Solvent Mixtures Oleg Borodin, Wesley A. Henderson, Eric T. Fox, Marc Berman, Mallory Gobet, and Steve Greenbaum The Journal of Physical Chemistry B 2013 117 (36), 1058110588 [13] NIST Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database Number 101, Release 16a, August 2013, Editor: Russell D. Johnson III http://cccbdb.nist.gov/ [14] Schmidt, J.R.; Polik, W.F. WebMO Pro, version 7.0; WebMO LLC: Holland, MI, USA, 2007; available from http://www.webmo.net (accessed January 2014). [15] The North Carolina High School Computational Chemistry Server, http://chemistry.ncssm.edu (accessed January 2014). [16] MOPAC Version 7.00, J. J. P. Stewart, Fujitsu Limited, Tokyo, Japan. [17] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. Volume 3 | 2013-2014 | 79
Physics and CompSci Research A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Wallingford CT, 2004. [18] Roncali, J., Blanchard, P. and Frére, P. 3,4- Ethylenedioxythiophene (EDOT) as a versatile building block for advanced functional π- conjugated systems. J. Mater. Chem., 15, 2005, 1598-610 [19] “aromatic compound”. Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2014. Web. 17 Jan. 2014 <http://www.britannica.com/EBchecked/topic/35 891/aromatic-compound>.
80 | 2013-2014 | Volume 3