Effects of Aromatic Trifluoromethylation, Fluorination, and Methylation

Jul 19, 2013 - Jeffery D. Mottishaw and Haoran Sun*. Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United State...
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Effects of Aromatic Trifluoromethylation, Fluorination, and Methylation on Intermolecular π−π Interactions Jeffery D. Mottishaw and Haoran Sun* Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States S Supporting Information *

ABSTRACT: Marcus theory states that the rate of charge transfer is directly proportional to the amount of intermolecular orbital overlap. Theoretically optimizing the electronic coupling through the orientation and distance which both can increase the frontier orbital overlap between molecules is an attractive route to potentially provide theoretical insight for discovering new high performance semiconductor materials. To investigate how these parameters qualitatively affect charge transfer of model systems, unconstrained dimer optimizations with MP2 and dispersion-corrected DFT methods were used to probe the π−π interactions of methylated, fluorinated, and trifluoromethylated benzene, pyridine, and bipyridine dimers. These systems can serve as simplified models representing weak noncovalent interactions in organic semiconductor materials. Enhanced intermolecular interaction energies, reduced π−π distances, and more favorable cofacial orientations were found with the trifluoromethylated dimers compared to fluorinated and methylated dimers studied. Similar effects were found with donor−acceptor pairs that represent organic p-n heterojunction systems. These enhanced π−π interactions are likely caused by increased molecular quadrupole moment and dispersion interaction associated with trifluoromethylation. This computational study illustrates the strong potential of trifluoromethylation and, possibly perfluoroalkylation of acenes and heteroacenes, leading qualitatively to enhanced electron transfer through better π−π stacked structures, making them viable candidates for use as n-type organic semiconductor materials. The findings also provide insight for fundamental interactions between drug molecules that include fluorinated and trimethylfluorinated aromatics binding to protein receptors.

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INTRODUCTION Weak noncovalent interactions, which include π−π stacking, are important in fields as diverse as protein engineering and materials chemistry.1 Of particular interest for materials scientists is how they can collectively influence the rate of intermolecular charge transfer.2 Organic semiconductors, which consist of polymers or small organic molecules weakly linked in the condensed state through noncovalent interactions,3−8 are semiconductor systems that depend heavily on intermolecular charge transport. These organic materials, once stability and mobility concerns are addressed, will have many advantages over traditional inorganic semiconducting materials such as the ability to be solution-processed,9 inexpensively printed onto various substrates,10 and easily tunable electronic properties through the addition of the appropriate functional groups.11 The addition of electron-donating groups to conjugated systems is often used for the synthesis of p-type organic semiconductor materials, while the addition of electron withdrawing groups potentially creates n-type organic semiconductor materials. This can be understood using fundamental organic chemistry,12 in which electron-withdrawing groups stabilize the radical anion formed upon acceptance of an electron, and electron donating groups stabilize the radical cation formed upon electron donation. While p-type materials (hole-conducting donors) are relatively well-developed,4−7,13,14 © XXXX American Chemical Society

n-type organic semiconductors (electron-conducting acceptors) suffer from poor stability under operating conditions,15 and their mobilities are lower by roughly an order of magnitude16 when compared to p-type organic semiconductors such as poly3-hexylthiophene17 (P3HT) and pentacene.18 Because of the enormous number of potential combinations of delocalized carbon backbones and functional groups, computational studies have been employed to screen large numbers of candidate molecules through their electronic properties. These studies have shown promise in the development of new p-type organic semiconducting materials.19 Criteria that have been typically examined through intensive computational investigation include: HOMO−LUMO levels (approximation of band gap in solid state), reorganization energy, and finally charge transport properties through calculated crystal structures. However, to be able to accurately predict crystal structure and arrive at a fundamental understanding of charge transport, the weak noncovalent interactions between neighboring molecules must first be determined. Once the role of weak noncovalent interactions are established, parametrization can then be applied to large systems such as a Received: April 15, 2013 Revised: July 12, 2013

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assembly and aggregation of the side chains,11 sterically hindering the approach by oxidizing agents to the radical anion formed during device operation. Another advantage of adding perfluoroalkyl groups is that orthogonal solutionprocessing can be easily done with fluorous solvents such as those from the hydrofluoroether family.34 Furthermore, introduction of perfluoroalkyl groups onto the aromatic ring also shows great potential in dictating self-assembly of the solid state structures of the corresponding aromatics leading to the formation of lamellar π−π stacked structures which can potentially improve charge transfer.30 Because of the increased thermodynamic and kinetic stability to oxidation under operating conditions, and orthogonal solution processing potential, the introduction of perfluoroalkyl groups to acene and heteroacene cores is an attractive route for the fabrication of new n-type organic semiconductors. The accurate and efficient calculation of weak intermolecular interactions in large systems is an active area of research from a broad range of theoretical perspectives.35−37 Typical Hartree− Fock and the widely used B3LYP functional used in DFT fail to account for electron correlation38 and cannot be used to probe weak interactions. However, when Hartree−Fock is corrected by a perturbative term such as is the case with second-order Møller−Plesset theory (MP2),39 noncovalent interactions can be successfully studied, and much progress has been made in the computational field by research groups such as those of Tsuzuki,24,40 Wheeler41,42 and of Sherrill.43−45 Because MP2 theory at the complete basis set limit (CBS) is known to overestimate the π−π interaction energies of aromatic dimer systems,46 it is often combined with a CCSD(T) correction to aid in obtaining results comparable to experiment. However, because of the large computational demands of CCSD(T), even medium sized systems with modest basis sets are too large for current computational resources.47 New types of MP2 theory combining empirical weights on the various spin states (SCS-MP2) have been successfully developed by Grimme.48 Recent long-range corrected DFT methods (such as DFT-D also pioneered by Grimme47,49−51 and the M06 family by Zhao and Truhlar52) have also shown utility in describing weak intermolecular interactions such as π-stacking. Of these, DFT-D which takes a pure functional and combines it with an empirical correction term to account for van der Waals interactions, shows excellent precision at a reduced computational cost. One problem inherent in the use of empirically corrected DFT methods is that when the systems studied contain atoms or interaction patterns not included in the original parametrization, the results may not be valid or close to benchmark results. Yet by applying the same methods consistently to all dimers studied, and making some comparisons with MP2 and available benchmark values, it is possible to make reasonable semiquantitative comparisons. SCS-MP2 was not used because of the desire to obtain pure ab initio results, while SCS-MP2 inherently has empirical character because of the different weights given to each spin component, and is sensitive to the system to which it is applied.53 Our goal is to first find a basis set combination that could be used with MP2 theory, with sufficient accuracy to allow for comparison of dimers of model aromatics (Figure 1). We also had interest in dimer systems that are too large for MP2 methods, and chose a dispersion-corrected DFT functional to probe the weak noncovalent interactions of these systems. While in most reported experimental cases large perfluoroalkyl tails are added to the acene cores, in this paper, we employed a

crystal through Monte Carlo or molecular mechanics methods.20 From both the perspective of crystal engineering and charge transfer principles, a potential strategy to improve the performance of organic semiconductors is to shorten the intermolecular distances and increase the amount of cofacial overlap present in the material in the solid state.21 One study of many that illustrates this principle is that of Liu, Kang, and Lee,22 who found computationally that cofacial stacking of βtrithiophenes, which is an ambivalent material, had increased electron transport as compared to hole transfer. To arrive at this conclusion, they used long-range-corrected density functional theory (DFT). From Marcus theory,23 they found that for electron transfer to be effective, both the distance between molecules and the molecular packing orientation (cofacial stacking versus herringbone24) are important parameters.25 A conclusion based on this concept is that if the component small molecules or polymers preferentially pack in a cofacial manner as compared to herringbone fashion, it is possible to create ntype organic semiconductors with higher electron mobilities. k CT =

4π 2 ⎛⎜ 1 ⎜ h ⎝ 4πλkBT

⎞ ⎛ ⎞ ⎟⎟V 2exp⎜ − λ ⎟ ⎝ 4kBT ⎠ ⎠

(1)

In eq 1, which is a formulation of Marcus−Hush theory for organic dimers taken from Liu et al.,22 kCT is the rate of charge transfer, V is the intermolecular coupling (transfer integral), λ = total reorganization energy (sum of inner-sphere and outersphere components), and T is the temperature in Kelvin. The intermolecular coupling for the case of electron transfer, is often taken to be the difference in energy between the orbital above the LUMO , commonly referred to as LUMO(+1), and the LUMO in organic dimers,26 Because of the square nature of the transfer integral in the Marcus formulation, optimizing this parameter has the potential to greatly increase the rate of charge transfer. In addition to charge transport and packing optimization, another important component of any semiconducting device is stability under operating conditions. Recent work by Strauss’s group27,28 and Sun’s group29 have proven that the addition of multiple trifluoromethyl groups to acene cores is an excellent strategy to modulate the LUMO energy level over a large range. Furthermore, addition of multiple perfluoroalkyl groups, which is an analogue to the addition of a trifluoromethyl group because of its similar electronic effects, leads to potentially controllable lamellar π−π stacked crystal packing30 and increased photostability.31 By systematic addition of perfluoroalkyl groups it is possible to create materials that are thermodynamically stable with respect to oxygen from the atmosphere when the radical anion is formed under operating conditions. One caveat to this however is that there is a slight penalty of increasing the inner-sphere reorganization energy (λi,) with the addition of multiple CF3 groups,29,32 particularly when the conjugation cores are small, such as the case with substituted benzene and naphthalene cores. With the use of large acene cores such as anthracene or pentacene, the innersphere reorganization gained by increased conjugation is optimal for electron transfer even with the addition of perfluoroalkyl groups. Perfluoroalkylation has been shown to induce n-type behavior in oligothiophenes by Usta et al.33 In addition to increasing chemical and photochemical stabilities,31 perfluoroalkylation also can provide a kinetic barrier to the approach of oxygen or water to the aromatic core of the semiconductor through fluorous interactions that lead to selfB

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distance of 3.5 Å between the aromatic cores was used to aid in convergence and enter the realm in which the weak interactions between the monomers would be maximized. We surveyed a series of nonsubstituted, methylated, fluorinated, and trifluoromethylated benzene cores to model large acenes. Along with these systems, heteroacenes such as pyridine and oligopyridine (i.e., bipyridine) dimers were used as model systems to investigate the effects of adding groups with various electron-donating or withdrawing capabilities. In particular, the properties we probed included interaction energies, orientations, and π−π stacking distances between two molecules, as these are the most relevant parameters that control the intermolecular charge transfer. Interaction energies are an important figure of merit that reflects the orbital interactions. The π−π distances are important to minimize in the rational design of n-type organic semiconductors because the shorter the distance, the more efficient the charge hopping which can be understood in the framework of Marcus theory.56 Cofacial orientations are also an excellent qualitative measure of how strong the molecular overlap is between the constituent monomers. Because sterics can also significantly impact packing, starting geometries for these unconstrained were chosen as to minimize these factors, which also aided in convergence of the calculations. Two types of correlated methods were used to elucidate the potential role of weak interactions in charge transport. The first type employed was the MP2 method at MP2/aug-cc-pVDZ//MP2/6-31g(d) level of theory using the Gaussian 09 suite of programs.57 MP2/631g(d) theory has been previously successfully used by Hutchison et al. to describe the influence of weak interactions on charge transport and packing,58 and also by RodriguezRopero to study π-stacking of thiophene dimers.59 Admittedly, this is a small basis for typical MP2 benchmark dimer calculations, but because of the number of heavy atoms in these molecules, it was the highest level calculation possible under current computing resources available. While MP2/631g(d) is unlikely to provide benchmark accuracy on distances or interaction energies, if all dimers are treated at the same level of theory, semiquantitative comparisons can be made. The early attempts using MP2/6-31g(d) for interaction energies led to severe under-binding. In an effort to rectify this, single point energy calculations using a higher level of theory (MP2/aug-ccpVDZ) were performed on the dimers and their constituent monomers. Another factor found to strongly influence the accuracy of the results was basis set superposition error which is due to overlap of the wave functions of the monomers. This was accounted for using the counterpoise correction60 as implemented in Gaussian 09. Frequency calculations were performed on all of the optimized monomers at MP2/631g(d). MP2/aug-cc-pVDZ theory has been successfully used

Figure 1. Aromatic compounds investigated in this study.

trifluoromethyl group to represent perfluoroalkyll groups since their electronic effects are similar.54 This simplification substantially reduces the computational cost because of the reduction of degrees of the vibrational and rotational freedom of the perfluoroalkyl groups.

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COMPUTATIONAL METHODS Unconstrained dimer optimizations55 were used to find stable conformational geometries of the systems studied. It has been documented that even for the simple case of the benzene dimer, there are multiple wells in the potential energy surface (PES) that lead to three types of dimer configurations.45 These are known as the sandwich (cofacial) dimer, the T-shaped dimer, and the parallel-displaced dimer. While the T-shaped orientation is nearly as stable as the parallel-displaced arrangement in pure benzene dimer because of electrostatic interactions, previous work22,25 has indicated that T-shaped orientation, which models typical packing in a herringbone configuration, often found with p-type organic semiconductor materials is not optimal for electron transport in n-type organic semiconductors. In these unconstrained dimer optimizations, the cofacial configuration was chosen for the initial geometry to find the local minima in the potential well corresponding to the cofacial or the parallel-displaced dimer, which maximizes frontier orbital interactions between the monomers. A starting

Table 1. Effect of Basis Sets and Methods on Intermolecular Interaction Energiesa dimer A (1,1)

dimer K (1,9)

methods

Eint/kcal/mol

dπ−π/Å

Eint/kcal/mol

dπ−π/Å

MP2/aug-cc-pVDZ//MP2/6-31g* ωb97xD/6-311G(d,p)//MP2/6-31g* ωb97xD/6-311++G(d,p)//MP2/6-31g* ωb97xd/aug-cc-pVDZ//MP2/6-31g* ωb97xd/aug-cc-pVTZ//MP2/6-31g*

−2.07 −0.85 −1.06 −1.38 −2.66

4.281 4.281 4.281 4.281 4.281

−4.22 −2.87 −3.09 −3.51 −3.51

3.984 3.984 3.984 3.984 3.984

In all cases, geometries were first optimized using MP2/6-31g(d) followed by single point energies at the given level of MP2 or DFT-D3 theory. BSSE was also corrected using the counterpoise method. a

C

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Table 2. Calculated Dimerization Energies (Eint/kcal/mol) and π−π Stacking Distances of Various Dimers between Benzene and Thiophene Derivatives at MP2/aug-cc-pvDZ//MP2/6-31g(d) Level of Theory

a

dimer

monomer pairs

Eint kcal/mol

π−π distance/Å

dimer

monomer pairs

Eint kcal/mol

π−π distance/Å

A B C D E F G H

1,1 1,2 1,3 1,4 5,5 6,6 2,2 3,3

−2.07 −4.41 −6.11 −7.76 −8.48 −8.67 −6.74 −8.05

4.281 a 3.606 3.601 3.665 3.626 3.646 3.665

I J K L M N O

1,7 1,8 1,9 10,10 11,11 12,12 7,7

−3.62 −5.01 −4.22 −5.60 −7.56 −6.79 −4.20

3.767 3.473 3.894 3.905 3.668 a 3.758

Optimized geometry was in T-shaped. Eint: calculated interaction energy of dimer.

were carefully chosen to not only reflect the systems of interest, but also to be computationally feasible. Substituted benzenes can serve as models for larger acene cores (such as anthracene or pentacene) while avoiding the high computational cost that the addition of these heavy atoms in the carbon framework require. Thiophene can potentially serve as a model for P3HT, and describing interactions between electron-deficient molecules and electron donors such as thiophene or benzene can represent the p-n junction of an organic solar cell or field-effect transistor. One problem with using small substituted acene or heteroacene cores is that their internal reorganization energies are much higher than with larger cores,29 making these systems simplified models of charge transfer. In addition, previous work with perfluoroalkylated aromatics has used long perfluoroalkyl (Rf) tails in the device fabrication.15 This is done to maximize the fluorous effect35 and provide a kinetic barrier to oxygen approaching the aromatic core.11 Because of the limitations inherent in MP2 calculations, we modeled these long fluorous tails with trifluoromethyl groups. The trifluoromethyl groups have similar influence on the electronic properties of the acene core when compared to the longer perfluoroalkyl chains, but have a reduced number of rotational degrees of freedom, making computations on them much more facile.

by other groups when there are a large number of heavy atoms which prohibit the use of larger basis sets.61 From the energy/ geometry combination used, rankings and trends can be seen, though the absolute energy and geometry values may differ from what could be obtained with coupled-cluster or MP2/ CBS. For comparison and to probe larger systems, DFT-D calculations were also performed. The chosen theory level was B97-D/TZV with no BSSE correction. This functional, which incorporates the pure Becke 97 functional62 with empirical corrections to the van der Waals term, has been successfully implemented in studying the interactions of electron-deficient dimers by Wheeler41 and also compared to other methods that treat dispersion by Granatier et al.47 To test this functional/basis set combination, interaction energies derived from MP2 and B97-D were compared, and an excellent correlation was found between the two methods with our models (R2 = 0.9931, Supporting Information, Figure S1). Table 1 shows how the interaction energy can vary widely depending on the method used. For example dimer K(1,9) varies by almost 1.5 kcal/mol. Given that the interaction energy of benzene dimer is just slightly larger than this range, the accuracy associated with the use of different methods can widely vary. However, the precision of the calculations, which is important for rankings and making semiquantitative predictions, remains intact. Table 1 shows full results for two dimers (dimer A(1,1), benzene and benzene, and dimer K(1,9), benzene and 1,3,5-trifluorobenzene). The ranking of the fluorinated dimer having a higher interaction energy than the benzene dimer is maintained, regardless of the theoretical method employeed. Therefore, because of the wide range of values, it is difficult to produce benchmark results under current resources, but it is possible to order compounds through semiquantitative comparisons. For more rigorous interaction energies, basis sets incorporating polarization functions such as those employed in the aug-ccpVXZ series of basis sets could have potentially been used. In addition, different dispersion functionals might influence the results and also, BSSE corrections could have been used in the DFT-D calculation. To test this, single-point energy calculations using the recent ωB97XD functional developed by Head-Gordon,63−65 coupled with aug-cc-pVTZ on the MP2/6-31g(d) geometries were performed on selected dimers and monomers. These calculations also included a counterpoise correction. While the values obtained from this higher level of theory, relative ordering of the selected dimers remain intact. Taken with the excellent correlation obtained from our initial calculations between MP2 and DFT-D methods, semiquantitative comparison can easily be made.66 Model dimers

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RESULTS AND DISCUSSION The MP2 and DFT-D computational results for the sandwich configuration of benzene compared well to the benchmark cofacial orientation results of Sinnokrot and Sherrill,44 even though in their work a much larger basis set and a CCSD(T) correction was applied to the benzene dimer system. Because larger basis sets increase the calculated intermolecular interaction energy with MP2 method, and MP2 method is known to overestimate the π−π noncovalent binding energy, through a cancellation of errors which has previously been reported in the literature,61 we found that our theory combination (MP2/aug-cc-pVDZ//MP2/6-31g(d)) is able to describe the dimer systems with respectable accuracy for comparison of systems. In most studies of the benzene dimer, constrained optimizations are employed where the distance between the rings is fixed, but all other parameters are allowed to relax. To test our method of using unconstrained dimer optimizations, we scanned the PES of the benzene dimer at the MP2/6-31g(d) level of theory and also at the B97-D/TZV theory by varying the distance between the dimers from 3.0 to 4.5 Å by 0.1 Å increments with a sandwich orientation. The local minimum obtained by this method (4.3 Å intermolecular distance) compares well with the value obtained through unconstrained optimization methods (4.281 Å). This illustrates D

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the utility of unconstrained dimer optimizations as a method to determine relative geometries and interaction energies when initial geometries are relatively close to the local minima. If one is considering interaction energies with MP2/6-31g*, we found that severe under-binding occurs and the interaction energy of the cofacial benzene dimer drops to −0.8 kcal/mol. This is substantiated in the literature that compares methods to determine aromatic interaction energies.46 However, when it is coupled with a single point energy calculation at MP2/aug-ccpVDZ level of theory, the interaction energies obtained are within the range of much larger benchmark results such as those done by Sinnokrot and Sherrill.45 Again, the goal of this work was to make comparisons, and by consistently using the same methods for geometry optimization and energy calculations on all of the compounds, trends and rankings of interaction energies will show, and the results can be compared to larger benchmark calculations for simpler systems to gauge the quality of the approximation. This approach of using semiquantitative methods to make comparisons on medium size systems has been employed by other researchers such as Wheeler.42 Table 2 shows the interaction energies and π−π distances for the simple aromatic compounds, benzene and thiophene derivatives, that were modeled. The first type of dimers studied had electron-deficient benzene cores which optimized dimer geometries are shown in Figures 2 and 3. These dimers are

Figure 3. Side and top views of optimized dimer geometries of benzene paired with various fluorobenzenes (dimers I(1,7), J(1,8), and K(1,9)). Geometry optimizations were done with MP2/6-31g(d).

Figure 4. Side and top views of optimized dimer geometries of thiophene (electron-rich) with benzene and various trifluoromethylated benzene derivatives (dimers P(17,1), Q(17,2), R(17,3), and S(17,4)). Geometry optimization was done with MP2/6-31g(d).

rich (Figure 5) to serve as a control study to compare the difference between electron-withdrawing CF3 substituent and

Figure 5. Side and top views of optimized geometries of methylated benzene dimers ranging from toluene dimers to xylenes (dimers L(10,10), M(11,11), and N(12,12)). Geometry optimization was done with MP2/6-31g(d).

electron-donating CH3 substituent. It is well-known that both electron-withdrawing and electron-donating groups contribute to increased intermolecular interactions,41,67,68 however with the advantages of these groups and their analogues, for example long alkyl and perfluoroalkyl substituents were widely used to improve the organic semiconductor solubility and stability, it is interesting to compare how these substituents influence π−π stacking in particular. The last type of dimer modeled was with pyridine as an aromatic core rather than benzene (Figure 6). Pyridine cores can potentially model oligopyridines69 and also heteroacene cores70 and have vastly different electronic properties than the corresponding models with benzene as the aromatic core. This portion of the work was then extended into bipyridine dimers, which may be useful 1D-stacking materials when paired with square-planar metal complexes, using DFT-D methods (coordinates of optimized geometries are given in the SI). MP2 could not be the type of method used to study the bipyridine dimers because of the large number of heavy atoms present in this system. Upon an examination of the frequency calculation on the benzene dimer in a cofacial

Figure 2. Side and top views of optimized dimer geometries of benzene and trifluoromethylated benzene derivatives (electron-rich with electron-poor molecules, A(1,1); B(1,2); C(1,3); and D(1,4)) and electron deficient dimers (electron-poor with electron-poor molecules, G(2,2); H(3,3); E(5,5); and F(6,6)). Geometry optimization was done with MP2/6-31g(d). A saddle point cofacial geometry of benzene dimer, A(1,1), was used for comparison.

important in that they can serve as models for the bulk n-type organic semiconductor devices. Determining the intermolecular distance and type of stable orientation is a potential route to screen for n-type semiconductor materials computationally. The second type of dimer we studied were donor−acceptor pairs created by the interaction of electron-deficient aromatic cores with electron-rich aromatic cores which optimized dimer geometries are shown in Figure 4. This type of dimer can potentially model the p-n junction of an organic device. The third type of dimer modeled was electron-rich with electronE

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dimer G(2,2). In all these four cases, the cofacial orientation was maintained. Previous work by Sherrill67 and Wheeler41 have shown that contrary to the Hunter-Sanders rules,44 both electron donating and electron withdrawing substituents increase intermolecular interaction energies, and that substituents can be modeled in a local interaction framework. While there exist powerful tools to elucidate which components of noncovalent interactions are most substantial such as symmetry-adapted perturbation theory,72 the goal of our work was to make semiquantitative comparisons and generalize the results in the form of charge transfer framework. Because of the strongly electron-withdrawing nature of the trifluoromethyl group, the initial hypothesis was that dipole−dipole interactions may play a significant role in the enhanced π−π interactions. Another area in which we wished to focus was the role of weak interactions due to different substitutions leading to different quadrupole moments. To test this hypothesis, different types of disubstituted trifluoromethylated dimers were explored. As expected, dimer H(3,3) with its meta arrangement of CF3 groups, has a slightly lower energy than the ortho arrangement of E(5,5). In both cases, cofacial stacking is maintained, though the π−π distance is slightly smaller for the meta arrangement. Interestingly, when the substituents are in a para arrangement F(6,6) the interaction energy is larger and the π−π distance is smaller than the ortho case. Because steric repulsion should be smaller for the 1,2-trifluoromethylbenzene dimer than the 1,4trifluoromethylbenzene dimer, electronic effects must be the deciding factor. The CF3 groups force the dimer into this favorable configuration throughout the course of the optimization. This has potential implications for the selfassembly properties of trifluoromethylated and potentially perfluoroalkylated acenes. As a control, we studied dimers in which the substituents are fluorine or methyl groups. Fluorine is commonly incorporated into n-type semiconductors through fluorination of the acene core. A first principles understanding of how each of these groups influence dimer configurations, π−π distances, and interaction energies relative to other dimers, is important to future device development. To explore the effect of fluorination on the type of dimers formed, we studied mono, di, and trifluorobenzene dimers with benzene to see how it compares to trifluoromethylation. Slightly reduced π−π distances were found, owing to the smaller steric bulk of fluorine versus the trifluoromethyl group. However, the interaction energies were significantly smaller when compared to the CF3 substituted dimers with benzene. Because of the other advantages of perfluoroalkyl groups over fluorine on sp2 carbon (i.e., fluorobenzene) such as relative chemical inertness73 and the ability to use fluorinated solvents (i.e., hydrofluoroether (HFE) solvents) in device fabrication as an orthogonal solvent when combined with the other organics

Figure 6. Side and top views of optimized geometries of pyridine derivative dimers. Geometry optimization was done with MP2/631g(d).

arrangement obtained with MP2 methods, two imaginary frequencies centered near −22.93 cm−1 were found with a trace amount of intensity. This calculation provides support that a cofacial arrangement of benzene monomers is in fact a saddle point under the methodology used.45 Basis set superposition error corrections were also necessary with the dimers to get accurate frequency results with MP2 method as had been previously noted.71 Because we wanted to study the cofacial potential well of our systems due to the enhanced charge transfer properties with this orientation,59 our goal was not to find the global minimum dimer configuration, which would have been very difficult for each type of dimer. Instead, we wanted a picture of the chemistry occurring with the sandwich dimer and the enhanced frontier orbital overlap that is present with this type of dimer. To make sure that the monomer energies were reliable, all monomer calculations were carried out at MP2/6-31g* converged with 0 imaginary frequencies. The implications of this are that the monomers were at local minima, while the benzene dimer for example, was at a saddle point in a sandwich arrangement, which is very much in line with calculations performed at a much higher level.44 Encouraged by the qualitative similarity of our results with the much more expensive CCSD(T) calculations, we extended this method to dimers with highly electron-deficient CF3 substituents. Under the conditions used, the benzenetrifluoromethyl benzene B(1,2) dimer adopts a T-shaped configuration with a higher interaction energy than the unsubstituted benzene dimer. Extending the number of CF3 groups, the interaction energy increases dramatically, and a cofacial arrangement of molecules is maintained, with a slight amount of slipping in dimer orientation. This indicates that it is possible to engineer the type of interaction that is most favorable by adding the proper number of trifluoromethyl groups. Surprisingly, the interaction of electron-deficient molecules with each other had stronger interaction energies than donor− acceptor compounds that consisted of electron-rich and electron deficient monomers. From calculated interaction energies, bis-CF3 substituted benzene dimers (H(3,3), E(5,5), F(6,6)) are more stable than mono-CF3 substituted benzene

Table 3. Calculated Dimerization Energies (Eint/kcal/mol) and π−π Stacking Distances of Various Dimers between Thiophene and Fluorinated and Trifluoromethylated Benzene Derivatives at MP2/aug-cc-pvDZ//MP2/6-31g(d) Level of Theorya

a

dimer

monomer pair

Eint kcal/mol

π−π distance/Å

dimer

monomer pair

Eint kcal/mol

π−π distance/Å

P Q R S

17,1 17,2 17,3 17,4

−3.00 −4.62 −6.37 −7.26

3.779 3.762 3.612 3.570

T U V

17,7 17,8 17,9

−3.84 −4.21 −5.02

3.618 3.678 3.561

Eint: calculated interaction energy of dimer. F

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quadrupole moment is positive owing to strong electron induction. By examining the interaction energies of these compounds with the prototypical benzene (which has a negative Θzz value as shown above), only a modest R2 value of 0.6161 was obtained when the quadrupole moment was compared with the interaction energy gained upon dimerization of the compound with benzene. This indicates that other components, such as dispersion, induction, or charge transfer may also play a role in these donor−acceptor systems. In all cases of trifluoromethylation, fluorination, and methylation of aromatics, trifluoromethylation leads to stronger π−π interactions based on the dimerization energies and the distances between the aromatic cores. Among all calculated trifluoromethylated dimers, dimer B(1,2) is the only exception showing a stable T-shaped structure. For trifluoromethylated benzene derivatives calculated here, it is likely that π−π stacked structures are more favorable over T-shaped dimer structures. In an effort to test our hypothesis that cofacial dimers are favored over T-shaped configuration for donor−acceptor pairs, we performed two fully relaxed calculations of benzene and 1,3trifluoromethylbenzene with one starting out in a cofacial arrangement, and the other in a T-shaped geometry. We found that both the T-shaped and cofacial arrangements of these molecules are in wells of the PES; however the cofacial arrangement (−6.11 kcal/mol) of these compounds was calculated to be more stable than the T-shaped arrangement (−4.20 kcal/mol) by nearly 1.9 kcal/mol. Because of the electron-withdrawing nature of nitrogen, heteroacenes and oligopyridines have vastly different properties than their phenyl counterparts. They have self-assembly properties that make them appealing for many host−guest applications.69 In addition, they have uses in dye-sensitized solar cells (DSSCs)75 because of the favorable metal to ligand charge transfer of ruthenium with substituted bipyridines. Understanding the fundamental interactions between pyridine and substituted pyridine molecules is important to further develop the potential applications of these systems. Table 5 shows the results of the calculations with nitrogenacene cores that optimized geometries are shown in Figure 6. Previous theoretical work by Castellano76 and others77,78 have found the energy of the pyridine dimer to be near −3 kcal/mol for the antiparallel dimer that we considered. Our MP2 results slightly overestimate this value to −4.83 kcal/mol. The π−π distance found by Castellano to be optimal (3.6 Å) compares well to our value (3.645 Å). This once again justifies our method of using unconstrained optimizations to probe π−π interactions. To extend these studies, we compared pyridines substituted with various groups at the para position. We chose a variety of R groups that are electron-donating12 (CH3 with a σp value of −0.17), electron-withdrawing (CF3 with a σp of 0.54), and a group intermediate between the two (F, σp value of 0.06). The methyl substituted pyridine dimer Y(15,15) had a much stronger interaction energy and reduced intermolecular distance than trifluoromethyl pyridine or the fluoropyridine has. In this case, it appears that both dipole−dipole and quadrupole−quadrupole interactions are much more significant than substituted benzene dimers. This perhaps is due to the electron-withdrawing effect of the in-ring nitrogen of the pyridine molecules. To better understand the characteristics of bipyridines, DFTD optimizations were performed with the B97-D/TZV level of theory as this has been previously used41,42,79 to study weak

used as solvents, perfluoroalkyl groups are more advantageous than simple fluorine groups for organic semiconductors. Figure 4 shows optimized dimer geometries of donor− acceptor pairs created by the interaction of electron-deficient aromatic cores (CF3-substituted benzenes) with an electronrich aromatic core (thiophene). This type of dimer can potentially model the p-n junction of an organic device. All the dimers involving thiophene and trifluoromethyl benzenes maintain a π−π stacked overlapping mode. The interaction energies increase as the benzene ring becomes more electron deficient resulting from multiple trifluoromethyl group substitution (Table 3). Though direct ring fluorination reduces the potential steric effect hindering π−π overlapping, the smaller quadrupole moments of direct ring fluorinated benzenes compared to those that are trifluoromethylated may explain why fluorinated dimers have lower interaction energies than their CF3 counterparts. To have a model for alkylated benzene, we considered the toluene dimer, ortho-xylene dimer, and meta-xylene dimer. Their optimized dimer geometries are shown in Figure 5. While the dipole moments and quadrupole moments created by the addition of a methyl group are smaller than that of the trifluoromethyl group, they are still significant, and we expected it to lead to increased interaction energies when compared to the benzene dimer. The toluene dimer L(10,10) adopted a cross configuration where the C−H bond of the methyl group interacts with the π-cloud of the opposing aromatic ring. This compares well to the work of Rogers et al.,74 though the energy we obtained for the dimer was significantly higher than the value that their group obtained. This can be attributed to different basis sets and the level of theory employed. Ortho-xylene and meta-xylene differ significantly in their calculated dipole moments calculated through MP2/aug-ccpvDZ (0.454 vs 0.256). If simple dipole−dipole interactions were the only factor, ortho-xylene should have a stronger interaction in dimer form. However, meta-xylene has a much stronger interaction energy and improved cofacial packing when compared to ortho-xylene. Similar to the toluene dimer, the ortho-xylene dimer is most stable in a cross configuration; however, the steric bulk of the opposing methyl groups destabilizes it. The methyl groups of meta-xylene lock the dimer into a configuration where the interaction energy, packing, and π−π distances are optimized. Table 4 shows that the quadrupole moments of the monomers chosen have a wide variation from negative to positive. Monomers 6, 9, and 19 are all cases where the Table 4. Traceless Quadrupole Moments Given in DebyeAngstroms Projected along the zz-Direction monomer

Θzz

monomer

Θzz

1 2 3 4 5 6 7 8 9 10 11

−6.2200 −5.8577 −1.7638 2.9729 −5.5556 0.2203 −4.4531 −2.1380 0.4945 −5.8071 −5.5333

12 13 14 15 16 17 18 19 20 21

−5.3330 −4.1320 −1.5693 −1.7496 −3.0962 −5.5296 −9.6354 2.0994 −2.2358 −10.2565

G

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Table 5. Calculated Dimerization Energies (Eint/kcal/mol) and π−π Stacking Distances of Various Dimers between Pyridine and Bipyridine Derivativesa dimer

monomer pair

Eint kcal/mol

π−π distance/Å

dimer

monomer pair

Eint kcal/mol

π−π distance/Å

W X Y Z

13,13 14,14 15,15 16,16

−4.83 −5.78 −4.69 −7.98

3.645 3.575 3.537 3.520

α β χ δ

18,18 19,19 20,20 21,21

−5.81 −16.65 −7.68 −11.32

3.679 3.132 3.608 3.355

Dimers W(13,13), X(14,14), Y(15,15), and Z(16,16) were calculated at the MP2/aug-cc-pvDZ//MP2/6-31g(d) level of theory. Dimers α(18,18), β(19,19), χ(20,20), and δ(21,21) were calculated with the DFT-D method at the B97-D/TZV//B97-D/TZV level of theory. a

Notes

interactions of systems whose size exceeds the computational capabilities of the MP2 method. The trifluoromethylated bipyridine had the strongest π−π interactions as measured by intermolecular distances and interaction energies (Table 5). These results indicate that pyridine oligomers have different properties than the single pyridine cores, and also indicate that they can be tuned by the addition of different functional substituents.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are thankful for the financial support from the U.S. Army Research Office (Grant W911NF-09-10472). J.D.M. thanks the NSF IGERT program for support of his graduate study. Authors acknowledge the University of South Dakota high performance computing facility and Mr. Douglas Jennewein for computing time.

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CONCLUSION Through systematic computational studies of weak noncovalent π−π interactions between trifluoromethylated, fluorinated, and methylated benzene, thiophene, and pyridine derivatives, we have demonstrated that introducing trifluoromethyl group onto aromatic cores increases the weak intermolecular π−π interaction energies comparing to direct ring fluorination and methylation. Our results illustrate that trifluoromethylation is a powerful tool in increasing π−π intermolecular interactions while at the same time it increases the electron-deficiency of the aromatic rings, which is critical for the n-type organic semiconductor materials, particularly the air-stable ones. In most cases, the addition of CF3 groups leads to reduced π−π distances, except in some cases where fluorinated aromatic dimers show the reduced intermolecular distances, most likely because of smaller steric effect. Though the CF3 group possesses a greater steric effect than the CH3 and F groups, simply partially substituting the CF3 group on the aromatic ring leads to stronger π−π interaction energies compared to CH3 and F substitutes, which is consistent with our previous observation in the solid state structures of perfluoroalkylated aromatics.30 Together with its chemical stability, we conclude that aromatic trifluoromethylation, perhaps including perfluoroalkylation, is a better choice for air-stable n-type organic semiconductor materials.

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ABBREVIATIONS DFT-D, dispersion-corrected density functional theory; MP2, second order Møller−Plessett perturbation theory

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ASSOCIATED CONTENT

S Supporting Information *

Coordinates of optimized geometries, total energies, full quadrupole moment data of all monomers studied, table of the geometry parameters of optimized dimers, complete list of reference 57, and comparisons between DFT-D and MP2 calculation results. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. H

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