Arene Trifluoromethylation: An Effective Strategy to Obtain Air-Stable n

Article pubs.acs.org/JPCA

Arene Trifluoromethylation: An Effective Strategy to Obtain AirStable n‑Type Organic Semiconductors with Tunable Optoelectronic and Electron Transfer Properties Haoran Sun,* Anjaneyulu Putta, and Michael Billion Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States S Supporting Information *

ABSTRACT: Modulation of organic semiconductor band gap, electron affinities (EA), ionization potentials (IP), and reorganization energies (λ) associated with charge transfer is critical for its applications. We report here that trifluoromethylation not only increases both IP and EA significantly as expected but also narrows the HOMO−LUMO band gaps and increases considerably the airstability of arene-based n-type organic semiconductors. The increased air-stability results from relatively high EA energies and a change in oxidation mechanism. Calculated EAs and IPs show that trifluoromethylated arenes are excellent candidates for n-type semiconductor materials; though a moderate increase of inner-sphere reorganization energy (λi) associated with charge transfer is the penalty for the improved performance of the trifluoromethylated compounds. However, since λi decreases as the π conjugation increases, a rational design to produce air-stable n-type semiconductor materials with reasonably small λi is simply to prepare trifluoromethylated arenes with extended π conjugation. Furthermore, we found that structural isomerization can finetune the optoelectronic and electronic transfer properties of the corresponding aromatics.

1. INTRODUCTION Air-stable and moisture-resistant organic semiconductors are essential for the development of organic solar cells (OSCs), organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), as well as molecular electronics.1 The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), HOMO−LUMO gaps, electron affinity (EA), ionization potential (IP), and reorganization energy (λ) associated with charge transfer are critical parameters for the construction and function of these electronic and optoelectronic devices. The ability to tune these parameters over a large range enhances the chemist’s opportunity to discover new chemical-, photochemical-, and operational-stable organic electronics and optoelectronics that operate in the presence of air. Particularly, HOMO−LUMO gaps are used, in conjunction with absorption spectra, to evaluate the optical properties of these semiconductor materials. EA and IP values are employed for selecting electrode materials with the appropriate work function in semiconductor devices.2 Charge mobility (electron mobility for n-type semiconductor or hole mobility for p-type semiconductor) is determined by the overall reorganization energy associated with charge transfer where inner-sphere reorganization energy (λi) is decided by the molecular structure, and outer-sphere reorganization energy (λo) is decided by the molecular packing in its crystal structure.3 Though direct tuning of overall reorganization energy (λi + λo) would be ideal for the rational design of new organic © XXXX American Chemical Society

semiconductor devices, it would be more practical to minimize the λi at the molecular level and then tune λo through crystal engineering where many challenges remain. Though many substituents can tune the HOMO and LUMO energy levels, IPs, and EAs of arene-based organic semiconductors, there are only a few functional groups (e.g., trifluoromethyl, perfluoroalkyl, and −SF5) that possess strong electron-withdrawing ability, superhydrophobicity, and superb chemical- and photochemical-stabilities. Because of its highest electronegativity, fluorine forms highly polar, relatively nonpolarizable, strong C−F bonds (bond dissociation energy = 485 kJ/mol (C6F6) to 547 kJ/mol (CF4)). Recent work, including high efficiency OSCs using partially fluorinated polymers4 and OFETs using fluorinated small molecules,5 shows that fluorinated aromatics exhibit profound n-type semiconductor behavior6 due to fluorine’s electron withdrawing ability and, perhaps, the excellent crystal engineering potential of fluorinated compounds.5a,7 Both direct ring fluorination and ring perfluoroalkylation including trifluoromethylation8 have been employed to stabilize arene-based organic semiconductors and to transform p-type semiconductors into n-type semiconductors.6f,9 However, a large dynamic range (i.e., over 1 eV changes on IP and EA) of electronic properties,10 superhydrophobicity, oleophobicity, and chemical-stability against reductive-defluorination11 and Received: February 21, 2012 Revised: June 30, 2012

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dx.doi.org/10.1021/jp301718j | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Calculated HOMO and LUMO energies and energy gaps for fluorinated and trifluoromethylated benzenes (a), trifluoromethylated anthracenes (corresponding structures are shown in Figure 2) (b), and per-trifluoromethylated acenes with the x axis is the total number of benzene rings in acenes (c). HOMO−LUMO energies of isomers of bis-, tri-, and tetra-substituted benzenes are also shown in panel a.

CF3 substitution position could fine-tune the HOMO, LUMO, IP, and EA energies,15 we found that the total number of CF3 substituents takes a dominant role in tuning the electronic properties over the positions of substitution (Figure 1a and Figure 3a). Given the many possible isomers for a large aromatic system we are dealing with, the computational works are only done with those representative molecules that are of interest either for practical applications or interest of theoretical works. We employed a reasonably accurate DFT method with the B3LYP level of theory and 6-311G(d,p) basis set for all computational works reported here unless otherwise specified in the text. All geometries were optimized under fully relaxed conditions and followed by Hessian calculations to check if local minima were located. IP, EA, and λi were calculated under adiabatic conditions.16 For comparison, vertical IPs and EAs were given in the Supporting Information as well. The coordinates of all optimized geometries were given in the Supporting Information. Unsubstituted benzene, naphthalene, anthracene, tetracene, and pentacene served as benchmarks to validate the method by comparing to reported experimental and computed values.16,17 These comparative studies validated that the computational method and basis set used here were sufficiently accurate. All calculations were performed on a 296 CPU cluster computer (USD HPC) at the University of South Dakota computing facility and EMSL Chinook supercomputer through a user access grant. GAMESS-US program,18 Avogadro and MacMolPlot19 interface programs were used for all the GAMESS calculations and results readout. The geometry optimization and frequency calculation for per-trifluoromethylated pentacene were done with Gaussian 09.20 For consistency and better comparison with the results of all other compounds, single-point energy calculations were also done with GAMESS program for per-trifluoromethylated pentacene using the geometry optimized with Gaussian. Though the absolute electronic energies computed by these two programs are significantly different, the calculated HOMO−LUMO energy gap and optical and electronic parameters with these two programs are within reasonable error range. IP, EA, and λ were calculated under adiabatic condition with cation, neutral, and anion species optimized under fully relaxed

nucleophilic aromatic substitution12 is not achieved by direct ring fluorination. In contrast, perfluoroalkyl groups possess stronger electron-withdrawing ability13 (σp value for fluorine on aromatic ring is 0.06, and σp value of perfluoroalkyl group is around 0.54) and higher stability against reductive-defluorination and nucleophilic aromatic substitution than direct ring fluorination. Perfluoroalkyl groups (CnF2n+1) provide greater hydrophobicity and oleophobicity to modified materials than alkyl groups14 and also provides potential benefit for crystal engineering5a and orthogonal solution-processing of the semiconductor materials. Here, we chose the trifluoromethyl group to represent the perfluoroalkyl groups in this study since electronic effect of trifluoromethyl and perfluoroalkyl groups are very similar.13 Thus, study of larger aromatic systems could be done computationally since the size of computational works will be significantly reduced after replacement of perfluoroalkyl groups with trifluoromethyl group. The dramatic difference between the electronic effects of CF3 and F substituents on the tuning aromatic system HOMO and LUMO energy levels is demonstrated in Figure 1. Interestingly, we further found that CF3 substitution also decreases HOMO− LUMO gap, while direct fluorination does not. Benzene and perfluorobenzene have almost the same HOMO−LUMO gap, but HOMO−LUMO gap of hexatrifluoromethylbenzene is reduced to 5.3 eV from 6.7 eV of unsubstituted benzene (Figure 1a), and the HOMO−LUMO gap for per-CF3 substituted anthracene is reduced to 2.56 eV from 3.56 eV of unsubstituted anthracene (Figure 1b). This initial observation of a decrease in HOMO−LUMO band gap upon trifluoromethylation encouraged us to further explore the electronic and steric effects of trifluoromethylation on arene-based organic semiconductor materials with a clear goal to obtain air-stable n-type organic semiconductors with tunable optoelectronic and charge transport properties.

2. GENERAL PROCEDURES AND COMPUTATIONAL METHODS The structures of all aromatic compounds reported here are listed in the Supporting Information (Scheme S-1 through S-6). All optimized geometries and their corresponding total energies are given in the Supporting Information as well. Though the B



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Figure 2. HOMO, LUMO, and electron density map of trifluoromethylated anthracenes. Electron density maps are plotted with the MacMolPlot program.

Table 1. Effects of Isomerization of Pertrifluoromethylation on Total Energies, HOMO and LUMO Energies, and HOMO− LUMO Energy Gap

a

Relative total energies of cis and trans isomers; unit is in kcal/mol. bUnits are in eV. C



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Figure 3. Adiabatic IP and EA of all trifluoromethylated benzenes (a), trifluoromethylated anthracenes (structures are shown in Figure 2) (b), and per-CF3 substituted acenes (from benzene to pentacene) (c). The IP and EA energies of isomers of bis-, tri-, and tetra-substituted benzenes are also shown in panel a.

gaps shown in Figure 1c. Further details will be discussed in the latter portion of this article. The LUMO maintains a similar shape for various CF3 substituted anthracenes; however, the HOMO shape changes as increasing CF3 substitution resulted in nonplanar distortion of the molecule. With the increasing numbers of substitution (compounds 5 and 6 in Figure 2), the HOMO crosses the nonplanar anthracene carbon frame from one side to the other to accommodate the structure change while maintaining the π conjugation. Such an unusual HOMO shape and nonplanar structure (see Figure S-4, Supporting Information) may be the reason that causes the HOMO energy increase that cancels out the electronic effect of the trifluoromethylation and further causes reduction of the HOMO−LUMO gap as LUMO energy is constantly decreasing followed by increasing the CF3 substitution (Figure 1b). The total electron density map clearly shows a dramatic increase of electron deficiency of the aromatic ring. The change of HOMO and LUMO energy levels is drastic via trifluoromethylation, more than 0.5 eV/CF3 group for anthracenes. Further comparison of pertrifluoromethylated acenes and unsubstituted acenes (from benzene to pentacene) found that the extent of decrease in HOMO−LUMO gap was reduced with increasing π conjugation (Figure 1c). 3.2. Effect of Trifluoromethylation on IPs, EAs, and Air-Stability. Significantly lowered HOMO and LUMO energies obtained from trifluoromethylation were also reflected on the increase of ionization potential (IP) and electron affinity (EA) (Figure 3a,b). For comparison, vertical IP and EA values are listed in the Supporting Information (Table S-6). Though the CF3 group is strongly electron withdrawing, the IP values do not increase monotonically, and a leveling effect is apparent once the total CF3 group reaches above 60% substitution on an aromatic compound. In contrast, the EA value continues to rise with increasing CF3 substitution (Figure 3a,b). Persistent increase of EA value after trifluoromethylation provides us an excellent opportunity for discovering air-stable n-type organic semiconductor materials. An EA threshold value for air-stable n-type organic semiconductor can be estimated based upon oxygen reduction potential and the electron transfer reaction pathways.

conditions, followed by Hessian calculation to check if a local minimum is located (judged by the absence of imaginary vibrational frequencies). Adiabatic IP and EA were calculated according to the following:

IP = Ecation − Eneutral

(1)

EA = Eneutral − Eanion

(2)

The adiabatic reorganization energy was calculated with the following: λ = λ1 + λ 2

λ1 = E±(Q N) − E±(Q ±) λ 2 = E N(Q ±) − E N(Q N)

E±(QN) is the charged state energy with neutral state structure, E±(Q±) is the charged state energy with charged state structure, EN(QN) is the neutral state energy with neutral state structure, and EN(Q±) is the neutral state energy with charged state structure.

3. RESULTS AND DISCUSSION 3.1. Effect of Trifluoromethylation on HOMO and LUMO Energies and HOMO−LUMO Energy Gaps. Exploration of larger aromatic systems including trifluoromethyl substituted naphthalenes, anthracenes, pyrenes, tetracenes, and pentacenes gives us consistent observation on HOMO− LUMO energy gap changes upon CF3 substitution. Figure 2 illustrates the HOMO and LUMO structures and total electron density map of various trifluoromethylated anthracenes. The data show that the HOMO and LUMO energy levels decrease dramatically, and HOMO−LUMO energy gap also decreases as the number of trifluoromethyl substituents increases (Figure 1b), though the magnitudes of such changes are less than those observed for benzene, perhaps due to the increase of π conjugation as shown in Figure 1c. We observed that different steric isomers of large acenes have different HOMO and LUMO energies and energy gaps (Table 1). These steric isomers lead to the variation of the HOMO−LUMO energy D



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Figure 4. Estimated first reduction potential of trifluoromethylated anthracenes from DFT calculated EA based upon the semiempirical correlation between EA and redox potential for polyaromatics, assuming that the solvation free energy is −1.99 eV.21a

Figure 5. Adiabatic λi of all trifluoromethylated benzenes (a), trifluoromethylated anthracenes (structures are shown in Figure 2) (b), and pertrifluoromethylated acenes and nonsubstituted acenes (from benzene to pentacene) (c). The solid lines in panels a and b are the linear fitting results for λih with R = 0.95, and the short dashed lines are employed to represent the λie changing trend in panels a and b.

of substituted anthracenes (Figure 4), we expect that compounds 3 (−0.22 V), 4 (0.32 V), 5 (0.92 V), and 6 (1.60 V) are air-stable n-type semiconductors provided that perfluoroalkyl chains block water access to the organic semiconductor core through crystal engineering, which is an ongoing effort in this lab. Directly calculating the reduction potential of organic semiconductor materials in solution is of significant interest of both theoretical and practical applications. Various solvation models, for example, polarizable continuum model (PCM) and conductor-like screening model (COSMO), together with the Born−Haber cycle have been used in calculation of absolute redox potential and correlated to experimental values. Though dramatic increase in calculation accuracy was observed recently with various solvation models, DFT method still underestimates the absolute redox potential in solutions. Our calculated IPs and EAs in CH2Cl2 with PCM model results (Tables S-4 and S-5, Supporting Information) show that, without using the Born−Haber cycle and considering the ionpairing effect, it is still somewhat challenge to estimate the thermodynamic redox potential in solution from calculated IPs or EAs with a solvation model, for example, the PCM model. These calculated IPs and EAs value may sometimes coincidently match well with onset redox potential, which is a common way to determine the redox potential, HOMO and

The commonly accepted EA threshold value of 2.79 eV for air-stable hydrocarbon-based semiconductor was estimated as 0.38 V (vs SHE) based upon Parker and Ruoff’s work on EA and reduction potential correlation.21 This semiempirical formula was obtained based upon the EA values obtained in thet gas phase and the formal reduction potentials obtained by electrochemistry in solutions. The linear correlation gives estimated solvation energy of 1.99 eV for poly aromatic hydrocarbons (PAH). Similar correlation between EA and reduction potentials of PAH has also been explored by the Gillmore group.22 E = EA + 1.99 − 4.48V

This estimated threshold is about 100 mV higher than the oxygen two-electron reduction potential of 0.281 V (vs SHE) at pH 7.23 Here, water, the proton source in air, with access to the aromatic core, is essential for this two-electron oxidation reaction of the radical anion (electron transporter in n-type organic semiconductor device) of the n-type semiconductor where oxygen was reduced to H2O2. With perfluoroalkyl group’s superhydrophobicity, water (proton source) could be eliminated from accessing the aromatic core, thus perfluoroalkyl groups block the multielectron reduction pathways for O2. Oxygen molecule is forced to undergo 1-electron reduction at −0.33 V (vs SHE).24 Thus, from estimated reduction potential E



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LUMO energy, and energy gap in the organic electronic field, and are widely used in organic semiconductor materials research.25 However, comparing to the formal redox potential defined from the thermodynamic viewpoint, the onset redox potential can be up to few hundreds of millivolts off from the formal redox potential and even more if the electron transfer kinetics are sluggish or if there are following chemical reactions.26 Similar to unsubstituted acenes, followed by increasing π conjugation, EA values increase and IP values decrease for CF3substituted acenes (Figure 3c). Both IP and EA almost show a leveling effect for substituted acenes. Therefore, one could expect that further changing the π system will have less impact on IP and EA. Though the calculated value may possess absolute error from the DFT method itself, the changing trend of EA values clearly indicates that trifluoromethylation of arenes is likely a potential pathway to discover new air-stable and moisture-resistant n-type organic semiconductors that could operate in the presence of air. 3.3. Effect of Trifluoromethylation on Reorganization Energy Associated with Charge Transport. EA and IP are the thermodynamic parameters that determine the air-stability, while charge mobility determines the semiconductor performance. To gain further insight of the effects of trifluoromethylation on semiconductor device performance, we further calculated the inner-sphere reorganization energy of abovementioned trifluoromethylated aromatics under adiabatic condition. Electron mobility is controlled by both inner-sphere and outer-sphere reorganization energies. The inner-sphere reorganization energy depends on the semiconductor molecule itself, while the outer-sphere reorganization energy depends on the crystal structure of semiconductor materials. In searching for new air-stable organic semiconductor materials, it is practical to evaluate the inner-sphere reorganization energy first before solving the much more complicated crystal engineering problems to minimize the outer-sphere reorganization energy. We observed that both λie and λih increase followed by increasing CF3 substitution on the same aromatic ring (Figure 5a,b). Although ring-fluorinated aromatic compounds generally have relatively smaller reorganization energy6b,7b,8a,10,27 associated with charge transfer, their chemical instability11,12 and relatively static electronic structure precludes direct fluorination of the aromatic ring as a winning strategy for obtaining new organic semiconductor materials bridging a large span of IP and EA. Further, we observe that the trifluoromethylation affects the reorganization energies differently on hole and electron transfer. Though the reorganization energy of hole transport (λih) rises linearly, λie levels out for electron transport (Figure 5a,b). This one-time-only penalty provides a chance that more perfluoroalkyl groups can be introduced without being concerned about the increasing of λie as the number of CF3 substitution further increases. Thus, one could turn this onetime unavoidable penalty into a strategy in the rational design of air-stable n-type organic semiconductors since generally mono- or bis-perfluoroalkylated aromatics are still not air-stable n-type semiconductors. Fortunately, this increased λi could be reduced by increasing π conjugation. Significant decrease of inner-sphere reorganization energy is observed followed by increasing π conjugation (Figure 5c). This result is in-line with the changing trend of reorganization energy of unsubstituted acenes (Figure 5c) that are consistent with reported experimental and computational

data.17 Thus, one could expect that, with a large aromatic system, trifluoromethylation could potentially provide air-stable semiconductor materials with reasonable λie and λih. The trend of reorganization energy changes for pertrifluoromethylsubstituted acenes is similar to what we observed in those unsubstituted ones, except λie of pertrifluoromethyl pentacene is 1.3 eV, which is 0.8 eV off from the extended trend line. The sterically hindered CF3 group on acenes provides a relatively high rotational barrier between neighboring CF3 groups. Thus, with this large system, the structure of optimized local minima could fall into different conformations of CF3 groups between neutral and charged states under fully relaxed geometry optimization. Though locating global minima would minimize this large uncertainty of the inner-sphere reorganization energy, finding the global minima for such large systems is still a significant challenge. 3.4. Nonplanar Effect of Trifluoromethylated Arenes. Further, nonplanar aromatic cores were observed for those with multiple CF3 substitutions, and the degree of nonplanarity increases as the number of CF3 groups increases. In the limiting case where all H atoms are replaced with a CF3 group, we observed a nonplanar structure on the aromatic core due to the steric effect of the CF3 group, particularly for the large acenes. The CF3 group is not within the average plane of the aromatic core in the optimized structure; this leads to different isomers when multiple CF3 groups are present and not present on the same plane. Table 1 shows examples of per-CF3-naphthalene, per-CF3-anthracene, and per-CF3-tetracene isomers and their corresponding optimized geometries, relative energies, HOMO and LUMO energies, and HOMO−LUMO energy gap. Comparing these two trifluoromethylated anthracene isomers, the cis isomer (defined as two para-CF3 groups at the same side of the phenyl ring plane) displays a lower total energy of 8.2 kcal/mol relative to its trans isomer (defined as two para-CF3 groups at opposite sides of the phenyl ring plane). Similarly, cis-per-CF3-tetracene is 35.7 kcal/mol lower energy than that of trans-per-CF3-tetracene. The corresponding naphthalene isomers, however, only possess 1.6 kcal/mol total energy differences. We further observed that the HOMO− LOMO energy gap of the trans isomer is lower than that of cis isomer in both per-CF3-anthracene and per-CF3-tetracene cases. However, the cis and trans isomers of naphthalene have the same HOMO−LUMO energy gap of 3.89 eV. The overall reduced HOMO−LUMO enegry gap is mainly due to the increase of HOMO energy caused by nonplanar aromatic core structure. This is in line with previously observed red shift of absorption spectra of nonplanar porphyrin.28 The isomerization-induced HOMO−LUMO energy gap change increases from naphthalene to tetracene, perhaps due to the increased degree of nonplanarity caused by steric hindrance. The two types of steric isomers we studied here are only a representation for a large number of possible steric isomers. It remains a significant challenge to exhaustively study all the steric isomers for these large systems, though it would be of theoretical interest to investigate systematically the isomerization effects on total energy, HOMO and LUMO energies and energy gap, IPs, EAs, and reorganization energies. 3.5. Effect of Substitution Position of Trifluoromethyl Group: Fine-Tuning the Electronic Properties. Significant changes of the HOMO−LUMO energy gap, IP, EA, and reorganization energies upon trifluoromethylation of aromatics provide an opportunity to search new organic semiconductor materials over a large range of potential landscape. Further fineF



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Table 2. Effects of Substitution Position on Electronic Properties of Selected Aromaticsa

a

acene_substitution

HOMO

LUMO

band gap

λih

λie

IP

EA

anthracene_2,6-CF3 anthracene_9,10-CF3 anthracene_2,3,6,7-CF3 anthracene_2,6,9,10-CF3 pentacene_2,6,9,13-CF3 pentacene_4,6,11,13-CF3 tetracene_6,12-CF3 pentacene_6,13-CF3

−6.149 −6.095 −6.775 −6.775 −5.823 −5.741 −5.754 −5.306

−2.530 −2.748 −3.156 −3.374 −3.700 −3.619 −3.045 −3.238

3.619 3.347 3.619 3.401 2.123 2.122 2.709 2.068

0.219 0.220 0.292 0.227 0.169 0.162 0.194 0.131

0.433 0.497 0.338 0.494 0.290 0.135 0.287 0.269

7.608 7.552 8.143 8.093 6.998 6.197 6.983 6.525

1.179 1.436 1.848 2.194 2.591 2.343 1.694 2.077

All units are in eV.

potential of the molecular surface, resulting in strong interaction with electron rich donor molecules, for example, p-type organic semiconductors, directly forming a molecular p− n junction. Thus, modulating charge mobility of organic semiconductors could potentially be done through careful control of intermolecular interactions where outer-sphere reorganization energy takes the dominant role.

tuning these optoelectronic and electron transfer properties are also important for optimization of organic semiconductor device performance. We observed that the position of CF3 substitution also affects those parameters in a smaller range, i.e., it fine-tunes the properties. Table 2 gives head to head comparison of different substitution positions of bis- and tetraCF3-substituted anthracenes, 2,6,9,13-tetratrifluoromethyl pentacene, and 4,6,11,13-tetratrifluoromethyl pentacene. 6,12-Bistrifluoromethyl tetracene and 6,13-bistrifluoromethyl pentacene shown in Table 2 could be potentially prepared, though with somewhat of a challenge. Slight changes on planarity of the aromatic core was observed on 6,12bistrifluoromethyl tetracene and 6,13-bistrifluoromethyl pentacene. Though 6,13-bis-trifluoromethylation does not make the pentacene thermodynamically air-stable, tetratrifluoromethylation (the EA value of 2,6,9,13-tetratrifluoromethyl pentacene is very close to the oxygen one-electron reduction potential) could make the pentacene air-stable through both thermodynamic and kinetic contributions of the trifluoromethyl groups. Recent development in the n-type organic semiconductor field, specifically perfluoroalkylation of the aromatic core, has been found useful to turn p-type semiconductor into n-type, some even show air-stability (perhaps kinetic stability caused by, at least partially, the superhydrophobic blocking effect of the perfluoroalkyl groups. Extending our computational results of trifluoromethylated arenes into perfluoroalkylated ones, both computationally and experimentally, is currently under way in this laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

Detailed computational procedures, HOMO and LUMO energy, IP, EA, and λi for additional compounds we studied, coordinates of optimized geometry, and total energy of all compounds in this article. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Army Research Office (grant number W911NF-09-10472) and the University of South Dakota high performance computing facility. M.B. thanks NSF Northern Plains Undergraduate Research Center seed grant for supporting his summer research. A portion of the computational work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

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CONCLUSIONS In summary, we have demonstrated, through systematic DFT calculations, that trifluoromethylation is a strategy that can modulate HOMO−LUMO energy gap, IPs, EAs, and innersphere reorganization energies (λi) of organic semiconductors over a large range. Though we have found that inner-sphere reorganization energy somewhat increases as the degree of trifluoromethylation on the same aromatic ring increases, it is recognized that λi is significantly reduced when the πconjugation is increased. Furthermore, the total reorganization energy comprises both outer- and inner-sphere reorganization energies. The outer-sphere reorganization energy depends on the intermolecular interactions, the distance between donor and acceptor in the crystal structure, and/or the polymeric setting. Though the outer-sphere reorganization energy calculation, which requires the crystal structure information, remains a challenge without molecular packing information, the innersphere reorganization energy values will guide the synthetic work to pursue the molecules with relatively small inner-sphere reorganization energy. Further, the strong electron withdrawing ability of the CF3 group significantly changes the electrostatic

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REFERENCES

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