and Intramolecular Interactions in Some Bromo- and Tricyanovinyl

Article pubs.acs.org/crystal

Inter- and Intramolecular Interactions in Some Bromo- and Tricyanovinyl-Substituted Thiophenes and Ethylenedioxythiophenes Phuong-Truc T. Pham† and Mamoun M. Bader*,‡ †

Department of Chemistry, Pennsylvania State University, Worthington Scranton, Pennsylvania 18512, United States Department of Chemistry, Alfaisal University, P.O. Box 50927, Riyadh 11533, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: We report herein on the competing inter- and intramolecular interactions in seven structurally related thiophene and ethyelenedioxythiophene (EDOT) molecules, substituted with bromine and/or tricyanovinyl (TCV) groups in various combinations, using single crystal structural analyses. Br···Br Interactions of less than 3.5 Å appear to be dominant in the crystal structures of the dibromo EDOT molecules and yet are almost nonexistent in 5,5″-dibromoterthiophene (shortest Br···Br distances are >4.2 Å), indicating a cooperative role involving the Br and the ethylenedioxy moiety. Short Br···Br distances of 3.5 Å within stacks and between adjacent stacks of molecules in crystalline dibromo EDOT dimer (6) could be utilized for the preparation of highly ordered polymers with the perfectly planar EDOT dimer as repeating unit, similar to the work reported by Wudl.18 On the other hand, new dimeric motifs are formed in Br-EDOT-TCV as strong S···N (3.03 Å) intermolecular interactions in TCV-EDOT are replaced by competing N···Br (2.99 Å) interactions. Short intramolecular N···S distances ranging from 3.2 to 3.3 Å are associated with small dihedral angles between the TCV and thiophene planes ranging from 0.80 to 4.3 deg. A slight enhancement of molecular planarity apparently has a profound impact on the extent of conjugation as evident from the CC bond lengths (1.34−1.40 Å) and C−C (1.37−1.44 Å) within the thiophene rings. These findings suggest that N···S, N··· Br, and Br···Br inter- and intramolecular interactions could be utilized as additional crystal engineering tools to promote molecular planarity and arrangement of higher oligomers in the solid state prior to polymerization of thiophene-based molecular materials. On the basis of the current study, these interactions appear to also enhance the stability of the structure and influence intramolecular charge transfer and π-stack formation patterns.

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molecules in single crystals and thin films.7 Improving our understanding of the intricate molecular/solid state structure property relations is crucial for enhancing the performance of organic electronic devices in which these materials are used.8 Our ability to introduce and selectively control noncovalent interactions in the solid state is somewhat limited and not as well understood or developed compared with our synthetic ability to make new molecules. Studies directed toward finding new clamps to hold molecules together in favorable arrangements is crucial in developing more efficient devices. Such studies are of paramount importance as they could help improve our ability to find new useful interactions, understand competing existing ones, and use them selectively to control how molecules pack in thin films and crystals. Promoting structural features that are conducive for better charge transport such as planarity and π-stack formation is especially important. Examination of the Cambridge Crystal Structural Database reveals that π-stacking is observed frequently for thiophenes endowed with one or more electron withdrawing groups.9 We

INTRODUCTION Thiophene-based materials have played a major role in materials research science and engineering as they have a wide range of interesting electrical and optical properties.1 Oligothiophenes serve as model compounds for the corresponding polymers as they have well-defined structures allowing for better understanding of structure/property relationships.2 Their solid state structures can be studied using X-ray crystallography, and device performance can then be related to molecular design as well as solid state packing.3 Furthermore, advances in the syntheses of thiophene and ethylenedioxythiophene chemistry has given this class of compounds a clear advantage over other organic competitors such as polyacenes.4 In particular, Stille and Suzuki coupling reactions have been widely and elegantly applied to access numerous oligo- and polythiophenes with a wide range of structural modifications and architectures.5 Better understanding of the relationship between molecular structure/macromolecular architecture and how these in turn impact device performance is important for advancing the field of organic electronics.6 Despite recent advances in this area, only qualitative predictions can be made on how some structural modifications will impact the solid state packing of these © 2014 American Chemical Society

Received: May 21, 2013 Revised: February 2, 2014 Published: February 6, 2014 916

dx.doi.org/10.1021/cg401513n | Cryst. Growth Des. 2014, 14, 916−922

Crystal Growth & Design

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have previously reported on the electrochemical and structural features of some tricyanovinyl (TCV) and dicyanovinyl (DCV)-substituted oligothiophenes.10,11 We found that these strong electron acceptors provide a rather simple approach to modifying the electrical properties of oligothiophenes as evident from their electrochemical data, presumably by lowering the lowest unoccupied molecular orbital (LUMO) levels and promoting molecular planarity and π-stack formation and in some cases favoring the syn conformations. They also play a crucial role in improving the thermal stability and solubility in organic solvents (compared with substituent-free thiophenes) of oligothiophene, both of which are vital characteristics for the fabrication of thin film and single crystal devices from both vapor phase and solution. Subsequently, TCV-substituted oligo-thiophenes were shown to have ambipolar transport properties.12 Marks and co-workers have recently reported on the use of inter- and intramolecular S···O interactions to enhance the planarity and crystallinity and extend conjugation and thus hole mobilities of dialkoxy bithiazole polymers, thieno[3,4-c]pyrrole-4,6-dione-based polymers, and thienyl-vinylene (TVT)-based polymers.13−15 We report herein the analyses of single crystal structural data for a series of closely related thiophene and ethylenedioxythiophene (EDOT) molecules endowed with bromo- and TCV substituents. These molecules were prepared as building blocks for higher oligomers as part of our continued efforts to synthesize longer oligothiophene derivatives suitable for the construction of thin film and single crystal organic semiconductor devices. We direct our focus on the inter- and intramolecular interactions present in each of these molecules to help elucidate the impact of substituents on the different types of competing interactions and packing patterns observed. Seven molecules are considered in this study (Figure 1): 1(thieno[3,4-d][1,3]dioxol-6-yl)ethene-1,2,2-tricarbonitrile,

Article

RESULTS AND DISCUSSION

Compounds 1−7 were prepared following published literature procedures.16 Two reactions were used in the preparation of these molecules: bromination using N-bromosuccinimide (NBS) in chloroform and the reaction of tetracyanoethylene with thiophene derivative in dimethylformamide (DMF). Both of these reactions were run at room tempertaure, rendering this type of chemistry suitable for undergraduate laboratory course experiments.17 Purification was achieved using column chromatography followed by crystallization from a suitable solvent. Crystallographic data are summarized in Table 1. We have previously reported the crystal structures of compounds 4 and 7 and include them herein for comparative purposes.10,18 Structure of compound 3 was reported by Wudl and co-workers (with slightly different unit cell parameters), who have successfully used close Br···Br interactions to prepare intrinsically doped, highly ordered, and conducting PEDOT.19 We first consider the impact of TCV and Br on structural features such as bond lengths, planarity, and inter- and intramolecular interactions of EDOT analogues, 1−3. To the best of our knowledge, the crystal structure of EDOT is not known. The structure of the EDOT dimer has been reported.20 In compound 1, we note that the nitrogen atoms of the TCV group are involved in several intermolecular interactions including N···H (2.71 Å), N···C (3.13 Å), and N···S (3.037 Å). Other interactions include O···H (2.44 Å) and close intramolecular N···S contact of 3.303 Å. These interactions are all well below the sum of the VDW radii for these atom pairs as reported by Bondi.20 The interplanar distance from the TCV plane of one molecule to the thiophene plane of another is approximately 3.39 Å with the shortest atom-to-atom distance (C···N) being 3.164 Å. Upon introducing bromine in compound 2, Br···N overtakes the N···S interaction forming a new dimeric motif (Figure 2), and the angle between the thiophene and TCV planes also slightly increased from 2.40° in 1 to 3.52° in 2 (Table 2). It is interesting to note that the opposite trend is observed for the thiophene counterparts (4 and 5) as the torsion between the TCV and thiophene planes decreases from 4.33° in 4 to 0.80° in 5 (Table 2) as bromine is introduced. The enhanced planarity in 5 is associated with significantly shorter C−C bond lengths within the thiophene rings (1.37−1.39 Å) indicating intramolecular charge transfer. Examining the bond lengths in both 4 and 5 reveals that the degree of charge transfer is less pronounced in 4 than it is in 5, suggesting an electron donor role for the bromine atom in 5. Similar findings on the charge transfer properties and theoretical study of 4 has been reported.17 Apparently a slight enhancement of planarity can have a profound impact on the effective conjugation as evident from the observed bond lengths. Selected bond lengths are listed in Table 3, while all bond lengths in compounds 1−7 are readily available in the CIF files (see Supporting Information). Other weak intermolecular interactions observed in compounds 1−3 are tabulated in Table 4, with those having less than the sum of the VDW radii in bold face. Relatively short Br···Br interactions of 3.46 and 3.52 Å are observed in dibromo EDOT derivatives 3 and 6 respectively (Figure 3). These distances are notably shorter than the sum of the van der Waals radii (3.70 Å). Compound 3 also exhibits close Br···S (3.63 Å) and S···S (3.58 Å) interactions; compound 6 on the other hand has short Br···C interaction (3.54 Å). Other short contacts observed involve the oxygen atoms in both 3 and 6 (O···H).

Figure 1. Structures of compounds 1−7.

EDOT-TCV (1); 2-(6-bromothieno[3,4-d][1,3]dioxol-4-yl)ethene-1,1,2-tricarbonitrile, Br-EDOT-TCV (2); 4,6dibromothieno[3,4-d][1,3]dioxole, Br-EDOT-Br (3); 1-(5thiophen-2-yl)thiophen-2-yl)ethene-1,2,2-tricarbonitrile, 2TTCV (4); 2-(5′-bromo-[2,2′-bithiophen]-5-yl)ethene-1,1,2-tricarbonitrile, Br-2T-TCV (5); 6,6′-dibromo-4,4′-bithieno[3,4d][1,3]dioxole, Br-(EDOT)2-Br (6); 5,5″-dibromo-2,2′:5′,2″terthiophene, Br-3T-Br (7). 917



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Table 1. Crystallographic Information and Selected Structural Features for Compounds 1−7 compound formula space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) R factor (%) compound formula space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) R factor (%)

EDOT-TCV (1) C11H5N3O2S C2/c 20.488(10) 5.153(3) 19.516(10) 90.00 91.815(8) 90.00 2059.36 4.63 2T-TCV (4) C13H5N3S2 Pna21 7.8613(10) 24.495(3) 6.1207(8) 90.00 90.00 90.00 1178.62 3.03

Br-EDOT-TCV (2)

Br-2T-TCV (5)

Br-EDOT-Br (3)

C11H4BrN3O2S P21/n 13.677(2) 5.3859(8) 16.245(3) 90.00 93.971(3) 90.00 1193.78 3.81 Br-(EDOT)2-Br (6)

C13H5BrN3S2 P21/n 9.1417 12.9641 10.8000 90.00 92.378 90.00 1278.85 2.99

C12H8Br2O4S2 C2/c 19.311(3) 5.1730(8) 15.885(3) 90.00 120.464(2) 90.00 1367.78 2.42

C6H4Br2O2S Cc 25.335(4) 5.0261(8) 15.713(2) 90.00 123.924(2) 90.00 1660.25 4.41 Br-3T-Br (7) C12H6Br2S3 Pcc2 7.622 30.003 5.884 90.00 90.00 90.00 1345.57 5.27

Table 2. Observed Bond Lengths, νCN IR Stretching Frequencies, and Torsion between the Thiophene Ring and the TCV Planes Observed in Compounds 1, 2, 4, and 5a

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 torsion, Φ0 νCN, cm−1

Figure 2. Dimers of 1 (A) and 2 (B) showing the impact of bromine on the intermolecular interactions dominating the dimer formation.

We also find significantly short O···S interactions (2.92 Å) in 6 which are similar to those reported by Roncali for bis-EDOT, resulting in the formation of rigid planar structures for this class of compounds.20 On the other hand, the closest Br···Br distances observed in dibromotert-thiophene 7 are 4.36 and 4.22 Å. In considering the bithiophene derivatives 4 and 5, we note that while the sulfur atoms assume syn conformation in 4 they have anti conformation in 5 with intramolecular N···S distances of 3.22 and 3.31 Å, respectively (Figure 4). In compound 4,

EDOT-TCV (1)

Br-EDOT-TCV (2)

2T-TCV (4)

Br-2T-TCV (5)

1.356 1.417 1.396 1.423 1.375 1.145 1.147 1.145 3.303 1.745 1.708 2.40 2224, 2237

1.353 1.411 1.387 1.425 1.386 1.136 1.140 1.131 3.281 1.752 1.719 3.52 2224

1.383 1.392 1.386 1.436 1.371 1.145 1.137 1.142 3.224 1.728 1.738 4.33 2223

1.376 1.378 1.394 1.422 1.373 1.139 1.147 1.165 3.304 1,725 1.734 0.80 2216, 2241

a

(1) Lower stretching frequency is due to stronger N···S and/or N··· Br interactions. (2) Comparison of R1, R2, and R3 reveals that in compounds 4 and 5 all three C−C bonds are nearly equal in length, whereas in 1 and 2 clearly R2 is single bond. (3) C−S bonds are almost identical in 4 and 5, whereas they are significantly different in 1 and indicate the involvement of sulfur in the conjugation in the latter.

short distances are observed for N···H (2.64 and 2.47 Å). When a bromine is introduced in 5 however, new interactions are observed, namely, N···Br (3.21 Å), C···Br (3.46 Å), and N···S 918



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Table 3. Short Inter- and Intramolecular Distances in Compounds 1−3 short distances

∑VDW radii21

1

2

N···S interN···S intraN···H N···C O···H Br···Br Br···N Br···S S···S C···H

3.35 3.35 2.75 3.25 2.72 3.70 3.40 3.65 3.70 2.90

3.037 3.303 2.710 3.130 2.442

7.857 3.281 2.592 3.082

3

5.712 2.991 11.252 5.748 2.864

2.477 3.464 3.633 3.575 2.885

Table 4. Short Inter- and Intramolecular Distances in Compounds 4−7 short distances

∑VDW radii

4

5

N···S interN···S intraN···H N···C O···H Br···Br Br···N Br···S S···S C···H

3.35 3.35 2.75 3.25 2.72 3.70 3.40 3.65 3.70 2.90

3.404 3.224 2.640 3.406

3.325 3.306 2.469 3.461

4.185 3. 411

8.435 3.207 4.134 3.914 3.649

6

7

2.707 3.516

4.223

4.115 3.784 5.094

4.046 3.934 2.853

(3.64 Å). Again, this shows that bromine induces new patterns of interactions in the presence of the electron acceptor TCV. Without TCV, such interactions are not seen as exemplified by compound 7. The molecule also becomes more planar as evident from the torsion angle of 0.80 deg. This enhanced planarity is associated with C−C bond lengths throughout the two thiophene rings being essentially the same ranging from 1.37 to 1.39 Å, suggesting the presence of intramolecular charge transfer, whereas the bromine atom acts as a donor group. A summary of short distances found in compounds 4−7, with those less than the sum of the VDW radii highlighted, is listed in Table 4. Both inter- and intramolecular interactions involving bromine appear to also play a crucial role in driving the packing of these molecules. For example, upon bromination of 4 to give 5, one notes the enhanced molecular planarity, intramolecular charge transfer (donor/acceptor interactions), and π-stack formation involving four molecules in 5 as opposed to two pairs of π-stacked dimers in 4 (Figure 5). In the cases of EDOT-containing molecules, the enhanced planarity is more evident, as in compound 6, due to intra molecular interactions such as S···O and the close contacts involving bromine between adjacent stacks. As previously mentioned, the interactions between adjacent stacks of perfectly planar molecular EDOT dimers 6 can be utilized in solid state polymerization to produce highly ordered PEDOT, where the repeat unit is the planar EDOT dimer rather than the EDOT monomer. The IR spectra of these molecules are also interesting and worth noting. The nitrile group stretching frequency shows two peaks at 2224 and 2237 cm−1 in compound 1, whereas only one is observed at 2224 cm−1 upon the introduction of bromine in 2. Again an opposite trend is observed in the IR characteristics

Figure 3. Short intermolecular distances involving Br, C, and H atoms in compounds 3 (A) , 6 (B) , and 7 (C), respectively.

of compounds 4 and 5. In compound 4, the nitrile group shows a peak at 2223 cm−1, whereas upon the introduction of bromine two peaks are observed for 5 (2216; 2241 cm−1). Similar IR shifts in CN group have been used as spectroscopic marker in a variety of biological systems.22 Together, these spectroscopic data and the accompanying structural features are indeed intriguing and warrant further investigation. It may be possible that some of these may be useful as biological markers since they are highly colored materials.22,23 We have also carried out DFT-B3LYP/6-31G* level calculations for compounds 1−7 as gas phase isolated molecules, using Spartan 10 software.24 The main features of these calculations are as follows: (1) Calculated molecular geometries and shapes were in good agreement with the X-ray experimental data, lending further confidence in this level of theoretical study. (2) The evolution of LUMO−highest occupied molecular orbital (HOMO) levels are as one expects with calculated band gaps ranging from 2.64 eV for compound 5 [compared with a band gap of 4.22 eV for bithiophene] to 5.28 eV for compound 3. The reduction in band gap is mainly due to the lowering of the LUMO levels (Figure 6). (3) The HOMO localizes on the thiophene moiety, and the LUMO localizes on the TCV group without overlapping the HOMO, 919



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Figure 4. Short intermolecular distances in 4 (A) and 5 (B) involving Br, N, S, C, and H atoms.

Figure 6. Density functional theory calculated HOMO−LUMO levels B3LYP level for compounds 1−7.

Trends obtained by these calculations are consistent with the measured UV−vis spectra. For example: both 2T-TCV (4) and Br-2T-TCV (5) had nearly identical UV−vis spectra (λmax = 500 nm in benzene), whereas upon bromination, the quantum yield (Φf) was enhanced by almost an order of magnitude, with Φf of 0.013 and 0.101 for compounds 4 and 5, respectively. Φf was measured in benzene using quinine sulfate in 0.1 M H2SO4 a reference (Φf = 0.58 at λ = 350 nm). We are currently investigating the optical properties of these and related materials. We also like to note that none of these molecules showed field effect in thin film or in single crystal device configurations

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CONCLUSIONS We have shown that bromine atoms become activated and get involved in new patterns of intermolecular interactions in the presence of strong electron donors (−OCH2CH2O−) and/or strong electron acceptors (TCV) with Br···N and Br···S being most significant. Intramolecular CN···S close contacts are thought to help force planarity of molecules as evident from the dihedral angles between the thiophene and TCV planes. Surprisingly, short Br···Br interactions are more prevalent in the brominated EDOT molecules (3 and 6), while they are almost negligible or nonexistent in the brominated thiophenes (compounds 5 and 7). Both type I and type II Br···Br interactions, described by Ramasubu and Desiraju, were observed in these molecules (Figure 3A,B).25 Bromine atoms tend to act as donor groups in the presence of the TCV group (5) while not so in the presence of the stronger donor the (−OCH2CH2O−) group (2). In the case of

Figure 5. Packing of 4 (A) and 5 (B) along the b-axis. Note the enhanced π-stack formation in 5 (B).

which illustrates that the HOMO and LUMO are completely separated. 920



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(5) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508−524. (b) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457−2483. (6) Fichou, D. J. Mater. Chem. 2000, 10, 571−588. (7) See for example: (a) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. C.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37−44. (b) Chang, S. K.; Engen, D. V.; Fan, E.; Hamilton, A. D. J. Am. Chem. Soc. 1991, 113, 7640−7645. (c) Zimmerman, S. C.; Duerr, B. F. J. Org. Chem. 1992, 57, 2215− 2217. (d) Lehn, J.-M. Pharm. Acta Helv. 1995, 69, 205−211. (8) See for example: (a) Nagamatsu, S.; Kaneto, K.; Azumi, R.; Matsumoto, M.; Yoshida, Y.; Yase, K. J. Phys. Chem. B 2005, 109, 9374−9378. (b) Yagodkin, E.; Xia, Y.; Kalihari, V.; Frisbie, C. D.; J. Douglas, C. J. J. Phys. Chem. C 2009, 113, 16544−16548. (c) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084−4085. (d) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Brédas, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436−4451. (9) ConQuest 2002: Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389−397. (10) Bader, M. M.; Custelcean, R.; Ward, M. D. Chem. Mater. 2003, 15, 616−618. (11) Bader, M. M.; Pham, P. T. T.; Elandaloussi, E. H. Cryst. Growth Des. 2010, 10, 5027−5030. (12) Cai, X.; Burand, M. W.; Newman, C. R.; da Silva Filho, D. A.; Pappenfus, T. M.; Bader, M. M.; Brédas, J.-L.; Mann, K. R.; Frisbie, C. D. J. Phys. Chem. B 2006, 110, 14590−14597. (13) Guo, X.; Quinn, J.; Chen, Z.; Usta, H.; Zheng, Y.; Xia, Y.; Hennek, J. W.; Ortiz, R. P.; Marks, T. J.; Facchetti, A. J. Am. Chem. Soc. 2013, 135, 1986−1996. (14) Xugang Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ortiz, R. P.; Butler, M. R.; Pierre-Luc T. Boudreault, P. L. T.; Strzalka, J.; Morin, P. O.; Leclerc, M.; Navarrete, J. T. L.; Ratner, M. A.; Chen, L..; Chang, R. P. H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2012, 134, 18427−18439. (15) Huang, H.; Chen, Z.; Ortiz, R. P.; Newman, C.; Usta, H.; Lou, S.; Youn, J.; Noh, Y. Y.; Baeg, K. J.; Chen, L. X.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2012, 134, 10966−10973. (16) (a) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem.Eur. J. 2005, 11, 3742−3752. (b) Nielsen, C. B.; Angerhofer, A.; Abboud, K. A.; Reynolds, J. R. J. Am. Chem. Soc. 2008, 130, 9734−9746. (c) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121−10125. (d) Jessing, M.; Brandt, M.; Jensen, K. J.; Christensen, J. B.; Boas, U. J. Org. Chem. 2006, 71, 6734−6741. (e) Steckler, T. T.; Abboud, K. A.; Craps, M.; Rinzler, A. G.; Reynolds, J. R. Chem. Commun. 2007, 4904−4906. (f) Pappenfus, T. M.; Burand, M. W.; Janzen, D. E.; Mann, K. R. Org. Lett. 2003, 5, 1535−1538. (17) Pappenfus, T. M.; Schliep, K. B.; Dissanayake, A.; Ludden, T.; Nieto-Ortega, B.; Navarrete, J.T. L.; Delgado, M.C. R.; Casado, J. J. Chem. Educ. 2012, 89, 1461−1465. (18) Bader, M. M. Acta Crystallogr. 2009, E65, o2119. (19) (a) Meng, H.; Perepichka, D. F.; Bendikov, M.; Wudl, F.; Pan, G. Z.; Yu, W.; Dong, W.; Brown, S. J. Am. Chem. Soc. 2003, 125, 1551−61. (b) Meng, H.; Peripichka, D. F.; Wudl, F. Angew. Chem Int. Ed. 2003, 42, 658−661. (20) Raimundo, J.-M.; Blanchard, P.; Frere, P.; Mercier, N.; LedouxRak, I.; Hierle, R.; Roncali, J. Tetrahedron Lett. 2001, 42, 1507−1510. (21) Bondi, A. J. Phys. Chem. 1964, 68, 441−51. (22) (a) Bagchi, S.; Fried, S. D.; Boxer, S. G. J. Am. Chem. Soc. 2012, 134, 10373−10376. (b) Zhang, S.; Zhang, Y.; Ma, X.; Lu, L.; He, Y.; Deng, Y. J. Phys. Chem. B 2013, 117, 2764−2772. (c) Layfield, J. P.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2013, 135, 717−725. (d) Fafarman, A. T.; Sigala, P. A.; Schwans, J. P.; Fenn, T. D.; Herschlag, D.; Boxer, S. G. Proc. Nat. Acad. Sci., U. S. A. 2012, 109, E299−E308. (e) Zimmermann, J.; Thielges, M. C.; Seo, Y. J.; Dawson, P. E.; Romesberg, F. E. Angew. Chem., Int. Ed. 2011, 50, 8333−8337. (23) Wong, B. M.; Piacenza, M.; Della Sala, F. Phys. Chem. Chem. Phys. 2009, 11, 4498−508.

compound 6, Br-(EDOT)2-Br, short Br···Br distances (3.516 Å) may allow for polymerization of this monomeric EDOT dimer. These findings may be expanded to other systems of interest, in which, tuning of intermolecular interactions involving bromine, nitrogen, and/or sulfur may help achieve band gap reduction as described by Reynolds and co-workers.26

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ASSOCIATED CONTENT

S Supporting Information *

CIF files for compounds 1−7 are available free of charge. 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 The MRSEC Program of the National Science Foundation under Award No. DMR-0212302 partially supported this work through faculty/students research team programs. Funding from the Pennsylvania State University Research Development Grants is also acknowledged. The authors also acknowledge B. E. Kucera, W. W. Brennessel, and V. G. Young, Jr., and the XRay Crystallographic Laboratory, Department of Chemistry at the University of Minnesota. M.M.B. also would like to acknowledge J. P. Maciejewski and W. Long for their help with some initial synthetic work. Valuable discussions, access to lab facilities, and support from Professor Michael D. Ward (NYU) are also highly acknowledged.

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REFERENCES

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