Intermolecular Forces and Functional Group Effects in the Packing

May 24, 2005 - ABSTRACT: Thiophene was derivatized at the β-carbon with isophthalic, dipicolinic, and four nitrobenzoic acids...
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Intermolecular Forces and Functional Group Effects in the Packing Structure of Thiophene Derivatives Ana de Bettencourt-Dias,* Subha Viswanathan, and Kathleen Ruddy Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received January 3, 2005;

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 4 1477-1483

Revised Manuscript Received March 24, 2005

ABSTRACT: Thiophene was derivatized at the β-carbon with isophthalic, dipicolinic, and four nitrobenzoic acids to yield 4-thiophen-3-yl-pyridine-2,6-dicarboxylic acid diethyl ester, I; 5-thiophen-3-yl-isophthalic acid diethyl ester, II; 2-nitro-4-thiophen-3-yl benzoic acid ethyl ester, III; 3-nitro-4-thiophen-3-yl benzoic acid ethyl ester, IV; 3-nitro2-thiophen-3-yl benzoic acid ethyl ester, V; and 2-nitro-5-thiophen-3-yl benzoic acid ethyl ester, VI. These resulting compounds have been isolated and characterized by X-ray single-crystal diffraction. The solid-state structures of these substances are discussed as a function of the position and nature of the functional groups and the intermolecular forces. Modeling of the thiophene-aromatic ring torsion angle allowed us to attribute large differences in torsion angles to packing effects in the structures, due to π-π interactions between the aromatic rings. Introduction Thiophene-based materials, due to the richness of thiophene chemistry and the general stability of its compounds, have found applications in fields ranging from antistatic coatings to polymer electronics.1-4 Films based on either crystalline oligothiophenes or partially crystalline polythiophenes derive their characteristics of conductivity and charge transport from the extension of the π-conjugation along the thiophene backbone. Experimentally it is not possible to measure the extent of the π-conjugation in the case of partially amorphous materials. However, the conjugation length is largely determined by the torsion angle between thiophene rings. Functional groups attached to the thiophene rings and solid-state packing alike can influence this torsion angle. The magnitude of the torsion angle and the factors that influence it have been studied experimentally and through theoretical calculations. Structural characteristics of oligothiophenes and oligothiophenes derivatized at the β-position with alkyl groups have been reported, while similar work on polythiophenes and derivatized polythiophenes is inherently more complicated. Thus, modeling torsion angles on monoand oligothiophene derivatives allows us to draw conclusions regarding conjugation length for polymeric materials.2,5 Bithiophene and longer oligomer chains show mostly planar structures in the solid state, and this behavior is confirmed through computational studies.2 The introduction of alkyl groups on the β-carbon of selected thiophene rings in longer chains induces a loss of planarity. Barbarella and co-workers have reported that the introduction of one or more methyl groups into sexithiophene can induce departures from planarity by up to almost 30° and even force molecules from their usual all trans conformation into a cis conformation of the rings bearing the methyl groups.6 It is most likely that larger substituents on the thiophene rings will induce even more extreme distortions and/or become part of the extended π-conjugated system. * To whom correspondence should be addressed. E-mail: debetten@ syr.edu.

Despite the extensive electrochemical work by Ferraris and co-workers on different thiophene-phenyl derivatives as potential materials for electrochemical capacitors,7-10 no experimental studies and only very few theoretical calculations have been reported so far on torsion angles of thiophene rings derivatized with aromatic functional groups. Ando and co-worker calculated the torsion angle of 3-phenylthiophene to be 32°,11 while De Almeida and co-workers have reported theoretical studies on the influence of phenyl groups present at the β-carbon on the torsion angle between thiophene rings.12 The latter authors point out the general importance of the phenyl group as a site of possible additional functionalization, of which we have taken advantage. In this paper, we present crystallographic and theoretical calculations of six different thiophene derivatives with aromatic acids at the β-carbon, the torsion angles between the thiophene and the aromatic ring, and its dependence on the nature of functional groups, their position on the aromatic ring, and solid-state packing effects.13 Carboxylic aromatic acids were used, and the presence of this functional group has the potential for complexation of metal ions into polymers or oligomers to yield inorganic-organic hybrid materials,14 allows for self-assembly through hydrogen bonding on polar substrates,15 or influences the solid-state organization through intra- and intermolecular interactions.16 Experimental Section Synthesis. The syntheses of the six compounds follow the same protocol. Stoichiometric quantities of 3-(tributyltin)thiophene and the halide of the aromatic acid of choice were mixed with 1 mol % of Pd(PPh3)4, and DMF was added under inert atmosphere. The mixture was heated to 110 °C overnight. The DMF was removed under reduced pressure, and the reaction mixture was eluted with petroleum ether/ethyl acetate over silica to afford the products. I: Yield: 75.1%, λmax ) 224 nm, 1H NMR (CDCl3, δ): 8.48 [2H, s], 7.87 [1H, dd, 3J ) 1.4, 1.5], 7.56 [1H, dd, 3J ) 1.5, 5.1], 7.50 [1H, dd, 3J ) 3, 5.1], 4.52 [4H, q, 3J ) 7.2], 1.50 [6H, t, 3J ) 7.2]; anal. (C15H15N1O4S1) MM ) 305.35 C 58.83 (59.00 calc.) H 4.99 (4.95 calc.) N 4.49 (4.59 calc.) S 10.37 (10.50 calc.); II: Yield: 91.2%, λmax ) 229 nm, 1H NMR (CDCl3, δ): 8.60 [1H, t, 3J ) 1.5], 8.45 [2H, d, 3J ) 1.5], 7.63 [1H, dd, 3J ) 1.5, 3.0], 7.49 [1H, dd, 3J ) 1.5, 5.1],

10.1021/cg0500021 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/24/2005

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Table 1. Crystallographic Data for Structures I-VI formula CCDC number T (K) crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Dc (g cm-3) µ (mm-1) crystal size (mm3) θ range (°) index ranges reflections collected independent reflections, Rint data/restraints/ parameters GOF R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]a largest diff peak, hole (e Å3) a

I

II

III

IV

V

VI

C15H15NO4S 229410 95(2) triclinic P1 h 8.1204(16) 9.6314(19) 9.900(2) 106.61(3) 103.79(3) 95.56(3) 709.2(2) 2 1.433 0.244 0.80 × 0.60 × 0.40 2.24, 27.50 -10 e h e 10, -12 e k e 12, -12 e l e 12 7099

C16H16O4S 229412 85(2) orthorhombic Pna21 6.7406(13) 18.795(4) 11.593(2) 90 90 90 1468.8(5) 4 1.376 0.233 0.90 × 0.10 × 0.04 2.06, 26.50 -8 e h e 8, -23 e k e 23, -14 e l e 14 10441

C13H11NO4S 229411 87(2) monoclinic P21/c 10.083(2) 17.551(4) 7.0968(14) 90 99.76(3) 90 1237.8(4) 4 1.488 0.271 1.30 × 0.60 × 0.30 2.05, 27.50 -13 e h e 12, -22 e k e 22, -9 e l e 9 11827

C13H11NO4S 258559 96(2) monoclinic P21/n 13.564(3) 6.3486(13) 16.054(3) 90 113.05(3) 90 1272.1(4) 4 1.448 0.263 0.24 × 0.18 × 0.02 2.52, 31.50 -19 e h e 19, -9 e k e 9, -23 e l e 23 15454

C13H11NO4S 258561 92(2) monoclinic P21/c 8.5142(17) 14.342(3) 10.284(2) 90 97.56(3) 90 1244.9(4) 4 1.479 0.269 0.20 × 0.10 × 0.06 2.41, 30.00 -11 e h e 11, -20 e k e 20, -14 e l e 14 14119

C26H22N2O8S2 258560 93(2) monoclinic Cc 22.204(4) 7.7068(15) 15.306(3) 90 104.89(3) 90 2531.2(9) 4 1.455 0.265 0.20 × 0.10 × 0.04 1.90, 28.38 -29 e h e 29, -10 e k e 10, -20 e l e 20 12926

3231, 0.0347

3027, 0.0250

2844, 0.0257

4224, 0.0180

3622, 0.0243

6257, 0.0271

3231/0/190

3027/11/188

2844/12/199

4224/0/199

3622/0/212

6257/2/349

1.156 0.0617 0.1326 0.433, -0.304

1.111 0.0325 0.0807 0.294, -0.398

1.080 0.0418 0.1106 0.722, -0.419

1.044 0.0485 0.1329 0.827, -0.554

1.048 0.0524 0.1633 1.151, -0.649

1.075 0.0522 0.1313 0.514, -0.387

R1 ) ∑|Fo| - |Fc|/∑|Fo|; wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

7.45 [1H, dd, 3J ) 3.0, 5.1], 4.45 [4H, q, 3J ) 7.2], 1.45 [6H, t, 3 J ) 7.2]; 13C NMR (CDCl3, δ): 14.37, 61.47, 121.79, 126.16, 126.87, 128.96, 131.37, 131.48, 136.46, 140.35, 165.82; anal. (C16H16O4S1) MM ) 306.36 C 62.98 (63.14 calc.) H 5.02 (5.30 calc) S 10.28 (10.54 calc.); III: Yield: 72.0%, λmax ) 263 nm, 1 H NMR (CDCl3, δ): 8.03 [1H, d, 3J ) 1.2], 7.83 [1H, dd, 3J ) 1.8, 8.1] 7.79 [1H, d, 3J ) 8.1], 7.63 [1H, dd, 3J ) 1.5, 3.0], 7.46 [1H, dd, 3J ) 2.7, 5.1], 7.41 [1H, dd, 3J ) 1.8, 5.1], 4.36 [2H, q, 3J ) 7.2], 1.35 [3H, t, 3J ) 7.2] 13C NMR (CDCl3, δ): 165.08, 149.48, 139.86, 138.96, 130.87, 129.84, 127.71, 125.90, 125.25, 123.42, 121.41, 62.57, 13.93; anal. (C13H11N1O4S1) MM ) 277.30 C 53.53 (56.31 calc.) H 3.18 (4.00 calc.) N 5.88 (5.05 calc.); IV: Yield: 30.0%, λmax ) 254 nm, 1H NMR (CDCl3, δ): 8.42 [1H, d, 3J ) 1.2], 8.21 [1H, dd, 3J ) 2.4, 8.1], 7.58 [1H, d, 3 J ) 8.1], 7.41 [2H, dd, 3J ) 3.0, 1.8], 7.10 [1H, dd, 3J ) 2.4, 3.6], 4.40 [2H, q, 3J ) 7.2], 1.41 [3H, t, 3J ) 7.2] 13C NMR (CDCl3, δ): 164.76, 149.44, 136.43, 134.89, 133.07, 132.15, 130.97, 127.47, 127.14, 125.38, 124.86, 62.28, 14.69; V: Yield: 43.0%, λmax ) 245 nm, 1H NMR (CDCl3, δ): 7.93 [1H, dd, 3J ) 1.5, 7.8], 7.83 [1H, dd, 3J ) 1.2, 8.1], 7.53 [1H, t, 3J ) 7.8], 7.35 [1H, dd, 3J ) 3.0, 4.8], 7.18 [1H, dd, 3J ) 1.5, 3.0], 7.06 [1H, dd, 3J ) 1.5, 5.1], 4.10 [2H, q, 3J ) 7.2], 1.05 [3H, t, 3J ) 7.2] 13C NMR (CDCl3, δ): 166.95, 151.22, 135.49, 134.06, 132.48, 130.43, 128.84, 128.65, 125.69, 125.57, 124.13, 61.95, 13.89; and VI: Yield: 76.3%, λmax ) 331 nm, 1H NMR (CDCl3, δ): 8.01 [1H, d, 3J ) 8.4], 7.87 [1H, d, 3J ) 2.1], 7.78 [1H, dd, 3 J ) 2.1, 8.7], 7.65 [1H, dd, 3J ) 1.5, 3.0], 7.47 [1H, dd, 3J ) 3.0, 5.1], 7.42 [1H, dd, 3J ) 1.5, 5.1], 4.40 [2H, q, 3J ) 7.2], 1.37 [3H, t, 3J ) 7.2] 13C NMR (CDCl3, δ): 165.00, 140.10, 139.22, 129.00, 128.65, 127.74, 127.32, 127.10, 126.10, 125.08, 123.82, 62.84, 14.01. Crystallography. Crystal data, data collection, and refinement details for all the compounds are given in Table 1. Suitable crystals were mounted on a glass fiber and placed in the low-temperature nitrogen stream. Data were collected on a Bruker SMART CCD area detector diffractometer equipped with a low-temperature device,17 using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Data were measured using omega scans of 0.3° per frame, and a full sphere of data was collected, for a total of 1850 frames. The first 50 frames were re-collected at the end of data collection to monitor for decay. Multiscan absorption corrections were applied. Cell

parameters were retrieved using SMART18 software and refined using SAINTPlus19 on all observed reflections. Data reduction and correction for Lp and decay were performed using the SAINTPlus19 software. Absorption corrections were applied using SADABS.20 The structures were solved by direct methods and refined by least-squares methods on F2 using the SHELXTL21 program package. All atoms were refined anisotropically. Hydrogen atoms were added geometrically, and their parameters were constrained to the parent site. Calculations. The DFT calculations were performed using the B3LYP method with the 6-31G(d) basis set. The geometry of the molecules was optimized without constraints. The initial conformations utilized the torsion angles obtained from the X-ray diffraction experiments. Gaussian 03 was utilized for all calculations.22

Results and Discussion Thiophene was coupled at the 3-position with 4-bromoisophthalic acid, 4-bromodipicolinic acid, and four different nitrobenzoic acid derivatives under Stille conditions,23 as displayed in Scheme 1. In this way, we successfully synthesized 4-thiophen-3-yl-pyridine-2,6dicarboxylic acid diethyl ester I, 5-thiophen-3-yl-isophthalic acid diethyl ester II, 2-nitro-4-thiophen-3-yl benzoic acid ethyl ester III, 3-nitro-4-thiophen-3-yl benzoic acid ethyl ester IV, 3-nitro-2-thiophen-3-yl benzoic acid ethyl ester V, and 2-nitro-5-thiophen-3-yl benzoic acid ethyl ester VI. These compounds were obtained in moderate-to-high yields and were easily purified by flash chromatography. In all cases, colorless to pale yellow diamond or platelike X-ray quality crystals were isolated by slow evaporation of solvent. All six substances were characterized by single-crystal X-ray diffraction and details are summarized in Table 1. The asymmetric units of I, II, III, IV, and V are displayed in Figure 1, while that of VI is shown in Figure 2. As often seen for thiophene-containing struc-

Packing Structure of Thiophene Derivatives

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Figure 1. ORTEP drawing of the molecules (thermal ellipsoids at the 50% probability level, hydrogen atoms omitted for clarity). Selected intramolecular torsion angles are (a) I C(4)-C(3)-C(5)-C(9) -11.9(2)°; (b) II, C(2A)-C(3A)-C(6)-C(7) 27.4(6)°; (c) III, C(4A)-C(3A)-C(5)-C(10) 25.3(3)°; (d) IV C(2A)-C(3A)-C(5)-C(6) -41.0(3)°; (e) V C(1)-C(2)-C(5)-C(6) 62.3(2)°.

Scheme 1.

Stille Coupling Strategy for the Synthesis of I-VI

tures, II, III, and IV exhibit disordered five-membered rings.13,24 Modeling of the disorder gives two different positions for the thiophene rings, in which they are rotated 180° around the C-C bond to the phenyl group, with occupancy factors for the major component of 68%

for II, 83% for III, and 56% for IV. The bond distances within all thiophene rings are consistent with reported literature values,24 with an average of 1.716 Å for S-C bonds, 1.380 Å for CdC double bonds, and 1.402 Å for C-C single bonds. The thiophene and phenyl or pyri-

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Figure 2. Thermal ellipsoid drawing of the asymmetric unit of VI (thermal ellipsoids at 50% probability, hydrogen atoms omitted for clarity). Selected intramolecular torsion angles are C(1)-C(2)-C(5)-C(10) 6.696(23)°, C(12)-C(13)-C(16)-C(21) 27.858(24)°.

Figure 4. Packing diagram of II (a) showing spacing between molecular planes, (b) down the a axis, where the lighter colored atoms correspond to the layer in the back. For clarity, hydrogen atoms were omitted.

Figure 3. Packing diagram of I, showing the cell edges and the distances between the thiophene (3.4014 Å) and the pyridine rings (3.3569 Å). Hydrogen atoms are omitted for clarity.

dine rings are not coplanar, with torsion angles between -41.0° and 62.3°. Very different spatial arrangements are found in all six structures. Structures of I through VI. 4-Thiophen-3-yl-pyridine-2,6-dicarboxylic acid diethyl ester I crystallizes in the triclinic space group P1 h with one molecule in the asymmetric unit. The main part of the molecule is relatively planar, as the torsion angle between pyridine and thiophene is -12°, and the two carboxylate groups are also in the plane of the molecule, with torsion angles of -2° and -5°. However, while one of the ethyl groups is also coplanar, the other one is almost perpendicular to the plane of the molecule. This planar arrangement leads to a compact packing structure arranged in parallel layers, shown in Figure 3. The two molecules in the unit cell are antiparallel to each other. The pyridine rings are within π-π-stacking distance from each other, at 3.357 Å, while the thiophene rings are at a π-π-interaction distance of 3.401 Å. The S-S distance between layers, which is important if crystal conductivity is a parameter to be optimized, is 3.637 Å. While on the low-end range of predicted S-S distances in oligothiophenes,2 it precludes any sulfursulfur interactions. 5-Thiophen-3-yl-isophthalic acid diethyl ester II, which is similar to I with the exception of the presence of a

phenyl instead of a pyridine ring, crystallizes in the orthorhombic space group Pna21 as a racemic twin. This molecule is less planar than I, with a torsion angle of 30° between the thiophene and phenyl rings. While one of the carboxylate ethyl ester units is nearly coplanar with the phenyl ring with a torsion angle of -4°, the other is slightly tilted, with a torsion angle of -13°. This compound packs in a wavelike arrangement with four molecules in the unit cell. As seen in Figure 4a, there is a spacing of 3.37 Å between adjacent planes, where the phenyl groups were used as markers to measure this distance. Looking down the a axis, as shown in Figure 4b, one can see the packing more in detail. All molecules in one plane point in one direction, while the molecules in the adjacent plane are rotated by approximately 45°, without overlap of the aromatic rings. In fact, none of the rings is parallel to the rings in the adjacent planes. The planes intersect at angles of 20.4° for the phenyl and 23.3° for the thiophene moieties. The shortest S-S distances here are 4.757 Å, longer than the 4.033 Å reported for example for 3,3′-bis(2-hydroxyethyl)-2,2′bithiophene.16 2-Nitro-4-thiophen-3-yl benzoic acid ethyl ester III crystallizes in the monoclinic space group P21/c. The nitro group is almost perpendicular to the phenyl ring, with a torsion angle of -73°, while the carboxylate group is almost planar with the phenyl ring, with an angle of -12°. The torsion angle between the thiophene and phenyl rings is 27°. Four molecules are present in the unit cell, which is displayed in Figure 5. The individual molecules are arranged in antiparallel stacks. The phenyl rings are parallel to each other, at a distance of 3.838 Å. The thiophene rings do not interact, as they are on opposite sides of the two molecules, but the planes that they span are also parallel and at a distance of 3.247 Å. The planes of the molecules in two adjacent stacks are almost perpendicular (86.5°) to each other. In this arrangement, each

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Figure 5. Packing diagram of III showing the cell edges and the distances between phenyl rings (3.8376 Å). Hydrogen atoms are omitted for clarity.

sulfur atom in a thiophene ring is at a long 3.910 Å from its nearest, perpendicularly arranged neighbor. 3-Nitro-4-thiophen-3-yl benzoic acid ethyl ester IV crystallizes in the monoclinic P21/n space group. The phenyl-thiophene torsion angle of -41° is the second largest in the group. The carboxylate ethyl ester is relatively coplanar with the phenyl ring, with a torsion angle of -7°, while the nitro group shows a torsion angle of -56°. The four molecules in the unit cell are arranged in columns along the b axis, as shown in Figure 6a. Two successive molecules have phenyl rings separated by 4.185 Å and thiophene rings that are 3.343 Å apart (Figure 6b). The shortest S-S distance is 6.182 Å, between sulfur atoms of neighboring stacks. 3-Nitro-2-thiophen-3-yl benzoic acid ethyl ester V crystallizes in the monoclinic P21/c space group. This molecule is the most twisted of all six, with a phenylthiophene torsion angle of 62°. This is not surprising, considering that both the carboxylate and the nitro groups are ortho to the thiophene ring. To further accommodate all three functional groups, both the nitro and the carboxylate groups are also nearly perpendicular to the phenyl ring, ca. 56° and 85°, respectively. The four molecules in the asymmetric unit interact through π-π stacking of the phenyl rings, which are 3.488 Å apart, as seen in Figure 7. The planes spanned by the thiophene rings are at a distance of 3.883 Å. The shortest S-S distance is here 4.552 Å. In Figure 7, short donor-acceptor (D-A) interactions are also displayed. The thiophene ring of the central molecule has two protons in short contact to oxygen atoms in adjacent molecules. C4-H4 is in close contact to a nitro group oxygen O4_1, while C1H1 is in close contact with a carbonyl oxygen O1_2. 2-Nitro-5-thiophen-3-yl benzoic acid ethyl ester VI has the most complex structure of all. It crystallizes in the monoclinic Cc space group as a racemic twin with two molecules in the asymmetric unit, as shown in Figure 2. One of the molecules is twisted, with a thiophenephenyl torsion angle of 28°, while the other one is more planar, with a torsion angle of 5°. Contrary to what might be expected, π-π interaction between the aromatic rings is limited. The phenyl rings are not parallel, as the planes they span intersect with an angle of 2.5°. The distances of the centroids of these rings to the plane

Figure 6. Packing diagram of IV (a) down the b axis and (b) showing distances between phenyl (4.1854 Å) and thiophene (3.3432 Å) rings (two molecules in the left stack as well as hydrogen atoms were omitted for clarity).

Figure 7. Packing diagram of V, showing the cell edges, the spacing between phenyl rings (3.4876 Å) and weak hydrogen bond interactions. Selected hydrogen bond distances (D‚‚‚A, D-H‚‚‚A; D ) donor, A ) acceptor) and angles are O4‚‚‚H4_3C4_3: 3.341, 2.405 Å, 168.4°; O1‚‚‚H1_4-C1_4: 3.449, 2.569 Å, 169.7°. Only hydrogen atoms involved in weak hydrogen bonding are shown for clarity.

spanned by the other phenyl average 3.378 Å. The planes spanned by the thiophene rings are ca. 4.140 Å apart. Figure 8 shows a view of the unit cell down the

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de Bettencourt-Dias et al. Table 2. Selected Ring-Ring Distances and Calculated and Experimental Phenyl-Thiophene Torsion Angles for Molecules I-VI ω1(exp) (°) I (pyridine- -11.89 thiophene) II 30.26 III 25.26 IV -1.48 V 62.34 VI 27.86

Figure 8. Packing diagram of VI viewed down the b axis. Hydrogen atoms omitted for clarity.

b axis, with its eight molecules. The shortest S-S distance is 3.552 Å, which is the shortest distance of all the structures characterized here. Additional Intermolecular Interactions. For all six molecules, despite what has been observed for other thiophene-based structures,6 no evidence of C-H‚‚‚S or C-H‚‚‚π intermolecular interactions is seen. There is also no evidence of strong classical hydrogen bonding interactions. However, several weak hydrogen bonding interactions, as discussed by Desiraju and Steiner,25 are present. These are between carbonyl or nitro oxygen atoms and thiophene or phenyl C-H in the range 3.1 to 3.7 Å with angles of 125 to 168°, as well as between carbonyl oxygen atoms and methyl group C-H, in the range 3.1 to 3.6 Å with angles between 125 and 166°. These distances and angles are comparable to others found in a survey of the Cambridge Structural Database26 of 32 structures including thiophene and carbonyl or nitro groups or ether linkages. In our case, these weak interactions seem to have little influence on the packing structure and the thiophene-phenyl torsion angle for all six compounds, compared to the π-π interactions (for more details on these weak hydrogen bonding interactions see Supporting Information Figures 1-5). Experimental and Calculated Torsion Angles. We believe that the spatial arrangement of these molecules is directly related to the torsion angle between the phenyl (or pyridine) and thiophene rings and that this torsion angle in turn is directly related to the position of the functional groups on the phenyl ring. To corroborate this assumption, systematic conformational analyses were performed and fully optimized using DFT calculations at the B3LYP level of theory using the basis set 6-31G(d) with Gaussian 03.22 The calculated and experimentally determined torsion angles are summarized in Table 2. Two other groups have utilized the same level of theory to perform similar calculations on related systems. Ando et al. reported an optimized torsion angle of 32° for neutral 3-phenylthiophene. In the radical cation, their calculations indicate that both rings are coplanar, which explains the general difficulty of coupling monothiophene derivatives with bulky groups at the β-carbon, which we have also observed for these small molecules.11 De Almeida and co-workers have reported for the 3-phenylthiophene dimer a torsion angle of 41.4° between thiophene and phenyl group for

ω2(exp) (°)

6.70

ωcalc (°)

S-S (Å)

ph-ph th-th (Å) (Å)

-25.65 3.637

3.357

3.401

25.26 28.77 -55.22 75.49 28.03

3.37 3.838 4.185 3.488 3.378

3.247 3.343 3.883 4.140

4.757 3.910 6.182 4.552 3.552

the more stable syn-gauche conformation.12 They concluded that the presence of phenyl substituents does not alter the band gap of the 3-phenylthiophene dimer with respect to the parent bithiophene to a significant extent, which is consistent with a torsion angle larger than 40°, indicative of little π-delocalization between the two rings.27 They therefore assumed that polymers from this compound will conduct in a manner similar to bithiophene. This conclusion is confirmed by the experimental observations of Ferraris and co-workers, who observed little change in the oxidation and reduction potentials of electrochemically deposited poly(3-phenylthiophene) compared to polythiophene.9 The experimentally determined torsion angle for compound I, which contains pyridine as the aromatic ring instead of phenyl, is twice as small as the calculated value. The solid-state structure shows the compound in a more planar conformation than the gas-phase model. This arrangement is fairly compact. In this case, we believe it is a consequence of significant π-π stacking interactions, which are not accounted for in the gasphase calculation, leading to the decreased torsion angle. In the cases of compounds II and III, we see a remarkable agreement between the theoretically calculated gas phase and the experimentally determined phenyl-thiophene torsion angle. Additionally, these angles are very similar to the angle of 32° calculated by Ando et al. for 3-phenylthiophene.11 In these two structures, the nitro and carboxylato substituents do not hinder the relatively bulky thiophene ring, and we do not see strong π-π interactions, so that the calculated gas-phase structure agrees with the solid-state conformation. The calculated gas-phase torsion angles for IV and V differ significantly from the experimentally determined values. These two structures have the more hindered phenyl rings, with the thiophene substituents adjacent to the nitro group in the first case and between the nitro and carboxylato in the second case. The calculated torsion angles allow for accommodation of all groups around the phenyl ring. However, in the solid state, to allow for a more efficient interaction of the molecules and therefore a more cohesive structure, packing effects in the form of π-π interactions between thiophene and phenyl rings lead to a decrease in the torsion angles. Structure VI also has a fairly unhindered phenyl ring, but in the solid state it shows two different torsion angles. When optimizing the gas-phase torsion angle, the final value is 28°, whether we utilize an initial angle of 27° or 7°. We thus conclude that for one of the molecules of the asymmetric unit the solid structure follows what is predicted by gas-phase calculations,

Packing Structure of Thiophene Derivatives

while for the other one crystal packing effects and weak π-π interactions between the two almost parallel phenyl rings allow for a decrease in the torsion angle. As discussed above, all six structures display to a small extent a moderate number of weak hydrogen bond close contacts. Because of their omnipresence and seeming lack of correlation with the torsion angle between the thiophene and the phenyl rings, we believe that the effect of these interactions on the packing structure is less than the effect of the π-π interactions. Conclusion We have isolated and characterized six different thiophene derivatives of aromatic acids. All six have very different solid-state structures, with different torsion angles between thiophene and phenyl ring. This torsion angle determines the extent of π-delocalization between the two rings. With the exception of structures IV and V, the experimentally determined torsion angles are all less than 40°, so that we expect non-negligible π-delocalization. By utilizing simple DFT calculations at the B3LYP level of theory, the calculated and experimentally determined thiophene-phenyl torsion angles showed a remarkable agreement for the structures with little hindrance around the thiophenephenyl bond. For the more hindered structures and for the pyridine aromatic ring, the calculated and observed torsion angles are significantly different. We believe that for these specific cases packing effects, visible in the form of π-π interactions, lead to more efficient packing than otherwise expected and therefore to smaller angles than calculated. Without performing extended calculations, we can summarize that as long as the functional groups around the phenyl rings are not crowded, this approach offers a good estimation of the torsion angle between thiophene and phenyl in the solid state and therefore of the π-delocalization. It is necessary now to extend the studied molecules to oligomeric systems, to allow predicting backbone conjugation in polymers based on these compounds. The effects of these same aromatic β-substituents on the torsion angles of bithiophene and terthiophene analogues are currently under study and will be reported elsewhere. Acknowledgment. We are grateful to Dr. Timothy M. Korter for assistance with the DFT calculations, to Mr. Nathan G. Armatas for experimental assistance, to Drs. Jacob Alexander and Karin Ruhlandt for preliminary crystallographic data on CCDC 229410, and to Syracuse University, NSF-REU, and PRF for support of this work. Supporting Information Available: X-ray crystallographic information files can be obtained free of charge via

Crystal Growth & Design, Vol. 5, No. 4, 2005 1483 www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail: [email protected]). CCDC 229410 contains the supplementary crystallographic data for I, CCDC 229412 for II, CCDC 229411 for III, CCDC 258559 for IV, CCDC 258561 for V and CCDC 258560 for VI in this paper. Figures detailing the weak hydrogen bonding interactions are available free of charge via the Internet at http://pubs.acs.org.

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