The Precarious Equilibrium between Different Types of Weak

The Precarious Equilibrium between Different Types of Weak Hydrogen Bonds in Methoxy-Substituted Distyrylbenzenes: The Hybrid Network in E,E-1,4-Bis[2...
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CRYSTAL GROWTH & DESIGN

The Precarious Equilibrium between Different Types of Weak Hydrogen Bonds in Methoxy-Substituted Distyrylbenzenes: The Hybrid Network in E,E-1,4-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene

2006 VOL. 6, NO. 1 241-246

Christophe M. L. Vande Velde, Herman J. Geise, and Frank Blockhuys* Department of Chemistry, UniVersity of Antwerp, UniVersiteitsplein 1, B-2610 Wilrijk, Belgium ReceiVed July 4, 2005; ReVised Manuscript ReceiVed August 17, 2005

ABSTRACT: E,E-1,4-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene, C32H38O10, forms a packing that consists of an intricate network of both CH‚‚‚n(O) hydrogen bonds and -OCH3-π interactions. The packing schemes of all known methoxysubstituted distyrylbenzenes are compared and explained on the basis of the observations made on the title compound. In general, intramolecular CH‚‚‚n(O) contacts are greatly preferred over intermolecular ones. Intermolecular CH‚‚‚n(O) contacts are more common than intermolecular -OCH3-π interactions. The latter are adopted only when the number of free methoxy groups, not involved in intramolecular CH‚‚‚n(O) contacts, is small enough. 1. Introduction Organic semiconductors are materials that enjoy considerable interest these days mainly because of the convenient and easy way in which their molecular structures and hence their properties can be tuned to a specific application. This is in stark contrast to the present-day complementary metal oxide semiconductor (CMOS) technology where such tuning is virtually impossible. Switching from polymeric to oligomeric organic materials is a worthwhile undertaking for the numerous optoelectronic applications for which these new compounds are used, as has been discussed in depth in Mu¨llen et al.1 and Segura and Martı´n.2 The most important reason for making this transition is the fact that it is far easier to produce and purify oligomers than polymers. Furthermore, oligomers are monodisperse materials, and as a result, they crystallize more easily than the corresponding polymers. For this reason, oligomers have been extensively used as model compounds for the polymers, but it is far more rewarding to study the properties of these oligomeric organic semiconductor materials for their own sake. One of these properties is electrical conductivity, and since this involves the hopping of electrons from one molecule to another, the contact distance and relative orientation of the semiconductor molecules in the solid are of prime importance. It is clear that accurate crystal structures and a detailed description of the packing of the molecules in the crystal are a first step toward understanding these conduction processes. In the initial phase of our investigations into these matters we have been looking at the crystal structures of the undoped, nonconducting native oligomers to try to gain a better understanding of the intra- and intermolecular interactions governing their crystal structures. To do this, we have determined a number of crystal structures of methoxy-substituted distyrylbenzenes, oligomeric derivatives of poly(p-phenylene vinylene) (PPV). A number of these display 3,4,5-trimethoxy substitution on the peripheral rings,3,4 while others have 2,4,6-trimethoxy-substitution on these rings.5,6 As an explanation of the packing of the 2,4,6-trimethoxysubstituted compounds, we proposed that it is determined by * Corresponding author. Fax +32.3.820.23.10; e-mail frank.blockhuys@ ua.ac.be.

-OCH3-π interactions. Because of the electron-donating methoxy substituents, the rings are relatively electron-rich and the methoxy groups are relatively acidic, which increases the importance of these interactions and makes them the most important ones in the formation of the lattice;6 for a review on CH-π interactions, see Nishio et al.7 Prior to our study of these 2,4,6-trimethoxy-substituted compounds, the only kind of packing observed for methoxy-substituted distyrylbenzenes was a network of CH‚‚‚n(O) hydrogen bonds.8 Indeed, we later confirmed that E,E-1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]2,5-dimethoxybenzene, a compound with 3,4,5-trimethoxy substitution, displays this type of intermolecular interaction.4 It is clear that substitution patterns (the number and relative positions of polar groups such as methoxy groups) largely determine the packing and the types of networks that are formed, as expected, and that basically two types of networks can be identified for these compounds. To further study the influence the positions of the methoxy groups have on the preference of one packing scheme over the other, we added two methoxy groups to the central ring of E,E1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,5-dimethoxybenzene,6 yielding the title compound E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6). It turns out that this new methoxy-substituted distyrylbenzene unites properties from the two classes of compounds, and hence the two types of networks, and sheds new light on the relative importance of the two different systems for lattice formation in these compounds and on the key factors influencing this preference. 2. Experimental Section 2.1. Syntheses. All starting materials were obtained from Acros or Aldrich and used as received. 1H NMR and 13C NMR spectra were recorded on a Varian Unity-400 apparatus. The molecular framework and atomic numbering of the title compound (6) are shown in Figure 1; hydrogen atoms are given the same number as the carbon atom on which they are substituted. The reaction sequence is given in Figure 2. 2.1.1. 2,4,5-Trimethoxyphenol (1). 2,4,5-Trimethoxyphenol was synthesized via the oxidation of 2,4,5-trimethoxybenzaldehyde with H2O2/H2SO4 as reported by Lin et al.,9 with the exceptions that all reagents were used diluted in tetrahydrofurane, the reaction was carried out below 5 °C in an icebath, and after adding water to the reaction

10.1021/cg050317g CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2005

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Figure 1. Molecular structure and numbering scheme of E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6). Displacement ellipsoids are at the 50% probability level; hydrogens are represented by spheres of arbitrary radius.

Figure 2. Reaction sequence for the synthesis of E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6). mixture, it was extracted with ethyl acetate without neutralization. The resulting organic phase was washed with dilute KOH until neutral, then dried and chromatographed on silica (50:50 hexane/ethyl acetate). The yield was 78%. Mp 55-56 °C. δ1H (CDCl3, 400 MHz, TMS): 6.60 (s, 1H, H6), 6.58 (s, 1H, H4), 5.27 (s, 1H, OH), 3.85 and 3.84 (s, 3H, 4-OCH3 and 5-OCH3), 3.80 (s, 3H, 2-OCH3). δ13C (CDCl3, 100 MHz, TMS): 144.1, 142.3, 139.8 and 139.7 (C1, C2, C4, and C5), 101.1 and 100.0 (C3 and C6), 57.4, 57.1, and 56.6 (OCH3). The identity of (1) was confirmed by an X-ray structure determination, but due to the low quality of the crystal, the data is unsuitable for publication. 2.1.2. 1,2,4,5-Tetramethoxybenzene (2). 2,4,5-Trimethoxyphenol (1; 9.4 g, 0.051 mol) was added to 35 mL of water and 2.9 g (0.051 mol) of KOH. To this mixture, 6.4 g (0.051 mol) of dimethyl sulfate was added over the course of 2 h. After the addition, the reaction mixture was refluxed for 2 h. The resulting solids were filtered and washed with water until neutral pH. After drying, the compound was recrystallized from petroleum ether (bp 80-110 °C) and isolated after decantation from the insoluble fraction. The yield was 4.1 g of pink needles (0.021 mol, 40%). Mp 95-97 °C. δ1H (CDCl3, 400 MHz, TMS): 6.61 (s, 2H, H3 and H6), 3.85 (s, 12H, OCH3). δ13C (CDCl3, 100 MHz, TMS): 143.3 (C1, C2, C4, and C5), 101.1 (C3 and C6),

57.12 (OCH3). The identity of (2) was confirmed by an X-ray structure determination yielding essentially the same structure as that published by von Deuten and Klar.10 2.1.3. 1,4-Bis(bromomethyl)-2,3,5,6-tetramethoxybenzene (3). 1,2,4,5-Tetramethoxybenzene (2; 3.2 g, 0.016 mol) was dissolved in 12 mL of glacial acetic acid with 1.9 g (0.064 mol) of paraformaldehyde. To this stirred mixture, 11.6 mL of HBr in acetic acid (45% w/v) was added dropwise. After the addition, the mixture was heated to 50 °C and kept at that temperature for 3 h. The resulting solution was poured in 300 mL of water; the precipitate was filtered off and washed with water until neutral pH. The resulting solids (3.4 g) were used in the next step without further purification. After the final step of this synthetic procedure, an NMR analysis was performed on these solids, which indicated that the 1,4-bis(bromomethyl)-2,3,5,6-tetramethoxybenzene (3) was heavily contaminated with about 90% of 1-bromomethyl-2,3,5,6-tetramethoxybenzene (4); this explains why the yield in the final step is so low. δ 1H (CDCl3, 400 MHz, TMS) (3): 4.60 (s, 4H, CH2Br), 3.96 (s, 12H, OCH3). δ 1H (CDCl3, 400 MHz, TMS) (4): 6.54 (s, 1H, H4), 4.65 (s, 2H, CH2Br), 3.92 (s, 6H, 2-OCH3 and 6-OCH3), 3.85 (s, 6H, 3-OCH3 and 5-OCH3). δ 13C (CDCl3, 100 MHz, TMS) (3): 147.6 (C2, C3, C5 and C6), 127.3 (CH2Br), 60.7

Equilibrium between Hydrogen Bonds in Distyrylbenzenes (OCH3), 22.21 (C1 and C4). δ 13C (CDCl3, 100 MHz, TMS) (4): 148.9 (C2 and C6), 141.4 (C3 and C5), 126.4 (CH2Br), 100.5 (C4), 61.1 (2OCH3 and 6-OCH3), 56.53 (3-OCH3 and 5-OCH3), 22.21 (C1). 2.1.4. 2,3,5,6-Tetramethoxy-p-xylylbis(triphenylphosphonium bromide) (5). Triphenylphosphine (4.8 g, 0.018 mol) was added to 3.3 g of the solids from the previous step in 30 mL of acetonitrile. The resulting mixture was refluxed for 4 h, the solvent was evaporated, and the solids were used in the next step without further purification. 2.1.5. E,E-1,4-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6). To the solids of the previous step was added 3.6 g (0.018 mol) of 2,4,6-trimethoxybenzaldehyde in a small amount of dry ethanol under nitrogen protection. Sodium metal (0.4 g, 0.018 mol) dissolved in a minimal amount of dry ethanol was added dropwise, and the reaction mixture was stirred at room temperature for 24 h, after which 2.3 g of a light green-yellow powder was filtered off. These solids proved to be quite impure [hence the NMR study on 3]. After the crude product was isomerized to the E,E isomer by dissolving it in p-xylene containing a catalytic amount of iodine and refluxing the solution for 4 h, E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6tetramethoxybenzene (6) was purified by recrystallization from ethanol. The yield was 190 mg (4%). Mp 202-204 °C. δ1H (CDCl3, 400 MHz, TMS): 7.88 (d, 2H, J ) 17.08 Hz, H7), 7.59 (d, 2H, J ) 17.08 Hz, H8), 6.18 (s, 4H, H33 and H35), 3.89 (s, 12H, H9 and H10), 3.87 (s, 12H, H42 and H46), 3.84 (s, 6H, H44). X-ray quality crystals of 6 were grown by slow evaporation of a CH2Cl2 solution. 2.2. X-ray Crystallography. The hydrogen atoms were located in a difference map and subsequently refined as riding for the ethenylic and phenylic positions. The methoxy hydrogens were constrained to their optimal geometry, but the hydrogen distances were left to refine freely, while the methyl groups were allowed to rotate. Data collection, CAD-4 EXPRESS;11 cell refinement, CAD-4 EXPRESS;11 data reduction, XCAD4;12 program used to solve structure, SHELXS-97;13 program used to refine structure, SHELXL-97;13 molecular graphics, ORTEP-3 for Windows14 and MERCURY (version 1.2.1);15 software used to prepare publication material, WinGX (version 1.64)16 and PLATON.17 Scattering factors were from the International Tables for Crystallography (Vol. C).18 The experimental details including the results of the refinement are given in Table 1.

Crystal Growth & Design, Vol. 6, No. 1, 2006 243 Table 1. Experimental Details A. Crystal Data chemical formula chemical formula weight (g‚mol-1) cell setting space group radiation type wavelength (Å) monochromator crystal form crystal size (mm3) crystal color no. of reflns for cell params θ-range (deg) T (K) a (Å) b (Å) c (Å) V (Å3) Z Dx (mg/m3) µ (mm-1)

orthorhombic Pbca Mo KR 0.71073 graphite prism 0.34 × 0.30 × 0.15 yellow 25 6.53-18.34 293(2) 7.309(2) 15.180(3) 27.115(4) 3008.7(10) 4 1.286 0.095

B. Data Collection diffractometer Enraf-Nonius Mach3 data collection method ω/2θ scans no. of measured reflns 5267 no. of independent reflns 2637 no. of obsd reflns 1262 criterion for obsd reflns I > 2σ(I) Rint. 0.0726 θmax (deg) 24.97 range of h 0-8 range of k -17 to 18 range of l 0-32 no. of standard reflns 3 frequency of standard 1 reflns (h) intensity decay (%) 7

3. Results and Discussion 3.1. Molecular Geometry. E,E-1,4-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6) crystallizes as thin six-sided prisms in the space group Pbca, and the crystallographic inversion symmetry is coincident with the molecular symmetry. The orthorhombic unit cell contains four molecules. From the molecular geometry in Figure 1, it is obvious that the additional methoxy substitution on the central ring B severely distorts the normal quasi-planar conformation of the distyrylbenzene skeleton, which is found in other substituted derivatives.3-6 The angle between the LS planes of the peripheral ring A and the central ring B is 39.37(13)°. The double bond vector makes an angle of 17.69(19)° with the central ring B and of 9.2(2)° with the peripheral ring A. A second effect of the substitution pattern of the central ring B is that its methoxy groups are nearly perpendicular to the ring plane: C9-O2-C2-C1 ) -98.3(3)° and C10-O6-C6-C1 ) -92.1(3)°. Usually these groups are located in the plane of the ring as has been observed in 1,2,4,5-tetramethoxybenzene10 and in ortho-dimethoxybenzene19 and a large number of its derivatives.20 3.2. Packing and Intermolecular Short Contacts. As mentioned in the Introduction, the packing of 6 displays a mixture of a -OCH3-π and a CH‚‚‚n(O) network. The CH‚‚ ‚n(O) network is expressed in the following contacts. The first, C10-H10a‚‚‚O2i, symmetry code i ) x - 1, y, z, D-H ) 1.00(2) Å, H-A ) 2.597(18), Å, D-A ) 3.481(4) Å, D-H-A ) 147.4(13)°, connects two molecules by means of the methoxy groups on the central ring B; it is represented by the dotted lines in Figure 3. D and A represent the hydrogen bond donor

C32H38O10 582.62

refinement on GOF wR(F2) R [F2 > 2σ(F2)] no. of reflns used in refinement no. of params used weighting scheme (∆/σ)max ∆Fmax ∆Fmin

C. Refinement F2 0.930 0.1307 0.0457 2637 200 w ) 1/[σ2(Fo2) + (0.0564P)2 + 0.2548P] where P ) (Fo2 + 2Fc2)/3 0.000 0.141 -0.198

and acceptor, respectively. Interestingly, the free electron pairs of these particular methoxy groups have become available for intermolecular hydrogen bonding because of the way in which they are twisted out of the plane of the benzene ring on which they are substituted. In contrast, methoxy groups in less sterically hindered PPV oligomers are normally located in the plane of the ring and are involved in intramolecular CH‚‚‚n(O) interactions, as has been discussed in Wu et al.21 and Vande Velde et al.4-6 The second contact, C44-H44b‚‚‚O6ii, symmetry code ii ) 1 - x, -1/2 - y, z - 1/2, D-H ) 1.000(16) Å, H-A ) 2.573(14) Å, D-A ) 3.529(4) Å, D-H-A ) 160.1(16)°, represented in Figure 3 by the dashed line, with a molecule that is at right angles to the central ring, additionally generates the following contact (not shown in Figure 3), H9c‚‚‚C44iii, 2.83(2) Å, C9-H9c-C44 ) 155.8(16)°, symmetry code iii ) x, 1/2 - y, 1/2 + z. In this way, the intermolecular CH‚‚‚n(O) network is built up nearly entirely from the four methoxy groups on the central ring B, only involving C44 on the peripheral ring A once as a donor.

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Figure 3. A view of the packing of 6 showing the sheets of molecules and their connection to each other. See text for details.

Indeed, the methoxy groups on the peripheral rings are involved in a completely different network of -OCH3-π interactions; these are represented in Figure 3 by the dasheddotted lines. The para-methoxy group contacts the centroid of the outer ring Cg(A), C44-H44a‚‚‚Cg(A)iv, symmetry code iv ) 1/2 + x, y, 1/2 - z, 2.86(2) Å, 2.714 Å perp, 135.1(12)°, in which “perp” indicates the perpendicular distance to the LS plane of the ring. This interaction is also clear from the following close contacts (not shown in Figure 3): C44-H44a‚‚‚C31iv, 2.83(2) Å, 163.8(14)°; C44-H44a‚‚‚C32iv, 2.81(2) Å, 138.3(16)°. In another -OCH3-π interaction one of the ortho-methoxy groups on the peripheral ring A contacts the centroid of the central ring B (not shown in Figure 3): C42-H42b‚‚‚Cg(B)v, symmetry code v ) 1/2 + x, 1/2 - y, -z, 2.95(2) Å, 2.889 Å perp, 127.8(16)°. Finally, two methoxy groups on the peripheral ring A contact two carbon atoms of the peripheral ring in what is presumably a three-center CH-π interaction, which is clearly not aimed at the centroid of the peripheral ring A: C44-H44c‚ ‚‚C33vi, symmetry code vi ) -1/2 + x, y, 1/2 - z, 2.80(2) Å, 145.5(11)°; C46-H46a‚‚‚C36vi, 2.78(2) Å, 146.7(12)°. 3.3. Intramolecular CH‚‚‚n(O) Interactions. In addition to the intermolecular CH‚‚‚n(O) network described in section 3.2, the title compound also displays a number of intramolecular CH‚‚‚n(O) interactions. These interactions between the olefinic hydrogen atom and the oxygen atom of the nearby orthomethoxy group have been described previously21 and have been demonstrated to be energetically favorable.5,6 Yet, the unique out-of-plane orientation of the methoxy groups on the central ring B in 6 prevents these intramolecular interactions because the lone pairs of the methoxy group point away from the ethenylic hydrogen. The in-plane orientation of the methoxy groups on the peripherals rings, on the other hand, does allow the interactions, and this leads to the following. In principle and disregarding any other substituents on the A and B rings other than the ortho-substituents, the substitution of the ethylene spacer is symmetric as can be seen from the line drawing of a fragment of 6 in Figure 4. However, since the methoxy groups on the central ring B are involved in the intermolecular CH‚‚‚n(O) network and are therefore not available for the intramolecular CH‚‚‚n(O) interactions, the substitution of the ethylene spacer becomes asymmetric. This is clearly seen from the different CH‚‚‚O distances in Figure 4. The

Figure 4. Schematic representation of the functional groups involved in the intramolecular CH‚‚‚n(O) interactions with the ethylene spacers in 6, including the nonbonded distances and CCO angles. Table 2. Geometrical Parameters (Distances in Å and Angles in deg) Relevant to the Intramolecular CH‚‚‚n(O) Interactions with the Ethylene Spacers in 6a

1 2 3 4 1′ 2′ 3′ 4′

D

H

A

D-H

H-A

D-A

D-H-A

C7 C8 C8 C7 C7 C8 C8 C7

H7 H8 H8 H7 H7 H8 H8 H7

O32 O36 O6 O2 O32 O36 O6 O2

0.93 0.93 0.93 0.93 1.083 1.083 1.083 1.083

2.20 2.32 2.34 2.45 2.123 2.263 2.293 2.427

2.818(3) 2.691(3) 2.938(3) 2.771(3) 2.818(3) 2.691(3) 2.938(3) 2.771(3)

123 103 122 100 119.38 99.36 118.54 96.74

a Donor and acceptor atoms are represented by D and A, respectively. Parameters 1-4 are based on the coordinates of the CIF (see also Figure 4). For parameters 1′-4′, the C-H distances have been normalized to 1.083 Å, and these have been used in the discussion.

differences in these distances should therefore be the net effect of the presence of the CH‚‚‚n(O) interaction, and this allows us to experimentally demonstrate their attractive nature. The values of the relevant geometrical parameters can be found in Table 2. The presence of the attractive interactions between the spacer and the peripheral ring A can be clearly seen from the values of three geometrical features. First, the CH‚‚‚O distances (in particular the normalized H-A distances 1′ to 4′ in Table 2) between peripheral ring A and the spacer are about 0.17 Å shorter than those between central ring B and the spacer, and

Equilibrium between Hydrogen Bonds in Distyrylbenzenes

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Figure 5. Structural formulas of methoxy-substituted PPV oligomers. Methoxy groups that are involved in an intramolecular contact are colored red; those that are free to participate in intermolecular interactions are colored green. See text for details.

this is true for both the five- and the six-membered ring interactions. Second, as was mentioned in section 3.1, the torsion angles around the single bonds connecting the ethenyl fragment to the two phenyl rings are markedly different: the double bond vector makes an angle of 9.2(2)° with peripheral ring A and of 17.69(19)° with central ring B. Third, the O32-C32-C31 and O36-C36-C31 angles of the peripheral ring have become significantly smaller than the corresponding angles in the central ring, that is, O2-C2-C1 and O6-C6-C1, this despite the steric hindrance from the vicinal methoxy groups (Figure 4). 3.4. Packing Schemes of Methoxy-Substituted Distyrylbenzenes. An analysis of the known structures of methoxysubstituted distyrylbenzene derivatives present in the CSD (v. 5.26)22 indicates that the regular packing mode involves intermolecular CH‚‚‚n(O) interactions. Evidence for this can be found in the crystal structures presented by Bartholomew et al.8 [CSD refcodes REDKUG, REDWUS and HESMUN], Wu et al.21 [NIVSOA], Verbruggen et al.3 [JACBIY], and Vande Velde et al.4 [GAKZAU]. The structural formulas of these compounds have been presented in Figure 5. In this figure, the methoxy groups that are involved in an intramolecular contact (i.e., those placed ortho to an ethenylic link) are colored red; those that are free to participate in intermolecular interactions are colored green. We expect that the compounds with methoxy groups that are occupied by intramolecular interactions will stack cofacially or in a herringbone pattern with the packing determined by PhH‚‚‚π interactions not involving the oxygen lone pairs. This is exactly what we see for HESMUN. As the number of “free” methoxy groups increases, their influence becomes clear, as in REDKUG and NIVSOA. In REDKUG, there are two intramolecular CH‚‚‚n(O) contacts present per molecule with the

methoxy groups in the 2-positions, as would be expected. The central phenyl ring of this compound becomes the hydrogen donor in the intermolecular interactions, and since it is not substituted, it introduces no steric issues when approaching the 5-methoxy group of a neighboring molecule. In NIVSOA, two of the six methoxy groups have free lone pairs, and a large number of intermolecular contacts shorter than the sum of the van der Waals radii are present. The 4-methoxy groups on the peripheral rings are contacted by the methoxy groups on the central ring in a typical CH‚‚‚n(O) contact. Oddly, the latter groups also contact the 2-methoxy groups on the peripheral ring in a CH‚‚‚n(O) contact. This is indeed surprising as in this case the intermolecular contact with a CH moiety of a methoxy group seems to be preferred over the intramolecular contact with the ethenylic CH fragment. Moving on to GAKZAU, we find that it has six free methoxy groups. The situation for this compound somewhat resembles the one for NIVSOA because it uses only one free methoxy group on the peripheral ring as an acceptor for CH‚‚‚n(O) contacts. The two other groups on this ring are CH‚‚‚n(O) contact donors for the methoxy groups on the central ring, which, in principle, do not have free oxygen lone pairs but do form two intermolecular CH‚‚‚n(O) contacts as acceptors, while still retaining a good geometry for the intramolecular CH‚‚‚ n(O) contacts with the ethenylic link. REDWUS and JACBIY have four and six methoxy groups, respectively, of which none are tied up in intramolecular CH‚ ‚‚n(O) interactions, and we would expect to see a heavy dependence of the packing scheme on intermolecular interactions. REDWUS displays a CH‚‚‚n(O) contact involving two of its four free methoxy groups; the other two are unused in the packing. JACBIY displays the expected pattern: of the six

246 Crystal Growth & Design, Vol. 6, No. 1, 2006

free methoxy groups, the two in the 3-positions are involved in CH‚‚‚n(O) interactions with a hydrogen atom on a benzene ring, the two in the 4-positions are involved in CH‚‚‚n(O) interactions with a hydrogen atom of a 4-methoxy group of a neighboring molecule, and the two in the 5-positions display no close contacts. The six structures described above strongly suggest that in general intramolecular CH‚‚‚n(O) contacts are greatly preferred over intermolecular ones; NIVSOA and GAKZAU present the two exceptions to this observation. Furthermore, CH‚‚‚n(O) contacts are clearly more common in methoxy-substituted distyrylbenzenes than -OCH3-π interactions, which are nearly completely absent [there is one in GAKZAU, see Vande Velde et al.4]. However, in the four E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,5-alkoxybenzenes that we presented earlier,6 the peripheral rings consist of 2,4,6-trimethoxyphenyl moieties; BEZYAH has been included in Figure 5 as a representative of this series. Four of the six methoxy groups present on these rings and the two on the central ring are involved in intramolecular CH‚‚‚n(O) interactions with the ethenylic hydrogen atoms, and these are strong enough to prevent the involved methoxy groups from further intermolecular networking. This, in combination with the unusually activated 2,4,6-trimethoxysubstituted aromatic ring, which results in acidic methoxy groups and electron-rich π-clouds, causes the structure to revert to a -OCH3-π network for stabilization. The title compound, E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6), is an example in which the two possible kinds of networks coexist. The four methoxy groups on the central ring are unavailable for intramolecular CH‚‚‚n(O) interactions due to steric reasons, as was explained above. Therefore, they readily form an intermolecular CH‚‚‚n(O) network. The peripheral electron-rich 2,4,6-trimethoxy-substituted ring A, on the other hand, has only one methoxy group that is not involved in an intramolecular CH‚‚‚n(O) interaction (the one in the para position) and this generates a -OCH3-π network. Thus the structure of 6 illustrates the effect of the intramolecular CH‚‚‚n(O) interaction on the crystal packing and also on the packing schemes of methoxy-substituted distyrylbenzenes in general. Three other compounds containing other functionalities besides methoxy groups can be found in the CSD, and these further strengthen our generalizations. E,E-1,4-Bis[2-(2-chlorophenyl)ethenyl]-2,5-dimethoxybenzene (HESNAU) and E,E1,4-bis[2-(3,4-dichlorophenyl)ethenyl]-2,5-dimethoxybenzene (HESNEY) both contain two methoxy groups on the central ring that can be involved in intramolecular CH‚‚‚n(O) interactions.23 As a consequence, the only short contacts present in the structures are due to the chlorine atoms. A third compound is E,E-1,4-bis[2-(3,4-methylenedioxyphenyl)ethenyl]-2,5-dimethoxybenzene (HESNIC) in which two methoxy groups on the peripheral rings have been condensed into a 3,4-methylenedioxy moiety.23 Of the six oxygen atoms, only the two on the central ring are ortho, and these are also involved in intramolecular interactions. HESNIC uses the lone pairs of the methylenedioxy group as acceptors in four CH‚‚‚n(O) interactions, two from hydrogen atoms on the other side of the peripheral phenyl ring and two from hydrogen atoms of the methoxy groups on the central ring. It is interesting to note that the packing efficiencies of these 11 compounds as calculated with PLATON vary in the limited range between 68.0% for E,E-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6) and 72.7% for

Vande Velde et al.

NIVSOA. These values appear to be due more to the general molecular shape of these compounds than to particular interactions or the lack thereof. 4. Conclusions E,E-1,4-Bis[2-(2,4,6-trimethoxyphenyl)ethenyl]-2,3,5,6-tetramethoxybenzene (6) serves to demonstrate, through the geometry around its ethenyl spacers, that an attractive intramolecular CH‚‚‚n(O) interaction indeed exists in PPV oligomers with methoxy groups placed ortho to the ethenylic bond in a favorable orientation. Furthermore, it illustrates that methoxysubstituted PPV oligomers form a class of compounds where intra- and intermolecular stabilizing contacts compete for the same stabilization energy. By farming out the intermolecular stabilization to the (weaker) -OCH3-π interactions, maximum stabilization can be obtained for these structures. Acknowledgment. The authors wish to thank Prof. Dr. R. Dommisse and J. Aerts for recording the NMR spectra. C.V.V. wishes to thank the Fund for Scientific Research (FWO Vlaanderen) for a grant as a research assistant. Supporting Information Available: X-ray crystallographic information files (CIF) for compound (6). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, Germany, 1998. (2) Segura, J. L.; Martı´n, N. J. Mater. Chem. 2000, 10, 2403-2435. (3) Verbruggen, M.; Yang, Z.; Lenstra, A. T. H.; Geise, H. J. Acta Crystallogr. 1988, C44, 2120-2123. (4) Vande Velde, C. M. L.; Geise, H. J.; Blockhuys, F. Acta Crystallogr. 2005, C61, o21-o24. (5) Vande Velde, C. M. L.; Baeke, J. K.; Geise, H. J.; Blockhuys, F. Acta Crystallogr. 2005, C61, o284-o287. (6) Vande Velde, C. M. L.; Chen, L. J.; Baeke, J. K.; Dieltiens, P.; Moens, M.; Geise, H. J.; Zeller, M.; Hunter, A. D.; Blockhuys, F. Cryst. Growth Des. 2004, 4, 823-830. (7) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction: EVidence, Nature and Consequences; Wiley-VCH: New York, 1998. (8) Bartholomew, G. P.; Bazan, G. C.; Bu, X. H.; Lachicotte, R. Chem. Mater. 2000, 12, 1422-1430. (9) Lin, Y. L.; Wu, C. S.; Lin, S. W.; Huang, J. L.; Sun, Y. S.; Yang, D. Y. Bioorg. Med. Chem. 2002, 10, 685-690. (10) von Deuten, K.; Klar, G. Cryst. Struct. Commun. 1979, 8, 10171021. (11) CAD4 Operations Manual; Enraf-Nonius: Delft, The Netherlands, 1994. (12) Harms, K. XCAD4; University of Marburg: Marburg, Germany, 1996. (13) Sheldrick, G. M. SHELXS-97 and SHELXL-97, release 97.2; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (14) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 535. (15) 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. (16) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838. (17) Spek, A. L. PLATON; Utrecht University: Utrecht, The Netherlands, 2003. (18) International Tables for X-ray Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol. IV. (19) Gerzain, M.; Buchanan, G. W.; Driega, A. B.; Facey, G. A.; Enright, G.; Kirby, R. A. J. Chem. Soc., Perkin Trans. 2 1996, 2687-2693. (20) Caillet, J. Acta Crystallogr. 1982, B38, 1786-1791. (21) Wu, G.; Jacobs, S.; Lenstra, A. T. H.; Van Alsenoy, C.; Geise, H. J. J. Comput. Chem. 1996, 17, 1820-1835. (22) Allen, F. H. Acta Crystallogr. 2002, B58, 380-388. (23) Irngartinger, H.; Lichtentha¨ler, J.; Herpich, R. Struct. Chem. 1994, 5, 283-285.

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