Lyomesophases of C3-Symmetrical Bipyridine-Based Discs in

Apr 23, 2009 - Rafael Martín-Rapún‡, Dmytro Byelov§∥, Anja R. A. Palmans‡, Wim H. de Jeu§∥ and E. W. Meijer*‡ ... E-mail: e.w.meijer@tue...
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Lyomesophases of C3-Symmetrical Bipyridine-Based Discs in Alkanes: An X-ray Diffraction Study† Rafael Martin-Rapun,‡ Dmytro Byelov,§, Anja R. A. Palmans,‡ Wim H. de Jeu,§, and E. W. Meijer*,‡ )

‡ Laboratory of Macromolecular and Organic Chemistry and §Department of Polymer Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and FOM-Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

Received January 23, 2009. Revised Manuscript Received March 23, 2009 The importance of the role of alkane solvents in the self-assembly process of π-conjugated molecules is well recognized but hardly understood. Here we present our results on the X-ray diffraction studies that we conducted to gain insight into the supramolecular structure of mixtures of a bipyridine-based molecule (1) with alkanes. Independent of the alkane used (linear or branched), above xw > 0.06 (with xw being the weight fraction of 1) the mixtures show lyotropic liquid-crystalline behavior. The nature of the lyomesophase depends only on xw and not on the nature of the alkane (linear or branched). A columnar rectangular phase is present when xw > 0.66. Upon dilution of 1, a columnar hexagonal phase is assigned first (0.50 < xw < 0.65), and finally a columnar nematic phase is observed when xw < 0.50. Concentration-dependent SAXD measurements revealed that the dilution of 1 can be viewed as a swelling process. First, solvent molecules occupy space between the columns formed by 1, which are not disrupted. This process can quantitatively be described by a 2D swelling model. Only at lower concentrations does 3D swelling start as the columns start breaking into shorter fragments.

Introduction Supramolecular chemistry gives access to a wide range of functional materials by use of noncovalent interactions such as π-π stacking, hydrogen bonding, and solvophobic interactions.1 A field of increasing interest is the construction of materials with applications in nanoelectronic devices because it allows for the relatively easy preparation of multicomponent functional materials out of a library of simpler building blocks. Among the organic materials, π-conjugated (semiconducting) polymers are by far the most promising for these applications.2 Self-assembled π-conjugated materials are typically obtained from molecularly dissolved building blocks under a certain stimulus i.e., interaction with a surface,3 addition of a template,4-6 or a change in concentration † Part of the Molecular and Polymer Gels; Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. E-mail: [email protected].

(1) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (2) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491–1546. (3) Gomar-Nadal, E.; Puigmarti-Luis, J.; Amabilino, D. B. Chem. Soc. Rev. 2008, 37, 490–504. (4) Gothelf, K. V.; LaBean, T. H. Org. Biomol. Chem. 2005, 3, 4023–4037. (5) Sugimoto, T.; Suzuki, T.; Shinkai, S.; Sada, K. J. Am. Chem. Soc. 2007, 129, 270–271. (6) Janssen, P. G. A.; Vandenbergh, J.; van Dongen, J. L. J.; Meijer, E. W.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2007, 129, 6078–6079. (7) Smulders, M. M. J.; Schenning, A. P H. J.; Meijer, E. W. J. Am. Chem. Soc. 2008, 130, 606–611. (8) Chen, Z.; Stepanenko, V.; Dehm, V.; Prins, P.; Siebbeles, L. D. A.; Seibt, J.; :: Marquetand, P.; Engel, V.; Wurthner, F. Chem.;Eur. J. 2007, 13, 436–449. :: (9) Wurthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.;Eur. J. 2001, 7, 2245–2253. :: (10) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mullen, K. J. Am. Chem. Soc. 2005, 127, 4286–4296. (11) Lahiri, S.; Thompson, J. L.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 11315–11319. :: :: (12) Stoncius, S.; Orentas, F.; Butkus, E.; Ohrstrom, L.; Wendt, O. F.; :: Warnmark, K. J. Am. Chem. Soc. 2006, 128, 8272–8285. (13) Tobe, Y.; Utsumi, N.; Kawabata, K.; Nagano, A.; Adachi, K.; Araki, S.; Sonoda, M.; Hirose, K.; Naemura, K. J. Am. Chem. Soc. 2002, 124, 5350–5364.

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or temperature.7-15 The solvent is often viewed as a continuous medium from which polarity,16 hydrophobicity, and protic/aprotic character are dominant in tuning the self-assembly by enhancing or preventing supramolecular interactions. Recently, we reported that more subtle changes in the solvent can give a pronounced effect in the self-assembly process of several compounds. For example, in chiral oligo(p-phenylenevinylene) derivatives, the temperature at which the self-assembly process started was found to depend on the length of the alkane used as the solvent (ΔTe = 10 °C when going from tetradecane to heptane at constant oligomer concentration).17 This implies that the solvent does not behave just as a continuous dielectric medium but that its co-organization at the periphery of the aggregates plays a direct role in the assembly process during the formation of the preaggregates.17 Another illustrative example highlighting the importance of the solvent during the self-assembly process was observed for bipyridine-based C3-symmetrical discotics. These molecules self-assemble into supramolecular polymers in alkanes as a result of π-π interactions.18 The conformation of the compound in the stack is propeller-like, and as a result, the stacks are inherently helical although there is not any preferred sense in the absence of a source of chirality. When using chiral (R)-(+)2,6-dimethyloctane as the solvent, a Cotton effect was induced.18 This suggested that the chirality of the solvent could be transferred to induce an excess of P over M helices in the self-assembled state. Moreover, CD experiments using n-heptane and iso-octane also proved the existence of a solvent shell surrounding the stacks.19 (14) van Herrikhuyzen, J.; Syamakumari, A.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021–10027. (15) Wang, W.; Han, J. J.; Wang, L. Q.; Li, L. S.; Shaw, W. J.; Li, A. D. Q. Nano Lett. 2003, 3, 455–458. (16) Rehm, T.; Schmuck, C. Chem. Commun. 2008, 801–813. (17) Jonkheijm, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Science 2006, 313, 80–83. (18) Palmans, A. R .A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Angew. Chem., Int. Ed. 1997, 36, 2648–2651. (19) Unpublished results.

Published on Web 04/23/2009

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Figure 1. (Left) Molecular structure of the bipyridine-based molecule (1) and (right) schematic representation of its self-assembly process in alkanes in the absence of a source of chirality. Segments of opposite helicity can also be present in the same stack.

The strength of the interaction between the alkane and the peripheral aliphatic tails of the building block can depend on the length of the alkane as occurs with the interactions of alkanes with phospholipid bilayers.20,21 In that case, X-ray measurements proved that longer alkanes align parallel to the lipid acyl chain whereas shorter alkanes increase the bilayer thickness by positioning in the hydrophobic region between monolayers.20 Moreover, concentration-dependent X-ray measurements have been widely employed to elucidate the lyomesophase structure of guanine derivatives in water22-25 and in organic solvents.26-30 In the latter case, these studies allowed the authors to differentiate the aggregation behavior of guanine ribbons in chloroform and in alkanes. It was found that the lattice parameters follow a powerlaw dependence with the volume concentration of the guanine derivative. The exponent is the fingerprint of the type of swelling process that is taking place whereas the other constant depends on the shape of the mesogenic element.31,32 In view of the powerful results obtained by X-ray diffraction (XRD) in the examples discussed above, we wondered if the

interaction of alkanes with self-assembled systems available in our laboratory could be unraveled by performing XRD studies. Here we concentrate on bipyridine-based discotic 1 bearing dodecyl tails in the periphery (Figure 1). Pure 1 exhibits a columnar liquidcrystal phase at room temperature (orthorhombic a = 6.9 nm, b = 4.0 nm).33 The helical structure of the stacks also imposes order in the direction of the column axis.33-40 Moreover, compound 1 shows lyotropic liquid crystallinity upon addition of alkanes,41 thus providing a periodic structure whose lattice parameters could change with the size of the alkane molecules. We now present a detailed study of the lyotropic behavior of compound 1 in alkanes and the assignment of the mesophases as a function of concentration based on the XRD results. We have varied the alkanes in length (CnH2n+2; n = 8, 12, 13, 14, 15, 16) but also in shape to compare the results obtained for the linear alkanes with highly branched 2,2,4,4,6,8,8-heptamethylnonane. By studying the system at different concentrations of 1, we rationalize how the swelling of compound 1 in alkane solvents is taking place.

Experimental Part (20) McIntosh, T. J.; Simon, S. A.; Macdonald, R. C. Biochim. Biophys. Acta 1980, 597, 445–463. (21) Aagaard, T. H.; Kristensen, M. N.; Westh, P. Biophys. Chem. 2006, 119, 61–68. (22) Spada, G. P.; Bonazzi, S.; Garbesi, A.; Zanella, S.; Ciuchi, F.; Mariani, P. Liq. Cryst. 1997, 22, 341–348. (23) Bonazzi, S.; Capobianco, M.; de Morais, M. M.; Garbesi, A.; Gotarelli, G.; Ponzi Bassi, M. C.; Spada, G. P.; Tondelli, L. J. Am. Chem. Soc. 1991, 113, 5809–5816. (24) Proni, G.; Spada, G. P.; Gottarelli, G.; Ciuchi, F.; Mariani, P. Chirality 1998, 10, 734–741. (25) Franz, H.; Ciuchi, F.; Di Nicola, G.; De Morais, M. M.; Mariani, P. Phys. Rev. E 1994, 50, 395–402. (26) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Spada, G. P.; Mariani, P. Liq. Cryst. 1999, 26, 965–971. (27) Giorgi, T.; Grepioni, F.; Manet, I.; Mariani, P.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Saturni, L.; Spada, G. P.; Gottarelli, G. Chem.;Eur. J. 2002, 8, 2143–2152. (28) Giorgi, T.; Lena, S.; Mariani, P.; Cremonini, M. A.; Masiero, S.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P.; Gottarelli, G. J. Am. Chem. Soc. 2003, 125, 14741–14749. (29) Lena, S.; Brancolini, G.; Gottarelli, G.; Mariani, P.; Masiero, S.; Venturini, A.; Palermo, V.; Pandoli, O.; Pieraccini, S.; Samori, P.; Spada, G. P. Chem. Eur. J. 2007, 13, 3757–3764. (30) Lena, S.; Cremonini, M. A.; Federiconi, F.; Gottarelli, G.; Graziano, C.; Laghi, L.; Mariani, P.; Masiero, S.; Pieraccini, S.; Spada, G. P. Chem.;Eur. J. 2007, 13, 3441–3449. (31) Hentschke, R.; Taylor, M. P.; Herzfeld, J. Phys. Rev. A 1989, 40, 1678–1680. (32) Taylor, M. P.; Hentschke, R.; Herzfeld, J. Phys. Rev. Lett. 1989, 62, 800–803.

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Preparation of the Liquid-Crystalline Phase. Compound 1 was prepared according to a previously described procedure.34 The mixtures of 1 with alkanes were prepared following the method previously described.41 A weighed amount of 1 (520 mg) was dissolved in about 0.4 mL of dichloromethane inside a vial. Subsequently, the appropriate amount of alkane was added (33) A detailed discussion on the helical order within pure 1 is given in Metzroth, T.; Hoffmann, A.; Martin-Rapun, R.; Smulders, M. M. J.; Pieterse, K.; Palmans, A. R. A.; Vekemans, J. A. J. M.; Meijer, E. W.; Spiess, H.-W.; Gauss, J. Manuscript in preparation. (34) Palmans, A. R. A.; Vekemans, J. A. J. M.; Hikmet, R. A.; Fischer, H.; Meijer, E. W. Chem.;Eur. J. 1997, 3, 300–307. (35) See refs 36-40 for other discotic liquid-crystalline systems with helical order along the columns due to the propeller-like organization of nonchiral molecules. (36) Fontes, E.; Heiney, P. A.; de Jeu, W. H. Phys. Rev. Lett. 1988, 61, 1202–1205. :: (37) Pisula, W.; Tomovic, Z.; Watson, M. D.; Mullen, K.; Kussmann, J.; Ochsenfeld, C.; Metzroth, T.; Gauss, J. J. Phys. Chem. B 2007, 111, 7481–7487. (38) Vera, F.; Tejedor, R. M.; Romero, P.; Barbera, J.; Ros, M. B.; Serrano, J. L.; Sierra, T. Angew. Chem., Int. Ed. 2007, 46, 1873–1877. (39) Barbera, J.; Puig, L.; Romero, P.; Serrano, J. L.; Sierra, T. J. Am. Chem. Soc. 2006, 128, 4487–4492. (40) Stals, P. J. M.; Smulders, M. M. J.; Martin-Rapun, R.; Palmans, A. R. A.; Meijer, E. W. Chem.;Eur. J. 2009, 15, 2071–2080. (41) Palmans, A. R. A.; Vekemans, J. A. J. M.; Hikmet, R. A.; Fischer, H.; Meijer, E. W. Adv. Mater. 1998, 10, 873–876.

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Article to the solution. Then the mixture was stirred at 30 °C in the open vial by use of an orbital shaker equipped with a Peltier SC20XT from Torrey Pines Scientific. After being concentrated, the mixture was kept under vacuum overnight at room temperature. The materials thus obtained were homogeneous, colorless, and stable for longer than 18 months. At higher concentrations, no flow is detected upon inversion of the vial containing the material, whereas at lower concentrations they behave as a viscous liquid. Concentration Determination. Because of the preparation procedure for the samples, the alkane may have been partially evaporated. Therefore, the real composition of all samples had to be determined by 1H NMR spectroscopy. The integration of signals of 1 corresponding to protons of the aromatic core and protons R to the oxygen of the alkyloxy tails of 1 were compared to the integration of signals corresponding to methylene and methyl groups that are present in the alkane and in 1. The weight fraction of 1 in the mixture, xw, was then calculated with the molecular masses of 1 and the alkane in hand (Supporting Information). The volume fraction of 1 (xν) in the mixture has been derived from xw as reported in the Supporting Information. We have used the densities provided by Aldrich for the alkanes (0.70 g mL-1 for octane, 0.75-0.77 g mL-1 for longer linear alkanes, and 0.79 g mL-1 for heptamethylnonane) and 0.95 g mL-1 for 1 as obtained from XRD measurements. Instruments. 1H NMR spectra were recorded at room temperature on a Varian Mercury (400 MHz for 1H NMR) NMR spectrometer using tetramethylsilane (TMS) as an internal standard. Differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere on a Perkin-Elmer Pyris 1 differential scanning calorimeter with Pyris 1 DSC autosampler and a Perkin-Elmer CCA7 cooling element. Polarizing optical microscopy studies were conducted using a Jeneval microscope equipped with crossed polarizers, a Linkam THMS 600 heating stage, and a Polaroid DMC Ie CCD camera. Small-angle X-ray diffraction (SAXD) measurements were made using an in-house setup at AMOLF with a rotating anode X-ray generator (Rigaku RUH300, 18 kW) equipped with two parabolic multilayer mirrors (Bruker, Karlsruhe), giving a parallel beam (divergence of about 0.025°) of monochromatic Cu KR radiation (λ = 0.154 nm). The SAXD intensity was collected with a 2D gas-filled wire detector (Bruker Hi-Star). A semitransparent beam stop placed in front of the area detector allowed for monitoring of the intensity of the direct beam. The SAXD intensities were corrected by subtracting a background that was normalized to the acquisition time. Samples were put in a capillary-type quartz cell with a diameter of 0.9 mm and a :: wall thickness of 0.01 mm (Mark-Rohrchen, Germany). Most of the samples presented flow-induced alignment as a result of the capillary filling. For measurement, the capillaries were fit to a custom-made brass sample holder that allowed for rotation of the capillary perpendicular to the beam. A Linkam THMS600 temperature-controlled system was employed as the sample stage. To minimize the background signal, the glass windows in the sample stage were left out for the measurements at room temperature and replaced by Kapton foil (20 μm) for the measurements at 100 °C. Datasqueeze software (version 2.1.2) was used to integrate all of the 2D data for the SAXD measurements.42 WAXD experiments were performed at the DUBBLE beamline (BM26B) at the European Synchrotron radiation facility (ESRF) in Grenoble, France. The 2D WAXD images were recorded using a CCD camera detector equipped with a Kodak KAI-11000 sensor. A 2  2 binning mode was used to acquire the images. The effective pixel size is 40.2  40.2 μm2, with the actual size of the detector being 53.4  79.4 mm2. The X-ray beam energy was 10 keV, and the sample-to-detector distance used (42) http://www.datasqueezesoftware.com/index.html

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Figure 2. (a) Optical microscope image of the schlieren texture exhibited by a nematic mixture of 1 (xw = 0.50) with hexadecane at 152 °C. (b) Crossed-polarized optical micrograph of a mixture of 1 (xw = 0.56) with tridecane at 170 °C (Tb,tridecane = 234 °C). The texture of the columnar hexagonal phase grows with 6-fold symmetry into the isotropic phase. was ∼56 mm. Diffraction signals from an HDPE standard sample were used for q-scale calibration. Corrections for dark current and detector flat field were applied to the 2D WAXD images of samples and before transformation into corrected 1D profiles by performing radial integration along the azimuthal angle using the FIT2D program developed by Dr. Hammersley of the ESRF.43,44 The WAXD intensities were corrected by subtracting a background (scattering by the empty cell) that was normalized to the acquisition time.

Results and Discussion Polarizing Optical Microscopy and Differential Scanning Calorimetry of Mixtures of 1 in Alkane Solvents. We prepared concentrated mixtures of 1 in a number of linear alkane solvents (CnH2n+2; n = 8, 12, 13, 14, 15, 16) and branched 2,2,4,4,6,8,8-heptamethylnonane. In the concentration range under study, 0.02 < xw < 0.80, the mixtures behave as highly viscous liquids to gel-like materials. All mixtures were analyzed by polarizing optical microscopy with variable temperature (POM). Independent of the alkane used, the mixtures are birefringent at room temperature when xw > 0.06 (∼0.02 M). POM observations clearly show that the system is homogeneous over the whole concentration range under study and no isotropic domains that could be ascribed to solvent-enriched regions resulting from a biphasic system were discerned.41 The clearing temperature exhibits a linear dependence on xw, as previously observed (Figure S1 in the Supporting Information).41 For xw = 0.15 and 0.30, mixtures of 1 in hexadecane isotropization occur around 100 and 140 °C, respectively. When xw > 0.50, the clearing point could not be precisely measured even for hexadecane as a result of evaporation of the solvent at the edges of the sample. The molecular mass of the hydrocarbon used as a solvent does not seem to affect either the texture of the phase or the clearing point. Upon cooling the isotropic mixture, we observed a clear schlieren texture (Figure 2a) for the lower concentration range (43) Hammersley, A. P.; Svensson, S. O.; Handfland, M.; Fitch, A. N.; Hausermann, D. High Pressure Res. 1996, 14, 235–248. (44) http://www.esrf.eu/computing/scientific/FIT2D/index.html

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Figure 3. Aligned sample (a-c) WAXD and (d-f) SAXD patterns registered at RT. The shearing direction and axis of the columns coincide with the direction of qz, whereas plane XY is perpendicular to the axis of the columns. (a, d) WAXD and SAXD patterns of the columnar nematic phase (NCol) of a mixture of 1 (xw = 0.33) and dodecane, respectively. (b, e) Ordered columnar hexagonal phase (Colho) of a mixture of 1 (xw = 0.55) and dodecane. (c, f) WAXD and SAXD patterns of the ordered columnar rectangular (Colro) phase of a mixture of 1 (xw = 0.70) and hexadecane. (e, f) Assignment of the peaks shown according to the corresponding lattice.

(xw < 0.56). This texture is typical for nematic liquid crystals. Despite the difficulty of growing textures when xw > 0.56 because the solvent starts to evaporate around the clearing point, we have been able to grow textures with defects that exhibit 6-fold symmetry, pointing to a columnar hexagonal phase (Figure 2b). For selected samples, the transition temperatures were confirmed using differential scanning calorimetry (DSC); the numbers were in good agreement with those observed in the POM measurements. The results clearly show that mixtures of 1 in alkanes form lyomesophases over a wide concentration range (xw > 0.06) and that the nature of the mesophase is determined by the weight fraction of 1 present in the mixture. Small-Angle (SAXD) and Wide-Angle (WAXD) X-ray Diffraction Experiments. To quantitatively analyze the influence of solvent addition to the packing of 1, we used X-ray diffraction. In addition, this allows for an unambiguous phase assignment in the case of the more concentrated mixtures. In Figure 3, we have gathered the WAXD and SAXD patterns of some representative aligned samples prepared with different solvents and concentrations. High-quality WAXS data were available only at room temperature whereas SAXS data were obtained at room temperature and at 100 °C. Alignment was achieved by flow when filling the capillaries for the lower concentrations and by shearing the sample inside the capillary for the samples of higher concentrations. WAXD. Irrespective of the alkane used, all mixtures exhibit the same features in the wide-angle region at room temperature. The broad diffuse halo corresponding to a distance of about 0.45 nm is caused by the liquidlike peripheral chains of 1 and the solvent molecules (Figure 3a-c). Outermost, there is a sharp maximum that gives a periodicity of 0.34 nm over the whole concentration range, which is the typical distance between consecutive discs inside a column in ordered columnar liquid crystals. The fact that this maximum lies on the meridian indicates that 1 forms columns that are aligned along the direction of flow. The reflection at 0.34 nm is still present at concentrations as low as xw = 0.06 but disappears when xw = 0.02. A weaker maximum corresponding to a distance of ca. 0.36 nm appears at concentrations of xw > 0.09 and is also present for pure 1. It is ascribed to Langmuir 2009, 25(15), 8794–8801

the helical order within the columns.33,45 In the propeller-shaped conformation of 1, the planes containing the central amides and bipyridine moieties are tilted about ∼-20° out of the plane of the central benzene units. Resulting from this tilt and from the rotation between consecutive molecules (see below), the perpendicular distance between bipyridine moieties is 0.36 nm while keeping the periodicity of 0.34 nm along the stack.33 SAXD. At higher concentrations of 1 in alkanes, additional split reflections in the small-angle area become visible in the WAXD patterns (Figure 3c). These were investigated in more detail by SAXD. Because no major variations were observed between SAXD measurements performed at room temperature (RT) and 100 °C (Figure S3 in the Supporting Information), here we discuss only the results obtained at RT. The SAXD patterns of all samples with a concentration above 6% w/w (xw > 0.06) exhibit an intense equatorial reflection that gradually becomes sharper and moves toward larger diffraction angles upon increasing the concentration (Figure 3d-f and SAXD profiles in Figure 4a). The quality of the alignment of the samples is shown through the azimuthal angle χ plots for that maximum in Figure 4b. Taking into account the other features of the SAXD patterns, we can distinguish three types of liquid crystal behavior of the mixtures. When the weight fraction of 1 in alkanes is above 0.66 (xw > 0.66) (Figure 3f), the SAXD pattern qualitatively resembles that of pure compound 1.33,34 We observe two equatorial reflections from which the inner one is the most intense in the pattern. In the absence of√any other feature, the reciprocal ratio between their spacings (1: 3) would point to a hexagonal 2D lattice in the plane perpendicular to the columns axis. However, there is a set of sharp reflections out of the equator and of the meridian, located in a plane with the same Miller index l 6¼ 0. The fact that these maxima lie out of the equator means that there is a modulation in (45) For all liquid crystalline mixtures with xw >0.09 the WAXD patterns exhibit two meridian aromatic diffuse scattering maxima corresponding to 0.34 and 0.36 nm. Both maxima are also present for the pure compound (see ref 34). The presence of both peaks also in the whole concentration range and also for pure compounds prompted us to discard the coexistence of two phases as the cause of both peaks.

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Figure 4. One-dimensional graphs extracted from the SAXD patterns shown in Figure 3: (dark gray) mixture of 1 (xw = 0.33) and dodecane, (gray) mixture of 1 (xw = 0.55) and dodecane, and (black) mixture of 1 (xw = 0.70) and hexadecane. (a) SAXD profiles. (b) Azimuthal angle χ plots for the maximum at the lowest angle, showing very good alignment of the samples. (c). qz plots in which the equatorial reflections are masked to take into account only the maxima exhibiting a four-spot pattern.

the electron density along the columns. The resulting SAXD pattern is reminiscent of those of highly concentrated phases of DNA,46 helical poly(phenylacetylene),47 and the helical organization of hexa-peri-benzocoronene derivatives.48 We attribute these maxima to the helical internal structure of one column and how it is correlated to that of the neighboring columns. They all correspond to a periodicity of about 3.7 nm along the axis of the columns. Given the C3 symmetry of 1, 3.7 nm corresponds to a 120° turn of the helix. The result of dividing 3.7 nm by the interdisc distance of 0.34 nm would give the number of molecules needed for a 120° turn of the helix, which is not an integer. In this context, additional off-equator and off-meridian reflections should theoretically appear but could not be observed. About 11 molecules are then needed for the 120° turn of the helix, meaning that one molecule is rotated approximately 11° with respect to its immediate neighbors in the same stack. It is remarkable that this rotational angle is far remote from that observed in single crystals49,50 and the liquid-crystalline state40 of smaller N,N0 ,N00 -trialkyl benzene-1,3,5-tricarboxamides, which is 60°. This fact would exclude for the bipyridine-based discs the same kind of intermolecular hydrogen bonding that is the driving force for the formation of stacks of the smaller trialkyl benzene1,3,5-tricarboxamides. In the case of 1, the π-π interactions between the aromatic cores could also account by themselves for the formation of the stacks. It is not possible to index the corresponding components of the split reflections within a 2D hexagonal lattice in the XY plane, perpendicular to the column axis. Thus, the lattice in that plane is pseudohexagonal and possesses rectangular symmetry. The equatorial maxima correspond to 110 and 310 reflections of the rectangular lattice (Figure 3f). For some of the reflections, h + k¼ 6 2n (n is an integer), thus the column in the center of the unit cell differs from the neighboring columns, probably in the phase of the helix; that is, it is vertically shifted with respect to them as occurs in highly concentrated phases of DNA.46 When the concentration is in the range of 0.50 < xw < 0.65 (Figure 3e), the SAXD pattern is different. Both equatorial √ reflections are still present and keep the same ratio of 1: 3 in reciprocal space, indicative of a 2D hexagonal lattice. However,

the discrete off-equator reflections become a diffuse maximum, also showing a four-spot pattern. We attribute the diffuse character of the maximum to a loss of correlation between the helix phases of neighboring columns.46 As a result, the symmetry increases, and a columnar hexagonal phase is obtained. This is consistent with the textures grown for mixtures in the lower part of this concentration range (Figure 2b). At lower concentrations (xw < 0.50), the diffuse four-spot pattern is still present, and its position does not change when compared to the previous group (0.50 < xw < 0.65). On the equator, only the inner maximum is left, but broader and at smaller angles, thus corresponding to larger intercolumnar distances in real space. Below 37% w/w concentration (xw < 0.37), the maximum is too broad for an accurate measurement. The nematic phase can be assigned straightforwardly in this concentration regime on the basis of POM observations. More precisely, it is an ordered columnar nematic phase taking the abovedescribed WAXD patterns into account. It is important to note that the periodicities along the column axis associated with the four-spot maxima are about the same, no matter their sharp or diffuse appearance. This is shown when we plot the integrated intensity as function of qz in the patterns excluding the most intense equatorial reflections (Figure 4b). The periodicity of ca. 3.7 nm in the lyomesophases of 1 is slightly higher than the 3.2 nm previously found for pure 1.33 No measurable difference in the SAXD and WAXD patterns could be detected between lyomesophases of 1 prepared in linear alkanes or branched heptamethylnonane. Concentration-Dependent Experiments. To gain insight into the interaction of the alkane solvent with 1, we then studied in detail the dependence of the intercolumnar and interdisc distances of 1 as a function of concentration for linear and branched alkanes. The intercolumnar distance a can be calculated from the SAXD profile according to eqs 1-3 for the nematic,51 hexagonal, and rectangular columnar phases, respectively:

(46) Durand, D.; Doucet, J.; Livolant, F. J. Phys. II France 1992,, 2, 1769–1783. (47) See for example:Nagai, K.; Sakajiri, K.; Maeda, K.; Okoshi, K.; Takahiro, S.; Yashima, E. Macromolecules 2006, 39, 5371–5380. (48) See for example:Feng, X.; Pisula, W.; Masayoshi, T.; Dou, X.; Enkelmann, :: V.; Wagner, M.; Ding, N.; Mullen, K. Chem. Mater. 2008, 20, 2872–2874. (49) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.; Warren, J. E. Chem. Commun. 1999, 1945–1946. (50) Bose, P. P.; Drew, M. G. B.; Dasa, A. K.; Banerjee, A. Chem. Commun. 2006, 3196–3198.

ð3Þ

8798 DOI: 10.1021/la9003017

a ¼ 1:117dnem pffiffiffi a ¼ 2d10 = 3 ¼ ahex pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi brec þ arec 2 þ brec 2 a ¼ 3

ð1Þ ð2Þ

(51) De Vries, A. Mol. Cryst. Liq. Cryst. 1970, 10, 219–236. The equation was proposed for calamitic nematic liquid crystals, but it has also been used to calculate, for instance, an approximate value of the mean interhelix distance in chiral nematic phases of DNA in water in ref 45. In that case, the mesogenic element is analogous to the system presented here.

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Figure 5. Intercolumnar distance a as a function of concentration xν for selected alkanes at room temperature. The phase of the mixture at each concentration is indicated by the symbol in use: (O) columnar nematic, (Δ) columnar hexagonal, (0) columnar rectangular, and (;) power law fit of the data. The absolute errors in the data are (0.3 nm for the columnar nematic phase and (0.1 nm for the columnar hexagonal and columnar rectangular phases. No dependence on the solvent can be observed above the resolution limit of the setup in use.

At high concentrations of 1, the intercolumnar distance a increases compared to that of the pure compound where a = 4.0 nm,33 which indicates that solvents are able to penetrate the structure. Upon addition of more solvent, the intercolumnar distances further increase in the concentration range under study, which reaches xw ≈ 0.29 as the maximum dilution as a result of the lower angle limit of our setup and the broadening of the nematic halo. Upon dilution of 1, the shift of the most intense equatorial peak in the SAXD pattern toward lower angles, and thus the increase in the intercolumnar distance, is gradual and continuous and constitutes a sign of homogeneity of the system that adds to POM observations, which already show that we have a homogeneous mixture over the whole concentration range under study. To rationalize this behavior, we decided to fit our data to a swelling model previously used for guanine derivatives.23-28 The model considers the system to be composed of a solvent and rods with a constant cross-sectional area, which constitute the mesogenic element in the material. If we do not consider the phase correlation between the helices of neighboring rods, which is not important for their overall shape, then they are packed into a 2D cell. Two particular cases are especially relevant: first, 2D swelling, in which infinite rods gradually separate from each other in the plane perpendicular to their long axis upon addition of solvent; second, 3D isotropic swelling where added solvent molecules take place isotropically around finite rods of constant length. For both situations, we can establish the power law dependence (eq 4) of the intercolumnar distance a on the volume fraction of rods xν:27 a ¼ mxμv

ð4Þ

The values of μ and m depend on the type of swelling process that is taking place. For instance, in the case of 3D isotropic swelling, μ = -1/3, whereas the fingerprint for a 2D swelling situation is μ = -1/2. From m, we can obtain the cross-sectional area of the rigid rods. The model assumes complete segregation between the medium and the molecules or parts of molecules contained in the rods. It is clear that the stacked mesogenic cores of molecules of 1 do not interact with the solvent medium. The end of the peripheral alkyl chains must be dissolved in the alkane solvent, but it is also reasonable to think that in the proximity of the mesogenic cores the alkyl chains are also stacked and so closely packed that they do not interact with the solvent. It is not possible to know to what extent the alkyl chains participate in the rods or in the alkane solvent, and in our simplification we propose complete segregation between the alkane solvents and the stacked molecules of 1, Langmuir 2009, 25(15), 8794–8801

Table 1. Fitting Parameters for the Dependence of the Intercolumnar Distance a on the Concentration of 1 According to the Power Law a = mxμv solvent

m(nm)

μ

R2

dodecane tridecane tetradecane pentadecane hexadecane heptamethylnonane octane

3.93 ( 0.06 4.04 ( 0.12 3.86 ( 0.07 3.84 ( 0.08 3.92 ( 0.07 3.97 ( 0.04 4.39 ( 0.17

-0.54 ( 0.02 -0.52 ( 0.03 -0.55 ( 0.02 -0.56 ( 0.02 -0.50 ( 0.02 -0.53 ( 0.01 -0.48 ( 0.05

0.9910 0.9704 0.9915 0.9902 0.9852 0.9968 0.8944

including the peripheral chains. Therefore, the volume fraction of rods equals the volume fraction of 1, xν, and is calculated as shown in the Supporting Information, taking the densities provided by Aldrich for the alkane solvents (0.70 g mL-1 for octane, 0.75-0.77 g mL-1 for longer linear alkanes, and 0.79 g mL-1 for heptamethylnonane) and 0.95 g mL-1 for 1 as obtained from XRD measurements. In Figure 5a-c, the intercolumnar distance a is plotted against the volume concentration (xν) of 1 in hexadecane, dodecane, and heptamethylnonane. Judging from these, there is no systematic trend when changing the alkane. Irrespective of the nature of the alkane (linear or branched), a seems to depend only on the volume fraction of 1 in the alkane solvent. The plots shown in Figure 5 were fit to the power law (eq 4), yielding the parameters gathered in Table 1. The results are equivalent to those obtained by considering that the peripheral alkyl chains of 1 are completely dissolved in the medium (not shown). The absolute value of μ is slightly above 0.50 for all solvents, which allows us to conclude that the rods are “infinitely” long and that the structure undergoes 2D swelling upon addition of alkane. Three-dimensional swelling can be excluded in the concentration range investigated here. The slight deviation of μ from the theoretical -0.50 can be explained by the densities that we have used to calculate xν from the NMR results. We have used the densities provided by Aldrich for the alkanes (0.70 g mL-1 for octane, 0.75-0.77 g mL-1 for longer linear alkanes, and 0.79 g mL-1 for heptamethylnonane) and 0.95 g mL-1 for 1 as obtained from XRD measurements. Larger differences between local densities of the regions in the material occupied by 1 and alkanes would lower the absolute values of μ in Table 1. From Equation 4, it follows that m is the distance that would separate the rods at xν = 1 if they could deform and pack in the absence of the medium with the same geometry as the mixtures. In our case, if xν = 1, then we have pure 1 because we had included the peripheral alkyl chains in the rods. Pure 1 is liquid crystalline DOI: 10.1021/la9003017

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Figure 6. Dependence on xν of the correlation distance D associated with the major equatorial maximum in the SAXD pattern as calculated with the Scherrer equation ()).

at RT and packs in an orthorhombic lattice with an intercolumnar distance of 4.0 nm as calculated using eq 3. This value coincides nicely with m values reported for all solvents in Table 1. Thus, the “infinite” helical rods that we observe have a cross section of only one molecule. The system is equivalent to previously reported G-quartet-forming guanine derivatives, in which one-G-quartet cross-section rods gradually separate from each other upon addition of water22-25 or an alkane.28 In those systems, columnar hexagonal and (chiral) columnar nematic phases undergo 2D swelling upon addition of solvent. Although the system of Gottarelli and Spada and ours give rise to gel-like or highly viscous mixtures, fibers originating this behavior are different from normal usual gels. In the normal case, fiberlike crystals of pure gelator, and thus bundles of one-molecule-thick fibers, are formed. To obtain more evidence that bipyridine-based discotics indeed behave as infinite helical rods that are gradually separated by the addition of more solvent, we estimated the correlation distance associated with the major equatorial maximum in the SAXD pattern (Figure 6) using the Scherrer equation.52 Above xν = 0.5, and thus in the columnar hexagonal and columnar rectangular range, the domain size is above the resolution limit of 100 nm of our setup (about 25 rods). A rapid decrease in the correlation distance of the peak takes place when going from the columnar hexagonal to the columnar nematic phase. Further addition of solvent moderately decreases that parameter, but at xν = 0.33, it is just about 10 nm, meaning that it extends only to two to three rods away. Correlation distances can also be calculated for the interdisc distance. As shown by WAXD measurements, the interdisc distance dπ-π is around 0.34 nm for all concentrations investigated; the length or geometry of the solvent does not have an influence (Figure 7a). The Scherrer equation was then used to estimate the correlation distance Dπ-π for the peak corresponding to the interdisc distance (Figure 7b). There is a large dispersion in our data as a result of the superposition of the peak with the halo arising from the alkyl chains. The data qualitatively show that above xν ≈ 0.20 the correlation distance Dπ-π is larger than the resolution limit of the setup, corresponding to about 7 nm, that is, more than 20 molecules long. A strong decrease in domain size occurs below that concentration, indicating that solvent molecules are disrupting the stacks. This result suggests that 3D swelling occurs in the concentration regime of xν < 0.20. Unfortunately, in the range of 0.20 < xν < 0.30 we do not have enough information to confirm whether 3D or 2D swelling takes place. (52) The Scherrer equation as used here is Dhkl = Kλ/(Δθhkl cos θhkl), where D is the correlation length/domain size associated with the peak; K is an empirical proportionality factor most often around 0.9, which is the value that we have used; λ is the wavelength of incident X-rays; θhkl is the center diffraction angle of the peak; and Δθhkl is the width of the peak at half-maximum intensity in radians.

8800 DOI: 10.1021/la9003017

Figure 7. Dependence on xν of (a) the π-π stacking distance dπ-π and (b) the correlation distance Dπ-π associated with it as calculated with the Scherrer equation. Both are represented for selected alkanes: (0) hexadecane, (b) dodecane, and (Δ) heptamethylnonane.

Swelling of 1 in Alkanes. Figure 8 summarizes the events that occur upon diluting compound 1 with alkane solvents as derived from POM, SAXD, and WAXD measurements described above. Pure 1 shows a Colro phase at room temperature with an intercolumnar distance of 4.0 nm. The rods pack in a pseudohexagonal lattice because of differences in phase between neighboring rods. Adding any alkane solvent increases the intercolumnar distance in the plane perpendicular to their long axis, and the Colro phase remains up to xν ≈ 0.61. Below xν ≈ 0.60, the correlation between the phases of the helices of neighboring rods is lost, and the symmetry of the lattice changes to hexagonal (Colho). It was shown previously that correlation between helices is compatible with a square or rectangular lattice but leads to frustration on a triangular lattice.36 Thus, the columnar hexagonal phase must lack correlation between helices, and this is possible here only because of the additional solvent between the helical columns. Further addition of solvent (xν < 0.45) prevents communication between the rods, and the regular packing is lost to give rise to a columnar nematic phase (NCol). With SAXD, we have studied the effect of the addition of solvent down to a volume concentration of 1 of xν ≈ 0.25. Below that concentration, the rods persist in the nematic phase as shown by the maximum as a result of the π-π stacking distance even at xν ≈ 0.06. This is not surprising because circular dichroism spectroscopy measurements have shown that helical columnar aggregates exists in isotropic alkane solutions at concentrations as low as 10-6 M.18 The results presented here are consistent with an isotropic orientation of solvent molecules around the aggregates of 1. This implies that XRD measurements have not given experimental evidence for an ordered co-organization of solvent molecules around the aggregates of 1.18 This in turn makes the preferred chirality of 1 in (R)-(+)-2,6-dimethyloctane even more fascinating and a subject for further study. Langmuir 2009, 25(15), 8794–8801

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Figure 8. Representation of the swelling process of 1 with alkanes at room temperature.

Conclusions Bipyridine-based disk 1 forms birefringent gel-like mixtures with several alkanes. We have used X-ray diffraction to investigate the supramolecular structure of the mixtures and to try to find the role of the solvent in the self-assembly of 1. All alkanes that we have tested, ranging from linear alkanes to highly branched 2,2,4,4,6,8,8-heptamethylnonane, behave in the same way, and the lyotropic behavior depends only on the volume concentration of 1. Thus, in the concentration range, xw > 0.02, that we have used to study the behavior of 1 in alkanes by XRD, the role of the solvent in the self-assembly process of bipyridinebased molecules is not evident.53 Concentrated mixtures maintain the columnar rectangular phase of pure 1, a pseudohexagonal phase in which symmetry breaking is due to the correlation of the phase of the helical structure of neighboring columns. The addition of solvent decreases the order of the phase and correlation is lost, giving rise first to a columnar hexagonal and then to a columnar nematic phase (Figure 8). The helical structure of the stacks persists over the whole concentration range. The concentration dependence of the lattice parameters can be fitted to a swelling model. According to SAXD measurements, at higher concentrations swelling takes place only in two dimensions in the plane perpendicular to the column axis. As a result, the correlation distance in that plane diminishes with concentration. The distance between stacks can be tuned by adding solvent to the (53) Palmans, A. R. A.; Meijer, E. W. Angew. Chem., Int. Ed. 2007, 46, 8948–8968.

Langmuir 2009, 25(15), 8794–8801

system, and all materials are easily aligned with flow. Solvent molecules disrupt the columns only when the concentration is below xν ≈ 0.30 as proven with WAXD experiments. As was previously pointed out, the structure of the columns in the lyomesophase is the same as in pure 1, thus their cross section is only one molecule and cannot be described as crystals. This fact distinguishes this system from most organogels, in which a network of fiberlike crystals of pure gelator entraps the solvent. Acknowledgment. We thank Martijn Veld for the artwork in Figure 1. Dr. J. A. J. M. Vekemans is acknowledged for fruitful discussions. We are indebted to the personnel of BM26/DUBBLE and especially to Dr. G. Portale for assistance during the WAXD experiments at ESRF. Furthermore, NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) and ESRF are acknowledged for granting the beam time. R.M.-R. acknowledges support from the EU through a Marie Curie IntraEuropean Fellowship (project MEIF-CT-2006-042044) within the EC Sixth Framework Programme. Supporting Information Available: Clearing temperatures observed by POM and a DSC trace, calculation of the volume fractions, indexing of SAXD data for columnar mixtures of 1 with alkanes, SAXD profiles at different temperatures, and plots of the dependence of the intercolumnar distance on the concentration for all of the alkanes. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la9003017

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