COMMUNICATION pubs.acs.org/cm
Lamello-Columnar Mesophase Formation in a Side-Chain Liquid Crystal π-Conjugated Polymer Architecture Ibtissam Tahar-Djebbar,† Fabien Nekelson,†,‡ Benoît Heinrich,§ Bertrand Donnio,§ Daniel Guillon,§ David Kreher,† Fabrice Mathevet,†,* and Andre-Jean Attias*,† †
UMR 7610 CNRS-UPMC, Laboratoire de Chimie Macromoleculaire, 4 Place Jussieu, 75252 Paris Cedex 05, France DRT/LETI/DIHS/LIMN, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 09, France § UMR 7504 CNR-UDS, IPCMS, 23 rue du Loess, 67034 Strasbourg Cedex 2, France ‡
bS Supporting Information KEYWORDS: polythiophene, liquid crystal polymers, lamello-columnar mesophase, conjugated polymers
he self-organization of π-conjugated organic materials forming highly ordered supramolecular architectures has been extensively investigated in the last two decades in view of optoelectronic applications.1 Indeed, the control of both the mesoscopic and nanoscale organization within thin semiconducting films is the key issue for the improvement of charge transport properties and achievement of high charge carrier mobilities. These well-ordered materials are currently either self-organized semiconducting polymers2 or liquid crystals.3 Indeed, on the one hand, previous studies have shown that welldefined polymer architectures such as a regioregular poly(3alkylthiophenes)4 promote self-organizations in two-dimensional sheetlike lamellar structures because of the high planarity of the polymer chains. More recently it has been reported that a semiconducting main-chain liquid-crystalline thieno[3,2b]thiophene polymer exhibited enhanced charge-carrier mobility, when crystallized from the mesophase, because of the formation of large, well-organized lamellar domains.5 On the other hand, it has been demonstrated that discotic mesogens self-assembling into columns can exhibit high charge carrier mobilities due to the efficient one-dimensional anisotropic charge transport along the columnar axis.6 However, the supramolecular architecture resulting from the phase separation at the nanoscale of two covalently linked π-conjugated systems able to self-assemble individually in a lamellar and a columnar nanostructure, respectively, has never been investigated, to the best of our knowledge. In this context, we endeavored to investigate the self-organization of a side-chain liquid crystal (SCLC) semiconducting polymer where (i) the backbone is a π-conjugated polymer and (ii) the side groups are π-conjugated discotic mesogens. Here, we present our preliminary results on the design, synthesis, and structural characterization of a new liquid crystal regioregular poly(3-alkylthiophene) polymer postfunctionalized with side-on discotic triphenylene moieties. As a result we will show that this strategy leads to the supramolecular self-assembly of this SCLC semiconducting polymer in a peculiar lamello-columnar mesophase where a 2D oblique columnar lattice and the lamellar piling coexist in the same structure.
T
r 2011 American Chemical Society
Scheme 1. Synthesis of Discotic Side-Chain Polymer PTTn
The target discotic side-chain polymer PTTn was prepared as depicted in Scheme 1 (see also the Supporting Information). The precursory polythiophene bearing bromide end-terminated lateral chains, PT, was substituted by Williamson ether reaction with a monohydroxy triphenylene derivative, namely 2,3,6,7,10-penta(pentyloxy)-11-hydroxytriphenylene to yield PTTn (number average molecular weight Mn = 35.2 kDa and polydispersity index PDI = 1.1 as determined by MALDITOF). The complete substitution of the polymer by the discs was evidenced by the size exclusion chromatogram shift to higher molecular weight region, and also by the shift of the 1 H NMR resonance of the methylene group (δ = 3.40 ppm) in α to the terminal bromine atom (see the Supporting Information). Prior to the functionalization, the regioregular 2,5-poly(3-(10bromodecyl)thiophene), PT, was prepared according to the McCullough’s procedure, based on Grignard methathesis7 (GRIM) leading to well-defined polymeric architectures, with a very narrow polydispersity and controlled molecular weights. PT was obtained with a PDI of 1.15, and Mn = 14.2 kDa (by gel permeation chromatography GPC in THF relative to polystyrene). The unsymmetrical triphenylene T was prepared according to adapted procedures described in the literature.8 Received: December 2, 2010 Revised: September 21, 2011 Published: October 18, 2011 4653
dx.doi.org/10.1021/cm2014187 | Chem. Mater. 2011, 23, 4653–4656
Chemistry of Materials
COMMUNICATION
Table 1. Phase Transition Temperatures and Enthalpy of Polymer PTTn compd
Tga (°C)
transitionb
Tc (°C)
ΔH (J g1)
PTTn
14.5
Lcol f I
107
7.5
If Lcol
86
7.2
Tg = glass transition determined during heating (rate 20 °C/min). b Lcol = lamello columnar mesophase, I = Isotropic phase. c Indicated temperatures are the onsets of the peaks on heating and on cooling (rate 2 °C/min). a
The thermal behavior of discotic side-chain polythiophene PTTn was examined by Polarized-light Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and temperature-dependent small-angle X-ray diffraction. The transition temperatures, phase sequence, and transition enthalpies are listed in Table 1. When observed under POM, the polymer PTTn did not develop characteristic optical textures, and consequently it is not possible to identify the mesophase. Nevertheless, on heating, a birefringent and homogeneous texture was observed from room temperature to around 100 °C, when the transition to the isotropic liquid occurred. Birefringence and high viscosity reappeared on cooling down in the mesophase. DSC experiments confirmed POM observations (Figure 1). DSC trace recorded on heating shows an endothermic peak at 107 °C associated to a quite large enthalpy change (ΔH = 7.5 J/ g); this transition is reversible (on cooling) but supercooled by a substantial 20 °C shift. The mesophase organization is kept below room temperature, the glass transition lying around 15 °C. Association of transition hysteresis, sudden variation of the viscosity and large transitional enthalpy at the isotropization are often encountered at transitions with mesophases with high degree of ordering, e.g., three-dimensional ordering or crystalline phases, due to nucleation. X-ray patterns of polymer PTTn registered as a function of temperature (Figure 2 and the Supporting Information, Scheme S7) contain several sharp and intense Bragg reflections in the small-angle region and two scattering halos in the wide-angle region confirming the liquid nature of the mesophase (see the Supporting Information, Table S2). Attempted indexation of the small-angle reflections in various planar (2D) and cubic (3D) lattices simply failed. However, a suitable solution was found by considering a combination of two interlocked systems, as demonstrated as follows: on the one hand, the patterns contain a first set of reflections L (number 1, 2 and 6) in a spacing ratio 1:2:4 characteristic of a well-developed layered structure, with a periodicity d of 50.8 Å, and on the other hand, a second set of reflections C (number 3, 4, 5 and 7, 8, 9, 10) indexed in a 2D oblique lattice9 with the parameters a = 31.5 Å, b = 21.1 Å, γ = 97°, S = 658 Å2 (Scheme S7, and Table S2). At this stage a thorough verification of the formation of a single mesophase instead of phase coexistence was performed. Although DSC thermograms contain a unique peak, corresponding to the isotropization transition, X-ray patterns (recorded at various temperature, various speeds, and on both heating and cooling) confirmed that the two sets of reflections (lamellar and columnar) vanish or appear just simultaneously with no significant evolution of the reflections intensity ratio within the mesophase temperature range, assessing the presence of a single mesophase. The sharpness of the small-angle reflections indicates the long-range correlation of the lamellar ordering and of
Figure 1. DSC traces for discotic side-chain polymer PTTn recorded at rates of 2 and 20 °C/min.
Figure 2. X-ray powder diffraction patterns of discotic side-chain polymer PTTn in the lamello-columnar Lcol phase.
the 2D arrangement of the columns. Regarding the wide-angle region, the diffuse and broad-scattering halo D centered at around 4.4 Å corresponds to the lateral short-range order of the molten chains characteristic of the liquid crystal state. The last broad reflections h0 observed at about 3.5 Å corresponds to the distance between the piled disk-like aromatic cores,10 indicative of the development of an intermolecular stacking into the columns with a possible contribution of the π-conjugated polymer chains. From the full width at half-maximum (fwhm) of this reflection, a correlation length of about 70 Å was estimated (Scherrer formula11), corresponding to the stacking of about 20 triphenylene units along the columns. Thus, this mesophase can be appropriately described as a 3D lamellar phase with a columnar order, i.e., a lamello-columnar phase12 and labeled Lcol phase. However, for classical lamello-columnar mesophases with the columns lying in the plane of the layers, the lamellar stacking direction coincides with one axis of the columnar lattice.13 These structures might be seen as strongly distorted columnar phases: with respect to the transverse columnar periodicities, the spacing coinciding with the piling direction is different and associated to reflections of abnormal intensity. But here lamellar and columnar first-order signals are distinct. The first-order reflections consist in a triplet of close peaks of similar intensity indicative of a slightly distorted hexagonal 4654
dx.doi.org/10.1021/cm2014187 |Chem. Mater. 2011, 23, 4653–4656
Chemistry of Materials
Figure 3. Possible perforated lamello-columnar model representing the mesophase Lcol of polymer PTTn: (a) top view along of the column axis, (b) three-dimensional view.
lattice. Nevertheless the double microsegregation of the discs and of the stiff conjugated polymer chains with the aliphatic tails and spacers may also develop in different directions. This case was previously reported for molecular compounds based on calamitic mesogens connected to ionic parts, the double segregation having given rise to various 3D organizations, which could be well-characterized by oriented patterns.14 However, the oriented patterns presented for these 3D phases would not fit the columnar triplet found here in the powder patterns. Moreover, the discrimination of the possible organizations can hardly be deduced from powder patterns. Indeed, the determination of the respective orientations of both lattices is here even impossible, because the patterns do not contain (hkl) reflections with l 6¼ 0 and, simultaneously, h 6¼ 0, and/or k 6¼ 0. X-ray results therefore just give insight to the piling within the columns, independently from the 3D structure containing them. Thus, the discs should be tilted with respect to the columnar axis due to the discrepancy between the distance separating stacked triphenylenes h0 ≈ 3.5 Å directly measured on the X-ray pattern and the intracolumnar periodicity h ≈ 4.5 Å obtained from the ratio of the molecular volume (i.e., Vm ≈ 1500 Å3 for d ≈ 1 g/cm3) and of the columnar area (i.e., Scol = S/2 = 329 Å2).10 The resulting tilt angle of about 39° characterizes the stacking within the columns independently of the lamellar lattice, itself characterized by the molecular area Slam = Vm/d ≈ 29 Å2. This tilting of the discs within the columns might be the trivial cause for the oblique symmetry, but the deviation from the hexagonal lattice (a/b = 31/2, γ = 90°) is small. Periodic variations of the tilt direction as in the smectic H/K phases15 are unlikely, because the patterns do not contain reflections with an odd h + k sum indicative of a noncentered lattice. Randomization from reduced correlation distances stay closer to the patterns and may also be an explanation for the absence of tridimensional correlations between both lattices. The further description of the molecular organization needs to conjecture the respective orientation of the layers with respect to the column. Actually the simplest case consists in columns formed by the piled triphenylene discs perforating orthogonally the layers formed by the alternation of segregating polythiophene and aliphatic tails and spacers (see Figure 3). In this model, both lattices are completely independent and the geometry is mainly constrained by the size and tilting of the discs and by the length of the spacers. With these dimensional constraints, the model could fit the experimental layer spacing
COMMUNICATION
of about 50 Å (but this does of course not prove the model). Going on this ideal case of columns perpendicular to the lamellae, the number of cores contained in a segment is deduced from the molecular areas according N = Scol/Slam = d/h ≈ 11. The confirmation of this basic model or of an alternative one would need some measurements on oriented samples informing upon the respective orientations of columns and layers. Unfortunately, all available alignment techniques for smectics and columnar mesophases failed and the phase structure could therefore not be completely elucidated. To summarize, we presented our preliminary results on the first example of an original lamello-columnar mesophase originating from a new π-conjugated side-chain polymeric architecture consisting of a regioregular polythiophene based backbone and discotic side groups. The structure was explained by the combination of the two simultaneous following phase separation processes between (i) the discotic mesogens on the one hand and the aliphatic and polymer chains on the other hand leading to a self-assembly into columns organized in a 2D oblique columnar lattice, and (ii) the alkyl and polythiophene chains leading to the lamellar structure. The respective orientations of both lattices could not be determined from powder patterns but different models of supramolecular organizations have previously been discussed. For a better knowledge on this new lamello-columnar mesophase, systematic variations will be introduced in the molecular geometry and charge transport properties will be investigated.
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthetic schemes, experimental procedures, and chemical characterizations of PT, T, and PTTn; temperature-dependent X-ray diffraction experiments for PTTn. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (F.M.); andre-jean.attias@ upmc.fr (A.-J.A.).
’ ACKNOWLEDGMENT This work has been supported by the CEA-Leti through the CARNOT project CC3M and by the French National Agency (ANR) in the frame of its program in Nanosciences and Nanotechnologies (TRAMBIPOLY project ANR-08-NANO051). The authors thank Dr. A. Brouzes for the model illustration. ’ REFERENCES (1) (a) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E Adv. Mater. 2006, 18, 1251. (b) Yamamoto, Y.; Fukushima, T.; Jin, W.; Kosaka, A.; Hara, T.; Nakamura, T.; Saeki, A.; Seki, S.; Tagawa, S.; Aida, T. Adv. Mater. 2006, 18, 1297. (c) Samorì, P.; Fechtenk€otter, A.; Reuther, E.; Watson, M. D.; Severin, N.; M€ullen, K.; Rabe, J. P. Adv. Mater. 2006, 18, 1317. (2) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 40, 685. (3) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902. 4655
dx.doi.org/10.1021/cm2014187 |Chem. Mater. 2011, 23, 4653–4656
Chemistry of Materials
COMMUNICATION
(4) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202. (5) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W. Nat. Mater. 2006, 5, 328. (6) Van de Craats, A. M.; Warman, J. M.; Fechtenk€otter, A.; Brand, J. D.; Harbison, M. A.; M€ullen, K. Adv. Mater. 1999, 11, 1469. (7) (a) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater. 1999, 11, 250. (b) Zhai, L.; Pilston, R. L.; Zaiger, K. L.; Stokes, K. K.; McCullough, R. D. Macromolecules 2003, 36, 61. (8) (a) Boden, N.; Bushby, R. J.; Cammidge, A. N.; El-Mansoury, A.; Martin, P. S.; Lu, Z. J. Mater. Chem. 1999, 9, 1391. (b) Kumar, S. Liq. Cryst. 2004, 31, 1037. (c) McKenna, M. D.; Barbera, J.; Marcos, M.; Serrano, J.-L. J. Am. Chem. Soc. 2005, 127, 619. (d) Kumar, S. Liq. Cryst. 2005, 32, 1089. (d) Zelcer, A.; Donnio, B.; Bourgogne, C.; Cukiernik, F. D.; Guillon, D. Chem. Mater. 2007, 19, 1992. (9) (a) Levelut, A. M. J. Chim. Phys. 1983, 80, 149. (b) Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.; Wilson, C.; Blake, A. J.; Schroeder, M.; Donnio, B. Chem.—Eur. J. 2003, 9, 2484. (10) Weber, P.; Guillon, D.; Skoulios, A. Liq. Cryst. 1991, 9, 369. (11) http://reinhold.kainhofer.com/Physics/Scherrer (12) Tschierske, C. Annu. Rep. Prog. Chem., Sect. C 2001, 97, 191. (13) Cardinaels, T.; Driesen, K.; Parac-Vogt, T. N.; Heinrich, B.; Bourgogne, C.; Guillon, D.; Donnio, B.; Binnemans, K. Chem. Mater. 2005, 17, 6589. (14) (a) Chen, B.; Zeng, X. B.; Baumeister, U.; Diele, S.; Ungar, G.; Tschierske, C. Angew. Chem., Int. Ed. 2004, 43, 4621. (b) Chen, B; Baumeister, U.; Pelzl, G.; Kumar Das, M.; Zeng, X.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2005, 127, 16578. (15) Gray, G. W.; Goodby, J. W. Smectic Liquid Crystals; Leonhard Hill: Glasgow, U.K., 1984.
4656
dx.doi.org/10.1021/cm2014187 |Chem. Mater. 2011, 23, 4653–4656
