Thermotropic Behavior, Packing, and Thin Film Structure of an

Article pubs.acs.org/Macromolecules

Thermotropic Behavior, Packing, and Thin Film Structure of an Electron Accepting Side-Chain Polymer Peter Kohn,† Lilit Ghazaryan,† Gaurav Gupta,† Michael Sommer,‡,§ Andŕe Wicklein,‡ Mukundan Thelakkat,*,‡ and Thomas Thurn-Albrecht*,† †

Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, 06099 Halle, Germany Angewandte Funktionspolymere, Makromolekulare Chemie I, Universität Bayreuth, 95440 Bayreuth, Germany

‡

S Supporting Information *

ABSTRACT: We report on the phase behavior and the structure of poly(perylene bisimide acrylate), an electron accepting semiconductor polymer with disclike side-chain units, in comparison to the corresponding low molecular weight perylene bisimide. By combination of DSC, optical microscopy, and temperature-dependent small-angle and wideangle X-ray scattering, we show that both compounds display a lamello-columnar packing. While the perylene bisimide model compound crystallizes, the polymeric architecture of poly(perylene bisimide acrylate) suppresses order, leading to a 2D lamello-columnar liquid crystalline phase. The structure of the side-chain polymer in thin films with different thermal treatments as observed by GIWAXS correlates well with previously observed largely different electron mobilities. Such a polymeric, liquid crystalline compound combines the advantages of molecular order and easy processability, together with the film forming properties of polymeric materials.

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polymeric backbone via flexible alkyl spacers20,21 (cf. Figure 1), are among the most promosing electron accepting polymers.

INTRODUCTION Perylene bisimide (PBI) derivatives with their high electron mobilities1 and strong absorption in the visible region2 are a promising class of electron accepting organic semiconductor materials3−5 with possible applications in electronic devices, as e.g. field effect transistors or solar cells.6−9 Generally, transport parameters in organic semiconductors are not a purely molecular property but depend strongly on packing. Structure is therefore important on a local scale as well as on a more global scale.10 While local order determines the hopping process between molecules, defects like grain boundaries between crystals limit charge transport over larger distances. For this reason, liquid crystalline materials are regarded as especially interesting as they combine local order and the possibility to induce macroscopic alignment by simple means. Depending on the architecture as well as the kind and length of the solubilizing groups which are attached to the aromatic core, low molecular weight PBIs show different crystalline and liquidcrystalline phases of different symmetry.5,11−14 In most cases columnar structures with a 1D stacking of the perylene bisimide (PBI) cores were reported; in some cases an additional lamellar arrangement of the PBIs in the plane perpendicular to the πstacking direction was found, leading to so-called lamellocolumnar phase.5,15,16 While in most cases low molecular weight PBIs have been used for electronic applications, polymers are interesting for their film forming properties and mechanical stability as compared to small molecule discotics. Thus, the search for electron accepting polymeric materials with high performance is an intensive field of research,17 and polymers containing PBI, either in the main chain18,19 or as side groups attached to a © 2012 American Chemical Society

Figure 1. Sketch of a discotic side-chain polymer: polymer backbone (solid black line), spacer (dotted black line), and swallow-tail substituents (dashed line).

Furthermore, side-chain polymers containing perylene bisimide were successfully incorporated as one block in donor−acceptor diblock copolymers with possible applications, e.g., in bulk heterojunction organic solar cells.22−26 In such a system the required donor−acceptor nanostructure could potentially be Received: May 19, 2012 Revised: June 22, 2012 Published: July 10, 2012 5676

dx.doi.org/10.1021/ma3010197 | Macromolecules 2012, 45, 5676−5683

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Differential Scanning Calorimetry. A DSC 7 from Perkin-Elmer was used to investigate the thermal properties of the samples. The first heating run after preparation was not considered. Background signals were subtracted, resulting in a measurement of the apparent heat capacity cp(T). Polarized Light Optical Microscopy (POM). An Olympus BX51 optical microscope was used to investigate the optical textures of the samples. The microscope was equipped with a Linkam hotstage. During temperature-dependent measurements the sample environment was flooded with nitrogen. X-ray Scattering. Transmission X-ray scattering experiments were performed with a laboratory setup consisting of a Rigaku rotating copper anode, a focusing X-ray optics device (Osmic confocal max flux), and a Bruker 2D-detector (HighStar). The optics also served as monochromator for Cu Kα radiation (λ = 0.154 nm−1). Aluminum discs with a central hole of 0.8 mm diameter were used as sample holders. An evacuated flight tube contains two sample stages with different sample-to-detector distances in order to access a broader qrange. (i) Temperature-dependent small-angle scattering experiments were performed with the sample mounted on a Linkam hotstage. Heat conducting paste was used to ensure good thermal contact. The largest scattering vector q accessible in these experiments was about q ≈ 6.8 nm−1. (ii) On the second stage at a smaller distance to the detector, qvectors up to q ≈ 20 nm−1 were accessible. In this position only patterns at room temperature could be recorded. For both chambers the q-scale was calibrated using standard samples. Details of the calibration procedure can be found in the Supporting Information. Grazing incidence diffraction experiments were performed at beamline ID10B at the European Synchrotron Radiation Facility (ESRF). A schematic of the setup is shown in Figure 3. An X-ray beam

realized as a self-assembling equilibrium morphology. The question of how the incorporation of discotic materials into a side-chain polymer influences packing was barely addressed. In contrast to polymers which have calamitic, rodlike molecules attached as a side chain, the number of detailed studies on the phase behavior and the variety of molecules used in the case of discotic side-chain polymers are rather limited.27−29 Most of the examples found in the literature deal with triphenylenebased side-chain polymers.28,30−32 Here, we present a detailed study of the structure of a poly(perylene bisimide acrylate) (PPerAcr) in comparison to the corresponding low molecular weight PBI analogue. PPerAcr shows a good n-type mobility of 1.2 × 10−3 cm2/(V s) in the ordered state.20 We used DSC, polarized light optical microscopy, and X-ray scattering to characterize the thermotropic behavior and to determine the microscopic structures. We show that for both samples the PBI units exhibit a lamello-columnar phase. While the low molecular weight PBI was crystalline, the polymer orders in a liquid-crystalline phase with a monoclinic unit cell and aligns easily in magnetic fields and thin films. The fact that order and orientation in thin films are suppressed directly after spincoating correlates well with previously observed large differences in electron mobility after different sample treatments.

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EXPERIMENTAL SECTION

Samples. The chemical structures of the investigated samples are shown in Figure 2. The side-chain polymer poly(perylene bisimide

Figure 3. Schematic of GIWAXS setup at beamline ID 10B at the ESRF. with a wavelength 0.155 nm and a small bandwidth ΔE/E = 6.1 × 10−5 (diamond (111) monochromator) was used. The beam was collimated using a combination of a horizontal and vertical slit, resulting in a beam size of 2 mm times 0.15 mm. An incidence angle αi = 0.18° (αc,PPerAcr < αi < αc,Si) was chosen. 2D scattering patterns were obtained by scanning a 1D vertical detector (Vantec) located at a distance of 30 cm from the sample. Reflectivity measurements were performed using an EMPYREAN diffractometer from PANalytical equipped with a graded multilayer Xray mirror (λ = 0.154 nm) and a parallel plate collimator coupled with a linear position-sensitive detector. Thin films of PPerAcr (concentration 10 mg/mL) were spin-coated (1000 rpm) from chloroform on Si wafers which were cleaned beforehand with sulfuric acid. The films were dried in vacuum for several hours and were then heated above the melting point on a Linkam hot stage (in nitrogen atmosphere) and cooled to room temperature. Magnetic Field Alignment. The aluminum discs with the sample described above were placed in the heating cell described in ref 34 and placed in a NMR magnet with B = 9.4 T. After heating to the isotropic phase the samples were slowly cooled to room temperature within the magnet.

Figure 2. Chemical structures of the investigated materials: (a) PBI side-chain polymer with polyacrylate backbone; (b) asymmetrically substituted low molecular weight PBI. acrylate) (PPerAcr) (cf. Figure 2a) had a molecular weight of Mn = 23 kg/mol (GPC in THF, polystyrene standard) and a polydispersity of PDI = 1.71. The monomer has a molecular weight of M = 0.825 kg/ mol. The synthesis of PPerAcr is described elsewhere.33 The low molecular weight perylene bisimide (PBI) (cf. Figure 2b) was asymmetrically substituted with a branched alkyl swallow tail at the one imide position and a linear C12 alkyl group at the other imide position and had a molecular weight of M = 0.769 kg/mol. A detailed account of the synthesis is available in the Supporting Information.

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RESULTS AND DISCUSSION The results of DSC measurements are shown in Figure 4. Both materials exhibited a single reversible phase transition with largely different values for ΔH of approximately 6.7 J/g (5.5 kJ/ 5677


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Figure 6. Small-angle X-ray scattering: patterns for (a) PPerAcr and (b) PBI recorded during stepwise first cooling. Temperatures are indicated.

Figure 4. DSC: (a) PPerAcr: first cooling (solid) and second heating (dashed) scan with a rate of 10 K/min. (b) PBI: first cooling (solid) and second heating (dashed) scan with a rate of 10 K/min. Temperatures given in (a) and (b) correspond to peak temperatures. Onset temperatures were 182.2 °C (heating) and 174.4 °C (cooling) for PPerAcr and 179.5 °C (heating) and 167.7 °C (cooling) for PBI.

Figure 5. Polarized light optical microscopy: PPerAcr (a) and PBI (b) at room temperature after cooling from the melt. Scale bar represents 200 μm in both cases.

mol of monomer) for PPerAcr and 26.5 J/g (20.0 kJ/mol) for PBI during heating. As the mass fraction of the aromatic core plus side chains and spacer in the polymer is ∼91%, the trivial effect of a reduction of the fraction of structure forming material in PPerAcr cannot account for the large difference in the values of ΔH. For several low molecular weight PBIs with different aliphatic substituents the ΔH values for a transition from a liquid-crystalline phase to the isotropic state were reported to be of the order of 10 kJ/mol, while for a transition from crystalline to isotropic transition enthalpies considerably larger values5,12,13 are common. The DSC results therefore

Figure 7. PPerAcr: powder X-ray scattering patterns at T = 20 °C in the (a) small-angle range and (b) wide-angle range (lower resolution). Dashed lines (marked 1, 2, 3) correspond to experimentally determined positions of Bragg reflections (1−4), while black bars indicate the positions expected for the suggested oblique lattice (cf. text). The position of the reflection associated with the π-stacking is indicated by the gray line in (b) (marked 4).

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Table 1. Positions of Bragg Reflections qobs [nm−1] Observed at T = 20 °C and Calculated Positions qcalc [nm−1] for the Suggested Lattices PPerAcr

PBI

i

(hkl)

qi,obs

qcal

di = 2π/qi,obs

i

qi,obs

di = 2π/qi,obs

1

2.11

2.11 2.96 3.35 4.22

2.98

2 3

(100) (010) (110) (200)

4

(001)

18.23

1 2 3 4 5 6 7

2.35 4.71 5.67 6.07 6.36 17.56 18.41

2.67 1.33 1.11 1.04 0.99 0.36

3.36 4.21

1.87 1.49 0.34

Figure 9. (a) Suggested structure for the polymer PPerAcr at T = 20 °C. At present, we cannot comment on the exact orientations of the molecules in the (a/b) plane. (b) Schematic summary of the results of a qualitative analysis of the X-ray data for PBI.

suggest that the low molecular weight PBI forms a crystalline phase while the structure in the polymeric PPerAcr is liquidcrystalline. As will be shown below, the results of all other experiments were consistent with that assumption. Let us now compare the optical micrographs of PBI and PPerAcr obtained from polarization microscopy at room temperature after cooling from the isotropic melt, shown in Figure 5. While the texture of the polymeric PPerAcr (cf. Figure 5a) is typical for a liquid-crystalline structure, the low molecular weight PBI in Figure 5b formed large spherulites with a size of a several hundred micrometers. Spherulites often form in crystalline compounds and were previously also observed for discotic systems.13,35,36 To investigate the structure of the two compounds on a microscopic scale, we performed temperaturedependent X-ray scattering experiments. Figure 6a,b shows the X-ray patterns for PPerAcr and PBI as obtained for the larger sample-to-detector distance during a stepwise cooling from the melt at T = 180 °C down to T = −90 °C. In accordance with the DSC results, at T = 160 °C both samples were in an ordered state as indicated by the appearance of Bragg

reflections. No additional phase transitions were observed during further cooling. A detailed analysis of the patterns will be given below, based on more extended measurements taken at room temperature. As mentioned in the Introduction, discotic molecules typically arrange in stacks which pack on a twodimensional lattice. While the separation of the molecules within a stack is of the order of 0.3−0.4 nm, the distances between the stacks are comparable to the lateral dimensions of the molecules, i.e., a few nanometers. Therefore, the (hk0) reflections resulting from the two-dimensional packing of the stacks usually appear at small q-values and are well separated from (00l) and (hkl) reflections. We start with the analysis of the PPerAcr side-chain polymer. The X-ray scattering patterns obtained at room temperature for the two sample-to-detector distances are shown in Figure 7a,b. The positions of the observed reflections in Figure 7a are given in Table 1. The WAXS pattern of PPerAcr in Figure 7b showed no Bragg reflections in the range q ≈ 7−15 nm−1. The absence of higher

Figure 8. Two-dimensional WAXS patterns of samples slowly cooled from the melt in a magnetic field of 9.4 T and azimuthal dependency of (hk0) reflections (white arrows) of (a) PPerAcr and (b) PBI. Here the azimuthal angle Ω is defined as shown in (b). The magnetic field direction is indicated. The four bright, radial lines which were visible in both patterns resulted from the wires that held the beam stop. 5679


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cooling the sample from the isotropic state in a magnetic field of 9.4 T. The 2D-scattering pattern at room temperature is shown in Figure 8a together with the azimuthal intensity distribution of the (hk0) reflections (q ≈ 5.5−7 nm−1), indicated by the white arrow. The reflection corresponding to the π-stacking showed equatorial intensity maxima demonstrating a preferred orientation of the π-stacking direction perpendicular to the magnetic field. The same alignment has been observed before for the case of low molecular weight discotics.39,40 The (hk0) reflections in Figure 8a showed meridional intensity maxima. Such a pattern is expected if the π-stacking direction is aligned perpendicular to the magnetic field but free to rotate in the corresponding plane.34 For PPerAcr the observed reflections (cf. Figure 6a) could be described by a two-dimensional oblique lattice with lattice constants a = 3.01 nm, b = 2.15 nm, and γ = 81°. For the πstacking distance c ≈ 0.34 nm was deduced from the peak at q = 18.23 nm−1 in Figure 7b. Based on these lattice parameters, only a unit cell containing two monomers gives a reasonable density around 1 g/cm3; here ρPPerAcr = 2Mmonomer/(ab sin γ cNA) ≈ 1.26 g/cm3 (NA is the Avogadro number). The lattice constant b = 2.15 nm compares well with twice the width of the PBI core of 0.92 nm.11 The structure of the unit cell which is suggested based on these considerations is schematically depicted in Figure 9a. Similar lamellar arrangements were found for perylene bisimides with linear substituents.5,15,41 In these studies, edge-to-edge packing distances (b-direction) of ≈1.03 nm for a unit cell containing one molecule5,15 and 2.20 nm for a unit cell containing two molecules41 were found. While the (010) reflection was nearly absent (cf. the weak shoulder indicated by an arrow in Figure 7a), the (110) reflection was clearly visible, indicating that a unit cell containing two molecules in b-direction was appropriate, caused e.g. by a slight displacement of every second core along the a-direction (cf. Figure 9a). Let us now discuss the crystal structure of the low molecular weight PBI for comparison. The respective scattering pattern at T = 20 °C is shown in Figure 10a. The experimentally determined positions of the Bragg reflections (dashed lines) are also given in Table 1. The corresponding WAXS pattern is shown in Figure 10b. In contrast to the polymeric PPerAcr, the large number of reflections visible in the range q ≈ 7−15 nm−1 is consistent with a well-ordered crystalline structure with only small fluctuations of the molecules around their lattice positions. The peak observed at q6 = 17.56 nm−1 (gray line) was assigned to the packing of the PBI molecules within the stacks, i.e., a (00l) reflection. A further peak was observable at q7 = 18.41 nm−1 (indicated by an arrow in Figure 10b). Such reflections are indicative for a 3D lattice as they correspond to mixed (hkl) reflections with h and/or k and additionally l nonzero. They generally occur in crystalline or in plastic phases.37,42,43 PBI was also cooled from the isotropic state in the magnetic field. The 2D pattern is shown in in Figure 8b. There is no indication for alignment, as exemplarily shown by the constant azimuthal intensity distribution of reflections indicated by a white arrow in Figure 8b. This is in line with the observed spherulitic crystallization as spherulites are overall isotropic objects unaffected by external fields.34 The detailed analysis of the exact crystal symmetry was made difficult by the isotropic crystal orientation in Figure 8b. Nevertheless, the following conclusions could be drawn, independent of the exact unit cell symmetry. The fact that q2 ≈ 2q1 in Figure 10 (cf. also Table 1), with no reflection visible in between q1 and q2, clearly

Figure 10. PBI: powder X-ray scattering patterns at T = 20 °C in the (a) small-angle range and (b) wide-angle range. Dashed lines correspond to experimentally determined positions of Bragg reflections, while black bars indicate the positions expected for the suggested oblique lattice (cf. text). The position of the reflection associated with the π-stacking is indicated by the gray line in (b). Note that the pattern in (b) was measured with a lower resolution than (a) as visible from the fact that the three peaks in the range q ≈ 5.5−6.5 nm−1 in (a) appear as one peak in (b).

order (hk0) reflections in this range is in line with the assumption of a liquid-crystalline structure, as a strong reduction of the intensity of the reflections at higher q, caused by lateral fluctuations of the stacked molecules, would be expected in this case. The broad halo at around q ≈ 10−17 nm−1 is usually assigned to the scattering from the liquid alkyl side chains.37,38 At q = 18.23 nm−1 a broad peak with a width of Δq ≈ 1.40 nm−1 was visible. This peak reflects the π-stacking. The small value of the correlation length of lcor = 2π/Δq ≈ 4.5 nm as determined from the peak width is an indication for liquidlike packing of the molecules within the stacks.37 No reflection was found at q-values larger than q = 18.23 nm−1; i.e. no (hkl) reflections with h or k and additionally l nonzero were observed, consistent with a liquid-crystalline structure without 3D crystalline packing. Extracting lattice parameters for the two-dimensional lattice of the stacks from the positions of the (hk0) Bragg reflections alone is only correct, if the stacking direction of the cores is perpendicular to the plane of the 2D lattice. This could be demonstrated by scattering patterns from oriented samples. Alignment of PPerAcr was achieved by slowly 5680


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Figure 11. GIWAXS scatttering pattern and corresponding reciprocal space map of a thin film of PPerAcr. Indices of relevant reflections are indicated.

(001)6, (011)7. The resulting crystallographic density of ρPBI ≈ 1.21 g/cm3 is smaller than the value obtained for the liquidcrystalline PPerAcr, ρPPerAcr ≈ 1.26 g/cm3. This at first sight counterintuitive result could either be caused by a densification due to polymerization or indeed indicate a triclinic unit cell. In any case we emphasize that the qualitative analysis of the positions of the Bragg reflections gave evidence for a lamellocolumnar arrangement of the PBI molecules, similar as in PPerAcr. In conclusion, the results of all experiments consistently indicated the same main structural difference of the two samples: while the low molecular weight PBI formed a crystalline phase, the polymeric PPerAcr formed only a 2D liquid-crystalline structure with liquidlike packing in stacking direction and liquid alkyl domains. This result is in line with observations by Boden et al.,32 who synthesized a series of sidechain polymers containing triphenylene units and reported that the polymers, in contrast to the monomers, did not form crystalline phases. Similar results were obtained for side-chain polymers with rodlike mesogens.46 Let us briefly discuss how the packing of the two compounds is related to their chemical structure. Generally, the curvature of the interface between core and side chains should be related to the number of side groups emanating from this interface. A reduction of the number of alkyl chains attached to the perylene core should induce a trend from 1D columnar stacking toward lamellar packing. Accordingly, the perylene bisimide derivative investigated in ref 14, with two branched alkyl side chains per molecule, forms a columnar phase. On the other hand, lamellar arrangements were found not only for perylene bisimides with linear substituents5,15,41 but also for the asymmetrically substituted PBI investigated here. Additionally, in the polymeric compound the acrylate main chain is atactic and can as such not participate in a periodic crystalline packing.47 Obviously, the effect of this quenched disorder is large enough to suppress crystallization

Figure 12. Reflectivity of the same film of PPerAcr as in Figure 11. The (100) peak around qz = 2.17 nm−1 corresponds to lattice planes oriented parallel to the substrate. Different from the measurement above refraction effects are not taken into account to calculate qz.

indicated a lamellar structure as for the PPerAcr, but with a lattice parameter b reduced by a factor of 2. Another point concerns the larger lattice parameter in c-direction of dπ−π ≈ 0.36 nm for the crystalline PBI in comparison to the liquidcrystalline PPerAcr (dπ−π ≈ 0.34 nm) (cf. Table 1). This result could indicate a tilt of the flat PBI core with respect to the (a/ b) plane. The results of this qualitative analysis are schematically depicted in Figure 9b. For perylene bisimides substituted symmetrically with short CnH2n+1 chains (n = 1, ..., 5) both monoclinic and triclinic crystal structures have been reported.44,45 Though we cannot completely exclude a triclinic, for further analysis we assume a monoclinic unit cell (a = 2.68 nm, b = 1.11 nm, γ = 86°, and c = 0.36 nm). In this case the reflections in Figure 10a and the reflection marked by the arrow in Figure 10b could the be indexed consistently by (hkl)i (i is the peak number in 10): (100)1, (200)2, (010)3, (110)4, (1̅10)5, 5681


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films is related to a lack of molecular order, while stacking of perylene units in edge-on orientation is likely to be responsible for the high mobility observed in OFET experiments using annealed samples.

and to reduce the order on the molecular scale to liquid crystalline. Having resolved the bulk structure of PPerAcr, we can now address the structure of the material in thin films as investigated by GIWAXS. Figure 11 shows the pattern recorded from a film cooled from the melt and the corresponding reciprocal space map where the coordinates were calculated according to the following equations taking into account refraction effects. qx =

2π [cos γ cos χ cos δ + sin γ sin χ cos δ − cos χ ] λ

qy =

2π [cos γ cos χ sin δ + sin γ sin χ sin δ] λ

qz =

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Additional information about the synthesis of PBI and the experimental setup and calibration of the SAXS experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected] (M.T.); [email protected] (T.T.-A.).

+ [sin 2 χ − sin 2 αc]1/2 } qp =

qx + qy

AUTHOR INFORMATION

Corresponding Author

2π {[(sin γ cos χ − cos γ sin χ )2 − sin 2 αc]1/2 λ

2

ASSOCIATED CONTENT

S Supporting Information *

Present Address §

Institut für Makromolekulare Chemie, Albert-Ludwigs-Universität, 79100 Freiburg, Germany.

2

Notes

The authors declare no competing financial interest.

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The strongest peak is the (100) close to the meridian, suggesting a preferred alignment of the corresponding lattice planes parallel to the substrate. As the GIWAXS geometry does not allow observation of the intensity directly on the meridian, a reflectivity curve measured on the same film is shown in Figure 12. A detailed analysis of the fringes at small q-values gives a thickness of 67 nm and a roughness of 2 nm. As in this scattering geometry only lattice planes parallel to the substrate contribute; the single (100) peak confirms the GIWAXS results. The peak of the (110) peak in Figure 11 is consistent with the lattice suggested above. The location of the (001) peak indicates π-stacking in lateral direction, i.e. edge-on, consistent with the orientation of the (100) planes and favorable for lateral transport. Note that the (001) is not a peak but rather a long vertical streak, also considerably broader than the other peaks. This is to be expected for a lamello-columnar liquid crystalline structure in which the stacking of PBI cores in neighboring layers is uncorrelated and liquidlike. In stark contrast, the pattern of the as spun film (not shown) is featureless corresponding to a quenched, completely amorphous structure. The overall result is very much in keeping with the mobilities measured in FET structures, which amounted to 9.6 × 10−6 cm2/(V s) for the as-spun film and much higher value, namely 1.2 × 10−3 cm2/(V s) for an annealed film,20 demonstrating how stacking order favors transport in comparison to a disordered arrangement of the PBI cores.

ACKNOWLEDGMENTS This work was supported by the DFG (SPP 1355) and the state Sachsen-Anhalt. We acknowledge the ESRF for provision of synchrotron radiation facilities, and we thank R. Nervo for assistance in using beamline ID10B. We thank W. Chassé for help in using the NMR-magnet and C. Müller, R. Brendel, M. Muth, and A.-K. Löhmann for their help during synchrotron experiments.

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REFERENCES

(1) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Ewbank, P. C.; da Silva Filho, D. A.; Bredas, J. L.; Miller, L. L.; Mann, K. R.; Frisbie, C. D. J. Phys. Chem. B 2004, 108, 19281−19292. (2) Li, J.; Dierschke, F.; Wu, J.; Grimsdale, A. C.; Müllen, K. J. Mater. Chem. 2006, 16, 96−100. (3) Würthner, F. Chem. Commun. 2004, 14, 1564−1579. (4) Horowitz, G.; Kouki, F.; Spearman, P.; Fichou, D.; Nogues, C.; Pan, X.; Garnier, F. Adv. Mater. 1996, 8, 242−245. (5) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, B.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudhölter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057−11066. (6) Yakimov, A.; Forrest, S. R. Appl. Phys. Lett. 2002, 80, 1667−1669. (7) Ling, M.-M.; Erk, P.; Gomez, M.; Koenemann, M.; Locklin, J.; Bao, Z. Adv. Mater. 2007, 19, 1123−1127. (8) Tatemichi, S.; Ichikawa, M.; Koyama, T.; Taniguchi, Y. Appl. Phys. Lett. 2006, 89, 112108−1 − 112108−3. (9) Yoo, B. W.; Jones, B. A.; Basu, D.; Fine, D.; Jung, T.; Mohapadra, S.; Facchetti, A.; Dimmler, K.; Wasielewski, M. R.; Marks, T. J.; Dodabalapur, A. Adv. Mater. 2007, 19, 4028−4032. (10) Pisula, W.; Zorn, M.; Chang, J. Y.; Müllen, K.; Zentel, R. Macromol. Rapid Commun. 2009, 30, 1179−1202. (11) Liu, S. G.; Sui, G.; Cormier, R. A.; Leblanc, R. M.; Gregg, B. A. J. Phys. Chem. B 2002, 106, 1307−1315. (12) Nolde, F.; Pisula, W.; Muller, S.; Kohl, C.; Mullen, K. Chem. Mater. 2006, 18, 3715−3725. (13) Wicklein, A.; Lang, A.; Muth, M.; Thelakkat, M. J. Am. Chem. Soc. 2009, 131, 14442−14453. (14) Marcon, V.; Breiby, D. W.; Pisula, W.; Dahl, J.; Kirkpatrick, J.; Patwardhan, S.; Grozema, F.; Andrienko, D. J. Am. Chem. Soc. 2009, 131, 11426−11432. (15) Zucchi, G.; Donnio, D.; Geerts, Y. H. Chem. Mater. 2005, 17, 4273−4277.

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CONCLUSIONS We investigated the thermotropic behavior of a discotic sidechain polymer PPerAcr containing perylene bisimide units in comparison to its corresponding low molecular weight perylene bisimide by means of DSC, optical microscopy, and X-ray scattering. Attachment of the perylene bisimde units to the polymer chain reduced order resulting in a lamello-columnar liquid crystalline structure, while the low molecular weight perylene bisimide was crystalline. The columnar stacking in the polymer is disordered. The liquid-crystalline nature of PPerAcr allowed alignment of the material by a magnetic field. Additionally, thin film GIWAXS experiments were performed to investigate the structural basis for previously observed largely different transport properties of as-spun and annealed films. The results indicate that the low electron mobility in as spun 5682


Macromolecules

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