Macroscopic Alignment of Graphene Stacks by ... - ACS Publications

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Langmuir 2004, 20, 4139-4146

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Macroscopic Alignment of Graphene Stacks by Langmuir-Blodgett Deposition of Amphiphilic Hexabenzocoronenes Bo W. Laursen, Kasper Nørgaard, Niels Reitzel, Jens B. Simonsen, Christian B. Nielsen, Jens Als-Nielsen, and Thomas Bjørnholm* Nano-Science Center, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

Theis I. Sølling and Martin M. Nielsen Danish Polymer Center, Risø National Laboratory, Frederiksborgvej 399, 4000 Roskilde, Denmark

Oliver Bunk and Kristian Kjaer Materials Research Department, Risø National Laboratory, Frederiksborgvej 399, 4000 Roskilde, Denmark

Natalia Tchebotareva, Mark D. Watson, and Klaus Mu¨llen Max Planck Institute for Polymer Research, Ackermannweg 10, 55129 Mainz, Germany

Jorge Piris Interfaculty Reactor Institute, Department Radiation Chemistry, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received January 7, 2004. In Final Form: February 26, 2004 We present structural studies of Langmuir (L) and Langmuir-Blodgett (LB) films of new amphiphilic hexa-peri-hexabenzocoronene (HBC) discotics, carrying five branched alkyl side chains and one polar group. The polar group is either a carboxylic acid moiety or an electron acceptor moiety (anthraquinone). Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity, both utilizing synchrotron radiation, show that these amphiphilic HBCs form well-defined Langmuir monolayers at the air-water interface, with a π-stacked columnar structure where the HBC cores are rotated around the surface normal and tilted relative to the water surface. The intercolumnar distance is 20 Å. The HBCs are confined to a layer lying on top of the layer of polar groups that are in contact with the water subphase. Efficient transfer of the monolayer of the anthraquinone-substituted HBC derivative to hydrophobic quartz substrates by vertical dipping gave well-defined multilayer Y-type LB films. Polarized optical spectroscopy, GIXD, and X-ray reflectivity measurements show that the LB films consist of at least two phases. Heating the films results in an irreversible rearrangement to a single macroscopically aligned phase of hexagonally packed columns oriented along the dipping direction with disk planes perpendicular to the columnar axes and stacked in a cofacial manner. This phase transition is analogous to the reversible transition observed in the bulk material.

Introduction In the ongoing search for semiconducting organic materials for field effect transistors (FET), light-emitting diodes (LED), and photovoltaic devices, it is well documented that structure and morphology are of major importance for material performance. One strategy for obtaining materials with improved order is to let the conductance path consist of planar π-electron-rich molecules that may self-assemble into columnar aggregates. Hexasubstituted hexa-peri-hexabenzocoronenes (HBC’s) display a strong tendency to form such quasi-onedimensional aggregates and columnar mesophases with unusually high charge-carrier mobilities along the π-stacking direction.1,2 Furthermore, this system is attractive * To whom all correspondence should be addressed: Ph (+45) 3532 1835; Fax (+45) 3532 0460; e-mail [email protected].

due to simple and easy synthetic procedures,3 which allows desymmetrization and functionalization of the peripheral substituents.4 The ability to synthesize nonsymmetric and functionalized compounds allows the air-water interface and LB technique to be used for organizing well-defined thin films of these organic semiconductors.5 Recently, we have shown that an amphiphilic HBC derivative carrying (1) van de Craats, A. M.; Warman, J. M.; Mu¨llen, K.; Geerts, Y. H.; Brand, J. D. Adv. Mater. 1998, 10, 36-38. (2) van de Craats, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. Adv. Mater. 1999, 11, 1469-1472. (3) Fechtenko¨tter, A.; Tchebotareva, N.; Watson, M.; Mu¨llen, K. Tetrahedron 2001, 57, 3769-3783. (4) (a) Grimsdale, A. C.; Bauer, R.; Weil, T.; Tchebotareva, N.; Wu, J. S.; Watson, M.; Mu¨llen, K. Synthesis-Stuttgart 2002, 1229-1238. (b) Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.; Rabe, J. P.; Mu¨llen, K. Chem.sEur. J. 2000, 6, 4327-4342. (5) Bjørnholm, T.; Hassenkam, T.; Reitzel, N. J. Mater. Chem. 1999, 9, 1975-1990.

10.1021/la049944i CCC: $27.50 © 2004 American Chemical Society Published on Web 04/09/2004

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Figure 1. Schematic representations of the packing structures found in the Langmuir films (at the air-water interface) and in the transferred LB films of anthraquinone-substituted HBC (1a). The black dots represent a polar group.

five n-dodecyl side chains and one acid-terminated decyl chain could be organized in monolayers at the air-water interface.6 It was shown that for this amphiphilic HBC the LB technique offered a unique possibility for modulating the packing mode of the aromatic cores, and hence the electronic properties, by control of the applied surface pressure. Yet, the high crystallinity of these monolayers apparently prohibits the efficient formation of multilayer structures by standard LB techniques. This difficulty was recently overcome by replacing the straight side chains with branched chains.7 In this present study we investigate the structural and electronic properties of a new group of amphiphilic HBC’s carrying branched side chains and variable-length tethers terminated by polar groups (carboxylic acids and esters), one of which (anthraquinone-substituted donor-acceptor dyad 1a) might satisfy the requirements for formation of rectifying monolayers.8 For these less crystalline and more flexible materials we have found that efficient formation of multilayer structures is easily achieved. Upon annealing, the obtained structures furthermore display macroscopic alignment along the dipping direction of the columnar aggregates evident in X-ray structure analysis, optical properties, and photoconductance. Figure 1 shows the resulting schematic structure model that we base on the sum of all the experimental data as explained in detail in the following sections as well as on photoinduced microwave conductivity measurements reported in a separate paper.9 The model shows that the HBC disk form columnar stacks in a Langmuir film on a water surface in which HBC is likely to be rotated by 30° around the surface normal and tilted 35° along the stacking direction (Figure 1). LB deposition of this Langmuir film to a hydrophobic Si wafer leads to a single macroscopically aligned phase in which the HBC moieties now stack in a (6) Reitzel, N.; Hassenkam, T.; Balashev, K.; Jensen, T. R.; Howes, P. B.; Kjaer, K.; Fechtenko¨tter, A.; Tchebotareva, N.; Ito, S.; Mu¨llen, K.; Bjørnholm, T. Chem.sEur. J. 2001, 7, 4894-4901. (7) Kubowicz, S.; Pietsch, U.; Watson, M. D.; Tchebotareva, N.; Mu¨llen, K.; Thu¨nemann, A. F. Langmuir 2003, 19, 5036-5041. (8) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950-957. (9) Piris, J.; Laursen, B. W.; Debije, M. G.; Tchebotareva, N.; Fischbach, I.; Watson, M. D.; Bjørnholm, T. Manuscript in preparation.

well-defined cofacial manner, resulting in columns oriented along the dipping direction. Layers of these columns are deposited on top of each other in a Y-type LangmuirBlodgett film. Transfer to hydrophobic quartz most likely results in a mixture of the two above phases in a Y-type LB film. Annealing, however, results in complete rearrangement into the same cofacial hexagonal phase as found on the Si substrate. Such macroscopic uniaxial alignment of discotic materials in thin films is of major importance for the use of these materials in electronic devices as well as for more fundamental studies of structure and properties. Thus, a simple means to obtain controlled uniaxial alignment is a prerequisite for exploiting the nanoscale one-dimensional charge transport of these systems in real devices, like field-effect transistors. Recently, significant progress in this field has been achieved for solution-processed films by use of PTFE alignment layers10,11 and zone-casting techniques12 as well as by zone-melting.13 Experimental Procedures Materials. The synthesis and bulk characterization of compounds 1a-d are reported elsewhere.14 Monolayers and Film Preparation. Monolayers of the amphiphilic HBC’s 1a, 1b, 1c, and 1d were spread from toluene solution (≈0.8 mg/mL) onto a Milli-Q purified water subphase (18.2 MΩ cm) in the LB trough. The subphase was thermostated to 21 °C. Mono- and multilayer films were transferred to hydrophobic solid supports (OTS-treated glass, quartz, and silicon) by the standard Langmuir-Blodgett technique with (10) (a) Bunk, O.; Nielsen, M. M.; Sølling, T. I.; van de Craats, A. M.; Stutzmann, N. J. Am. Chem. Soc. 2003, 125, 2252-2258. (b) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. Adv. Mater. 2003, 15, 495-499. (11) Zimmermann, S.; Wendorff, J. H.; Weder, C. Chem. Mater. 2002, 14, 2218-2223. (12) Tracz, A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; Mu¨llen, K.; Pakula, T. J. Am. Chem. Soc. 2003, 125, 1682-1683. (13) Liu, C. Y.; Bard, A. J. Chem. Mater. 2000, 12, 2353-2362. (14) (a) Samorı`, P.; Yin, X.; Tchebotareva, N.; Wang, Z.; Pakula, T.; Ja¨ckel, F.; Watson, M. D.; Venturini, A.; Mu¨llen, K.; Rabe, J. P. J. Am. Chem. Soc., submitted for publication. (b) Tchebotareva, N.; Fischbach, I.; Watson, M. D.; Schnell, I.; Mu¨llen, K.; Spiess, H. W. Manuscript in preparation.

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Figure 2. Structures of the four amphiphilic HBC’s used in this study. vertical dipping. All transfers were performed with dipping rates of 1 mm/min and constant barrier pressure of 20 mN/m. X-ray Measurements. The X-ray measurements on Langmuir monolayers of 1a and 1c were performed at the X-ray undulator beamline BW1 at the synchrotron facility HASYLAB at DESY in Hamburg, Germany. The X-ray wavelength was λ ) 1.304 Å, monochromated by Bragg reflection from a Be(002) crystal. Two different X-ray techniques were used:6 grazingincidence X-ray diffraction (GIXD) and specular reflectivity (XR). For the GIXD, the grazing angle of incidence (Ri) was slightly below the critical angle (Rc) for total reflection (Ri ) 0.85Rc), thus increasing the surface sensitivity by minimizing the penetration depth of the incident X-rays into the water subphase. The horizontal scattering angle (2θxy) was resolved by scanning a Soller collimator while the vertical exit angle Rf was resolved by a position-sensitive detector. Then the vertical scattering vector component is Qz = (2π/λ) sin Rf while the horizontal component is Qxy ≡ (Qx2 + Qy2)1/2 = (2π/λ)[1 + cos2(Rf) - 2 cos(Rf) cos(2θxy)]1/2. The XR experiments probe the vertical electron density profile across the interface by varying the incident angle (Ri) and the exit angle Rf simultaneously (Rf ) Ri ≡ R), recording the intensity pattern resulting from interference between rays reflected at different depths. The experimental data are shown as a function of the purely vertical scattering vector Qz ) (2π/λ)[sin(Ri) + sin(Rf)] ) (4π/λ) sin(R), as the measured reflectivity (R(Qz)) normalized to the Fresnel reflectivity (RF(Qz)) from a theoretically sharp water surface. The data were inverted to yield the electron density profile normal to the water surface, F(z), by expressing F(z) in terms of cubic splines and fitting the corresponding R(Qz)/RF(Qz) to the data with a smoothness constraint on the F(z) curve.15 Transferred Multilayers. X-ray reflectivity experiments were carried out on LB multilayers at the beamline D4 at the HASYLAB. Grazing incidence X-ray diffraction (GIXD) experiments were carried out on a 40-layer LB film of 1a on an OTStreated Si wafer. λ ) ca. 1.386 Å. In a separate set of experiments we employed the z-axis diffractometer at the wiggler beamline BW2 at HASYLAB. A monochromatic beam of 10.00 keV (λ ) 1.240 Å) impinged on the sample surface under a grazing incidence angle of Ri ) 0.16°; this value is below the critical angle for total reflection of Si, RcSi ) 0.178°, but above the critical angle for the LB film, Rcfilm ) 0.123°, and therefore enhances the sensitivity to the organic material. The sample was kept in a helium atmosphere during the experiments to prevent radiation damage and to minimize air scattering. The experimental setup offers the possibility of rotation around the sample normal. The sample was positioned in such a way that the dipping direction and the y-axis coincide. The results showed (cf. below) that this direction is the most abundant direction for the columnar π-stacking (cf. Figure 8). Optical Measurements. UV-vis spectra of thin films on glass and quartz substrates were recorded with a Perkin-Elmer Lambda 9 with a polarizer unit. Optical microscopy was (15) Pedersen, J. S.; Hamley, I. W. J. Appl. Crystallogr. 1994, 27, 36-49.

performed on a Zeiss Axiotech microscope. Transmitted/reflected light through/from the LB films when examined with crossed polarizers was quantified by attaching a diode array spectrometer (Avaspec-2048) to the microscope via a collimating lens and an optical fiber. A Linkam LTS-350 heating/cooling stage equipped with quartz windows and connected to a Lambda 17 spectrometer through optical fibers allowed thermal annealing and variable temperature spectral measurements. Atomic Force Microscopy (AFM). AFM investigations of multilayer films of 1a on silicon substrates were done in contact mode using a Nanoscope III from Digital Instrument.

Results Of the four monofunctionalized amphiphilic HBCs (1a1d, Figure 2) studied here, anthraquinone-substituted HBC 1a received the most detailed attention, while the three others (1b, 1c, and 1d) serve as reference compounds. All four amphiphilic HBCs carry five C8 chains with methyl branches in the 3- and 7-positions, while the sixth side group consists of a linker of 3-10 methylene units terminated by the hydrophilic subunit (acetoxy 1b, carboxylic acid 1c, 1d, or the electron acceptor anthraquinone 1a). The branched side chains were introduced in order to obtain softer, more flexible materials with increased solubility in organic solvents and lower phase transition temperatures of the mesomorphic materials. Monolayers at the Air-Water Interface. The four amphiphilic HBCs 1a-1d all form stable monolayers at the air-water interface (Figure 3). The four compounds display similar compression isotherms, with mean molecular areas close to 95 Å2 for the compressed monolayers. At surface pressures above 40 mN/m the monolayers collapse and beginning formation of bilayers is observed at approximately 45 Å2. The molecular areas are similar to an analogous HBC with nonbranched C12 side chains, yet the collapse pressure is much lower (ca. 65 mN/m for the amphiphilic HBC with nonbranched side chains).6 The structure of monolayers formed from anthraquinone-substituted 1a and the acid-substituted 1c was studied at the air-water interface by grazing-incidence X-ray diffraction (GIXD)16-18 using synchrotron radiation. No (16) Kjaer, K. Physica B 1994, 198, 100-109. Jensen, T. R.; Kjaer, K. Structural properties and interactions of thin films at the air-liquid interface explored by synchrotron X-ray scattering. In Novel Methods to Study Interfacial Layers; Studies in Interface Science Vol. 11; Mo¨bius, D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; pp 205-254. (17) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 252-313.

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Figure 3. Compression isotherms obtained with a barrier speed of 5 mm/min.

Figure 4. GIXD data (integrated over 0 < Qz < 1.0 Å-1) for monolayers of 1a and 1c at the air-water interface, at surface pressures of ca. 1 mN/m, corresponding to mean molecular areas of 107 and 100 Å2, respectively.

significant structural rearrangements with increasing pressure were observed from 1 to 20 mN/m, indicating that the molecules self-assemble into a very cohesive phase already when spread at the water surface. Figure 4 shows diffractograms of the loosely compressed monolayers of the two compounds. The diffraction data for the two compounds are virtually identical, showing only three peaks at Qxy values of 0.320, 0.647, and 1.275 Å-1, corresponding to spacings d ) 2π/ Qxy of 19.7, 9.8, and, 4.9 Å. The first two peaks are assigned as first- and second-order reflections from the columnar packing distance of 19.7 Å while the main contribution to the broad peak at 4.9 Å most likely arises from the intracolumnar stacking of the HBC cores. Since the preferred cofacial π-π distance for the aromatic HBC cores is 3.5 Å,19 this leads us to conclude that the columns are aligned parallel to the surface and that the molecular HBC cores within the each column must be rotated around the surface normal and/or tilted relative to the surface plane. Comparisons of the preferred column width of (18) Als-Nielsen, J.; Mo¨hwald, H. Handbook on Synchrotron Radiation; 1991; p 3. (19) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. Adv. Mater. 1996, 8, 510.

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Figure 5. Electron density profiles of monolayers of 1a (solid line) and 1c (dashed line) at the air-water interface, with schematic representation of 1a and the water subphase.

cofacially stacked HBCs (23 Å) to the width of columns floating on water (19.7 Å) strongly indicate that HBC is rotated around the surface normal by cos-1(19.7/23) ) 30° in order to allow a column width of only 19.7 Å, as illustrated in Figure 1. To further reduce the π-stacking distance, we infer an additional tilt along the stacking direction of about 35° (see Figure 1). The combined effect of tilt and rotation give rise to a π-π stacking distance of 3.5 Å, and this is in good agreement with tilt values of 49° found in thin films of HBC peripherally substituted with six 3,7-dimethyloctanyl side chains10 and of 48° in bulk HBC25 which give rise to similar intracolumnar repeat distances. In this 2-D crystal the coherence lengths along the π-stack is estimated from the peak widths (by use of the Scherrer formula) to be 73 Å (see Table 1), which corresponds to segments of 15 coherently packed molecules. For the intercolumnar peak the coherence lengths is found to be 350 Å, corresponding to approximately 17 columns. Compared to the nonbranched system, this gives a picture of a system with less intracolumnar order but well-defined packing of the columnar aggregates.6 For compounds 1a and 1c the vertical electron density profile of the Langmuir monolayer was determined by X-ray reflectivity measurements16-18 performed at the same surface pressure as the diffraction experiment described above. The resulting electron density profiles (normalized to water) are shown in Figure 5. The measured data along with the corresponding fit to the data are given as Supporting Information. From the reflectivity data the height of both monolayers are found to be ≈27 Å. Like the GIXD data, the electron density profiles of the Langmuir films of 1a and 1c are quite similar, except at the water interface where 1a displays increased electron density compared to 1c. This increased electron density is attributed to the anthraquino(20) Biasutti, M. A.; Rommens, J.; Vaes, A.; De Feyter, S.; De Schryver, F.; Herwig, P.; Mu¨llen, K. Bull. Soc. Chim. Belg. 1997, 106, 659-664. (21) (a) Minari, N.; Ikegami, K.; Kuroda, S. I.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222-231. (b) Minari, N.; Ikegami, K.; Kuroda, S. I.; Saito, K.; Saito, M.; Sugi, M. Solid State Commun. 1988, 65, 1259-1262. (22) (a) Wegner, G. Thin Solid Films 1992, 262, 105. (b) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 25132525. (23) Jones, R.; Hunter, R. A.; Davidson, K. Thin Solid Films 1994, 250, 249-257. (24) Samori, P.; Yin, X.; Tchebotareva, N.; Wang, Z.; Pakula, T.; Ja¨ckel, F.; Watson, M. D.; Venturini, A.; Mu¨llen, K. Manuscript in preparation. (25) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kru¨ger, C.; Zander, M. J. Am. Chem. Soc. 1995, 117, 30-41.

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Table 1. Summary of the Characteristic X-ray Peaks/Distances for the Different Thin Films of 1a

Langmuir film, 1af LB film, 1a on Sig LB film, 1a on glassh peaks assigned to tilted structure peaks assigned to cofacial structure LB film, 1a on glassi

packing distance along HBC columna (Å)

coherence lengthb (Å)

intercolumnar distance in plane of filmc (Å)

coherence lengthb (Å)

interlayer distanced (Å)

calcd densitye (g/cm3)

4.9 (Qxy) 3.5 (Qy)

73 37

19.7 (Qxy) 23.3 (Qx)

347 80

≈27j 22.7 (Qz)

≈1.0 1.38

4.8 (Qy) 3.6 (Qy)

21 26

19.9 (Qx) 23.3 (Qx)

134 94

22.3 (Qz) 22.3 (Qz) 22.9 (Qz)

1.20 1.37

a Peaks assigned to the HBC repeat distance along the columnar stack; the scan direction is given in parentheses (refer to Figure 8). The coherence lengths, in Å, calculated by the Scherrer formula (0.9 × 2π/fwhm). c Peaks assigned to the intercolumnar spacing; the scan direction is given in parentheses (refer to Figure 8). d Peaks assigned to the intercolumnar spacing perpendicular to the film (interlayer distance); the scan direction is given in parentheses (refer to Figure 8). e Density of the packing; calculated from the repeat distances and the molar weight. f GIXD data for Langmuir film of 1a at the air-water interface (see Figure 4). g GIXD data for LB film of 1a on Si (see Figure 8). h GIXD data for unannealed LB film of 1a on glass (see Figure 9 and Supporting Information). i X-ray reflectivity data for unannealed LB film of 1a on glass (see Figure 7). j Estimated from the electron density given in Figure 5.

b

Figure 6. UV-vis spectra of LB films of 1a on quartz substrate (1, 10, 14, 30, and 40 layers). Inset: absorbance at 220 and 360 nm as a function of number of layers.

ne subunit in 1a and clearly shows that the anthraquinone units actually are located in a well-defined region below the HBC columns. All attempts to transfer the Langmuir films directly to hydrophilic glass and silicon failed due to little or no adhesion. However, mono- and multilayer Langmuir-Blodgett films of compounds 1a, 1b, and 1d were transferred effectively to hydrophobic solid supports (OTS-treated glass, quartz, and silicon) by standard Langmuir-Blodgett technique with vertical dipping. Films of 1a with 1, 10, 14, 30, and 40 layers (on both sides of the slide) were prepared on quartz slides and their absorbance spectra recorded (Figure 6). The spectra of these LB films show two main absorbance bands at 360 and 220 nm typical of the HBC system. However, the peaks are broadened and the fine structure is lost compared to the solution spectra, similar to what is observed for aggregates in solution.20 The absorbance increases linearly with the number of layers (Figure 6, inset), without any signs of decreasing transfer efficiency up to 40 layers, underlining the very efficient transfer of 1a. Comparable transfer efficiencies were observed in the transfer profiles of 1b,d from the LB trough, yet the absorbance spectra were found to be much less reproducible, showing highly variable absorbance per monolayer ranging from the values found for 1a and down to 60% of this. Well-defined X-ray reflectivity data collected for a 40layer film of 1a on OTS-treated glass (Figure 7) exhibit three Bragg peaks, and with Kiessig fringes (due to the

Figure 7. X-ray reflectivity curve a 40-layer LB film of 1a on glass substrate.

total film thickness) in between the Bragg peaks, attesting to the efficient buildup of high-quality multilayer films. From the Bragg peak positions we calculate spacings of d ) 45.8 Å and its harmonics, 22.9 and 15.3 Å. The 22.9 Å peak is by far the strongest, indicating that the main vertical packing motif has a 22.9 Å spacing, with a slight pairing of the layers giving the doubled spacing of 45.8 Å as might be expected for a Y-type LB film. The interlayer spacing is in fair agreement with contact mode AFM investigations of a scratched 40 layer film of 1a that revealed a total thickness of 95 nm, corresponding to a average layer thickness of 23.7 Å. Diffraction from a 30-layer LB film of 1a on an OTStreated Si wafer was studied by means of GIXD employing synchrotron radiation. Diffraction data were collected for x, y, and z directions, with the film mounted such that the y-axis was parallel to the dipping direction, as shown in Figure 8 (inset). Diffraction intensities as a function of Q in the three directions of the film are shown in Figure 8, and the characteristic peaks are tabulated in Table 1. In the in-plane directions x and y broad peaks are observed at ≈1.3 Å-1, corresponding to a repeat distance of 4.8 Å. An additional peak corresponding to a d spacing of 3.5 Å is found only in the dipping direction of the LB film (y direction). This clearly indicates a macroscopic anisotropic film dominated by a structure with columns of 3.5 Å spaced disks aligned along the dipping direction. In agreement with this, the intercolumnar peak is most intense and narrow in the z and x directions, while only a relatively weak and broad diffraction peak is observed in the y direction where the 3.5 Å π-stack signal is

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Figure 8. GIXD of a 30-layer LB film of 1a on Si wafer. Inset: orientation of the sample relative to the directions of the GIXD experiment. Qx and Qy scans are integrated over 0 < Qz < 0.3 Å-1. The “Qz” scan was along a line making a 0.3° angle with the specular direction.

Figure 9. GIXD of a 40-layer LB film of 1a on OTS-treated quartz, showing part of the Qx scan. For comparison, the corresponding part of the Qx scan for the multilayer film on Si (Figure 8) is included.

dominant. The observed 3.5 Å repeat distance and the nearly symmetric column width (≈23 Å) are attributed to a nontilted (cofacial) hexagonal packing of the disks (see Figure 1).

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Further GIXD experiments performed on a 40-layer film on OTS-treated quartz showed the same peaks, however with a relatively more intense 4.8 Å peak in the Qy scan and an additional 19.9 Å peak clearly seen in the Qx scan shown in Figure 9 (see also Table 1 and Supporting Information). These peaks are assigned to a packing structure similar to what was found in the Langmuir layer at the air-water interface. However, this phase completely vanishes upon heating to 140 °C for 5 min. Annealing, Phase Transitions, and Optical Anisotropy. The in-plane anisotropy of the LB films was investigated by polarized microscopy/spectroscopy. It was found that some of the multilayer LB films of 1a and 1d were highly birefringent. Thus, optical microscopy using crossed polarizers shows major differences in transmitted and reflected light as a function of the orientation of the LB film relative to the axes of the microscope polarizers, as shown by the micrographs in Figure 10. This optical polarization effect, caused by the electronic anisotropy of the LB film, was quantified by spectroscopic measurements of the relative intensity of reflected light as the LB film was rotated relative to the microscope polarizers. For the 40-layer LB film of 1a on silicon, which was investigated by GIXD, a ratio of 1:50 was found between minimum (dipping direction parallel to one of the polarizers) and maximum (dipping direction 45° relative to both polarizers). The anisotropy of LB films of 1a on quartz and glass substrates was much lower (ratios of 1:10 or less). Furthermore, these LB films showed hardly any dichroic effect in the linear polarized absorption spectra. The different structure and the difference in the degree of optical anisotropy in the LB films of 1a on silicon and on quartz/glass substrates further led us to suspect the possible existence of two or more phases in the LB films. To investigate this, LB films of 1a on quartz substrates were annealed by heating to 150 °C for 5 min (with heating/cooling rate of 10 °C/min). The annealing results in significant changes of optical properties. Thus, UV-vis spectra show a considerable reduced absorption per monolayer along with a small blue shift of the two main peaks (Figure 11). Applying exciton coupling theory, this is consistent with a transition from tilted packing to cofacial packing of the HBC cores.26 Even more significant is the change in optical anisotropy, where the annealing gives rise to an increase in dichroic ratio from 1.06 to 3.66, with the lowest absorbance

Figure 10. Micrographs of highly birefringent 40-layer LB film of 1a on an OTS-treated Si wafer, viewed through crossed polarizers in reflection mode; arrows indicate the dipping direction and white lines the polarizer directions.

Amphiphilic Hexabenzocoronenes

Figure 11. UV-vis spectra of a 2 × 40-layer LB film of 1a on OTS-treated quartz glass, before (full line) and after annealing (dashed line) (150 °C for 5 min; heating/cooling rate: 10 °C/ min).

Figure 12. Polarized UV-vis spectra, with light polarized parallel (|) and perpendicular (⊥) to the dipping direction. Dotted lines represent the nonannealed sample and full lines the spectra after annealing.

observed when the light is polarized along the dipping direction (Figure 12). At the same time the off-resonance polarizer efficiency increased from 1:5 to 1:200 (Figure 13). The annealing process was monitored by measuring the change in absorbance at 360 nm as a function of temperature (Figure 14), clearly showing that an irreversible phase transition sets in at ca. 50 °C (peak at 64 °C). The significant change in absorbance observed during the first heating cycle proves that this phase transition involves a major reorganization of the HBC cores, which is not observed during cooling or in the second heating. For the acid derivative 1d the pristine films show pronounced optical anisotropy, which is further enhanced upon annealing. After annealing, LB films of 1d show a much more well-defined absorbance per monolayer, similar to what is found for the annealed films of 1a. These observations led to the conclusion that the cofacial phase is more dominant in the pristine LB films of 1d and that the variation in optical density of these LB films is a result of variations in the relative fraction of the two structural phases. It is noticed that even though compound 1a in its bulk state14 and in spin-cast thin films displays a reversible (26) Piris, J.; Pisula,W.; Tracz,A.; Pakula, P.; Mu¨llen, K.; Warman, J. M. Manuscript in preparation.

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Figure 13. Intensity of reflected light (at 500 nm), before and after annealing, from a 2 × 40-layer LB film of 1a on quartz glass as a function of angle between the microscope polarizers (crossed polarizers) and the dipping direction of the LB film.

Figure 14. Absorption at 360 nm of a 2 × 40-layer LB film of 1a on quartz glass as a function of temperature T, during thermal annealing. Inset: first derivative of the A vs T plot.

crystal to liquid crystal phase transition at 133 °C, no such transition is observed in the LB films, even when heating to 250 °C. Discussion and Conclusions Attaching branched side chains to the HBC system clearly results in more liquid and less crystalline Langmuir films as can be seen from the lower collapse pressure and reduced coherence in the X-ray data compared to amphiphilic HBC systems with nonbranched side chains.6 At the same time this increased flexibility allows the buildup of multilayer LB films by simple vertical dipping, which was not possible for the more crystalline nonbranched analogues. After annealing, LB films with a well-defined hexagonal cofacial packing are obtained. These multilayer films display macroscopic uniaxial alignment of the columnar aggregates along the dipping direction, resulting in significant optical and electronic anisotropy. Thus, besides being birefringent and dichroic, the films display anisotropic photoconductance.9 The observed alignment of the columnar aggregates along the dipping direction of the LB film may be of major importance for the utilization of these well-ordered films in electronic devices. Dipping-induced flow orientation of rigid-rod-like polymers or small molecules, induced by

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the nonuniform force gradients upon pulling the substrate and monolayer from the water subphase, is wellknown.21-23 Here the HBC aggregates at the water surface may be considered as rodlike particles and thus susceptible to flow orientation. In a previous study of an amphiphilic HBC derivative indication of alignment was also observed,7 suggesting that this may be a general trend for these strongly aggregating discotics. In the solid films of 1a we find two different packing structures: one with the HBC cores tilted and rotated relative to the stacking direction and one cofacial structure with the disks perpendicular to the stacking direction (see Figure 1). The latter is the thermodynamically stable structure obtained after annealing, while the tilted structure corresponds to the preferred packing of the monolayer at the air-water interface. The GIXD investigations of the monolayer of 1a do not allow us to unambiguously assign the direction of the tilting in this structure. However, the observation of the narrow column width (approximately 20 Å) in the xy-plane of the Langmuir film and the unannealed LB film on quartz suggests that the disks are rotated around the z-axes, as illustrated in Figure 1. The cofacially packed structure display symmetric column dimensions (23 Å) characteristic for a hexagonal structure. In this respect the cofacially packed phase corresponds to the hexagonal liquid crystalline Dh phase observed for bulk 1a above 133 °C.14 The unannealed tilted phase, on the other hand, seems rather similar to the room temperature crystalline phase, where bulk 1a displays a monoclinic structure with tilted stacks (≈45°) and columnar spacings of 23 and 20 Å.14 For compound 1a the tilted phase formed at the airwater interface is sufficiently stable to be retained through the transfer process, and this phase seems to dominate the pristine LB films. However, it is a delicate balance, and while this is true for glass and quartz substrates, nearly complete rearrangement to the cofacial structure apparently takes place when the monolayer is transferred to silicon wafers. For the acid derivative 1d the re-

Laursen et al.

arrangement takes place also on glass and quartz substrates, and films dominated by cofacial packed domains are obtained in most cases, as can be seen from the absorbance spectra of the pristine LB films. For both compounds complete and irreversible rearrangement to the more stable cofacial structure take place at only slightly elevated temperature (see Figure 1 and Supporting Information). It is interesting to notice that the LB films not only provide an in-plane macroscopic alignment of the HBC aggregates but also favors and stabilizes the cofacial liquid-crystal-like packing, which for the bulk material only is found at elevated temperatures. In conclusion, we have shown that well-defined thin films of amphiphilic HBC derivatives 1a and 1d carrying branched side chains may be obtained by LB techniques. In the pristine films cofacial packing as well as tilted packing is found. However, annealing results in highly anisotropic films with cofacial packed columns aligned along the dipping direction. These findings show that the LB technique may provide a high degree of structural control over thin film formation and suggest that molecular design and LB technique together may contribute to the development of organic self-assembling electronic devices, based on mesogenic discotics. Acknowledgment. This work was supported by the EU growth program, DISCEL Contract G5RD-CT-200000321, the Danish Natural Science Research Council’s DANSYNC program and the Danish Technical Research Council, and the German Ministry of Education and Research (“Nanocenter”) and the German Science Foundation (SFB625 and special program “Organische Fieldeffecttranistoren”). Supporting Information Available: Figures showing X-ray reflectivity measurements of Langmuir films of 1a and 1c, annealing of multilayer LB film of 1d, and GIXD of 40-layer LB film of 1a on quartz glass. This material is available free of charge via the Internet at http://pubs.acs.org. LA049944I