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Notes Surface Morphology and Molecular Ordering in Thin Films of Polymerizable Triphenylene Discotic Liquid Crystals on HOPG Revealed by Atomic Force Microscopy H. Scho¨nherr,*,† M. Manickam,‡ and S. Kumar‡ MESA+ Research Institute and Faculty of Chemical Technology, University of Twente, Department of Materials Science and Technology of Polymers, P.O. Box 217, 7500 AE Enschede, The Netherlands, and Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore 560 013, India Received April 2, 2002. In Final Form: June 28, 2002
Introduction Discotic liquid crystals1 constitute a unique class of materials and since their discovery 25 years ago by Chandrasekhar2 have led to numerous groundbreaking developments in the areas of photovoltaics,3 light emitting diodes (photoluminescence),4 organic photoconductors,5 displays,6 and other devices.7 A well-characterized group of discotic liquid crystals are the hexaalkoxy-triphenylenes.8 Similar to other groups of materials, the anisotropy of the molecules, that is, flat disklike core and surrounding alkyl substituents, leads to the formation of * Corresponding author. E-mail:
[email protected]. Tel: ++31 53 489 3170. Fax: ++31 53 489 3823. † University of Twente. ‡ Centre for Liquid Crystal Research. (1) (a) Chandrasekhar, S. Liq. Cryst. 1993, 14, 3. (b) Chandrasekhar, S.; Kumar, S. Sci. Spectrosc. 1997, 8, 66. (2) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 9, 471. (3) (a) Gregg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1990, 94, 1586. (b) Wo¨hrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129. (c) SchmidtMende, L.; Fechtenkotter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; Mackenzie, J. D. Science 2001, 293, 1119. (4) (a) Bacher, A.; Erdelen, C.; Paulus, W.; Ringsdorf, H.; Schmidt, H. W.; Schuhmacher, P. Macromolecules 1999, 32, 4551. (b) Stapff, I. H.; Stumpflen, V.; Wendorff, J. H.; Spohn, D. B.; Mobius, D. Liq. Cryst. 1997, 23, 613. (c) Seguy, I.; Destruel, P.; Bock, H. Synth. Met. 2000, 111-112, 15. (d) Christ, T.; Glusen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stumpflen, V.; Tsukruk, V.; Wendorff, J. H. Adv. Mater. 1997, 9, 48. (5) (a) Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.; Schuhmacher, P.; Siemensmeyer, K. Phys. Rev. Lett. 1993, 70, 457. (b) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (c) Ochse, A.; Kettner, A.; Kopitzke, J.; Wendorff, J. H.; Bassler, H. Phys. Chem. Chem. Phys. 1999, 1, 1757. (d) Simmerer, J.; Glusen, B.; Paulus, W.; Kettner, A.; Schuhmacher, P.; Adam, D.; Etzbach, K. H.; Siemensmeyer, K.; Wendorff, J. H.; Ringsdorf, H.; Haarer, D. Adv. Mater. 1996, 8, 815. (e) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.; Paulus, W.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Adv. Mater. 1995, 7, 276. (6) Kawata, K. Chem. Rec. 2002, 2, 59. (7) (a) Boden, N.; Borner, R. C.; Bushby, R. J.; Clements, J. J. Am. Chem. Soc. 1994, 116, 10807. (b) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater. Chem. 1999, 9, 2081. (c) Boden, N.; Bissell, R.; Clements, J.; Movaghar, B. Liq. Cryst. Today 1996, 6, 1. (d) Chandrasekhar, S. In Hand Book of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: New York, 1998; Vol. 2B, Chapter VIII. (e) Boden, N.; Movaghar, B. In Hand Book of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: New York, 1998; Vol. 2B, Chapter IX. (8) Destrade, C.; Mondon, M. C.; Malthete, J. J. Phys. Colloq. 1979, C3, 17.
columnar mesophases. Recent synthetic progress opened the way to a wide variety of symmetrically and unsymmetrically substituted derivatives.9-11 The anisotropic shape and the self-assembly of triphenylenes (TPs) and similar molecules have inspired numerous studies at interfaces. Pioneering scanning tunneling microscopy (STM)12 work by the group of Rabe showed the two-dimensional (2-D) arrangement of TP molecules at the highly oriented pyrolytic graphite (HOPG)-solution interface.13 In submolecular resolution STM images, the aromatic cores could be differentiated from the n-alkyl substituents. Similarly, the surfaceinduced chirality of some TP derivatives was unraveled by Charra and Cousty using STM.14 These STM studies are limited to the interfacial structure of the first layer of TP molecules on the HOPG substrate. Compared to STM, atomic force microscopy (AFM)12,15 does not suffer from a fundamental filmthickness-dependent limitation, that is, the tunneling current. Since there is also no need for a conductive substrate in AFM, virtually any substrate material can be used. As shown in numerous studies, AFM is one of the prime techniques to study surface morphologies and molecular ordering on length scales of >10 µm to the angstrom level.16-18 In our previous work, we have addressed the arrangement and the control of the orientation of triphenylene (9) (a) Kumar, S.; Manickam, M. J. Chem. Soc., Chem. Commun. 1997, 1615. (b) Kumar, S.; Varshney, S. K. Liq. Cryst. 1999, 26, 1841. (c) Kumar, S.; Manickam, M.; Balagurusamy, V. S. K.; Scho¨nherr, H. Liq. Cryst. 1999, 26, 1453. (d) Kumar, S.; Varshney, S. K. Synthesis 2001, 305. (10) (a) Goodby, J. W.; Hird, M.; Toyne, K. J.; Watson, T. J. Chem. Soc., Chem. Commun. 1994, 1701. (b) Cross, S. J.; Goodby, J. W.; Hall, A. W.; Hird, M.; Kelly, S. M.; Toyne, K. J.; Wu, C. Liq. Cryst. 1998, 25, 1. (c) Closs, F.; Ha¨ussling, L.; Henderson, P.; Ringsdorf, H.; Schuhmacher, P. J. Chem. Soc., Perkin Trans. 1 1995, 829. (d) Henderson, P.; Ringsdorf, H.; Schuhmacher, P. Liq. Cryst. 1995, 18, 191. (e) Stewart, D.; McHattie, G. S.; Imrie, C. T. J. Mater. Chem. 1998, 8, 47. (f) Borner, R. C.; Jackson, R. F. W. J. Chem. Soc., Chem. Commun. 1994, 845. (11) (a) Boden, N.; Borner, R. C.; Bushby, R. J.; Cammidge, A. N.; Jesudason, M. V. Liq. Cryst. 1993, 15, 851. (b) Boden, N.; Borner, R. C.; Bushby, R. J.; Cammidge, A. N. J. Chem. Soc., Chem. Commun. 1994, 465. (c) Boden, N.; Bushby, R. J.; Cammidge, A. N.; Headdock, G. Synthesis 1995, 31. (d) Boden, N.; Bushby, R. J.; Cammidge, A. N. J. Am. Chem. Soc. 1995, 117, 924. (e) Boden, N.; Bushby, R. J.; Lu, Z. B. Liq. Cryst. 1998, 25, 47. (f) Boden, N.; Bushby, R. J.; Lu, Z. B.; Cammidge, A. N. Liq. Cryst. 1999, 26, 495. (12) (a) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178. (b) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (13) Askadskaya, L.; Boeffel, C.; Rabe, J. P. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 517. (14) Charra, F.; Cousty, J. Phys. Rev. Lett. 1998, 80, 1682. (15) (a) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (b) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996. (16) For recent reviews, see: (a) Scanning Probe Microscopies in Polymers; Ratner, B. D., Tsukruk, V., Eds.; ACS Symposium Series 694; American Chemical Society: Washington, DC, 1998. (b) Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., Wahl, K. J., Eds.; ACS Symposium Series 741; American Chemical Society: Washington, DC, 1999. (c) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 385. (d) Durbin, S. D.; Feher, G. Annu. Rev. Mater. Sci. 1996, 47, 175. (e) Sheiko, S. S.; Mo¨ller, M. Chem. Rev. 2001, 101, 4099. (f) Jandt, K. D. Surf. Sci. 2001, 491, 303. (17) In addition to topographic imaging of virtually any surface in various media, materials properties, such as normal and friction forces (compositional mapping), elasticity, etc., can be assessed.
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derivatives and liquid crystalline tricycloquinazolines on various substrates, including gold surfaces, using spectroscopic and microscopic methods.19,20 While for instance mercaptoalkyl- or dithioalkyl-derivatized pentaalkoxytriphenylenes were shown to self-assemble in edge-on orientation onto Au(111) substrates, hexathioether triphenylenes assembled into layers with flat-on orientation.19 The ultimate proof for the validity of the original interpretation, which was based on grazing angle reflection Fourier transform infrared spectroscopy (GIR-FTIR), was AFM data of the monolayers in which the columnar ordering in the plane of the monolayer was clearly resolved.19 In this paper, we present results of an AFM study on the surface morphology of the quenched mesophase of the novel polymerizable 2-[(3,6,7,10,11-pentabutoxy-1-nitro2-triphenylenyl)oxy]ethyl acrylate 1 in 50-100 nm thin films on HOPG. In particular, we address the molecular level ordering of the hexagonal columnar mesophases at the surface of these films and contrast the arrangement of this triphenylene with that of the similar 2,3,6,7,10,11-hexakis(pentyloxy)-1,5-dinitrotriphenylene derivative 2.
Figure 1. POM photograph of the quenched mesophase of 1 imaged at 25 °C. Chart 1. Structures of Triphenylenes 1 and 2
Experimental Section Synthesis. The synthesis and characterization of the dinitro TP derivative 2 has been described previously.9c The monoacrylate TP derivative 1 was synthesized as follows: A solution of 2-[(3,6,7,10,11-pentabutoxy-1-nitro-2-triphenylenyl)oxy]ethyl alcohol9c (500 mg, 0.72 mmol) and triethylamine (87 mg, 0.87 mmol) in dichloromethane (10 mL) was cooled to 0 °C under nitrogen. To this, a solution of acryloyl chloride (79 mg, 0.87 mmol) in 2 mL of dichloromethane was added dropwise. The reaction mixture was stirred at room temperature for 15 h under nitrogen, and after that it was poured over ice-water. The organic phase was separated by extraction with dichloromethane (25 mL × 4). The combined extracts were washed with water and dried over anhydrous sodium sulfate, and the solvent was removed under vacuum. The product was crystallized with ethermethanol to yield 320 mg (59%) of the product. 1H NMR (250 MHz, CDCl ): δ 7.93 (1H, s), 7.79 (3H, m), 7.47 3 (1H, s), 6.49 (1H, d), 6.22 (1H, dd), 5.87 (1H, d), 4.47 (4H, brs), 4.25 (m, 8H), 4.08 (t, 2H), 1.92 (m, 10H), 1.6 (m, 10H), and 1,1 (m, 15H). MS: m/z 747.3 (100%). Sample Preparation. Thin films were cast from toluene (Merck, p.a. grade) solution onto freshly cleaved HOPG. The complete evaporation of the solvent was achieved by heating the film to the isotropic phase. Finally, the films were quenched rapidly to ambient temperature. Typical film thicknesses of at least 50-100 nm were measured by AFM in depressions of the films. Atomic Force Microscopy. The contact mode AFM data were acquired with a NanoScope II AFM (Digital Instruments (DI), Santa Barbara, CA) equipped with a short tube 1 µm (A) scanner and a DI liquid cell filled with Milli-Q water. Silicon nitride cantilevers and tips (DI, nominal spring constant of 0.06 N/m) were used. The AFM liquid cell setup was assembled and equilibrated for 1 day without liquid. After the cell was filled with water, the experiments were carried out over a period of typically 3-5 days in order to obtain optimum stability (matching of up and down scans). If needed, the Milli-Q water in the cell was refilled. Polarized Optical Microscopy (POM). The POM images of the mesophase of films of 1 on glass were captured with a Leitz (18) For AFM work on LB films of TP derivatives, see: (a) Josefowicz, J. Y.; Maliszewskyj, N. C.; Idziak, S. H. J.; Heiney, P. A.; McCauley, J. P.; Smith, A. B., III Science 1993, 260, 323. (b) Tsukruk, V. V.; Reneker, D. H.; Bengs, H.; Ringsdorf, H. Langmuir 1993, 9, 2141. (19) Scho¨nherr, H.; Kremer, F. J. B.; Kumar, S.; Rego, J. A.; Wolf, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051. (20) Hiesgen, R.; Scho¨nherr, H.; Kumar, S.; Ringsdorf, H.; Meissner, D. Thin Solid Films 2000, 358, 241.
Table 1. Phase Behavior of TP Derivatives 1 and 2 upon Cooling TP-1 TP-2
Ttransition POM [°C]
Ttransition DSC [°C]
I 139.4 Colh I 94.6 Colh
I 139.3 Colh I 94.0 Colh
DMRXP polarizing microscope (crossed polarizers) operated in transmission mode in conjunction with a Mettler FP 82 hotstage. Differential Scanning Calorimetry (DSC). The thermal transitions and transition enthalpies of 1 were determined by differential scanning calorimetry (DSC7 Perkin-Elmer). The temperature data obtained from the heating and cooling cycles of DSC are collected in Table 1. Data for compound 2 were taken from ref 9c. The peak temperatures are given in °C.
Results and Discussion The mesophase behavior of thin films of triphenylene 1 (Chart 1) on glass was investigated first by POM. The compound melts at about 68 °C and clears completely at 141 °C. The classical texture of columnar mesophases appeared at 139.4 °C upon cooling and remained stable down to room temperature (Table 1). No crystallization was observed upon keeping 1 at room temperature for >24 h. Figure 1 shows a POM photograph of compound 1 obtained by cooling from the isotropic liquid to room temperature. These textures are very similar to the known textures for the Colh phases shown by several wellcharacterized discotic liquid crystals. Independent DSC experiments confirmed the presence of the mesophase transition at 139.3 °C (Table 1). The first heating run of the solvent-crystallized compound 1 shows a broad melting transition at 67.7 °C (∆H ) 26.6 J/g) and a clearing transition at 140.5 °C (∆H ) 10.5 J/g). The first cooling run exhibits only one isotropic to mesophase transition at 139.3 °C (∆H ) 10.2 J/g). The compound remains in this mesophase until room temperature, and the second heating run gives only a mesophase to isotropic transition at 140.5 °C (∆H ) 10.4 J/g). As seen already in the POM investigation, the mesophase could be frozen by quenching the sample to
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Figure 2. Surface morphology of thin films of 1 on HOPG as visualized by contact mode AFM in water.
ambient temperatures, while the dinitro derivative 2 is a room-temperature liquid crystalline material.9c The surface morphology of quenched films of 1 on HOPG was characterized by contact mode AFM. Initial measurements were carried out in air; however, large imaging forces due to pronounced adhesion prevented noninvasive imaging of the surface morphology. Hence, to reduce the adhesive forces and thus minimize imaging forces, the AFM experiments were carried out in an aqueous medium.21,22 As shown in Figure 2, the surface of the fairly smooth films showed typical defects in the form of depressions. During the course of the AFM experiments in the liquid cell (several days), the surface morphology and in particular the location of these defects were unaltered. Noninvasive imaging conditions were verified by imaging the film over the whole accessible scan size at regular intervals. At higher magnification, a periodic nanometer scale structure was resolved for both triphenylene derivatives 1 and 2. The structures were usually found in one orientation over length scales of several hundreds of nanometers. This long-range orientational ordering may reflect the effect of the interactions of the triphenylenes in contact with the HOPG since the atomically smooth terraces on the HOPG have similar extensions. The periodic structure found for 1 is shown in Figure 3. The raw data suggest a near-hexagonal lattice structure. The periodicity is more obvious in the 2-D fast Fourier transform (2-D FFT) of the image and the Fourier filtered section (insets in Figure 3). From the analysis of many autocorrelation filtered images, as well as the corresponding 2-D FFTs, the lattice constants were determined quantitatively. As shown in the histograms of measured hexagonal lattice constants in Figure 4, the pentabutoxy-substituted mononitromonoacrylate triphenylene 1 (d1 ) 18.5 ( 0.7 Å) and the hexapentyloxy-substituted dinitro derivative 2 (d2 ) 20.3 ( 0.6 Å) have significantly different spacings.23 The lattice constants of 1 and 2 differ by 1.8 ( 0.9 Å, which agrees with the difference of ∼1.5 Å expected based on X-ray data on bulk specimens of very similar compounds.9c (21) Weisenhorn, A. L.; Maiwald, P.; Butt, H.-J.; Hansma, P. K. Phys. Rev. B 1992, 45, 11226. (22) In addition, we carried out tapping mode AFM measurements in air (no data shown) which confirmed the morphology observation obtained by contact mode AFM in water.
Notes
Figure 3. Unprocessed high-resolution contact mode AFM nanograph of the quenched mesophase of 1 imaged in water (insets: 2-D FFT (left) and Fourier filtered section (right)).
Furthermore, the spacings are comparable to lattice constants determined for related compounds: the intercolumnar spacing determined by X-ray diffraction for a mononitro-substituted hexapentyloxy-TP is 20.1 Å, and that for the corresponding mononitro-substituted hexabutoxy-TP is 18.6 Å.9c Thus, we can conclude that the columnar spacings at the surface of thin TP films on HOPG are similar to typical bulk values. These data also lead to the conclusion that for 1 and 2 in the quenched mesophase and the room-temperature mesophase, respectively, the close-packed columns of triphenylene molecules are oriented parallel to the surface normal direction; the molecules are hence oriented with their planes in the plane of the HOPG substrate. This flat-on orientation is in contrast to the observed edge-on orientation in self-assembled monolayers (SAMs) on gold,19 thin films obtained by Langmuir-Blodgett (LB) techniques,18 and films of discotics on various alignment layers.6 The differences in orientation can be understood if the anchoring, film thickness, and the surface free energy at the free surface are considered. In both SAMs and LB films, the monolayers are tightly packed owing to a maximized number of surface-bound thiolates and the applied pressure, respectively. These constraints lead to the tightest surface coverage in the monolayers and hence to edge-on orientation of the discotic cores. As discussed in a recent review article, the effects of alignment layers and surface free energy can be utilized to manipulate the orientation of discotic liquid crystals at the surface of rather thick (>100 µm) films.6 Both flat-on and edge-on orientations have been achieved using the corresponding alignment layers and surface free energies. In the case of the triphenylenes 1 and 2 studied here, HOPG serves as an aligning substrate which forces the discotic liquid crystals into a flat-on orientation.13,14 The orientation at the surface of the film may be in principle preferentially flat-on or edge-on, depending on surface energy and film thickness. Since we have investigated films with a (23) The determination of lattice constants is based on a careful calibration of the AFM scanner. For this purpose, images of the periodicity of the underlying graphite lattice were evaluated. These images were captured on the same HOPG substrate after removing the TP film and a section of graphite by cleaving with pressure sensitive adhesive.
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Figure 4. Lattice constants of the hexagonal lattices observed by contact mode AFM for triphenylenes 1 (A, left) and 2 (B, right) in thin films of HOPG.
thickness of 50-100 nm in contact with water, the observed flat-on orientation can be understood. Thus, we can assume that the hexagonal columnar mesophase extends from the HOPG interface to the surface of the films. These data imply a homeotropic alignment of the molecules in the 50-100 nm thin films on HOPG. Hence, in addition to “molecular engineering” through introduction of functional groups for controlled chemisorption,19 the orientation of triphenylene discotic liquid crystals throughout thin films can be controlled by “physical” interfacial interactions and film thickness. Using UV-light-initiated free radical polymerization, we have successfully polymerized the monoacrylate derivative 1 in the (bulk) mesophase. Gel permeation chromatography (GPC) results on polymerized samples showed evidence for polymers with molar masses of up to 8000 g/mol (relative to polystyrene). The low degree of polymerization may be related to the known retardation effect of nitro groups in free radical polymerization.
In summary, we have shown that contact mode AFM in an aqueous medium revealed the hexagonal periodicity of the mesophase at the surface of thin films of a novel polymerizable TP derivative, thus indicating the homeotropic alignment of the discotic liquid crystalline molecules. The observed lattice constants agree well with literature data on similar bulk systems. Currently, we investigate phase transitions and in situ polymerization of these triphenylene discotics using temperature-dependent AFM.24 Acknowledgment. The authors are grateful to Professor Dr. Helmut Ringsdorf and Professor Dr. G. Julius Vancso for invaluable discussions and support. The financial support of the University of Twente is acknowledged. LA025797H (24) (a) Pearce, R.; Vancso, G. J. Macromolecules 1997, 30, 5843. (b) Scho¨nherr, H.; Bailey, L. E.; Frank, C. W. Langmuir 2002, 18, 490.
