Discotic Twin and Triple Molecules with Charge-Transfer Interactions

X-ray and atomic force microscopy observations reveal features of the surface morphology of Langmuir−Blodgett (LB) films from discotic donor−accep...
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Langmuir 1996, 12, 754-757

Discotic Twin and Triple Molecules with Charge-Transfer Interactions in Langmuir-Blodgett Films Vladimir V. Tsukruk* College of Engineering and Applied Sciences, Western Michigan University, Kalamazoo, Michigan 49008

Holger Bengs† and Helmut Ringsdorf Institut fu¨ r Organische Chemie, Universita¨ t Mainz, 55122 Mainz, Germany Received July 24, 1995X X-ray and atomic force microscopy observations reveal features of the surface morphology of LangmuirBlodgett (LB) films from discotic donor-acceptor molecules with strong core-to-core interactions caused by a charge-transfer (CT) complexation. The compounds are designed to be twin and triple molecules composed of chemically connected donor (triphenylene) and acceptor (trinitrofluorenone) fragments which display liquid crystalline structure in the bulk state. The LB films from these compounds possess edge-on orientation of molecules within monolayers with a smooth surface on a submicron scale but with substantial macroscopic imperfections. These imperfections are represented by the micron size fractures and ruptures apparently caused by local deformations and tensions during transfer of the discotic monolayers from the air-water interface to a solid substrate. It is suggested that this is due to the excessive rigidity of the monolayers caused by the stiffness of the columns with positional intracolumnar ordering enhanced by the strong intracolumnar CT interactions.

Introduction Discotic liquid crystalline (LC) materials attract attention by their ability to form columnar phases with anisotropic transport properties.1 For example, photoconductivity has been observed in discotic LCs which is anisotropic with respect to transport properties and characterized by a high mobility of charge carriers.1,2 In the past few years, understanding of the rich polymorphism of these compounds in bulk and in molecular films has significantly progressed. Various discotic LCs were used for fabrication of molecular films: amphiphilic discotics and discotic side chain polymers,3 asymmetric and starlike triphenylenes,4 heteroatomic salts,5 pentaalkylbenzenes,6 as well as phthalocyanine dimers, trimers, and polymers.7 Additionally, mixtures of electron rich discotic molecules (donors) with flat acceptor molecules were employed to build molecular films with chargetransfer (CT) interactions between rigid cores.8 It was further observed that the CT complexation enhances core* To whom correspondence should be addressed. † Present address: Hoechst AG, Central Research G 830, 65926, Frankfurt a. M., Germany. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Pershan, P. Structure of Liquid Crystal Phases; Word Science: Singapore, 1988. Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.; Schuhmacher, P.; Siemensmeyer, K. Phys. Rev. Lett., 1993 70, 457. Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 341, 141. (2) Bengs, H.; Closs, F.; Frey, T.; Funhoff, D.; Ringsdorf, H.; Siemensmeyer, K. Liq. Cryst. 1993, 15, 565. (3) Suresh, K. A.; Blumstein, A.; Rodhelez, F. J. Phys. France 1985, 48, 453. Karthaus, O.; Ringsdorf, H.; Urban, C. Makromol. Chem., Macromol. Symp. 1991, 46, 347. Karthaus, O.; Ringsdorf, H.; Tsukruk, V.; Wendorff, J. H. Langmuir 1992, 8, 2279. (4) Josefowicz, J. Y.; Maliszewskyj, N. C.; Idziak, S. H.; Heiney, P. A.; McCauley, J. P.; Smith, A. B. Science 1993, 260, 323. Maliszewskyj, N. C. Ph.D. Dissertation, University of Pennsylvania, Philadelphia, PA, 1994. (5) Albouy, P. A.; Vandevyver, M.; Perez, X.; Ecoffet, C.; Markovitsi, D.; Veber, M.; Jallabert, C.; Strzelecka, H. Langmuir 1992, 8, 2262. (6) Reiche, J.; Dietel, R.; Janietz, D.; Lemmetyinen, H.; Brehmer, L. Thin Solid Films 1992, 226, 265. (7) Nostrum, C. F.; Nolte, R. J.; Devillers, M. A.; Oostergetel, G. T.; et al. Macromolecules 1993, 26, 3306.

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to-core interactions of discotic donor and acceptor molecules and leads to alternating regular packing of the molecules in the bulk LC phase. On the other hand, the CT complexation promotes edge-on orientation of discotic molecules in ultrathin films. A firm confirmation of edgeon orientation of discotic molecules in Langmuir-Blodgett (LB) films was recently obtained.3-6 It became clear that strong core-to-core interactions or asymmetric edge-toedge interactions are necessary for the fabrication of LB films with edge-on orientation of discotic molecules. In this type of LB film, the columns lie parallel to a solid substrate, which gives rise to anisotropic transport properties (e.g. photoinduced conductivity) in the plane of the films. The quality of these films, as measured by surface roughness, in-plane homogeneity, and intermolecular correlations, is critical for the design of molecular films with proper transport properties. In our previous X-ray studies, we observed that LB films from mixtures of donor and acceptor discotics are substantially nonhomogeneous with a high surface roughness.8 It was assumed that the high level of surface roughness of these LB films is caused by an nonhomogeneous distribution of the components in the plane of the films as a direct result of phase separation of donor and acceptor compounds. Indeed, phase separation is frequently observed in bulk mixtures of the donor discotic molecules and acceptor compounds.9 Obviously, the tendency of different discotic components to phase separate substantially affects the ability to fabricate homogeneous molecular films from these promising compounds. Therefore, the goal of this study is to understand the structural behavior of donor-acceptor discotic CT complexes with suppressed ability to phase separate. We focus on the structural organization of these compounds in LB films with a limited number of molecular layers. To implement this goal, several different discotic donoracceptor compounds are designed (for chemical formulas (8) Tsukruk, V. V.; Wendorff, J. H.; Karthaus, O.; Ringsdorf, H. Langmuir 1993, 9, 614. (9) Kraning, W.; Bo¨effel, C.; Spiess, H. W.; Karthaus, O.; Ringsdorf, H.; Wu¨stefeld, R. Liquid Crystals 1990, 8, 375.

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Figure 1. Chemical structure of twin (1 and 2) and triple (3) discotic compounds under investigation.

see compounds 1, 2, and 3 in Figure 1). The compounds studied here are composed of chemically connected donor (triphenylene) and acceptor (trinitrofluorenone (TNF)) fragments. These twin and triple discotic molecules form homogeneous columnar mesophases in the bulk state as have been demonstrated earlier.10 For these compounds, the pressure-area behavior of the monolayers at the airwater interface is studied and LB films with a limited number of the molecular layers are fabricated. Surface morphology and microstructure of these films are studied by X-ray reflectivity and atomic force microscopy. Experimental Section The synthesis, thermal behavior, and structure in the bulk mesophase of the donor-acceptor twin molecules 1 and 2 are described elsewhere.10 The donor-acceptor-donor triple molecule 3 is a condensation product of the corresponding acceptor diethyl ester and the free alcohol derivative of the corresponding triphenylene (Figure 1).10,11 The condensation takes place under vacuum with a 15% molar excess of the triphenylene derivative by means of titan catalysis.12 The product is purified by 2-fold flash chromatography. After final recrystallization from CH2Cl2-ethanol, the product is collected with a centrifuge. The yield of a brown solid product is 22%. 1H-NMR data are in accordance with the given chemical structure. Elemental analysis for C114H151N3O22 (molecular weight is 1915.45); chemical composition [found (calculated)]: C7, 1.24 (71.48); H, 7.92 (7.95); and N, 1.97 (2.19). Differential scanning calorimetry, polarized optical microscopy, and X-ray analysis of bulk material reveal the columnar mesophase for compound 3 within the temperature range of 119176 °C. The properties of monolayers at the air-water interface are studied on a KSV-5000 trough installed in a laminar flow hood. Solutions are prepared by dissolution of the components in spectroscopically pure chloroform (concentration of about 0.5 mg/ L). The solutions are spread over a pure Milli-Q water subphase. Pressure-area diagrams of the monolayers at the air-water interface are recorded at various compression speeds in the range of 0.02-0.06 nm2/min. The monomolecular films are transferred to the surface of a silicon wafer (1 in. diameter, {100} orientation, Virginia Semiconductor, Inc.). The silicon wafers are cleaned in a piranha solution (H2SO4:H2O2, 70:30) at elevated temperature and hydrophobized by octadecyltrichlorosilane. A vertical deposition of the monolayers at the solid substrate is done at a stroke speed 3-5 mm/min. LB films with the number of layers from two to six are prepared at surface pressures in the range of 2030 mN/m, that is slightly below the collapse pressure (as indicated by arrows in Figure 2). (10) Mo¨ller, M.; Tsukruk, V.; Wendorff, J. H.; Bengs, H.; Ringsdorf, H. Liq. Cryst. 1992, 12, 17. Bengs, H. Ph.D. Dissertation, Mainz University, Germany, 1993. (11) Kreuder, W.; Ringsdorf, H.; Herrmann-Scho¨nherr, O.; Wendorff, J. H. Angew. Chem. 1987, 99, 1300. (12) Reck, B.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1985, 6, 291.

Figure 2. The π-a diagrams for the compounds 1 (a) and 3 (c); the arrows indicate the collapse pressure. The π-a diagram in a compression-expansion cycle at the pressures below the collapse pressure for compound 2 (b); the arrows indicate direction of the compression and the expansion of the monolayer. Atomic force microscopy (AFM) in the contact mode and noncontact (the “tapping”) mode is used to probe surface morphology of the films in accordance to the well-established procedure.13,14 AFM images of the film surfaces at ambient temperature are obtained with an atomic force microscope, the Nanoscope III (Digital Instruments, Inc.), using a silicon-etched tip. A scanner D is used for scanning from 15 µm to 50 nm with applied forces in the range of several nanonewtons. The scanning rate is selected to be as low as 0.6-2 Hz. The AFM images are obtained with minimal feedback gains and without any additional input filters. Raw AFM data are processed only using a flattening procedure. All the images presented here are obtained repeatedly and are stable under experimental conditions. X-ray reflectivity measurements are performed over the range of scattering angles 0 < 2θ < 6° on the Philips-MRD instrument using step-by-step scanning with a step of 0.01° and monochromatized copper radiation. The geometrical sizes of molecular fragments are estimated from molecular models built by means of the BIOSYM package.

Results and Discussion The π-a diagrams for the compounds under investigation are presented in Figure 2. All the compounds form stable monolayers at the air-water interface. The π-a diagrams display different regions of the pressure-area behavior. The decrease of an area per molecule, A, in the range of 100-250 Å2 for the different compounds causes a gradual increase of the surface pressure to the level of 2 mN/m. The sharp increase of the surface pressure to 30 mN/m shows up at the second stage, at much smaller areas A ) 70-150 Å2. A collapse of the monolayers occurs at surface pressures higher than 30 mN/m and the areas per molecule below 60-130 Å2 for the different compounds. The upper limits of the area per molecule shown above were observed for compound 3 while the lowest limit corresponds to compound 1. (13) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791. (14) Frommer, J. Angew. Chem., Int. Ed. Engl. 1991, 31, 1.

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Reversible compression-decompression cycles can be repeatedly obtained at surface pressures below 10-20 mN/m as demonstrated for compound 2 in Figure 2b. At higher pressures close to the collapse pressure, the irreversible compression of the monolayers is observed for all three compounds. This behavior indicates the stiffness of the monolayers and the presence of strong cohesive intermolecular forces within monolayers with predominant attractive interactions. The areas per molecule in the compressed state, Ao, are determined from π-a diagrams by extrapolating to zero pressure.15 Such an evaluation leads to Ao values of 76, 95, and 154 Å2 for the monolayers of compounds 1, 2, and 3, respectively. These values fit closely to the areal surfaces of the edges of the discotic molecules evaluated from the molecular models (see Figure 3 for the dimensions of the different molecules and their estimated areal surfaces, Ao). Thus, according to the well-established structural behavior of discotic compounds within molecular films,3-8 all the compounds studied here form monolayers at the air-water interface with the discotic molecules in an edge-on position. The variations of the maximum area per molecule in a set of compounds studied are consistent with the edge-on arrangement of the molecules and their chemical structures. The difference of 20 Å2 between the area occupied by molecules 1 and 2 matches quite fairly to the additional area occupied by the spacer incorporated between the donor and acceptor fragments in compound 2. The experimental area per molecule, Ao, deduced from the pressure-area diagrams for compound 3 is about 2 times larger than the area per molecule for compound 1. That is consistent with the presence of an additional triphenylene fragment in compound 3 (compare the chemical structure of compounds 1, 2, and 3 in Figure 1 with the schemes of their molecular packing in Figure 3). The corresponding schemes of molecular packing for the different discotic compounds that are consistent with the monolayer behavior discussed above as well as with the X-ray and AFM observations discussed below are shown in Figure 3. The vertical deposition of the monolayers on the hydrophobized surface of the silicon wafers turned out to be problematic. The transfer ratios fluctuate widely for different layers, stroke speeds, and dipping directions. X-ray reflectivity from the LB films with six layers shows only very weak modulations (the Kiessig fringes) caused by reflection from the organic film on top of the solid substrate.16 No sharp Bragg reflections are observed for the LB films. The fast damping of the Kiessig fringes reflects the significant distortion of layer order and the high roughness of the film surfaces that makes further detailed analysis of the X-ray data difficult. However, more details of a surface morphology of the LB films can be collected by AFM (see below). As an example, the X-ray reflectivity data are presented for the LB film with six molecular layers for compound 2 in Figure 4. The periodicity of weak modulations on the X-ray curves (as indicated by arrows in Figure 4) corresponds to a total thickness of the film for compound 2 of 110 ( 12 Å. The monolayer thickness evaluated from the total thickness divided by the number of layers is about 19 ( 2 Å. This value corresponds nicely to the expected thickness of the monolayers for compound 2 estimated from the molecular models (about 21 Å as demonstrated in Figure 3). Similar thicknesses of the molecular layers are obtained for compounds 1 and 2. (15) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (16) Foster, M. Crit. Rev. Anal. Chem. 1993, 24, 179.

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Figure 3. Schemes of the edge-on arrangement of the discotic molecules within the molecular layers for compounds 1 (a), 2 (b), and 3 (c), consistent with the experimental observations. Small shaded disks represent the TNF fragments and large disks represent the triphenylene fragments. All the geometrical sizes and the areal surfaces, Ao, are estimated from the molecular models assuming the extended conformation of the flexible segments and partial overlapping of the flexible tails.10 Intracolumnar (core-to-core) distance of 3.4 Å is taken from ref 10.

Figure 4. X-ray reflectivity curve from the LB film for compound 2 with six molecular layers.

The representative AFM images of the LB films for the compounds 1 and 2 are shown in Figure 5. The LB films possess an odd morphology with disrupted layers, cracks, and holes at various scales. The flat areas of the layers between surface defects reach 0.5-2 µm across. The microroughness of the films, rms, averaged over 2 µm × 2 µm areas that includes holes and cracks is in the range of 10-20 Å. This is much higher than the surface roughness usually observed for LB films made from discotic triphenylenes (rms is in the range of 4-5 Å).4 Nevertheless, the local roughness of the film surfaces within the flat areas (excluding macroscopic holes, cracks, and bumps) is much lower and falls in the range of 4-6 Å. This relatively small value reflects the smoothness of the molecular layers on a local, nanometer scale. Thus, the high level of the surface roughness and imperfection of the LB films, which reveals itself in the X-ray reflectivity data, is caused by macroscopic (as compared to the

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fairly well (within the precision of the measurements) to the molecular sizes of the compounds in the plane of the disks perpendicular to the spacers connecting the triphenylene and TNF fragments as is estimated from the molecular models (Figure 3). Within the experimental precision, these values agree with the X-ray data discussed above. Therefore, the AFM observations confirm the formation of the layered structures of the molecular layers of the LB films with the edge-on orientation of the discotic molecules (Figure 3). Conclusions

Figure 5. Typical surface morphology of the LB films studied: the AFM images of the LB film for compound 2 (a, 3 µm × 3 µm) and compound 1 (b, 100 nm × 100 nm) at various magnifications. Note the flat, smooth areas of a micron across and deep cracks in-between (a).

molecular scale) disruptions of the rather smooth molecular layers composed of discotic molecules. Obviously, the phase separation of the chemically connected donor and acceptor components cannot be a reason for the imperfections of the LB films from discotic materials because of the specific molecular design implemented. Apparently, the large-scale imperfections observed for the films transferred to the solid substrate are caused by local deformations and tensions arising during the transfer procedure of the monolayers from the airwater interface to the solid substrate. As is known, the bending deformations are due to the 90° angle between the planes of the air-water interface and the solid surface and the local tensions arise because of fast water removal from a meniscus area.15 The combination of these stimuli can lead to the significant mechanical deformation of the domain structure in the solid molecular films during deposition. For the films studied here, these deformations might result in microcracking due to the excessive rigidity of the compressed monolayers composed from stiff columns. This stiffness is a direct consequence of the expanded positional intracolumnar ordering promoted by the strong core-to-core CT interactions between the rigid flat fragments along the columnar axes and additional chemical linkages of the neighboring columns via spacers (see Figure 3). The thickness of the layers in the LB films can be determined from the height histograms for the selected surface areas around the surface defects. All the LB films studied have very similar thicknesses of a single molecular layer in the range of 21-25 Å. This value corresponds

In conclusion, the observations of the LB films from donor-acceptor discotic molecules reveal some features of the surface morphology of the columnar structures with strong core-to-core CT interactions. All the films fabricated display the edge-on orientation of the discotic molecules within the molecular layers. The LB films on the silicon surfaces possess a smooth local surface morphology but show large nonhomogeneity on the micron scale. The imperfections of the LB films are represented by the fractures and disruptions of the molecular layers with flat areas between the surface defects up to several microns across. The phase separation of the chemically connected donor and acceptor components can be excluded as a reason for the surface imperfections of the LB films because of the specific molecular design. Apparently, the large-scale imperfections observed for the films at the solid substrates are caused by the bending deformations and local tensions arising during the transfer of the compressed monolayers composed of the stiff columns with strong intracolumnar correlations from the air-water interface to the solid surfaces. One can speculate that possible ways to improve the transfer properties of the stiff monolayers from discotic materials with strong CT core-to-core interactions are the “dilution” of the columnar ordering with amphiphilic compounds of “elastic” type or changing the balance of intermolecular forces by the attachment of the discotic molecules to a polymer backbone with controlled architecture. The first attempts to improve the deposition by mixing the discotic compounds studied here with a stearic acid resulted in relatively smooth monolayers and multilayers. In addition, the attachment of the discotic triphenylene groups to a polymer backbone seems to be a promising way to fabricate smooth, perfect LB films from discotic compounds. These results will be published elsewhere.17 Acknowledgment. V.T. acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the NSF-DMR Research Opportunity Award for financial support, the Center for Materials Research, Stanford University, for use of their facilities, C. Frank for helpful discussions, T. Einloth, Jean Li, M. Laskowski, and G. Wayshunas for technical assistance, and Nihon University, Tokyo, and Wright Laboratories, WPAFB, for their hospitality during the preparation of the paper. The authors would like to thank A. Laschewsky and T. Bunning for reading the manuscript and providing helpful suggestions. LA9506133 (17) Janietz, D.; Festag, R.; Schmidt, C.; Wendorff, J. H.; Tsukruk, V. V. Thin Solid Films, submitted for publication.