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Ordered Monolayers of Nanographitic Sheets Processed from Solutions via Oxidative Cyclodehydrogenation Paolo Samorı´,†,§ Christopher D. Simpson,‡ Klaus Mu¨llen,*,‡ and Ju¨rgen P. Rabe*,† Department of Physics, Humboldt University Berlin, Invalidenstrasse 110, 10115 Berlin, Germany, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received December 22, 2001. In Final Form: March 4, 2002 We describe a new approach for solution processing of crystalline monolayers of an alkyl-substituted polycyclic aromatic hydrocarbon molecule on the basal plane of graphite. These layers are prepared by first adsorbing a soluble oligophenyl precursor at a solid-liquid interface and subsequently performing the oxidative cyclodehydrogenation reaction to the final fused product in-situ. Scanning tunneling microscopy imaging with submolecular resolution allowed visualization of the ordered nanostructures of both the precursors and the final organic adlayer, proving that the reaction had occurred.
An important goal in materials chemistry is the preparation of highly ordered mono- and multilayers from solutions of large organic (macro)molecules which, due to their size, cannot be processed by vacuum sublimation.1 The major disadvantage in employing this approach to π-conjugated molecules is their low solubility in organic solvents.2 This problem may be solved by adsorbing a soluble precursor on a surface and subsequently transforming it into the final product via a chemical reaction carried out on the surface or in the region near to it; using this strategy, one may produce organized thin films of nonsoluble materials from solutions. Scanning tunneling microscopy (STM) allows direct real-space imaging with a submolecular resolution of the structures of organic monolayers at different interfaces, for example, solidvacuum, solid-air, or solid-liquid interfaces.3 Polycyclic aromatic hydrocarbons (PAHs) are a widely investigated class of compounds,4,5 with interesting charge-transfer properties of their supramolecular assemblies.6 Recently, it has been shown that alkylated hexa-peri-hexabenzocoronenes, which are PAHs consisting of 42 carbon atoms * Corresponding authors: Prof. Dr. J. P. Rabe, Department of Physics, Humboldt University Berlin, Invalidenstrasse 110, 10115 Berlin, Germany; e-mail,
[email protected]. Prof. Dr. K. Mu¨llen, Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. † Humboldt University Berlin. ‡ Max-Planck-Institut for Polymer Research. § New address: Istituto per la Sintesi Organica e la Fotoreattivita `, C.N.R. Bologna, via Gobetti 101, 40129 Bologna (Italy). (1) (a) Feast, W. J.; Parker, D.; Winter, J. N.; Bott, D. C.; Walker, S. N. In Electronic Properties of Polymers and Related Compounds; Kuzmany, H., Mehring, M., Roth, S., Eds.; Springer Series in SolidState Science, Vol. 63; Springer: Heidelberg, 1985; p 45. (b) Prokhorova, S. A.; Sheiko, S. S.; Ahn, C.-H.; Percec, V.; Mo¨ller, M. Macromolecules 1999, 32, 2653-2660. (c) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A.-D. J. Am. Chem. Soc. 1998, 120, 7691-7695. (d) Grell, M.; Bradley, D. D. C. Adv. Mater. 1999, 11, 895-905. (e) Zhang, L.; Huo, F.; Wang, Z.; Wu, L.; Zhang, X.; Ho¨ppener, S.; Chi, L.; Fuchs, H.; Zhao, J.; Niu, L.; Dong, S. Langmuir 2000, 16, 3813-3817. (2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 110, 416-443; Angew. Chem., Int. Ed. Engl. 1998, 27, 402-428. (3) Gimzewski, J. K.; Joachim, C. Science 1999, 283, 1683-1688. (4) (a) Clar, E. Aromatische Kohlenwasserstoffe - Polycyclische Systeme; Springer: Berlin, 1952. (b) Clar, E. The Aromatic Sextet; John Wiley: London, 1972. (5) Fechtenko¨tter, A.; Watson, M. D.; Mu¨llen, K. Chem. Rev. 2001, 101, 1267-1300. (6) van de Craats, A. M.; Warman, J. M. Adv. Mater. 2001, 13, 130133.
in the aromatic core, can be produced from their precursor via an oxidative cyclodehydrogenation reaction on a flat surface. This reaction was accomplished by thermal heating at 710 K in an ultrahigh vacuum environment. Unfortunately, under these harsh conditions the structural stability of the molecules is very poor and they get easily de-alkylated.7 We describe here a STM investigation on the selfassembly at the graphite-solution interface8 of a soluble precursor of hexadodecyl-hexa-peri-hexabenzocoronene (2 in Figure 1), namely, hexadodecyl-hexaphenylbenzene (1 in Figure 1).9 The oxidative cyclodehydrogenation reaction of 1 made it possible to obtain ordered monolayers of 2 which have been visualized with STM, again with a submolecular resolution. Albeit that 2 and other alkylated hexa-peri-hexabenzocoronenes do exhibit enough solubility that permits them to be processed as thin films from solution, we have chosen this molecule as a model compound for bigger and consequently less soluble PAHs, which are to date synthetically affordable (3-7 in Figure 1).5 In addition, 2 is a well-known compound which has been intensely studied regarding its self-assembly behavior at surfaces.9 The STM study has been accomplished by making use of a home-built beetle type STM10 with STM probes which have been produced mechanically by cutting a 0.25 mm thick Pt/Ir (80%, 20%) wire. Almost-saturated solutions of 1 in 1,2,4-trichlorobenzene (TCB) have been applied to the basal plane of the freshly cleaved highly oriented pyrolytic graphite (HOPG) substrate. The underlying (7) Weiss, K.; Beernick, G.; Do¨tz, F.; Birkner, A.; Mu¨llen, K.; Wo¨ll, C. H. Angew. Chem. 1999, 111, 3974-3978; Angew. Chem., Int. Ed. Engl. 1999, 38, 3748-3752. (8) (a) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (b) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747-13759. (c) Claypool, Ch. L.; Faglioni, F.; Goddard, W. A., III.; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978-5995. (d) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2326-2342. (e) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550-5556. (f) Gesquie`re, A.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; De Schryver, F. C.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Langmuir 2000, 16, 10385-10391. (9) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem. 1995, 107, 1768-1770; Angew. Chem., Int. Ed. Engl. 1995, 34, 1609-1611. (10) Hillner, P. E.; Wolf, J. F.; Rabe, J. P. Humboldt University Berlin, Berlin, Germany. Unpublished results.
10.1021/la015760h CCC: $22.00 © 2002 American Chemical Society Published on Web 04/25/2002
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Figure 1. Chemical formulas.
Figure 2. (a,b) Scanning tunneling microscopy of 1 at the solution-HOPG interface. In (a), the black and white arrows indicate single or double missing molecules, respectively. In (c), 2 at the solid-liquid interface on graphite is shown. The two-dimensional Fourier transform is shown in the upper-right insets in (b) and (c). Tunneling parameters: (a,b) Ut ) 0.70 V and average It ) 50 pA; (c) Ut ) 0.50 V and It ) 50 pA.
HOPG lattice has been resolved during the measurements by varying the tunneling parameters which enabled us to calibrate the piezo in the xy plane. Average unit cells have been determined after their correction for the piezo drift (using SPIP Scanning Probe Image Processor, Version 1.720, Image Metrology ApS, Lyngby, Denmark). STM current images with a submolecular resolution have been recorded using scan rates of ∼20-50 lines/s. After having studied with STM the structure of the monolayer of 1 at the HOPG-solution interface using TCB as the solvent, this wet surface has been treated ex situ by adding a drop of FeCl3 in CH3NO2 on its top, which is known from solution experiments to induce the cyclodehydrogenation reaction yielding 2.5 After the reaction occurred (ca. 10 min), the sample has been rinsed several times with CH3NO2 in order to wash away all the ions (FeCl3) from the surface, and finally it was dried under a gentle stream of N2. The obtained films have been studied once more with STM applying a drop of TCB which allowed protection of the investigated area from the condensation of H2O in the air. A drop of the solution of the hydrophobic molecule of type 1, which can be regarded as the first generation of
a phenylene-based dendrimer, was cast on the hydrophobic surface of HOPG. This caused the molecular self-assembly at the solid-liquid interface of a polycrystalline monolayer. Two different types of molecular arrangements are shown in the STM image in Figure 2a. Structure I in Figure 2a denotes a domain with a hexagonal packing. In this crystal, several missing molecules can be recognized. Two different types of these defects are visible: the first one, marked with white arrows, consists of only one molecule missing; the second one, indicated with black arrows, exhibits two nearby molecules missing. These defects unambiguously prove that the observed contrast is the real picture of the single molecules adsorbed at the interface and it is not due to an artifact in the STM imaging or to a superstructure of the hexagonal HOPG lattice underneath. A zoom into a defect-free area is shown in Figure 2b. The unit cell corresponds to a ) (2.47 ( 0.10) nm, b ) (2.43 ( 0.10) nm, and R ) (59 ( 3)°, and thus its area is AI ) ab sin R ) (5.2 ( 0.6) nm2, which indicates that the unit cell is filled by only one molecule. The bright spots in the hexagonal packing of structure I in Figure 2a can be attributed to the π-conjugated cores of the molecules, because in STM
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current images of alkylated aromatic molecules on HOPG, the aromatic moieties appear brighter than the alkyl chains, due to the smaller energy difference between their frontier orbitals and the Fermi level of the substrate.11 This assignment is consistent with the fact that the area AI of the unit cell corresponds to the area within the van der Waals contour of a single molecule flattened on the surface. Due to their high conformational mobility on the time scale of the STM imaging, it was not possible to resolve the structures of the alkyl side chains. The second type of packing of 1 on HOPG is marked with II in Figure 2a. The molecules are arranged in nanorods. The unit cell of this crystal structure amounts to a ) (3.5 ( 0.2) nm, b ) (6.3 ( 0.2) nm, and R ) (76 ( 4)°, and therefore its area is AII ) (21 ( 2) nm2. Comparing this latter one with AI, it appears evident that the unit cell of type II is made up of more molecules, at least four, which would be the case of an arrangement of 1 lying as much as possible flat on HOPG. The molecular physisorption from solutions at the solidliquid interface is governed by the interplay of the enthalpic gain upon adsorption and the entropic loss due to the reduction of the dimension from 3D to 2D.12 It is very likely that structure I is characterized by a maximum overlap of the electronic orbitals of the molecule with the ones of the HOPG substrate which provides the maximization of the enthalpic gain upon the maximization of the packing density at the interface, while structure II exhibits an inferior overlap of the electronic states of molecules and substrate. Nevertheless, this type of arrangement appears to be equally energetically favored, most probably because (i) it allows a maximization of the intermolecular interactions and (ii) it reduces the entropic loss. Another type of defect that can be observed in Figure 2a is the fuzzy interface which separates domains I and II. In this region, the molecules are likely to be weakly bound to the HOPG surface, thus characterized by molecular dynamics which is faster than the rate of the STM imaging. For this reason, their structures have not been properly resolved. Figure 2c displays the STM images at the solid-liquid interface of the organic adsorbate after the cyclodehydrogenation reaction. The 2D structure is completely (11) Lazzaroni, R.; Calderone, A.; Bre´das, J. L.; Rabe, J. P. J. Chem. Phys. 1997, 107, 99-105. (12) Samorı´, P.; Francke, V.; Mu¨llen, K.; Rabe, J. P. Adv. Mater. 2000, 12, 579-583.
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different from the one observed for 1 (Figure 2a,b). It is characterized by an oblique unit cell: a ) (1.84 ( 0.10) nm, b ) (2.71 ( 0.10) nm, R ) (79 ( 3)°, and an area AIII ) (4.9 ( 0.5) nm2. The presence or absence of the covalent bonds between the phenylene units in the first generation of the dendrimer, for 2 and 1, respectively, induces the molecule to adopt a disklike or propeller-like conformation, which is reflected in the symmetry of the packing on solid surfaces. In fact, while in the first case the flat molecular shape enables an ideal overlap of the π molecular orbitals with the HOPG states near the Fermi level, in the latter case this overlap is more hindered by the three-dimensional structure of the molecule. Since the here-determined values of the unit cell of III are in very good agreement with previous studies on 2,9 it is concluded that the cyclodehydrogenation reaction has taken place. STM being a local probe technique, it is difficult to gain direct insights into the yield of the reaction. Nevertheless, during all the measurements which were carried out it was not possible to observe coexisting 2D crystals of 1 and 2 suggesting a reasonably high yield of the reaction. Unfortunately, due to the presence of ions as reactant (FeCl3) it was not possible to visualize the reaction of 1 leading to 2 in situ by means of STM. This did not allow us to cast light onto the mechanism of the reaction, namely, whether it took place at the surface or in the region near to it, via the desorption of 1 followed by a readsorption of 2. However, the similar size of the unit cells of AI and AIII suggests that the reaction may proceed at the surface, although no conclusive evidence can be provided so far. In conclusion, we have shown a new practical approach which makes it possible to process from solutions crystalline layers of large functionalized PAHs. This result opens perspectives for the growth of ordered layers of insoluble even larger PAHs which exhibit physicochemical properties which approach those of graphite.5 Acknowledgment. This work was supported by TMR project SISITOMAS, the Volkswagen-Stiftung (Elektronentransport durch konjugierte molekulare Scheiben und Ketten), the European Science Foundation through SMARTON, and the German “Bundesministerium fu¨r Forschung und Technologie” as part of the program “Zentrum fu¨r multifunktionelle Werkstoffe und miniaturisierte Funktionseinheiten”. LA015760H
