Two-Dimensional Self-Assembly of Linear Molecular Rods at the

Jan 12, 2011 - Faculty of Chemistry, Northeast Normal University, 130024 Changchun, Jilin, PR China. ∥ Karlsruhe Institute of Technology (KIT), Inst...
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Two-Dimensional Self-Assembly of Linear Molecular Rods at the Liquid/ Solid Interface† ,^,#

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Zhongcheng Mu,‡,§ Lijin Shu,

Harald Fuchs,‡ Marcel Mayor,*,

and Lifeng Chi*,‡

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Physikalisches Institut, Westfaelische Wilhelm-Universitaet Muenster, Wilhelm-Klemm-Strasse 10, 48149 Muenster, Germany, §Faculty of Chemistry, Northeast Normal University, 130024 Changchun, Jilin, PR China, Karlsruhe Institute of Technology (KIT), Institute for Nanotechnology, Postfach 3640, 76021 Karlsruhe, Germany, ^DFG Center for Functional Nanostructures (CFN), 76028 Karlsruhe, Germany, and #Universit€ at Basel, Departement Chemie, St. Johannsring 19, 4056 Basel, Switzerland Received October 5, 2010. Revised Manuscript Received December 9, 2010 We report on the synthesis and scanning tunneling microscopy (STM) studies of a series of linear molecular rods (1-5) comprising different numbers and/or spatial arrangements of perfluorinated benzene and benzene subunits interlinked with diacetylenes in the para position and decorated with or without terminal dodecyl chains. The molecules organize themselves into well-ordered 2D crystal structures at the liquid/solid interface through intermolecular and molecule-substrate interactions. Whereas the molecules substituted by dodecyl chains form the lamellar structures with alternating rigid core rows and alkyl chain rows, the unsubstituted ones change the orientation of the rigid backbones with respect to the lamellar axis. The molecular arrangement is not influenced by fluoro substituents on any phenyl ring of the backbone, which suggests that the interactions between the π-conjugated backbones are dominated by close packing rather than by the dipole moments of the rods or fluorine-based intermolecular interactions.

Introduction π-conjugated oligomers and polymers serve as organic semiconducting materials, which have attracted increasing interest because of their potential applications as active layers in optoelectronic devices1-3 and as molecular nanowires in electronic devices.4 The device performances of organic semiconductors strongly depends on the molecular spatial arrangement and the morphology of the solid material in addition to the molecule’s intrinsic properties.5 Therefore, an understanding of the conformation, orientation, and alignment of the molecules is of great importance in the improvement of the materials’ physical properties. Among many molecule-based devices, molecular wires are basic components that are normally rigid-rod-like molecular structures comprising extended electronic conjugation to convey either charge or excited energy from one active unit to another. The ability of various molecular wires to transport electrons or excited states was initially investigated by spectroscopic and/or † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding authors. E-mail: [email protected]; chi@ uni-muenster.de.

(1) Bourroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741–1744. (4) (a) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864–11873. (b) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, G.; Smith, P. Science 1998, 279, 835–37. (c) Montali, A.; Bastiaansen, G.; Smith, P.; Weder, C. Nature 1998, 392, 261– 264. (d) Moroni, M.; Le Moigne, J.; Pham, A.; Bigot, J. Y. Macromolecules 1997, 30, 1964–1972. (e) Samorí, P.; Severin, N.; M€ullen, K.; Rabe, J. P. Adv. Mater. 2000, 12, 579–582. (f) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 5885–5886. (g) Kim, J.; Swager, T. M. Nature 2001, 411, 1030–1034. (h) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705–1707. (5) (a) Garnier, F.; Yaasar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716–8721. (b) B€auerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 303–307. (c) Neher, D. Adv. Mater. 1995, 7, 691–702. (6) Molecular Wires; De Cola, L., Ed.; Topics in Current Chemistry; SpringerVerlag: Berlin, 2005; Vol. 257.

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electrochemical methods with tailor-made model compounds.6 More recently, the transport features of typical π-conjugated rigid-rod-type molecular wires such as oligo(p-phenyleneethynylene)s (OPEs),4h,7 oligo(p-phenylenevinylene)s (OPVs), and oligo(p-phenylene)s (OPs)9 comprising terminal sulfur anchor groups were investigated on a single-molecule level. In spite of the singlemolecule character of these experiments, intermolecular stacking of molecular rods comprising electron-rich and -poor subunits has been observed in molecular junctions.10 Furthermore, the dipole moment of molecular rods comprising electron-poor perfluorinated subunits enabled single-molecule rectification.11 Quadrupole stacking interactions between benzene and perfluorobenzene subunits have been used as the structural motive for arranging the molecular packing in the solid state,12 in liquid crystals,13 and in the aggregates in solution. In particular, the aggregation properties of macrocycles consisting of these subunits interlinked by diacetylenes were investigated.14 Whereas OPEs as shape-persistent oligomers were proposed as molecular wires in electronic devices,15 less attention was given to oligo(p-phenylenediethynylene)s (OPDEs). The self-assembly of π-conjugated molecules on solid supports was investigated by utilizing scanning tunneling microscopy (STM),16which can provide structural details of a self-organized (7) (a) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. B.; Mayor, M.; v. L€ohneysen, H. Phys. Rev. Lett. 2002, 88, 176804-1–176804-4. (b) Mayor, M.; Weber, H. B.; Reichert, J.; Elbing, M.; von H€anisch, C.; Beckmann, D.; Fischer, M. Angew. Chem., Int. Ed. 2003, 42, 5834–5838. (8) (a) Hassenkam, T.; Moth-Poulsen, K.; Stuhr-Hansen, N.; Nørgaard, K.; Kabir, M. S.; Bjørnholm, T. Nano Lett. 2004, 4, 19–22. (b) Huber, R.; Gonzalez, M. T.; Wu, S.; Langer, M.; Grunder, S.; Horhoiu, V.; Mayor, M.; Bryce, M. R.; Wang, C.; Jitchati, R.; Sch€onenberger, C.; Calame, M. J. Am. Chem. Soc. 2008, 130, 1080–1084. (9) L€ortscher, E.; Elbing, M.; Tschudy, M.; von H€anisch, C.; Weber, H. B.; Mayor, M.; Riel, H. ChemPhysChem 2008, 9, 2252–2258. (10) Wu, S.; Gonzalez, M. T.; Huber, R.; Grunder, S.; Mayor, M.; Sch€onenberger, C.; Calame, M. Nat. Nanotechnol. 2008, 3, 569–574. (11) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; von H€anisch, C.; Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8815–8820.

Published on Web 01/12/2011

DOI: 10.1021/la1040079

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Mu et al. Scheme 1. Synthesis of Molecular Rods 1-5a

a (a) NBS, AgNO3, CH3COCH3, ∼20C. (b) Pd2(dba)3 3 CHCl3, CuI, C6H5CH3, EtN(i-prop.)2, ∼20C. (c) TBAF, wet THF, ∼20C. (d) PdCl2(PPh3)2, CuI, THF, EtN(i-prop.)2, ∼20C. (e) DIBAL-H, C6H5CH3, ∼20C. (f) CBr4, Zn, PPh3, CH2Cl2, ∼20C. (g) KOH(aq), BTEAC, THF, ∼20C.

adlayer. Previous investigations have addressed the self-assembly of π-conjugated oligomers, such as oligo(thiophene)s,17OPEs,18 OPVs,19 and phenylene-ethynylene-butadiynylenes (PEBs).20 Most of these molecules were substituted with alkyl groups in the middle of a rigid backbone, which is mainly used to improve their solubility but also served as flexible intermolecular glue that is able to compensate for minor geometrical mismatches of the rigid motive. However, the introduction of the side chains may lead to a dilution of the electronically active function of the molecule and also may decrease the degree of conjugation due to the torsion of the backbone induced by the additional steric (12) (a) Patrick, C. R.; Prosser, G. S. Nature 1960, 187, 1021–1021. (b) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs, R. H. Angew. Chem., Int. Ed. 1997, 36, 248–251. (c) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini, M.; Siegel, J. S. Angew. Chem., Int. Ed. 1995, 34, 1019–1020. (d) Kendall, J.; McDonald, R.; Ferguson, M. J.; Tykwinski, R. R. Org. Lett. 2008, 10, 2163–2166. (e) Feast, W. J.; Lovenich, P. W.; Puschmann, H.; Taliani, C. Chem. Commun 2001, 505–506. (f) Collings, J. C.; Batsanov, A. S.; Howard, J. A. K.; Dickie, D. A.; Clyburne, J. A. C.; Jenkins, H. A.; Marder, T. B. J. Fluorine Chem. 2005, 126, 515–519. (g) Smith, C. E.; Smith, P. S.; Thomas, R. L.; Robins, E. G.; Collings, J. C.; Dai, C.; Scott, A. J.; Borwick, S.; Batsanov, A. S.; Watt, S. W.; Clark, S. J.; Viney, C.; Howard, J. A. K; Clegg, W.; Marder, T. B. J. Mater. Chem. 2004, 14, 413–420. (h) Meejoo, S.; Kariuki, B. M.; Harris, K. D. M. ChemPhysChem 2003, 4, 766–769. (i) Collings, J. C.; Roscoe, K. P.; Robins, E. G.; Batsanov, A. S.; Stimson, L. M.; Howard, J. A. K.; Clark, S. J.; Marder, T. B. New J. Chem. 2002, 26, 1740–1746. (j) Jankowski, W.; Gdaniec, M.; Polonski, T. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2006, C62, o492– o494. (k) Ponzini, F.; Zagha, R.; Hardcastle, K.; Siegel, J. S. Angew. Chem., Int. Ed. 2000, 39, 2323–2325. (l) Zhu, S.; Zhu, S.; Jin, G.; Li, Z. Tetrahedron Lett. 2005, 46, 2713–2716. (13) (a) Watt, S. W.; Dai, C.; Scott, A. J.; Burke, J. M.; Thomas, R. L.; Collings, J. C.; Viney, C.; Clegg, W.; Marder, T. B. Angew. Chem., Int. Ed. 2004, 43, 3061– 3063. (b) Kishikawa, K.; Oda, K.; Aikyo, S.; Kohmoto, S. Angew. Chem., Int. Ed. 2007, 46, 764–768. (c) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E. B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38, 2741–2745. (14) (a) Shu, L.; Mayor, M. Chem. Commun. 2006, 4134–4136. (b) Shu, L.; M€uri, M.; Krupke, R.; Mayor, M. Org. Biomol. Chem. 2009, 7, 1081–1092. (15) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–804. (16) Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. Chem. Soc. Rev. 2009, 38, 402–421. (17) (a) B€auerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. 1995, 34, 303–307. (b) Stecher, R.; Gompf, B.; M€unter, J. R. S.; Effenberger, F. Adv. Mater. 1999, 11, 927–931. (c) Azumi, R.; Gotz, G.; Debaerdemaeker, T.; Bauerle, P. Chem.;Eur. J. 2000, 6, 735–744. (d) Mena-Osteritz, E. Adv. Mater. 2002, 14, 609–616. (18) (a) Samorı´ , P.; Francke, V.; M€ullen, K.; Rabe, J. P. Chem.;Eur. J. 1999, 5, 2312–2317. (b) Samorí, P.; Francke, V.; Enkelmann, V.; M€ullen, K.; Rabe, J. P. Chem. Mater. 2003, 15, 1032–1039. (c) Gong, J.-R.; Zhao, J.-L.; Lei, S.-B.; Wan, L.-J.; Bo, Z.-S.; Fan, X.-L.; Bai, C.-L. Langmuir 2003, 19, 10128–10131. (d) Mu, Z.; Yang, X.; Wang, Z.; Zhang, X.; Zhao, J.; Bo, Z. Langmuir 2004, 20, 8892–8896. (e) Yoosaf, K.; James, P. V.; Ramesh, A. R.; Suresh, C. H.; George Thomas, K. J. Phys. Chem. C 2007, 111, 14933–14936. (19) Gesquiere, A.; Jonkheijm, P.; Schenning, A. P. H. J.; Mena-Osteritz, E.; B€auerle, P.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. J. Mater. Chem. 2003, 13, 2164–2167. (20) Jester, S.-S.; Shabelina, N.; Le Blanc, S. M.; H€oger, S. Angew. Chem., Int. Ed. 2010, 49, 6101–6105.

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requirements of the alkyl chain.21 There are a few π-conjugated oligomers without a side chain on the middle repeat units, which present better properties of electronics and optoelectronics as recently reported.22 So far, less attention has been paid to the properties of these compounds at interfaces. By investigating the self-assembly of OPDE dimer 5 comprising a terminal dodecyl chain on one end and a pentafluorobenzene terminal unit on the other end, we observed the formation of stripes over large areas. The lateral pattern that emerged from the pairwise antiparallel arrangement of the rigid rod which might be induced by its dipole moment. Furthermore, the close packing seemed to be supported by an intermolecular electrostatic fluorine diacetylene attraction.23 The intermolecular packing of rigidrod-type molecular architectures comprising perfluorinated subunits was further improved with Ar-H 3 3 3 F hydrogen bonds24 in star-shaped structures, resulting in porous chiral networks.25 In the present work, we compare the self-assembly of various OPDE derivatives in order to identify the structural motives dictating the resulting lateral pattern on highly oriented pyrolytic graphite (HOPG). In particular, OPDE rods with or without perfluorinated subunits and/or an alkyl chain located at the end of the π-conjugated backbones (Scheme 1) are considered. The selfassembled structures of these molecules directly reflect the interactions between π-conjugated backbones, the effects of peripheral substituents, and the interaction between the molecule and the substrate. In particular, the adsorption of a π-conjugated molecule without further substituents such as functional groups or alkyl chains at the liquid/solid interface provides us with a valuable clue in developing supramolecular self-assemblies not mediated by alkyl groups and H bonds under ambient conditions. Furthermore, the comparison of similar rigid rods with and without perfluorinated benzene subunits allows us to investigate the role of the rigid-rod dipole and intermolecular interactions for the emerging lateral pattern. The results provide helpful information for the comprehension of molecular self-assembly associated with chemical structures and thus for the future design of (21) M€ullen, K.; Wegner, G. Electronic Materials: The Oligomeric Approach; Wiley-VCH: Weinheim, Germany, 1998. (22) Fenenko, L.; Shao, G.; Orita, A.; Yahiro, M.; Otera, J.; Svechnikov, S.; Adachi, C. Chem. Commun. 2007, 2278–2208. (23) Shu, L.; Mu, Z.; Fuchs, H.; Chi, L.; Mayor, M. Chem. Commun. 2006, 1862–1863. (24) Barrena, E.; de Dteyza, D. G.; Dosch, H.; Wakayama, Y. ChemPhysChem 2007, 8, 1915–1918. (25) Mu, Z.; Shu, L.; Fuchs, H.; Mayor, M.; Chi, L. J. Am. Chem. Soc. 2008, 130, 10840–10841.

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π-conjugated polymers for organic optoelectronic devices on solid surfaces.

Experimental Section Synthesis and Characterization of Molecular Rods 1-5. The synthesis of molecular rods 1-5 is displayed in Scheme 1. The assembly strategy for 1-5 is based on diacetylene formation reactions26 such as Pd/Cu-catalyzed variations of the oxidative acetylene coupling reaction or of the Cadiot-Chodkiewitz coupling reaction (Pd/Cu CCCR).27 The required ethynyl-functionalized building blocks have been synthesized by either Sonogashira coupling reactions28 or by a Corey-Fuchs-type reaction sequence.29 Bromoethynyl-functionalized building blocks have been obtained by the bromination of the corresponding acetylenes. Acetylenes 6 and 10 have been synthesized according to a literature procedure.23,30 The bromination of 6 by NBS in the presence of catalytic amounts of AgNO3 in acetone provided bromoethynyl derivate 7 as a yellow oil in 97% yield. The Pd/Cu CCCR allowed us to elongate 7 with phenylethynyl to silylprotected ethynyl 8. Thus 7, phenylethynyl, and catalytic amounts of Pd2(dba)3 3 CHCl3 and CuI in a mixture of toluene and ethyldiisopropylamine (EtN(i-Prop)2) have been stirred at room temperature (∼20 C). After workup and column chromatography (CC), 8 has been isolated as a yellow oil in 57% yield. Subsequent deprotection by tetrabutylammoniumfluoride (TBAF) provided free acetylene 9, which has been subjected to Pd/Cu-catalyzed homocoupling conditions. Treatment with catalytic amounts of Pd(Ph3)2Cl2 and CuI in a mixture of tetrahydrofurane (THF) and EtN(i-Prop)2 provided linear tetramer 1 as a white solid in 55% yield after CC and size exclusion chromatography (SEC). Pd/Cu CCCR also allowed us to elongate dodecyl-functionalized phenylethynyl 10 with bifunctional bromoethynyl 7 to silyl-protected ethynyl rod 11 in 54% yield. Subsequent deprotection by TBAF (93%) followed by bromination with NBS provided bromoethynyl 13 in 90% yield, which was elongated with phenylethynyl again by applying Pd/Cu CCCR to provide dodecyl-functionalized trimer 2 as a beige solid in 38% yield. Dodecyl-functionalized dimer 3 was obtained as a white solid in 62% yield from bromoethynylbenzene and 10 exposed to Pd/Cu CCCR. 1,4-Dibromoethynyl-2,3,5,6-tetrafluorobenzene 17 has been synthesized from 2,3,5,6-tetrafluoroterephthalonitrile 14 in three steps with an overall yield of 35%. The reduction of 14 with DIBAL-H in toluene provided dialdehyde 15 as a white solid in 43% yield. In a Corey-Fuchs reaction sequence, dialedehyde 15 was converted to di(dibromoethenyl) 16 in 85% yield by treatment with CBr4, zinc, and PPh3 in dichloromethane (DCM). Subsequent HBr elimination provided required dibromoethynyl 17 as a white solid in 96% yield. The treatment of acetylene 10 with a 4-fold excess of bifunctional dibromoethynyl 17 under Pd/ Cu CCCR conditions provided elongated bromoethynyl 18 as a yellow solid in 56% yield. Similar reaction conditions applied to the 2,3,4,5,6-pentafluoro-bromoethynylbenzene23 and ethynyl 6 provided partially fluorinated molecular rod 19 comprising a silylprotected acetylene in 73% yield. Deprotection by TBAF provided molecular rod 20 comprising a terminal pentafluorobenzene and a free acetylene on opposite ends in 68% yield. Two partially fluorinated molecular rods 18 and 20 have again been coupled under Pd/Cu CCCR conditions to provide tetramer 4 with (26) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632–2657. (27) (a) Chodkiewicz, W. Ann. Chim. (Paris) 1957, 13, 819. (b) Ghose, B. N.; Walton, D. R. M. Synthesis 1974, 12, 890–891. (c) Wityak, J.; Chan, J. B. Synth. Commun. 1991, 21, 977–979. (28) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467–4470. (b) Sonogashira, K. In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, Germany, 1998; p 203. (29) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769–3772. (30) Weil, T.; Reuther, E.; Beer, C.; M€ullen, K. Chem.;Eur. J. 2004, 10, 1398– 1414.

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alternating fluorinated and unfluorinated benzene subunits in 54% yield. As already reported, comparable reaction conditions also provided molecular rod 5.23 The molecular structures of molecular rods 1-5 and their precursors 6-20 have been confirmed by 1H NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, compounds 2-20 have been characterized by 13C NMR spectroscopy. Unfortunately, the limited solubility of unfunctionalized tetramer 1 did not allow its characterization by 13C NMR spectroscopy. However, the expected values in its 1H NMR, MALDI-ToF-MS, and elemental analysis together with its STM profile leave no doubt concerning the molecular structure. In addition, 19F NMR spectroscopy of the fluorinated compounds further corroborated their structural identity. Scanning Tunneling Microscopy. STM investigations were performed by using a commercial multimode Nanoscope III scanning tunneling microscope (Digital Instrument Co., Santa Barbara, CA) with mechanically cut Pt/Ir (90:10) tips at ambient temperature. The images shown were recorded in constantcurrent mode if not otherwise indicated. For measurements at the solution/substrate interface, a saturated solution of the rigid-rod derivatives in 1-phenyloctane (purchased from Aldrich and used without further purification) was applied to a freshly cleaved surface of highly orientated pyrolytic graphite (HOPG; MaTeck GmbH). Measurement conditions are given in the corresponding figure captions. Different tips and samples were used to check for reproducibility and to ensure that there are no image artifacts caused by the tips or samples. Unit cell parameters and measurements obtained from STM images were calibrated in situ using the HOPG substrate. Flattening of the images was carried out to compensate for the tilting of the substrate and scan line artifacts, and a low-pass filtered transform was employed to remove scanning noise in the STM images.

Results and Discussion Figure 1 shows the STM image of a monolayer of molecule 1 physisorbed at the liquid/graphite interface from 1-phenyloctane solution. From the large-scale image (Figure 1a), it is clearly observable that molecule 1 spontaneously forms a homogeneous, ordered lamella pattern with an area of more than 4  104 nm2. This large-area ordered structure can be preserved during the STM experiment, indicating a stable, robust monolayer.16 The white arrow in Figure 1a indicates the direction of the lamella. The lack of alkyl chains or additional functional groups in rigid rod 1 compared with typical STM investigations on the molecular assembly at the liquid/solid interface is noteworthy. The stable monolayer of 1 even allows us to perform high-resolution STM imaging as shown in Figure 1b, which reveals that the molecules orient with their long axes parallel to the graphite surface. The width of a lamella is about 2.58 ( 0.1 nm, which is shorter than the length of a molecule (3.34 nm according to MM2 calculations), indicating that the molecule should be tilted relative to the direction of a lamella. To recognize an individual molecule in the image, we measured the distance (0.95 ( 0.1 nm) between two neighboring aryl rings and the contour length (3.25 ( 0.1 nm) of four adjacent aryl rings in a line within a lamella, which are consistent with the length of corresponding moieties of a molecule. An angle of 51 ( 2 between the long molecular axis and the direction of a lamella is found. After obtaining the pattern of 1, we also imaged the underlying substrate, which reveals that the long molecular axis is not parallel to the direction of one of the main graphite axes but has an angle of 9 ( 2, as shown in Figure 1c. On the basis of the above analysis, a packing model was superimposed in the image, as shown in Figure 1b. The molecular backbones of 1 are parallel to each other in one lamella, and the neighboring molecule in the same lamella is shifted along its long DOI: 10.1021/la1040079

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Figure 1. (a) Large-scale STM constant-current image of 1 physisorbed onto a graphite surface (50 nm  50 nm, Vbias = 0.5 V, Iset = 0.5 nA), with an arrow indicating the direction of the lamellae. (b) High-resolution STM constant-current image of 1 (12.0 nm  12.0 nm, Vbias = 0.4 V, Iset = 1.6 nA). The proposed packing model is superimposed. (c) High-resolution STM constant-current image of 1 (12.0 nm  12.0 nm, Vbias = 0.4 V, Iset = 1.6 nA), where the inset is an STM image of the underlying substrate (4.0 nm  4.0 nm, Vbias = 0.05 V, Iset = 1.8 nA). Arrow A indicates the direction of the long molecular axis of 1, and arrow B indicates the direction of one of the main graphite axes.

Figure 2. (a) STM constant-current image of 2 physisorbed onto a graphite surface from 1-phenyloctane solution (18.0 nm 18.0 nm, Vbias = -0.8 V, Iset = 0.1 nA). (b) STM constant-current image of 3 physisorbed onto a graphite surface from 1-phenyloctane solution (15.0 nm  15.0 nm, Vbias = -0.95 V, Iset = 0.55 nA).

axis by the length of a gearing repeat unit, leading to the formation of close-packing structure with a distance between two adjacent molecular axes of about 0.54 nm. The unit cell parameters were determined to be a = 3.44 ( 0.1 nm, b = 0.71 ( 0.1 nm, and R = 52.8 ( 2, as indicated in Figure 1b. The dimensions of the unit cell also allowed us to determine the length of the gearing repeat unit as the projection of the short side of the rhombus onto its long side. Because the short side and its projection on the long side define a right-angled triangle from which the length of the short side (b) and its angle with the long side (R) are known, the length of the projection can easily be calculated by the geometric rules of a right-angled triangle. In general, the structure of a physisorbed monolayer is governed by adsorbate-substrate and adsorbate-adsorbate noncovalent interactions, such as hydrogen bonding, electrostatic forces, and van der Waals and dipolar interactions. In the present case, the molecule does not bear the functional groups enabling the formation of hydrogen bonds or the variation of the electronic distribution within the backbone, thus only van der Waals forces remain as interactions responsible for the pattern formation. Obviously, the densely packed molecular pattern is entropically driven by the large number of released solvent molecules from the surface upon pattern formation. Because the molecules are confined on the surface, π-π stacking interactions can be observed only between the molecules in the monolayer and the conductive substrate in 2D,18b which becomes a potential driving force responsible for the formation of the monolayer. Therefore, the formation of monolayer 1 probably arises from a combination 1362 DOI: 10.1021/la1040079

of van der Waals forces and π-π stacking interactions. Although the later is expected only between the molecules and the substrate, the first interaction may also develop between the parallel-arranged molecular backbones. Because the stable lateral monolayer pattern of “naked” unsubstituted molecule 1 observed above was to some extent surprising, we wondered to what extent the molecular arrangement will be influenced by an additional alkyl chain or by substituting a benzene subunit with a perfluorobenzene subunit. To investigate the interaction mechanism between rigid cores and the influences of terminal alkyl substituents on the 2D structures, the following experiments were performed. First, we introduced only a dodecyl chain at one end of the rigid backbones with different repeat units (molecules 2 and 3) to examine the arrangement of the rigid backbone and the effects of the alkyl chain. As seen in the STM image (Figure 2), both 2 and 3 form lamellar structures with alternating molecular backbone rows and alkyl rows. In a molecular backbone row of either 2 or 3, the rigid moieties are interlocked by means of matching a phenyl ring to a diacetylene moiety of a neighboring molecule, which is very similar to the arrangement of 1. The arrangement of rigid backbones does not depend on the number of unsaturated repeat units. This structural feature suggests the presence of affinities between backbones. In contrast to the structure of 1, the distinction is the angles between the long axes of rigid moieties and the lamellar direction, which are 87 ( 2 for 2 and 73 ( 2 for 3. It is indubitable that this difference results from the introduction of an alkyl chain. For molecules 2 and 3, the angles between the long Langmuir 2011, 27(4), 1359–1363

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Article

Figure 3. (a) STM constant-current image of 4 physisorbed onto a graphite surface from 1-phenyloctane solution (21.74 nm  21.74 nm, Vbias = -0.95 V, Iset = 0.6 nA). (b) STM constant-current image of 5 physisorbed onto a graphite surface from 1-phenyloctane solution (20.0 nm  20.0 nm, Vbias = -0.90 V, Iset = 0.6 nA).

axis of rigid moieties and the direction of the alkyl chain are 156 ( 2 and 155 ( 2, respectively, which are considerably larger than 120 for the lattice symmetry of HOPG and 108 for the sp3 C atom, suggesting that molecules 2 and 3 lie flat on the graphite surface with planar conformations. Usually, alkyl chains align along one of the main axes of HOPG, from where we can deduce that the long axes of rigid backbones of 2 and 3 do not align along one of the symmetry axes of HOPG. In this regard, the arrangement of rigid moieties of 2 and 3 is similar to that of 1. For the large-area lateral pattern of molecular rods 1-3, the main design rule seems to be the optimization of the density of the flat-lying molecules in order to maximize the covered surface area. The 2D unit cell parameters of the 2 monolayer are a = 4.06 ( 0.1 nm, b = 0.76 ( 0.1 nm, and R = 59 ( 3, whereas the same parameters of the 3 monolayer are a = 2.50 ( 0.05 nm, b = 0.80 ( 0.03 nm, and R = 50 ( 3. The above experimental results indicate that the introduction of an alkyl group at the end of the rigid backbone changes only the orientation of the rigid cores with respect to the lamellar direction rather than changing the alignment of the rigid moieties. Because we hypothesized concerning the additional stabilization of lateral patterns due to fluorine-diacetylene interactions23 and Ar-F 3 3 3 H hydrogen bonds25 in earlier studies, the effects of perfluorinated aryl subunits in the rigid rod on the resulting selfassembled pattern were of particular interest. Thus, molecular rods 4 and 5 displayed in Scheme 1 were investigated. Attempts to synthesize a fluorinated structural analogue of 1 without a terminal dodecyl chain failed because of the miserable solubility features of the precursors, indicating increased quadrupole stacking interactions between the differently functionalized benzene subunits.12 Upon adsorption on a graphite surface, both 4 and 5 formed typical lamellar structures, as presented in Figure 3. The alignment of molecular rigid moieties is almost identical to what we observed above for the rigid rods lacking peripheral fluorine atoms. Also, the interdigitation of the alkyl chains strongly resembles the lateral pattern of 2 or 3. The slight difference is that the contrast of fluorine-substituted benzene subunits is lower than others because of the different electron density induced by the electron-withdrawing F atom. The distance between two adjacent moieties of rigid rods is 0.55 nm, which is almost equal to that of 1, indicating that fluoro groups do not influence the arrangement of rigid moieties. The unit cell parameters of the monolayer formed by 4 were determined to be a = 4.87 ( 0.1 nm, b = 0.77 ( 0.1 nm, and R = 44 ( 3. The adsorbed structure of molecule 5 with unit cell parameters of a = 2.54 ( 0.1 nm, b = 0.83 ( 0.1 nm, and R = 131 ( 2, which are almost the same as Langmuir 2011, 27(4), 1359–1363

those of 3, suggests that molecules 5 and 3 form very comparable 2D supramolecular patterns. The electron-poor terminal pentafluorobenzene of 5 increases the dipole moment of the rigid rod structure considerably compared to that of 3, thus an antiparallel arrangement of the rod is expected. However, because 3 already self-assembles in an antiparallel manner in order to optimize the surface coverage, the similarity of the packing motives of 3 and 5 is not surprising. Because both effects, the surface covering and antiparallel arrangement of the rods’ dipole, are steering toward the same surface pattern, the experiment is not suited to quantify their contributions. Investigated molecules 2-5 have their rigid-rod OPDE backbone and their terminal dodecyl chain in common, and the obtained 2D structures also displayed striking similarities. In all cases, the rigid rod subunits arrange in an antiparallel manner with the more spacious aryl subunits penetrating the thinner diacetylene pocket of the neighboring rod, resulting in long rows consisting of lateral staples of geared rods. These extensive rows are decorated on both sides with dodecyl chains, and their intedigitation results in the parallel arrangement of the rows. The striking similarity of the patterns obtained from the fluorinated and the unfluorinated rods suggests that mainly lateral arrangement features dominate the self-assembly. The close packing by geared rigid rods forming lateral stacks has also been found in the self-assembly of unfunctionalized rigid rod 1.

Conclusions We have shown that linear molecular rods on a graphite surface can spontaneously form well-defined monolayers under ambient conditions. Unsubstituted molecule 1 forms a stable monolayer driven by van der Waals interactions and molecule-substrate interactions. The introduction of an alkyl chain at the end of the backbone influences the orientation of the rigid backbone with respect to the lamellar axis, instead of varying the arrangement of rigid moieties. Our results also demonstrate that the introduction of perfluorinated phenyl units does not change the arrangement of the linear rigid backbone. The results confirm that the supramolecular self-assembly of unsubstituted π-conjugated oligomers at the liquid/solid interface is possible. Supporting Information Available: Synthesis protocols and characterization of rigid rod structures 1-5. This material is available free of charge via the Internet at http:// pubs.acs.org. DOI: 10.1021/la1040079

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