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C-H···F Hydrogen Bonding: The Origin of the Self-Assemblies of Bis(2,2’-difluoro-1,3,2-dioxaborine) Dirk Rohde, Cun-Ji Yan,† and Li-Jun Wan* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China ReceiVed NoVember 20, 2005. In Final Form: March 17, 2006 The effect of the molecular structure on the self-assembly of specially designed two-core 1,3,2-dioxaborines has been studied with various techniques. It was found that the molecules spontaneously adsorbed on HOPG surfaces and self-organized into well-ordered two-dimensional (2D) monolayers. The structural details of the 2D assemblies were investigated by scanning tunneling microscopy (STM). From X-ray analysis of the corresponding three-dimensional (3D) crystal and from theoretical calculation, we were able to reveal the driving force behind the specific self-assembly. The C-H‚‚‚F hydrogen bonding between the ortho carbon of the phenyl ring and the fluorine of the BF2 group plays an important role in the formation of the adlayers. The different electron affinities and geometries of the molecules affect the intermolecular interactions which further lead to different properties in the bulk materials.
Introduction Self-assembly or self-organization is a “bottom-up” strategy for nanofabrication and a form of nanotechnology.1 In recent years, self-assembly techniques and the mechanisms behind selfassembly have been studied intensively due to their importance in both fundamental research and industrial application.2 The natural, spontaneous, and effective process involved in selfassembly leads to functional materials and living organisms. As a result of the intensive study, various self-assemblies have been successfully fabricated on different surfaces and interfaces. On a solid surface, the formation of a self-assembled monolayer (SAM) is usually dependent on the intermolecular, molecule/ substrate, and intramolecular interactions. These interactions are responsible for the formation of a SAM. Understanding the driving forces behind the molecular arrangement can help us to realize molecular engineering3 and control self-assembly, which is critical for the realization of the “bottom-up” strategy.2a The intermolecular interaction is clearly observed in the formation of SAMs.4 The typical intermolecular interaction is the so-called noncovalent interaction which includes hydrogen bonding as well as hydrophilic/hydrophobic forces, electrostatic attraction, and π-π stacking. To obtain a desirable material by self-assembly, it is a prerequisite to precisely control the SAM formation. For a designable procedure, it is important to study the formation mechanism or driving forces that will dominate the structure of a self-assembly. From this background, researchers have made great efforts to understand how the chemical structures of the molecules affect the structure of their self-assemblies. * To whom correspondence should be addressed. E-mail: wanlijun@ iccas.ac.cn. † Also in Graduate School of CAS, Beijing, China. (1) (a) Lehn, J. M. Science 2002, 295, 2400. (b) Lehn, J. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (2) (a) Whitesides, G. M.; Gryzybowski, B. Science 2002, 295, 2418-2421. (b) Requicha, A. A. G. Proc. IEEE 2003, 91, 1922. (3) (a) Lehn, J. M. Supramolecular Chemistry: Concept and PerspectiVes; VCH: Weinheim, Germany, 1995. (b) Desiraju, G. R. Crystal engineering, the design of organic solids; Elsevier: Amsterdam, 1989. (4) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (b) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903. (c) Noh, J.; Ito, E.; Nakajima, K.; Kim, J.; Lee, H.; Hara, M. J. Phys. Chem. B 2002, 106, 7139. (d) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1492.
Scanning tunneling microscopy (STM) is a powerful tool for investigating surface microstructures with atomic resolution in ambient atmosphere, in ultrahigh vacuum (UHV), and even in electrolyte solutions.5 By using STM to image molecules adsorbed on surfaces, the orientation of the adsorbed molecules relative to each other and to the underlying substrate can be determined. Furthermore, intermolecular interactions and the interactions between the molecule and the substrate surface can be studied in detail for various types of organic molecules and multicomponent composites, including alkane derivatives,6 alkylated aromatic compounds,7 oligomers and polymers,8 as well as various metal complexes9 and molecular cages.10 For example, Hipps et al. investigated the structure of bifunctional 2D adlayers on a Au (111) surface on which NiTTP (nickel(II)tetraphenyl-21H,23Hporphyrine) and F16CoPc (cobalt(II)hexadecafluoro-29H,31Hphthalocyanine) form well-ordered and stoichiometric 1:1 structures. In contrast to adlayers of compounds without fluorine (5) (a) Bai, C. L. Scanning Tunneling Microscopy and its Applications; Springer: Shanghai, 1995. (b) Chiang, S. Chem. ReV. 1997, 97, 1083. (c) Moffart, T. P. In Scanning Tunneling Microscopy of Metal Electrodes in Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1999; Vol. 21. (d) Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1998, 28, 117. (e) Wano, H.; Uosaki, K. Langmuir 2005, 21, 4024. (6) (a) Xu, S.-L.; Yin, S.-X.; Liang, H.-P.; Wang, C.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2004, 108, 620. (b) Yin, S.; Wang, C.; Qiu, X.; Xu, B.; Bai, C.-L. Surf. Interface Anal. 2001, 32, 248. (c) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (d) Rabe, J. P.; Buchholz, L. Science 1991, 253, 424. (7) (a) Zhang, J.; Gesquiere, A.; Sieffert, M.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C.; De Feyter, S. Nano Lett. 2005, 5, 1395. (b) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (c) Gong, J.-R.; Lei, S.-B.; Wan, L.-J.; Deng, G.-J.; Fan, Q.-H.; Bai, C.-L. Chem. Mater. 2003, 15, 3098. (d) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Su¨ling, C. J. Phys. Chem. 1995, 99, 8690. (8) (a) Lei, S.-B.; Wan, L.-J.; Wang, C.; Bai, C.-L. AdV. Mater. 2004, 16, 828. (b) Han, M.-J.; Wan, L.-J.; Lei, S.-B.; Li, H.-M.; Fan, X.-L.; Bai, C.-L.; Li, Y.-L.; Zhu, D.-B. J. Phys. Chem. B 2004, 108, 965. (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. (d) Mena-Osteritz, E. AdV. Mater. 2002, 14, 609. (9) (a) Ruben, M. Angew. Chem., Int. Ed. Engl. 2005, 44, 1594. (b) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Commun. 2005, 1294. (c) Gong, J.-R.; Wan, L.-J.; Yuan, Q.-H.; Bai, C.-L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971. (d) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem B 2003, 107, 2903. (10) (a) Lu, J.; Lei, S.-B.; Zeng, Q.-D.; Kang, S.-Z.; Wang, C.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2004, 108, 5161. (b) Pan, G.-B.; Liu, J.-M.; Zhang, H.-M.; Wan, L.-J.; Zeng, Q.-D.; Bai, C.-L. Angew. Chem., Int. Ed. Engl. 2003, 42, 2747. (c) Pan, G.-B.; Bu, J.-H.; Wang, D.; Liu, J.-M.; Wan, L.-J.; Zeng, Q.-D.; Bai, C.-L. J. Phys. Chem B 2003, 107, 13111. (d) Pan, G.-B.; Wan, L.-J.; Zeng, Q.-D.; Bai, C.-L. Chem. Phys. Lett. 2003, 367, 711.
10.1021/la053138+ CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006
Self-Assemblies of DOB
functionalization, it was shown that the higher intermolecular interaction via the fluorinated species is an important factor for the remarkable ordering.4b De Feyter and De Schryver demonstrated that hydrogen bonding has a strong effect on the supramolecular architecture. Oligo-p-phenylen vinylenes functionalized with different hydrogen bonding groups can form linear or hexagonal assemblies, depending on the type of hydrogen bonding group. By functionalizing the oligomeres, the ordering can be controlled.7b Dodecyl-substituted 1,3,2-dioxaborines (DOB) and their selfassemblies on highly ordered pyrolytic graphite (HOPG) surfaces are the topic of this article. Although they received relatively little attention, 1,3,2-dioxaborines possess a number of interesting and potentially beneficial properties.11 By combining the highly electron-accepting 1,3,2-dioxaborine moieties12 with electron donors, it is possible to create charge-transfer dyes that show a number of useful effects such as fluorescence,13 solvatochromy,14 lasing ability,15 and electroluminescence.16 Certain dioxaborine dyes show large nonlinear optic (NLO) effects of 2nd and 3rd order17 as well as absorption in the near-infrared.18 DOBs are also useful as electron transport materials in electronic devices such as organic light emitting diodes (OLEDs)19 and organic field effect transistors (OFETs).20 Especially in the case of twocore DOBs, several interesting applications have recently been demonstrated. These materials possess remarkably high electron mobilities not only in crystalline films but also in amorphous layers.19 Finally, owing to their large electron-affinity, DOBs are found to be unusually stable even under ambient conditions in the presence of oxygen.20 The focus of this article is on two-core DOBs. The molecules were employed to prepare SAMs on HOPG surfaces. The effect of the molecular structure of specially designed DOBs on the structure of the so-prepared self-assemblies was investigated by STM, theoretical calculation, and X-ray diffraction. It was found that the molecules form well-defined SAMs on HOPG. The C-H‚‚‚F hydrogen bonding between the ortho carbon of the phenyl ring and the fluorine of the BF2 group of neighboring molecules plays an important role in the formation of the adlayers. The results in this study will provide important information for the optoelectronic applications of DOB derivatives and on the mechanisms that control the formation of well-organized 2D adlayers with fluorine-containing molecules on solid surfaces. Experimental Section The chemical structures of DOB 4a-c are shown in Scheme 1. They were synthesized by a conversion of the corresponding tetraketones 3a-c with BF3‚Et2O in acetic acid/toluene as the solvent. The tetraketones 3a-c were prepared by a standard Claisen (11) (a) Morgan, G. T.; Tunstall, R. H. J. Chem. Soc. 1924, 125, 1963. (b) Mikhailov, B. M. Pure Appl. Chem. 1977, 49, 749. (c) Lappert, M. F. Chem. ReV. 1956, 56, 959. (12) Fabian, J.; Hartmann, H. J. Phys. Org. Chem. 2004, 17, 359. (13) Go¨rlitz, G.; Hartmann, H.; Kossanyi, J.; Valat, P.; Wintgens, V. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1449. (14) Go¨rlitz, G. Ph.D. Thesis, University Halle-Wittenberg, Germany, 1997. (15) Hartmann, H.; Hultzsch, R.; Ilge, H. D.; Friedrich, B.; Hebenstreit, J.; Fassler, D.; Meinel, U. DD 225884, 1985; Chem. Abst. 1986, 104, 139107. (16) (a) Hunze, A.; Kanitz, A.; Hartmann, H.; Rohde, D. WO 02/065600; Chem. Abstr. 2003, 137, 187010. (b) Halm, J. M.; DeLorme, J. H. Photogr. Sci. Eng. 1979, 23, 252. (17) (a) Kammler, R.; Bourhill, G.; Yin, Y.; Bra¨uchler, C.; Go¨rlitz, G.; Hartmann, H. J. Chem. Soc., Faraday Trans. 1996, 92, 945. (b) Halik, M.; Wenseleers, W.; Grasso, C.; Stellacci, F.; Zojer, E.; Barlow, S.; Bre´das, J.-L.; Perry, J. W.; Marder, S. R. Chem. Commun. 2003, 1490. (18) Halik, M.; Hartmann, H. Chem. Eur. J. 1999, 5, 2511. (19) Domercq, B.; Grasso, C.; Maldonado, J.-L.; Halik, M.; Barlow, S.; Marder, S. R.; Kippelen, B. J. Phys. Chem. B 2004, 108, 8647. (20) Sun, Y.-M.; Liu, Y.-Q.; Rohde, D.; Wan, L.-J. AdV. Funct. Mater. 2005, submitted.
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Figure 1. Molecular geometry of 4a determinate from X-ray diffraction data. condensation from p-dodecyloxy acetophenone 1 and dicarbonic acid esters 2a-c with sodium hydride in toluene.16 For the sample preparation, the corresponding compound was dissolved in toluene (HPLC grade, Aldrich) with a concentration of less than 10-4 M. A drop of the solution was deposited on a freshly cleaved surface of HOPG (quality ZYB, Digital Instruments) and dried in air prior to STM imaging. The samples were imaged by STM in ambient atmosphere at room temperature without any annealing or treatment. The experiments were performed on a Nanoscope III SPM (Digital Instruments, Santa Barbara, USA). STM tips were prepared by cutting Pt/Ir wire (90/10). All STM images were recorded in the constant current mode. The specific tunneling conditions are given in the figure captions. All images are shown without further processing such as Fourier transformation. The calculation of the molecular geometries in the gas phase was performed using Gaussian 03 (Gaussian Inc., Pittsburgh, USA). To simulate the molecular interactions in the 3D and 2D crystals Materials Studio 3.1 (Accelrys, San Diego, USA) was used. The packing models of the molecules were built by using Hyperchem (Hypercube Inc., Florida, USA).
Results and Discussion For the various DOBs substituted with dodecyloxy groups, we have studied the 2D adlayers on HOPG. In addition, we have also investigated the 3D crystal structure of 4a. Owing to the poor solubility of 4b and 4c we were unable to produce crystals with the quality sufficient for X-ray diffraction experiments of two compounds. 3D Crystal Structure of 4a. X-ray diffraction experiments on 4a show that the molecule is almost perfectly flat, as shown in Figure 1. The two dioxaborine moieties are trans connected, and the π-conjugated system has a dimension ∆π of 1.679 nm (O3-dodecyl-O3-dodecyl). The two inner dioxaborine rings are in an identical plane with a dihedral angle of 0°. The two outer phenyl rings are also in an identical plane. The two outer phenyl rings have a dihedral angle of 3° with respect to the two inner dioxaborine rings. The two dodecyl chains are almost coplanar with respect to the π-conjugated system. As shown in Figure 2A, the molecules form a 3D crystal in layers. The unit cell dimensions of the 3D crystal of 4a are x ) 0.723 nm, y ) 0.813 nm, z ) 1.894 nm, χ ) 81.37°, ψ ) 87.23°, and ω ) 78.44°. The analyses of the X-ray data using the program Platon21 reveal the nature of the intermolecular interactions in the crystal. The inter-planar (21) Spec, A. L. J. Appl. Crystallogr. 2003, 36, 7.
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Scheme 1. Synthesis and Chemical Structure of the Bis DOBs
distance between adjacent layers is 0.361 nm, suggesting the presence of a vertical π-π interaction between the conjugated DOB backbones. The molecules are oriented in such a way that the dioxaborine moieties of each layer are positioned directly over the phenyl groups of adjacent layers. As shown in Figure 2B, an additional short-distance intermolecular interaction linking the layers diagonally (inter-planar) between F1-C9 with a distance of 0.311 nm (shown as a dashed red line) was discovered. The molecules within each layer (intraplanar) are linked by an interaction involving the fluorine atoms F1 and F2 of the BF2 group and the ortho carbon atom of the phenyl ring C5, with a distance of 0.345 nm shown as a dashed black line in Figure 2B. Thalladi et al. studied intermolecular interactions of the C-F group in fluorobenzenes and showed its hydrogen bonding acceptor capability.22 However, the interaction strength in C-H‚‚‚ F-C hydrogen bondings is very weak.22-24 Brammer et al. demonstrated that H‚‚‚F-C hydrogen bondings are clearly longer (22) Thalladi, V. R.; Weiss, H.-C.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702. (23) (a) Steiner, T. Angew. Chem., Int. Ed. Engl. 2002, 41, 48. (b) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565. (c) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond. In Structural Chemistry and Biology; Oxford University Press: Oxford, U.K., 1999. (24) (a) Chan, M. C. W.; Kui, S. C. F.; Cole, J. M.; McIntyre, G. J.; Matsui, S.; Zhu, N.; Tam K.-H. Chem. Eur. J. 2005, 12, 2607. (b) Kui, S. C. F.; Zhu, N.; Chan, M. C. W. Angew. Chem., Int. Ed. 2003, 42, 1628.
than their counterparts involving H‚‚‚F-metal. A metal-bonded fluorine atom is a better acceptor and forms stronger hydrogen bondings.25 For 2,2’-difluoro-1,3,2-dioxaborines, the polar B-F bonding activates the fluorine as a hydrogen bonding acceptor and increases the C-H‚‚‚F-B interaction. Crystal structures are already reported for a small number of DOBs.26 The different authors do not specify the formation of hydrogen bondings. However, for simple 2,2’-difluoro-1,3,2-dioxaborines, they describe an intermolecular fluorine hydrogen contact of length 0.237 nm. The C-H‚‚‚F angle is about 140°.26e To obtain specific information on the molecular interactions, we performed DFT calculations to understand the intermolecular forces.27 The DFT calculations show that the interaction forces in the vertical and diagonal directions are almost identical, whereas (25) (a) Brammer, L.; Zordan, F.; Espallargas, G. M.; Purver, S. L.; Marin, L. A.; Adams, H.; Sherwood, P. ACA Trans. 2004, 39, 114. (b) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1, 277. (c) Brammer, L.; Bruton, E. A.; Sherwood, P. New J. Chem. 1999, 23, 965. (d) van den Berg, J. A.; Seddon, K. R., Cryst. Growth Des. 2003, 3, 643. (26) (a) Mirochnik, A. G.; Bukvetskii, B. V.; Gukhman, E. V.; Karasev, V. E. J. Fluorescence 2003, 13, 157. (b) Dromze´e, Y.; Kossanyi, J.; Wintgens, V.; Valat, P.; Hartmann, H.; Go¨rlitz, G. Z. Kristallogr. 1997, 212, 372. (c) Bo¨hme, U.; Hartmann, H.; Go¨rlitz, G. Z. Kristallogr. 1996, 211, 133. (d) Bo¨hme, U.; Hartmann, H.; Go¨rlitz, G. Z. Kristallogr. 1996, 211, 953. (e) Hanson, A. W.; Macaulay, E. W. Acta Crystallogr. 1972, 28, 1961. (27) (a) Rossmeisl, J.; Hinnemann, B.; Jacobsen, K. W.; Nørskov, J. K.; Olsen, O. H.; Pedersen, J. T. J. Chem. Phys. 2003, 118, 9783. (b) Tsuzuki, S.; Lu¨thi, H. P. J. Chem. Phys. 2001, 114, 3949.
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Figure 2. 3D crystal packing of 4a. (a) View along the a axis. (Because of the clearness, the dodecyloxy groups are not shown here.) (b) Intermolecular interaction. (Because of the clearness, the dodecyloxy groups are not shown here.) (c) View to 2D cut through the 3D crystal in the (7 2 1) plane.
the horizontal intra-plane force exceeds the π-π interaction by 30%. The calculated partial atomic charges (see the Supporting Information) for the link show a typical charge distribution that is expected for hydrogen bonding C(δ-)-H(δ+)‚‚‚F(δ-). The main structural feature of the hydrogen bonding different from van der Waals interaction is the preference of linearity.23 The C-H‚‚‚ F angle in the 3D crystal of 4a is 163° and thus in the range as reported for many other C-H‚‚‚F hydrogen bondings in the literature.25 Therefore, we assume bifurcated hydrogen bondings between the fluorines of the dioxaborine electron-acceptor moiety and the ortho carbon atom C5 of the oxy-phenyl donor moiety of the molecule.23 This hydrogen bonding has a significant impact on the resulting crystal structure. The electrostatic potential map of 4a shown in Figure 3 calculated by using the DFT method, reveals the highly electrostatically negative F-atom. In the (7 2 1) plane of the crystal shown in Figure 2C, the C-H‚‚‚F link between the molecules is highlighted with black dashed lines. The molecules in this plane are ordered in the form of lamellae. The distance ∆M between the π-conjugated DOB backbones in the (7 2 1) plane is 0.723 nm. The distance ∆A between the neighboring alkyl chains is about 0.5 nm. 2D Adlayer of 4a on HOPG. Figure 4A shows the 2D adlayer of 4a adsorbed on HOPG. The molecules form domains in a size up to 100 nm. A linear lamella structure with alternating bright and dark stripes can be seen in the STM image. The bright stripes consist of the well-ordered DOB π-backbones of 4a, whereas the dark stripes are formed by the parallel packing of the alkyl (28) Rohde, D. Ph.D. Thesis, University Halle-Wittenberg, Germany 2002.
Figure 3. Electrostatic potential contours around 4a calculated using DFT method in Materials Studio 3.1.
chains. The inter-lamella distance ∆L is measured to be about 3.9 nm. Figure 4B shows a high-resolution STM image acquired on 4a adlayer. Within the lamella the π-conjugated backbones of the molecules have a spacing ∆M of 0.76 ( 0.05 nm. The π-conjugated system illustrated in Figure 4B as a white ellipse has a size ∆π of about 1.7 nm. These results are in good agreement with the geometry calculated by using Gaussian and with the X-ray diffraction data. On the basis of the STM observation, a structural model is proposed in Figure 4C. The possible bifurcated hydrogen bondings are indicated by dashed lines. The molecular ordering in the model is similar to the molecular arrangement in the (7 2 1) plane of its 3D crystal. Compared the (7 2 1) plane
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Figure 4. (a) Large scale STM image of 4a (75 nm × 75 nm). Tunneling conditions: Iset ) 478 pA, Vbias ) 500 mV. (b) High-resolution image of 4a. The image size is 19.5 nm × 19.5 nm. Tunneling conditions: Iset ) 471 pA, Vbias ) 498 mV. (c) Structural model of 4a determinate from the STM image.
with the 2D adlayer on HOPG, it can be seen that the unit cell parameters are almost identical. The dimensions of the unit cell on HOPG are a ) 0.76 ( 0.05 nm, b ) 3.96 ( 0.05 nm, and R ) 91° ( 2° and those of the (7 2 1) plane in Figure 2C are a ) 0.723 nm, b ) 3.93 nm, and R ) 90.28°. The angle in the 2D adlayer of Figure 4 between the π-conjugated system and the alkyl chain Π is 135°, consistent with that in the 3D crystal (see Figure 2C). The analysis of the Moire pattern7d found that the spacing ∆A is about 0.448 nm in the adlayer of 4a on HOPG. The periodicity of 0.448 nm indicates a parallel packing of the dodecyl chains with respect to the graphite lattice.6b Note that the spacing ∆A in (7 2 1) of the 3D crystal is substantially larger (0.5 nm in Figure 2C). The influence of the substrate is considered to be responsible for the difference in 3D crystal and 2D adlayer on the HOPG surface. 2D Adlayer of 3a on HOPG. To obtain more information about the intermolecular interactions, the adlayer of tetraketon molecule 3a on HOPG was also investigated by STM. Owing to the complexation of the tetraketone with boronic acid derivatives, the electronic properties of the molecule are changed dramatically.11,12 Figure 5A shows a large-scale STM image of the 3a adlayer. The molecules form a 2D assembly in lamella configuration. The spacing ∆L between two adjacent lamellae
is about 3 nm. This space is too small for an alkyl chain to take a tail-to-tail configuration as observed for 4a. The high-resolution image in Figure 5B provides the structural details about the molecular arrangement. In contrast to molecule 4a, the tetraketone 3a molecules take a comb arrangement with their alkyl chains, resulting in a large angle of Φ ) 53° between the molecules within the lamella axis. The distance ∆M between the neighboring molecules within the lamella is 0.91 ( 0.05 nm. The specific orientation of the molecules also provides important information about the keto-enol-tautomerie of 3a. In polar solvents such as DMSO, the enol form is dominant, as shown by NMR investigations. However on the surface, we only observe the keto form. The calculation results of the two different tautomeric forms adsorbed on HOPG suggest the existence of hydrogen bonding between neighboring molecules and a denser packing in the enol form. For the keto form, on the other hand, we were able to propose a structural model for Figure 5A,B. Figure 5C is the illustration of the model. A unit cell can be defined from the adlayer. The parameters of the unit cell are a ) 1.24 ( 0.05 nm, b ) 3.06 ( 0.05 nm, and R ) 103° ( 2°. Compared the adlayer structures of molecules 3a and 4a, it can be seen that the lateral molecular interactions between the DOB molecules are much larger than those for the tetraketones.
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Figure 5. Large scale STM image of 3a (50 nm × 50 nm). Tunneling conditions: Iset ) 415 pA, Vbias ) 600 mV. (b) High-resolution image of 3a. The image size is 11.5 nm × 11.5 nm. Tunneling conditions: Iset ) 368 pA, Vbias ) 670 mV. (c) Structural model of 3a determinate from the STM image.
The intermolecular distance ∆M along a lamella in the 3a adlayer is 0.91 ( 0.05 nm, larger than that in the 4a adlayer with 0.76 ( 0.05 nm. The electrostatic repulsion between the molecules apparently increases the distance ∆M. In the 3D crystal of 4a, there exist bifurcated C-H‚‚‚F hydrogen bondings between C5 and the fluorine atoms. This bonding was also observed in the molecular 4a adlayer on HOPG shown in Figure 4. The distance F1-C5 in the 4a adlayer is about 0.37 nm, and the C-H‚‚‚F angle is 152°, determined from the structural model in Figure 4C. In Figure 4C, the bondings are indicated by black dashed lines. The C-H‚‚‚F interaction exceeds the electrostatic repulsion of the molecules, demonstrating the importance of the hydrogen bonding for the self-assembly of DOBs. As a result of the increased molecular interaction due to the complexation, the tetraketone 3a and the DOB 4a show different properties in their bulk crystals. For example, π-conjugated molecular systems with long alkyl chains are known to form liquid crystals. For molecule 3a, we observed the formation of liquid crystal phases after melting at temperatures between 127 and 140 °C. As a result of the strong intermolecular interactions, the melting point of 4a is 120 K higher than the melting point of 3a. For 4a, we did not observe the formation of liquid crystals after melting. 2D Adlayer of 4b on HOPG. To investigate the influence of the molecular structure on the self-assembly, two other bisdioxaborines were synthesized and their adlayers on HOPG were studied by STM. By introducing a phenyl bridge between the
DOB moieties, the electronic structure and the charge distribution within the molecule are modified. The electron-density in the dioxaborine moieties increases in the sequence 4a-4b-4c, resulting in a decrease of the carbon acidity in the phenyl ring in the same sequence, which was previously confirmed using 13C, 11B, and 19F NMR, cyclic voltammetric measurements, optical measurements, and theoretical calculations for different DOB dyes.12,16,27 Thus, the strength of the lateral hydrogen bonding between the molecules is determined by the molecular structure. In addition to the above-mentioned decrease of the carbon acidity in the phenyl ring, the bridges between the DOB moieties also induce different molecular multipole moments. Although 4a and 4b show C2h symmetry and a quadrupole moment, 4c shows a C2V symmetry and a remarkable dipole moment. Figure 6A shows a 2D self-assembly of the para-phenyleneconnected bis-DOB 4b on HOPG. As in the case of 4a, a lamella structure is observed in the 4b molecular adlayer. The distance ∆L between two lamellae is measured to be about 4.7 nm. The intermolecular distance ∆M along a lamella is 0.79 ( 0.05 nm. The π-conjugated part in the molecule is extended in size ∆π of about 2 nm. The geometric parameters of the adlayer can be seen in the image. On the basis of the observation, a structural model is shown in Figure 6B, showing that the π-conjugated part is at an angle of about Φ ) 87° with respect to the lamella axis. The angle Π between the alkyl chain and the DOB backbone is about 132° and thus slightly smaller than that in the 4a adlayer. The average distance between the fluorine atom and the ortho
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Figure 6. (a) High-resolution STM image of 4b (15 nm × 15 nm). Tunneling conditions: Iset ) 463 pA, Vbias ) 405 mV. (b) Structural model of 4b determinate from the STM image. (c) High resolution STM image of self-assembly of 4c (18 nm × 18 nm). Tunneling conditions: Iset ) 688 pA, Vbias ) 456 mV. (d) Structural model of 4c determinate from the STM image. Table 1. Unit Cell Parameters of the Adlayers Formed by the Investigated Compounds ∆L [nm] 3a HOPG 4a 2D cut through the 3D crystal 4a HOPG 4b HOPG 4c HOPG
∆M [nm]
F [°]
a [°]
a [nm]
b [nm]
3 ( 0.05
0.91 ( 0.05
53 ( 2
103 ( 2
1.24 ( 0.05
3.06 ( 0.05
3.93
0.723
91.8
90.28
0.723
3.93
3.9 ( 0.05
0.76 ( 0.05
84 ( 2
91 ( 2
0.76 ( 0.05
3.96 ( 0.05
4.7 ( 0.05
0.79 ( 0.05
87 ( 2
92 ( 2
0.79 ( 0.05
4.7 ( 0.05
4.9 ( 0.05
0.86 ( 0.05
85 ( 2
92 ( 2
0.86 ( 0.05
4.9 ( 0.05
carbon atom of the outer phenyl ring is 0.42 nm. The average distance between the fluorine atom and the ortho carbon atom of the middle phenyl ring is 0.39 nm. The C-H‚‚‚F links are shown as black dashed lines in Figure 6B. Further geometrical data of the C-H‚‚‚F hydrogen bond are given in the Supporting Information. The unit cell parameters are a ) 0.79 ( 0.05 nm, b ) 4.7 ( 0.05 nm, and R ) 92° ( 2°. 2D Adlayer of 4c on HOPG. In molecule DOB 4c, the dioxaborine moieties are connected by a meta-phenylene. Figure 6C is an STM image of the 4c adlayer on HOPG. The distance ∆L between two neighboring lamellae is about 4.9 nm. In the image, the phenyl groups appear as bright circles. The intermolecular distance ∆M along the lamella direction is 0.86 ( 0.05 nm. The angle Φ between the π-conjugated part and the lamella axis is about 85°. The structural model for the 4c adlayer deduced from STM observation is shown in Figure 6D. The results of theoretical calculation for molecule 4c using Gaussian show that the molecule has a C2V symmetry and that the angle Π between the alkyl chain and the DOB backbone is 140°. However, the conformation of the molecule is asymmetric on
HOPG, with Π1 ) 124° and Π2 ) 133°. The distances between the fluorine atoms and the ortho carbon atoms of the outer phenyl rings are about 0.45 and 0.5 nm, respectively. The distances between the fluorine atoms and the ortho carbon atoms of the middle phenyl ring are 0.40 and 0.42 nm (further geometrical data such as C-H‚‚‚F angles are given in the Supporting Information). The possible hydrogen bondings are indicated by black dashed lines in Figure 6D. The dimensions of the unit cell are a ) 0.86 ( 0.05 nm, b ) 4.9 ( 0.05 nm, and R ) 92° ( 2°. A special feature of compound 4c is the C2V symmetry. In the molecule, the two BF2 groups point in the same direction. Therefore, the molecule has an electrostaticlly negative head (BF2 groups) and an electrostaticlly positive tail (phenyl groups; see Figure 6D). The C-H‚‚‚F bondings as well as the dipoledipole interaction are the origin of the head-to-tail configuration of the molecules in the lamella. The geometry of the molecule results in a dipole moment pointing from the tail to the head of the molecule. The 4c self-assembly results in a dipole of the individual lamella.
Self-Assemblies of DOB
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The investigations show that the DOB adlayers on HOPG are affected by C-H‚‚‚F bifurcated hydrogen bonding. The parameters obtained from X-ray diffraction and STM experiments on the investigated molecules are listed in Table 1. Comparison of the adlayer parameters of the DOB 4a with those of the respective tetraketone 3a reveals the importance of the C-H‚‚‚F link for the self-assembly of the DOBs. In the DOB adlayer, the hydrogen bonding leads to a close packing of the molecules and to a significantly smaller intermolecular distance ∆M.
Conclusion We have synthesized and investigated several different connected two-core 1,3,2-dioxaborines to analyze the influence of the molecular structure on their self-assembly. The different arrangements of the tetraketone 3a and DOB 4a indicate an enhanced lateral intermolecular reaction due to the complexation. This is also expressed in the properties of the bulk materials. The studies on the 3D crystal structure and 2D monolayer on HOPG
reveal the nature of the intermolecular interactions. This interaction is from C-H‚‚‚F hydrogen bonding. The intermolecular reaction is an important aspect with regard to the applicability of DOBs in electronic devices. Therefore, the tuning of the intermolecular interaction is significant in fabricating selfassembly. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (Nos. 20520140277, 20121301, and 20575070), National Key Project on Basic Research (Grant G2000077501), and the Chinese Academy of Sciences. Supporting Information Available: The preparation procedure and experimental data of the compounds, the crystal data of compound 4a, the geometrical data of the C-H‚‚‚F hydrogen bonding, and charge distribution computed by DFT method. This material is available free of charge via the Internet at http://pubs.acs.org. LA053138+
