Communication pubs.acs.org/crystal
Synthesis and Molecular Structures of BINOL Complexes: An STM Investigation of 2D Self-Assembly Xianghua Zeng,†,# Shaoqing Chang,‡,§,# Ke Deng,*,‡ Junyong Zhang,† Hongbin Sun,§ Qingdao Zeng,*,‡ and Jingli Xie*,†,∥ †
College of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing 314001, P. R. China CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, P. R. China § Department of Chemistry, College of Science, Northeastern University, Shenyang 110819, P. R. China ∥ State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China ‡
S Supporting Information *
ABSTRACT: Three new (R)-BINOL complexes have been prepared. 2D self-assembly has been observed in the system consisting of crystalline (R)-BINOL derivatives (guest) and 1,3,5-tris(10-carboxydecyloxy)-benzene (host) by means of the scanning tunneling microscopy (STM) technique. Density functional theory (DFT) calculations help to reveal the assembly on the HOPG surface.
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losupramolecular helicates or metal−organic frameworks (MOFs).15 In 1997, Wuest reported that a motif of 2,4diaminotriazinyl (DAT) was used to create predictably ordered three-dimensional networks linked by hydrogen bonds, whose structures were determined by X-ray crystallography.16 Recently, Chen and co-workers incorporated the 2,4-diaminotriazinyl hydrogen-bonding motif into the (R)-BINOL scaffold to synthesize a chiral organic building block for the selfassembly of porous hydrogen-bonded organic frameworks (HOFs).17 It is worth noting that the hydrogen-bonding interactions within HOFs are quite weak, typically not strong enough to stabilize the frameworks and establish their permanent porosities.18−20 By contrast, because of the ability to immobilize functional guest molecules, two-dimensional networks acting as the host system are essential for the fabrication of highly ordered arrays of (R)-BINOL derivatives.21 Among the various host systems, the intermolecular hydrogen-bonded network of 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) is a typical template system, which allows a large variety of guest molecules to be trapped in their cavities, such as fullerene (C60), phthalocyanine, NN-Macrocycles, and so on.22,23 However, to the best of our knowledge, exploration of the unique assembly process of crystalline (R)-BINOL derivatives down to the nanoscale remains very limited. In this contribution, TCDB is exploited as template molecule and (R)BINOL derivatives (Scheme 1) as guest molecules. The host− guest architectures of TCDB/(R)-BINOL are achieved when they are coadsorbed on HOPG and clearly observed by means
nderstanding the role of the organization process of molecules at the atomic and molecular level is one of the most enduring challenges in supramolecular science, and extensive research efforts have been devoted to submolecular observation of a host−guest network on the surface through the use of 2D hydrogen-bonded networks as the host.1,2 It is envisaged that the fundamental information gained from the self-assembly process, which incorporates different guest molecules in ordered arrays by virtue of noncovalent interactions such as hydrogen bonds, van der Waals forces, and so on, could help to fabricate nanostructure and develop nanodevices.3 In this particular area, hydrogen bonds are highly selective and directional; though moderately strong, they have been extensively applied to direct the molecular ordering on surfaces, both under UHV conditions and at the air−solid or liquid−solid interface, as revealed by the scanning tunneling microscopy (STM) technique.4−6 It is said that gaining control of two-dimensional pattern formation might turn out to be crucial to the design and properties of functional surfaces.7,8 Indeed, ordered self-assembled 2D structures are a clearly emerging area of interest in the field of supramolecular science, and this can be achieved when the mobility of molecules is sufficiently high to allow them to diffuse, and consequently obtain a configuration desirable for attracting and stabilizing intermolecular interactions.9 Currently, 1,1′-binaphthyls represent a class of important chiral molecules that have found extensive application in molecular recognition, asymmetric synthesis, and functional materials.10,11 In the past decades, complexes containing −pyridine,12,13 −carboxyl,14 −phosphonates,12 and so on were scaffolded on organic backbone of 1,1′-bi-2-naphthol (BINOL), then reacted with metal ions to form metal© XXXX American Chemical Society
Received: February 1, 2015 Revised: June 4, 2015
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DOI: 10.1021/acs.cgd.5b00143 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Scheme 1. Chemical Structures of the Host and Guest Moleculesa
a
(a) TCDB and (b) L1; (c) L2; (d) L3.
of scanning tunneling microscopy (STM) technique. Furthermore, density functional theory (DFT) calculations24,25 contribute to explaining the assembly of TCDB/(R)-BINOL system on the surface. An X-ray single-crystal diffraction study revealed that L1 crystallized in a orthorhombic space group P212121 and displayed a 3D supramolecular structure via C−H···O and C−H···π interactions. This ligand has a dihedral angle θ (ca. 65.9°) between the two naphthalene nucleus of binaphthyl, i.e., plane a and plane b, indicates that the ligand is non-coplanar (Figure 1a). There are weak nonclassical C−H···S and O−H···
Figure 2. (a) O−H···O (red dashed lines) and C−H···π (blue dashed lines) interactions in L1; (b) extended 3D supramolecular network via C−H···O (red dashed lines) and C−H···π (blue dashed lines) interactions.
interactions between C−H group of naphthalene and another naphthalene nucleus of adjacent ligand (3.7019 Å). Similar to L1, the structural analysis of L3 reveals that there is a dihedral angle (ca. 67.7°) between those two naphthalene rings and this value is similar to other binaphthyl ligands,26 which again indicate the noncoplanar character of the molecule (Figure S1a). Details about the structural characters of L3 could be found in the Supporting Information. It is interesting to investigate the unique assembling process of crystalline (R)-BINOL derivatives on the surface. In this work, we employ TCDB as the template molecule and (R)BINOL derivatives as the guest molecules to exploit the properties of the host−guest architectures of TCDB/(R)BINOL by STM technique. 1,3,5-Tris(10-carboxydecyloxy)-benzene (TCDB), which can form two-dimensional networks with nearly tetragonal cavities on the HOPG surface, has been well studied.3 Figure 3a shows the large-scale STM image of assembly of TCDB. Detailed assembly structures are shown in Figure 3b. It could be noted that each tetragonal cavity is formed by two TCDB molecules. Careful analysis reveals that two TCDB molecules form two pairs of hydrogen bonds via the terminal carboxylic groups (see the red dashed circles in Figure 3d). Figure 3c is the corresponding molecular model calculated by DFT method on the basis of STM observations. The measured unit cell is superimposed on the molecular model with a = 3.9 ± 0.1 nm, b = 2.2 ± 0.1 nm, α = 73 ± 1°. Obviously, this type of network is large enough to be used as a template to accommodate various functional molecules as guest species. A large-scale STM image of L1/TCDB is shown in Figure 4a. Presented in Figure 4b is a high-resolution image emphasizing the details of assembled structure. As shown in the image, two small white dots correspond to the benzene cores of TCDB. The size of filler is measured to be L1 = 1.6 ± 0.1 nm,
Figure 1. (a) Non-coplanar structure viewed along a-axis and (b) intramolecular hydrogen bonding in L1.
N intramolecular hydrogen bonds between the benzothiazole sulfur atom and the C−H group of naphthalene group, and the benzothiazole nitrogen atom and O−H group of phenolato group (C4−S1 = 3.1360 Å, C25−S2 = 3.0873 Å, O2−N1 = 2.6107 Å, O1−N1 = 2.5901 Å), respectively (Figure 1b). Interestingly, there are planes which are constructed via O− H···O (between the phenolato oxygen atom and O−H group of neighboring phenolato group) (O···O = 2.9890 Å) and C− H···π (between C−H group of benzothiazole and phenolato ring of adjacent ligand) interactions (3.6868 Å) (Figure 2a). As shown in Figure 2b, 2D planar structure of L1 extends to a 3D supramolecular network via intermolecular C−H···O (between phenolato oxygen atoms and C−H group of benzothiazole group of adjacent ligand, C···O = 3.3408 Å) and C−H···π B
DOI: 10.1021/acs.cgd.5b00143 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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nm (length) and 1.01 nm (width) of two neighboring L1 molecules, which is slightly longer, indicating that dense packing could be achieved on the surface within the host system. The measured unit cell is superimposed on the molecular model with a = 4.4 ± 0.1 nm, b = 2.1 ± 0.1 nm, and α = 79 ± 1° (Figure 4c). Molecules L2 and L3 have similar structure as L1 except for the substituted groups on the naphthalene ring. Figure 5a
Figure 3. (a) Large scale STM image of TCDB adsorbed on the HOPG surfaces. Iset = 269.9 pA; Vbias = 688.2 mV. (b) Higher resolution STM image of TCDB on the HOPG surface. Iset = 289.9 pA; Vbias = 688.2 mV. (c,d) Suggested molecular models for the observed area in (a).
Figure 5. (a) Large scale STM image of the 2D self-assembled structure of L2/TCDB adsorbed on the HOPG surfaces. Iset = 299.1 Pa; Vbias = 699.8 mV. (b) Higher resolution STM image of L2/TCDB on the HOPG surface. Iset = 299.1 pA; Vbias = 699.8 mV. (c) Proposed molecular model of L2/TCDB. (d) STM image of L3/TCDB on the HOPG surface. Iset = 289.9 pA; Vbias = 699.1 mV. (e) Proposed molecular model of L3/TCDB.
shows a large-scale STM image of a well-ordered self-assembled structure of L2/TCDB. The high-resolution STM images for the self-assembled structure of L2/TCDB (Figure 5b) and L3/ TCDB (Figure 5d) display the similar assembled structures in manner to L1/TCDB. By carefully analyzing the high-resolution STM images, we found that each cavity of TCDB network is filled with two guest molecules. As marked in Figure 5b, the tetragonal feature can be ascribed to the two L2 molecules lying side by side to form a dimer, while the dispersed small spots (drawn as white dots) can be ascribed to the benzene cores of the TCDB molecule. The length of the tetragon of L2 dimer is measured to be L3 = 2.0 ± 0.1 nm, L4 = 0.9 ± 0.1 nm in Figure 5b, which is about twice the width of a naphthalene ring, indicating that each tetragon is composed of two L2 molecules. The measured unit cell parameters are a = 4.4 ± 0.1 nm, b = 2.0 ± 0.1 nm, and α = 78 ± 2° (cf. Figure 5b/c). Similar host− guest structure for L3/TCDB molecule (L5 = 1.9 ± 0.1 nm, L6 = 0.9 ± 0.1 nm, a = 4.4 ± 0.1 nm, b = 2.0 ± 0.1 nm, α = 79 ± 2°) is observed in Figure 5d. From the structural analysis of L3, the dimeric packing of two L3 molecules gave values of 2.12 nm (length) and 0.99 nm (width), in agreement with the experimentally observed values. To show the adsorption structures more vividly, models for the array structures are presented in Figure 4c and Figure 5c and e, respectively. To better illustrate the self-assembled architecture of crystalline (R)-BINOL derivatives, simulation was performed by density functional theory (DFT) calculations. As a computational quantum mechanical modeling method, DFT is among the most popular and versatile methods available in condensed-matter physics, computational physics, and computational chemistry. We have successfully used this methodology in other systems.25,27−29 In this work, theoretical calculations
Figure 4. (a) Large scale STM image of the 2D self-assembled structure of L1/TCDB adsorbed on the HOPG surfaces. Iset = 289.9 Pa; Vbias = 648.2 mV. (b) Higher resolution STM image of L1/TCDB on the HOPG surface. Iset = 289.9 pA; Vbias = 648.2 mV. (c) Proposed molecular model of L1/TCDB.
L2 = 0.9 ± 0.1 nm, respectively, which is similar to the length of two naphthalene ring of L1 molecules, indicating that the cavity of TCDB networks is occupied by two closely packed L1 molecules. Crystal structure analysis revealed the values of 2.14 C
DOI: 10.1021/acs.cgd.5b00143 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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were performed using DFT provided by DMol3 code.30 For DMol3 code, only the system with total atoms
