Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Balancing Noncovalent Interactions in the Self-Assembly of Nonplanar Aromatic Carboxylic Acid MOF Linkers at the Solution/ Solid Interface: HOPG vs Au(111) Kristen N. Johnson,† Matthew J. Hurlock,† Qiang Zhang,† K. W. Hipps,†,‡ and Ursula Mazur*,†,‡ †
Department of Chemistry, Washington State University, Pullman, Washington 99163-4630, United States Materials Science & Engineering Program, Washington State University, Pullman, Washington 99163-2711, United States
‡
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S Supporting Information *
ABSTRACT: This study explores directed noncovalent bonding in the self-assembly of nonplanar aromatic carboxylic acids on gold and graphite surfaces. It is the first step in developing a new design strategy to create two-dimensional surface metal−organic frameworks (SURFMOFs). The acid molecules used are tetraphenylethene-based and are typically employed in the synthesis of three-dimensional (3D) MOF crystalline solids. They include tetraphenylethene tetracarboxylic acid, tetraphenylethene bisphenyl carboxylic acid, and tetraphenylethene tetrakis-phenyl carboxylic acid. The twodimensional structures formed from these molecules on highly ordered pyrolytic graphite (HOPG) and Au(111) are studied by scanning tunneling microscopy in a solution environment. The process of monolayer formation and final surface linker structures are found to be strongly dependent on the combination of the molecule and substrate used and are discussed in terms of intermolecular and molecule−substrate interactions, bonding geometry, and symmetry of the acid molecules. In the case of linker self-assembly on HOPG, the molecule−substrate interactions play a significant role in the resulting surface structure. When the acid molecules are adsorbed on Au(111), the intermolecular interactions tend to dominate over the weaker molecule−substrate bonding. Additionally, the interplay of π−π interactions and hydrogen bonding that directs the surface selfassembly on different supports can be modified by varying the linker concentration. This is particularly applicable for the case of the acid molecules adsorbing on the Au(111) substrate. Precise control over predesigned surface structures and orientation of the nonplanar aromatic carboxylic linkers open up an exciting prospect for manipulating the direction of SURFMOF growth in two dimensions and potentially in 3D.
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INTRODUCTION Metal−organic frameworks (MOFs) are porous materials selfassembled from metal ions or clusters and organic linkers.1−6 The characteristic high surface areas of MOFs are the basis for their many applications including gas storage,7−9 molecular separation,10−12 catalysis,13−15 and sensors.16−19 For usage in sensors and nanoscale devices, controlled deposition of defectfree, homogenous, and highly oriented porous crystalline MOF thin films (so-called surface-mounted MOFs, SURFMOFs) is of great importance.20−27 A typical approach to growing MOFs on solid supports (metals or oxides) is to first covalently functionalize the surface with a self-assembled molecular monolayer that is terminated with −OH, −NH2, or −COOH moieties, for example. The functionalized surface is then exposed to sequential flow of fluid solutions containing a metal precursor and an organic linker with solvent washings between cycles. This process is referred to as layer-by-layer (LBL) growth. During the MOF deposition process, ligand exchange reactions take place at the interface between the solid and liquid phases, allowing the © XXXX American Chemical Society
metal ions to bind to linker groups at the surface and vice versa.28−31 Alternatively, deposition can be accomplished by employing the Langmuir−Blodgett (LB) method.32−35 Film growth has been monitored by surface plasmon resonance18 or a quartz crystal microbalance.28,33 Optical spectroscopic methods and microscopic techniques are also employed in MOF film analysis.30,33,36,37 It is difficult to prepare uniform defect-free thin MOF films, especially monolayers using LBL and LB methods. Two component metal−organic networks have been synthesized on crystalline metal surfaces by vapor deposition of organic molecules and metal atoms provided either by the substrate or through vapor deposition.38−47 These experiments were performed in a UHV environment, and scanning tunneling microscopy (STM) was employed for analyzing the SURFMOF monolayers. In most cases, it was reported that Received: January 21, 2019 Revised: March 22, 2019
A
DOI: 10.1021/acs.langmuir.9b00204 Langmuir XXXX, XXX, XXX−XXX
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cules are conformationally flexible, and we demonstrate that an appropriate choice of substrate and experimental parameters can provide precise control over their self-assembled surface structures. The ability to control the orientation of the linkers can potentially lead to the construction of SURMOFs with prearranged structures at the solution−solid interface.
the vapor deposition method produced islands or chains rather than continuous MOF films on the surface.38−48 Our long-term goal is to first synthesize and characterize highly ordered monolayer films of flexible aromatic carboxylic acid MOF linkers at the solution−solid interface and then incorporate metal ions into them under mild experimental conditions to create SURFMOFs. We anticipate that the linkers can be adsorbed on a solid support in controlled geometries that will lead to the construction of MOFs with predetermined structural topologies. SURFMOF synthesis carried out in this fashion will be monitored in situ by STM. STM has the unparalleled advantage of monitoring real-time growth of MOFs under actual experimental conditions (i.e., in situ, in solution), giving unique information about the kinetics and thermodynamics as a function of multiple experimental variables (e.g., concentration, temperature, metal ion, and time). Here, we present an STM study that explores different experimental pathways to create highly ordered defect-free templates of self-assembled nonplanar aromatic carboxylic acids. (There is substantial STM data on the self-assembly of carboxylic acids at the solution−solid interface to serve as a guide for the roles of noncovalent intermolecular, solvent, and molecule−substrate interactions.48−55) The flexible aromatic carboxylic acid employed in this work is based on the tetraphenylethene (TPE) chromophore (Figures 1 and S1 in
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EXPERIMENTAL SECTION
Materials. Synthesis and characterization of tetraphenylethene tetracarboxylic acid (ETC), tetraphenylethene tetrakis-phenyl carboxylic acid (ETTC), and tetraphenylethene bisphenyl carboxylic acid (BPDC) are described in the Supporting Information. Octatonic acid (99%) and heptanoic acid (99%) were purchased from Alfa Aesar. Highly ordered pyrolytic graphite (HOPG) substrates (Grade 1 or 2, a 1 cm2 size) were acquired from SPI supplies and freshly cleaved before use. Au(111) substrates were prepared by epitaxial growth of gold on mica using the vapor deposition technique.62 Gold (99.999%) and mica were purchased from Cerac Inc. and Ted Pella Inc., respectively. Freshly made gold substrates were hydrogen flame annealed and imaged to ensure that surface reconstruction lines could be seen. Samples were deposited only on those gold substrates which had distinct reconstruction lines. STM Imaging. STM images were recorded using a Molecular Imaging (currently Keysight) Pico 5 STM equipped with a 1 μm2 scanner. The sample and scanner were enclosed in an isolation chamber that was held in ambient air or an argon environment. STM tips were made by cutting or electrochemically etching Pt0.8Ir0.2 wire (California Fine Wire Company). Images were typically obtained at 25 °C with the sample potential being set between −0.5 and −0.7 V or +0.5 and +0.7 V and a tunneling current of approximately 20−100 pA. Typical scan rates were 4.7 lines per second, resulting in a total image time of 2.0 min. All images were background subtracted using SPIP image processing software. Drift correction procedures were applied to images selected for extracting unit cell information. Solutions of different carboxylic acids were prepared by dissolving solid compounds in octanoic or heptanoic acids. Their concentrations ranged from 10−4 to 10−6 M. Typically, a 10 μL aliquot of a carboxylic acid compound solution was placed onto HOPG or Au(111). A custom-made solution cell sample holder was available to accommodate the solution in contact with the substrate surface. DFT Calculations. Density functional calculations were performed on the free molecules beginning with a minimum energy structure obtained from molecular mechanics performed in Avogadro using the MMFF94 force field and steepest descent algorithm. This structure was used as a starting point for the DFT geometry optimization obtained using the B3LYP functional and 6-311G basis set in Gaussian 09.63 Time-dependent DFT was also used to calculate the electronic spectra and to obtain a more accurate value for the band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
Figure 1. Molecular structures and the corresponding density functional theory (DFT)-optimized structures (shown as CPKs) of MOF linkers: tetraphenylethene tetracarboxylic acid (ETC), tetraphenylethene tetrakis-phenyl carboxylic acid (ETTC), and tetraphenylethene bisphenyl carboxylic acid (BPDC). Labels 1 and 2 identify the phenyl rings.
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RESULTS AND DISCUSSION Linker Organization on HOPG. High-resolution STM images of the three acid linkers show that the molecules selfassemble at either the heptanoic or octanoic acid/graphite interface into well-ordered two-dimensional (2D) networks. Imaging results in Figure 2 were obtained using heptanoic acid as solvent. The observed molecular domains are uniform, highly stable, and extend over hundreds of nanometers (see Figure S2 in the Supporting Information). Domains meet at grain boundaries and otherwise form complete monolayers on the HOPG surface. The bright submolecular features observed in Figure 2a can be readily attributed to the phenyl groups in the ETC linker. We note that the ETC images (and those of BPDC and ETTC) were found to be qualitatively independent of polarity of the bias voltage in the range of −0.5 to −0.7 and +0.5 to
the Supporting Information). These molecules have not been previously studied by STM. Aliphatic acids are used as solvents. Tetraphenylethene itself can be easily functionalized and its derivatives are known for their strong emission and high quantum efficiencies when aggregated.56−58 TPE-based linkers have been recently used to synthesize luminescent MOFs19,58−61 for chemical sensing of species such as nitroaromatics and metal ions.19 Highly ordered pyrolytic graphite (HOPG) and Au(111) are used as solid supports in our studies as these conductors typically serve as electrodes in devices and are very appropriate for STM studies. The TPE-based MOF linkers used in this study include tetraphenylethene tetracarboxylic acid (ETC), tetraphenylethene bisphenyl carboxylic acid (BPDC), and tetraphenylethene tetrakis-phenyl carboxylic acid (ETTC). These moleB
DOI: 10.1021/acs.langmuir.9b00204 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. High-resolution images of (a) 5 × 10−4 M ETC, (b), 5 × 10−4 M BPDC, and (c) 5 × 10−4 M ETCC solutions in heptanoic acid deposited on HOPG. CPK models of the respective acid linkers are superimposed on the molecules observed in the images. Hydrogens in the CPK structures (except for the carboxylic acid protons) are omitted for clarity. The images were collected under ambient conditions with −0.4 V bias voltage and 50 pA tunneling current (a), +0.6 V and 20 pA (b), and −0.6 V and 35 pA (c).
Table 2 along with the area per molecule. These values were obtained from multiple drift corrected STM data, with a representative shown in the Supporting Information, Figure S3. Also given in Table 2 (and further described in Table S1 in the Supporting Information) are the unit cell parameters of an epitaxial cell having the correct orientation to the unit cell of the underlying substrate. Well-resolved phenyl groups of the TPE core dominate the STM image of the self-assembled BPDC molecules in Figure 2b. Tetraphenylethene cores in ETC and BPDC are electronically nearly identical and are therefore expected to be stabilized with similar probability upon adsorption on the HOPG surface. This expectation is confirmed by the STM images and reinforced by the comparable values of calculated twist angles of phenyl rings (labeled 1 in Figure 1) relative to the CC bond for the gas phase ETC and BPDC, which are 50 and 55°, respectively. This angle supports strong π−π interaction between the unsubstituted phenyls of adjacent BPDC synthons that form parallel rows of close-packed surface structures (see the later sections on adsorption to Au). Interestingly, the BPDC carboxylic-acid-terminated phenyl groups labeled 2 in Figure 1 appear very “dim” in the STM image consistent with their nearly perpendicular orientation relative to the substrate surface and the ethene center. Accordingly, the molecular conformation of BPDC on the HOPG surface is nearly the same as the one calculated for the gas phase molecule, where the angle subtended by phenyl ring 2 and the ethene plane is approximately 83° (Table 1). Neighboring molecular rows of BPDC are held together by hydrogen bonds between carboxylic acid groups of adjacent linkers. As in the case of the ETC self-assembled monolayer, the acid groups in BPDC are not seen in the STM image of the
+0.7 V. This is to be expected since the Fermi energy of HOPG (5.0 eV) lies intermediate between the calculated HOMO (6.43−5.76 eV) and LUMO (3.12−2.56 eV) of all three compounds as determined from TD-DFT. In the gas phase, according to our DFT calculations, the phenyl rings of the ETC synthon are rotated 50° relative to the ethene plane (Table 1 and Figure 1). On the surface, ETC Table 1. Angles Subtended by the Phenyl Rings Relative to the Ethene Plane in ETC, BPDC, and ETTC Based on DFTOptimized Gas Phase Molecular Structuresa phenyl ring angles relative to the ethene plane molecule
ethene and ring 1
ethene and ring 2
rings 1 and 2
ETC BPDC ETTC
50.0° 54.9° 54.8°
N/A 82.7° 84.5°
N/A 43.7° 37.5°
a
Phenyl ring numbering scheme is identified in Figure 1.
likely retains the C2 symmetry of its TPE core, which interacts strongly with the underlying graphite substrate via phenylgroup π−π interactions. The self-assembled framework is further directed by the intermolecular hydrogen bonds between adjacent COOH groups, which are not seen in the STM image. The factors that make them difficult to identify are their orientation (upright) relative to the surface and electronic effects. The calculated highest occupied MO and lowest unoccupied MO have very little density on the carboxylic acid functionalities, rendering them “invisible”, at least at the bias voltages employed in this study. The distances between the adjacent ETC molecules and the angle in the unit cell (nearly rectangular and occupied by one ETC) are listed in
Table 2. Unit Cell Dimensions, Epitaxial Lattice Parameters, and Areas/Molecule for ETC, BPDC, and ETTC Monolayers Adsorbed on HOPG in Alkyl Acidsa HOPG substrate unit cell parameters
epitaxial lattice parameters
molecule
a (nm)
b (nm)
α (deg)
area/molecule (nm )
a (nm)
b (nm)
α (deg)
area/molecule (nm2)
ETC BPDC ETTC
1.29 ± 0.04 2.3 ± 0.1 2.47 ± 0.05
1.48 ± 0.07 1.91 ± 0.04 2.34 ± 0.03
80 ± 2 64 ± 4 61 ± 3
1.88 ± 0.2 1.9 ± 0.2 5.0 ± 0.2
1.28 2.35 1.98
1.37 1.92 1.98
81.0 60.6 59.8
1.79 1.98 5.31
2
a
All adsorbates are arranged in a planar configuration on the substrate. BPDC has two molecules per unit cell. C
DOI: 10.1021/acs.langmuir.9b00204 Langmuir XXXX, XXX, XXX−XXX
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Langmuir surface framework (see Supporting Information Figure S2). Overall, the robust stability of the BPDC monolayer can be attributed to strong adsorbate−substrate interactions of the TPE core, the linker intermolecular π−π (e.g., the nonacid bearing phenyl groups) interactions, combined with the intermolecular hydrogen bonding. Two molecules occupy an oblique unit cell (Table 2 and Figure S3 in the Supporting Information) with the area per molecule being almost identical to that of ETC. The STM image of the ETTC (Figure 2c) monolayer reveals that its molecular alignment is analogous to that of the ETC linker (Figure 2a), where the molecular network is supported by intermolecular hydrogen bonding of the carboxylic acid groups. Because of larger molecular dimensions, ETTC assemblies have larger pore size and occupy more than twice the surface area per molecule compared to either ETC or BPDC. One molecule is assigned to an oblique unit cell (Table 2 and Figure S3 in the Supporting Information). In addition to the prominent tetraphenylethene core, the terminal phenyls (labeled 2 in Figure 1) are also clearly visible in the image of the adsorbed ETTC synthons. In the gas phase structure, the phenyl rings 2 in ETTC are nearly perpendicular (84.5°) to the CC plane, whereas phenyls marked 1 are rotated 54.8° relative to the same molecular moiety (Table 1 and Figure 1). At the surface, however, the flexible ETTC linkers distort driven by the terminal phenyls’ affinity to adsorb in a planar conformation for optimum π−π interaction with the HOPG surface. This change in the orientation of the terminal phenyls compresses the ETTC dihedral angle (angle between adjacent biphenyl arms attached to the same carbon of the CC bond, resembling scissors) to 54 ± 2° (Figure 3a)
The interfacial self-assembly of both BPDC and ETTC appears to be thermodynamically controlled with a welldefined single free energy minimum. At room temperature in the 10−4−10−6 M range of linker solution concentration, molecular organization and local geometry of BPDC and ETTC adsorbates remain unchanged (Figures 2 and S2 in the Supporting Information). A stable periodic arrangement is favored because of sufficiently strong directional adsorbate− substrate and adsorbate−adsorbate interactions. The hydrogen bonding distances for the three linkers averaged to 0.26 ± 0.03 nm. This value is in good agreement with the reported value ranging from 0.26 to 0.31 nm for the hydrogen bond length of carboxylic acids self-assembled on HOPG and Au(111).45,54,55,64 ETC adsorbed onto HOPG shows two different concentration-dependent polymorphs. Unlike the single structure seen at higher concentration (Figure 2a), a complex structure is observed upon adsorption from a 5 × 10−5 M heptanoic acid solution. This structure (Figure 4) is a pure 2D Kagome
Figure 4. High-resolution image of ETC 5 × 10−5 M solutions in heptanoic acid deposited on HOPG. CPK models of the acid linker are superimposed on the Kagome pattern of molecules observed in the monolayer. The image was acquired under ambient conditions with 0.6 V bias voltage and 20 pA tunneling current.
structure with a p6m symmetry, composed of regularly arranged triangular and hexagonal networks of pores stabilized by van der Waals adsorbate−substrate interactions and hydrogen bonding. Interestingly, Kagome lattices have been reported in three-dimensional (3D) MOFs with tunable cavities.60,65 Linker Organization on Au(111). Dramatically different self-assembled linker structures are observed when gold substrates are exposed to the same synthon solution concentration used in HOPG adsorption studies. On the Au(111) surface at a solution concentration in the 5 × 10−4 to 5 × 10−5 M range, all three linkers adopt vertical orientation directed by intermolecular van der Waals and π−π interactions and form parallel coherent columns of synthons (Figure 5). Although the π-orbital overlap between the adjacent phenyl groups in the linkers is weak, judging by the long interplanar distance of 0.35 nm (cross sections in Figure 5), it likely contributes to the overall columnar structure and stability of the linker assemblies. This columnar arrangement is confined to ≤30 nm2 close-packed islands densely covering gold terraces (Figure S4 in the Supporting Information). As in the case of HOPG, the observed molecular features for the three linkers on Au(111) were found to be essentially independent of bias
Figure 3. Measured dihedral angle in the ETTC linker adsorbed on (a) HOPG and on (b) Au(111). Both images were aquired under ambient conditions with −0.6 V bias voltage and 35 pA tunneling current and are drift corrected.
from the calculated gas phase linker scissor angle of 65°. The scissor angle on a gold support where the graphite−phenyl interaction is absent is indeed close to the gas phase at 66 ± 2° (Figure 3b). Similar scissor angle contraction in ETTC was reported when the linker was incorporated into MOF PCN128.58 In this MOF matrix, the TPE core has limited flexibility, whereas the outer phenyls distort such that the angle between adjacent biphenyl arms decreased to 55°, relative to that in a free ETTC molecule (∼65°).58 In contrast, the ETTC scissor angle increased to 86° in MOF PCN-94.60 D
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Figure 5. High-resolution images of 1 × 10−6 M ETC (a), 5 × 10−5 M BPDC (b), and 5 × 10−5 M ETTC (c) solutions in heptanoic acid deposited on Au(111). Vertical (lower right) and horizontal (lower left) cross-sectional profiles of the π−π stacked molecules accompany each image. CPK models of the organized linkers are overlaid on the linker assemblies observed in the images. Hydrogens in the CPK structures (except for the carboxylic acid protons) are omitted for clarity. The images were collected under ambient conditions with −0.7 V bias voltage and 50 pA tunneling current (a) and (c) and −0.8 V and 105 pA (b).
voltage polarity. The similarity in interplanar spacing for all three systems argues strongly for π−π stacking forces in the common core being the primary factor in determining the molecular structure. There is one significant difference between the vertical structures of the species with four carboxylates (ETC and ETTC) and the species with two carboxylates (BPDC): the spacing between the lamellae. We now consider a possible explanation of this difference. Previous STM studies have shown that aromatic carboxylic acids can form highly organized monolayers of upright standing molecules on metal surfaces such as gold, copper, and silver albeit under different experimental conditions than those reported in this study.66−70 For example, benzoic acid, isophthalic acid, and trimesic acid adsorbed on an Au(111) single crystal in 0.1 M HClO4 changed their orientation from planar to perpendicular under applied positive potential in ECSTM experiments.67 Several different aromatic carboxylic acids adsorbed onto silver- and copper-modified gold substrates from ethanolic solutions were imaged after being rinsed and dried.69,70 The images indicated that these molecules adopted an orientation perpendicular to the substrate. In each of the above studies, the acid molecules in the upright orientation were considered to be chemisorbed to metal surfaces via deprotonated carboxyl groups. The adjacent acid molecules were thought to be hydrogen bonded to their undissociated COOH groups.67,69,70 Since aromatic acids are stronger than aliphatic acids, ETC and ETTC dissolved in heptanoic acid at 10−4−10−5 M must become at least partially ionized and coordinate to the gold surface via two COO− groups (Figure 5a,c). The adsorbed linkers with their perpendicular orientation to the surface have a large dipole perpendicular to the Au(111) substrate because of the formation of image dipoles in the metal (ETTC example in Figure 6). This large dipole influences (repulsively) the adsorption of adjacent rows of stacked synthons. Some contribution from the image charges may operate even in the HOPG system.71 However, it seems that its role is slightly different from that observed in metals, judging from
Figure 6. Schematic of the organization of ETTC at the Au(111)/ heptanoic acid interface. Shown also are dipoles formed between the conducting substrate and a charged adsorbate.
the reports thus far reported. For example, the surface dielectric response of graphite is very different from a freeelectron metal.72 It is also found that semiconductor nanocrystals luminesce on graphite but not metals.73 Additionally, Brus and co-workers, through the use of electrostatic force microscopy, have found that perfect image charges are not formed in HOPG.74 They have argued that this is due to the large interlayer separation and the low density of states near the Fermi level. The vertical cross sections in Figure 5 indicate that the separation between the rows of ETC and ETTC molecules adsorbed on Au(111) is much larger than expected if it was solely due to hydrogen bonds between the undissociated COOH groups on the linkers. The average distance between the columns of ETC and ETTC was nearly identical and averaged to 0.9 ± 0.12 nm. The hydrogen bonds for the same linkers adsorbed on HOPG in a planar conformation (Figure 2) averaged to 0.26 ± 0.03 nm. Therefore, the large separation between the rows of chemisorbed ETC and ETTC linkers on Au(111) may be attributed to the interfacial dipole−dipole repulsion between the adjacent columns of chemisorbed synthons (Figure 6). This scenario is further supported by the structure of the BPDC linker on the gold surface. Figure 5b demonstrates that like the ETC and ETTC synthons, BPDC also forms ordered columns, approximately 2 E
DOI: 10.1021/acs.langmuir.9b00204 Langmuir XXXX, XXX, XXX−XXX
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Table 3. Unit Cell Dimensions, Epitaxial Lattice Parameters, and Areas/Molecule for ETC, BPDC, and ETTC Monolayers Adsorbed on Au(111) from Alkyl Acid Solutions More Concentrated Than 5 × 10−5 Ma Au(111) substrate unit cell parameters
epitaxial lattice parameters
molecule
a (nm)
b (nm)
α (deg)
area/molecule (nm2)
a (nm)
b (nm)
α (deg)
area/molecule (nm2)
ETC BPDC ETTC
2.20 ± 0.05 2.2 ± 0.1 2.4 ± 0.3
0.5 ± 0.2 0.5 ± 0.2 0.6 ± 0.1
77 ± 4 81 ± 6 74 ± 5
1.03 ± 0.2 1.1 ± 0.3 1.3 ± 0.4
2.08 2.29 2.36
0.58 0.58 0.58
72.9 78.9 72.2
1.15 1.25 1.30
All adsorbates are arranged in π−π stacked configuration on the substrate as shown in Figure 5.
a
Figure 7. Surface coverage of ETTC on Au(111) as a function of solution concentration in heptanoic acid: (a) 5.0 × 10−6 M, (b) 6.5 × 10−6 M, and (c) 1.0 × 10−5 M. Different polymorphs (I, II, and III) are identified in each figure as appropriate. The images were collected under ambient conditions with (a) −0.6 V bias voltage and 40 pA tunneling current, (b) −0.6 V and 35 pA, and (c) −0.7 V and 50 pA.
Figure 8. ETTC, 6.5 × 10−6 M solution, in heptanoic acid deposited on Au(111) forms three different polymorphs. High-resolution images of the polymorphs labeled I, II, and III, and the proposed models. High-resolution images of the polymorphs and the proposed models of monolayer organization and orientation are also shown. The structure of polymorph III is the same as that depicted in Figure 5c. The molecular models are larger than the molecules in the STM images. Hydrogens in the CPK structures (except for the carboxylic acid protons) are omitted for clarity. The images were collected under ambient conditions with −0.6 V bias voltage and 35 pA tunneling current.
nm wide of stacked synthons (Table 3 and Figure S5 in the Supporting Information). However, the thickness (a axis) of the columns is consistent with the BPDC synthons laying with their two trans biphenyl arms bearing the carboxylic acid groups parallel and one of the unsubstituted phenyls being perpendicular to the substrate. In this orientation, the BPDC synthons cannot effectively adsorb to the gold surface via deprotonated carboxyl groups. Without the interfacial dipole repulsion proposed for ETC and ETTC, the separation between the rows of BPDC stacked synthons is approximately half the distance (0.63 ± 0.14 nm), separating the columns of ETC and ETTC (0.9 ± 0.12 nm). The terminal COOH functionalities on the adsorbed BPDC most likely interact with the aliphatic acid solvent rather than with each other. In a
model structure where two solvent molecules bridge two BPDC synthons (Figure S6 in the Supporting Information), the calculated separation between the molecular columns is 0.6 nm, nearly the same as the experimental distance. One final important observation about the orientation of BPDC on Au(111) is that the CC bond of its TPE core is not parallel to the substrate. Based on the molecular dimensions (Figure S1 in the Supporting Information) and the unit cell parameters (Table 3), it is most likely that the ETC and ETTC tetraphenylethene cores are nearly normal to the gold surface. The two-dimensional lattice parameters in Table 3 (Figure S5 in the Supporting Information) confirm that for all three linkers, the unit cell is oblique and contains one molecule. F
DOI: 10.1021/acs.langmuir.9b00204 Langmuir XXXX, XXX, XXX−XXX
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Figure 9. BPDC, 5 × 10−6 M solution, in heptanoic acid deposited on Au(111) forms three different polymorphs identified as I′, II′, and III′ in the figure on far left. High-resolution images of the polymorphs and the proposed models of monolayer organization and orientation are also shown. The molecular models are larger than the molecules in the STM images. Hydrogens in the CPK structures (except for the carboxylic acid protons) are omitted for clarity. The images were collected under ambient conditions with −0.5 V bias voltage and 60 pA tunneling current.
close-packed ETTC synthons engaged in π−π bonding between terminal phenyls (labeled 2 in Figure 1). The structure of polymorph III in Figure 8 is identical to the one shown in Figure 5c. At low solute concentration (
