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Hexagonal Molecular Tiling by Hexagonal Macrocycles at the Liquid/Solid Interface: Structural Effects on Packing Geometry Kohei Iritani, Motoki Ikeda, Anna Yang, Kazukuni Tahara, Keiji Hirose, Jeffrey Moore, and Yoshito Tobe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03007 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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Hexagonal

Molecular

Tiling

by

Hexagonal

Macrocycles at the Liquid/Solid Interface: Structural Effects on Packing Geometry Kohei Iritani,† Motoki Ikeda,† Anna Yang,§ Kazukuni Tahara,†,‡ Keiji Hirose,† Jeffrey S. Moore,§,* Yoshito Tobe†,#,*

†

Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University,

Toyonaka, Osaka 560-8531, Japan,

§

Departments of Chemistry and Beckman Institute for

Advanced Science and Technology, University of Illinois at Urbana Champaign, Urbana, Illinois 61801, United States, ‡Department of Applied Chemistry, School of Science and Technology, Meiji University, Kawasaki, Kanagawa 214-8571, Japan, #The Institute of Scientific and Industrial Research, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan

*To whom correspondence should be addressed. E-mail: [email protected] (J.S.M.); [email protected] (Y.T.).

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KEYWORDS Self-assembly, Liquid-solid interface, Scanning tunneling microscopy, Hexagonal macrocycles, Molecular tiling

ABSTRACT We

present

here

hexagonal

tiling

using

hexagonal

phenylene-ethynylene

and

phenylene-butadiynylene macrocycles attached by alkyl ester groups, PEM-C6 and PBM-C8, respectively, or triethylene glycol ester groups, PEM-TEG and PBM-TEG, respectively, at each vertex of the macrocyclic periphery at the liquid/solid interface. In this study, we focused on the effects of macrocyclic core size and the chemical properties of side chains attached to macrocyclic cores as well as solute concentrations on hexagonal geometry of self-assembled monolayers. STM observations at the 1,2,4-trichrolobenzene/graphite interface revealed that PEM-C6 formed a honeycomb structure by van der Waals interactions between the interdigitated alkyl chains. However, upon increasing solute concentration, it changed to more dense hexagonal structure (tentatively called loose hexagonal structure I). In contrast, PBM-C8 formed loose hexagonal structure II of a slightly different packing mode at low concentration, while at high concentration, it formed a high density hexagonal structure in which alkyl chains are not adsorbed on the surface (dense hexagonal structure). In the dense hexagonal structure, macrocyclic cores are linked by

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hydrogen bonds between the ester carbonyl oxygen and aromatic hydrogen atoms of the neighboring macrocycles. The packing geometries of loose hexagonal structures of PEM-C6 and PBM-C8 are different due to the different distance between the attachment of alkyl ester groups which are located in confined space. On the other hand, PEM-TEG and PBM-TEG formed dense hexagonal structures, similar to PBM-C8 at high concentration, with their TEG units not adsorbed on the surface.

INTRODUCTION Two-dimensional (2D) monolayers formed by self-assembly of organic molecules on solid surfaces have attracted a great deal of interest in connection with fundamental insight into crystallization process and prospects for potential applications in molecular-scale electronics and tailor made catalysis.1–4 Among various kinds of patterns, tiling of surfaces by regular polygons is a topic of particular interest because of its diverse relevance ranging from art, mathematics, material physics to molecular science.3,5–7 According to the systematic treatment by Kepler, of the 11 tessellations in the Euclidean plane, three consists of regular polygons (i.e., regular tilings with triangles, squares, or hexagons) and the other eight known as semiregular Archimedean tilings are constructed by combination of two or more different polygons.8,9 In contrast to abundant regular tilings, examples of semiregular tilings are limited6,10,11 except for Kagomé pattern which has been

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reported more frequently.12–16 Most of the tilings have been constructed by supramolecular assembly of diagonal, trigonal, or tetragonal building blocks bearing functional groups designed to form noncovalent intermolecular linkages by hydrogen bonding, van der Waals interaction, or coordination to metal centers.1–4,17–19 In spite of the facile formation of such molecular assembly, due to reversible nature of the intermolecular linkages, such space created by self-assembly is fragile and can be modified by a variety of factors such as solute concentration, temperature, solvent polarity, and co-adsorption of solvent or guest molecules.20,21 In contrast, shape-persistent polygonal macrocycles are advantageous as building blocks of molecular tiling, because the shape and size of polygons are defined by robust covalent bonds. The shape-persistent porous space of the macrocycles can also be used as selective binding sites to guest molecules or clusters on surfaces.22– 28

However, because the synthesis of macrocycles is in general more laborious than that of acyclic

building blocks, the use of macrocycles for molecular tiling is much less studied. In studies on molecular tilings, scanning tunneling microscopy (STM) is used to visualize the molecular assemblies on surfaces with sub-molecular resolution under ultrahigh vacuum conditions or at the liquid/solid interfaces.29–31 Shape-persistent macrocycles are broadly used in supramolecular chemistry32–35 not only in 3D space but also in 2D space forming self-assembled monolayers on surfaces.12,36–39 As building blocks for regular tiling, whereas a number of triangle macrocycles have been developed,12,40–42

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much less has been studied for square24,26,43 and hexagonal44–46 macrocycles because of their lower accessibility. Similar to the triangle macrocylces,19,47 the 2D arrangement of square and hexagonal macrocycles must be affected by not only the structural parameters of macrocycles, i.e., the size of macrocyclic core and substituents attached to the macrocyclic periphery, but also conditions of monolayer formation such as solute concentration, temperature, and solvent types. Previously we demonstrated the effect of the constituent elements of macrocyclic core and substituents on the packing patterns of square macrocycles.26 However, to the best of our knowledge, there is no systematic study on the effects of core size, peripheral substituents, and concentration of hexagonal macrocycles on the packing geometry and density of their hexagonal tiling. In order to clarify these effects, we investigated hexagonal tiling using four hexagonal macrocycles as tile components (Figure 1). These include phenylene-ethynylene macrocycles attached by six hexyl ester groups PEM-C6 or triethylene glycol (TEG) ester groups PEM-TEG,48 and phenylene-butadiynylene macrocycles bearing six octyl ester groups PBM-C8,49 or TEG ester groups PBM-TEG.49 For alkyl ester derivatives PEM-C6 and PBM-C8, we expected that the alkyl chain would play a major role for the formation of self-assembled monolayers due to intermolecular van der Waals interactions. On the other hand, for PEM-TEG and PBM-TEG, such intermolecular interactions between the TEG chains would be less favorable than the alkyl chains because of electrostatic repulsion between the lone pairs of oxygen atoms.50,51 Indeed, it has been reported that a dendron

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having TEG units formed self-assembled monolayers in which the TEG units are not adsorbed on graphite surface.52 In view of our previous work on square macrocycles,26 however, in both alkyl and TEG esters, hydrogen bonds between the carbonyl oxygen atoms and the aromatic hydrogen atoms would also participate to assemble molecules in 2D. Another purpose of this study is to elucidate the effect of the core size. Since the distance (ca. 1 nm) between the attachment of a pair of alkyl chains of PEM-C6 is suitable for them to interdigitate with those of a neighboring molecule maximizing van der Waals interactions, as demonstrated for linear phenylene-ethynylene molecules53–55 and triangle macrocycles,12,19,47 we expect that PEM-C6 would form such intermolecular linkages. On the other hand, such interaction pattern is not expected for PBM-C8 because of larger distance between the attachment of adjacent alkyl chains.40 Moreover, the location of alkyl chains laid on the surface within confined space created by the macrocycles would be affected by their core size which defines the shape and size of the pore space. Experimentally, we use 1,2,4-trichlorobenzene (TCB) as a solvent throughout the work because of its high solubilizing ability to relatively insoluble macrocycles, which made us possible to investigate the effect of concentration on the structural patterns of self-assembled 2D monolayers. Finally, formation of multilayers is possible. Recently, studies on the multilayer formation of macrocycles were reported. 28,56,57

For example, Flood and Tait reported that a polarized macrocycle, which strongly aggregates

in solution, formed multilayers of up to five molecular layers at the liquid/solid interface.28

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Accordingly, we paid attention for the formation of stacked multilayer, although the self-aggregation powers of the macrocycles in this study are not very strong.48,49,58

Figure 1. Chemical structures of PEM-C6, PEM-TEG, PBM-C8, and PBM-TEG.

EXPERIMENTAL SECTION Synthesis. Synthesis of PEM-TEG,48 PBM-C8,49 and PBM-TEG49 are reported previously. PEM-C6 was prepared by the same method reported previously (See Supporting Information). Details of STM Observations at the Liquid/Solid Interface. All experiments were performed at 23–27 °C using a Nanoscope V (Bruker AXS) with an external pulse/function generator (Agilent 33220A) with negative sample bias. All STM images were acquired in the constant current mode. Tips were mechanically cut from Pt/Ir wire (80%/20%, diameter 0.25 mm). During scanning,

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electric pulses of −1.6 to −2.8 V with a pulse width of 880 nsec were occasionally applied using a pulse/function generator to clean the point end of an STM tip. Prior to imaging, compounds were dissolved in commercially available 1,2,4-trichlorobenzene (TCB) at various solute concentrations. Immediately before use, the HOPG substrate (grade ZYB, Momentive Performance Material Quartz Inc., Strongsville, OH) was cleaved using adhesive tape. A homemade liquid cell placed on the HOPG substrate was employed to minimize the effect of solvent evaporation using a sample solution of 30 µL. All STM observations of the monolayers were performed at the interface between TCB and HOPG using the HOPG substrate fitted with the liquid cell after annealing treatment at 70 °C for 1 h in an oven to avoid kinetically trapped metastable structures.59 By changing the tunneling parameters during the STM imaging, namely, the voltage applied to the underlying HOPG substrate, it was possible to switch from the visualization of the adsorbate layer to that of the substrate. This enabled us to correct for drift effects by the use of SPIP (scanning probe image processer) software (version 6.2.4., Image Metrology A/S, Hørsholm). Unit cell parameters are determined from more than 30 experimental values of at least two calibrated STM images. Details of Molecular Mechanics Simulations. Molecular mechanics (MM) simulations were performed with the Materials Studio 8.0 using the Forcite module with COMPASS force field. The initial structures of PEM-C6, PBM-C8, and their derivatives having ethyl ester groups were built

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from the respective molecular models which were optimized by the semiempirical PM3 method. Then, the orientation of the alkyl chains relative to the π-system was adjusted based on that observed in the STM images. The molecules were placed 0.35 nm above the first layer of a periodic two-layer sheet of graphene with an interlayer distance of 0.335 nm to mimic graphite. The two-layer graphene structure was frozen during the simulations, and a cutoff of 2.0 nm was applied for the van der Waals interactions (Lennard-Jones type). Periodic boundary conditions (PBC) for simulations are described in the figure captions.

RESULTS AND DISCUSSION STM Observations of Self-Assembled Monolayers of PEM-C6. Figures 2a and S1 show typical STM images of a monolayer formed by PEM-C6 at concentration of 1.0 × 10−6 M in TCB. Bright cyclic features correspond to conjugated macrocyclic cores of PEM-C6 and they align to form a porous hexagonal pattern of a honeycomb shape. The darker area between the cores are assigned to four alkyl chains which are interdigitated to each other. These results indicate that PEM-C6 forms a honeycomb structure in which the molecules are connected by van der Waals interactions between alkyl chains which are interdigitated, as expected. Unit cell parameters are a =

b = 4.7 ± 0.1 nm, γ = 60 ± 1º. Similar to the triangle building blocks bearing six alkyl chains,60 homochiral domains arising from different orientation of the alkyl chains, tentatively called

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clockwise or anticlockwise orientation depending on the direction of alkyl chains located at inner periphery of the porous network,60 were observed. Figure 2a shows a domain of anticlockwise honeycomb structure, which was judged by the relative displacement of adjacent macrocyclic cores. A network model built by molecular mechanics simulations is displayed in Figure 2b. To distinguish two enantiomeric honeycomb structures accurately, we determined angle α between the unit cell vector a and one of the normals of main symmetric axes of graphite.61,62 In anticlockwise domains, the tilt angle of the unit cell vector is +18 ± 1º (clockwise) with respect to the reference axis (Figure 2a), whereas in clockwise domains of the corresponding angle is anticlockwise, −18 ± 1º (Figure S2). In all networks described hereafter, which are formed by alkyl chain interactions, we observed enantiomeric domains arising from different orientation of the alkyl chains.

Figure 2. (a) STM image of a monolayer formed by PEM-C6 (1.0 × 10−6 M, Iset = 20 pA and Vbias = –490 mV) at the TCB/graphite interface. Unit cell parameters are a = b = 4.7 ± 0.1 nm, γ = 60 ± 1º. White arrows indicate the directions of main symmetry axes of underlying graphite. Yellow line is 10


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parallel to one of the normals of main symmetric axes of graphite. The angle α is +18 ± 1º (anticlockwise domain). (b) A molecular model of an anticlockwise honeycomb structure of PEM-C6 on graphene bilayer (optimized by MM simulations under periodic boundary conditions (PBC): a = b = 4.67 nm, γ = 60.0º). Color code for atoms: blue; C of PEM-C6 molecules, grey; C of the graphene bilayer, red; O, white; H.

By increasing concentration to 1.0 × 10−5 M, a high density area composed of clear circles and fuzzy circles appeared in addition to the porous hexagonal areas exclusively observed at lower concentration (Figures 3a and S3). The location of the fuzzy features matches that of the pore space of the porous area as indicated by green solid circles in Figure 3b, indicating that the fuzzy features are due to the macrocyclic cores of PEM-C6 molecules weakly adsorbed in the honeycomb pores. We assume that co-adsorbed PEM-C6 molecules in the pores adopt a windmill-like conformation, similar to that of the triangle building block with chiral side chains.63 The co-adsorbed PEM-C6 molecules can adopt enantiomeric geometries, one in which the alkyl chains orient in the same direction as that of the inner periphery of the honeycomb network (Figure 3c), and the other in the opposite direction (Figure 3d). Unfortunately, it was not possible to determine experimentally which enantiomeric form the co-adsorbed PEM-C6 molecules adopted. However, on the basis of molecular mechanics simulations under PBC, the monolayer in Figure 3d is estimated to be more

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stable than that in Figure 3c by 3.4 kcal/mol. The same sense of diastereomeric preference was observed for co-adsorption of a triangle building block in its windmill conformation into a molecular matrix.63 The reason for the fuzzy appearance of the co-adsorbed PEM-C6 molecules is their lateral mobility owing to its weak intermolecular interaction with the surrounding molecules. Unit cell parameters of the porous honeycomb area are a = b = 4.7 ± 0.1 nm, γ = 60 ± 1º, whereas those of the high density area are a = b = 2.7 ± 0.1 nm, γ = 60 ± 1º.

Figure 3. (a) STM image of a monolayer formed by PEM-C6 (1.0 × 10−5 M, Iset = 77 pA and Vbias = –770 mV) at the TCB/graphite interface. Unit cell parameters of the porous honeycomb area and high density area are a = b = 4.7 ± 0.1 nm, γ = 60 ± 1º, and a’ = b’ = 2.7 ± 0.1 nm, γ’ = 60 ± 1º, respectively. White arrows indicate the directions of main symmetry axes of underlying graphite.

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(b) The same STM image as (a), in which the fuzzy circles corresponded to co-adsorbed PEM-C6 molecules are marked by green solid circles. (c, d) Molecular models of honeycomb structures with co-adsorbed PEM-C6 molecules (green) whose alkyl chains adopting the same orientation as that of inner periphery of the honeycomb network (c) or the opposite orientation (d) on graphene bilayer (optimized under PBC: a = b = 4.67 nm, γ = 60.0º). Color code for atoms: blue; C of PEM-C6 molecules forming the honeycomb structure, green; C of co-adsorbed PEM-C6 molecules in the pores, grey; C of the graphene bilayer, red; O, white; H.

The molecular network observed at further higher concentration (4.6 × 10−5 M) is shown in Figures 4a and S4. Unlike the case of lower concentrations, porous network was not observed. Moreover, all macrocyclic cores exhibit uniform contrast, suggesting the packing mode is different from that of the high density pattern observed at 1.0 × 10−5 M (Figure 3a), although the unit cell parameters, a = b = 2.7 ± 0.1 nm, γ = 60 ± 1º, are identical. These results indicate that PEM-C6 formed a hexagonal structure (tentatively called loose hexagonal structure I) in which alkyl chains align in a side-by-side fashion as shown in Figure 4b. Even at the highest attainable concentration (ca. 5 × 10−5 M) due to limited solubility, no apparent sign of multilayer formation was noticed; bright spots and streaky images, indicative of multilayers (vide infra), were seldom observed (Figure S4).

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Figure 4. (a) STM image of a monolayer formed by PEM-C6 (4.6 × 10−5 M, Iset = 22 pA and Vbias = –440 mV) at the TCB/graphite interface. Unit cell parameters are a = b = 2.7 ± 0.1 nm, γ = 60 ± 1º. White arrows indicate the directions of main symmetry axes of underlying graphite. (b) A molecular model of loose hexagonal structure I of PEM-C6 on graphene bilayer (optimized under PBC: a = b = 2.66 nm, γ = 60.0º).

As described above, the monolayer morphology is critically dependent on the concentration of PEM-C6. As a measure of relative stability, we calculated surface densities of the three different monolayer patterns by dividing the number of adsorbed heavy atoms (non-hydrogen atoms) by the unit cell area used for molecular mechanics simulations (Table 1). As a result, the surface densities of the porous honeycomb structure, the same structure containing co-adsorbed PEM-C6 molecules, and loose hexagonal structure I were calculated to be 10.8, 16.2, and 16.6 nm−2, respectively, indicating the last pattern observed at highest concentration has the highest density.

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Table 1. Summary of Experimental Unit Cell Parameters, Those Used for Simulation under Periodic Boundary Conditions (PBC), Unit Cell Area, and Monolayer Density for the Monolayers Formed by PEM-C6 Monolayer Structure Porous Honeycomb Honeycomb with Co-Adsorption Loose Hexagonal I a

Experimental Unit Cell Parameters

PBC

a = b = 4.7 ± 0.1 nm, a = b = 4.67 nm,

γ = 60 ± 1º

γ = 60.0º

a = b = 4.7 ± 0.1 nm, a = b = 4.67 nm,

γ = 60 ± 1º

γ = 60.0º

a = b = 2.7 ± 0.1 nm, a = b = 2.66 nm,

γ = 60 ± 1º

γ = 60.0º

Unit Cell 2 a

Area (nm )

Zb

Monolayer Density (nm−2)c

18.9

2

10.8

18.9

3

16.2

6.13

1

16.6

Area per unit cell calculated for PBC. b Number of molecules in a unit cell. c Monolayer density

calculated by (Number of adsorbed heavy atoms per unit cell)/Area.

STM Observations of Self-Assembled Monolayers of PEM-TEG. Next, to examine the effect of chemical property of peripheral substituents on packing geometries, self-assembly of PEM-TEG having TEG groups was investigated. At concentration of 2.6 × 10−5 M, two different packing patterns were observed (one is shown in Figures 5a and S5a, the other is shown in Figures 5b and S5b). Though the domain size of each structure was too large to determine the area ratio, typically larger than a hundred square nanometer (Figure S5), the hexagonal structure (Figure 5a) was observed more frequently. In Figure 5a, bright hexagonal features corresponding to macrocyclic cores pack in a hexagonal arrangement (hereby tentatively called dense hexagonal pattern) with unit 15


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cell parameters of a = b = 2.2 ± 0.1 nm, γ = 60 ± 1º. The small unit cell vectors indicate that the whole TEG ester group cannot be adsorbed on the surface and its terminal should orient in the solution phase. A reasonable structural model can be constructed by a PEM derivative in which the TEG ester group is replaced by ethyl ester as shown in Figure 5c. Note that the terminal methyl groups orient up with respect to the graphite surface, meaning that CH2OCH2CH2OCH2CH2OCH3 portion of the TEG unit are not be adsorbed on the surface and dissolved in solution phase. The model also indicates that the adsorbed molecules are connected by hydrogen bonds between the ester carbonyl oxygen and aromatic hydrogen atoms of neighboring molecules. Besides the dense hexagonal structure, a non-hexagonal pattern (hereby called zigzag structure), in which macrocyclic cores in a row are tilted with respect to those in the neighboring rows, was observed (Figure 5b). Unit cell parameters are a = 2.2 ± 0.1 nm, b = 4.5 ± 0.1 nm, γ = 90 ± 1º. As shown in a structural model (Figure 5d), we assume the macrocycles are linked by hydrogen bonds only between those adopting the same orientation within a row. In all networks which are formed by hydrogen bonds, we observed enantiomeric domains arising from different orientation of the carbonyl groups, similar to those formed by van der Waals interactions.

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Figure 5. (a, b) STM images of a monolayer formed by PEM-TEG (2.6 × 10−5 M, Iset = 62 pA and Vbias = –450 mV for Figure 5a, Iset = 93 pA and Vbias = –488 mV for Figure 5b) at the TCB/graphite interface. Unit cell parameters for dense hexagonal and zigzag structures are a = b = 2.2 ± 0.1 nm, γ = 60 ± 1º and a = 2.2 ± 0.1 nm, b = 4.5 ± 0.1 nm, γ = 90 ± 1º, respectively. Magenta and cyan arrows in (b) show molecular rows in which macrocyclic cores are tilted to each other. White arrows indicate the directions of main symmetry axes of underlying graphite. (c, d) Molecular models of dense hexagonal (c) and zigzag (d) structures formed by a PEM derivative bearing ethyl ester groups on graphene bilayer (optimized under PBC: a = b = 2.13 nm, γ = 60.0º for dense hexagonal structure, a = 2.13 nm, b = 4.43 nm, γ = 90.0º for zigzag structure). Yellow circles indicate hydrogen bonds between carbonyl oxygen and phenylene hydrogen atoms.

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At higher concentration (2.0 × 10−4 M), only dense hexagonal structure was observed (Figures 6a and S6). As shown in Table 2, surface density of dense hexagonal structure (18.3 nm−2) is higher than that of zigzag structure (16.1 nm−2), in accord with the general tendency of preferred formation of the more densely packed form at higher concentration. In addition, we noticed the presence of some bright round features as well as deformed and sometimes streaky features which may be due to stacking of the macrocycle. Figure 6b shows a height profile along the blue line in Figure 6a. Because the apparent height of the bright features is approximately twice as those of the PEM monolayer, we assume that the bright features is due to the formation of stacked dimer in which the macrocycle overlays on the PEM core of the monolayer. Though it has been reported that a strongly self-aggregating macrocycle forms multilayers ranging from double to quintuple layers by self-assembly at the liquid/solid interface,28 the scattered appearance of the bright features must be due to lower aggregation property of PEM-TEG in solution.48,58

Figure 6. (a) STM image of a monolayer formed by PEM-TEG (2.0 × 10−4 M, Iset = 50 pA and Vbias = –500 mV) at the TCB/graphite interface. Unit cell parameters are a = b = 2.2 ± 0.1 nm, γ = 18


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60 ± 1º. White arrows indicate the directions of main symmetry axes of underlying graphite. (b) An apparent height profile along the blue line in (a).

Table 2. Summary of Experimental Unit Cell Parameters, Those Used for Simulation under Periodic Boundary Conditions (PBC), Unit Cell Area, and Monolayer Density for the Monolayers Formed by PEM-TEG Monolayer Structure Zigzag Dense Hexagonal a


PBC

a = 2.2 ± 0.1 nm,

a = 2.13 nm,

b = 4.5 ± 0.1 nm,

b = 4.43 nm,

γ = 90 ± 1º

γ = 90.0º

a = b = 2.2 ± 0.1 nm, a = b = 2.13 nm,

γ = 60 ± 1º

γ = 60.0º

Unit Cell 2 a

Area (nm )

Zb


9.44

2

16.1

3.93

1

18.3



STM Observations of Self-Assembled Monolayers of PBM-C8. Self-assembly of PBM-C8 having larger macrocyclic core was investigated to study the effect of the ring size, more specifically the distance between the attachment of alkyl ester substituents on the macrocyclic core. Figures 7a and S7 display typical STM images obtained using TCB solution of 1.0 × 10−6 M. On the basis of the unit cell parameters, a = b = 3.1 ± 0.1 nm, γ = 60 ± 1º, we deduced that PBM-C8 formed a loose hexagonal structure (loose hexagonal structure II) shown in Figure 7b. Note that the 19


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alkyl chains of PBM-C8 most likely do not align parallel to those of neighboring molecules, in either interdigitated or side-by-side manner, because of the limited space between the macrocyclic cores, although the position of the alkyl chains could not be determined experimentally. The different behavior of PBM-C8 from that of PEM-C6 is ascribed to different distance between attachment of alkyl chains on the macrocyclic frameworks as described later.

Figure 7. (a) STM image of a monolayer formed by PBM-C8 (1.0 × 10−6 M, Iset = 200 pA and Vbias = –130 mV) at the TCB/graphite interface. Unit cell parameters are a = b = 3.1 ± 0.1 nm, γ = 60 ± 1º. White arrows indicate the directions of main symmetry axes of underlying graphite. (b) A molecular model of loose hexagonal structure II of PBM-C8 on graphene bilayer (optimized under PBC: a = b = 3.14 nm, γ = 60.0º).

When the solute concentration was increased to 1.0 × 10−5 M, in addition to domains of loose hexagonal structure II, a dense hexagonal structure in which the octyl chains point toward the solution phase was observed in spite of the presence of alkyl chains which tend to adsorb on 20


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graphite (Figure S8, lower-right). At further higher concentration (4.6 × 10−4 M), the proportion of the dense hexagonal structure increased, though domains of loose hexagonal structure II coexisted (Figure 8a). Unit cell parameters of the dense hexagonal structure are a = b = 2.5 ± 0.1 nm, γ = 60 ± 1º. A network model of the dense hexagonal structure composed of a PBM derivative having ethyl ester groups is shown in Figure 8b. We assume that the dense hexagonal structure of PBM-C8 was stabilized by hydrogen bonds between the ester carbonyl oxygen and the aromatic hydrogen atoms similar to the same network structure of PEM-TEG.

Figure 8. (a) STM image of a monolayer formed by PBM-C8 (4.6 × 10−4 M, Iset = 50 pA and Vbias = –350 mV) at the TCB/graphite interface. Unit cell parameters of the loose and dense hexagonal structures are a’ = b’ = 3.1 ± 0.1 nm, γ’ = 60 ± 1º and a = b = 2.5 ± 0.1 nm, γ = 60 ± 1º, respectively. White arrows indicate the directions of main symmetry axes of underlying graphite. (b) A molecular model of a dense hexagonal structure of a PBM derivative having ethyl ester groups on graphene bilayer (optimized under PBC: a = b = 2.50 nm, γ = 60.0º). Yellow circle indicates hydrogen bonds between the ester carbonyl oxygen and aromatic hydrogen atoms. 21


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Surface densities of the loose and dense hexagonal structures of PBM-C8 were calculated to be 14.8 and 15.5 nm−2, respectively (Table 3). For loose hexagonal structure II of PBM-C8, we consider that van der Waals interactions between alkyl chains are not favorable due to space confinement unlike the case of PEM-C6. As the result, higher density structure, namely the dense hexagonal structure, was also observed in the monolayer of PBM-C8.

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Table 3. Summary of Experimental Unit Cell Parameters, Those Used for Simulation under Periodic Boundary Conditions (PBC), Unit Cell Area, and Monolayer Density for the Monolayers Formed by PBM-C8 Monolayer Structure Loose Hexagonal II Dense Hexagonal a


PBC

a = b = 3.1 ± 0.1 nm, a = b = 3.14 nm,

γ = 60 ± 1º

γ = 60.0º

a = b = 2.5 ± 0.1 nm, a = b = 2.50 nm,

γ = 60 ± 1º

γ = 60.0º

Unit Cell 2 a

Area (nm )

Zb


8.54

1

14.8

5.41

1

15.5



STM Observations of Self-Assembled Monolayers of PBM-TEG. Finally, monolayer formed by PBM-TEG was investigated. At both concentrations of 2.0 × 10−6 M and 2.0 × 10−5 M, only dense hexagonal structure was observed (Figures 9a and S9 for 2.0 × 10−5 M and Figure S10 for 2.0 × 10−6 M). On the basis of the unit cell parameters, we deduce that, similar to the dense hexagonal pattern of PEM-TEG, only the CO2CH2 portion of the side chain is adsorbed on the surface and the molecules are connected by hydrogen bonds between the ester carbonyl oxygen and the aromatic hydrogen of a neighboring molecule as shown in Figure 9b.

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Figure 9. (a) STM image of a monolayer formed by PBM-TEG (2.0 × 10−5 M, Iset = 50 pA and Vbias = –190 mV) at the TCB/graphite interface. Unit cell parameters are a = b = 2.4 ± 0.1 nm, γ = 60 ± 1º. White arrows indicate the directions of main symmetry axes of underlying graphite. (b) A molecular model of a dense hexagonal structure of a PBM derivative having ethyl ester groups on graphene bilayer (optimized under PBC: a = b = 2.50 nm, γ = 60.0º). Yellow circle indicates hydrogen bonds between the ester carbonyl oxygen and aromatic hydrogen atoms.

Summary of STM Observations. The results of STM observations are summarized in Table 4. Five network patterns, porous honeycomb, loose hexagonal I and II, dense hexagonal, and zigzag structures were observed. In general, more densely packed structures were observed with increasing solute concentration principally because of increasing substrate-molecule interaction. For PEM-C6, porous honeycomb structure was observed at relatively low concentration with its alkyl chains interdigitated to those of the neighboring molecule, whereas at high concentration loose hexagonal structure I of higher density was observed predominantly. On the other hand, PBM-C8 formed loose hexagonal structure II at low concentration, while at high concentration, in addition to the 24


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loose hexagonal structure, dense hexagonal structure was observed. For both macrocycles bearing TEG ester groups, PEM-TEG and PBM-TEG, the dense hexagonal structure was observed in which most of the TEG group is not adsorbed on the surface. In addition, for PEM-TEG, zigzag structure was observed at low concentration. In both dense hexagonal and zigzag patterns, the molecules are connected by hydrogen bonds between the carboxyl oxygen and aromatic hydrogen atoms.

Table 4. Summary of STM Observations of Monolayers Formed by PEM and PBM Derivatives

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Effect of Macrocyclic Ring Size. The most important outcome of this work is the effect of the macrocyclic ring size on the packing geometry of alkyl ester-substituted PEM-C6 and PBM-C8. At low

concentration,

PEM-C6

formed

porous

honeycomb

structure,

because

the

phenylene-ethynylene unit offers a suitable distance of attachment for alkyl chains to adopt the interdigitated geometry forming a van der Waals linkage.12,53–55 On the other hand, since the butadiynylene spacer is too large for alkyl chains attached to the phenylene units to interdigitate,40 the corresponding honeycomb structure of PBM-C8 is not favorable. Moreover, at higher concentration, whereas phase transition of PEM-C6 resulted in loose hexagonal structure I in which the alkyl chains interact in a side-by-side manner, in loose hexagonal structure II of PBM-C8 the alkyl chains do not seem to interact to each other. As a result, the macrocyclic cores of PEM-C6 are arranged so that all edges of neighboring molecules face to each other (Figure 4: loose hexagonal structure I), whereas those of PBM-C8 are arranged so that all vertices of neighboring molecules face to each other (Figure 7: loose hexagonal structure II). In order to elucidate the reason for the different packing patterns of the loose hexagonal structures of PEM-C6 and PBM-C8, the relative energies of loose hexagonal structures I and II were calculated for both PEM-C6 and PBM-C8 by molecular mechanics simulations under PBC. Figure 10 shows optimized structures of single PEM-C6 and PBM-C8 molecules in pattern I displayed in gray and those of pattern II overlaid in blue. Apparently, the discrepancy between the molecular structures in patterns I and II of PBM-C8

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is larger than that of PEM-C6. The alkyl chains of PBM-C8 in pattern I are significantly distorted from a straight geometry. As shown in Table 5, since the total energy for packing pattern I of PEM-C6 is smaller than that of pattern II, and for PBM-C8 the total energy of pattern II is smaller than that of I, the result of simulations is consistent with the experimental observations. More specifically, whereas the internal energy of hypothetical molecule of PEM-C6 in packing pattern II is slightly more favorable, the intermolecular energy favors pattern I much more, resulting in the preference of pattern I. On the other hand, for PBM-C8, the large disadvantage in the internal energy of pattern I due to significant distortion of the alkyl chains is not compensated by the favorable intermolecular energy, favoring pattern II. It is thus revealed that the position of attachment of the alkyl chains on the macrocyclic framework plays a significant role in the 2D packing patterns of hexagonal macrocycles.

Figure 10. Molecular models of (a) PEM-C6 and (b) PBM-C8 optimized by MM simulations (gray for pattern I and blue for pattern II)

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Table 5. Total, Intermolecular, and Internal Energies for Monolayers Formed by PEM-C6 and PBM-C8 Calculated by MM Simulations

Compound

PEM-C6

PBM-C8

a

intermolecular

Internal

Energy

Energy

(kcal/mol)

(kcal/mol)

(kcal/mol)

I

−288.0

−199.1

−88.9

γ = 60.0º

II

−284.8

−194.0

−90.8

a = b = 3.14 nm,

I

−387.3

−269.6

−117.7

γ = 60.0º

II

−392.7

−264.0

−128.7

PBC for

Packing

Simulations

Pattern

a = b = 2.66 nm,

Total Energy a

Sum of the intermolecular energy and the internal energy.

Effect of Chemical Property of Side Chains. As expected, STM observations revealed that the surface density of monolayers was dependent on the chemical property of the side chains. Namely, in the case of macrocycles bearing TEG chains, only dense hexagonal structures were observed, because the TEG groups are not adsorbed favorably on graphite surfaces. In such case, the molecules are stabilized on surfaces by hydrogen bonds between the ester carbonyl oxygen and aromatic hydrogen atoms. On the basis of the data base for three-dimensional crystals,64 it has been clarified that, for hydrogen bonds between a carbonyl oxygen and hydrogen atoms, the favorable H‧‧‧O-C angle is 180º. The H‧‧‧O-C angles estimated from the optimized network models of dense hexagonal and zigzag structures of PEM-TEG and dense hexagonal structure of PBM-TEG (PBM-C8 as well) are 158º, 167º, and 175º, respectively (Figure S11), which indicates that all 28


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structure can be stabilized by hydrogen bonds. The most favorable angle for PBM-C8 may contribute to the formation of its dense hexagonal structure at high concentration in spite of the presence of alkyl chains, in addition to the aforementioned distortion in its loose hexagonal structure II. The slightly more favorable H‧‧‧O-C angle for the zigzag structure of PEM-TEG than that of loose hexagonal structure may also account for its formation at low concentration in spite of lower density than the dense hexagonal structure.

SUMMARY AND CONCLUSIONS We have investigated the effects of the chemical properties of side groups and size of macrocyclic cores, in addition to solute concentration on the geometry and density of self-assembled monolayers on hexagonal tiling using hexagonal macrocycles, PEM-C6, PEM-TEG, PBM-C8, and PBM-TEG. It was revealed that the chemical properties of side chains affect the driving forces for the construction of the monolayers; whereas the 2D networks of PEM-C6 and PBM-C8 were driven mainly by van der Waals interactions between alkyl chains adsorbed on the surface, for PEM-TEG and PBM-TEG, molecular tiling was achieved by hydrogen bonds. We found that the size of conjugated cores of the macrocycles also plays a significant role on the tiling patterns. Namely, at low concentration, only PEM-C6 formed the porous honeycomb structure because of the suitable distance of the attachment of the substituents

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for interdigitated interaction pattern. Moreover, the packing geometries of the loose hexagonal structures of PEM-C6 and PBM-C8 are different due to the different confined space created by the macrocyclic cores in which alkyl ester groups are folded in. The present results provide useful insight into control of packing geometries of regular molecular tiling using regular polygon building blocks.

SUPPORTING INFORMATION Additional STM images and models, and synthetic details, 1H and 13C NMR spectra of PEM-C6. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * Tel. +81 6 6879 8476. Fax: +81 6 6879 8479. E-mail: [email protected] (Y.T.) * Tel. +1 217 544 5289 E-mail: [email protected] (J.S.M.) Notes The authors declare no competing financial interest.

ABBREVIATIONS 2D, two-dimensional; STM, scanning tunneling microscopy, TEG, ω-methyltriethylene glycolyl;

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TCB, 1,2,4-trichlorobenzene

ACKNOWLEDGMENT This work is supported by JSPS KAKENHI Grant Numbers 2435004 and 15H02164 and by the National Science Foundation CHE under Grant Number 16-10328. The authors would like to thank the School of Chemical Sciences NMR Lab and Mass Spectrometry Lab at University of Illinois at Urbana Champaign.

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Hexagonal Molecular Tiling by Hexagonal ... - ACS Publications

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