HostâGuest Chemistry in Integrated Porous Space Formed by

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Host-Guest Chemistry at Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces Kohei Iritani, Kazukuni Tahara, Steven De Feyter, and Yoshito Tobe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00083 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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Host-Guest Chemistry at Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces Kohei Iritani,† Kazukuni Tahara,*,†,§ Steven De Feyter,*,‡ and Yoshito Tobe*,† †

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

University, Toyonaka, Osaka 560-8531, Japan §

Department of Applied Chemistry, School of Science and Technology, Meiji University,

Kawasaki, Kanagawa 214-8571, Japan ‡

Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven –

University of Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium

ABSTRACT Host-guest chemistry in two-dimensional (2D) space, that is physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been

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investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles, called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by self-assembly of DBAs can be controlled from 1.6 nm to 4.7 nm by simply changing the alkyl chain length from C6 to C20. Single molecules, as well as, homo-and heteroclusters are capable of co-adsorbing within the host matrix using shape- and size-complementarity principles. Moreover, based on the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups, such as azobenzenedicarboxylic acid for photo-responsive guest adsorption/desorption, or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions, and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space which is modified for specific purposes.


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1. INTRODUCTION Host-guest chemistry is at the center of supramolecular chemistry.1–3 It has advanced enormously during the last several decades to become not only an established field in organic chemistry but also a still-growing field relevant to multidisciplinary science and technology. A major effort in host-guest chemistry has been devoted to design and synthesis of new host molecules which exhibit highly sensitive, guest-selective, and stimuli-responsive guest-binding properties, in most cases with covalently bound cyclic or pseudo-cyclic frameworks, studied mainly in solution, i.e., three-dimensional (3D) space, by spectroscopic methods.4,5 Examples of these systems include crown ethers, cryptands, cyclodextrins, calixarenes, calixresorcinarenes, carcerands, cucurbiturils, and pillararenes to name a few.6,7 Another aspect of host-guest chemistry is its development and applications in various phases and matters such as crystals, solids, zeolites, metal organic frameworks (MOFs), liquid crystals, polymers, and soft matters and even biomolecules and cells, as well as at various interfaces including those between different phases (gas/liquid, gas/solid, liquid/liquid, and liquid/solid) and membranes.8,9 Though a variety of real-life applications have already been realized, further technological advances are expected to develop more sophisticated applications.10,11 With regard to interfacial host-guest chemistry occurring at gas/solid or liquid/solid interfaces, alkanethiol-modified Au surfaces, have been extensively investigated partly because of their relevance for applications such as microcontact printing12–14 and dippen lithography.15–17 Flat lying physisorbed monolayers are also investigated intensively. In these physisorbed monolayers, non-covalent interactions between a


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molecule and substrate surface, which are absent in host-guest chemistry in 3D space, play an important role. Moreover, once a molecule is adsorbed on a atomically flat surface, its rotational and translational mobilities are significantly reduced, thereby displaying molecular behaviors different from those in fluid solution wherein a molecule can move more freely. Consequently, host-guest chemistry on surfaces is expected to exhibit characteristics specific to two-dimensional (2D) space. Another aspect of physisorbed molecular layers concerns stereochemistry; a prochiral molecule becomes chiral upon adsorption on a solid surface, because enantiotopic faces which are identical and indistinguishable in achiral 3D space become unidentical in 2D space, generating surface-specific 2D chirality.18 Note that 2D space in this context refers to single-molecule thick films. In addition, thanks to the recent advances in technology of scanning probe microscopies (SPMs), molecular behaviors on a conducting substrate surface can be visualized in both static and dynamic sense by images obtained with these microscopic tools.19,20 These allow us to directly count molecules to elucidate statistics for a given host-guest system, which are obtained as averaged signals in 3D systems such as solution and crystals by, for example, NMR spectroscopy and X-ray diffraction, respectively. They also help our understanding of molecular behaviors by seeing the molecules, to develop new designs or concepts for host-guest systems. As a consequence, host-guest chemistry in 2D space has become a topic of intense interest during the last decade. In this Feature Article, we describe first general aspects of molecular self-assembly on surfaces and at liquid/solid interfaces, followed by the formation of porous molecular networks by self-assembly of specifically designed molecular building blocks. Then we focus on our own work relating to host-guest


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chemistry of integrated nanoporous spaces which are modified with a specific functionality.

2. MOLECULAR NETWORKS FORMED BY SELF-ASSEMBLY ON SOLID SURFACES AND AT LQUID/SOLID INTERFACES 2.1 Molecular Networks Formed by Self-Assembly. Molecular self-assembly in 2D space on solid surfaces or at liquid/solid interfaces has become a subject of strong interest because of variety of prospects as bottom-up approach of fabrication of molecules

for

molecular-scale

electronics,21

selective

sensors,

and

tailored

catalysis.19,20,22–24 These periodic self-assembled monolayers, referred to as 2Dcrystals, are formed via non-covalent interactions between molecular building blocks. Research aimed at constructing 2D crystal structures by controlling intermolecular interactions is therefore called “2D crystal engineering”25–27 in analogy with the wellestablished field known as “crystal engineering” for design and control of molecular crystal structures in three-dimensional (3D) systems.28–30 In 2D crystal engineering, scanning tunneling microscopy (STM) is a vital tool to visualize the molecular assemblies in real space on a conducting substrate surface with sub-molecular resolution.31,32 STM is often operated under ultrahigh vacuum (UHV) conditions, or at air/solid or liquid/solid interfaces on an atomically flat conducting solid surfaces. Imaging under UHV is advantageous in terms of a large temperature window ranging from a few to several hundreds Kelvin; at very low temperatures imaging with high resolution and measurements of electric and magnetic responses of molecules are possible.19


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Alternatively at high temperatures highly reactive intermediates such as radicals can be generated. On the other hand, advantages of STM measurements at the liquid/solid interface are a wide range of molecules that can be used in terms of molecular weight and functional groups33 and the dynamic nature of the system due to a supernatant solution phase that serves as reservoir of solute molecules.34,35 This reservoir fosters repair of defects in a monolayer or reorganization of a whole network structure in response to the chemical potential of the solute molecule depending on concentration and temperature. However it can sometimes cause complications, due to co-adsorption of solvent molecules. The resulting dynamic behavior produces rich chemistry in STM experiments at the liquid/solid interface. Additionally, the relative ease of manipulation at ambient conditions, together with the moderate investments needed for the set-up of such facilities, led many researchers from outside the surface science community to this field. Because of the above-mentioned aspects, investigation of the control and synthesis of 2D molecular networks at the liquid/solid interface has become an active research field, and numerous 2D structures formed by molecular self-assembly have been reported during the past decades.20,22–24,34 In this section, factors that govern the structure of 2D molecular networks at the liquid/solid interface are briefly surveyed. In general, molecular self-assembly at the liquid/solid interface is governed by noncovalent forces between (i) molecule-molecule, (ii) molecule-substrate, (iii) moleculesolvent, and (iv) solvent-substrate.22–24,34,35 For molecule-molecule interactions through which building blocks are assembled on surfaces, hydrogen bonds, metal-ligand coordination bonds, and van der Waals interactions are commonly employed. Because


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of the strength and directionality which make the resulting network structure reliable and predictable, hydrogen bonds are most frequently used to promote assembly on surfaces. Though metal-ligand coordination is used for self-assembly at the liquid/solid interface, it is more frequently employed for constructing grid-like structures on metal surfaces under UHV conditions, where the substrate metal may participate to the formation metal-coordination bonds. Unlike the above non-covalent forces, van der Waals interactions are weak and undirectional per structural unit such as CH2 or CH. However, when they act together in a cumulative manner they serve as strong and directional forces for assembly. As substrates for formation of self-assembled monolayers at the liquid/solid interface, graphite and Au(111) are commonly employed because of ready availability and chemical stability under ambient conditions, though under UHV conditions other metals such as Ag(111), Cu(111) are used, as well. At liquid/solid interfaces, highly oriented pyrolytic graphite (HOPG) is most commonly used due to ready availability, easy handling, and facile imaging. At the interface of organic solvent and graphite, alkanes are adsorbed favorably due to matching of lattice registry between the methylene units of an alkane and the honeycomb lattice of graphite substrate. For this reason, attachment of alkyl groups to a planar core framework is commonly used as a method to stabilize self-assembled monolayers on graphite surfaces. At the liquid/solid interface, solvent molecules play a significant role in the formation of self-assembled monolayers through molecule-solvent and solventsubstrate interactions.35,36 Not only the type of solvents (i.e. polarity and hydrogen bond formation) but also concentration and temperature affect the chemical potential of


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the system, thereby controlling the thermodynamics of the monolayer structures. Frequently solvent molecules are co-adsorbed in nanoporous molecular networks on surfaces to stabilize the network structure. Although at times they are not visualized by STM due to high mobility. Mobility of solute molecules depends on strength of solvation and solvent viscosity. These properties therefore directly affect the adsorption-desorption dynamics at the liquid/solid interface. In addition, the thermodynamic vs. kinetics issue should be mentioned.37,38 Namely, any observed 2D structures can be either a kinetic or thermodynamic result. Care must be taken in discussing the thermodynamic stabilities of different phases. Indeed, phase transitions via adsorption-desorption processes are frequently observed over time. Transitions induced by the STM tip itself have also been reported. These phase transitions from kinetically trapped states into a thermodynamically favored arrangement can be accelerated by annealing treatments. Most of the recent reports pay attention to this aspect.

2.2 Porous Molecular Networks Formed by Self-Assembly. By self-assembly of molecular building blocks, a variety of patterns can be formed on surfaces or at liquid/solid interfaces. Among them, porous 2D molecular networks, in which guest molecules can be embedded through guest-substrate and host-guest interactions, are most interesting to us for many potential applications. In addition, understanding the factors controlling on-surface host-guest systems would help establish guiding principles to construct complex systems composed of multiple components. Thus far only a few examples of a maximum of four component self-assemblies are known.39–45


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Moreover, if on-demand modification of the porous space becomes possible in terms of not only the physical environment (namely the size and shape of pores) but also the chemical environment, fabrication of guest-selective sensory devices or substratespecific tailor-made catalysts may become possible. These so called nanoreactors in which catalytic chemical transformations take place by putting substrate molecules in confined space. Additionally, because a single functional molecule can be isolated in confined space, changes in its optical or electrical properties depending on orientation or mobility may lead to high density data storage systems. Similar to host-guest chemistry in 3D space, there are two possible approaches to construct nanoporous spaces for guest binding on surfaces; the use of intrinsic macrocyclic structures as building blocks, or extrinsic porous space formed by self-assembly of acyclic building blocks which are programmed to assemble by design.35,44,46,47 Though the former approach is advantageous in view of strict control over the physical and chemical environment of binding space, it is not flexible toward chemical modifications, and in most cases, it needs tedious synthetic efforts. By contrast, the latter approach is favorable because it allows facile modification of the size, shape, and functionality of the binding space. As a consequence, the latter approach is more frequently employed. Because matching of the shape and size of the guest and the surrounding host matrix plays an important role in guest binding on surfaces, control over the pore dimension is the most important aspect in designing host building blocks. The majority of the porous spaces, therefore, have a hexagonal shape as a result of its regular tilings and have large space to accommodate guest molecules. Moreover, hexagonal space can be constructed by self-assembly of trigonal molecular building blocks which are conveniently prepared by


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derivatization of benzene itself or other aromatic compounds with a π-conjugated framework. There are indeed three prototypical classes of trigonal building blocks corresponding to the three major driving forces of on-surface self-assembly described in the previous section, hydrogen bonds, metal-ligand coordination bonds, and van der Waals forces. In these systems, the dimension of the isotopological porous space is regulated by simply changing the size of the building blocks. An archetypical molecule of the first class of building blocks is trimesic acid (1,3,5benzenetricarboxylic acid, TMA) bearing three carboxy groups arranged in a trigonal geometry. Though carboxylic acids are known to form a variety of hydrogen bond patterns as shown in Figure 1, the commonly observed motifs in the solid state and in surfacesupported monolayers48 is the cyclic dimer because of its high stability. Connection of each of the three carboxy groups of TMA (Figure 2a) with neighboring molecules by this hydrogen bonding motif would give rise to hexagonal patterns on surfaces. Indeed, Griessl et al. reported co-existence of two types of hexagonal patterns originally called chickenwire (called honeycomb hereafter) and flower formed by self-assembly of TMA on graphite under UHV (Figure 3).49 In the honeycomb pattern TMA molecules are connected by the cyclic dimer motif. On the other hand in the flower pattern the molecules are bound by the cyclic trimer motif. For self-assembly of TMA at solvent/graphite interface, aliphatic acids are mainly used as solvents because of the limited solubility of TMA in non-polar solvents. It was found, however, that such subtle difference as the number of methylene units in the alkyl part of the aliphatic acids (i.e. octanoic, heptanoic, and hexanoic acids) affects the polymorph distribution. The densely-packed flower pattern is favored with shorter chain acids, whereas the less dense chicken-wire (honeycomb) pattern


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persists with longer chain acids used as the solvent.50 Though the diameter of the honeycomb pattern of TMA is about 1.1 nm (Figure 4a),51 by increasing the length of the connectors between the carboxy group and the trigonal 1,3,5-trisubstituted benzene core by inserting rigid rod-like units, such as 1,4-phenylene and ethynylene, the size of the resulting hexagonal pore would be enlarged. 1,3,5-Benzenetribenzoic acid (BTB, Figure 2b), whose arms are elongated by one phenylene unit compared to TMA, also forms the honeycomb network with a diameter of 2.8 nm (Figure 4b) at solvent/graphite interfaces by the same hydrogen bonding motif, together with dense-packed polymorphs depending on the solvent.51–53 These polymorphs are also dependent on concentration,54 temperature,55,56 and the polarity of applied bias voltage.54,57 Moreover, 1,3,5-tris(4-carboxyphenylethynyl)2,4,6-trimethylbenzene (TCPETMB, Figure 2c) with extended arms by further insertion of an ethynylene unit to BTB formed isotopological honeycomb patterns with a diameter of ca. 3.5 nm (Figure 4c) at the nonanoic acid/graphite interface, demonstrating successful control over the hexagonal pore dimension by the size of hydrogen bonding building blocks.51,58 On the other hand, by extension of the building block by additional phenylene units, i.e., in the case of 1,3,5-tris[4’-carboxy(1,10’-biphenyl-4-yl)]benzoic acid (TCBPB, Figure 2d), an isotopological regular honeycomb network was not formed. Instead, a deformed hexagonal network due to Ar-CH…O= hydrogen bonding was formed together with other polymorphs, presumably because of the higher packing density of this form compared to the regular hexagonal pattern, revealing a limitation of this approach.59


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Figure 1. Three typically observed hydrogen bonding motifs between carboxylic acids in the solid state and in surface-supported monolayers.

Figure 2. Chemical structures of (a) trimesic acid (TMA), (b) 1,3,5-benzenetribenzoic acid (BTB), (c) 1,3,5-tris(4-carboxyphenylethynyl)-2,4,6-trimethylbenzene (TCPETMB), and (d) 1,3,5-tris[4’-carboxy(1,10’-biphenyl-4-yl)]benzoic acid (TCBPB).

Figure 3. STM images of self-assembled monolayers of TMA on graphite under UHV. (a) Chicken-wire (honeycomb) and (b) flower patterns. (Reproduced with permission from ref. 49. Copyright 2002 John Wiley and Sons.)


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Figure 4. STM images of the honeycomb networks formed by (a) TMA, (b) BTB, and (c) TCPETMB with the corresponding models of a single pore depicted below. (Reproduced with permission from ref. 51. Copyright 2009 American Chemical Society.)

The porous matrix with a diameter of ca. 1.3 nm formed by self-assembly of TMA provides an ideal space to accommodate planar organic molecules of similar size such as corornene (COR, Figure 5a)60 and sulflower (Figure 5b)61 as well as spherical molecules such as C60 fullerene (Figure 5c).62 As an example, an STM image of a TMA network containing COR molecules in the nanoporous space is shown in Figure 6.60 In all cases, dynamic behaviors of the guest molecules were observed, such as lateral rotation within the pore, removal from the cavity by increased reference current, and hopping to the neighboring pore by tip manipulation. Because the guest molecules are co-adsorbed via van der Waals interactions with the graphite surface and host matrix, size complementarity plays an important role for successful guest inclusion. Thus, in the larger porous space formed by BTB, three molecules of COR are accommodated in each cavity, but only one molecule of a larger guest, tentatively called triangular nanographene (TNG, Figure 5d).59


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Figure 5. Chemical structures of guest molecules. (a) coronene (COR), (b) sunflower, (c) C60, (d) triangular nanographene (TNG).

Figure 6. STM image of a TMA network accommodating COR molecules in the nanoporous space. (Reproduced with permission from ref. 60. Copyright 2004 American Chemical Society.)

As an example of the second class of building blocks which form isotopological hexagonal

networks

based

on

metal-ligand

coordination

bonds,

a

series

of

dicycanooligophenylenes (Figure 7) are briefly described here. Construction of 2D self-


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assembled networks, which are also regarded as 2D metal-organic frameworks, is typically undertaken under UHV conditions by vaporization of metal atoms or using the substrate metal for coordination. Ligands such as carboxylates and pyridine derivatives are commonly used. α,ω-Dicyanooligophenylenes of the phenylene units of 3 to 6 were investigated on the Ag(111) surface under UHV to construct hexagonal networks, because of their tendency to form a trigonal geometry by coordination to a Co atom.63,64 As shown in Figure 8, all dicycanooligophenylenes formed the honeycomb patterns with cavity diameters ranging from 4.2 to 6.7 nm. Despite of the high fidelity for the formation of isotopological patterns, the use of self-assembled molecular networks formed under UHV for applications may be limited compared to networks formed at the liquid/solid interface under ambient conditions.

NC

CN n (n = 1–4)

Figure 7. Chemical structures of α,ω-dicyanooligophenylenes with the phenylene units of 3 (n = 1) to 6 (n = 4).


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Figure 8. STM images of honeycomb networks formed by coordination of α,ωdicyanooligophenylenes with phenylene units of 3, 4, and 5 to Co atoms under UHV. (Reproduced with permission from ref. 63. Copyright 2007 American Chemical Society.)

The third class of building blocks which form isotopological hexagonal networks by van der Waals interactions is the major topic of this Article and is described in the sections that follow.

3. POROUS NETWORKS FORMED BY SELF-ASSEMBLY OF TRIANGULAR BUILDING BLOCKS VIA VAN DER WAALS INTERACTIONS BETWEEN ALKYL CHAINS 3.1 Porous Networks Formed by van der Waals Interactions. For more than a decade, we have been working on 2D self-assembly of triangular building blocks consisting of a phenylene-ethynylene macrocyclic core (dehydrobenzo[12]annulenes hereafter called DBAs) and six flexible alkoxy chains at liquid/solid interfaces (Figure 9).65–67 DBAs bearing C6 to C20 alkyl chains were revealed to self-assemble at the interface of organic solvent and graphite to form porous 2D molecular networks of hexagonal lattices via van der Waals interactions between interdigitated alkyl chains as the directional intermolecular linkages. Figure 10 shows an STM image of a nanoporous self-assembly of DBA-OC12 with C12 alkyl chains formed at the 1-phenyloctane/graphite interface, as an example.68–71 The size of the DBA core, which defines the distance of ca. 1 nm between a pair of alkyl chains attached on one edge of a DBA molecule, is crucial for the formation of the interdigitating motif of the alkyl chains to maximize van der Waals interactions with


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neighboring molecules. Similar trigonal building blocks are also shown to form honeycomb patterns via alkyl-alkyl interactions.72–76 Densely-packed linear patterns in which two alkyl chains of a DBA are exposed to the solution phase were observed under certain conditions. We elucidated favorable conditions for the formation of the porous 2D molecular networks with respect to alkyl chain length,69,70 kind of solvent,69 solute concentration,70 and temperature.77 In short, DBAs with shorter alkyl chains in good solvent for DBAs such as 1,2,4-trichlorobenzene (TCB) and 1-phenyloctane (1PO) at low concentration (typically ca. 10−6 M) and high temperature favor the porous structure over the dense-packed polymorph.67 We then took advantage of adaptability for the synthetic design of the DBA building blocks for the systematic studies on the modification of their alkyl chains. Namely, we modified DBAs from the following aspects; (i) pore size control by changing the alkyl chain length,69,70 (ii) even/odd effect of the number of alkyl chains,78 (iii) supramolecular chirality on surfaces induced by stereocenters on the alkyl chains,79,80 (iv) chemical modification of the pore interior for binding guest molecules via electrostatic interactions. These systems are described in the next section.


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Figure 9. (a) Schematic representation for the formation of porous molecular network by selfassembly of DBAs on a surface and (b) chemical structures of DBAs of C6 to C20 alkyl chains.


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Figure 10. (a) STM image of porous network formed by DBA-OC12 at the 1phenyloctane/graphite interface, showing two domains of different supramolecular chirality (arbitrarily named counterclockwise domain (top) and clockwise domain (bottom)). The white line indicates a domain boundary. (b) Molecular models showing two alkyl chain interdigitation patterns (arbitrarily assigned + (top) and – (bottom)) and the resulting honeycomb nanowells formed from six molecules of DBA-OC12 (arbitrarily assigned counterclockwise (top) and clockwise (bottom)). (Reproduced with permission from ref. 67. Copyright 2016 the Chemical Society of Japan.)


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The diameters of the hexagonal nanowells in the porous networks of DBAs can be controlled by changing the alkyl chain length within a range of ca. 1.6 nm (C6 chain) to 4.7 (C20 chain) nm. In the absence of other molecules, solvent molecules are adsorbed to occupy the nanowells, as they are occasionally visualized by STM.77 DBA molecules, presumably adopting conformations in which the alkyl chains are folded, may also be selfadsorbed in the cavity and observed as fuzzy features.81 When guest molecules co-exist in the solution, however, they can be immobilized in the nanowells formed by DBAs by replacing the solvent molecules, depending on the size- and shape-complementarity between the guests and nanowells. The guest co-adsorption therefore is governed by van der Waals interactions between the guest surface and underlying graphite surface, as well as interactions between the guest periphery and the rim of the nanowell interior.

3.2 Host-Guest Chemistry at Simple Nanowells Formed by Self-Assembly of DBAs. When we started 2D host-guest chemistry using DBAs, since we were unware of the crucial effect of solute concentration on the polymorphs of the 2D networks, i.e. porous vs. non-porous, dense-packed patterns, the experiments were carried out at relatively high concentration (ca. 10–4 M in TCB), under which conditions short alkyl chain DBAs (DBAOC10 and DBA-OC12) form the porous pattern whereas longer alkyl chain DBAs (DBAOC14 and DBA-OC16) favor the non-porous form. The first guest molecule we examined was coronene (COR, Figure 5a).82 Though we demonstrated later that a single molecule of COR fits in the nanowell of the honeycomb network of DBA-OC6 (Figure 11a),71 the porous space of the above DBAs are larger for a single COR molecule. For example, in the


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honeycomb network of DBA-OC10 bright but fuzzy spots are visible as shown in Figure 11b, indicating that undefined number of mobile COR molecules are adsorbed in the nanowells. Interestingly, in the case of DBA-OC14, a phase transition from the non-porous to porous structure was observed by addition of excess COR. Molecular modeling indicates seven COR molecules fit in the cavity (Figure 11c) though the number of COR guests was not determined experimentally. However, this guest-induced phase transition did not occur as efficiently for the larger DBA-OC16, indicating the balance point between the thermodynamic stabilities of guest-containing porous vs. dense-packed structures.

Figure 11. (a) STM image of the porous network of DBA-OC6 containing a COR molecule in the nanowell, (b) STM image of the porous network of DBA-OC10 containing COR molecules in the nanowell, and (c) a model of a nanowell formed by DBA-OC14 containing seven molecules of COR. (Reproduced with permission from ref. 71, copyright 2012 the American Chemical Society and ref. 82. Copyright 2007 John Wiley and Sons.)

Next, we employed larger triangle-shaped hydrocarbon TNG (Figure 5d)83 as a guest, which was expected to fit better the hexagonal cavity of the network due to shape complementarity and to be more strongly adsorbed on the surface compared to COR.83 In the honeycomb network of DBA-OC10, a single molecule of TNG was observed as a


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large fuzzy spot owing to the mobility (Figure 12a). In the nanowells of DBA-OC12, bright rhombus-shaped features ascribed to TNG dimers were occasionally observed in addition to fuzzy features due to mobile single TNG molecules (Figure 12b). Notably, the long axis of the rhombus-shape features is always pointing to the facing corners of the hexagonal pore, because the long axis size of the TNG dimer fits the corner-to-corner distance of the nanowell. In contrast, in the case of DBA-OC14, although the non-porousto-porous phase transition took place similar to the case with COR guest, only large dim features are imaged in the nanowells. TNG molecules were not observed at submolecular resolution due to their high mobility in the pore (Figure 12c). For DBAs with longer alkyl chains, DBA-OC16, DBA-OC18, and DBA-OC20, phase transition to the porous structures was observed by addition of excess TNG, indicating higher ability of TNG than COR to induce structural transformation due to shape complementarity and enhanced affinity to graphite. Indeed, the individual TNG molecules are identified clearly. As shown in Figure 12d for DBA-OC16, though the number of guest molecules observed in the nanowells varies from 2 to 5, the most frequently counted number was four. In addition, the shape of the nanowells is deformed from regular hexagon and the orientation of TNG molecules is not uniform. In the case of DBA-OC18, five molecules of TNG were observed most frequently (70%) as shown in Figure 12e. In the case of DBA-OC20, because a close-packed cluster composed by six molecules of TNG nicely occupies the nanowell, six TNG molecules shown in Figure 12f were most dominantly observed (85%). Thus the number of guest molecules embedded in each nanowell shows a clear dependence on the size of the pores.


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Figure 12. STM images of nanowells containing TNG molecules formed by DBAs with different alkyl chain length, (a) DBA-OC10, (b) DBA-OC12, (c) DBA-OC14, (d) DBAOC16, (e) DBA-OC18, and (f) DBA-OC20. (Reproduced with permission from ref. 66. Copyright 2010 The Royal Society of Chemistry.)

In addition to molecular clusters consisting of one kind of guest molecule co-adsorbed in the nanowells, those formed by two components can also be immobilized in the hexagonal nanowells formed by DBAs. Similar to the nanowell formed by self-assembly of TMA (Figure 6), a cyclic array formed from six molecules of isophthalic acid (ISA) by hydrogen bonds accommodates a COR molecule (Figure 13a). A host-guest self-assembly at the octanoic acid (OA)/graphite interface was formed with a dense-packed pattern, similar to the flower network of TMA (Figure 3), by van der Waals interactions between the periphery of the hexagonal complex (Figure 13b). When mixed with DBA-OC10, this hexagon-shaped host-guest complex is embedded in the honeycomb network of the DBA via van der Waals interactions, leading to a three-component 2D crystal.84 As shown in Figure 13c, a host-guest complex consisting of one COR molecule and six ISA molecules is immobilized in each nanowell formed by DBA-OC10. In the nanowell of the higher


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homologue DBA-OC12, the majority of the cavities appear fuzzy and featureless due to mobility of the host-guest complex which does not fit to the nanowell. The fact that only DBA-OC10 immobilizes the hexagonal heterocluster of COR/ISA in its cavities demonstrates the strict complementarity in size and shape for 2D host-guest complex formation.

Figure 13. (a) A molecular model for a host-guest complex formed by one COR molecule with six molecules of ISA. (b,c) STM images of host-guest complex consisting of one COR surrounded by six ISA molecules and that of DBA-OC10 containing the hostguest complex in each nanowell, respectively. In (c) the white line and curved arrows indicate a domain boundary and CW/CCW chirality of the honeycomb structure in each domain. (Reproduced with permission from ref. 84. Copyright 2008 American Chemical Society.)


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4. INTEGRATED POROUS NETWORKS FORMED BY SELF-ASSEMBLY OF TRIANGULAR BUILDING BLOCKS VIA VAN DER WAALS INTERACTIONS 4.1 Host-Guest Chemistry at Integrated Nanowells Formed by Self-Assembly of Functionalized DBAs. As described above, the nanowells formed by self-assembly of alkylsubstituted DBAs serve as 2D space to co-adsorb planar guest molecules, which are confined in a size- and shape-complementary fashion via van der Waals interaction between the interior rim of the nanowells and the periphery of guest molecule(s). In view of the advances in host-guest chemistry in 3D space, we sought (1) reversibility and (2) programmed selectivity in host-guest chemistry within 2D nanopores. Reversible control over the nanowell size in response to external stimuli using photochromic molecules adsorbed on surfaces has been extensively studied.85 On the other hand, size control of 2D pores in response to external stimuli at a single molecule level was scarcely achieved.86,87 Previously the size- and shape-modification of 2D pores with spacecontrolling groups has been reported.88–91 However, the construction of 2D pores equipped with functional groups capable of binding neutral guest molecules via electrostatic interactions was not achieved. We sought modification of the chemical environment of the nanowells to bind guest molecules via non-covalent electrostatic interactions such as hydrogen bonds and dipoledipole interactions. We conjectured that both objectives could be achieved by 2D self-assembly of DBAs bearing appropriate functional groups at the end of three alternating alkyl chains. The concept is schematically illustrated in Figure 14 with DBA molecules bearing functional groups alternately (represented by magenta arrows) that self-assemble via alkyl chain interdigitation. The functionalized alkyl chains were designed to occupy the outer positions of the interdigitation pattern and the non-substituted alkyl groups would thereby locate the inner positions in the


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interdigitated alkyl chain motif. As a consequence, the functional groups can be placed in the interior space of the nanowells. The key to this molecular design is then the selective introduction of different alkyl groups to the DBA core in an alternate fashion. Since we have developed a synthetic protocol for the key intermediate DBA-OTBS/OMOM79 bearing two different protecting groups alternately on the DBA core in connection with 2D chirality induction (Scheme 1), we thought it would be possible to prepare differentially substituted DBAs (with R1 and R2) to achieve the above objectives. The pivotal step of the synthesis is the selective protection of catechol derivative 1 based on different acidity of the two hydroxy groups to furnish iodoalkyne 2 which was subject to Sonogashira cross-coupling reaction for cyclotrimerization.79

Figure 14. Schematic representation of on-surface host-guest binding in a functionalized nanowell formed by self-assembly of six molecules of an alternately functionalized DBA. The light blue dots represent solvent molecules which are adsorbed on the surface and solvate the guest molecules.


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Scheme 1. Preparation of differentially substituted DBAs in an alternate fashion via key intermediate DBA-OTBS/OMOM.

After synthesis, we reported the first reversible control over the nanowell size in response to photo-irradiation. Azobenzene was used as a photoresponsive group on which two carboxy groups are introduced to create a confined space by hydrogen bonds forming a cyclic hexamer, similar to that of ISA in which one COR molecule is immobilized (Figures 13a,b). On the basis of this design, we synthesized DBA-AZ, in which a meta-phenylene linker connects a C12 alkyl chain and the dicarboxyazobenzene unit to form a hydrogen-bonded cyclic hexamer in the nanowell (Figure 15a).92 We conjectured that the azobenzene units would be adsorbed on the graphite surface initially in its planar trans-configuration and form a cyclic hexamer in which cavity a single molecule of COR would be accommodated. The expectation was that upon UV light irradiation, at least some azobenzene units would isomerize to cis-configuration and desorb from the surface, thereby producing another space for COR molecules to occupy in the cavity. Irradiation of visible light was shown to revert the cis- to trans-configuration, closing back the cyclic hexamer structure to reduce space for COR adsorption (Figure 15b). The reversible photoswitching process was then assessed by monitoring the number of adsorbed COR molecules.


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Figure 15. (a) Chemical structure of DBA-AZ and (b) a schematic representation of photoresponsive pore-size control in the self-assembled network of DBA-AZ.

At the interface of OA and graphite, DAB-AZ (1.0 × 10–5 M) formed a honeycomb network, in which the azobenzene units are directed to the center of the nanowell, suggesting the formation of the cyclic hexamer of the dicarboxyazobenzene units as expected (Figure 16a). In contrast, a photostationary cis/trans mixture of DBA-AZ containing 57% of cis-azobenzene moieties formed mostly disordered structures with only small proportion of the honeycomb domains, indicating that the hydrogen bonding between the dicarboxyazobenzene units forming the cyclic hexamer in the trans-configuration plays a significant role for the formation of the honeycomb structure. In the presence of excess COR, a single COR molecule was observed in


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majority (86%) of the pores (Figure 16b); two CORs were found in 4% of the pores and 9% of the pores contained fuzzy feature. In-situ irradiation of UV light (320 nm) to a monolayer formed by DBA-AZ followed by addition of excess COR, the number of the pores containing two COR molecules increased to 16%, whereas the pores containing one COR decreased to 73% (Figure 16c). Pores containing three and four COR molecules are also observed in 3% and 0.3%, respectively. By irradiation of this mixture with long-wavelength light (λ > 400 nm), the distribution of the number of COR molecules was recovered nearly to that of the initial stage (one COR, 92%; two CORs, 3%; fuzzy features, 5%), due to the reduction of the pore size by cis-to-trans isomerization of the azobenzene units. These results demonstrate the reversibility of the free space control in the nanowell by photo-irradiation of UV and visible light.

(a)

(b)

(c)

Figure 16. STM images at the OA/graphite interface of (a) the honeycomb structure formed by DBA-AZ, (b) a self-assembled monolayer formed from a mixture of DBA-AZ and excess COR, and (c) a self-assembled monolayer formed by in-situ UV irradiation of DBA-AZ followed by


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addition of excess COR. The red, yellow, and blue hexagons in (c) indicate the pores containing four CORs, two CORs, and those with fuzzy images, respectively. (Reproduced with permission from ref. 92. Copyright 2013 John Wiley and Sons).

The above experiments clearly indicate the important role of hydrogen bonds between the dicarboxyazobenzene units by forming a cyclic hexamer to stabilize the honeycomb structure. We conjectured that this non-covalent motif can be used to periodically functionalize the nanowells, forming a self-assembled 2D network of a higher level periodicity, and synthesized DBA-AZ1 bearing only one dicarboxyazobenzene unit.93 As shown in Figure 17, DBA-AZ1

would form Pattern A network with the

dicarboxyazobenzene units distributed periodically if the hydrogen bonds are effective enough to put all six carboxy groups into the same nanowell, whereas Pattern B network with random distribution would result otherwise.


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Figure 17. Chemical structure of DBA-AZ1 and schematic representation for periodically functionalized porous network (Pattern A) and randomly functionalized porous network (Pattern B).

Self-assembled monolayer of DBA-AZ1 was formed at the TCB/graphite interface at 2.1 × 10−6 M. Whereas only small domains of a honeycomb structure were observed without formation of the cyclic hexamer (Pattern B) at room temperature, by annealing treatment at 100 °C for 11 h large domains of a honeycomb structure were formed (Figure 18a). Some nanowells contain six spoke-like features ascribed to the azobenzene units, indicating the formation of hydrogen bonded cyclic hexamers. There are nanowells which do not contain the spoke-like features. These nanowells occasionally contain dim features which may be due to mobile self-adsorbed DBA-AZ1. Most importantly, the locations of the two different type nanowells are periodically arranged; the nanowells with spoke-like features are always surrounded by vacant nanowells. These results demonstrate the formation of Pattern A with long range periodicity, verifying our


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working hypothesis for construction of a periodic functionalization of the porous molecular network.

Figure 18. (a) STM image of monolayer of DBA-AZ1 formed at the TCB/HOPG interface. The blue arrows show fuzzy features in the open pores. (b) STM image of monolayer of a mixture of DBA-AZ1, COR and HPEPEB at the TCB/graphite interface. Inset shows a zoom-in image of the pore containing HPEPEB. (c) A molecular model of the three component monolayer. (d) Chemical structure of HPEPEB. (Reproduced with permission from ref. 93. Copyright 2016 American Chemical Society).

Because two different nanowells are formed, one with a small pore space formed by hydrogenbonded dicarboxyazobenzene units and the other open pore without functional groups, the selective adsorption of two guest molecules, COR and HPEPEB (Figure 18d) was examined. Under optimized conditions regarding the relative and absolute concentrations of each


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component, and after annealing treatment at 100 °C for 11 h, site selective co-adsorption was achieved at the TCB/graphite interface as shown in Figure 18b. Bright disk-like features observed at the center of the spoke containing pores are due to COR molecules, whereas asterisk-like features consisting of dim dots found at the large unfunctionalized nanowells are HPEPEB molecules. Close inspection revealed co-adsorption of solvent TCB molecules in the space between the spokes of radial HPEPEB molecule (Figure 18b inset). These results reveal the formation of highly ordered three-component network of long range order by selective adsorption of two different guest molecules at the respective nanowells (Figure 18c). The controlling principle based on hydrogen bonding, coupled with size-selective adsorption of guest molecules to the periodically distributed pores as demonstrated here, must be useful for construction 2D architectures of high complexity.

4.2 Host-Guest Chemistry at Integrated Nanowells via Electrostatic Interactions. Next, we focused on construction of nanowells containing binding functionalities which interact with guest molecules by fluorophilicity, electrostatic dipole-dipole or charge-transfer interactions as depicted in Figure 14. For the first two purposes, we used a similar principle as above to functionalize the inner rim of the nanowell with fluoroalkane or oligoethylene glycol units. We synthesized DBA-F94 bearing C8F17 chains and GBA-TeEG95 bearing tetraethylene glycol (TeEG), respectively. The functional chains are linked by a p-phenylene unit at three alternating alkyl chain terminals (Figure 19a). These DBAs, together with a reference compound DBA-H with C10 alkyl chains instead of the functional chains, were synthesized in a similar manner as that for DBA-AZ from DBA-OTBS/OMOM (Scheme 1). Formation of nanowells by selfassembly of a DBA bearing zinc-porphyrin unit and complexation with C60 by charge-transfer


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interaction are described later. We expected that these DBAs would form honeycomb structures in which the C8F17, TeEG, or C10 alkyl groups are positioned around the perimeter of the hexagonal pores as shown in the model for DBA-F in Figure 19b. Because fluorophilicity has been utilized as a key driving force in various supramolecular materials such as liquid crystals96–98 and lipid bilayers,99 and host-guest complexes,100 we conjectured the nanowell outlined by fluoroalkane groups of DBA-F would favorably bind a fluorinated guest over the corresponding hydrocarbon with the same backbone. Accordingly, as guest molecules whose size and shape would fit to the nanowells of DBA-F and DBA-H, we used octafluoro-substituted hexakis(phenylethynyl)benzene HPEB-F with 18 fluorine atoms at the exterior edge and the corresponding hydrocarbon HPEB (Figure 19c).

Figure 19. (a) Chemical structures of DBA-F, DBA-TeEG, and DBA-H, (b) a molecular model of a honeycomb structure formed by DBA-F, and (c) chemical structures of HPEBF and HPEB.


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STM observations revealed that self-assembled monolayers of DBA-F and DBA-H were formed at the 1PO/graphite interface after annealing treatment at 70 °C for 30 min. As expected in the nanoporous space the fluoroalkyl or alkyl groups are positioned around the perimeter as observed as dim features (Figures 20a,b). The host-guest binding experiments were carried out for four combinations of BDA-F/HPEB-F, DBA-F/HPEB, DBA-H/HPEB-F, and DBAH/HPEB. A constant DBA/guest mole ratio (2.4/1) and a total concentration (4.4 × 10−6 M) were set after optimization of conditions so that the results of different combinations can be compared. As a result, varying guest occupancy in the nanowells formed by the DBAs was observed. In order to assess the binding ability, the presence/absence of the guest in the nanowells are classified on the basis of the appearance in the STM images into three categories; bright pores containing immobilized guest, fuzzy pores with mobile guest, and dark pores without guest as shown in Figures 20c-f where the pores are indicated by red, green, and blue hexagons. Unidentified nanowells are colored in pink. However, to exclude arbitrariness, the relative heights of the pore space with respect to that of the DBA core were measured. As a result, averaged relative nanowell height was 1.14, 1.08, and 0.82 for bright, fuzzy, and dark pores, respectively, supporting the assignment based on appearance. The similar relative height of the fuzzy pores to that of bright pores suggests lateral dynamics of the guest in the nanowell. The statistical distribution of the classified pores is listed in Table 1.


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Figure 20. STM images of honeycomb structures of (a) DBA-F and (b) DBA-H formed at the PO/graphite interface. STM images of monolayers formed at the PO/graphite interface by (c) DBA-F and HPEB-F, (d) DBA-H and HPEB-F, (e) DBA-F and HPEB, and (f) DBA-H and HPEB with red, green, blue, and pink hexagons indicating the bright pores, fuzzy pores, dark pores, and pores which were not assigned, respectively. Tentative network models of (g) DBA-F and HPEB-F and (h) DBA-F and HPEB on a bilayer graphene sheet by molecular mechanics


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simulations. (Reproduced with permission from ref. 94. Copyright 2014 American Chemical Society).

Table 1. Distributions of the Bright, Fuzzy, and Dark Pores. Fraction of different type pores (%)a

a

DBA

guest

DBA-F

bright pore

fuzzy pore

dark pore

HPEB-F

20

36

40

DBA-F

HPEB

65

22

7

DBA-H

HPEB-F

7

9

81

DBA-H

HPEB

59

31

6

The balance being 3–7% unidentified pores.

As shown in Table 1, whereas the host network of DBA-F accommodates HPEB-F moderately, it binds HPEB more efficiently. Conversely, the host network of DBA-H exhibits little affinity toward HPEB-F in contrast to high affinity to HPEB. The observed affinity is consistent with the appearance of the guests which were visualized as asterisk-like features (Figures 20e,f). Molecular mechanics calculations using COMPASS force field for the hostguest complexes on a bilayer graphene sheet under periodic boundary conditions indicate favorable host-guest affinity between DBA-F and HPEB guest due to H···F electrostatic interactions between the peripheral hydrogen atoms of the guest and the fluoroalkyl groups of the 2D pores (Figures 20h). By contrast, the host-guest pair of opposite relationship, DBA-H and HPEB-F, exhibited the least binding affinity due to the wrong hydrogen/fluorine substitution pattern for electrostatic interaction. The observed moderate degree of binding between DBA-F and HPEB-F is ascribed to the balance between fluorophilicity and steric compression.


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Principally, the cavity formed by DBA-F is small to accommodate HPEB-F (Figure 20g). The triple bonds are likely deformed slightly from linearity. Unexpectedly, DBA-H and HPEB exhibit significant binding affinity in spite of weak interactions (only van der Waals force) and the slightly too large cavity size for the guest. Careful inspection revealed that the dark and fuzzy pores, which do not contain a confined guest, are always surrounded by the bright pores containing the asterisk-like features (Figure 21a), indicating the formation of a superlattice structure. The periodic distribution of the dark and fuzzy pores over the hexagonal lattice was confirmed by their nearest neighbor analysis. Moreover, the cavities containing the confined guest molecules are deformed from regular hexagons and consist of alternating edge length (2.7 nm and 2.9 nm) and angles (114º and 126º), to maximize van der Waals contact with the guest via an induced fit mechanism as illustrated in Figure 21b. The vacant, regular hexagonal pore is surrounded by six guest-confined nanowells, thus generating a superlattice structure.

Figure 21. (a) STM image of the superlattice structure formed by DBA-H and HPEB. The red and green double-headed arrows indicate the distances between the next nearest neighbors (5.0 ± 0.1 nm and 4.7 ± 0.1 nm for red and green arrows, respectively). (b) A schematic representation


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of a superlattice formed by DBA-H and HPEB. The regular hexagon in the center is surrounded by six distorted hexagons. The red and green dotted lines correspond to the next nearest neighbor distances in the regular and distorted hexagons. The lengths of the hexagon edges indicated by the yellow and white lines are 2.7 and 2.9 nm, respectively. (Reproduced with permission from ref. 94. Copyright 2014 American Chemical Society).

4.3 Host-Guest Chemistry at Integrated Nanowells via Charge-Transfer Interaction. In the 2D host-guest systems so far described, direct adsorption of the guests by van der Waals interactions with the underlying surface are playing a principal role. An alternative method to immobilize guest molecules in nanowells is to construct an ordered array of binding sites which interact with guests using electrostatic interactions. For this purpose, we planned to use Znporphyrin (ZnP) as the binding functionality, because of its binding ability via π-π interactions,96 donor-acceptor interactions,102 and coordination bonding,103 and surface adsorption properties on the graphite surface104–106 Therefore, we designed and synthesized DBA-ZnP having three ZnP units linked by m-phenyleneethynylene units at the end of three alkyl chains in an alternating fashion (Figure 22a).107 By using the meta-phenylene linker at the end of the C14 alkyl chains together with the complementary C16 alkyl group, six ZnP units would be located in a cyclic arrangement at the hexagonal nanowell formed by the van der Waals interdigitation of the alkyl groups. A molecular model of DBA-ZnP and its hexamer model, formed by self-assembly of six molecules of DBA-ZnP, bearing a cyclic array of the ZnP units in the nanowell are shown in Figure 22b. We also expected that C60 molecules would be immobilized on the ZnP units to form a cyclic array on the surface.


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Figure 22. (a) Chemical structures of DBA-ZnP and bromoalkyl-ZnP. (b) A schematic representation for the formation of a honeycomb structure by self-assembly of six molecules of DBA-ZnP and complexation with C60. ZnP units, zinc atoms, and C60 are shown in green, brown, and gray, respectively. (Reproduced with permission from ref. 107. Copyright 2016 The Royal Society of Chemistry).

STM observation of self-assembled monolayer of DBA-ZnP (5.8 × 10−6 M) at the TCB/graphite interface at room temperature revealed the formation of a hexagonal 2D molecular network shown in Figure 23a. Approximately six bright features are observed in the nanowells, which are attributed to the ZnP units forming a cyclic array. A network model is shown in Figure 23b. The small bright features located between the DBA cores and the neighboring ZnP units are due to the phenylene linkers (marked by yellow arrow in Figure 23a). We noticed that not all nanowells contained six ZnP units; some of the ZnP units are oriented to the solution phase rather than being adsorbed on the surface. The proportion of the adsorbed ZnP units was determined by analysis of apparent height


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profiles along lines drawn between two ZnP units located at the diagonal positions of a nanowell, as shown in Figures 23c,d. The line is inclined with respect to one of the main axes of graphite by ±7° on the basis of molecular modelling. A histogram for the distribution of the maximum heights at the ZnP units shows a bimodal distribution, to which normal distribution curves centred at 106 pm (red line) and 38 pm (green line) are fitted as shown in Figure 23e. The red and green curves correspond to the distribution of the ZnP units adsorbed on the surface and desorbed from it, respectively. From the area ratio of the fitting curves, the fraction of ZnP units adsorbed on the graphite surface was estimated to be 98%, indicating that most of the ZnP units are embedded in the molecular matrix of DBA-ZnP.


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Figure 23. (a) STM image of a honeycomb network of DBA-ZnP at the TCB/graphite interface containing approximately six ZnP units in each nanowell and (b) its molecular model. The yellow arrow in (a) marks a phenylene linker. (c) Enlarged STM image of the monolayer of DBA-ZnP in which the white and blue lines indicate the directions of one of the main symmetry axes of graphite and a representative line for the height profile measurement. The blue line is rotated by +7° with respect to the white line. (d) A representative apparent height profile along the blue line in (c). (e) A histogram for distribution of the apparent heights. (Reproduced with permission from ref. 107. Copyright 2016 The Royal Society of Chemistry).


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Next, guest binding ability of the template network to C60 fullerene was examined. A mixture of DBA-ZnP (2.9 × 10−6 M) and five-time excess C60 (1.4 × 10−5 M) in TCB was used. Figure 24a shows a typical STM image of the mixture at the TCB/graphite interface after annealing treatment at 50 °C for 40 min. Very bright features arranged in a cyclic pattern were observed, which were not visualized without C60. In order to confirm if C60 molecules are immobilized on the ZnP units, apparent height profiles were measured along a line drawn between two ZnP units located at the diagonal positions of a nanowell (Figure 24b). A histogram for the distribution of heights shows a relatively broad and scattered bimodal distribution as shown in Figure 24c, which is fitted by two normal distribution curves centred at 260 pm (orange line) and 117 pm (grey line). While the latter is assigned to the distribution of free ZnP units, the former curve centred at 260 pm is attributed to that of C60 molecules immobilized on the ZnP units. The scattered distribution of the C60 sites is attributed to the bright streaky features observed in the STM images, probably due to removal of C60 molecules from the ZnP units during scanning by the STM tip.103,104 From the area ratio of the grey and the orange curves, the fraction of the ZnP units hosting an immobilized C60 was estimated to be 96%.


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Figure 24. (a) STM image of a mixture of DBA-ZnP and C60 at the TCB/graphite interface. (b) Enlarged STM image of the monolayer of DBA-ZnP with C60 in which the white and blue lines indicate the directions of one of the main symmetry axes of graphite and a representative line for the height profile measurement. (c) A histogram for distribution of the apparent heights. (Reproduced with permission from ref. 107. Copyright 2016 The Royal Society of Chemistry).

The binding affinity of C60 to the ZnP unit located in the honeycomb matrix of DBAZnP on the surface was compared with that in solution, using bromoalkyl-ZnP (Figure 22a) representing a ZnP unit. The binding constant of bromoalkyl-ZnP with C60 in TCB (25 °C) was determined to be (1.9 ± 0.6) × 103 M−1 by the fluorescence quenching experiment, from which the complexation ratio of ZnP in solution was estimated to be 21% at the same concentration as that of the STM measurement. These results indicate a


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significantly higher binding affinity of C60 to the ZnP unit adsorbed on the surface, probably due to an entropy effect of the ZnP units immobilized on the surface.105 In summary, DBA-ZnP self-assembles forming a honeycomb molecular network in which most of the six ZnP units are placed in each nanowell, and six C60 molecules are immobilized at most of the ZnP units, forming cyclic arrays of ZnP-C60 complexes at the liquid/graphite interface.

5. CONCLUSION AND OUTLOOK In contrast to host-guest chemistry in 3D space which has advanced enormously during the last several decades, much less has been done in 2D space, that is physisorbed monolayers of a single atom or a single molecular thickness on surfaces. Once a molecule is adsorbed on a flat surface, its rotational and translational mobilities are significantly reduced, thereby displaying behaviors different from those in 3D space such as fluid solution. Because of this fundamental interest and perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis, and thanks to the technological advances in scanning tunneling microscopy which is a vital tool for the study of molecular self-assembly on surfaces, host-guest chemistry in 2D space has been becoming a subject of intense current interest. We have revealed that a series of triangleshaped

molecules

with

a

phenylene-ethynylene

macrocycle,

called

dehydrobenzo[12]annulenes (DBAs), substituted by six alkyl chains, self-assembled at the interface of graphite and an organic solvent, forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains. The dimension of the nanoporous space formed by self-assembly of DBAs can be controlled from 1.6 nm to 4.7 nm by simply changing the alkyl chain length from C6 to C20. In this


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nanoporous space, not only single molecules but also its homo-and heteroclusters are coadsorbed in a shape- and size-complementary manner via van der Waals interactions. Moreover, taking advantage of the versatility of the DBA molecules for chemical modification at the alkyl chain terminals, we were able to construct nanoporous networks wherein the interior space is modified by functional groups for specific purposes. These include azobenzenedicarboxylic acid for photo-responsive guest adsorption/desorption, fluoroalkane and tetraethylene glycol groups for selective guest binding by electrostatic interactions, and zinc-porphyrin units for complexation with a guest by charge-transfer interaction. We have demonstrated that it is possible to construct nanoporous space on surfaces for guest binding by specific host-guest interactions by designing building blocks which form self-assembled monolayers equipped with binding sites, by reflecting the fruits of host-guest chemistry in 3D space into 2D. These results provide a significant insight toward the synthesis of functional materials on the basis of surface-confined porous networks. There are a number of challenges in host-guest chemistry in 2D space. These include, for example, the use of host-guest matrices for on-surface reaction for the synthesis of 2D polymers111,112 and epitaxial multilayer growth induced by guest accommodation.113 Though the steps toward our ultimate goals, such as tailor-made catalysis, are not clearly visible yet, advances in the above-mentioned topics would certainly contribute step-by-step to realize the goals. To this end, however, new designs based on immobilization of molecules by physisorption on surfaces, together with further advances in microscopy technologies, would be very advantageous for 2D systems.


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AUTHOR INFORMATION Corresponding Authors * Tel: +81- 44 934 7212. Fax: +81-44 934 7212. E-mail: [email protected] (K. T.) * Tel: +32 16 32 79 21. E-mail: [email protected] (S. D. F.) * Tel. +81 6 6850 6225. Fax: +81 6 6850 6229. E-mail: [email protected] (Y. T.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant numbers 24350046, 26620063, and 15H02164 and the Fund of Scientific Reserch-Flanders (FWO), KU Leuven – Internal Funds, the Belgian Federal Science Policy Office (IAP-7/05). The research leading to these results has also received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 340324.

ABBREVIATIONS 3D, three-dimensional; MOF, metal organic framework; 2D, two-dimensional; SPM, scanning probe microscopy; STM, scanning tunneling microscopy, UHV, ultrahigh vacuum; DBA, dehydrobenzo[12]annulene; TCB, 1,2,4-trichlorobenzene; 1PO, 1-phenyloctane; OA, octanoic acid

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Author Biographies

Kohei Iritani Kohei Iritani received B.S. (2012) and M.S. (2014) degree from School and Graduate School of Engineering and Science, Osaka University. He has been a PhD student in the group of Professor Yoshito Tobe at Osaka University since 2014. He spent two months in the research group of Professor Iris Oppel at RWTH Aachen University as a visiting researcher in 2016. His current research focuses on molecular self-assembly and host-guest chemistry at the liquid-solid interfaces, and synthesis of 2D polymers.

Kazukuni Tahara Kazukuni Tahara received B.S. degree from School of Science and Technology, Meiji University in 2000 and then studied at Graduate School of Science, The University of Tokyo, obtaining the Ph.D. in 2005 under the direction of Professor Eiichi Nakamura. After postdoctoral work (JSPS research fellow) with Professor Yoshito Tobe at the Graduate School of Engineering Science, Osaka University, he became an assistant professor at same institute in 2006. He became an associate professor at School of Science


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and Technology, Meiji University in 2016. He is a recipient of the Chemical Society of Japan Award for Young Chemists (2012). His current interests include synthesis of πconjugated molecules for on-surface self-assembly, and functionalization of graphitic surfaces.

Steven De Feyter Steven De Feyter is full professor, since 2011, at KU Leuven – University of Leuven in Belgium. After starting up scanning tunneling microscopy during his PhD in the group of Prof. Frans C. De Schryver at KU Leuven he moved for a postdoctoral position to the group of Prof. Ahmed Zewail (Caltech) where he was involved in ultrafast organic femtochemistry. His current interests include the study of supramolecular chemistry and self-assembly phenomena at surfaces, including modification of 2D materials, with a focus on liquid-solid interfaces. In 2013, he was awarded an ERC advanced grant.

Yoshito Tobe


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Yoshito Tobe received PhD (1979) from Osaka University under the supervision of Professor Y. Odaira. He was appointed as Assistant Professor of Osaka University and was promoted to Lecturer (1983), Associate Professor (1992), and Professor (1998). He was Visiting Professor from 1987 to 1988 with Professor P. E. Eaton at the University of Chicago. He is a recipient of the Chemical Society of Japan Award for Young Chemists (1985), Society of Synthetic Organic Chemistry Japan Award (2012), and the Chemical Society of Japan Award (2015). His current research focuses on exotic aromatic compounds and on-surface supramolecular chemistry.


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