b CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China
Self-assembly refers to a technology of the basic structural units (molecules, nano-materials, microns or larger size of the material) of spontaneous formation of structures . In the self-assembly process, the basic structural units spontaneously organize or aggregate into a stable regular geometry of the structure. Whether the self-assembly can be achieved is dependent on the characteristics of the basic structural elements, such as surface topography, shape, surface functional groups and surface potentials, etc. After assembly, the final structure has the lowest free energy. The results show that the internal driving force is the key to self-assembly, including van der Waals force, hydrogen bond, electrostatic force . The self-assembly system from the molecule to the macro objects of different scales has been a hot spot for scientists. The molecular self-assembly is self-assembling single molecules by using inter-molecular short-range force to achieve the nanometer or micron scale ordered structure. Recently, some research groups have used molecular units to self-assemble a variety of nanomaterials with different geometrical shapes by different means. Many scientists have made outstanding work in this field [3-13]. This is a hot topic in the research of molecular self-assembly. In this way, the molecules from the bottom of the self-assembly for a variety of different shapes of nano-materials, and show some special physical and chemical properties. Molecules can also be self-assembled under the induction of the template into a regular and orderly patterned surface.
Host-guest chemistry, as one of the defining concepts of supramolecular chemistry, describes the formation of unique structural complexes between two or more molecules or ions via non-covalent interactions. Host-guest chemistry in two dimensions deals with various types of noncovalent interactions, such as hydrogen bonding, dipole-dipole and van der Waals interactions, metal-ligand coordination, or a combination of several of them [14-17]. So far, the most important application and property of 2D nanoporous networks are as the accommodation of guest molecules. The organic molecules are usually self-assembled to the solid surface to obtain a surface-confined host network structure. If the size and shape of the guest molecule match those of the host network, the nanopores of the host network (single molecular thick) can adsorb guest molecules. The host network formed at the solid-liquid interface is believed to be stabilized by (dynamic) co-adsorption of the solvent molecules . The immobilization of the guest molecule at the solution-solid interface takes place at the cost of solvent desorption, since adsorption capacity of the guest molecule is higher than that of the solvent molecule in most instances . The stability of guest molecules usually occurs via an attractive dispersion interaction with the host network and the underlying surface. Thus, the hostguest chemistry on solid surface is usually "surface-assisted". Alternatively, the host and the guest molecules can be brought onto the surface simultaneously. In the review, coronene (COR) is chosen as the guest molecule due to its potential property. Other than benzene, COR is the simplest six-fold symmetrical aromatic molecule. It is a blue fluorescent material and molecular semiconductor, which have been used to set up organic transistors, solar cells, or information storage systems, etc. Its imide and carboxylate derivatives are regarded to be liquid crystalline with high fluorescence emission in solution as well as in the solid state. Although COR is electron rich, its derivatives are known to be electron deficient.
Scanning tunneling microscopy (STM) has been extensively used in surface science and technology because it has the ability to directly observe and study the properties of molecular structures on solid surfaces in ultra-high vacuum (UHV) conditions, solidliquid interfaces or in ambient environmental [20, 21]. At present, STM is mainly used to determine the physical adsorption monolayer at single molecular level and to detect reactivity, chirality and kinetics of bounded molecules in the definite atomic plane or interface [22-25]. Furthermore, STM can be used to detect the electronic structure of the physical adsorption monolayer at the single molecular level when combined with the scanning tunneling spectrum (STS) [26-31]. This review summarizes the influence of the guest molecule COR on the host networks, which have been studied by our research team. All kinds of molecules used in the review are shown in Scheme 1.
|Scheme 1. Chemical structures of the adsorbate molecules: (a) molecules with three carboxylic groups (TMA, TCDB and StOF—COOH3). (b) HPB and HPB derivatives HPB-6a, HPB-6pa. (c) polycyclic aromatic diimide derivatives PAI1, PAI2, PAI6, PAI8. (d)4, 4'-bipyridine (Bpy) and 1, 3, 5-tris(4-pyridyl)-2, 4, 6-triazine (TPTZ) (e) macrocycle formed by phenanthrene molecules: PBM1, PBM2. (f) macrocycles formed by diazobenzene groups: 4NN-macrocycles and 3 N N-macrocycles. (g) guest molecules: COR.|
2. COR embedding in the host networks
Trimesic acid (TMA, 1, 3, 5-benzenetricarboxylic acid) has been extensively investigated on HOPG surface during the ten years. Robust honeycomb networks are formed by TMA, and the cyclic hexamers (1.5 ± 0.1 nm) provide the potential to seize small guest molecules [32-37]. The cavity size and shape of TMA can be controllable by introducing different length of alkyl chains along the three directions.
1, 3, 5-Tris(10-carboxydecyloxy)-benzene (TCDB) is one of TMA derivatives (TMAs) with the long alkyl chains. TCDB self-assembles into the comparatively large and tunable rectangular cavity via O—H…O hydrogen bonds on HOPG as shown in Fig. 1a. When mixed with COR, it can be seen that one tetragonal cavity of TCDB can immobilize either one (in domain A) or two (in domain B) COR molecules, according to the molar ratio of TCDB with COR (Fig. 1b). As compared to the indistinct structure of COR itself, each individual COR molecule can be clearly resolved in the TCDB/ COR architecture. Fig. 1d reveals one COR enbeded in one TCDB cavity and not fully occupied the cavity of TCDB networks (inner width is 2.3 nm × 1.3 nm). Fig. 1e displays two COR molecules lie side by side in one TCDB host network and the dimer size of two COR is commensurate with the cavity size of TCDB networks. Therefore COR can be stabilized firmly on HOPG by the networks .
|Fig. 1. (a) High-resolution STM images for self-assembled structures of TCDB. (b) The molecular model corresponding to (a). (c) Large-scale STM image of coexistence of two kinds of COR/TCDB host-guest architectures. (d) An STM image of COR/TCDB architecture. (e) A high-resolution image of COR/TCDB architecture. (f) Suggested molecular model of (e). Reproduced with permission . Copyright 2004, American Chemical Society.|
Another example of TMAs is StOF—COOH3, an oligodontia endcapped with three carboxylic groups [39, 40]. By diluting the concentration of the solution, StOF—COOH3 molecules selfassemble from a disorder structure into honeycomb networks at the octanoic acid/HOPG interface marked in Figs. 2a–c. The diameter of the honeycomb networks is 4.3 ± 0.2 nm. After a thermal annealing process of 60℃ for 30 min, the networks become more regular with full view of well-ordered hexagons (Fig. 2d). When introducing COR in the StOF-honeycomb network, COR molecules aggregate into triangle-shaped trimers and are accommodated in the honeycomb cavities as shown in Fig. 2f. While the introduction of COR can not make the disordered StOF—COOH3 structure regular. Heating the COR/StOF-honeycomb system and COR/StOF-disorder system at 60℃ for 1 h, the disorder system arranged in a well-ordered paralleled ladders pattern as shown in Fig. 2h, COR/StOF-honeycomb system remained the perfect hexagonal network with COR trimers included in the cavities .
|Fig. 2. Self-assembled structural transition with decreases of solvent concentration: (a) STM image of the StOF-disorder structure (undiluted). (b) STM image of the coexistence of disordered and well-ordered structure (5× dilution). (c) STM image of the StOF-honeycomb structure (5 × 5 dilution). (d) High-resolution image of StOF-honeycomb structure after heating. (e) High-resolution STM image of COR/StOF-honeycomb self-assembly after heating. (f) High-resolution STM image of COR/StOF–honeycomb self-assembly after heating. (g), (h), (i) supposed model based on (d), (e), (f), respectively. Reproduced with permission . Copyright 2016, American Chemical Society.|
4NN-macrocyclic is well-designed macrocyclic compounds consisting of four diazobenzene photosensitive units which can process reversible cis-trans isomerization and cause drastic changes in structures and chemical properties. Thus, 4NNmacrocycle has unique photoisomerization properties because of the four photosensitive diazobenzene groups without alkyl chains. 4NN-macrocycle cannot be observed on surface solely under the STM investigation but can be stabilized in the TCDB network with its thermally stable trans-trans-trans-trans (t, t, t, t) state at heptanoic acid/HOPG interface, which can form wellordered 2D networks with flexible nanoscale pores through hydrogen bond (Fig. 3a). With the TCDB/4NN-macrocycle mixing ratio differ, two kinds of TCDB/4NN-macrocycle self-assembling structures were observed at heptanoic acid/HOPG interface before light irradiation, including (t, t, t, t) 1 and (t, t, t, t) 2. When the 4NNmacrocycle/TCDB co-assembled network was irradiated by UV light, it was observed to undergo hotoisomerization from the two former structures to three (t, t, t, t) photoisomers, three (t, t, t, c) photoisomers, and one (t, c, t, c) photoisomer, which provides a facile approach to study photoisomerization process of the azobenzene derivatives and the conformation of their photo-isomers in ambient environment . It should be concluded that the photoisomers are also observed to appreciably affect the guest-host network characteristics during the photocycled switch process . Prior to irradiation, 4NN-macrocycles in the TCDB/4NN-macrocyclic/COR system adopts (t, t, t, t) photoisomer, COR is absorbed on the 4NN-macrocycle (Fig. 3c). When ternary structure is irradiated with ultraviolet light, the 4NN-macrocycle adopts the (t, c, t, c) photoisomer, and its inner cavity is deformed so that the COR molecule is trapped in the ellipse of the 4NN-macrocyclic isomer (Fig. 3e). On the other hand, the approximate system energy indicates that the transition between (t, t, t, t) and (t, c, t, c) photoisomer can occur easily when the sample is irradiated with ultraviolet light and visible light. It is worth noting that there is a choice of isomers in the self-assembly process, and the surface network will affect the kinetic process of the isomerization of the 4NN-macrocycles. Such photoisomerization-dependent guest immobilization has also been observed for the adsorption of dicarboxyazobenzene substituted DBA. After irradiation of the surface, the number of cavities contains more than two COR molecules increased .
|Fig. 3. (a) High-resolution STM image of the TCDB1/4NN-macrocycle (t, t, t, t) network structure. (b) The molecular model for TCDB1/4NN-macrocycle (t, t, t, t) structure. (c) STM image of the ternary adlayer of TCDB1/4NN-macrocycle (t, t, t, t)/ coronene before the UV irradiation. (d) The molecular model for the TCDB1/4NNmacrocycle (t, t, t, t)/COR adlayer. (e) STM image of the TCDB2/4NN-macrocycle (t, c, t, c)/COR architecture after the UV irradiation.(f) The molecular model for the ternary TCDB2/4NN-macrocycle (t, c, t, c)/COR network. In all the molecular models, the red ball represents oxygen atom, the blue for carbon, and the purple for nitrogen. Reproduced with permission . Copyright 2011, American Chemical Society.|
Similar to 4NN-macrocycles, the single component of 3 N Nmacrocycle does not self-assemble on the HOPG surface, however it can be stabilized in the TCDB network as shown in Fig. 4a. The sizes of cavity A and B are about 1.3 nm and 0.8 nm, respectively. It is notable that the size and geometry commensurability of the guests with these cavities are believed to play a substantial role in the observed site selectivity [45, 46]. Thus, COR here can only be entrapped in the chiral empty pores (A type cavity) due to the size of matching between the A type cavity (1.3 nm) and the coronene molecule (diameter is 0.9 nm). Meanwhile, the B type cavity can immobilize serval smaller guest molecules like fullerene molecules  and heptanoic acid  as shown in Fig. 4c . This fourcomponent networks indicate that the size matching between guest molecules and network pores is very important for forming a well-defined multi-component network.
|Fig. 4. (a) A high-resolution STM image of the TCDB/3 N N macrocycle hybrid networks (21 nm × 21 nm). (b) Tentative network model of the hybrid networks. (c) A four components architecture STM image of a mixture of TCDB, 3 N Nmacrocycle, coronene and heptanoic acid solvent molecules (16.4 nm × 16.4 nm). (d) Tentative network model showing the four component networks. Reproduced with permission . Copyright 2013, The Royal Society of Chemistry.|
3. COR covering the host networks
PAI1, PAI2, PAI6 and PAI8 are polycyclic aromatic diimide derivatives (PAIs), which have the same alkyl chains but different backbone blocks . The four PAIs can assemble into densely packed monolayers on HOPG. PAI1 molecule displays a bowl (Figs. 5b–d). After deposition of COR, coronene selectively adsorbs on the helicene-typed PAI1 monolayer strongly, depending on the conjugated cores of these PAIs. Thus, the V-shaped protrusion of PAI1 becomes circular spots, while other monolayers display no evidence of immobilization of COR molecules. The cross-sectional profiles of COR/PAI1 system show that the distance between bright stripe and darker stripe has enlarged from around 0.1 nm to around 0.2 nm (Fig. 6). The results indicate that the COR/PAI1 system is a bilayer system, and the DFT calculation has identified the adsorbed COR molecule is located on the PAI1 conjugated cores. The density of states (DOS) is calculated to study the electronic interactions between the COR molecules, PAI1 monolayer and the substrate. The strong interaction between COR molecules and PAI1/HOPG system is obviously improved due to the increased total DOS structures and the adsorption of COR molecules have the property to induce the change of energy-level alignment, which is looking forward to being used in organic electronics .
|Fig. 5. High-resolution STM images (15 nm × 15 nm) of molecule PAI1 (a), PAI2 (b), PAI6 (c), and PAI8 (d) at the 1-phenyloctane/HOPG interfaces. The unit cells are inserted on the STM images. Reproduced with permission . Copyright 2017, American Chemical Society.|
|Fig. 6. (a) High-resolution STM image of the self-assembled densely packed structure of Cor/PAI1/HOPG with the chemical structure of COR. (b) Line profile of the PAI1 and Cor/API1 monolayer on the HOPG surface corresponding to the STM images. (c) Molecular model in image (a), (d) side view of the molecular model in (c). (e) Density of states (DOS) for the HOPG slab and PAI/HOPG system before and after adsorption of COR molecules. Fermi energy is aligned to 0 eV. Reproduced with permission from . Copyright 2017, American Chemical Society.|
The assembly behavior of hexaphenylbenzene (HPB) and its derivative molecules have been extensively studied due to its' high guest selectivity. The HPB-6pa and HPB-6a molecules contain six carboxyl groups in the peripheral phenyl group, thus these molecules form large-scale network structures by hydrogen bonding between carboxyl groups [52-54]. At the octanoic acid/HOPG interface, HPB-6a and HPB-6pa can form triangle networks with different sizes through hydrogen-bonded interactions. The triangle cavities' side length is approximate 2.0 nm (Fig. 7a). The addition of COR to HPB-6pa systems leads to a very different arrangement. The coronene molecules assemble into a honeycomb structure on top of the HPB-6pa monolayer as presented in Fig. 7b. While no COR is found on the top of HPB-6a after introducing coronene, which is possibly due to the smaller size of the cavities compared with HPB-6pa. Further research has confirmed the HPB-6pa/COR is a bilayer system. By the bias changing as shown in Fig. 7c, the hexagonal COR pattern can be visualized at higher bias voltage as the second organic layer, while in the case of lower bias voltage, the network structure of HPB-6pa could be observed as the first organic layer. The six inserted COR molecules re-form a new hexagonal cavity, and then form a stubborn Kagome network that can also adsorb other COR molecules .
|Fig. 7. (a) STM image of the HPB-6pa adsorbed monolayer on the HOPG surface, I = 298 pA and V = 598 mV. (b) STM image of the HPB-6pa/COR co-adsorbed monolayer on the HOPG surface. (c) STM image of the HPB-6pa/COR molecular assembly structure at alternate bias voltages, I = 248 pA. The change of bias value is indicated on the image by the dashed line. (d) Suggested molecular model for the HPB-6pa architecture on HOPG. (e) COR molecules insert into the cavity formed by itself. Inset: the suggested molecular models of the COR enter into assembly structures. (f) Suggested molecular model for the HPB-6pa/COR architecture on HOPG). Reproduced with permission . Copyright 2011, The Royal Society of Chemistry.|
4. COR templating the host networks
HPB molecule is a large π-conjugate system with high electronic state density [56-59]. HPB molecules self-assemble into two kinds of structures. One is the snowflake structure whose size of the triangular cavity is about 1.2 nm (length of the edge of the triangle) as shown in Fig. 8a and another structure is the honeycomb structure with a central pore size of 4.0 ± 0.1 nm in diameter disappearing mostly in phase boundary and soon disappearing during scanning (Fig. 8c). Both of the HPB-snowflake and the HPB-honeycomb structures attract each other via van der Waals between the side alkyl chains of HPB molecules and HOPG. When introducing COR into a snowflake HPB system, the assembly undergoes a dramatic transformation. COR molecules aggregate into heptameric COR cluster and in order to trap these COR molecule clusters, snowflake networks turn into the honeycomb cavities, as shown in Fig. 8e .
|Fig. 8. (a) High-resolution STM images of the snowflake-like structures. (b) Highresolution STM images of the honeycomb structure. (c) High-resolution image of HPB—COR co-adsorption assembly. (d), (e), (f) Supposed molecular model of (a), (b), (c). Reproduced with permission . Copyright 2016, American Chemical Society.|
PBM1 and PBM2 are three- and four-membered macrocycle formed by phenanthrene molecules. PBM1 can self-assemble into a linear network structure on HOPG and the size of triangular macrocycle is about 0.8 nm (Fig. 9a), while PBM2 presents a nanoporous structure with a good flatness and the square cavity is around 1.3 nm (Fig. 9c). After introducing the increasing concentrations of COR solutions (at C1 (1/4 of saturated concentration), C2 and C3), the self-assembled structures of PBM1 remains unchanged, implying that the guest molecules cannot be immobilized into the cavities. However, the co-deposition structures of PBM2/COR system changes with the concentrations of COR solution (at C1, C2 and C3). At the C1 COR solution, COR molecules are entrapped into the square shaped cavities and the lattice parameters of PBM2 host nanoporous networks remain unchanged as shown in Fig. 9e. When increasing the concentration of COR (C2 and C3), the lattice parameters are enlarged and the PBM2/COR networks undergo a structural transformation. At the C2 COR solution, COR molecules are immobilized in the cavities around PBM2 molecules and the square shaped cavities of PBM2 as shown in Fig. 9g. After depositing a droplet of a higher concentration (C3), the cavities around PBM2 are expanded in order to entrap more COR molecules (Fig. 9i). The results will help us to control molecular pattern formation through concentration, size and shape .
|Fig. 9. (a) High-resolution STM image (25 nm × 25 nm) of the self-assembled structures of PBM1. (c) High-resolution STM image (24 nm × 24 nm) of the self-assembled structures of PBM2. (e) A high resolution STM image (20 nm × 20 nm) of the self-assembled structure of PBM2/COR(I) networks on the HOPG surface. (g) STM image (21 nm × 21 nm) of the PMM2/COR (Ⅱ) networks. (i) STM image (21 nm × 21 nm) of the PBM2/COR(Ⅲ) networks. (b), (d), (f), (h), (j) Schematics corresponding to the molecular models of all assemblies are presented on the right hand side. Reproduced with permission . Copyright 2016, the Royal Society of Chemistry.|
2-Fold symmetric Bpy mixed with TMA can self-assembly at the 1-heptanoic acid/HOPG interface and form a well-ordered rectangular network structure. The Bpy molecule acts as a bridge to connect two TMA molecules on both sides via intermolecular N…H–O hydrogen bonds as shown in Fig. 10a. Introduction of the guest molecules, the Bpy/TMA rectangular structural re-organize into hexagonal TMA networks, and COR molecules entry into the re-constructed hexagonal TMA networks rather than the rectangular binary networks, pictured in Fig. 10c. This phenomena illustrates that COR/TMA complex on graphite is the minimum Gibbs free-energy system compared with COR/TMA/Bpy complex .
|Fig. 10. (a) A STM image of the binary assembly structure. (b) Suggested molecular model of Bpy/TMA binary assembly. (c) A STM image of COR/TMA and TMA/Bpy assembly structures. (d) Schematic diagram for this competitive adsorption process. Reproduced with permission . Copyright 2014, The Royal Society of Chemistry.|
TPTZ is a 3-fold symmetric ligand molecule. Similar to Bpy system, TPTZ is usually self-assembled into a uniform hexagonal network structure via N…H—C hydrogen bonding, as shown in Fig. 11a. Mixing TMA and TPTZ, a 3-fold symmetry hexagonal network is formed at the liquid/solid interface, as shown in Fig. 11b, which is similar to the work reported by Lackinger et al. . The TMA/TPTZ mixture introduces the COR molecule in the same manner as in the case of TMA/Bpy. TPTZ molecules are desorbed from the binary assembled network, meanwhile the COR/TMA complex structure re-assemble and cover more surface with time, presented in Fig. 11d . These results allow us to better understand the interactions between the guest molecules and the template and substrate, and give us a better understanding of the concept of competitive adsorption for the manufacture of functional molecular arrays.
|Fig. 11. (a) STM image of an ordered hexagonal network of TPTZ at 1-heptanoic acid/HOPG interface. (b) STM image of TPTZ/TMA network structure. (c) Suggested molecular model of TPTZ/TMA binary assembly. (d) The COR/TMA host-guest structure reconstruction when COR molecules were added intoTMA-TPTZ assembly system. (e) Schematic diagram for this dynamic layer reconstruction. Reproduced with permission . Copyright 2014, the Royal Society of Chemistry.|
5. Summary and outlook
The host-guest chemistry based on supramolecular networks has been discussed in the review. The nonporous uniform network has a tunable cavity that can be used as a good molecular template for stably immobilizing COR or COR clusters to form complex binary or ternary structures whose structural properties are susceptible to both concentration and thermal effects as well as guest molecules. With the introduction of COR to intrinsically nonporous networks, the orientation of the guest species in host-guest architecture can be controlled due to the confinement of cavities, like pore size and shape characteristics of networks. When the guest molecule enters the self-assembled lattice structure, COR may cause the main selfassembly structure transformation and form a new structure. The latest trends in host-guest chemistry suggest that molecular design, supramolecular synthesis and surface science principles are combined to achieve a host-guest system designed for a particular function. The recently reported newly design strategy provides an approach to the sophisticated host networks which are the selective and responsive to external stimuli of guestbinding behavior. There are two main directions for the hostguest binding in solid surface studies: the molecular separation in quantitative measurement (e.g., the use of high surface area powder materials) and the detection of small quantities of chemical molecule in qualitative measurements. Looking forward to the future, the host-guest chemistry will benefit from more avant-garde design strategies that allow modification of the chemical/chiral environment within the 2D cavity. This modification will allow selective identification of the host network and the guest molecules based on chemical and/or chiral complementarity. The interesting possibility is to use these confined spaces in these nano-sized cavities for chemical conversion. Such limiting induced chemicals may allow access to reaction paths and products that are not available in solution, or on large steps of the solid surface.
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