Guiding the self-assembly of different types of functional molecules into well-defined structures on surfaces is beneficial for both fundamental surface and interface study and emerging application fields, especially molecular and organic electronics. This review focuses on understanding the two-dimensional self-assembly process of telechelic organics, which feature alkoxylene chains terminated with carboxyl groups. With the combined flexibility of alkyl chains and directionality of carboxyl groups, telechelic organics show unique assembly behaviour on two-dimensional surfaces. By increasing the length of the alkoxylene chains, the cavities in the nanoporous networks of telechelic trimesic acid (1,3,5-benzene tricarboxylic acid) derivatives change from hexagonal cavities to irregular cavities on a highly oriented pyrolytic graphite surface. The nanoporous networks provide a flexible host template for host–guest supramolecular chemistry because the cavities framed by the flexible alkoxylene chains can be changed in accordance with the sizes/shapes of the guest molecules. Furthermore, the terminal carboxylic group can form a hydrogen bond with another hydrogen bond partner, leading to multi-component structural motifs and hierarchical assemblies. The unique assembly behaviour of telechelic organics makes them promising structures as important building blocks for the design and construction of complex self-assembled nanoarchitectures.
Analogous to crystal engineering, two-dimensional surface molecular engineering is aimed at constructing well-defined structures on surfaces using different types of functional molecules [1–4]. As the building blocks of two-dimensional crystal engineering, varieties of organic molecules have been used to fabricate supramolecular structures under the control of multiple interactions between neighbouring molecules, solvent and substrates [5–7]. The impetus for the application of two-dimensional surface molecular engineering is from organic and molecular electronics. The formation of ordered supramolecular structures of functional organic molecules via self-assembly processes is critical for high-performance organic electronic devices [8–10]. In addition, the controllable arrangement and patterning of organic functional molecules on surfaces is an important step towards integrating the functional molecules as molecular electronic devices [11,12]. Fundamentally, the spontaneous formation of surface molecular nanostructures is a self-assembly process. Understanding the principle of the self-assembly process of organic molecules on surfaces is beneficial for many surface and interface processes such as heterogeneous catalysis, electron transfer and wetting.
Non-covalent bond interactions between molecules are mainly responsible for the formation of perfectly ordered supramolecular structures on surfaces [13–15]. Non-covalent bond interactions, for instance van der Waals forces [16–20], hydrogen bonds [21–24], π–π stacking interactions [25,26], dipole–dipole interactions [27–29] and electrostatic interactions [30,31] between functional groups in organic molecules, play an important role in the self-assembly process. Long alkyl chains have been widely applied to immobilize organic molecules to obtain two-dimensional periodic arrangements on surfaces through van der Waals forces between alkyl chains and a substrate [16,17]. In addition, two-dimensional crystallization energy resulting from van der Waals forces between long alkyl chains can induce full interdigitation of alkyl chains and leads to the densely packed lamellar structures on surfaces . Owing to the inherent conformation flexibility of alkyl chains, the interesting structural evolution of functional molecule homologues decorated with alkyl chains with different lengths has been observed in a number of systems [13–16].
Hydrogen bonding is another important intermolecular interaction involved in the self-assembly process. The most important features of hydrogen bonding are directionality, selectivity and relatively high bond strength (1∼40 kcal mol−1) . For example, hydrogen bonds between carboxylic groups can form different binding patterns such as cyclic dimers, trimers and catemeric motifs, which can drive the formation of hierarchical assemblies . In addition, carboxylic groups can form hydrogen bonds with other functional groups acting as hydrogen bond donor and acceptor [33–36], which further enriches the kaleidoscopic assembly patterns driven by hydrogen bonding.
In recent years, surface host–guest chemistry has received great attention [14,26,36–39]. Well-defined molecular networks with appropriate nanopatterns can serve as surface molecular templates to drive the selected molecular organization of guest molecules in a precisely controlled position, orientation and interdistance . From the viewpoint of molecular engineering, the separation of patterning and functionalization into independent steps can enable the programmable assemblies of nanoarchitectures. Generally, the highly directional intermolecular interactions, typically hydrogen bonding, are employed to frame the nanoporous network [40–42], whereas the rigid acetylene or phenyl moieties are employed to obtain isoreticular networks with different pore sizes [23,42–44]. The size and/or shape complement between the host cavity and the guest molecules is frequently proposed as a key factor responsible for the formation of the desired pattern assembly. Until now, various two-dimensional molecular templates as well as designed host–guest systems have been constructed on solid supports and studied at the molecular scale by means of scanning tunnelling microscopy (STM) [36–39]. Steered by two-dimensional molecular templates, the desired patterns of C60, porphyrin and phthalocyanine derivatives, and many other functional molecules, have been accomplished [26,38,39].
This review focuses on the self-assemblies of a class of telechelic organic compounds which bear an aromatic core decorated with multiple alkyl chains with terminal functional groups, for instance a carboxyl group. Trimesic acid (1,3,5-benzene tricarboxylic acid; TMA) decorated with alkoxylene chains terminated by carboxylic groups, as a kind of telechelic molecule, will be extensively discussed in this review. With the combined flexibility of alkoxylene chains and directionality of carboxyl groups, telechelic organics show unique assembly behaviour on two-dimensional surfaces. Intriguingly, the nanoporous networks formed by these molecules are flexible to accommodate guest molecules of different sizes and shapes. This review is organized as follows. In §2, the two-dimensional self-assembly structure of TMA derivatives on highly oriented pyrolytic graphite (HOPG) and Au(111) surfaces is discussed. Section 3 focuses on two-dimensional host–guest chemistry by using templates of TMA derivatives. Section 4 is devoted to the multi-component assembly of TMA derivatives with other functional molecules. Finally, a brief conclusion and outlook concludes this review.
2. Two-dimensional crystal structure of telechelic TMA derivatives on HOPG and Au(111)
The chemical structures of TMA and its derivatives decorated with alkoxy chains are illustrated in scheme 1. Three carboxylic groups and the central benzene core are linked by the flexible alkoxylene arms. The threefold symmetry of TMA is preserved. The length of the alkoxylene arms can be varied by the number of –CH2–moieties in the alkoxy chains. 1,3,5-tris(carboxymethoxy)-benzene (TCMB), 1,3,5-tris(3-carboxypropoxy)-benzene (TCPB), 1,3,5-tris(4-carboxybutoxy)-benzene(TCBB), 1,3,5-tris(5-carboxyamyloxy)-benzene (TCAB) and 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) have 1, 3, 4, 5 and 10 –CH2–moieties in their alkoxylene chains, respectively.
TMA molecules can self-assemble into two-dimensional hexagonal networks of chicken wire structure or flower structure with periodic-ordered cavity voids on a HOPG [45,46] or Au(111) substrate via intermolecular hydrogen bonds at the liquid–solid interface or under an ultrahigh vacuum (UHV) environment [47–51]. Wu and co-workers  found that the molecular coverage can induce the evolution of TMA self-assembling structures on Au(111) at room temperature under UHV. The interpore distance was tuneable from 1.6 nm onwards at a step size similar to 0.93 nm. To elaborately fabricate the isoreticular networks, rigid spacers, such as a benzene spacer or carbon–carbon triple bond, have been extensively adopted to enlarge hexagonal cavities [42–44]. The symmetry of the hexagonal networks of TMA with rigid spacers is similar to that of a single TMA molecule. By contrast, TMA derivatives decorated with alkoxylene chains, as a flexible spacer, can form nanoporous networks with unique features on solid surfaces [52–55].
Two-dimensional crystal structures are the balance of intermolecular interactions and molecule–substrate interactions . The evolution of self-assembly of telechelic TMA derivatives is expected to be affected by the following three interactions. First, as mentioned above, the chain–chain van der Waals interactions between long alkyl chains may induce the formation of densely packed lamellar structures on the surface . Second, the carboxyl groups at the end of alkoxylene chains tend to form dimers via hydrogen bonds [23,32]. Third, the adsorption energy increases proportionally with the chain length . Stronger interactions between the molecule and substrate may decrease the mobility of molecules on the substrate and induce epitaxial structures on the substrate [56–58]. Interestingly, different assembly patterns of TMA derivatives are found on HOPG and Au(111) surfaces [52–55].
(a) Self-assembly structures of telechelic TMA derivatives on HOPG
The adlayer of telechelic TMA derivatives was fabricated by depositing a droplet (approx. 2 μl) of approximately 1 mM toluene solution of the targeted molecules on a freshly cleaved HOPG surface. STM measurement was carried out after evaporation of the solvent. The obtained self-assembly structures of TCMB, TCPB and TCDB on the HOPG surface under ambient conditions are illustrated in the STM images in figure 1 [53,54,59]. All three of the TMA derivatives self-organize into well-ordered nanoporous structures on the HOPG surface. As displayed in the STM image in figure 1a, TCMB molecules fabricate into a hexagonal network with sixfold symmetry [53,59]. In the TCMB hexagonal network, each hexagonal cavity is framed by six TCMB molecules which are interconnected via hydrogen bonds between neighbouring carboxylic groups, as shown in the simulated model in figure 1b. The diameter of the hexagonal cavities in the nanoporous networks of TCMB is 1.9 nm. Similar to the chicken wire structure of TMA , hydrogen bonds are the main driving force for the fabrication of TCMB hexagonal networks on the HOPG surface because the chain–chain interactions and molecule–substrate interactions induced by the shortest alkoxylene chains are too weak to affect the self-assembly process. Hence, the threefold symmetry of TMA derivatives is maintained in the two-dimensional hexagonal networks.
In case of TCPB and TCDB, well-ordered columns without sixfold symmetry are formed on the HOPG surface owing to the increased chain–chain interactions and molecule–substrate interactions with the increasing length of the alkoxylene chains [54,59]. According to the previous result, the two-dimensional crystallization energies for each CH2 in the alkyl chains are calculated to be 4∼6 kJ mol−1 . With the increase of alkoxylene chain length, the alkyl chains tend to form closely packed structures. At the same time, the directionality of the hydrogen bonds between the terminal carboxylic acid groups drives the formation of nanoporous structures. Owing to the flexible conformation of the alkyl chains, distorted non-porous structures are observed. In the case of TCPB, the two carboxylic groups of a TCPB molecule form a hydrogen-bonded cyclic dimer with the neighbouring TCPB molecule. However, the irregular cavities formed by the cyclic dimer are too small to be observed in the STM image in figure 1c. The structural model in figure 1d illustrates the small and irregular cavities. The neighbouring cavities are packed to maximize the van der Waals interactions between the alkyl chains. Similarly, TCDB loses its threefold symmetry in the assembly and forms rectangular cavities with an inner width of 2.3×1.3 nm, as shown in the STM image in figure 1e . Each rectangular cavity is built by two TCDB molecules connected via four alkoxylene chains. The structural model in figure 1f clearly demonstrates that two carboxylic groups of a TCDB form four hydrogen bonds with the two carboxylic groups of another TCDB molecule, which results in two hydrogen-bonded cyclic dimers of the TCDB molecule. Also, two additional hydrogen bonds are formed by the left carboxylic groups linked with two interdigitated alkoxylene chains, as indicated by the black ellipse in the structural model of TCDB in figure 1f. Obviously, the interdigitation of two alkoxylene chains contributes to the stronger chain–chain interactions between the alkoxylene chains in TCDB. It should be noted that the left carboxylic group of TCPB also forms a hydrogen bond with the neighbouring TCPB molecule in a head-to-head manner, which is shown by the black ellipse in figure 1d. Hence, chain–chain interactions between alkoxylene chains in TCDB molecules is stronger than those of TCPB, which is in agreement with previous results [16,18]. The parameters of the self-assembly structures for different TMA derivatives are listed in table 1.
(b) Self-assembly structures of telechelic TMA derivatives on Au(111)
In the case of the Au(111) surface, the adlayer of TCMB, TCPB and TCAB is prepared by immersing the gold surface in the alcohol solution containing TMA derivatives for approximately 30 s [52,55]. As shown in the high-resolution STM images in figure 2b,e, the triangular appearances of the TCMB and TCPB molecules are consistent with the chemical structure of the individual molecules, which demonstrates that telechelic TMA derivatives adopt a flat-lying geometry on Au(111). The adlayers of the three TMA derivatives self-assemble into completely different patterns on the Au(111) surface. First, TCMB forms hexagonal networks on the Au(111) surface with threefold symmetry, similar to that on the HOPG surface . The diameter of the hexagonal cavity is measured to be 2.0±0.1 nm. Each hexagonal cavity is composed of six molecules which are not arranged with a head-to-head geometry of the carboxyl groups. H-bonding is formed between the hydrogen atoms in the carboxyl groups and the oxygen atom of another carboxyl group, as shown in the model in figure 2c. Therefore, the formation of H-bonds in the TCMB honeycomb adlayer on the gold surface is different from the head-to-head dimer of the carboxyl group in the honeycomb networks on HOPG. A unit cell of (6×6) concluded from the obtained STM data means that the TCMB molecular adlayer structure is completely commensurate with the Au(111) substrate. Hence, the molecule–substrate interaction plays a key role in the hexagonal network structure of TCMB on a gold surface.
As shown in figure 2d–i, the adlayer fabricated by TCPB and TCAB molecules on the gold surface is a close-packed structure without any cavities [52,55]. Apparently, the chain–chain interactions and the molecule–substrate interactions play a more and more important role in the formation of the TMA derivatives adlayer with increasing length of the alkoxylene chains. TCPB molecules adopt antiparallel orientations to form close-packed molecular rows, and the alkoxylene chains are arranged interdigitatively with each other along their preferred 〈110〉 direction on the gold surface. A (4×4√3) structure is deduced according to the STM data. Similar to TCMB, the head-to-head H-bonds cannot be formed by carboxyl groups because of limited space, as shown in the model of the TCPB adlayer in figure 2f.
The self-assembly structure of TCAB molecules is highlighted by the four-blade propeller-like bright protrusions periodically appearing in the STM images . An in-depth analysis demonstrates that the close-packed TCAB monolayer structure is collaboratively stabilized by the tetramers and dimers of the carboxylic groups, and the propeller-like motifs are formed by the alkoxylene chains from four neighbouring molecules respectively, which are bonded together by the cyclic tetramer of the terminal carboxylic groups. The structural model for the TCAB adlayer in figure 2i clearly demonstrates the cyclic tetramers and dimers of the hydrogen bonds, as indicated by the bars and windmills, respectively. Furthermore, the interdigitated alkoxylene chains can be seen from the structural model of the TCAB adlayer.
It is obvious that the differences in the adlayer structures of the TMA derivatives on the Au(111) surface should be closely related to the increasing length of the alkoxylene chains and the gold substrate. It is well acknowledged that there is a strong interaction between organic molecules and a gold substrate [56–58]. The enhanced molecule–substrate interactions and chain–chain interactions drive the TCPB and TCAB molecules to adopt close-packed structures without any cavities on the gold substrate. Only hydrogen bonds can dominate the self-assembly structure of TCMB with the shortest alkoxylene chains to form hexagonal networks on the Au(111) surface.
3. The host–guest system of telechelic TMA derivatives as a template
The nanoporous networks of TCDB on the HOPG surface have been used as templates to construct host–guest architectures [54,60–64]. First, the long alkoxylene chains of the TCDB molecules provide large enough cavities in the TCDB self-assembly structure to accommodate diverse functional guest molecules. Second, owing to the flexibility of the alkoxylene chains, the large cavities in the TCDB self-assembly structure are shape-variable spaces according to the entrapped guest molecules of different sizes and symmetry. A large number of guest molecules, such as metal phthalocyanine (MPc, M=H, Cu, Cu, VO, ClGa), and their derivatives [54,60], coronene , supramolecular metallacyclic polygon  and 4,4′-bipyridyl (Bpy) , have been entrapped by the nanoporous networks of TCDB to construct a series of host–guest architectures on a HOPG surface. Here, three representative guest molecules with different sizes and symmetry, CuPc, coronene and a supramolecular metallacyclic rectangle, are highlighted in this review. The host–guest architectures on the HOPG surface are obtained by depositing a drop of volatile solution (toluene or ethanol) containing TCDB and the guest molecules.
The high-resolution STM images of three kinds of host–guest architectures and the parameters of the unit cell are listed in table 2. The large cavities of the TCDB molecules can entrap one or two guest molecules, which induce the shrinkage or enlargement of the large cavities according to the sizes and shapes of the guest molecules, as shown in table 2. For example, a coronene molecule with a small diameter of 0.9 nm and a large supramolecular metallacyclic rectangle with the dimensions of 2.1×1.2 nm can be entrapped by the cavities (table 2) [54,61]. Accordingly, the lattice parameters of the unit cell are changed: when each cavity entraps a guest molecule, the lattice parameter a can be shrunk to 2.8±0.1 nm; when two molecules are entrapped, parameter a can be enlarged to 4.5±0.1 nm. Compared with the dramatic variations of lattice parameter a, the alteration (from 2.0 to 2.4 nm) of lattice parameter b is much smaller. It is obvious that the flexibility of the cavities in the direction of parameter a should be closely related to the four flexible alkoxylene chains linked by the hydrogen bonds of a TCDB cyclic dimer.
According to theoretical analysis of MPc/TCDB architectures, the O−H⋯ O length of four hydrogen bonds linking a TCDB cyclic dimer is changed from approximately 2.53 Å for unfilled TCDB cavities to approximately 2.55 Åfor filled TCDB cavities owing to the intrusion of the guest molecules . The intermolecular interaction between the host and guest molecules depends on the nature of the guest molecules. In the case of MPc/TCDB architectures, the calculated result shows that van der Waals forces are the main intermolecular interactions because of the rather weak interaction between a TCDB dimer and MPc (about 7.8 kcal mol−1 for CuPc/TCDB architectures). However, in the composite architecture F16CuPc/TCDB(II) structures (shown in figure 3a), two F16CuPc molecules are entrapped by each cavity of the TCDB nanoporous network. The electronegative characteristics of the fluorine groups in F16CuPc molecules lead to a higher dipole moment and induce the intermolecular mutual repulsion between two adjacent F16CuPc molecules in each cavity . As a result, the central Pc–Pc distance between two F16CuPc molecules in a cavity varies from 1.5 to 1.7 nm and the cavities are expanded. When the Pc–Pc distance is approximately 1.5 nm, the O–H⋯ O hydrogen bonds linking the interdigitated alkoxylene chains are formed, and this is known as the ‘on’ structure (as shown by the black rectangle in figure 3b). When the distance between the central Pc–Pc distance is approximately 1.7 nm, the distance between the carboxyl groups linked by the interdigitated alkoxylene chains is too large to form O–H⋯ O hydrogen bonds (the ‘off’ structure, as shown by the black rectangle in figure 3c). The van der Waals forces and electrostatic interactions caused by the asymmetrical electronic distribution of MPcs are responsible for the special ‘on and off’ host–guest architectures .
4. Multi-component assembly of telechelic TMA derivatives with other functional molecules
The terminal carboxylic group can form a hydrogen bond with another hydrogen bond partner. For example, Wu and co-workers  reported the construction of TMA–BPBP(4,4′-bis(4-pyridyl)biphenyl) binary networks via hydrogen bonding between the terminal carboxylic group and pyridine. The cavity size and shape of the network can be modulated by changing the surface coverage ratio of the two components and the substrate temperature . In the case of telechelic TMA derivatives, the introduction of alkylene chains causes intriguing effects in the formation of the hydrogen bond motif (or vice versa) and results in interesting multi-component assemblies.
Recently, we reported size-tuneable nanoporous networks on the basis of bicomponent assembly of TMA derivatives with a truxenone derivative (2,3,7,8,12,13-hexahexyloxy-truxenone; TrO23) [65,66]. The chemical structures of TrO23 are shown in figure 4a. The carbonyl groups of TrO23 provide a new acceptor for the hydrogen of the carboxylic acid groups of the TMA derivatives, leading to a new hydrogen bond between the carboxylic acid groups of the TMA derivatives and the carbonyl groups of TrO23, as shown in figure 4b. The directionality of the hydrogen bonding is confined firmly to the pocket of the two side-hexyloxyl chains around each of the three carbonyl groups. At the same time, under the duplex interactions of the hydrogen bonds and the chain–chain van der Waals interactions between the alkoxylene chains of the TMA derivatives and the two side-hexyloxyl chains of TrO23, the flexible alkoxylene chains with carboxylic end groups of the TMA derivatives are tightly held between the two side-hexyloxyl chains of TrO23, which makes the alkoxylene chains of the TMA derivatives lose their flexibility and function as rigid spacers, just like the phenyl rings and triple bonds [43,44].
A series of two-component hexagonal networks of TMA derivatives co-adsorbed with TrO23 are fabricated at the HOPG/1-phenyloctane interface. As shown in the STM image in figure 4c, TCDB and TrO23 form a threefold symmetrical two-component network with ordered large hexagonal cavities with a diameter of 3.0 nm . The hydrogen bonding between the carboxylic acid groups of TCDB and the carbonyl groups of TrO23 is responsible for the topology of the two-dimensional hexagonal network. The structural model superimposed on the STM image clearly demonstrates that each hexagonal cavity is composed of three TCDB molecules and three TrO23 molecules arranged alternately. Each alkoxylene chain of TCDB is fixed between two hexyl chains of TrO23, which together serve as a side of a hexagon. An interesting feature of these bicomponent networks is that the size of the hexagonal cavities can be tuned by changing the length of the alkoxylene chains of the TMA derivative molecules with an accuracy of one carbon atom per step. As expected, a series of hexagonal cavities with a diameter of 1.4, 1.5 and 3 nm can be formed by the two-component hexagonal networks of TMA derivatives and TrO23 . The duplex interactions of the chain–chain van der Waals interactions and hydrogen bonds make the two-component hexagonal networks very robust.
The two-component hexagonal networks with different cavity sizes are used as molecular templates to accommodate functional guest molecules such as coronene and copper phthalocyanine (CuPc) . After the template was examined by STM, a drop of either the coronene solution or the CuPc solution is introduced onto the same surface to obtain the final host–guest structure. As shown in the STM images in figure 5, the hexagonal cavities in two-component hexagonal networks capture one or more coronene molecules according to the size match between guest molecules and hexagonal cavities. As shown in figure 5a,b, the small cavities of the TCBB/TrO23 template can accommodate only one coronene molecule; as for the large cavities in the TCDB/TrO23 template shown in figure 5c,d, six coronene molecules are entrapped . Owing to duplex interactions of chain–chain van der Waals interactions and hydrogen bonds, the capture of guest molecules does not change the direction of the extended ‘rigid’ alkoxylene chains of TMA derivatives. Hence, the hexagonal cavities are preserved in the three-component host–guest architecture. Engineering the size of the accurately tuneable molecular templates enables the construction of guest molecules with different aggregation states.
Furthermore, a series of flower-like assembly structures are obtained by using trinary assembly of CuPc, TCDB and TrO23 on the HOPG surface . The assemblies are prepared by sequential deposition of CuPc, TCDB and TrO23 solution on HOPG and imaged at the solid–liquid interface. As shown in the STM images in figure 6a–e, three kinds of molecules fabricate a series of flower-like assembly structures on the HOPG surface. Each flower in the flower-like assembly structures is composed of six lobes, which are formed via the hierarchical assemblies of three components: CuPc, TCDB and TrO23. First, three TrO23, one TCDB and three CuPc form a triangle-shaped building unit (TBU) through hydrogen bonding and van der Waals interaction, as shown in scheme 2. Second, six TBUs, just as the six lobes, can further attach to each other in an edge-to-edge configuration to form the structure of flower 0, as displayed in figure 6a,f. The centre-sitting TCDB provides extra hydrogen bonding to stabilize the structure of flower 0. Third, two TBUs arrange themselves in an edge-to-edge configuration by sharing common CuPc lines as separators to form a rhombus unit cell, which induces the structure of flower 1 shown in figure 6b,g. Fourth, as shown in figure 6c,h,d,j, the TBUs can grow into bigger lobes in a vertex-sharing manner, which results in two flower-like structures named flowers 2 and 3, respectively. Finally, flower in figure 6e is entirely composed of TBUs by sharing the vertex of the triangle shape without the CuPc separator. It can be expected that flower is thermodynamically favoured because the close-packed TBUs can simultaneously maximize the intermolecular interactions and molecule–substrate interactions. The molar ratio of the three components in each flower structure is calculated, as presented at the bottom of figure 6. A formula for the molecular ratio of TrO23, CuPc and TCDB in flower n (except n=0) is deduced as follows: (n2+3n+2):(3n2+9n+3): (n2+n).
The relative molecular ratios between TrO23, CuPc and TCDB in each self-assembled structure are shown below each STM image. The ternary hierarchical assemblies can be tuned by controlling the amount of the individual components in the solution phase. When the concentration of TCDB and TrO23 is at medium level, the TBUs cannot deplete all the CuPc molecules. As a result, the CuPc prefers to stay at the interface and organize itself into segregated domains between the TBU lobes, which induce the flower structure of low generations (1, 2). Correspondingly, high concentrations of TCDB and TrO23 lead to the appearance of flower 0 or the flower structures of high generation.
In this review, the assembly behaviour of TMA derivatives was chosen as a typical example to demonstrate the interesting assembly features of telechelic organics. First, telechelic organics provide an interesting model to understand the balance between van der Waals interactions and hydrogen bonding to shape the two-dimensional assembly structures. By increasing the length of the alkoxylene chains, the driving force for the self-assembly structure can be varied from hydrogen bonds to chain–chain interactions, leading to the change in the cavities in the non-porous networks of the TMA derivatives from hexagonal cavities to irregular cavities on the HOPG surface. Second, the cavities framed by the flexible alkyl chains behave as a flexible host template for surface supramolecular chemistry. This flexible host–guest nanoarchitecture is a marriage of the typical surface host–guest chemistry, where the host cavities are kept robust before and after guest molecular inclusion, and the typical bi- or multi-component assembly, where the assembly is a result of the collaborative interactions between each component in the system. Recently, we have shown a similar flexible host–guest assembly in another system with the contribution of both alkyl interdigitation and hydrogen bonding, although not telechelic organics. Third, we have shown that TMA derivatives can form hierarchical assemblies via multi-component assembly, demonstrating that the combination of multiple interaction motifs in the same building blocks can benefit the design and construction of complex self-assembled nanoarchitectures.
We want to point out that our understanding of the assembly behaviour of telechelic organics is only just beginning. Systemic investigations into the effect of different terminal groups and the number and position of alkoxylene chains are lacking. Also theoretical molecular dynamic simulation could contribute to the quantitative understanding of the contribution of different weak interactions in the assembly process. With the combined features of flexibility of alkyl chains and directivity of hydrogen bonding, telechelic organics may find an important niche in two-dimensional surface molecular engineering.
This work is supported by the National Key Project on Basic Research (grant nos. 2011CB808700 and 2011CB932300), the National Natural Science Foundation of China (grant nos. 91023013, 20905069, 21233010, 21127901 and 21121063), the Beijing Municipal Education Commission (20118000101) and the Chinese Academy of Sciences.
One contribution of 17 to a Theme Issue ‘Molecular nanostructure and nanotechnology’.
- © 2013 The Author(s) Published by the Royal Society. All rights reserved.