Functionalization of two-component molecular networks: recognition of Fe³⁺

Abstract

Two-component supramolecular networks have been constructed with a symmetric triphenylene derivative with three carboxyl groups (sym-TTT) and melamine. Two kinds of hydrogen bonds with different strength are involved in the multi-component self-assembly, one is H-bond between carboxyl group of sym-TTT and melamine, the other is intermolecular H-bond between melamine molecules. These interactions drive a structural transformation from close-packed network to hexagonal network with active amino groups inside of the cavity. Scanning tunneling microscopy (STM) measurements reveal that the functionalized network of sym-TTT/melamine could recognise Fe³⁺. These results could be helpful for designing functionalized molecular networks by multi-component self-assembling strategy.

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Introduction

The nanoporous network on solid surfaces has drawn much interest in the area of nanotechnology.^1–3 In general, self-assembly leads to the formation of nonoporous networks via non-covalent interactions, such as van der Waals interactions,^4–8hydrogen bonds,^9–11 metal–ligand coordination,^12–14etc. Especially, two-dimensional (2D) hydrogen-bonded porous networks on surfaces have demonstrated unique potential of tailoring supramolecular assemblies, such as networks formed by 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA),⁹perylene tetra-carboxylic di-imide (PTCDI) and melamine,¹⁰ and 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB).¹¹

Such 2D porous networks offer the possibility to immobilize functional guest molecules depending on the characteristics of the porous networks, such as size, shape, and symmetry. For most of the reported host–guest architectures, the interactions between guest molecules and inner cavity of the networks are typically van der Waals interactions or relatively weak hydrogen bonds with evenly distributed interaction sites within the cavity. With nanoporous networks as templates, various two-dimensional (2D) molecular structures, such as fullerene (C₆₀),¹⁰metallophthalocyanines (MPc),¹¹coronene,¹¹etc, have been constructed due to site-selective,^15,16 size-selective,⁶ and shape-responsive adsorptions.^17,18

However, this kind of guest selectivity is limited because of the relatively weak interaction between host and guest. It can be expected that a functionalized inner cavity would facilitate enhanced selectivity of the networks for guest molecules. For example, Beton et al. have demonstrated that nanoporous networks can be functionalized through inclusion of additional chemical groups within the framework molecules.¹⁹ Therefore, designing the cavities of functionalized nanoporous networks with active groups may increase the guest selectivity. Thus, the functionalized nanoporous networks with active groups would be expected to provide guest selectivity.

Herein, we report a two-component self-assembly strategy to functionalize the inner cavity of 2D molecular networks, upon simple mixing of the two molecular components at room temperature. Multi-component assembly on surfaces has been demonstrated as a promising potential method for the rational design of supramolecular structures with high complexity and reversibility.²⁰ Functionalized nanoporous networks in a multi-component self-assembly strategy would largely reduce the difficulties related to molecular design and synthesis.

We have designed two molecular building blocks, 2,6,10-tricarboxydecyloxy-3,7,11-triundecyloxy triphenylene (sym-TTT) with symmetric carboxyl groups and melamine, to construct nanoporous networks with active groups by a multi-component self-assembly strategy. The results of STM measurements reveal that functionalized nanoporous networks could identify Fe³⁺.

Experimental section

The chemical structures of sym-TTT and melamine are shown in Fig. 1. The sym-TTT was synthesized according to the reported method.²¹Melamine and Fe(ClO₄)₃·6H₂O were purchased from Aldrich and used without further purification. STM measurements were performed by using a Nanoscope IIId (Veeco Metrology, USA) with mechanically formed Pt/Ir (80/20) tips. The assemblies were formed by subsequent deposition of the components onto a freshly cleaned HOPG (grade ZYB, Veeco Metrology, USA) surface. The mole ratio was controlled both by the concentration and volume deposited. All images were recorded in constant current mode. The specific tunneling conditions are given in the corresponding figure captions.

Fig. 1 Chemical structures of (a) sym-TTT and (b) melamine.

We performed a DFT optimization for the proposed models. For theoretical calculations, we have used DFT provided by DMol3 code.²² The Perdew and Wang parameterization²³ of the local exchange–correlation energy are applied in local spin density approximation (LSDA) to describe exchange and correlation. We expand the all-electron spin-unrestricted Kohn–Sham wave functions in a local atomic orbital basis. For the large system, the numerical basis set is applied. All calculations are all-electron ones, and performed with the medium mesh. Self-consistent field procedure is done with a convergence criterion of 10⁻⁵ a.u. on the energy and electron density.

Results and discussions

The self-assembled adlayer structure of pure sym-TTT has been reported at the octanoic acid–graphite interface.²⁴ In a previous report, it can be seen that every six sym-TTT molecules form a hexagonal network with the seventh molecule entrapped in the center. It is plausible to attribute the assembly structures to van der Waals interactions between the side-chains, while the end-capped carboxyl groups remain in a free state. The packing structures indicate the possibility that other functional guest molecules could replace the center sym-TTT molecule.

We choose melamine, with its three-fold symmetry, as the functional guest molecule to modify the supramolecular network of sym-TTT. The melamine represents a good module, which is capable of forming multi-dimensional H-bonded host–guest architectures. Previous studies revealed that the melamine could be mixed with perylene tetracarboxylic di-imide (PTCDI) to form highly ordered 2D networks for arranging fullerenes or Lu@C₈₂.²⁵

When the solution including the sym-TTT and melamine molecules is deposited on graphite surface, two distinct domains can be observed on the same surface, as shown in Fig. 2(a). The random lines indicate phase separated boundaries. Interestingly, a new kind of porous network is formed and observed in Fig. 2(a). It is clear that the new nanoporous network is different from that formed by pure sym-TTT²⁴ or melamine.^26–28 When the molar ratio of sym-TTT : melamine is adjusted to 1 : 1, the bi-component nanopores cover nearly the whole surface and only several small defects can be found (Fig. 2(b)). More details of the structure are shown in the high resolution STM image in Fig. 2(c), where six small bright spots are trapped within the center of hexagons formed by six big bright spots which represent six sym-TTT molecules. Each sym-TTT molecule also shapes like a triangle with dark lines. This is similar to the adsorption pattern of pure compound sym-TTT, where six bright spots form a hexagonal structure.²⁴ The diameter of the hexagonal structure formed by six small bright spots is measured to be 1.0 ± 0.1 nm, which is similar to that of the melamine self-assembled structures at room temperature and under UHV conditions.^26–28 It is obvious that the small bright spots are attributed to melamine molecules. A unit cell is determined for the adlayer. The measured unit cell parameters are a = 4.5 ± 0.1 nm, b = 4.5 ± 0.1 nm and α = 60 ± 1.0°, respectively. According to the STM images, the corresponding molecular model is proposed in Fig. 2(d).

Fig. 2 (a) STM images of the assembling structure of sym-TTT/melamine on graphite surface. STM image shows both species coadsorbed on the surface (sym-TTT : melamine = 2 : 1) with scan size = 58.7 nm × 58.7 nm, V = 625.7 mV, I = 358.6 pA. The random lines indicate phase separated boundaries between sym-TTT and sym-TTT/melamine self-assembled network. (b) STM image of sym-TTT/melamine self-assembled nanoporous network (48.7 nm × 48.7 nm, I = 227.7 pA, V = 1146.6 mV). (c) High-resolution STM image of sym-TTT/melamine network (19.7 nm × 19.7 nm, I = 398.5 pA, V = 924.8 mV). The white circle highlights functionalized nanopores formed by the six-melamine cluster. (d) Proposed molecular model for sym-TTT/melamine network structures.

In the molecular model of the sym-TTT/melamine nanoporous network (Fig. 2(d)), six melamine molecules connect to each other by a pair of hydrogen bonds (N⋯H–N) to form a hexagonal structure. In addition, each melamine forms two hydrogen bonds (O⋯H–N (amino group of melamine) and O–H⋯N (nitrogen atom of melamine) with one carboxyl group of the neighboring sym-TTT molecule. Thus, sym-TTT/melamine can self-assemble into perfect nanoporous networks due to sym-TTT with three symmetric carboxyl groups. All carboxylic acid groups participate in the formation of the H-bonding. The driving force for the cocrystal structure may mainly be attributed to intermolecular H-bonds between melamine molecules, van der Waals interactions between the substituted alkyl chains of sym-TTT and intermolecular H-bonds between sym-TTT and melamine. Since the 2D supramolecular networks of sym-TTT are linked together by the weaker van der Waals interactions and the end-capped carboxyl groups remain in a free state, these would lead to the possible inclusion of functional guest molecules through stronger multiple hydrogen-bonded interactions. By multi-component self-assembly strategy, the functionalized nanoporous networks will be constructed.

According to the above result, it is noted that the amino-groups inside of the melamine cluster make it a functionalized nanopore, which could be used to capture a guest molecule. Therefore, we have chosen target guest metal ion Fe³⁺. The solution including target guest was added to the already formed supramolecular functional networks of sym-TTT/melamine. Surprisingly, upon addition of Fe³⁺, it is observed that many bright sphere-shaped structures are surrounded by six sym-TTT molecules as shown in Fig. 3(a). From high resolution STM image in Fig. 3(b), it can be clearly observed that there are seven bright dots in the sphere-shaped structure. The center bright spot may be attributed to Fe³⁺. Previously, it has been reported that Fe³⁺ can be coordinated by amine nitrogen.²⁹ Thus it reveals that one ion is trapped within six melamine molecular clusters. A unit cell is determined for the adlayer. The measured unit cell parameters are a = 4.5 ± 0.1 nm, b = 4.3 ± 0.1 nm and α = 56 ± 1.0°, respectively. Careful inspection of the high resolution image reveals that the structure is in fact a distorted hexagonal rather than a six-fold symmetric structure. The results further indicate that the structure is only four-ligand but not six-coordinated structure. Fig. 3(c) is the proposed molecular model based on the STM observations. In this model of sym-TTT/melamine/Fe³⁺ trimers, six N atoms of six melamine molecules are connected to six H–O groups in carboxyl groups of six sym-TTT molecules via N⋯H–O hydrogen bonding, forming the bi-component H-bonded functional nanopores. The other amino-groups are linked with one iron(III) ion through metal–ligand coordination. Once the metal–ligand process is limited to the 2D surface, a dramatically different coordinated behavior could happen. Such four-coordinated structures are beneficial to reduce the surface free energy at liquid–solid interfaces.

Fig. 3 (a) Low-magnification STM image of sym-TTT/melamine/Fe(III) ion structure (64.7 nm × 64.7 nm, I = 227.7 pA, V = 1146.6 mV). (b) High-resolution STM image showing sym-TTT/melamine/Fe(III) ion structure (19.7 nm × 19.7 nm, I = 398.5 pA, V = 924.8 mV). The circle indicates the sym-TTT/melamine structure, still an empty cavity. (c) Proposed molecular model for the sym-TTT/melamine/Fe(III) ion structure. (d) The schematic diagram of [Fe(melamine)₄]³⁺ model. The red dashed lines indicate N⋯H–N H-bonds and the blue dashed lines represent the metal–ligand coordination between Fe(III) ion and four amino-groups of four melamine molecules, respectively.

Conclusion

In summary, by using scanning tunneling microscopy technique, it reveals that functional porous networks have been fabricated through co-adsorption of triphenylene derivatives attached by three symmetrical carboxylic acid groups with melamine on HOPG surface. The multi-component H-bonded nanoporous networks with active groups could provide a template to capture iron(III) ions. These results demonstrate that this kind of network could be promising templates for selectively capturing functional guest molecules.

Acknowledgements

This work was supported by the National Basic Research Program of China (2007CB936503, 2011CB932303). Financial support from the National Natural Science Foundation of China (Grant Nos. 21073048, 51173031), and the Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2010KL0010),, ia also gratefully acknowledged.

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