Supramolecular self-assembly has been widely applied in the preparation of diverse functional nanomaterials with specific nanostructures spontaneously formed by building blocks through intermolecular non-covalent bonds [
1–
3]. Recently, researches on two-dimensional supramolecular self-assembly studied by scanning tunneling microscopy (STM) have gained considerable attention, which helps to intuitively reveal intermolecular interaction modes and explore the laws of supramolecular self-assembly [
4–
7]. Currently, the fabrication of host-guest systems has received much concern, as it is an effective approach to achieve specific distribution of heterogeneous molecules and prepare various composite assembly structures [
8–
11]. Due to the selectivity and directionality of hydrogen bonds, the self-assembly nanostructures driven by hydrogen bonds are more predictable when compared with other non-covalent bonds. And this makes the self-assembly structures of aromatic acid derivatives widely used as host structures in host-guest systems [
12–
16]. Two-dimensional assembly structures are the result of the intermolecular interactions, molecule-solvent interactions, and molecule-substrate interactions [
17]. And in the bi-component or multi-component systems, more types of molecules are involved, so further researches are needed for the controllable preparation of bi-component and multi-component structures.
In the previous reports, it is found that pyridine derivatives can break the original O–H···O hydrogen bonds between aromatic acid derivatives and form stronger O–H···N hydrogen bonds with aromatic acid derivatives, which can regulate the self-assembly structures of aromatic acid derivatives and show broad application prospects in the fabrication of abundant binary nanostructures [
18–
25]. But few researches about the pyridine regulation of dimeric building blocks formed by hydrogen bonds between aromatic acid molecules have been reported. For instance, the introduced pyridine derivatives could regulate the self-assembly structure aggregated by 5′-(4-carboxyphenyl)-[1,1′:3′,1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid (H
5BHB) dimer building blocks, while H
5BHB still appeared as dimer in its bi-component systems with pyridine derivatives [
26]. For 5′-(4,5-bis(4-carboxyphenyl)-2,5-dihydro-1
H-imidazole-2-yl)-[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid (BCDTDA) molecule containing an imidazole group, BCDTDA molecules self-assembled into ordered structures in the form of dimeric building blocks
via intermolecular N–H···O hydrogen bonds. Both coronene (COR) and 4,4′-bipyridine (BP) could induce the structural transformation of BCDTDA, but had no effect on the BCDTDA dimers [
27]. However, the addition of pyridine derivative with alkoxy chains (PEBP-C4, PEBP-C8) was able to destroy BCDTDA dimers, and multiple co-assembly structures were prepared. In this work, the self-assembly dimeric structure of aromatic carboxyl acid derivative regulated by various guest molecules including pyridine derivatives and COR was investigated. Differently, dimeric building blocks formed by aromatic carboxyl acid could be disrupted by any of these introduced guest molecules.
As shown in
Scheme 1, three types of pyridine derivatives and COR were utilized to regulate the self-assembly structure of 5′,5″″-([2,2′-bithiophene]-5,5′-diyl)bis(([1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid)) (H
4BDETP) respectively. And pyridine derivatives included (
E)-1,2-di(pyridin-4-yl)ethene (DPE), 4,4′-di(pyridin-4-yl)-1,1′-biphenyl (BPYB) and 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (TYPY). COR was introduced into H
4BDETP/TYPY co-assembly system to fabricate multi-component structure. Combined with STM experimental results and DFT calculations, the formation mechanisms of these co-assembly structures were studied, which might be helpful for deepening insight into the formation of co-assembly structures.
H
4BDETP molecules formed dimeric building blocks
via two pairs of hydrogen bonds between carboxyl groups and self-assembled into linear structure at 1-heptanoic acid/HOPG interface as previously reported [
28]. The introduced COR induced the structural transformation of H
4BDETP's self-assembly system, and three types of H
4BDETP/COR co-assembly structures were obtained in Fig. S1 (Supporting information).
Figs. 1a–
c were their high-resolution STM images and provided more details. The adsorption conformation of H
4BDETP was changed by adding COR molecules. In H
4BDETP/COR-Ⅰ co-assembly structure (
Fig. 1a), H
4BDETP molecules appeared as H-shaped bright spots, and each COR molecule appearing as circular bright spot was captured by the cavity formed by four H
4BDETP molecules. Minor difference existed in the tilt angle of H
4BDETP molecules between adjacent rows. A unit cell was depicted in
Fig. 1a and its experimental parameters for H
4BDETP/COR-Ⅰ structure (Table S1 in Supporting information) were:
a = 3.3 ± 0.1 nm,
b = 4.2 ± 0.1 nm,
α = 90° ± 1°. The ratio of H
4BDETP to COR was 1:1. Based on STM experimental results, the optimized assembly model for H
4BDETP/COR-Ⅰ structure was built by DFT calculations and presented in
Fig. 1d. H
4BDETP dimers disintegrated and all carboxyl groups of each H
4BDETP molecule formed O–H···O hydrogen bonds with those of four H
4BDETP molecules. The calculated lattice parameters for H
4BDETP/COR-Ⅰ co-assembly structure in Table S1 were consistent with the corresponding experimental lattice parameters, indicating the optimized structural model was reasonable.
In H
4BDETP/COR-Ⅱ structure (
Fig. 1b), each COR molecule was surrounded by four H
4BDETP molecules and the tilt angles of neighboring H
4BDETP molecules in the same row were slightly different, which were similar to the H
4BDETP/COR-Ⅰ structure. However, the difference between the orientations of H
4BDETP molecules in adjacent rows was significant. The ratio of H
4BDETP to COR was 1:1. And the unit cell parameters for H
4BDETP/COR-Ⅱ structure were measured to be:
a = 5.5 ± 0.1 nm,
b = 5.4 ± 0.1 nm,
α = 95° ± 1°. As shown in
Fig. 1e, all H
4BDETP dimers were destroyed. Two carboxyl groups of H
4BDETP molecule formed O–H···O hydrogen bonds with those of neighboring H
4BDETP molecules in the same row. Along axis
b direction, one of H
4BDETP's remaining carboxyl groups formed hydrogen bond with H
4BDETP molecule in the neighboring row while the other was exposed on the substrate, or both combined with heptanoic acid to form hydrogen bonds, respectively.
Compared to the above co-assembly structures, H
4BDETP molecules in H
4BDETP/COR-Ⅲ structure (
Fig. 1c) appeared as arch-shaped or H-shaped bright spots, implying that a part of H
4BDETP dimers were kept despite the participation of COR. And H
4BDETP monomers distributed between the adjacent columns of H
4BDETP dimers. Hexagonal and rectangular cavities were formed by H
4BDETP molecules to accommodate one or two COR molecules respectively. The measured lattice parameters for H
4BDETP/COR-Ⅲ co-assembly structure were:
a = 6.9 ± 0.1 nm,
b = 6.9 ± 0.1 nm,
α = 86° ± 1°. And the ratio of H
4BDETP to COR was 1:1 in the co-assembly structure. The structural model (
Fig. 1f) indicated that two H
4BDETP dimers were connected by two H
4BDETP monomers through hydrogen bonds between carboxyl groups, which constituted a rectangular cavity and could be considered as the building block. And the remaining carboxyl groups of each building block combined with those of heptanoic acid molecules.
Total energies including intermolecular interactions and molecules-substrate interactions for H4BDETP/COR co-assembly structures were given in Table S2 (Supporting information). To better compare the thermodynamic stability of assembly structures, total energy per unit area instead of total energy was employed because of different lattice parameters for the unit cells of these assembly structures. Table S2 showed that total energies per unit area of three kinds of H4BDETP/COR structures were −0.273, −0.273 and −0.281 kcal mol−1 Å−2, respectively. They were approximately equal, and this was energetically beneficial for their co-existence on HOPG.
When DPE was added to H
4BDETP's mono-component system, an ordered co-assembly structure similar to H
4BDETP's linear structure was observed (Fig. S2 in Supporting information). It was well distinguished in high-resolution image (
Fig. 2a) that arch-shaped and short rod-shaped bright spots corresponded to H
4BDETP and DPE molecules respectively. H
4BDETP molecules distributed in a face-to-face manner were slightly staggered between each other, indicating that DPE molecules have disrupted H
4BDETP dimers. And DPE molecules acted as bridges to connect H
4BDETP molecules. The unit cell was superimposed on the STM image and the measured parameters were:
a = 4.2 ± 0.1 nm,
b = 2.4 ± 0.1 nm,
α = 88° ± 1°. The ratio of H
4BDETP to DPE was 2:1.
Different from H
4BDETP's self-assembly structure, H
4BDETP molecule in H
4BDETP/DPE lamellar structure (
Fig. 2b) interacted with the adjacent face-to-face H
4BDETP molecule
via a pair of O–H···O hydrogen bonds marked by black circle. Another carboxyl group of H
4BDETP molecule formed O–H···N with DPE molecule and O–H···O hydrogen bonds with the neighboring H
4BDETP molecule possessing the same orientation, which was indicated by yellow circle. And the remaining carboxyl groups of the H
4BDETP molecule were saturated by those of two adjacent H
4BDETP molecules. The energy per unit area of H
4BDETP/DPE structure (−0.296 kcal mol
−1 Å
−2) displayed in Table S2 was lower than that of H
4BDETP's self-assembly structure (−0.260 kcal mol
−1 Å
−2) [
28], which supported the structural transformation of H
4BDETP's self-assembly system induced by DPE from a thermodynamic perspective, and agreed well with the experimental observation.
To investigate the regulatory effect of different bridging units in bipyridine on H
4BDETP's assembly structure, BPYB with a longer length was adopted. Unlike DPE, terminal pyridine groups in BPYB were connected by the biphenyl unit. In Fig. S3 (Supporting information), two kinds of H
4BDETP/BPYB lamellar structures were observed and marked as H
4BDETP/BPYB-Ⅰ and H
4BDETP/BPYB-Ⅱ respectively. In both types of lamellar structures, the ratio of H
4BDETP to BPYB was 1:1. In the high-resolution STM image of H
4BDETP/BPYB-Ⅰ structure (
Fig. 3a), H
4BDETP molecules (arched-shaped bright spots) with the same orientation were arranged into a column, and BPDYB molecules (bar-shaped bright spots) were inserted between two columns of the H
4BDETP distributed face-to-face. The measured lattice parameters were:
a = 5.8 ± 0.1 nm,
b = 2.4 ± 0.1 nm,
α = 68° ± 1°. The calculated model (
Fig. 3b) showed that two H
4BDETP molecules arranged into face-to-face were connected through O–H···N hydrogen bonds formed with two BPYB molecules, which could be taken as the building block. And each building block interacted with four adjacent ones through O–H···O hydrogen bonds between carboxyl groups of H
4BDETP.
Compared with H
4BDETP/BPYB-Ⅰ structure, in H
4BDETP/ BPYB-Ⅱ structure (
Fig. 3c) H
4BDETP molecules appearing as H-shaped bright spots adsorbed on HOPG in a different configuration owing to the isomerism of sulfur atoms positions in thiophene groups. H
4BDETP and BPYB molecules were arranged alternately, which was similar to the co-assembly results of aromatic acids and pyridine in earlier reports [
29,
30]. The experimental lattice parameters were:
a = 1.6 ± 0.1 nm,
b = 3.6 ± 0.1 nm,
α = 84° ± 1°.
Fig. 3d showed that O–H···O hydrogen bonds existed between the neighboring H
4BDETP molecules along the direction of lattice parameter
a, and O–H···N hydrogen bonds were formed between H
4BDETP and BPYB molecules in adjacent rows. In H
4BDETP/BPYB co-assembly system, the dimeric structure of H
4BDETP was completely disrupted by BPYB. Table S2 showed that total energies per unit area of H
4BDETP/BPYB co-assembly structures (−0.294, −0.296 kcal mol
−1 Å
−2) were lower than that of H
4BDETP's mono-component structure, which was energetically beneficial for the formation of H
4BDETP/BPYB co-assembly structures.
Distinct from DPE and BPYB, the triazine core in TYPY was modified with three pyridine groups. Fig. S4 (Supporting information) showed H
4BDETP/TYPY structure differed from H
4BDETP's co-assembly structures with DPE or BPYB. It was seen from
Fig. 4a that the dimeric structure of H
4BDETP was destroyed. Each H
4BDETP appearing as H-shaped bright spot was surrounded by six triangle bright spots representing TYPY molecules. Every two H
4BDETP molecules and two TYPY molecules formed a hexagonal nanopore. Along axis
b direction, adjacent hexagonal nanopores shared one H
4BDETP molecule. The ratio of H
4BDETP to TYPY was 1:2 in the co-assembly structure. The experimental lattice parameters were:
a = 3.2 ± 0.1 nm,
b = 2.9 ± 0.1 nm,
α = 69° ± 1°. According to the assembly model in
Fig. 4b, the original O–H···O hydrogen bonds between H
4BDETP dimers were totally broken by TYPY. All carboxyl groups of H
4BDETP molecules formed new O–H···N hydrogen bonds with the pyridine groups of four TYPY molecules. And benzene rings of H
4BDETP molecule interacted with pyridine groups of another two TYPY molecules through C–H···N hydrogen bonds and
π-
π interactions.
When COR molecules were added to H
4BDETP/TYPY system, two kinds of multi-component co-assembly structures were formed (Fig. S5 in Supporting information). In these co-assembly structures, both the ratio of H
4BDETP to TYPY to COR was 1:2:2. High-resolution STM images (
Fig. 5a and
c) showed that COR could trigger the structural transformation of H
4BDETP/TYPY, and the subtle difference between two kinds of H
4BDETP/TYPY/COR structures lay in the arrangement of H
4BDETP molecules marked by blue H-shaped pattern. In H
4BDETP/TYPY/COR-Ⅰ structure (
Fig. 5a), two TYPY molecules with opposite orientations that were signed as yellow triangle pattern served as the bridge to connect neighboring H
4BDETP molecules along the direction of lattice parameter
b. COR molecules labelled by red circular pattern entered the cavities surrounded by H
4BDETP and TYPY molecules. The experimental lattice parameters were:
a = 2.8 ± 0.1 nm,
b = 3.9 ± 0.1 nm,
α = 95° ± 1°. The structural model (
Fig. 5b) revealed that two carboxyl groups of each H
4BDETP molecule formed O–H···O hydrogen bonds with those of H
4BDETP molecules in the same row, and the remaining two carboxyl groups formed O–H···N hydrogen bonds with the pyridine groups of two TYPY molecules respectively.
π-
π interactions existed between the benzene rings of neighboring TYPY molecules. Each COR molecule was encapsulated by the cavity constituted by H
4BDETP and TYPY molecules, and formed
π-
π interaction with TYPY molecule. As shown in
Figs. 5c and
d, the intermolecular interactions in H
4BDETP/TYPY/COR-Ⅱ structure were the same as those in H
4BDETP/TYPY/COR-Ⅰ structure. Differently, H
4BDETP molecules in adjacent columns adsorbed on HOPG in two orientations, which led to the difference in the lattice parameters (Table S1). DFT results in Table S2 showed that the total energies per unit area of H
4BDETP/TYPY/COR assembly structures (−0.280, −0.273 kcal mol
−1 Å
−2) were smaller than that of H
4BDETP/TYPY (−0.265 kcal mol
−1 Å
−2), which indicated that H
4BDETP/TYPY/COR assembly structures were more energetically stable on HOPG.
In conclusion, H4BDETP molecules self-assembled into linear structure in the form of hydrogen-bonded dimeric building blocks, which could be regulated by guest molecules. With the addition of COR, H4BDETP transformed into nanoporous assembly structures to accommodate COR. H4BDETP dimers in the H4BDETP/COR-Ⅲ structures were partially retained and formed new hydrogen bonds with H4BDETP monomers, while all dimers in H4BDETP/COR-Ⅰ and Ⅱ structures were destroyed. For pyridine derivatives, bipyridine molecules (DPE and BPYB) co-assembled with H4BDETP into lamellar structures, while tripyridine molecules TYPY co-assembled with H4BDETP into nanoporous structure owing to the increased binding sites brought by the additional pyridine group. And the difference in the central bridging unit between DPE and BPYB led to the discrepancy in their co-assembly structures with H4BDETP. Besides, H4BDETP/TYPY structure changed with the introduction of COR and two kinds of multi-component structures were obtained. This work may give insights into the regulatory behaviors of guest molecules on the self-assembly structures formed by hydrogen-bonded dimeric building block of aromatic acid derivatives.
PDF
Full
59
34
关闭全屏
No. 86 Xueyuan South Road, Haidian District, Beijing
010-62662699(Operation and Technical Support)
qkjq@cast.org.cn
