Matsuhiro
Maeda
a,
Ruri
Nakayama
a,
Steven
De Feyter
b,
Yoshito
Tobe
*cd and
Kazukuni
Tahara
*a
aDepartment of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, 214-8571, Japan. E-mail: tahara@meiji.ac.jp
bDivision of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan 200 F, 3001 Leuven, Belgium
cThe Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan. E-mail: tobe@chem.es.osaka-u.ac.jp
dDepartment of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30030, Taiwan
First published on 6th August 2020
Self-sorting of multiple building blocks for correctly positioning molecules through orthogonal recognition is a promising strategy for construction of a hierarchical self-assembled molecular network (SAMN) on a surface. Herein we report that a trigonal molecule, dehydrobenzo[12]annulene (DBA) derivative having three tetradecyloxy chains and three hydroxy groups in an alternating manner, forms hierarchical triangular clusters of different sizes ranging from 2.4 to 16.4 nm, consisting of 3 to 78 molecules, respectively, at the liquid/graphite interface. The key is the dynamic combination of three different conformational states, which is solvent and concentration dependent. The present knowledge extends design strategies for production of sophisticated hierarchical SAMNs using a single component at the liquid/solid interface.
The most common strategy to construct large hierarchical structures is the use of building blocks of C3 or D3h symmetry capable of assembling through strong non-covalent interactions such as hydrogen bonding and metal coordination, and they are typically prepared under ultrahigh vacuum (UHV) conditions.14–17 Under these conditions, the size of unit cells can be modified by surface coverage. By fully exploiting this approach, a hierarchical structure with a large unit cell of 45 nm was reported recently.18 Another strategy for the construction of hierarchical SAMNs with long-range periodicity is to optimize multiple intermolecular interactions using elaborated building blocks or multi-component building block(s).19–24 Such experiments are typically undertaken at the liquid/solid interfaces or occasionally in air.25 Compared to UHV conditions, the unit cell size and the number of building blocks for constructing such hierarchical structures are limited, because the presence of a supernatant solvent renders the self-assembly and system complicated. Solvation and surface wetting compete with intermolecular and molecule–substrate interactions, and solvent molecules are often co-adsorbed in SAMNs.26–28
In hierarchical superstructures, the constituent clusters consist of three different parts, i.e., vertices, edges and internal core, each bearing different coordination numbers (Fig. 1). In most cases, a single building block forms these different parts by adapting the modes of intermolecular interactions as enforced by an external factor, i.e., surface density. Such self-assembly behavior may be regarded as a kind of self-sorting, even though this term is defined as “mutual recognition of complementary components in artificial self-assembly”.29–31 On a surface, there exists only one example of such self-sorting through dynamic combination of conformational states of a single molecular building block, yet the structural controlling factors were not discussed in detail.32
Here, we extend the above concept to produce hierarchical SAMNs using a C3h-symmetric building block with orthogonal coordination sites for van der Waals interaction and hydrogen bonding. By changing its conformation on the surface depending on the solvent polarity, the building block self-assembles to form hierarchical structures made of triangular clusters with size ranging from 2.4 to 16.4 nm, and consisting of 3 to 78 molecules, respectively. The molecules form the vertices, edges and core positions of the clusters as well, thanks to balanced orthogonal intermolecular interactions. Each triangular cluster comprises the feature of Pascal's triangle which is a triangular array of binomial coefficients in mathematics. Scanning tunneling microscopy (STM) is our method of choice, since it provides high quality structural information in real space of SAMNs on atomically flat conductive surfaces such as graphite, even at the liquid/solid interface.33,34
Over the past decade, we studied the self-assembly of dehydrobenzo[12]annulene derivatives DBA-OCns having six long alkoxy groups at the liquid/graphite interfaces (Fig. 2a).35DBA-OCns form low density porous structures and high density non-porous structures. The length of the alkoxy groups, solute concentration and temperature determine the relative abundance of the polymorphs.36 The main driving forces for the formation of these SAMNs are the intermolecular interaction through van der Waals interactions between the alkoxy groups and molecule–substrate interactions between the alkoxy groups and graphite. Of further relevance is the fact that in the non-porous structures, four alkoxy groups are physisorbed on the surface while the remaining two are probably solvated, whereas in the porous SAMNs all six alkoxy groups are adsorbed.
To further extend the self-assembling ability of DBA derivatives for the formation of various 2D patterns, we reduced the symmetry of the building blocks from D3h to C3h, thereby rendering them more flexible. Indeed, we found that DBA-OC14-OC1 having three long alkoxy groups and three methoxy groups in an alternating manner exhibits a rich structural polymorphism due to the increased mobility of the alkoxy groups and the variable number of physisorbed alkoxy groups (hereby denoted as m: m = 3–1). This structural variety depended on the type of solvent and solute concentration.37 This led us to hypothesize that by dynamic self-sorting, based on the ability of the molecule to adsorb with a different number of alkoxy chains (m) in contact with the surface, in combination with extending the type and number of intermolecular interactions, it may become possible to assemble a C3h-symmetric DBA molecule in a hierarchical manner. Therefore, we designed DBA-OC14-OH bearing three OC14 and three hydroxy groups in alternating positions on the DBA core (Fig. 2a and b). Indeed, we found that (i) this DBA derivative forms hierarchical patterns, consisting of triangular clusters ranging from 2.4 (3 molecules) to 16.4 nm (78 molecules) in size, (ii) each cluster is formed by hydrogen bonding through self-sorting of DBA conformers, (iii) clusters are connected via van der Waals driven alkyl chain interactions, and (iv) the size of the cluster could be modulated by changing the solvent polarity, demonstrating the dynamic feature of the self-sorting process (Fig. 2c: see Fig. S1† for more details).
At the TCB/graphite interface, DBA-OC14-OH exclusively forms a hexagonal porous structure with four bright triangles at each vertex of the hexagonal pore at all concentrations examined (Fig. 3a and S2†). Since π-conjugated cores are typically resolved as bright features in the STM images because of their relatively higher tunneling efficiency,43 the bright triangle is assigned to a tetramer of DBA-OC14-OH with three physisorbed alkoxy groups (m = 3) forming the triangular shape by both van der Waals and hydrogen bonding interactions. The neighboring tetramers are connected by seven alkyl chains (Fig. S3†).44 A molecular model optimized by molecular mechanics (MM) simulation (COMPASS force field) is shown in Fig. 3b. The mean O⋯H distance of the hydroxy groups between the central and surrounding DBA-OC14-OH molecules is 2.2 ± 0.2 Å, supporting the hypothesis that the molecules are clustered through hydrogen bonding interactions (Fig. 3b).45,46 The presence of enantiomorphous domains was identified by the orientations of the alkyl chains located at the rims of the pore (Fig. S4–S9†). The details of the chirality aspects both at single and supramolecular levels are described in the ESI.†
In contrast to the results in TCB, the self-assembly of DBA-OC14-OH at the HA/graphite interface exhibits a sharp concentration dependence. At 6.0 × 10−6 M, a hexagonal assembly of triangular clusters consisting of three molecules of DBA (n = 2: n refers to the number of DBA-OC14-OH molecules forming each edge of the triangular cluster) with two physisorbed alkyl groups (m = 2) was exclusively observed (Fig. 3c and S10†). The total number of DBA-OC14-OH molecules (N) per cluster, represented by N = n(n + 1)/2, is three in this case. Based on the unit cell parameters and the STM image, it is safe to conclude that the triangular cluster is formed by hydrogen bonding mediated by one HA molecule per adjacent DBA pair. The triangular clusters are bridged by four interdigitated alkyl chains of DBA-OC14-OH. Note that two HA molecules stick in between the interdigitated alkoxy groups (Fig. 3d). In the model, the mean O⋯H atomic distances between the hydroxy group of DBA-OC14-OH and the carboxy group of HA measure 1.70 ± 0.02 and 1.75 ± 0.01 Å, respectively, suggesting the presence of hydrogen bonding interactions (Fig. S11†). Similar to the hexagonal porous structure, there are both antipodal domains differentiated by the alkyl chain orientations located at the rims of the hexagonal pore (Fig S12 and S13†). Again, the details are discussed in the ESI.†
By increasing the concentration to 1.0 × 10−5 M, in addition to small domains of the triangular cluster (n = 2), irregular areas consisting of triangular clusters of various sizes (n = 2 up to 6) and a densely packed phase cover the surface (Fig. S14†). Further increase of the concentration to 1.0 × 10−4 M led to the formation of a dense structure consisting of DBA-OC14-OH without physisorbed alkoxy groups (m = 0); all alkoxy groups orient to the solution phase (Fig. 3e, S15 and S16†). There exist two chiral domains of the dense structure, which could be differentiated by the molecular orientations with respect to the substrate axes. In the molecular model, all intermolecular O⋯H distances between DBA-OC14-OH are 2.0 Å, indicating hydrogen bonding (Fig. 3f). Thus, at the HA/graphite interface, though several hierarchical structures of n = 2 to 6 emerged, concentration control over the formation of clusters of specific size was not achieved.
Next, we examined the effect of solvent polarity by changing the ratio of TCB and HA in the solvent mixture on the size control of the hierarchical triangular clusters.47 To compare the affinity of the solvents to DBA-OC14-OH, its solvation energies in TCB and HA are estimated by molecular dynamics (MD) simulations to be −54.22 and −0.26 kcal mol−1, respectively (see the ESI†). The larger negative value in TCB indicates smaller adsorption probability than in HA, which is consistent with the favorable formation of the low density hexagonal porous network by the DBA with three physisorbed alkoxy groups (m = 3). In contrast, the less favorable solvation in HA is consistent with the formation of the densely packed structure by the DBA with none of the alkoxy groups (m = 0) physisorbed at high concentration. By varying the ratio of these two extreme solvents, we hypothesized that the distribution of the DBA molecules with a different number of physisorbed alkoxy groups m may be controlled to form triangular clusters of a specific size. Based on this hypothesis, solutions of mixtures of TCB and HA at different molar fractions (XHA; 0.020–0.50 and XTCB; 0.98–0.50) were prepared keeping the total DBA concentration constant (1.0 × 10−4 M), and the resulting SAMNs were analyzed. The overall results are summarized in Table 1.
a The surface coverages and standard deviations were determined from more than 20 images in three independent experimental sessions. b Collapsed triangular clusters. c The relative ratios of different clusters at an XHA of 0.20 are listed in Table 2. d The error was not determined due to its small area ratio. | |||||||
---|---|---|---|---|---|---|---|
X HA | Hexagonal porous structurea (%) | Triangular clustersa (%) | Dense structurea (%) | ||||
Defectsb | n = 2 | n = 4 | 4 < n < 10 | 9 < n < 21 | |||
0.020 | 56.2 ± 6.9 | 0.7 ± 0.1 | 43.1 ± 6.8 | 0.0 | 0.0 | 0.0 | 0.0 |
0.049 | 0.5d | 2.9 ± 0.8 | 96.6 ± 0.4 | 0.0 | 0.0 | 0.0 | 0.0 |
0.066 | 0.5d | 2.7 ± 0.7 | 96.6 ± 0.7 | 0.2d | 0.0 | 0.0 | 0.0 |
0.070 | 1.1 ± 0.7 | 1.5 ± 0.5 | 84.6 ± 13.2 | 22.1 ± 12.5 | 0.0 | 0.0 | 0.0 |
0.076 | 0.0 | 1.0 ± 0.8 | 54.8 ± 6.7 | 44.3 ± 7.4 | 0.0 | 0.0 | 0.0 |
0.082 | 0.0 | 0.3 ± 0.2 | 44.7 ± 13.6 | 55.0 ± 13.6 | 0.0 | 0.0 | 0.0 |
0.090 | 0.0 | 0.7 ± 0.4 | 45.1 ± 3.7 | 23.7 ± 4.8 | 2.8 ± 1.7 | 27.6 ± 5.9 | 0.0 |
0.20c | 0.0 | 0.0 | 0.0 | 0.1d | 9.8 ± 1.1 | 90.2 ± 2.5 | 0.0 |
0.50 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 100 |
At XHA = 0.020, both a hexagonal porous structure and a triangular cluster co-exist (n = 2, Fig. S17a†). At the mixing ratio XHA ranging from 0.049 to 0.066, DBA-OC14-OH mainly forms the triangular cluster of n = 2, similar to the conditions in the pure HA solution of low concentration (Fig. S17b†). Upon increasing XHA to 0.070–0.082, the n = 4 cluster appears and its surface coverage increases up to about 50% with increasing XHA (Fig. 4a, S17c, d and S19†), reaching a maximum at XHA = 0.082, forming large domains over 80 × 80 nm2 (Fig. S18d†). The cluster of n = 4 consists of DBAs in three different conformational states (three of m = 2, six of m = 1, and one of m = 0) and 9 molecules of HA attached to the edges linked by hydrogen bonding interactions (Fig. 4b and S20†). The surrounding six DBA molecules are bound to the central core DBA of m = 0 via hydrogen bonds. The adjacent clusters are connected by van der Waals interactions between interdigitated alkoxy groups of DBA-OC14-OH with HA molecules stuck in between. We were unable to find clusters of n = 3.
Hierarchical structures consisting of larger triangular clusters emerge upon further increasing XHA to 0.090 (Fig. S21†). At an XHA of 0.20, DBA-OC14-OH produces a mixed phase of hierarchical clusters containing the n = 12 cluster as the major component (ca. 27%) together with other clusters of similar sizes (n = 10, 11, 13 and 14, Fig. 5 and Table 2). Note that the length of the edge of the cluster of n = 12 reaches 16.4 nm and the cluster is composed of 78 DBA molecules (three DBAs of m = 2, 30 DBAs of m = 1, and 45 DBAs of m = 0). In the hexagonal assembly of the large clusters, even if the sizes of the triangular clusters are not all uniform, the hexagonal packing is sustained by small gaps (Δn) in between adjacent clusters (typically Δn is smaller than or equal to 2) by distorting the central hexagonal pore. When the size difference (Δn) becomes larger than 2, the triangular clusters are often chipped out from the vertex to maintain the overall hexagonal packing (triangles with red numbers in Fig. 5). These domains are stable against STM tip scanning. Moreover, a longer annealing period from 3 to 6 h led to no notable structural change and the hierarchical structure remains even after 1 day at room temperature, indicating that the hierarchical structure is thermodynamically stable. The formation of the triangular clusters rather than other clusters with different shapes, i.e. hexagonal shape clusters would be related to the favored intermolecular interactions (Fig. S25†).48
Cluster size | Ratioa (%) | Cluster size | Ratioa (%) |
---|---|---|---|
a 22 large area STM images (80 × 80 nm2 or larger) obtained in three independent experimental sessions were used. Some imperfect triangular clusters in which the number of missing DBA molecules is less than 10% are included in the statistics. b The error was not determined due to its small occurrence. | |||
n = 4 | 0.05b | n = 13 | 12.0 ± 2.1 |
n = 5 | 0.22 ± 0.14 | n = 14 | 15.1 ± 6.7 |
n = 6 | 0.77 ± 0.56 | n = 15 | 5.2 ± 1.3 |
n = 7 | 0.88 ± 0.74 | n = 16 | 4.3 ± 1.7 |
n = 8 | 3.2 ± 1.9 | n = 17 | 0.20 ± 0.10 |
n = 9 | 4.7 ± 3.3 | n = 18 | 0.43b |
n = 10 | 13.2 ± 4.3 | n = 19 | 0.05b |
n = 11 | 13.1 ± 5.2 | n = 20 | 0.05b |
n = 12 | 26.7 ± 4.9 | — | — |
We emphasize that each n = 12 cluster is formed by 78 molecules through a dynamic combination of its three different conformational states, each being a part of vertices, edges and core, even though the control is not perfect probably due to thermal fluctuations. This process can be regarded as a dynamic version at the solid/liquid interface of integrative self-sorting, which is a new aspect of self-assembly mainly observed in solution and in crystals.29–31 Jester and Höger reported a related preliminary case in which a six-alkyl-bearing building block capable of adopting several conformations with a different coordination number at the solvent/graphite interface is a part of vertices, edges and core, forming a hierarchical structure of variable sizes.32 In our case, the hierarchical structures are formed through orthogonal van der Waals and hydrogen bond interactions not only between the DBA molecules but also between the DBA and solvent molecules. Moreover, the cluster size, namely the number of different dynamic conformers can be modulated to some extent by changing the degree of solvation. The present work improves the design strategy for the construction of hierarchical SAMNs.
Further increase of XHA to 0.50 leads to the dense structure exclusively covering the whole surface (Fig. S23†). At all mixing ratios, domains of n with small odd numbers of 3 or 5 were never or scarcely observed. Though this may be related to the epitaxy with the substrate lattice4–17 or formation mechanism of the clusters (vide infra), it is not understood yet. In contrast, both odd and even clusters coexist for the larger clusters (n = 10–14), which is attributed to the small differences in size and molecular density among these large clusters. To shed light on the formation mechanism of the clusters, monolayers formed without annealing treatments were observed. At XHA = 0.076, the surface is covered with the hexagonal porous structure and the smallest triangular cluster (n = 2, Fig. S24a†), implying that the triangular cluster of n = 4 is formed by the lateral replacement of the smallest triangular clusters through the annealing treatment. This may also account for the absence of the triangular cluster of n = 3. At XHA = 0.20, the DBA molecules form a disordered (non-uniform) structure consisting of incomplete large triangular clusters and smaller clusters of n = 2 and 4 (Fig. S24b†), implying that the larger triangular clusters are grown from the smaller clusters by the ripening process.
The average height profiles (11 lines, width of each line is one pixel) of the alkyl chains and DBA cores were measured from more than three different calibrated images (typically 30 × 30 nm2, 512 pixels) by the use of SPIP software. In each cross section of the hexagonal porous structure, two structurally inequivalent DBA cores in the tetramer were included because the apparent height of the DBA core is slightly different depending on its position. The apparent heights of the central DBA core parts and the surrounding DBA core parts were averaged.
The triangular cluster sizes and its distribution were statistically analyzed at a HA molar fraction (XHA) of 0.20. The size of triangular clusters is classified by the number of DBA-OC14-OH molecules (n) at each triangular side. To determine size distribution statistically, 22 large area images (80 × 80 nm2 or larger) were used. There were also imperfect triangular clusters as defects. Some imperfect triangular clusters in which the number of missing DBA molecules was less than 10% were included in the statistics. In the case where the side lengths of the triangular clusters were different, the number of DBA-OC14-OH molecules at the longest side was used as n.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, additional STM images, and details of MM simulations. See DOI: 10.1039/d0sc03163a |
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