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From Foldable Open-Chains to Shape-Persistent Macrocycles: Synthesis, Impact on 2D Ordering, and Stimulated Self-Assembly Soobin Kim, Henry D. Castillo, Milim Lee, Riley D. Mortensen, Steven L. Tait, and Dongwhan Lee J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01805 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Journal of the American Chemical Society

From Foldable Open-Chains to Shape-Persistent Macrocycles: Synthesis, Impact on 2D Ordering, and Stimulated Self-Assembly Soobin Kim,†,|| Henry D. Castillo,‡,|| Milim Lee,† Riley D. Mortensen,‡ Steven L. Tait,*,‡ and Dongwhan Lee*,† † ‡

Department of Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington, IN 47405, United States

ABSTRACT: Small molecule self-assembly at surfaces offers an efficient route to highly-ordered organic films that can be programmed for a variety of chemical and electronic applications. The success of these materials depends on the ability to program intermolecular interactions to guide precise structural ordering. Toward this objective, we have designed and synthesized a series of bis(triazolo)benzene-based π-conjugated molecules. Our synthesis exploits a last-stage C–C cross-coupling reaction to close up zigzag-shaped linear precursors to cyclized products, so that direct side-by-side comparisons can be made for their structure-dependent self-assembly behavior at surfaces and response to external stimuli. Indeed, scanning tunneling microscopy (STM) analysis revealed distinct differences as the conformational flexibility of the molecular backbone and the chemical structure of the peripheral groups are varied. Specifically, alkyl chains adsorb and form interdigitated structures, whereas oligo ethylene glycol (OEG) chains remain desorbed and thus shift self-assembly to more densely packed π-conjugated cores. While the macrocycles self-assemble immediately and spontaneously, their linear precursors exhibit slower self-assembly kinetics, which could be attributed to the difference in the degree of conformational freedom. We also found that perturbation by the STM tip and the addition of co-solutes profoundly impacted the kinetics of self-assembly and surface patterning. This highly unusual behavior highlights the importance of non-covalent interactions that are inherently weak in solution but can be made strong for symmetric and conformationally restricted molecules confined within 2D surfaces.

Introduction Molecular self-assembly is a spontaneous organization of unit molecules to form energetically more favorable structural patterns via non-covalent interactions. This process allows formation of supramolecular architectures from small molecular components.1-12 In particular, molecular self-assembly at surfaces offers the potential to build tailored, functional materials through the design and synthesis of customized molecular building blocks.13-22 This technique has been applied as a bottom-up approach to achieve surface-patterning of nanomaterials. By appropriately designing the chemical structures and intermolecular interactions of the molecular units, it is possible to program the surface with desired 2D crystalline patterns.17-19, 23-

Scheme 1. Chemical Structures of Macrocycles, Open-Chain Precursors, and Building Blocks for Their Synthesis

24

Within this context, shape-persistent macrocycles emerge as ideal candidates to advance molecular design strategies and to study self-assembly on the surface. These molecules are advantageous because of key interaction points available for molecular design around the conformationally rigid backbone,25-33 peripheral interactions to direct lateral/2D organization,30-35 and facial interactions with a surface or to direct the formation of 3D structures.30-32, 34, 36-38 These properties contribute to the success of many macrocycles for long-range structural ordering at surfaces. In this paper, we describe the synthesis, 2D ordering, and stimulated self-assembly of shape-persistent macrocycles, M1 and M2, and their zigzag-shaped precursors, L1 and L2 (Scheme 1).

Our structure design exploits a bis(triazolo)benzene-based polyheterocyclic motif to define consecutive turns along the πconjugated molecular backbone (Scheme 1). A limited number of torsional bonds of the open-chain precursor molecules facilitated intramolecular C–C coupling to produce the corresponding macrocycles in > 55% yields in preparative scale, which is remarkable for cyclization chemistry under standard conditions. At liquid–solid interfaces under ambient conditions, these molecules self-assemble onto graphitic surfaces. A side-by-side comparison of four molecules that share essentially identical πconjugated molecular backbone has revealed dramatic effects

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of (i) backbone conformational restriction, (ii) peripheral solubilizing groups, and (iii) co-solutes on the morphology, stability, and dynamics of molecular self-assembly onto 2D surfaces (Figure 1).

Figure 1. Schematic representations of molecule–surface interactions and prominent characteristics observed by STM studies on macrocycles (M1 and M2) and open-chain precursors (L1 and L2).

Intriguingly, we found that scanning tunneling microscope (STM) tip-induced electric field is essential for achieving longrange structural ordering of L2. This is the first report of using a conformationally flexible precursor to create a single ordered packing structure by STM-tip stimulus. There are several examples of tip-induced switching from one molecular assembly structure to another packing structure,39-43 but examples of tipinduced assembly are rare.44-45 Our ability to guide self-assembly by STM tip under ambient conditions now significantly expands the scope of molecularly “editable” surfaces at the nanoscopic level. Detailed in the following sections are the advent and current progress of this chemistry.

Background and Design Principles Shape-Persistent Macrocycle from Open-Chain Precursor. A combination of conformational rigidity and extended πconjugation for electronic tunneling makes shape-persistent macrocycles ideal candidates for STM studies of molecular packing, especially for (i) direct visualization of molecular shapes;35, 46-47 (ii) 2D host-guest interactions;48-50 and (iii) 2D supramolecular crystal engineering.21, 50-51 As outlined in Scheme 2, we wished to prepare a zigzagshaped molecule L that undergoes an end-to-end ring closure reaction at the last-stage in the synthesis to form a shape-persistent macrocycle M. A stepwise construction of the open-chain precursor should allow for precise placement of specific functional groups along the linear molecular backbone. As a tradeoff, however, the last step of ring closure could potentially suffer from uncontrolled intermolecular reactions to produce oligomeric byproducts, rather than the desired cyclic product. Without appropriate structural pre-organization,48 macrocyclization step often suffers from low yields. In addition, isolation of the macrocycle product from a mixture of oligomeric byproducts poses a practical challenge in preparative scale synthesis. One strategy to overcome this challenge is exploiting non-covalent interactions, such as hydrogen bonding,52-56 or metal coordination,57-59 to guide structural pre-organization. Alternatively, wire-spoked wheel-like cores are employed to bring

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reactive ends in close proximity to build circular architectures.31, 60,61

Scheme 2. Bis(Triazolo)Benzene as a Turn Motif for the Linear Precursor L en route to the Macrocycle M.

In the absence of such secondary interactions or auxiliary scaffolds, molecular design based on first-principles is needed to limit the number of possible conformations to be taken by the open-chain precursor L en route to the macrocyclic product M (Scheme 2). For this purpose, we decided to employ rigid structural motifs that enforce consecutive turns and allow for only a finite number of “rotatable” bonds along the molecular backbone. Within this context, bis(triazolo)benzene A (Scheme 2)62 emerged as an attractive synthetic target. Bis(Triazolo)Benzene as a Rigid Turn Motif. A contiguous repeat of 120-degree turns establishes a semicircular shape. Aryl-substituted bis(triazolo)benzene A (Scheme 2) thus serves as a structurally more rigid yet geometrically equivalent surrogate of the privileged oligo(meta-phenylene ethynylene) unit B (Scheme 2).27, 56, 63-66 With appropriate linker groups that run in parallel fashion to connect two half-ring A, a pseudo-hexagontype macrocycle M (Schemes 1 and 2) is anticipated. Modular synthesis from readily available starting materials is one salient advantage of bis(triazolo)benzene chemistry. As outlined in Scheme 3, bis(triazolo)benzene can be prepared by consecutive azo-coupling and oxidative cyclization reactions onto 1,3-diaminobenzene. A stepwise construction of N-aryl triazole unit on each side of the benzene ring is also ideal for desymmetrizing the molecule by varying the Ar and Ar' group of the aniline precursors in Scheme 3. Scheme 3. Stepwise Construction of Bis(Triazolo)Benzene via Consecutive Azo Coupling and Oxidative Cyclization Reactions


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Peripheral Functionalization with Solubilizing Groups. A limited solubility of typical aromatic-rich compounds often poses practical challenges in large-scale preparation. One intuitive approach is appending long and flexible tethers as solubilizing groups. In addition to facilitating the synthesis and purification, such conformationally non-rigid aliphatic groups can also participate in non-covalent interactions to assist molecular self-assembly.22, 43, 48, 67 For example, long alkyl chains interdigitate to form compact 2D molecular assemblies on highly oriented pyrolytic graphite (HOPG).40, 50, 68-70 Tightly packed alkyl chains can also align well with the underlying graphite surface to guide the direction and packing of the assembly. While oligo ethylene glycol (– (OCH2CH2)n– = OEG) chains could also engage in intermolecular interactions for molecular packing on graphite,71 or selfassembled monolayers on gold,72 they have received much less attention than alkyl chains in 2D assembly. In this study, we compare self-assembly behavior of linear and cyclic molecules that share identical π-conjugated skeletons yet differ in peripheral solubilizing groups, i.e. OEG (L1 and M1; Scheme 1) vs alkyl (L2 and M2; Scheme 1) of comparable length. A significant body of research and literature is already available for the self-assembly behavior of macrocycles examined by STM.21, 33-35, 43, 46-51, 63, 68-69, 73-82 However, there has been no direct side-by-side comparison between macrocycles and their chemically equivalent open-chain precursors. Most molecules studied at the liquid–solid interface have only one preferred flat conformation that maximizes substrate–adsorbate interactions. Unlike such systems, the open-chain precursors L1 and L2 can adopt up to 20 unique conformations when laid on flat surface (vide infra), which are potentially accessible for 2D packing. It remained to be answered how such degree of freedom would converge to an ordered array in reality, and what functional role if any would be played by the peripheral tethering groups in this process.

Results and Discussion Modular Synthesis for Desymmetrization. By straightforward functional group transformations, bromonitrobenzene derivatives 1 (R = –O(C2H4O)3CH3 for 1a; –C12H25 for 1b) were prepared from readily available starting materials. Each of these two basic building blocks were carried through the reaction sequence outlined in Scheme 4, which involves installation of an ethynyl fragment, and reduction of the nitro group to the amine group to produce 2. Under standard reaction conditions,83-84 2 was converted to the corresponding diazonium salt, and coupled to m-phenylenediamine to prepare 3. In parallel, a portion of 2 was converted to 2´ by removing the acetone “capping” group. Oxidative cyclization of the ortho-amino azo compound 3 furnished the benzotriazole 4, which was subjected to a second round of azo-coupling with 2´ to furnish the key synthetic intermediate 5 (Scheme 4). Cross-Coupling and Oxidative Cyclization: One-Pot SixElectron Chemical Transformation. Both the oxidative N–N coupling (to build a triazole ring from an ortho amino-azo group)83-84 and oxidative C–C coupling (to connect two terminal alkynes into a diyne linkage)85-86 reactions are known to proceed under basic conditions using Cu(II)/O2 as oxidant. Taking advantage of this fortuitous situation, we carried out a one-pot

synthetic operation to convert 5 to 6. This net six-electron process cleanly effected bimolecular oxidative coupling and cyclization (Scheme 5), all in a single reaction vessel, to produce the desired product 6 up to 70% yield. Subsequent deprotection under basic conditions exposed the free alkyne ends of L, which were ready for the final ring closure step. Intramolecular Macrocyclization. Typical macrocyclization reaction conditions require low concentration (i.e. high-dilution)87-90 or extremely slow delivery of reactants (i.e. pseudodilution)25, 91-95 under inert conditions to suppress the formation of undesired oligomeric/linear byproducts. In our case, however, simple addition of L (ca 0.5 mM concentration) to a stirred basic solution of CuCl2 and Ni(NO3)2·6H2O,96 over a period of 5–6 h, was sufficient to obtained the desired macrocycle M under ambient conditions. Moreover, the macrocyclic product is essentially insoluble in acetone. Simple washing with acetone was thus sufficient to remove the unreacted L and unidentified byproducts to isolate M in pure form (57% yield for M1; 56% yield for M2). Structural Confirmation by Single-Crystal X-Ray Crystallography. Diffraction-quality single-crystals of the macrocycle product was obtained for M3 (Scheme 4, and Figure S1). This compound was prepared by adapting the synthetic route shown in Scheme 4, but by using shorter hexyloxy tethers to improve crystallinity. The X-ray structure of M3 unambiguously confirmed the chemical connectivity as well as the shapepersistent nature of the macrocycle (Figure S1). While the macrocycle M is conformationally locked as far as the π-conjugated molecular backbone is concerned, its openchain precursor L can “fold” into many different structures. For example, a maximum number of 20 different conformations are theoretically accessible for L on a 2D surface (Figures S2 and S3), even when the symmetric and unbendable bis(triazolo)benzene “turn” and linear arylene ethynylene “linker” units greatly reduce the rotamer diversity. We were intrigued to see how such a difference in structural properties between the macrocycle M and its open-chain analogue L would be manifested when the molecule is placed at the liquid–solid interface and allowed to find the energetically most preferred shape and packing pattern. STM Studies of Molecular Self-Assembly: Effects of Peripheral Tethering Groups on 2D Packing. By functionalizing the macrocycle M or precursor L with either OEG (for M1 and L1) or long alkyl chains (for M2 and L2), sufficient solubility is achieved in octanoic acid. The 2D self-assembly of each molecule was thus tested by STM at the interface of solutions (50 µM – 1 mM) in octanoic acid and a clean graphite surface (Figure 2). In each experiment, 2 µL of solution was placed on a freshly-cleaved graphite surface. High-resolution imaging allowed for precise measurement of the supramolecular packing unit cells. In each case, a periodic packing structure was observed (Table 1; see Figure S4 for long-range order, Figure S5 for detailed packing models of M1, and Figure S6 for alternative packing models of L1 and L2). Each of the molecules was tested over a range of concentrations (M1, 100 – 500 µM; L1, 74 µM to 1 mM; M2, 3 µM to 1 mM; L2, 100 µM to 1 mM). Only M2 showed any variation in packing structure at different concentrations (Figures 3b and S7). A summary of the STM results is provided in Figure 2.



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Scheme 4. Synthetic Route to Macrocycles and X-Ray Structure of a Model Compound

Scheme 5. One-Pot Reaction for Simultaneous Oxidative C–C and N–N Cross-Coupling R R

N

N

N

N

N N

NH2 R

N

N N

N

R

N

– 6 e–

2×

– 6 H+

OH HO

HO

H

N

R

Scheme 5


N

N

N

N N

R

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Figure 2. High resolution STM images at the octanoic acid/HOPG interface, and the corresponding molecular models. See Table 1 for unit cell metrics. (a) M1; unit cell representing a two molecule basis. Sample concentration = 200 µM; It = 0.15 nA, Vsample = –0.9 V. (b) M2; high density phase (left) and low density phase (right). Sample concentration = 50 µM; It = 0.05 nA, Vsample = –0.8 V. (c) L1. Sample concentration = 74 µM; It = 0.08 nA, Vsample = –1.0 V. (d) L2. Sample concentration = 1 mM; It = 0.04 nA, Vsample = –0.8 V. See Figure S4 for additional STM images at larger length scales. The experiments shown in (a) and (c) were conducted in the presence of the co-solute aminopyrazine: 200 µM for (a); 7.4 mM for (c).

Table 1. Unit Cell Parameters of 2D Lattices of Macrocycles and Linear Precursors on HOPG Surfaces.a M1b

M2 Low Density

a (Å)

42.8 ± 1.7

b (Å)

20.8 ± 0.7

γ (Degrees)

Packing Density (molecules/nm2)

M2 High Density

L1

L2

35.9 ± 1.2

22.3± 1.1

23.0 ± 1.3

30.7 ± 0.8

21.8 ± 0.8

22.9 ± 1.2

13.5 ± 1.4

16.2 ± 0.2

93 ± 7

75 ± 2

82 ± 4

75 ± 6

88 ± 1

0.22 ± 0.02

0.13 ± 0.01

0.20 ± 0.02

0.33 ± 0.04

0.20 ± 0.01

a

Each measurement represents an average of measurements from 6 to 11 images that are each calibrated to atomic-resolution HOPG images. b Unit cell has two molecule basis.

To evaluate the effects of the peripheral tethering groups, we compared on-surface assembly of M1 or L1, which have OEG chains, with the assembly of M2 or L2, which have alkyl chains. For both M2 and L2, alternating high contrast and low contrast regions are observed in the STM images (Figures 2b and 2d), which correspond to aromatic and aliphatic groups, respectively. In each case, features between the molecules indicate interdigitation of the alkyl units of neighboring rows to form a lamellar packing. Although the two peripheral groups were designed to be nearly the same length, we observe striking differences in the packing structure for the molecules functionalized with OEG chains. For example, the unit cell size for L1 (ρ = 0.33 molecules/nm2) is significantly smaller than that for L2 (ρ = 0.20 molecules/nm2). While the L2 unit cell size allows for co-adsorption of alkyl chains, it is clear from the L1 unit cell size that the OEG chains are not adsorbed to the surface (vide infra). For the macrocycles, the unit cell size of M1 (ρ = 0.22 molecules/nm2) is similar to that of the high-density phase of M2 (ρ = 0.20 molecules/nm2). However, the orientation of M1 within

the packing structure does not allow for lamellar packing of the OEG chains. Based on the unit cell size, some of the OEG chains could be adsorbed on the surface, but not all of them (see Figure S5). We also note that the STM images do not resolve OEG chain packing between the macrocycles, nor do they show rows that could accommodate lamellar packing of the OEG chains. For these reasons, we conclude that the OEG chains are not co-adsorbed for L1 and M1, whereas the alkyl chains of L2 and M2 co-adsorb in a lamellar packing. There are many reports on interdigitated alkyl packing to drive assembly at surfaces, but examples of glycol-type units desorbed into solution are less common.63 As seen in the self-assembly of M2 and L2, the alkyl chains play a crucial role in molecular organization due to their tendency to maximize van der Waals (vdW) interactions by adsorption and interdigitation. Compound L1 produces a more dense packing (compared to L2), consisting of only aromatic cores with OEG chains desorbed (Table 1). Similarly, the packing orientation of M1 prevents full adsorption of OEG chains on the surface. With OEG functionalization, the design of the



π-conjugated molecular core, not the peripheral chain interactions, determines the molecular arrangement. Depending on the peripheral chains, self-assembly can thus be tuned between (i) a looser packing of aromatic cores separated by lamellar structures with alkyl functionalization, and (ii) a tighter packing of aromatic cores with (desorbed) OEG functionalization. This type of control in molecular packing, achieved by alkyl vs glycol functionalization, was recently demonstrated by six-fold symmetric macrocycles.63 In the absence of strong interaction with solvent, OEG and alkyl chains of comparable length should have similar entropies, when they lie on the surface (Figure 3a) or back-fold in solution (Figure 3b). Therefore, the striking difference in the behavior of the two types of chains is primarily due to enthalpic effects. We postulate that solvation provides a strong driving force (in spite of some loss of solvent entropy) for back-folding of the hydrophilic OEG chains of M1 into octanoic acid (Figure 3b). For OEG chains to achieve interchain dipole interactions to favor on-surface interdigitation, a specific relative chain alignment would be required to avoid repulsion between oxygen lone pair electrons;63 this may not be compatible with other packing considerations. In contrast, the alkyl chains of M2 have a weaker, but consistently attractive, vdW interaction, which is less specific in relative chain alignment. For alkyl-tethered molecules, vdW interactions with the surface and adjacent alkyl chains in a lamellar organization (Figure 3a) is thus preferred over interactions with the solvent (Figure 3b).

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As M1 and L1 have comparable mass, solubility, interaction with the graphite surface, and intermolecular interactions, we suggest that the poor surface assembly of L1 compared to M1 is due to the large conformational space sampled by L1. While a maximum of 20 distinct flat conformations are accessible by folding L1 in 2D space (see Figures 4, S2, and S3), the surface assembly consists of a single packing structure that could be interpreted by either of two conformations (Figure S6). Not only does this present a larger than usual entropic penalty for adsorption,97 it also presents a significant kinetic impediment as the collision rate of the preferred conformer with the surface would be relatively low. As discussed in the following sections, we have discovered two ways to improve the rate of assembly by reducing the conformational space.

Figure 4. Linear precursor L having five rotatable bonds (designated as 1–5) could access a total of 36 rotamers (including 4 nonchiral and 16 enantiomeric pairs of planar-chiral structures) that can sit on 2D surface (see Figures S2 and S3 for details). Without considering chirality, a maximum number of 20 unique flat conformers are available for L1 having back-folded OEG chains (left). For L2 having alkyl chains lying on surface (right), however, only two conformations can maximize intermolecular digitation of alkyl tethering groups (see Figure S6).

Figure 3. Space-filling models of (a) low density packing of M2 and (b) M1 (top: face-on view, bottom: side-on view) along with schematic representations (right).

Conformational Effects on Self-Assembly. For M1, large domains of ordered structures are observed by STM within tens of minutes after deposition onto HOPG (Figure 2a). This timespan is from sample deposition to successful STM visualization, so self-assembly of M1 likely occurs much more quickly. In contrast, its precursor molecule L1 does not form ordered structures in the same timescales as M1. Several hours (4–12 h) are required before ordered domains of L1 are observed.

STM Tip-Induced Self-Assembly. Stable domains of M1 or M2 spontaneously and immediately form after deposition onto HOPG, and domains of L1 spontaneously form several hours after deposition. Unlike these species, however, L2 does not spontaneously assemble, but requires perturbation from the STM tip to form large ordered domains (Figure 5). Scanning over the sample continuously in the same area (Vsample = –2.0 to –0.7 V) leads to gradual formation of a complete and well-ordered domain of the adsorbate in the local vicinity of the scanning probe. Small domains (< 5 nm2) were observed in the first STM images of an area, but after the span of 10–30 minutes during which the STM tip is rastering across the same area, the ordered domains grow to fill the scanned area. This effect was observed each time we moved to a new (i.e. previously unscanned) region on the surface. Even after 12 hours since deposition, no large domains were seen in the first images of a previously unscanned area, but after several minutes of scanning, domains were observed to grow.


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Figure 5. STM tip-induced self-assembly of L2 at the octanoic acid/HOPG interface. Self-assembly only occurs in the scanned area. Initially, small domains are present (a) and then grow over time (b). Drift is present as seen with the surface defect. Upon scanning a larger area, ordered domains are localized around the originally scanned area (c). Domains further develop and ripen over time (c-d). Conditions: [L2] = 1 mM; It = 0.05 to 0.10 nA, Vsample = –0.9 to –0.7 V.

The tip-induced ordered self-assembly of L2 could occur either through interaction with the STM electric field, or by mechanical perturbation in the tight tip-surface gap. Scanning at lower magnitude bias (Vsample = –0.3 V) for two hours resulted in no observable domain formation. However, when the tip was set in a stationary location ca. 400 nm from the surface with a sample bias of –3 to –5 V for one hour, ordered self-assembly was observed (Figure S8). For those biases, domains were observed spanning an area greater than 200 nm × 200 nm. When the tip was held in a stationary position at smaller biases (e.g., –0.3 V) for one hour, smaller domains were observed (ca. 10– 30 nm). With zero bias on the tip, no domain formation occurred. These control experiments have unambiguously established that the major effect for tip-induced assembly is the electric field, rather than the scanning action of the tip itself. This result represents the first observation of STM tip-induced ordering from a conformationally flexible species. Tipinduced assembly has only been seen previously with simple, rigid molecules.44-45 Other works at the UHV/metal interface revealed tip-induced re-orientation and polymorphism due to either the STM tunneling electron current or molecular alignment with the STM’s electric field.98-102 At the solution/HOPG interface, it was demonstrated that an external electric field is able to alter the self-assembly from a face-on arrangement to an edge-on arrangement.103 Additional studies at the solution/HOPG interface investigated the effects of the electric field of the STM tip on two molecular systems that exhibited induced polymorphism and host–guest complexation/de-complexation events.40, 42, 70, 104-105 Unlike these previous works, we present a system at the solution/HOPG interface, in which a conformationally flexible molecular component is induced to self-assemble by perturbations by the STM tip. The electrical field is essential to the ordered self-assembly of molecules; without the electric field,

self-assembly does not occur. We postulate that the electric field in the tip-surface gap may interact with the local dipoles of L2 to shift the conformational potential energy landscape in a way that facilitates self-assembly, presumably by reducing the number of possible conformations. This tip effect is not a reversible or temporary perturbation. Once the STM tip has induced ordering of the molecular layer, it is stable indefinitely. Apparently, the STM is aiding a kinetically rapid access to a thermodynamically stable adsorption state. The differences in the behavior between OEG-functionalized L1, which does not exhibit any tip-induced acceleration of assembly, and alkyl-functionalized L2 might be due to the onsurface intermolecular interactions. Alkyl chain interdigitation between L2 molecules requires a specific orientation of the peripheral alkyl chains, in addition to the conformational constraint on the molecular core (Figures 2d and 4), whereas selfassembly of the OEG-functionalized L1 is dependent only on the molecular core conformation (Figure 2c). Co-Solute Facilitated Self-Assembly. The addition of small aromatic co-solutes also facilitates the self-assembly of L1 and L2. As shown in Figure 6, L1 exhibits faster self-assembly with the addition of aminopyrazine, tetrafluorobenzene, hydroxybenzyl alcohol, benzoic acid, aminobenzoic acid, or phenol as co-solute (≥ 10 equiv). However, no adsorption of L1 was observed within 8 h when diaminobenzene was added.

Figure 6. STM images taken just after deposition of L1 (left), and domains that form within the indicated time range (t) in the presence of varying amount of aminopyrazine co-solute (right). Conditions: [L1] = 74 µM; (a) It = 0.05 nA, Vsample= –0.8 V; (b) It = 0.06 nA, Vsample = –0.8 V; (c) It = 0.05 nA, Vsample= –0.7 V.



The extent to which the co-solute facilitated L1 self-assembly depends on the molar ratio between the two species (Figure 6). Without aminopyrazine, ordered domains were observed within 4–12 h. At a 1:2 ratio of L1:aminopyrazine, ordered domains were observed within 2–6 h, whereas a 1:100 ratio resulted in ordered domains within 30 min. For the concentration ranges tested ([L1] = 74–125 µM in the presence of co-solute; 74 µM – 1 mM in the absence of co-solute), no differences were observed in the self-assembly time at given L1: co-solute ratio. In a similar fashion, L2 spontaneously and immediately forms ordered domains throughout the surface when deposited in the presence of aminopyrazine co-solute (L2:aminopyrazine = 1:10; [L2] = 400–500 µM). We note that the structures of L1 or L2 self-assembled in the presence of co-solute match exactly with those observed without co-solute, indicating that the co-solute does not adsorb within the ordered layer. If these co-solutes were indeed involved in a co-adsorption, we anticipate a difference in the unit cell dimension and/or packing structure. Moreover, one would expect that different co-solutes would have different effects on the 2D packing. Presumably, the co-solutes interact with the open-chain molecules in the solution phase, either assisting in pre-organization or altering solvation to facilitate adsorption. A large body of literature exists, in which supramolecular chemistry at interfaces produces unique host–guest structures that are not found in solution. For example, previous works at the solution/HOPG interface reported guest co-adsorption, either by guest binding within porous structures,48 or by inducing new self-assembly structures.40, 42, 70, 104-105 In those cases, the guest is clearly imaged by STM. In contrast, L1 and L2 exhibit expedited adsorption and self-assembly in the presence of a non-co-adsorbing co-solute. Our observation calls into question the role of the co-solute, and the mechanism by which it facilitates ordered self-assembly. As discussed above, the conformational entropy is likely to play a significant role in impeding the ordering of L1 and L2 at the surface. It is conceivable that the interaction of the co-solute with the open-chain molecules reduces the conformational space, thereby accelerating the kinetics of self-assembly on surfaces. In solution phase, however, no significant changes were observed by either UV–vis or 1H NMR spectroscopy that indicate strong and slowly exchanging host–guest complexation. Another explanation for the effects of co-solute on ordered selfassembly is a “salting out” effect of the co-solute. However, it usually requires much higher concentrations of co-solute.106 It is also noted that we do not see a strong influence on improved assembly by increasing the concentration of L1 or L2. In any case, the overall impact of the co-solute seems to be shifting the free energy landscape in favor of the adsorbed and ordered state. The stability of self-assembled M1 is also improved with cosolutes. With or without co-solute, the macrocycle immediately and spontaneously forms observable domains. Without co-solute, however, some domains that exhibit dynamic behavior on the time scale of STM imaging are often observed (Figure 7a). The addition of aminopyrazine co-solute in a 1:1 or 1:10 ratio (M1:co-solute) or other co-solutes (tetrafluorobenzene, hydroxybenzyl alcohol, benzoic acid, aminobenzoic acid, or phenol) in a 1:10 ratio, however, resulted in improved stability of the domains (Figure 7b). Understandably, co-solutes have neg-

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ligible effects on the self-assembly of M2 since it is driven primarily by interdigitation of long-alkyl chains lying on the HOPG surface (Figure 2b).

Figure 7. STM images of (a) non-stable domains of M1 without co-solute, and (b) stable domains of M1 obtained in the presence of aminopyrazine. Conditions: (a) [M1] = 250 µM, It = 0.10 nA, Vsample = –0.9 V. (b) [M1] = [aminopyrazine] = 125 µM, It = 0.14 nA, Vsample = –0.9 V.

Summary and Outlook Molecules adapt their shapes in response to external stimuli. To study the effects of backbone design on such dynamic processes, p-conjugated molecules comprising unbendable turns and rotatable struts were prepared. Central to our approach is high-yielding synthesis of macrocycles by modular construction and last-stage ring-closure. This strategy allowed for direct side-by-side comparison between flexible open-chain precursors and rigid cyclization products. In a manner reminiscent of protein folding, interactions among backbone, side chain, and solvent collectively and profoundly impacted both the conformations and assembly patterns of these molecules. Notably, STM tip-induced electric field and co-solutes stimulated the formation of ordered 2D domains at liquid–solid interfaces. This intriguing behavior has significant implications for non-contact and non-covalent manipulation of molecular assemblies, which invites further investigation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xxxxxxx. Synthesis and characterization; additional spectroscopic and STM imaging data (PDF) X-ray crystallographic data (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]

Author Contributions ||

S.K. and H.D.C. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work at Seoul National University was supported by the Basic Research Grant (2017R1A2B2006605) and Creative Materials Discovery Program (2017M3D1A1039558) through the National Research Foundation of Korea (NRF). Support for the work at Indiana University was provided by the National Science Foundation (DMR 1533988).

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