Chiral Transition of the Supramolecular Assembly by Concentration

Article pubs.acs.org/JPCC

Chiral Transition of the Supramolecular Assembly by Concentration Modulation at the Liquid/Solid Interface Li Xu,†,‡ Xinrui Miao,*,† Lihua Cui,† Pei Liu,† Kai Miao,† XiaoFeng Chen,*,†,‡ and Wenli Deng*,† †

College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China National Engineering Research Centre for Tissue Restoration and Reconstruction, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: Global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by concentration modulation. Scanning tunneling microscopy investigations reveal that a heterochiral supramolecular double rosette-like structure was fabricated for the first time via hydrogen bond interactions with achiral 1-octanoic acid under low concentrations. When the solution concentration was increased, the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern was observed. Interestingly, these two metastable structures ultimately could transform into a stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only an achiral octamer arrangement could be obtained, owing to the changes of intermolecular hydrogen bonding, van der Waals force, and dipole−dipole interactions. The present results provided an important impetus for the induction and control of polymorphous chiral structural transformation through modulated solution concentration of achiral 1-octanoic acid.

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INTRODUCTION Fabricating and manipulating surface chirality and understanding its mechanism from the single molecule to supramolecular level has aroused intensive attention in past years because of its importance to chiral molecular recognition, enantioselective hoterogeneous catalysis, chemical sensors, and so on.1−4 At the supramolecular level, the underlying driving forces for the formation of chirality building blocks arise from noncovalent interactions such as van der Waals (vdWs) interactions,5 hydrogen bonds,6−9 electrostatic forces,10 and metal−ligand interactions.11 Weak intermolecular interactions would have a dramatic influence on molecular adsorption configuration and cluster in the monolayers. For chiral structure, the change of the weak interactions may also drive chiral phase transition.8,9,12 Böhringer and co-workers reported the results of a coverage-driven chiral phase transition from a conglomerate to a racemate for the chiral 1-nitronaphthalene molecule on Au (111) at different molecular coverage by the intermolecular hydrogen bonds interactions.9 Recently, in order to further research the formation and transformation mechanism of chiral structure on the surface, some chiral guided methods can be adopted. One important strategy is the use of solvent to induce and control the configurations of twodimensional (2D) surface chiral assembly. Scientists have reported a number of studies about solvent-induced chirality emerge of achiral or chiral molecules on achiral surfaces.5,13−17 Bai et al. studied the solvent effects on the expression of supramolecular chirality in self-assembled monolayers of achiral © 2015 American Chemical Society

bent-shaped molecules through tuning hydrogen bonding configuration.18 The solvent can play a significant role of the formation and expression of chirality by tuning of intermolecular interactions between the adsorbed molecules. However, solution-concentration-induced structural transformation from chiral to achiral phase has seldom been demonstrated. In this work, we report a detailed investigation of the formation and transformation of chirality in a 2D assembly of 2hydroxy-7-(pentadecyloxy)-9-fluorenone (HPF, Figure 1a) at the liquid−solid interface. The fluorenone derivatives have a well-defined chemical structure, which is on one side by a fluorenone end group, to favor association via H-bonding, and on the other side by alkoxy chains, to promote adsorption on the graphite surface. Thus, the structural asymmetry and the complex intermolecular interactions showed the expected handedness. Inspired by the hydrogen bonds, we additionally selected the 1-octanoic acid as the solvent, which is acidic and is able to be involved in hydrogen bonding with HPF. Here, the global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by controlling the HPF concentration in 1-octanoic acid. The complementary hydrogen bonds between HPF and 1octanoic acid were anticipated to be an attractive approach to Received: May 20, 2015 Revised: July 15, 2015 Published: July 16, 2015 17920

DOI: 10.1021/acs.jpcc.5b04799 J. Phys. Chem. C 2015, 119, 17920−17929

The Journal of Physical Chemistry C

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RESULTS Fabrication of Heterochiral Pattern at the 1-Octanoic Acid/HOPG Interface (Concentration 1.0 × 10−6 mol L−1 ∼ 3.0 × 10−5 mol L−1). First, we investigated the self-assembly of HPF at the 1-octanoic acid/HOPG interface under a low concentration (1.0 × 10−6 mol L−1 ∼ 3.0 × 10−5 mol L−1). Large-scale STM images (Figure 1b and Figure S1) reveal that

fabricate and induce the chiral architectures at the liquid/solid interface. A heterochiral supramolecular double rosette-like structure was perfectly fabricated at low concentrations. The scanning tunneling microscopy investigations and molecular mechanic calculations demonstrate that the complementary hydrogen bonding interaction between the carboxyl and hydroxy in adjacent conjugated moieties and the molecule− solvent hydrogen bonding interaction are anticipated to be important ways to create and control the formation of heterochiral pattern. Moreover, we are able to control and induce the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern through modulating the solution concentration. A dramatic change in the 2D architecture and an intriguing complex expression of surface chirality can be observed. Interestingly, these two structures ultimately could transform into a zigzag pattern by the perturbation prompted by the STM tip. At high concentrations, only octamer arrangement can be obtained owing to the changes of intermolecular hydrogen bonding, vdWs force, and dipole−dipole interactions. Molecular modeling simulations provided deeper insight in the formation of hetero- and homochiral patterns.

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EXPERIMENTAL SECTION Synthesis and Scanning Tunneling Microscopy. Detail of the synthesis of 2-hydroxy-7-pentadecyloxy-9-fluorenone (HPF) molecules has been reported previously.19 HPF molecule was dissolved in 1-octanoic acid (TCI) with the concentration from 1.5 × 10−4 mol L−1 to 1.0 × 10−6 mol L−1. The samples were prepared by depositing a droplet (about 2 μL) of the solution onto a freshly cleaved atomically flat graphite surface (HOPG, quality ZYB; Bruker). The STM experiments investigating solution concentration dependence were carried out at 20−24 °C in the atmospheric environment. In order to confirm the temperature effect on the structure transformation, the same experiments were carried out at 25− 30 °C in the atmospheric environment again. STM measurements were performed on a Nanoscope IIIa SPM (Bruker, U.S.A.) at the liquid/solid interface. STM tips were prepared by mechanically cutting Pt/Ir wire (80:20). Except for flattening to remove the tilting effect of the substrate plane, the images were recorded under constant current mode and shown without further processing. The specific tunneling conditions were given in the figure captions. Different tips and samples were used to check the reproducibility of the results and exclude image artifacts that were caused by the tips or the samples. Molecular Modeling. Molecular models of the observed assembled structures were constructed by using Materials Studio 4.4. The model of the monolayer was constructed by placing the molecules according to the intermolecular distances and angles that were obtained from analysis of the STM images. Molecular Mechanics were applied for the structural optimization. Calculation of energies, dipole moments, and orbitals were conducted using density functional theory (DFT) as implemented in the Dmol3 package in Materials Studio 4.5 (Accelrys Inc.). The Perdew−Burk−Ernzerhof function is used to describe exchange and correction. All of the computations are all-electron, spin-restricted, and performed with minimal basis set and medium integration mesh. The convergence thresholds for energy and electron density in self-consistent iterations are 1.0 × 10−5 a.u. and 1.0 × 10−3 a.u. for gradient and displacement in geometry optimizations.

Figure 1. (a) Chemical structures of 2-hydroxy-7-(pentadecyloxy)-9fluorenone (HPF) and 1-octanoic acid. (b) Large-scale STM image of HPF showing a double rosette-like pattern on the HOPG surface in the 1-octanoic acid under a low concentration (1.2 × 10−6 mol L−1). Tunneling parameters: It = 494 pA, Vb = 745 mV. The green and blue open-circle arrows indicate the double rosette-like pattern of clockwise (CW) and counterclockwise (CCW) according to the propagation direction of architecture, showing exclusively a CW fluorenone unit in the inner rosette-like hexamer, respectively. (c) High-resolution STM image for CW double rosette-like pattern: It = 566 pA, Vb = 846 mV. (d) High-resolution STM image for CCW double rosette-like pattern: It = 490 pA, Vb = 865 mV. Insets in (c) and (d) show the main symmetry axes of HOPG. The red solid arrow lines indicate the direction of the main symmetry axes of the graphite. The black solid lines run parallel to the unit cell vector a, and a unit cell is indicated. (e, f) Proposed models for the double rosette-like fashion with CW and CCW, respectively. The enlarged insets in (e) and (f) show the possible optimized hydrogen bonds of the inner hexamer rosette core and the arrangement of outside hexamer with CW and CCW, respectively. The combined white arrows display the stretching direction of the fluorenone cores.

the HPF molecules form a kind of double rosette-like pattern with the opposite direction of rotation arrangement, relating by mirror image symmetry. The bright rods are attributed to the conjugated fluorenone moieties, whereas the side chains appear as less bright lines. We identify the double rosettes as clockwise (CW) and counterclockwise (CCW), which are denoted in accordance with the stretching direction of the fluorenone cores in the inner rosette-like hexamer. Figure 1c,d are the high-resolution STM images, showing the details of the 2D heterochiral double rosette-like structure in left and right domains in Figure 1b, respectively. As shown in Figure 1c, close inspection of the STM image reveals there are 17921


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The Journal of Physical Chemistry C two arrangements of the HPF molecule; the inner rosette core is composed of six fluorenone cores with a head-to-abdomen rotation pattern, whereas one of peripheral rosette petals appears as a triangular building shape composed of three pairs of molecules with a back-to-back fashion in a rotary manner, and six laboratories formed the outside rosette-like structure displaying exclusively CCW rotation pattern. The rosette core and petals with an opposite rotation direction built the double rosette-like pattern in the 2D monolayer. The unit cell parameters of Figure 1c can be defined as a = 6.5 ± 0.1 nm, b = 6.6 ± 0.1 nm, and γ = 60 ± 1°. The rotation direction of double rosette-like pattern in Figure 1d, by contrast, was the opposite direction. The unit cell parameters are a = 6.5 ± 0.1 nm, b = 6.5 ± 0.1 nm, and γ = 60 ± 1°. The result demonstrates that the double rosette-like building unit simultaneously exhibits the heterochiral phenomenon. The HPF molecule contains a hydrogen donor (hydroxyl hydrogen) and a hydrogen acceptor site (carbonyl on the fluorenone unit), which could form a self-complementary effect. With theoretical simulation and calculation (Figure S2 and S3), for the inner rosette, it is understood that six −OH groups in HPF molecules form continuous hydrogen bonding with −C O in the neighboring molecules (illustrated in the insets of Figure 1e,f). The side chains of HPF stretch into six directions with respect to the main symmetry axes of underlying HPOG substrate. The estimated diameter of the center cavities of chiral inner rosette is approximately 1.0 nm. Small bright round dots in the voids can be observed, as indicated by the green arrows in Figure 1c,d, which should be ascribed to the coadsorbed 1octanoic acid molecules with a head-down fashion onto the substrate. Previous studies have shown that a relatively strong dipole− dipole interaction among the fluorenone moieties was an important factor in the formation of self-assembled patterns.20,21 HPF molecules in the peripheral rosette petals forming a dimer with the back-to-back fashion could eliminate the polarity of a single molecule and lead to the lowest energy (Figure S4). No intermolecular hydrogen bonds are formed in the peripheral rosette. It is found that a bright short rod indicated in pink color tends to align parallel to the side chain of each pair of HPF molecules in the peripheral rosette petals as shown in the upper insets of Figure 1e,f. On the basis of the length (∼0.9 nm) and shape of the short chains, they are attributed to the coadsorbed 1-octanoic acid molecules. The relatively strong hydrogen bonds between −COOH in 1octanoic acid and −OH in HPF stabilizes the peripheral rosette petal structure (Figure S5). In addition, the head-to-middle intermolecular hydrogen bonding between hydroxyl groups and carboxyl of fluorenone motif induces formation of the inner rosette. These hydrogen bonds are sensitive to the handedness of the adsorbed molecule and result in the formation of a C6symmetric rosette-like pattern. In fact, low-density structure is not optimal from the free-energy point of view. However, HPF molecules can overcome the energy cost to form the lowdensity pattern by the formation of cyclic rosette-like polymorph and the stability of solvent molecules coadsorption due to the hydrogen bonding. Solution Concentration Induced the Chiral Structural Transformation. As the research continued, we found that the heterochiral configuration of HPF self-assembly could transform into a homochiral pattern at increased concentrations (1.0 × 10−4 mol L−1 ∼ 4.0 × 10−6 mol L−1), as shown in Figure 2a and S6. This tiling pattern is chiral because the windmill-like

Figure 2. (a) Large-scale STM image showing the windmill-like tetramer architecture of HPF in the 1-octanoic acid (solution concentration: 3.0 × 10−5 mol L−1). Tunneling parameters: It = 497 pA, Vb = 785 mV. The green open-circle arrows indicate the windmilllike tetramer architectures of CW and CCW fashions according to the propagation direction of the carbonyl in the fluorenone unit, respectively. (b) CW windmill-like pattern: It = 585 pA, Vb = 775 mV. (c) CCW windmill-like pattern: Iset = 485 pA, Vset = 782 mV. (d, e) Proposed models for tetramer architectures with CW and CCW windmill-like patterns, respectively. The enlarged insets in (d) and (e) show the possible optimized hydrogen bonds with CW and CCW windmill-like tetramer architectures, respectively. The blue arrows display the stretching direction of the carbonyl in the fluorenone unit.

motif with opposite propagation direction appeared in enantiomer domains and is denoted with CW and CCW by irregular borderlines. Four bright rods consisting of fluorenone cores form a windmill-like tetramer and then act as the basic unit of the adlayer. There are two types of tetramers, CW fashion shown in Figure 2b and CCW fashion shown in Figure 2c. The unit cell parameters can be defined with a = 2.1 ± 0.1 nm, b = 3.4 ± 0.1 nm, γ = 87 ± 1° for CW tetramer, and a = 2.2 ± 0.1 nm, b = 3.2 ± 0.1 nm, γ = 88 ± 1° for CCW tetramer, respectively. The homochiral tetramer is an essential chiral unit with different rotations. According to the molecular arrangement and chirality, structural models are proposed in Figure 2d,e for CW and CCW chiral assemblies. After careful observation, it is found that two fluorenone cores of HPF molecules on one diagonal displaying normal size are flat-lying on the HOPG surface with an opposite orientation; whereas the other two cores on the other diagonal looked slightly smaller, indicating that they are tilted-lying on the HOPG surface as indicated by pink and blue crescents in Figure 2b,c. From the chemical structure of HPF and the molecular arrangement, it is believed that reticular hydrogen bonds exist among the tetrameric HPF molecules and are responsible for the formation of the chiral windmill-like pattern, which results in the existence of two noncoplanar fluorenone cores. The reticular hydrogen bonds in a molecular cluster are illustrated in the insets of Figure 2d,e. Four hydroxyl groups in neighboring HPF molecules interact with each other, which leads to the formation of cyclic −OH··· O− hydrogen bonds. Notably, because all of the alkyl chains in the double rosettelike and windmill-like patterns keep their interdigitated arrangement and lie flat on the surface, the strength of the 17922


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of octamer II structure cross side by side. Due to the conformational flexibility of the ether group located between the fluorenone cores and the side chains,24 HPF can form various self-assembled patterns through different fashions of side-chain interdigitations. The result suggests that the mobility of the side chains may also affect the 2D structural formation and the outcome of the self-assembly process. In addition to the intermolecular hydrogen bonding and vdWs force, the intermolecular dipole−dipole interactions also play an important role in determining the self-assembled structure of octamer I. A dipole alignment in the structural model is shown in Figure 3d. The orientation and dipole direction of adjacent fluorenone cores align in an antiparallel fashion, which is viewed as a stabilized state and gives rise to a low energy. The arrows with one color depicted in Figure 3d present a pair of dipoles. From the energy point of view, polarized molecules tend to align themselves in a way that minimizes the overall polarity. This paired arrangement induced by dipole−dipole interactions extinguishes the polarity of single HPF molecule. For the adjacent octamers in neighboring lamellae as indicated by green arrows in Figure 3b are also aligned in an antiparallel fashion, which indicates that the overall dipole moment around the self-assembled area is actually zero. Thus, the intermolecular hydrogen bonding, vdWs force, and dipole−dipole interaction result in a balance of polarity of the whole assembly and the formation of octamer I pattern. Figure 4 shows the change in surface of double rosette-like pattern, windmill-like structure, octamer, and defects. The

vdW forces does not obviously change in the self-assembly process. Therefore, the actual mechanism of chiral selection that favors the formation of windmill-like pattern with a particular handedness during 2D crystallization can be ascribed to chiral desolvation processes and the change of intermolecular hydrogen bonds. Although no solvent coadsorption was observed in the windmill-like arrangement, the nature of the solvent, such as the polarity, must play an important part in the formation of windmill-like structure. The single HPF molecule in its linear configuration remains achiral. Chirality just arises in the double rosette-like and windmill-like tetramer structures because of the close-packed self-assembly on the surface. In addition, a tilt of the molecular axis a with respect to the adlattice vectors would make the whole layer chiral.22 For the particular mirror domains (Figure 1c,d and Figure 2b,c), the title angle ψ turns either clockwise or counterclockwise. While further increasing the HPF concentration (4.5 × 10−5 mol L−1∼ saturated), a typical close-packed structure was observed in 1-octanoic acid as revealed in Figure 3a. Eight

Figure 3. (a) Large-scale STM image showing the octamer structure of HPF in the 1-octanoic acid under a high concentration (solution concentration: 1.2 × 10−4 mol L−1). Tunneling parameters: It = 460 pA, Vb = 680 mV. The red and green rectangles indicate the two kinds of octamer structures, respectively. (b,c) High-resolution STM images of HPF self-assembled monolayer showing two kinds of octamer structures, octamer I and octamer II, respectively. Tunneling parameters: (b) It = 485 pA, Vb = 805 mV; (c) It = 465 pA, Vb = 755 mV. (d,e) Proposed models for the two kinds of octamer structures, respectively.

Figure 4. Concentration dependence of the surface coverage of different patterns at the 1-octanoic acid/graphite interface. The labels refer to the different patterns of HPF in the drop-casting solution.

surface phases display a clear dependence on the solution concentration. In the low concentrations (1.0 × 10−6 mol L−1 ∼ 4.0 × 10−6 mol L−1), only the double rosette-like pattern can be observed (Figure S8a). In the concentration range (4.0 × 10−6 mol L−1 ∼ 1.0 × 10−4 mol L−1), the double rosette-like pattern disappears and the windmill-like pattern increases gradually (Figure S8b−e). In the higher concentrations (4.5× 10−5 mol L−1 ∼ saturated), the octamer structure gradually covers the whole surface instead of the windmill-like pattern (Figure S8f−h). By consecutive STM scanning, no phase transformation was observed among these three self-assembled structures. The surface coverage of self-assembled pattern does not reach 100% because of the existence of some monolayer defect. Instantaneous Nature of Structural Transformation. One interesting result in the assembly of HPF molecules was

molecules form an octamer. These octamers stack into a lamella, which are packed through the interdigitation of the side chains. For the molecules with long alkyl side chains, 2D assembly often leads to mirror-symmetry-broken structures. In addition to the steric hindrance and superlattice, the mechanism relates to conformational mobility of the side chain.23 Close inspection of the high-resolution images (Figure 3b,c) reveal that there are two kinds of octamer, named as octamer I and octamer II, respectively. In most cases, octamer I is the main packing pattern. It is worth noticing that the octamer II structure, as indicated by a red rectangle in Figure 3c, comprises two similar tetramers in a slightly different way compared with that in Figure 2c and S7. As shown in Figure 2d and 3e, different color markers indicate that the alkyl chains of tetramer structure interdigitate each other, whereas alkyl chains 17923


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It is well-known that self-assembled systems prefer to fabricate low-energy close-packed pattern in a 2D layer.29 Table 1 summarizes the adsorption structures and characteristic parameters of the 2D self-assembled patterns of physisorbed adlayers of HPF in different concentrations used in this study. From the surface coverage of these patterns, we can calculate that with the increase of solution concentration, the adlayer on the surface becomes increasingly dense, and the zigzag pattern has the highest packing density of these structures. In addition, the density of the vdWs force and the hydrogen bond has a similar variation trend with the surface coverage. It is noteworthy that once the zigzag pattern transformed completely, the whole scanning area was covered by one domain (Figure S10). No domain boundary was observed, which demonstrates that the formation process of the zigzag pattern is gradually completed and the orientation of molecular arrangement is commensurate with the graphite lattice. In addition, the intermolecular and molecule−substrate vdWs force and intermolecular −OH···O hydrogen bonds are so strong that all the conjugated moieties and the side chains lie in a line. These results indicate that the zigzag pattern is the most thermodynamically stable structure. A similar trend for higher packing densities of the final structures was commonly observed for irreversible structure transition of surfacesupported adlayers.30−32 Decreased intermolecular distances provide an entropy and enthalpic advantage through stronger intermolecular interactions. The observation of both low- and high-density monolayer structures for HPF molecule on HOPG may display a competition between entropy and enthalpy as the important driving force for the self-assembly process. Entropy may favor a low-density structure, whereas enthalpy favors a high-density structure on the graphite surface.33 Significant enthalpic compensations also result from molecule−substrate interactions that enhance with packing density. Thus, in most cases, more densely packed monolayers are thermodynamically more favorable.34,35 In addition, structural changes that are associated with higher packing densities could also affect or impair intermolecular bonds.36 For example, the adlayer structure of 1,3,5-tris(carboxybiphenyl)benzene is compromised in favor of an increased packing density rather than optimized intermolecular hydrogen bonds.37 In our system, no phase transformation from the octamer to zigzag fashion was observed within several hours. The reasonable explanation for this phenomenon is that the packing density of the octamer fashion (0.7762 nm−2 and 0.8095 nm−2) is close to that of the zigzag fashion (0.8117 nm−2) in the unit area (Table 1). The octamer cannot overcome the energy barrier to transform to zigzag fashion. Temperature Effect on the Chiral Structure Formation. The stability of the HPF self-assembly was explored by conducting the experiments at different ambient temperatures. At high concentrations, the octamer pattern was observed at the ambient conditions (20−30 °C). The driving force for this structural formation should be a maximized packing density, and intermolecular bonding seems not to be preponderant in this process. At moderate concentrations, the windmill-like pattern was also obtained at the ambient conditions (20−30 °C). When the room temperature was above 25 °C at low concentrations, the windmill-like pattern was often observed, while the double rosette-like structure was not obtained (Figure S11). In other words, the double rosette-like structure could be obtained only under low concentrations, and the temperature ranged from 20 to 24 °C. Once the windmill-like pattern was

that both the chiral double rosette-like structure and the windmill-like tetramer structure would finally transform into a homogeneous domain of an achiral zigzag structure after being scanned dozens of times by the STM tip as shown in Figure 5,

Figure 5. (a,b) STM images depicting the instantaneous nature of phase transition. (a) Double rosette-like pattern to zigzag pattern. Tunneling parameters: It = 525 pA, Vb = 626 mV. (b) Windmill-like tetramer architecture to zigzag pattern. Tunneling parameters: It = 490 pA, Vb = 665 mV. (c) High-resolution STM image of the zigzag pattern. Tunneling parameters: It = 485 pA, Vb = 675 mV. The red arrows show the direction of the scanning, which indicate the sequence of lines scanned by the STM tip. The dotted yellow line indicates the position at which the structure changed. (d) Proposed model for the zigzag pattern. The enlarged inset indicates the possible optimized hydrogen bonds.

which demonstrates that this supramolecular system has sensitivity to changes in the substrate bias. It can be noticed that the transformation happens almost instantaneously and the zigzag structure appears in the lower part of the image (Figure 5a,b). Voltage pulses applied to the STM tip reduce the energy barrier between the two supramolecular configurations, or they may directly provide enough energy for the molecules to cross over the barrier.25 The high-resolution image (Figure 5c) shows that the HPF molecules arrange in an ordered zigzag fashion, in which the conjugated moiety of HPF molecule interacts with those of neighboring molecules through sequential −O−H···O hydrogen bonds. Based on an energy minimization of this conformation, the most stable zigzag conformation displaying that the alkoxy side chains run parallel to the long side of fluorenone cores was found to adopt the closest-packing structure, which was favorable in terms of enthalpy.26,27 The unit cell parameters were measured as a = 3.8 ± 0.2 nm, b = 1.3 ± 0.1 nm, and γ = 86 ± 1°. A model for the zigzag configuration was proposed on the basis of a previous report (Figure 5d).28 Whereafter, the sample was placed under ambient conditions for more than 24 h. The self-assembly of HPF displayed that only the zigzag pattern existed on the HOPG surface (Figure S9), which indicates that the zigzag pattern was the most stable configuration. 17924


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2.487 4.744 5.673 5.440 6.578

0.2528 0.8901 1.0598 1.0162 1.1313

obtained with the increase of room temperature (above 26−28 °C), the adsorbed area of HPF would decrease with the continuous scanning (Figure S12). This result also indicates that the molecule−substrate interaction is relatively weak and that the temperature influences the adsorption of HPF. One fundamental question is whether these phase formations are under kinetic or thermodynamic control. The formation of octamer and windmill-like patterns are independent of temperature. Only under low concentrations is the formation of a nanopattern sensitive to the temperature. At low temperature, the HPF molecule will be in its ground state and immobile. In order to adsorb on the HOPG surface, the molecule must overcome the activation barrier ΔEdiff. When the lateral forces between molecules become more prominent and have an influence on the molecule−substrate interaction, the close-packed patterns are often observed. In addition, individual 1-octanoic acid molecules could not adsorb on the HOPG surface at ambient conditions. The relatively low temperatures favor the solvent to form hydrogen bonding with HPF molecules and coadsorb on the surface, which are the dominated factors for the formation of double rosette-like structure. Thermal energy at room temperature is sufficient for full desolvations of the adsorbed HPF molecule. At relatively low temperature, the origin of this activation barrier is the favorable interactions of coadsorbed solvent molecules with both graphite lattice and adsorbed HPF molecules. Once the solvent molecules have desorbed under relatively high temperature, the molecule−molecule interaction is increased by decreasing the mutual distance and changing the intermolecular interactions. Thus, we believe that the formation of double rosette-like structure favored at low temperatures mainly attributes to the solvent coadsorption. In addition, once three kinds of arrangements are formed at given temperatures, their respective molecular orderings do not evolve with time without voltage stimulus by the STM tip. This suggests that these temperature-induced patterns correspond to different potential minima in the phase formation process. These results indicate that at low temperature, three kinds of patterns are thermodynamically stable, whereas at relatively high temperature, the double rosette-like structure is metastable.

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18 4 16 8 4 60 88 69 85 86

P6 P4 P4 P4 P2

0.5032 0.5684 0.7762 0.8095 0.8117 ± ± ± ± ±

Per unit cell. bCW pattern.

rosette tetramerb octamer I octamer II zigzag

6.5 2.2 3.2 3.1 3.8 1.0 ∼ 4.0 4.0 ∼ 100 45 ∼ saturated 45 ∼ saturated −

0.1 0.1 0.1 0.1 0.2

6.5 3.2 6.9 3.2 1.3

± ± ± ± ±

0.1 0.1 0.1 0.1 0.1

DISCUSSION Molecular Model (MM Simulation). We propose that the process for the formation of globally heterochiral structure is determined by the induction and propagation of heterochirality. The induction of heterochiral assemblies is attributed to the intermolecular and molecule−solvent hydrogen bonding. Indeed, hydrogen bonds have been extensively utilized to achieve homo- and heterochiral 2D molecular assembly, leading to the formation of very stable porous monolayers.38,39 In the heterochiral double rosette-like structure, there are two kinds of hydrogen bonds. The intermolecular −OH···OC hydrogen bond is contributed to the formation of inner rosette. The molecule−solvent −OH···COOH hydrogen bond stabilize the outer rosette. Special complementary geometry that fits exactly to the pockets is required for the guest molecules to interact with the pockets. The −COOH in 1-octanoic acid forming a hydrogen bond with −OH in HPF molecule fits this chiral pocket. Our attempt to use other hydrogen bond donors, such as 1-octanol, to fabricate a similar double rosette-like pattern failed (Figure S13). This control experiment further illustrates the important of molecule−solvent −OH···COOH hydrogen bond, a close fit of the coadsorber and chiral pocket in the outer

a

hydrogen bond density (kcal/mol·nm2) vdW density (kcal/mol·nm2) coverage (nm−2) no. of molecule space group γ (±1 °C) b (nm2) a (nm2) concentration (μM)

b

pattern

Table 1. Summary of Structures and Packing Parametersa of HPF Adlayers at the 1-Octanoic acid/HOPG Interface under Different Concentrations


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results in globally heterochiral rosettes with CW (or CCW) handedness. In common, an energetically favored self-assembled pattern at low coverage may switch to a different binding fashion at high packing density. Such subtle balance between lateral intermolecular interaction, molecule−solvent interaction, and molecule−substrate interaction determines the formation and transformation among different structures. Compared to the heterochiral double rosette-like fashion, the circulating hydrogen bonds strength greatly enhanced the cooperation effect as resonance-assisted hydrogen bonding,41 so tetramer structures exhibited cyclic O−H···O hydrogen bonding. By DFT calculations, we considered and calculated the possible existence of four kinds of hydrogen bonding in the unit cell, separately (Figure 7).

HPF trimer in the heterochiral surface formation. Although different electrostatic intermolecular forces, such as dipole− dipole interaction, vdWs force, acceptor−donor interaction, do contribute to chiral recognition, the most important factors are hydrogen bonding and steric interactions, especially for the heterochiral double rosette-like structure.40 The heterochiral of HPF double rosette-like structure is further transferred and propagated through a hierarchical supramolecular self-assembly process to 2D surface. The adsorbed molecules were allowed to interact through a short-range interaction potential only with the nearest-neighbor molecules. Both fluorenone moiety and alkoxy chains would choose a preferred orientation commensurate with the underlying graphite lattice (Figure S14). Alkyl substitution is beneficial in stabilizing molecules on solid surfaces through vdWs interactions, which could increase the desorption barrier of the combined molecules. Thus, each molecule in the internal rosette would be distorted with the −OH group in order to maximize the stretched interchains vdWs interactions and match well with the graphite lattice. Although the orientation of fluorenone core in the heterochiral double rosette-like pattern is different, it is energetically favorable to align all the long alkyl substituent along the directions of the graphite lattice due to the registry between alkyl chain and graphite lattice. Compared to the head-toabdomen hydrogen-bonding and the role of spatial closely packed principles, we inferred that −CO groups of each dimer in the outer rosette arrange with a back-to-back configuration, whereas three dipolar pairs formed a unit resulting in a triangular cyclic arrangement with the angles of 120°. The result indicates that the dipole−dipole interactions exist among triangular dipole pairs. Moreover, once the inner CW or CCW rosette is formed, the outer dimers must pack with an opposite chiral arrangement as shown in Figure 6a,d, in

Figure 7. Possible molecular arrangement, hydrogen bonding, and total energy per unit cell for the windmill-like tetramer pattern. (a,b) “Back-to-back” carbonyl group generated repulsive antiparallel alignment of proximate dipoles in the fluorenone cores. (c,d) “Face-toback” carbonyl group generated stabilizing, cyclically arranged, parallel alignment of proximate dipoles. (e) Side view of hypothetical model for tetramer pattern of (d). In the simulation models, the binding energy per unit cell is listed and all single molecule formed in each tetramer is the same one. Blue arrows indicate the dipolar interactions of fluorenone cores.

The achiral HPF molecules have been obtained by replacing the pentadecyloxy group with an ethoxy group. The four molecular models in our calculations adopted the reasonable values, which were in accord with the experimental results. According to the dipole orientations inferred from −CO and −OH functional groups, there are two pairs of possible configurations in one tetramer. We performed DFT calculations with PBE functional at the GGA level to give the total energy. Figure 7a,b show that the “back-to-back” carbonyl group generated repulsive antiparallel alignment of proximate dipoles in the fluorenone core, while the “face-to-back” carbonyl group in Figure 7c,d generated stabilizing, cyclic arranged, parallel alignment of proximate dipoles. The calculation results reveal that the binding energy of cyclic O− H···O hydrogen bonding of tetramer configuration (Figure 7d) was much less than the other geometry configurations. Furthermore, the cyclic hydrogen bonding results in two noncoplanar fluorenone cores, as shown in the inset of Figure 7e, which is in accordance with those shown in the STM image

Figure 6. (a,b) Possible molecular models for the CW rosettes pattern and (c,d) for the CCW rosettes pattern, respectively. Experimentally, the heterochiral double rosette-like pattern (a) and (c) are observed for HPF. Arrows indicate the propagation direction of the fluorenone units. The blue crosses represent the structures that are unfavored.

order to get the densest packing. Then, the heterochiral interactions are formed between the core segment and the six outer dipolar pairs. Through the intermolecular dipole−dipole interaction and the molecule−molecule and molecule− substrate vdW forces, the induced CW (or CCW) heterochiral structure transfers and propagates to the 2D assembly and 17926


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gen bonds, the intermolecular dipole−dipole interaction of HPF plays an important role for the structural formation.

in that two bright spots are slightly smaller than the other two spots. It is probably because of the special arrangement that the chiral tetramer could be formed and recognized. Nature of the Solvent (Polarity and Hydrogen Bonding). As previously reported, the assembled structure of HPF molecule was studied in a 2D system, where 1phenloctane was used as solvent.28 The obvious differences exist in the HPF adlayers at 1-phenloctane/HOPG and 1octanoic acid/HOPG interfaces (Figure 8). Particularly, HPF

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CONCLUSION In summary, by controlling the solvent concentration method, we minutely investigated the structural stability and transition of chiral domains in self-assembly of HPF molecules at the 1octanoic acid/HOPG interface. It is found that the HPF molecules could form two distinct self-assembled structures of the double chiral rosette-like and chiral windmill-like patterns under different concentrations. Our results reveal that the achiral 1-octanoic acid solvent, which acts as a participating counterpart and dispersing agent, can play an important role in the chiral structural formation and transitions. At low concentrations, HPF molecules with the coadsorption of 1octanoic acid form the double chiral rosette-like pattern with an opposite rotation direction arrangement. With the increase of HPF concentration, the HPF molecules produce homochiral windmill-like structures. At the high concentrations, the octamer structure without obvious chirality is formed. Owing to the different intermolecular hydrogen bonding and van der Waals interactions of intermolecular and molecule−substrate, a transformation from chiral rosettes or chiral tetramer patterns to zigzag fashion was observed during the scanning process, whereas no phase transformation from chiral octamer to zigzag fashion was observed. The zigzag structure of HPF molecules, which survived under the tested conditions, was the most favorable configuration. In general, solvent-induced chiral expression of molecules on achiral surfaces provided a promising approach for the design and fabrication of chiral nanoporous surfaces using an achiral solvent.

Figure 8. High-resolution STM image of HPF assembly displayed a flower-like hexamer pattern at the1-phenyloctane/HOPG interface. Tunneling parameters: It = 450 pA, Vb = 710 mV.

molecules formed a unique heterochiral double rosette-like structure in low concentration of 1-octanoic acid. Although similar tetramers could be observed in these two solvents, as marked by a red circle in Figure 8, a dimer exists between adjacent tetramers, as marked by a green circle at the 1phenloctane/HOPG interface, which does not occur at the 1octanoic acid/HOPG interface. The results indicate that different solvents could produce diverse molecular assembling in a 2D crystalline structure. Usually, 1-octanoic acid is considered to be a pretty good solvent for hydrogen bonding, and it is also a polar solvent. The 1-octanoic acid solvent acting as dispersant and counterpart in the system could not adsorb with HPF molecules or anticipate in the formation of the molecular adlayer. In addition to the size and shape of 1-octanoic acid, the another important driving force for the coadsorption of 1-octanoic acids in the double rosette-like pattern is the hydrogen bond between 1-octanoic acid and the HPF molecule. Such hydrogen bonding could limit the adsorption orientation of the HPF molecule, lead to the generation of stereoselectivity, and further result in the heterochiral formation probability and stability of the selfassembly monolayer.42 Therefore, the heterochiral formation is conducted by changing the role of the solvent and codependent solute−solvent interaction. It is supposed that the interactions between 1-octanoic acid molecules weakly adsorbed in the outer rosette petal or diffused in the liquid phase, and the HPF double rosette-like structure may have a crucial role in the chiral transformation. Under moderate concentrations, although the chiral windmill-like tetramer is formed without the solvent coadsorption, the 1octanoic acid providing an acidic and polar environment induces the HPF molecules to form tetramers resulting from the intermolecular cyclic −O−H···O− hydrogen bonds. When the concentration increases further, the intermolecular interactions of HPF are enhanced, and the solvent−molecule interactions are weakened accordingly. Thus, in the octamer structure, except for partial intermolecular −O−H···O− hydro-

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ASSOCIATED CONTENT

* Supporting Information S

Detailed description of experimental section, STM images, and molecular modeling. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04799.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Program on Key Basic Research Project (2012CB932900, 2012CB619100), the National Natural Science Foundation of China (21103053, 51373055, 21403072, 51172073), the China Postdoctoral Science Foundation (2014M552189), and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged.

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