Article pubs.acs.org/Langmuir
Enantiomeric Excess-Tuned 2D Structural Transition: From Heterochiral to Homochiral Supramolecular Assemblies Shu-Ying Li,†,‡ Ting Chen,† Lin Wang,†,‡ Bing Sun,†,‡ Dong Wang,*,† and Li-Jun Wan*,† †
Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *
ABSTRACT: Spontaneous resolution of enantiomers is an intriguing and important phenomenon in surface chirality studies. Herein, we report on a two-dimensional (2D) structural transition from the heterochiral to homochiral assembly tuned by changing the enantiomeric excess (ee) of enantiomers in the solution phase. Enantiomers cocrystallize as racemates on the surface when the ee of the R-enantiomer (or S-enantiomer) remains below a critical value, whereas chiral segregation is achieved, and globally homochiral surfaces composed of exclusively one enantiomer are obtained as the critical ee is exceeded. The heterochiral−homochiral transition is ascribed to the formation of energetically unfavored homochiral molecular dimers under the control of the majority-rules principle at high ee values. Such results present an intriguing phenomenon in chiral ordering at surfaces, promising a new enlightenment toward understanding chiral resolution and the evolution of chirality.
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INTRODUCTION Chiral self-assembly of adsorbed organic molecules on twodimensional (2D) surfaces is one of the most fascinating topics in chirality studies and has attracted enormous interest in recent years. Constructing a chiral surface contributes significantly to functional surface nanoarchitectures and technical applications such as asymmetric catalysis1 and nonlinear optical materials.2 Organic molecules can selfassemble on surfaces and form nanoensembles with a rich library of 2D supramolecular chirality. Using scanning tunneling microscopy (STM), recent investigations have provided detailed insights into chiral phenomena such as chiral recognition,3 chiral switching,4 chiral resolution,5−8 chiral amplification,9−11 and the loss of chirality12 in 2D chiral systems. Chiral resolution from mixed enantiomers is an intriguing chiral phenomenon and was discovered during the crystallization of sodium ammonium tartrate by Louis Pasteur in 1848. Spontaneous chiral resolution occurs even more frequently in monolayers of mixed chiral molecules upon adsorption on surfaces.13,14 Eckhardt and co-workers have reported spontaneous separation of chiral phases in an oriented assembly of chiral amphiphiles deposited on mica, which is considered to be a 2D analogue of Pasteur’s aggregate formation.15 The formation of chiral conglomerates from mixed enantiomers has been observed in numerous molecular assemblies, presumably because of the favorite intermolecular interactions between the enantiomers with the same handedness.7,13−16 Furthermore, an intriguing chiral amplification © 2016 American Chemical Society
process has been observed for the assembly built from unbalanced enantiomeric mixtures. Raval and co-workers reported that the homochiral surface is achieved from the mixed enantiomer system of tartaric acid (TA).17 A small chiral perturbation with an enantiomeric excess (ee) of 0.2 generates a homochiral assembly preferred by the TA enantiomers in majority. Such an effect is called the “majority-rules” principle18,19 and proposed to be related to the initial emergence of homochirality in the prebiotic world.20 On the other hand, enantiomers may also interact with each other as racemates and further assemble into an ordered monolayer instead of forming separate chiral domains.21−25 However, to date, few reports have dealt with a transition from cocrystallization to chiral resolution for enantiomers in the assemblies of inherently chiral molecules on the surface. Herein, we present a 2D structural transition from a heterochiral lattice to a homochiral conglomerate by controlling the ee in a solution of 4-[(1-methylheptyloxy)carbonyl]phenyl 4′-octyloxy-4-biphenylcarboxylate, molecule 1, often referred to as MHPOBC (depicted in Figure 1).26 The ee-dependent assembly of chiral 1 at the interface between 1-octanol and highly oriented pyrolytic graphite (HOPG) has been studied by means of STM. At lower ee values, a coassembly of both enantiomers into a heterochiral adlayer is observed. When further increasing the ee to higher values, the mixed Received: April 13, 2016 Revised: May 31, 2016 Published: June 10, 2016 6830
DOI: 10.1021/acs.langmuir.6b01418 Langmuir 2016, 32, 6830−6835
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substrate, a long-range ordered structure, which is distinct from the assembly of enantiopure molecules, was observed (Figure 2a). R-1 and S-1 coassemble into a regular lamella on the
Figure 1. STM images of monolayers assembled by enantiopure molecule 1 at the liquid/solid interface. (a) High-resolution STM image (I = 2 nA and Vbias = 0.900 V) of the S-1 adlayer. The upper is the molecular structure of S-1. (b) High-resolution STM image (I = 0.7 nA and Vbias = 0.900 V) of the R-linear structure. The upper is molecular structure of R-1. The packing models for the S-dimer and Rdimer are superimposed in (a) and (b), respectively. The white arrows and green-dashed lines indicate the direction of the unit-cell direction b and the axis of molecular dimers, respectively. θ is the angle between the axis of the dimer and the unit-cell direction b. Concentration of S-1 or R-1 = 7 × 10−3 M.
Figure 2. STM images of the self-assembly of racemic mixtures at the 1-octanol/HOPG interface. (a) Large-scale STM image (I = 1.3 nA and Vbias = 0.900 V) of the racemic-mixture adlayer on the surface. A unit cell is indicated in white lines. (b) High-resolution STM image (I = 1.5 nA and Vbias = 0.900 V) of the heterochiral lamellar structure. One dot stands for one benzene circle in molecule 1, and contiguous three dots correspond to one molecule of 1. The enantiomeric adsorption conformations are expressed by blue and pink patterns, respectively. The total concentration of S-1 and R-1 = 7 × 10−3 M.
enantiomers undergo spontaneous resolution, generating globally homochiral surfaces with the use of the majorityrules principle.
substrate rather than being separate in different domains. Ten molecules are accommodated in one unit cell. Unit-cell parameters are characterized as a = 4.5 ± 0.1 nm, b = 6.3 ± 0.1 nm, and α = 99 ± 1°. Molecular densities in the linear structure and lamellar structure are very close (0.352 vs 0.357 nm−2). The high-resolution STM image in Figure 2b illustrates that the lamella is composed of pairs of molecular rods. Two adjacent bright rods interdigitate with each other and act as a structural unit in the lamellar structure. Given the chemical structure of 1, the two connected dots are assigned to the diphenyl group, whereas the disjunctive bright dots correspond to the benzene ring separated by the ester group in the molecule. Alkyl chains in 1 reside in the dark region between two neighboring molecular lines. Two molecules in pair adsorb on the surface in enantiomeric conformations, as outlined by blue and pink patterns, respectively, in Figure 2b. A chiral feature of the adsorption conformation is identified according to the radian in one molecule combined with molecular orientation. Moreover, a tiny dislocation exists between two neighboring lamellae, which is responsible for the formation of the dark circular holes. Note that the STM appearance and contrast for the phenyls in molecule 1 are varied in the linear and lamellar motifs. It may be a result of the different orientation and registry of the aromatic cores relative to the HOPG substrate (see Figure S1 for details). Assembly Structure at Different ee Values. We then investigated the assembly structures applied from a mixed enantiomer solution with different ee values keeping the total concentrations of R-1 and S-1 constant (7 × 10−3 M). A rather sharp phase transition is found in which the heterochiral phases are present exclusively until a critical ee value and the transition to the new homochiral phase is completed within a fairly narrow ee range. A lamellar structure instead of a linear structure emerges with ee ranging from −0.43 to 0.43, and the unit-cell parameters are the same with the lamellar motif presented above. Enantiomers tend to coadsorb on the substrate and further assemble into a heterochiral lamellar structure. At the critical ee (±0.43), either a linear monolayer
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RESULTS AND DISCUSSION Homochiral Linear Assembly of Enantiopure Molecules. Because of the introduction of a methyl group into the alkyl chain, a stereogenic center is generated in chiral molecule 1. When enantiopure S-1 is applied on the substrate, highly ordered 2D linear structure emerges (Figure 1a). The unit-cell parameters are measured to be a = 2.2 ± 0.1 nm, b = 2.6 ± 0.1 nm and α = 97 ± 1°. Bright rods in STM images are assigned to the aromatic moiety of S-1, on account of the size match between the bright rods (1.3 ± 0.1 nm) and the dimension of aromatic cores (1.4 nm) in the S-1 molecule. Alkyl chains in S-1 are located at the lower contrast areas between bright molecular lines. Careful inspection reveals that two enantiopure S-1 molecules form a molecular pair and act as the basic structural unit in one unit cell of the linear structure. Moreover, the axis of the aromatic backbone of the molecular dimer is rotated clockwise (θ = +36 ± 1°) relative to the unit-cell direction b, as outlined in Figure 1a. On the basis of this, the assembly is named S-linear structure. That is, enantiopure S-1 molecules assemble into an asymmetric linear structure, and the asymmetry is manifested by the oblique orientation of the dimers with respect to unit-cell direction b. Furthermore, we analyzed 200 STM images obtained at different positions, and all of the domains were composed of the S-linear structure. For comparison, we have also investigated the assembly of R-1 under the same experimental conditions. R-1 forms similar linear structures, as depicted in Figure 1b. In contrast to the assembly of S-1, the vector of the dimer is rotated by −36 ± 1° away from the orientation of molecular alignment for R-1, which is referred as R-linear structure. As expected, all analyzed domains possess the same handedness. The chirality of the monomer is transferred and manifested to the supramolecular level and leads to the enantioselective formation of globally homochiral linear structures. Lamellar Assembly of the Racemic Mixture. When a droplet of a racemic solution of R-1 and S-1 was applied on the 6831
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assembles into a heterochiral lamellar structure similar to the assembly formed by the racemic mixture of enantiomers (Figure 3b). Unit-cell parameters are measured to be a = 4.3 ± 0.1 nm, b = 7.4 ± 0.1 nm, and α = 99 ± 1°. Two adjacent molecules form a molecular dimer and serve as a structural unit in the assemblies. Within a molecular dimer, two molecules adopt mirror conformations, leading to the formation of a lamellar assembly. Additionally, even though the solution of achiral molecule 2 is doped with chiral S-1, the lamellar structure assembled by achiral molecules remains unchanged until the concentration of S-1 in solution reaches 86%. A globally homochiral S-linear monolayer preferred by S-1 emerges as the concentration of S-1 exceeds 86%. That is, the “sergeant and soldiers principle”29,30 is almost inefficient in the present system. Molecular Modeling for the Assembling Process. When molecules adsorb on the solid substrate, the substrate− molecule interaction imposes strong restrictions to the adsorption conformation of chiral molecules. Because of the steric hindrance, methyl groups on stereogenic centers generally point upturned relative to the substrate.16,31,32 As a result, enantiomers adapt enantioselective conformations upon adsorption. On the other hand, prochiral molecules can adopt both enantiomeric conformations with an equal probability as there is no steric effect. Thus ,when achiral molecule 2 adsorbs on the surface, there are two enantiomeric conformations to be adopted, denoted as δ-2 and ϕ-2, respectively, as shown in Figure 4a. Consequently, achiral 2 is able to form homochiral building units for the linear structure (composed of enantiopure δ-2 or ϕ-2) as well as heterochiral building units for the lamellar structure (containing racemic δ-2 and ϕ-2). However, STM images demonstrate that achiral 2 can only assemble into a lamellar motif rather than a linear structure, implying that the heterochiral building unit is more stable than the homochiral unit when mirror conformations can both be adopted simultaneously. As for the mixed enantiomer system with ee ranging from −0.43 to 0.43, the lamellar structure, very similar to the assembly of achiral molecule 2, is observed (Figure 2). There are two possible adsorbed species within the adlayer, that is, S-1 and R-1. Both species tend to adopt the conformations with methyl upturned, which are mirror images to each other. The molecular adsorption conformation on the surface is consistent with its conformation in the liquid crystal phase.33,34 On the basis of this, enantiomers can be directly recognized according to their enantioselective adsorption conformations on STM images, which are illustrated in Figure 4b. By comparing the molecular models in Figures 4a,b, it turns out that S-1 and R-1 in Figure 4b just correspond to the two kinds of molecules (δ-2 and ϕ-2) assembled in enantiomeric conformations in the monolayer of achiral 2, respectively. R-1 and S-1 coassemble into heterochiral building units and further form the lamellar structure. This conformation makes sure that all of the methyl branches bent away from the substrate surface and the preferred molecular interactions in the heterochiral assembly of achiral 2 are preserved. That is, at lower ee values, owing to the abundance of both enantiomers as well as the energetic advantage, R-1 and S-1 prefer to coassemble into a racemic structural unit and further form a heterochiral lamellar structure on the surface with an equal proportion. Commonly, enantiomers from mixed enantiomer systems spontaneously separate into mirror-type domains which are identical to those of enantiopure molecules.13,14,17,35 However, in the present
or a lamellar motif could be formed when a droplet from the same solution is applied on the substrate. Upon increasing the ee of S-1 to higher than 0.43, the lamellar pattern disappears from the substrate. Instead, a globally homochiral linear monolayer, the same as the structure formed by enantiopure S-1, exists throughout the assembling process. The statistical analysis of over 200 images recorded at different positions and different sets of samples demonstrates that all investigated domains possess the S-linear structure. For comparison, experiments for ee of R-1 higher than 0.43 have also been carried out. Only R-linear structures can be observed, and no Slinear domains emerge in the adlayer. It demonstrates that chiral separation of S-1 and R-1 can be realized during 2D crystallization when the ee in the solution phase is above the critical value. The handedness of the excess enantiomer determines the chirality of the 2D homochiral linear structure, following the majority-rules principle. That is, not only chiral resolution but also chiral amplification occurs at higher ee values. Statistics showing the correlation between numbers of each kind of domain and ee in solution are supplemented in Figure S2. Once linear structure emerges when modulating ee values in solution, only one of the enantiomorphs can be observed and globally homochiral surfaces are obtained. Additionally, it is noteworthy that the molecular surface coverage is fairly low. Even with a rather high molecular concentration of approximately 7 × 10−3 M, only one-sixth of the substrate is covered with a molecular monolayer and the domain area is particularly large (Figure S3), implying a fast growth rate with respect to the slow nucleation rate that contributes to the sharp phase transition when the ee in solution varies.27,28 To evaluate the role of concentration in the present structural transition, similar experiments were performed by lowering the total concentration of R-1 and S-1 in solution to 1 × 10−3 M. The assembled patterns remain unchanged at a given ee value (see details in the Supporting Information). Lamellar Structure Assembled by Achiral Molecules. Because achiral 2 was not subject to a steric effect, the organizations of achiral molecule 2 at the liquid/solid interface were also explored to have an additional insight into the assembly mechanism. In contrast to chiral molecule 1, there is no stereogenic center in achiral 2 (Figure 3a). Achiral 2 self-
Figure 3. STM images of the self-assembly of achiral 2 at the liquid/ solid interface. (a) Chemical structure of achiral 2. (b) High-resolution STM image of 2 adlayer. One dot stands for one benzene circle in molecule 2, and contiguous three dots correspond to one molecule of 2. Tunneling conditions: I = 1.5 nA and Vbias = 0.900 V. 6832
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Figure 4. Molecular modeling for the assembling process from a monomer to structural unit to ordered monolayer and the final motifs we observed by STM. (a) Illustration of the assembly of achiral 2 on HOPG (tunneling conditions: I = 1.5 nA and Vbias = 0.900 V). (b) Modeling for the formation of a heterochiral lamellar structure in the mixed enantiomer system (tunneling conditions: I = 1.5 nA and Vbias = 0.900 V). (c) Illustration of the assembly of the S-linear structure composed of enantiopure S-1 (tunneling conditions: I = 2 nA and Vbias = 0.900 V). (d) Illustration of the formation of the R-linear structure constituted by enantiopure R-1 (tunneling conditions: I = 0.7 nA and Vbias = 0.900 V). For clarity, the molecular models of enantiopure S-1, enantiopure R-1, achiral δ-2, and ϕ-2 are colored by blue, pink, pale gray, and dark gray, respectively. Methyl groups at the stereogenic center are circled by black-dotted lines and painted green. The red circles indicate the structural unit of the linear structure existing in the lamellar monolayer. The handedness of the building units is superimposed on STM images.
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work, enantiomers crystallize together as a racemic conglomerate at low ee values. As revealed, for the situations in which ee of S-1 or R-1 exceeds 0.43, spontaneous chiral resolution of enantiomers is achieved and globally homochiral surfaces are obtained. As shown in Figure 4c, the conformation with the methyl branch pointing away from the substrate is supposed to be favored for S-1 because of the steric effect. Two adjacent molecules possessing the same chirality organize into a dimeric structural unit and further assemble into the S-linear structure. Upon increasing ee in solution, the great imbalance in enantiomer ratios benefits the formation of a homochiral dimeric building unit and disfavors the heterochiral unit, which can be understood from the well-established chiral amplification effect.10,11,17,36−38 Moreover, we note that two molecules at the border of adjacent unit cells arrange in an antiparallel manner (circled in Figure 4a,b), which is structurally reminiscent to the structural unit in the linear structure (Figure 4c,d). Coupled with the similar packing density of lamellar and linear structures, it is reasonable that the thermodynamic energy difference between these two structural motifs is very subtle, which makes it possible for the emergence of a linear motif at higher ee values.
CONCLUSIONS
In essence, this work presents an intriguing structural transition in which cocrystallization and chiral segregation of enantiomers take place selectively according to the ee in solution. When deposited in the mixed enantiomer system, R-1 and S-1 cocrystallize on the surface in an equal proportion at lower ee values, in spite of the imbalance in the ratios of enantiomers in solution. Two adjacent enantiomers constitute a staggered dimeric structural unit and further organize into a heterochiral monolayer just like the assembly behavior of achiral 2. Once the ee of S-1 or R-1 exceeds a critical value, the chiral resolution of enantiomers occurs and the majority-rules principle operates, which results in the formation of a parallel dimeric unit composed of enantiopure molecules and further the globally homochiral surfaces. The occurrence of the unexpected structural transition is attributed to the compositional transition from racemic to enantiopure enantiomers in structural units as the ee in solution is modulated. The observation of the eedependent structural transition provides a new example for 2D chiral assembly phenomena, which is important for chiral resolution and the propagation of chirality. 6833
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01418. Composite STM images of the adlayer and the HOPG substrate, the role of concentration in the structural transition, and relevance between numbers of each kind of domain and ee in solution as well as experimental details (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +86 10 62558934 (D.W.). *E-mail:
[email protected]. Phone: +86 10 62558934 (L.J.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21233010, 21373236, 21433011, 21573252, and 21127901) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100).
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