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Temperature-Triggered Chiral Self-Assembly of Achiral Molecules at the Liquid-Solid Interface Linxiu Cheng, Yibao Li, Chunyu Zhang, Zhong-Liang Gong, Qiaojun Fang, Yu-Wu Zhong, Bin Tu, Qingdao Zeng, and Chen Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10883 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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ACS Applied Materials & Interfaces

Temperature-Triggered Chiral Self-Assembly of Achiral Molecules at the Liquid-Solid Interface Linxiu Cheng,† ┴, ‡ Yibao Li,┴, ‡ Chunyu Zhang, § Zhongliang Gong, § Qiaojun Fang, † Yuwu Zhong, §,* Bin Tu, †,* Qingdao Zeng, †,* and Chen Wang †,* †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center

for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, P. R. China. §

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ┴

Key Laboratory of Organo-pharmaceutical Chemistry, Gannan Normal University, Ganzhou

341000, P. R. China. ‡

These authors contributed equally to this work.

KEYWORDS. Chiral self-assembly, temperature-triggered, scanning tunneling microscopy, flower-like structure, achiral molecules.

ABSTRACT. Temperature triggered chiral nanostructures have been investigated on two dimensional surfaces by means of scanning tunneling microscopy (STM). Achiral molecules 1 and 2 tend to self-assemble into strip structures on graphite before heating. However, R and S flowerlike structures are observed when heating to certain temperature. The transition

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temperatures of 1 and 2 systems are 55℃, 60℃, respectively. The DFT calculations demonstrate that R and S flowerlike structures are more stable than strip structures. The co-existence of flowerlike structures and strip structures demonstrates the thermodynamic equilibrium. Further, when adding chiral solvent to the sample with other conditions remaining the same, the racemic phenomenon disappears and the homochirality emerges. This is an efficient method to control the chirality of 2D molecular assemblies.

INTRODUCTION

Two-dimensional (2D) chiral surfaces possess great value in many research and application fields, which has drawn attention of researchers in the recent years.1-6 The 2D chiral surfaces mostly achieved by molecular adsorbing process. However, varieties of factors such as substrate interactions, 7, 8 conformation of molecules9 and the stimuli from the environment10-14 can significantly influence the molecular adsorbing process. Among these three factors, the stimuli from the environment often give rise to extraordinary chiral nanostructures with newfangled morphologies or functionalities. For instance, S. De Feyter et al. successfully obtained enantiomorphous nanowells with a solvent-mediated chiral induction effect during the self-assembled process. The nanowells could act as template networks to host-guest clusters to construct chiral multicomponent surfaces at the liquid-solid interface.15 T. R. Linderoth and co-workers reported a globally homochiral molecular structure achieved via co-adsorption of a molecular chiral switch with a complementary, intrinsically chiral induction seed on the Au (111) surface.16 Moreover, L. J. Wan et al. have realized hierarchical amplification of the global

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homochirality of 2D networks by co-assembly of achiral molecules with chiral coabsorbers.17 Charming 2D chiral nanostructures triggered by other stimuli were also observed.18-21All these achievements pave the way to further applications in chiral separation, chromatography and asymmetric heterogeneous catalysis via efficient chiral surface functionalization. Apart from the stimuli mentioned above at the liquid-solid interface for surface chiral self-assembly, temperature is another important stimulus which is utilized to control and modify the 2D arrangement.22-24 Temperature plays a significant role in self-assembly process, which could obviously influence thermodynamics and kinetics.25 Meanwhile, the absorption behaviors of molecules on liquid-solid interface can be different when taking temperature into consideration. However, the effect of temperature is seldom investigated on surface for 2D chiral self-assembly.26 Herein, to explore how temperature influences the chiral self-assembly, we focus on the investigation of chiral self-assembly behaviors of two achiral molecules 1 and 2 (Scheme 1) at liquid-solid interface by means of scanning tunneling microscopy (STM). When triggered by temperature, structures with chirality morphology start to emerge. Combining with DFT calculations, we try to investigate the relationship between temperature the resulted chiral structures. EXPERIMENTAL SECTION STM investigation. Molecules 1 and 2 were dissolved in octanoic acid with concentration less than 1.0 × 10-3 M, respectively. The samples were prepared by depositing a droplet (0.4 µL) of the above solution on freshly cleaved HOPG surface.

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(Sample 1) Then target temperature was set after turning on heating stage. After that, heating process was to put the sample on a heating stage with desired temperature. 30 mins later, the sample was allowed to be taken out and cooled to room temperature. (Sample 2 was obtained) To obtain homochiral structure, the sample 2 was allowed to heat to 80℃ for 45 mins after adding a drop of (S)-2-octanol (0.4 µL), then allowed to cool to ambient temperature. (Sample 3 was obtained) STM tips were prepared by mechanical cutting of Pt/Ir wire (80%/20%). STM measurements were performed by means of a Nanoscope IIIa (Bruker, Germany). All STM images were recorded in constant-current mode at room temperature. The specific tunneling conditions were given in the corresponding figure captions. DFT calculations. Provided by DMol3 code,27 Density functional theory(DFT) were utilized to perform theoretical calculations. Periodic boundary conditions (PBC) was utilized to display the 2D periodic arrangement on graphite in this research. The Perdew and Wang parameterization28 and Perdew-Burke-Ernzerh parameterization29 of the local exchange correlation energy were applied in local spin density approximation (LSDA) to display exchange and correlation. All-electron spin-unrestricted Kohn-Sham wave functions were expanded in a local atomic orbital basis. Numerical basis set was applied for the large system. The calculations equipped with the medium mesh, and were allelectron ones. Self-consistent field procedure was done with a convergence criterion of 10-5 au on the energy and electron density. The unit cell parameters and geometry of adsorbates have been optimized combined with experimental data. The optimized parameters and the interaction energy between adsorbates were obtained when the density convergence criterion and energy reached to desired degree. 4 / 22


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Model system showed the interactions between the adsorbates and highly oriented pyrolytic graphite(HOPG). In this investigation, the adsorption of adsorbates with πconjugated benzene-ring on graphite is similar to graphene, which lead to performing calculations on infinite graphene monolayers using PBC. Graphene layers were divided by the normal direction of 35 Å in the superlattice. Graphene supercells were used, and the Brillouin zone were sampled by a 1 x 1 x 1 k-point mesh when modeling adsorbates on graphene. The interaction energy (Einter) of adsorbates on graphite is Einter = Etot(adsorbates/graphene) - Etot(isolated

adsorbates in vacuum)

- Etot(graphene). The adsorption energy per

molecule is given by Eads=(Etot(adsorbates/graphene)-nE(per molecule of adsorbates)-Etot(graphene))/n. RESULTS AND DISCUSSION Scheme 1 illustrates the structures of these two compounds. Compound 1 possesses an alkyne group, with a benzene ring and amino functionalized triazine structures on each side of the alkyne group. Compound 2 is similar to 1, except with one additional alkyne group and benzene ring in the middle. 1 generates well-ordered linear nanostructure, as shown in Figure S1a. The large-scaled linear structure is composed of ordered bright strips with neighbouring molecules arranged in certain angle. The molecules are bright enough to be recognized as nanorod structure. To study the strip structure in more details, high-resolution STM image is obtained (Figure 1a). In the image, the nanorods are tightly packed with the unit cell parameters measured to be: a1 = 1.5 ± 0.1 nm, b1 = 4.3 ± 0.1 nm and α1 = 73 ± 1.0°, which is analogous to the calculated parameters in Table 1. The bright rods are assigned to molecules 1.

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As shown in Figure 1b, the arrangement of molecular model of compound 1 nanostructure is constructed from the STM images. Proper intermolecular hydrogen bonds formed by amino groups of adjacent molecules (Amplified in Figure 1c) are confirmed by simulation. Two amino groups and two nitrogen atoms of one molecule form four pair of

hydrogen bonds with adjacent molecules. According to the DFT

calculations, the stabilization energy of the structure yields to be -0.371 kJ mol-1Å-2(Table 2). Considering van der Waals interaction exists between adsorbates and substrate, dispersion corrections should be introduced to the calculation and therefore density functional theory with dispersion corrections (DFT-D3) method is employed. According to the DFT-D3 calculations, the stabilization energy turns out be about -0.416 kJ mol-1Å-2 (Table 2). The self-assembly morphology of compound 2 on graphite is different from that of 1 to some extent (Figure S1b, Figure 1d). The Compound 2 molecules generate linear structure with the adjoining strips interlaced slightly. The unit cell parameters of the linear structure are measured to be a2 = 1.8 ± 0.1 nm, b2 = 4.5 ± 0.1 nm and α2 = 70 ± 1.0°, which is similar to the calculated parameters listed in Table 1. For compound 2, although the number of amino groups participating in the formation of hydrogen bonds is identical to that of molecules 1, the positions of intermolecular hydrogen bonds are different as shown from the molecular models in Figure 1e and Figure 1f. The adsorption energies of the linear structure of compound 2 are about -0.324 kJ mol-1Å-2 (DFT), -0.355 kJ mol-1Å-2 (DFT-D3), respectively. The results indicate that the stability of the linear structure of compound 2 is lower than that of 1, which may be due to the structure complexity of compound 2. 6 / 22


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Interestingly, new kinds of self-assembled networks named flowerlike structures appear when the sample is heated to a certain temperature as shown in Figure 2a. Through elaborative inspection of the STM image, it is found that molecules 1 self-assemble into two types of hexamers on graphite. One arranges in clockwise (R) rotation, while the other adopts counterclockwise (S) rotation, which are marked by red circles and green circles, respectively. The high-resolution STM image in Figure 2b shows that six molecules 1 gather to form a hexamer that has a cavity with a diameter of 0.8 ± 0.1 nm. The unit cell of the flower R structure is identical to that of the flower S structure with parameters estimated to be a = b = 2.5 ± 0.1 nm and α = 60 ± 1.0°, respectively. The packing models of flowerlike structure are shown in Figure 2c and Figure 2e. The resulting network structures are constructed via formation of hydrogen bonds between the amino groups at the terminal, indicating the powerful intermolecular hydrogen bonding interactions after absorbing heat. As schematically shown in the inset of Figure 2d and Figure 2f, the hydrogen bonds existing in the flowerlike structures turn out to be NH···N hydrogen bonds between terminal amino groups of two neighboring molecules, which is different from the site of the hydrogen bonds existing in strip nanostructures before heating. As a result, four amino groups of one molecule take part in the construction of hydrogen bonds with neighboring molecules, which may lead to the higher stability of flower-like structure compared to the strip structure. Meanwhile, the nanostructure and the hydrogen bonding of compound 2 with the parameters to be a = b = 3.0 ± 0.1 nm and α = 60 ± 1.0°, are analogous to the nanostructure of 1 (Figure S2). Using DFT-D3, the absorption energies of flower R and flower S structures of molecules 1 and 2 are -0.787 kJ mol-1Å-2, -0.784 kJ mol-1Å-2, -0.665 kJ mol-1Å-2 and -0.658 kJ mol-1Å-2,

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respectively.(Table 2) The energies of both flower-like structures of 1 and 2 are much favorable than those of the strip structures of 1 and 2 before heating, manifesting that the flower-like structures are more stable than the strip structures. Moreover, the energies of structures of 1 are much favorable than those of 2, displaying that molecules 1 contribute more significantly to the formation and stability of the self-assembled nanostructures. To investigate the transition temperatures of the nanostructures, the STM images of molecule 1 system are obtained after being heated to different temperature. No transformations of morphology are obtained after heating to 40℃ and even to 50℃. From room temperature to 50℃, exclusively the strip structure is observed (Figure 3a-c). As shown in Figure 3d, in some area, flower-like structures appear when the temperature reaches ~55℃. When the temperature is 80℃, the sample turns almost entirely to flowerlike structure (Figure 3e). Above this temperature, the morphology does not change any more (Figure 3f)(Distribution of compound 1 seeing in Figure S4). Analogously, molecules 2 self-assemble into flower-like structure when the sample is heated to 60℃ (Figure S3c). The transition temperature is 60℃. However, when the temperature is up to 80℃ and even 100℃ (Figure S3d), the area of flower-like structure is small. That is to say, only a few molecules convert into flower-like structure. It is speculated that molecules 2 possess one more phenyl group and an alkynyl group, which are much longer than molecules 1. This lead to difficulties for molecules 2 to move and reassemble on graphite, which may hinder the rotation of molecules to generate flower-like structure.

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According to the results mentioned above, the flower R and S structures are obtained at higher temperatures, and no other structure forms when further increasing the temperature. This illustrates that flower R and S structures are thermodynamically stable. Moreover, the recorded STM images of flower structures exhibit no change after 30 mins or 60 mins of thermal annealing, which demonstrates that the temperature-triggered phase transitions of molecules 1 and 2 are irreversible. To understand the influence of the concentration of compound 1 and 2 in solution on the graphite at various temperatures, STM images are recorded under different concentration. It turns out that no new morphology emerges even when the concentration is diluted to 100 times. More interestingly, the transition temperatures do not show any change, which unambiguously manifests that there is no obvious relationship between concentration and transition temperature.24 (Figure S5) However, when the solvent is changed to (S)-2-octanol with other conditions remaining the same, only flower R structure is observed from the STM images (Figure 4a and 4b). The flower S structure disappears. The emergence of homochirality at interfaces demonstrate that the chiral induction effect occurs.15

CONCLUSION In conclusion, we have investigated the temperature triggered chiral nanostructures on 2D surfaces by means of scanning tunneling microscopy (STM). Achiral molecules 1 and 2 tend to self-assemble into strip structures on graphite before heating. However, R and S flowerlike structures appear when heating the 1 and 2 systems to 55℃, 60℃, respectively.

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The DFT calculations of strip and flower-like structures show that the absorption energies of R and S flowerlike structures are more favorable than those of strip structures, which illustrates that R and S flowerlike structures are more stable than strip structures. The structure transitions are directed by temperature and bear no obvious relation to concentrations. FIGURES

Figure 1. (a) High resolution image (23.85 × 23.85 nm2) of assembly structure of compound 1. The imaging conditions are I = 409.9 pA and V = 560.8 mV. (b) Molecular models for the structure of compound 1. (c) The intermolecular hydrogen bonds for the corresponding structure indicated by dotted lines. (d) High resolution image (20.51 × 20.51 nm2) of compound 2 adsorbed assembly structure. (e) Molecular models for the strip structure of compound 2. (f) The intermolecular hydrogen bonds for the structure of

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compound 2 indicated by dotted lines. The imaging conditions are I = 443.5 pA and V = 700.0 mV. These STM images are performed at room temperature. Table 1. Parameters of monolayers of molecules 1 and 2 with different self-assembled nanostructures.

Unit cell parameters

Molecules 1 before heat

Molecules 1 after heat Flower R and S

Molecules 2 before heat Molecules 2 after heat Flower R and S

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a (nm)

b (nm)

α(deg)

Expt.

1.5 ± 0.1

4.3 ± 0.1

73.0 ± 1.0

Calcd.

1.50

4.18

73.0

Expt.

2.5 ± 0.1

2.5 ± 0.1

60 ± 1

Calcd.

2.84

2.84

60.0

Expt.

1.8 ± 0.1

4.5 ± 0.1

70.0 ± 1.0

Calcd.

1.80

4.40

70.0

Expt.

3.0 ± 0.1

3.0 ± 0.1

60.0 ± 1.0

Calcd.

3.26

3.26

60



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Table 2. Energy (kJ mol-1) parameters of monolayers of molecules 1 and 2 with different selfassembled nanostructures.a Interactions between adsorbates (kJ mol-1)

Total energyb (kJ mol-1)

Interactions between adsorbates and substrate (kJ mol-1)

Total energy per unit area (kJ mol-1 Å-2)

Adsorption energy per molecule (kJ mol-1)

Method

DFT

DFT-D3

DFT

DFT-D3

DFT

DFT-D3

DFT

DFT-D3

DFT

DFT-D3

1 before heat

-77.735

-69.525

-144.851

-179.780

-222.586

-249.305

-0.371

-0.416

-111.293

-124.653

1 flower R

-178.225

-134.099

-337.132

-415.344

-515.357

-549.443

-0.738

-0.787

-171.785

-183.147

1 flower S

-178.159

-139.933

-329.626

-407.430

-507.785

-547.363

-0.727

-0.784

-169.262

-182.454

2 before heat

-60.538

-12.058

-209.045

-283.472

-269.583

-295.530

-0.324

-0.355

-134.792

-147.765

2 flower R

-388.104

-239.899

-171.588

-372.193

-559.692

-612.092

-0.608

-0.665

-186.564

-204.031

2 flower S

-388.158

-234.405

-170.079

-371.615

-558.237

-606.020

-0.606

-0.658

-186.079

-202.007

a

Here, the more negative energy means the system is more stable. bThe total energy (including the interaction energy between adsorbates and the interaction energy between adsorbates and substrate) and the energy per unit area for adsorbates on the HOPG surface.

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Figure 2. (a) STM image (38.16 × 38.16 nm2) of compound 1 adsorbed monolayer on the HOPG surface (flower R and S). (b) STM image (29.27 × 29.27 nm2) of flower S structure structure of compound 1. The imaging conditions are I = 409.9 pA and V = 560.8 mV. (c) Molecular models for the flower R structure of compound 1. (d) The intermolecular hydrogen bonds for the flower R indicated by dotted lines. (e) A suggested structural model for the flower S structure on HOPG. (f) The intermolecular hydrogen bonds for the flower S indicated by dotted lines. The imaging conditions are I = 409.9 pA and V = 560.8 mV.

Figure 3. STM images of molecules 1 adsorbed monolayer on the HOPG surface at different temperatures: (a) at room temperature (~25℃, 298 K), at the scale of 78.90 ×

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78.90 nm2. (b) at 40℃ (313 K), at the scale of 70.14 × 70.14 nm2. (c) at 50℃ (323 K), at 76.20 × 76.20 nm2. (d) at 55℃ (328 K), at 78.11 × 78.11 nm2. (e) at 80℃ (353 K), at 60.10 × 60.10 nm2. (f) at 100℃ (373 K), at 38.20 × 38.20 nm2. The imaging conditions are I = 409.9 pA and V = 560.8 mV.

Figure 4. (a) STM image (73.82 × 73.82 nm2) of flower R structure of molecules 1 adsorbed monolayer on the HOPG surface after treated with (S)-2-octanol, the heating temperature is 80 ℃ . (b) Higher resolution image (20.20 × 20.20 nm2) of assembly structure of flower R structure. The imaging conditions are I = 409.9 pA and V = 560.8 mV.

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SCHEMES Scheme 1. Chemical structures of molecules (a) 1, (b) 2.

a H2N N H2N

b

N

N

N

N

NH2 N NH2

H2N N H2N

N

N

N

N

NH2 N NH2

ASSOCIATED CONTENT Supporting Information 1. Synthesis of molecule 1 and 2 2. STM images of self-assembled structures of molecules 1 and 2 3. STM images and molecular models of molecules 2 adsorbed monolayer 4.

STM images of molecules 2 adsorbed monolayer on the HOPG surface at different temperatures This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]; [email protected]; [email protected]. ACKNOWLEDGMENT

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This work was supported by the National Natural Science Foundation of China (Nos. 21472029, 21472196) and the National Basic Research Program of China (Nos. 2016YFA0200700, 2013CB934203). REFERENCES 1. Pérez-García, L.; Amabilino, D. B. Spontaneous Resolution, Whence and Whither: From Enantiomorphic Solids to Chiral Liquid Crystals, Monolayers and Macro-and Supra-Molecular Polymers and Assemblies. Chem. Soc. Rev. 2007, 36, 941-967. 2. Li, C. J.; Zeng, Q. D.; Wang, C.; Wan, L. J.; Xu, S. L.; Wang, C. R.; Bai, C. L. Solvent Effects on the Chirality in Two-Dimensional Molecular Assemblies. J. Phys. Chem. B. 2003, 107, 747750. 3. Walba, D. M.; Stevens, F.; Clark N. A.; Parks, D. C. Detecting Molecular Chirality by Scanning Tunneling Microscopy. Acc. Chem. Res. 1996, 29, 591-597. 4. Hu, F. Y.; Zhang, X. M.; Wang, X. C.; Wang S., Wang, H. Q.; Duan, W. B.; Zeng Q. D.; Wang, C. In Situ STM Investigation of Two-Dimensional Chiral Assemblies through SchiffBase Condensation at a Liquid/Solid Interface. ACS Appl. Mater. Interfaces 2013, 5, 1583-1587. 5. Raval, R. Chiral Expression from Molecular Assemblies at Metal Surfaces: Insights from Surface Science Techniques. Chem. Soc. Rev., 2009, 38, 707-721. 6. Elemans, J. A. A. W.; De Cat, I.; Xu, H.; De Feyter, S. Two-Dimensional Chirality at LiquidSolid Interfaces. Chem. Soc. Rev. 2009, 38, 722-736.

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20. Ernst, K. H. Supramolecular Surface Chirality. Springer Berlin Heidelberg 2006, 209-252. 21. Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. Manipulating Self-Assembly with Achiral Molecules: An STM Study of Chiral Segregation by Achiral Adsorbates. J. Phys. Chem. B. 2000, 104, 7627-7635. 22. Ruben, M.; Payer, D.; Landa, A.; Comisso, A.; Gattinoni, C.; Lin, N.; Collin, J. -P.; Sauvage, J. -P.; De Vita, A.; Kern, K. 2D Supramolecular Assemblies of Benzene-1, 3, 5-triyl-tribenzoic Acid: Temperature-Induced Phase Transformations and Hierarchical Organization with Macrocyclic Molecules. J. Am. Chem. Soc. 2006, 128, 15644-15651. 23. Stöhr, M.; Wahl, M.; Galka, C. H.; Riehm, T.; Jung, T. A.; Gade, L. H. Controlling Molecular Assembly in Two Dimensions: The Concentration Dependence of Thermally Induced 2D Aggregation of Molecules on a Metal Surface. Angew. Chem. 2005, 117, 7560-7564. 24. Blunt, M. O.; Adisoejoso, J.; Tahara, K.; Katayama, K.; der Auweraer, M. V.; Tobe, Y.; De Feyter, S. Temperature-Induced Structural Phase Transitions in a Two-Dimensional SelfAssembled Network. J. Am. Chem. Soc. 2013, 135, 12068-12075. 25. Gutzler, R.; Cardenas, L.; Rosei, F. Kinetics and Thermodynamics in Surface-Confined Molecular Self-Assembly. Chem. Sci. 2011, 2, 2290-2300. 26. Fang, Y.; Ghijsens, E.; Ivasenko, O.; Cao, H.; Noguchi, A.; Mali1, K. S.; Tahara, K.; Tobe, Y.; De Feyter, S. Dynamic Control over Supramolecular Handedness by Selecting Chiral Induction Pathways at the Solution-Solid Interface. Nat. Chem. 2016, 8, 711-717.

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27. Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756-7764. 28. Perdew, J. P.; Wang, Y. Pair-Distribution Function and Its Coupling-Constant Average for the Spin-Polarized Electron Gas. Phys. Rev. B. 1992, 45, 13244. 29. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865.

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Table of Contents Graphic

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Temperature-Triggered Chiral Self-Assembly of Achiral Molecules at

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