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Odd-Even Effects in Chiral Phase Transition at the Liquid/Solid Interface Hai Cao, Kazukuni Tahara, Shintaro Itano, Yoshito Tobe, and Steven De Feyter J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Odd-Even Effects in Chiral Phase Transition at the Liquid/Solid Interface Hai Cao,† Kazukuni Tahara,‡,§ Shintaro Itano, ‡ Yoshito Tobe,*, ‡ and Steven De Feyter*,† †

Division of Molecular Imaging and Photonics, Department of Chemistry, KU

Leuven−University of Leuven, Celestijnenlaan 200F, B3001, Leuven, Belgium. ‡

Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka

University, Toyonaka, Osaka 560-8531, Japan. E-mail: [email protected]. §

Department of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1

Higashimita, Tama-ku, Kawasaki, 214-8571, Japan.

ABSTRACT: Chiral selection on inorganic crystalline surfaces represents one of the most promising avenues to the separation of enantiomers. However, there are competing influences at play: on the one hand, the confinement to the surface seems to enhance chiral discrimination between enantiomers; on the other hand, racemic patterns tend to possess higher packing density therewith higher stability. A clear picture on the delicate balance between these two opposing factors is missing. We address this issue in monolayers of alkylated dehydrobenzo[12]annulene (DBA) derivatives at the liquid/solid interface, by a detailed investigation of the relationship

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between packing density and 2D chirality. We report on chiral phase transitions, evolving from homochiral low density porous networks to enantiomeric excess, racemic or homochiral denselypacked structures, by using scanning tunneling microscopy (STM) as a visualization tool. The changes in monolayer chirality in response to increased packing density, however, are strongly correlated with molecular structural features such as the length of the alkyl chains, and in particular their parity. While heterochiral lattices are indeed denser than its enantiomorphous counterparts, close packing does not necessarily favor racemic crystallization: the azimuthal orientation of building blocks in a domain may play a decisive role. In light of the popularity of using alkyl chains to adhere molecules onto a surface, we believe that our findings may have implications for predictive chiral recognition and resolution processes.

Introduction The adsorption and organization of molecules on inorganic crystalline surfaces is a critical step in many chemical processes, for instance, crystallization, separation and catalysis.1-3 Spontaneous ordering of adsorbates on a surface is foremost determined by their chemical structures and intermolecular interactions, but rests as well with the non-covalent adsorbatesubstrate interactions. A manifestation of the complexity of supramolecular interactions at interfaces is 2D polymorphism, that is, the formation of multiple crystalline phases from the same building block.4 In recent years, there has been a dramatic proliferation of research concerned with the kinetics and thermodynamics of monolayer polymorphs, using scanning tunneling microscopy (STM) as visualization tool.5,6 Molecules in physisorbed layers are typically lying flat on a surface therefore exposing their largest facet to the STM tip, facilitating the monitoring of subtle


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variations in intermolecular bonding patterns,7,8 as induced by for instance hydrogen bonding, metal coordination and van der Waals interaction between interdigitated alkyl chains. The impact of concentration,9,10 surface coverage,11 temperature12,13 and type of solvent14,15 on polymorphism is well-documented. By monitoring the structural similarities and differences, and the transition between different crystalline phases, one can get insight into the mechanism controlling molecular arrangements on a surface. Another intriguing topic that has gathered a tremendous wave of interest over the last few decades is 2D chirality on surfaces.16-20 Upon confinement of molecules on a surface, chiral monolayers can be easily achieved, even from intrinsically achiral building blocks.21 On the one hand, chiral discrimination, i.e. the separation of enantiomers in separate domains, seems to be enhanced upon confinement to a surface.18,22 On the other hand, the expected higher stability of racemic crystals due to a denser packing, known as Wallach’s rule, opposes the trend for chiral discrimination.23,24 So far, however, there is barely any system devised to reveal the subtle balance between the two phenomena, i.e. chiral discrimination vs formation of racemic crystals. One approach to bring insight into the competition or balance between chiral discrimination on the one hand, and racemic crystal formation on the other hand, is by investigating surfacecoverage dependent chiral phase transition. Few studies address this issue, all of them under UHV conditions.25-28 It has been pointed out in all these cases that homochiral aggregation of chiral or prochiral molecules is energetically favored at low coverages, but racemic nucleation becomes more favorable with increased surface coverage. Nonetheless, for a given molecular system, at most three phases are reported, and a systematic and comparative study on how subtle changes to the system affect chiral phase transitions is lacking. Therefore, there is a pressing


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need to identify the ‘variables’ that may affect 2D chiral crystallization in view of the development of predictive models and, moreover, to implement chiral resolution at interfaces.29 Here we present a systematic study of concentration-driven phase transitions in monolayers of prochiral alkylated dehydrobenzo[12]annulene (DBA) derivatives (DBA-OCn, n = 11-17, 20, Figure 1a and 1b) at the liquid−solid interface, the liquid being octanoic acid and the substrate highly oriented pyrolytic graphite (HOPG), aiming at bringing insight in how monolayer chirality and structure evolve in response to molecular structural variation (Figure 1c). Selfassembly of such alkylated DBAs has been extensively investigated.9,13,30-35 At the lower solute concentration, they typically form enantiomorphous nanoporous networks on the basal plane of HOPG, while homochiral linear phases in which one or two alkyl chains are desorbed start to dominate at higher solute concentration. In this study, additional phases were revealed following a hot deposition protocol.35 We found that DBAs with even-numbered side alkyl chains experience transitions from homochiral low density phases to racemic or even enantiomeric excess close-packed structures (Figure 1d), which is in contrast to monolayers formed by DBAs with an odd number of carbon atoms in the alkyl chains: in the latter case, chiral discrimination still prevails. Based on a comparative analysis of many distinct phases, we essentially seek to compare the packing density of closepacked heterochiral lattices with their homochiral counterparts to uncover the origin of odd-even discrepancy in chiral crystallization and ultimately to implement chirality control and chiral resolution in surface-confined monolayers. Results and discussion Description of molecular and monolayer chirality


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Before proceeding to the discussion of different polymorphs, we would like to bring up two issues that have been addressed before;34 one concerns how chirality is bestowed to otherwise D3h symmetric DBAs, while the other is the correlation between monolayer chirality and parity of alkyl chains. The combination of a triangular rigid aromatic core and six long alkyl chains endows DBAs with great flexibility while on surface. Though D3h symmetric in gas and liquid phase, DBAs become chiral upon planar adsorption on HOPG surface to match the substrate registry,36,37 as illustrated in Figure 2a. The alkyl chains of a DBA molecule are slightly tilted with respected to the aromatic core such that they match the graphite lattice while at the same time are ideally spaced to accommodate an alkyl chain of an adjacent molecule, called alkyl chain interdigitation. As a result of graphite’s threefold symmetry, the triangular core may “spin” either clockwise (CW) or counterclockwise (CCW). In such a way, the molecular symmetry is reduced to C3 upon adsorption, thereby chirality is bestowed to intrinsically achiral DBAs. We arbitrary refer to the mirror-image adsorption motifs of these DBAs as S-DBA and R-DBA, respectively (see Figure 2a). The intermolecular interactions that keep DBA molecules immobilized in a 2D network is nothing but the two possible mirror image alkoxy chain interdigitations, denoted as (+)-type and (−)-type. Considering the molecular chirality of DBAs upon surface confinement, so S-DBA and R-DBA, there are four possibilities for intermolecular interdigitations, namely RR(+), RR(−), SS(+) or SS(−), as displayed in Figure 2b. Even-numbered alkylated DBAs tend to adopt RR(+) and SS(−) interdigitations while odd-numbered alkylated DBAs prefer RR(−) and SS(+). By doing so, steric repulsion between terminal methyl groups and DBA cores can be minimized. Figure 2c shows a STM image of a homochiral (+)-type DBA-OC13 porous network


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presumably consisting of S-DBA-OC13 enantiomers. The aromatic cores are recognized as bright triangles while dim stripes in between are alkyl chains.

Polymorphism and chiral phase transition revealed by hot deposition protocol In the course of the investigation of concentration- and temperature-triggered structural evolutions in DBA monolayers, polymorphism was revealed in monolayers of DBA-OC12 following a protocol in which about 15 microliter of its octanoic acid solution is dropcasted onto a hot HOPG surface, which we refer to as “hot deposition” protocol. Specifically, DBA solutions (not preheated) were applied to an 80 °C hot HOPG surface and held at that temperature for 3 min. The samples were left to cool down in the ambient for few minutes after which STM data were recorded at room temperature. Some distinct phases emerged with varying packing densities, as displayed in Figure 3a-3c and Figure S1a-1d. Above-mentioned linear phase (denoted as phase I, Figure 3a) dominates the surface at 5 × 10-5 M, but it converts into a trimeric hexagonal phase (phase II, Figure 3b), and then into a chevron phase (phase III, Figure 3c) and a herringbone phase (phase IV, Figure 3c) at increased concentrations (Figure 3d). In particular, the latter two phases are observed for DBA compounds for the first time, and up to four phases were sometimes observed in a single STM image (Figure S1d). We ascribe the observed polymorphism to the combined effect of high concentration and tempering. DBAs furnish nanoporous networks at low concentrations in octanoic acid at room temperature, (Figure S2), but rather disordered densely-packed structures when at higher concentrations (Figure S3). Previously we have found the gradual structural transition from disordered densely-packed structures into a uniform linear phase by increasing the deposition


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temperature up to 80 °C.35 We believe that tempering provides the requisite activation energy for rearranging randomly adsorbed molecules into regular structures or possibly thermodynamically more stable phases,38 manifested as the co-occurrence of distinct phases. It should be noted that hot deposition in different solvents may lead to distinct outcomes (Figure S4), which is probably correlated to the solubility of DBAs in the solvent being used.31 In order to gain insight into how monolayers evolve from low density to high density structures, we examined the arrangement of DBA molecules in each phase. Phase I (Figure 3e, 3i) and II (Figure 3f, 3j) typically appear at low concentrations. In the latter phase two of the alkyl chains per molecule are not visible. Both phases are homochiral at the level of individual domains, that is, composed of either (+) or (−) interdigitations, corresponding to the enantiomorphous organization of R- or S-DBA enantiomers. In addition, monolayer chirality is also manifested by the oblique angle of unit cell with respect to underlying substrate lattice. The oblique angles are small though. Naturally, mirror image domains coexist for both phases on the surface. Phase III is revealed at relatively higher concentrations as compared to phase I and II. It can be viewed as every two rows of phase I being separated by a close-packed DBA row (Figure 3g, 3k, S5). Two different DBA configurations, both having four alkyl chains on surface, can be identified. The handedness between two neighboring close-packed rows is identical, but alternates in a regular chevron fashion. The unit cell, containing 8 molecules, is rectangular rather than oblique. The longer vector b runs along the axis of graphite lattice while the shorter unit cell vector a is aligning parallel to the axis, suggesting that this structure is truly racemic in terms of monolayer handedness and as well with respect to the underlying substrate. As a consequence, no mirror domain can be found. Close examination of the close-


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packed rows reveals that oppositely oriented R- and S-enantiomers arrange alternatively along vector a, but in between the two close-packed rows are enantiomers with the same chirality (Figure 3k). In such a way, R- and S-DBA coexist in a domain with equal probability. Besides phase III, the herringbone phase IV, is also racemic (Figure 3h, 3l, S6). The corresponding unit cell is rectangular as well and contains eight molecules. Likewise, the shorter unit cell vector a is aligned parallel to the main axis of graphite lattice while the longer unit cell vector b is running along the normal. (+)-Type segments of R-enantiomers and (−)-type segments of S-enantiomers alternatively arrange in a domain. No mirror- image domains are formed. It is worth noting that the density of two racemic phases is identical within experimental error (Table S1). Despite the emergence of distinct phases, chiral transition in monolayers of DBA-OC12 from low density homochiral phases (phase I, II) to high density racemic phases (phase III, IV) so far appears to be one additional evidence of chiral phase transition on 2D surface. Still little is known about the driving force that controls the transformation, and moreover to what extent the packing density of racemic lattice is increased in comparison to its homochiral counterpart. Nevertheless, a merit of alkylated molecules is that structurally similar homologues can be formed. We decided to vary the alkyl chain length, and paid in particular attention to the impact of odd-numbered vs even-numbered alkyl chains.39 Homologous DBA derivatives with both oddand even-numbered alkyl chain lengths (referred to as odd-DBA and even-DBA afterwards) were selected and then as well dissolved in 1-octanoic acid at various concentrations for study.

Odd-even effect in phase transition


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Most of the DBA homologues experience concentration-driven phase transitions resembling those of DBA-OC12, starting from the linear phase I, as displayed in Figure S7-S17. For comparison, similar structures from different DBAs are grouped and sorted according to their structural characteristics and packing densities even though not every phase is available for all the DBAs (Figure 4). As one may expect, the structural diversity and stability is strongly parityrelated. For n = 11-16, the coexistence of multiple phases has been observed for all the evenDBAs at 5 × 10-4 M (Table S2), but these phases are sensitive to concentration changes. In contrast, monolayers of all odd-DBAs are characterized by only one structure at any given concentration, and the surface structures barely or slowly changes upon dilution. So, even-DBAs have strong tendency to form multiple phases, whereas phases from odd-DBAs possess higher stability towards concentration changes. However, this trend becomes less clear for DBA–OC17 (Figure S13) and DBA-OC20 (Figure S15). Very likely, longer intermolecular distance lessens the odd-even discrepancy. Note that for all these compounds, no structural transition was perceived during hours of STM imaging at room temperature. One in-situ experiment carried out at 80 °C (Figure S16) and one additional experiment performed at higher deposition temperature with longer heating time (Figure S17) reveal identical monolayer structures as those obtained following the hot deposition protocol, indicating the excellent stability of those phases. Phase I of all the DBAs is homochiral in a domain, yet three distinct chiral arrangements in phase III, namely racemic, homochiral and enantiomeric excess, can be identified for different DBAs. Surprisingly there is an odd-even effect in monolayer chirality. We found that phase III of all the odd-DBAs is homochiral, and enantiomers of opposite handedness are segregated and organized into enantiomorphous domains, indicating that chiral discrimination still prevails. The


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unit cell is oblique and contains four molecules. Figure 5a and 5b show (−)-type phase III of DBA-OC13 comprised exclusively of R-enantiomers. When it comes to phase III of DBA-OC14, the situation is distinct, as shown in Figure 5c and 5d. It also has four molecules in its oblique unit cell, but the monolayer chirality is neither racemic nor homochiral but a manifestation of enantiomeric excess. Monolayer chirality varies in a periodic way: two thirds of a domain is opposite to the rest. As a consequence, there are (+)rich and (−)-rich domains (Figure S10). While it might sound counterintuitive, it is remarkable that all the DBAs in a domain share the same chirality, i.e. either R or S. The enantiomeric excess comes from the interdigitations that are not the preferential type. It is therefore a rare example showing heterochiral structure formation by an enantiopure building block. Interestingly, both racemic and enantiomeric excess phase III can be obtained for DBA-OC16, even within a single domain (Figure S12c (mid)).

Higher packing density or preferential chiral discrimination Integrating the otherwise oblique S- and R- enantiomorphous motifs into a rectangular unit cell and mismatched intermolecular interdigitations are responsible for the racemic and enantiomeric excess phase III of even-DBAs, respectively. Both are not energetically favorable in view of adsorbate-substrate registry or intermolecular chiral recognition, accordingly there must be an underlying driving force that compensates the energy loss. In the light of existing examples showing coverage-driven conversion of homochiral structures into racemic close-packed networks, we interpret the odd-even effect in monolayer chirality as the interplay of absorbatesubstrate registry, chiral discrimination and packing density. Higher density means that the


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surface is able to accommodate more molecules thereby the adsorption energy per unit area is increased. However, the difference in packing density of racemic crystals and their homochiral counterparts is probably subtle, as a comparative study has shown that racemic crystals are packed on average merely 1% more tightly than the enantiomorphous ones for resolvable enantiomers, even less for achiral molecules and interconverting enantiomers.24 Such an insignificant variation in dimensions is barely resolvable by STM imaging. We used the oblique angles of a unit cell with respect to the substrate lattice to estimate the variation in dimensions. Using phase III of DBA-OC12 as a reference, we compared the enantiomorphous and conceived racemic lattice of DBA-OC13 (Figure 6a, S18) and calculated their packing densities. Based on the average values of α = 8o and β = 3o measured for DBAOC13, we found that the area per molecule of the hypothetical racemic phase of DBA-OC13 is about 0.7% larger than that of the existing homochiral phase, which is in line with the expected higher density for a racemic packing. But the uncertainty on this value is high, and at this point, it remains unclear why odd-DBAs adopt enantiomorphous arrangements as opposed to evenDBAs. One hint lies at the level of dimerization, as illustrated by preferential SS(+) and RR(+) interdigitations for odd-DBAs and even-DBAs, respectively (Figure 2c). As a result of the tilted alkyl chains, odd-DBAs exhibit a larger oblique angle than the even-DBAs. A previous study on porous DBA networks revealed average values of 13o for odd-DBAs and 4.5o for even-DBAs (n = 11-16).34 This value decreases slightly for longer alkyl chains, but the discrepancy between odd- and even-DBAs is solid and robust. A mismatched interdigitation, SS(+) for even-DBAs, as depicted in Figure 6b, is therefore able to bring some changes in length along the long vector b of its unit cell, which further affects the packing density. From the schematic illustration, one can


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easily see that the mismatched interdigitation brings neighboring DBAs closer. More precisely, the intermolecular distance is shortened by around 4%, as calculated by using DBA-OC14 as an example (details shown in Figure S19), increasing packing density by approximately 4% in comparison with its homochiral counterpart, which is much larger than the gain for a racemic arrangement of DBA-OC13. Considering that the enantiomeric excess and racemic phase III of DBA-OC16 coexist on surface, we assume that the packing density of these two heterochiral phase are very close, which may also explain the co-occurrence of phase III and racemic phase IV of DBA-OC14. Therefore, the imperfect organization of DBA-OC14 is pursued also for the sake of higher packing density. It is obvious that odd-DBAs cannot form such an enantiomeric excess structure as the mismatched interdigitation enlarges intermolecular distance rather than shrink it. The reason for the lack of stable racemic phase III for odd-DBAs may as well lie in their dimerization preference and large oblique angles. On the one hand, the enantiomorphous oddDBA phase is to some extent already dense enough as compared to the even-DBAs considering their relatively shorter intermolecular distance. On the other hand, larger azimuthal angle of two mirror-image motifs means that there might be a larger activation barrier to overcome to form a racemate. However, one odd-DBA, namely DBA-OC17, is able to find its way to form a heterochiral phase. Figure 7a presents the enantiomeric excess phase (termed as phase V) of DBA-OC17, in which most of the molecules have four visible alkyl chains, but some only have two adsorbed on surface. This phase is nearly racemic as there is only a very small bias towards (+) or (−) chirality within a domain. The way DBA-OC17 arranges is distinct from all the above cases. Heterochiral dimerization of DBA-OC17 is adopted with the same purpose of increasing packing


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density (Figure 7b), yet no long-range periodicity can be found within a domain despite the monolayer chirality alters in a regular fashion.

Densely-packed phase Only DBAs with long alkyl chains, i.e. DBA-OC16, -OC17 and -OC20 (Figure 7c, 7d and S12-S15) are able to form even more densely-packed phase (phase VI). This phase normally coexists with phase I or III without distinct domain boundary. The monolayer chirality of this phase is linked to the type of phase III a DBA may form. In the case of DBA-OC17, only a homochiral phase VI is observed whereas both racemic and homochiral phase can be expected for DBA-OC16 (Figure S12). No other phase with even higher packing density appears.

Chiral resolution by diastereomeric interactions Phase transition is therefore interpreted in view of monolayer chirality. The bias towards one nucleation over the other at high surface coverage is steered by subtle differences in chiral discrimination between enantiomers, which results from a slight variation in molecular structure, specifically the parity of side alkyl chains. There are in principle two ways to implement chiral resolution in otherwise heterochiral phase, by imposing some changes to the system that is able to affect either the adsorbate-adsorbent interaction or the intermolecular interaction between adsorbates. For the former, substrate registry might be affected by different substrates or a buffer layer. Some observations have already demonstrated distinct nucleation behaviors of a


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compound on different facets of a metal crystal,40,41 different domains of Au(111) surface25 and the top of physisorbed alkane monolayers.42 Alternatively, some variation or additive is needed to influence intermolecular interactions in a racemic lattice. To this end, we tested the possibility of using diastereomeric interaction between odd- and even-DBAs to segregate enantiomers into different domains. An equimolar mixture of DBA-OC15 and DBA-OC16 at the concentration of 5 × 10-4 M in octanoic acid was selected. Phase V, which is merely found in the monolayer of DBA-OC17, was unexpectedly observed for the binary mixture of DBA-OC15 and DBA-OC16 (Figure S20, S21). Considering the possible diastereomeric interactions between these two molecules (Figure 8a) and the nature of phase V (Figure 7a), the emergence of phase V can be considered as an indication of the good mixing of two DBAs. Phase III was observed as well, but the monolayer chirality within a domain is exclusively homochiral. Careful investigation of the homochiral (−)-phase III (Figure 8b, 8c) reveals the alternating arrangement of S- and R-enantiomers in a domain. We tentatively ascribe the S-enantiomer to DBA-OC15 and the R-enantiomer to DBA-OC16, as depicted in Figure 8d. In such a way, the segregation of two enantiomers of DBA-OC16 into homochiral domains is realized by taking advantage of diastereomeric interactions.

Conclusions In this study, we have shown that a small variation in the length of the alkyl chains of alkylated dehydrobenzo[12]annulene (DBA) derivatives, i.e., alkyl chain parity, is able to steer the monolayer chirality in close packing structures. It is known that a racemate can crystallize in different ways, as conglomerate, or as a racemic compound being the most important ones. In


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case of a conglomerate both enantiomers crystallize separately. This process is also called spontaneous resolution as it allows pure or nearly pure enantiomers to be obtained by simply sorting them. A racemic compound means that, within the crystal, both enantiomers are present in equal amounts in a well defined arrangement. Chiral discrimination prevails in the former case, while it is often suggested that a racemic lattice is preferred because it is denser than its homochiral counterpart. By means of a hot deposition protocol, we demonstrated a structural transition in DBA networks, from nanoporous networks to linear, high-density and close-packed structures, and simultaneous evolution of monolayer chirality, from homochiral to enantiomeric excess and racemic. Based on a comparison of six distinct phases obtained from different DBAs, an evident odd-even effect in chiral phase transition is revealed. Despite a small difference in structure, even-numbered DBAs exhibit a strong tendency to form more phases that are enantiomeric excess or racemic as compared to their odd-numbered analogues. Quantitative analysis on a particular high density phase that can be formed by both odd-DBAs and evenDBAs unambiguously illustrated that heterochiral lattices are denser than their enantiomorphous counterparts, yet the efficiency is parity-related. This odd-even discrepancy originates from symmetry breaking of DBAs to match surface registry, and is manifested as different intermolecular chiral interdigitations. The determination of the factors that steer the course of crystallization in one direction rather than the other is of both fundamental and practical interest, yet systematic studies to that end are lacking. On the basis of a study on alkylated DBA systems, we conclude that close packing does not necessarily favor racemic crystallization, but the azimuthal orientation in their adlattices plays a decisive role in 2D chiral crystallization. The results described here represent an effort to ascertain the driving forces that may affect chiral resolution on surfaces, which is one of the


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more intriguing issues prevailing throughout the last few decades. In particular, on the basis of the observed odd-even effect, we have also shown the potential of control towards desirable nucleation by taking advantage of diastereomeric interactions. Given the abundance of selfassembling systems of alkylated molecules, we believe that our findings are relevant for a broad range of compounds.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details of sample preparation, STM measurements, calibration of STM images (Scheme S1) and surface coverage analysis; DBA porous networks at the octanoic acid (OA)/HOPG interface; The influence of solubility, solvent and hot deposition protocol on monolayer structure formation at high concentrations; Supplementary STM images and detailed information for different phases of DBA-OCn (n = 11-17, 20); In-situ STM measurement at 80 °C. AUTHOR INFORMATION Corresponding Authors Yoshito Tobe ([email protected]) Steven De Feyter ([email protected])


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is supported by the Fund of Scientific Research–Flanders (FWO), Internal Funds KU Leuven, Belgian Federal Science Policy Office (IAP-7/05) and JSPS KAKENHI grant nos. 10252628 and 26620063. This research has also received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC Grant Agreement No. 340324. H. C. is a FWO Pegasus Marie Curie Fellow.

REFERENCES 1. Hazen, R. M.; Sholl, D. S. Chiral Selection on Inorganic Crystalline Surfaces. Nat. Mater. 2003, 2, 367–374. 2. Vang, R. T.; Lauritsen, J. V.; Lægsgaard, E.; Besenbacher, F. Scanning Tunneling Microscopy as a Tool to Study Catalytically Relevant Model Systems. Chem. Soc. Rev. 2008, 37, 2191–2203. 3. Gellman, A. J. Chiral Surfaces: Accomplishments and Challenges. ACS Nano 2010, 4, 5–10. 4. Ahn, S.; Matzger, A. J. Six Different Assemblies from One Building Block: TwoDimensional Crystallization of an Amide Amphiphile. J. Am. Chem. Soc. 2010, 132, 11364– 11371.


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Figure 1. (a) Molecular structure of the DBA derivatives used in this study. (b) Definition of the two enantiomers, referred to as S-DBA and R-DBA, of a surface-confined prochiral DBA molecule based on the tilting of its alkyl chains versus the DBA core. S-DBA and R-DBA are differentiated by blue and golden colors. (c) Parity-related discrepancy in the oblique angles.34 The red solid lines connect the center of two triangular cores while the black dashed lines indicate the orientation of main symmetry axis of the underlying graphite lattice. (d) Schematic illustration of a conventional pathway for chiral phase transition in DBA monolayers. ‘Homochiral’ and ‘racemic’ refer to the monolayer chirality in a domain. Given step 1 has been well demonstrated in previous studies,9,13,30,35 our focus will be on the latter two steps, in particular step 2.


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Figure 2. (a) Schematic illustration of the origin of molecular chirality upon surface confinement (R and S): structural transformation of a DBA molecule, from D3h symmetrical in vacuo to C3 symmetrical on a surface, to match the substrate registry. Ideally the offset of parallel aligned alkyl chains by one methylene group will bring a tilting angle of about 7.5o (b) Definition of monolayer chirality ((+) and (−)) by four possible chiral interdigitations that can be adopted in an enantiomorphous DBA domain. The black arrows indicate the orientation of terminal methyl groups. (c) STM image (size: 12.5 nm × 12.5 nm) and a structural model of a (+)-type nanoporous phase obtained from DBA-OC13, presumably formed by the S-DBA enantiomer.


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Figure 3. (a, b, c) Representative STM images showing regular DBA-OC12 surface networks upon hot deposition at different concentrations. Phases I-IV are indicated, and the white curving line in c highlights the domain border that separates two different phases. Large scale and supplementary STM images in which molecular chirality is assigned are displayed in Figure S1S3. (d) Surface coverage variation of different phases in response to DBA concentration in the supernatant solution. (e-h) STM images of phases I–IV. Symmetry axes of the underlying graphite lattice and the unit cell of each phase are indicated. (i-l) Tentative structural models for phases I–IV. S- and R-enantiomers are highlighted in blue and gold, respectively. In the latter two models, red and green translucent parallelograms mark SS(−) and RR(+) chirality interdigitations, respectively.


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Figure 4. Schematic illustration of all the phases observed for DBA-OCn (n = 11-17, 20). RDBA and S-DBA are differentiated with golden and blue colors. The monolayer chirality of each phase in a domain is indicated as homochiral (H in green circles), enantiomeric excess (E in blue circles) and racemic (R in red circles). The black parallelograms outline unit cells of each phase. Note that the phase V is short of long-range periodicity, therefore an idealized structural model is provided. For all the phases exist as two mirror image forms, only one chiral lattice is displayed for clarity.


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Figure 5. (a) High-resolution STM image and (b) corresponding structural model of enantiomorphous phase III of DBA-OC13. (c) High-resolution STM image and (d) structural model of enantiomeric excess phase III of DBA-OC14. Symmetry axes of underlying graphite lattice and unit cell of each phase are indicated. S- and R-enantiomer are highlighted in blue and golden colors, respectively. In the structural models, green and red translucent parallelograms are employed to indicate preferential RR(−) (for odd-DBAs) and SS(−) (for even-DBAs) chirality,


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while the black translucent parallelograms are used to map out the mismatched SS(+) interdigitations of DBA-OC14.

Figure 6. (a) Cartoon illustration of the homochiral lattice of DBA-OC13 and racemic lattice of DBA-OC12. α and β are oblique angles of vector a and b with respect to substrate lattice. (b) Illustration of the discrepancy of preferential (SS(−)) and mismatched (SS(+)) interactions of even-DBAs. The black bold lines connect the centers of two triangular cores and the green dashed lines indicate the graphite reference axis. d1 and d2 are the distances between two triangular cores along the main symmetry axis of graphite lattice.


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Figure 7. (a) STM image of phase V of DBA-OC17. The red and green ellipses outline the (+)type and (−)-type interdigitations, respectively. Handedness of the molecules involved are also indicated. (b) Schematic illustration of the heterochiral interdigitations of DBA-OC17. The substitution of R-enantiomers (in golden, top image) with S-enantiomers (in blue), or vice versa (bottom image), reduces the distance between two substituted enantiomers while at the same


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time avoids repulsion between their terminal methyl groups. (c, d) STM image and structural model of close-packed phase VI formed by DBA-OC17.

Figure 8. (a) Illustration of the diastereomeric interactions between DBA-OC15 and DBAOC16. (b, c) Close-up and large scale STM images of homochiral (−)-type phase III of an equimolar mixture of DBA-OC15 and DBA-OC16. (d) Tentative structural model for the binary monolayer of DBA-OC15 and DBA-OC16. S-DBA-OC15 and R-DBA-OC16 are highlighted in blue and golden color, respectively.


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TOC Graphic:


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