Complex Chiral Induction Processes at the Solution/Solid Interface.

Two-dimensional supramolecular chirality is often achieved by confining molecules against a solid surface. The sergeants-soldiers principle is a popular strategy to fabricate chiral surfaces using predominantly achiral molecules. In this method, achiral molecules (the soldiers) are forced to assemble in a chiral fashion by mixing them with a small percentage of structurally similar chiral molecules (the sergeants). The full complexity of the amplification processes in chiral induction studies is rarely revealed due to the specific experimental conditions used. Here we report the evolution of chirality in mixed supramolecular networks of chiral and achiral dehydrobenzo[12]annulene (DBA) derivatives using scanning tunneling microscopy (STM) at the solution/solid interface. The experiments were carried out in the high sergeants-soldiers mole ratio regime in relatively concentrated solutions. Variation in the sergeants/soldiers composition at a constant solution concentration revealed different mole ratio regimes where either amplification of supramolecular handedness as defined by the sergeant chirality or its reversal was observed. The chiral induction/reversal processes were found to be a convolution of different phenomena occurring at the solution-solid interface namely, structural polymorphism, competitive adsorption and adaptive host-guest recognition. Grasping the full complexity of chiral amplification processes as described here is a stepping-stone toward developing a predictive understanding of chiral amplification processes.

Introduction

One of the critical steps in the evolution of chiral surface science involves reproducible fabrication of homochiral surfaces. Such chiral surfaces have important applications in separation of enantiomers,[1,2] enantioselective heterogeneous catalysis[3−5] and nonlinear optics.[6] A desirable attribute is to have chiral corrugations on a nanometer scale to match with the molecular dimensions. Self-assembly of chiral molecules on solid substrates has emerged as a straightforward way to produce such nanostructured homochiral surfaces.[7,8] Self-assembly of prochiral molecules also leads to formation of enantiomorphous mirror image domains but the monolayer remains overall racemic due to equal surface coverage of opposite handed domains. The chiral balance of such racemic monolayers can be altered by merging a small percentage of structurally similar chiral molecules in them. This strategy is widely known as the sergeants–soldiers principle wherein the chirality of the sergeants determines the handedness of the supramolecular network predominantly made up of achiral molecules.[9−12]

Although the pioneering work[13] on the sergeants–soldiers principle was carried out using macromolecular assemblies in solution, the first of such experiments on a solid surface was accomplished under ultrahigh vacuum (UHV) conditions using relatively small molecules namely, tartaric acid (sergeants) and succinic acid (soldiers).[14] The improvement in the tools of surface science over the past decade has enabled the study of larger and relatively complex molecular systems. A number of such investigations have been made possible by the widespread use of scanning tunneling microscopy (STM), both under UHV conditions[15−17] as well as at the solution-solid interface,[18,19] which allows imaging of surfaces with high spatial resolution.

The complexity of the chiral assembly process increases significantly when working at the solution-solid interface due to the dynamic nature of the interface.[12,20] Because of the possibility of in-plane and out-of-plane dynamics facilitated by the presence of an organic solvent, molecular systems self-assembling at the solution-solid interface often exhibit a variety of unique phenomena such as concentration dependent structural polymorphism,[21−25] competitive adsorption,[26−29] and guest induced structural transitions.[30−33] Since self-assembling systems have access to multiple evolution pathways at the solution-solid interface, deciphering experimental factors that influence the formation of chiral surfaces at this interface is vital to the overall understanding of chiral induction phenomena.[34]

In this contribution, we describe the results of a series of chiral induction experiments encompassing complex processes that are unique to the solution-solid interface. The experiments were essentially carried out by varying the stoichiometric balance of a sergeants–soldiers mixture and monitoring the changes in the morphology (Table ) and chiral composition of the self-assembled network on a solid surface. In contrast to previously reported systems,[14,15,35] here we have studied high sergeants to soldiers mole ratios in relatively concentrated solutions. We identify three mole ratio regimes where either amplification of the supramolecular handedness (as defined by the sergeant chirality) or its reversal was observed. Furthermore, the chiral amplification (reversal) processes described here span two different structural patterns and are a convolution of three factors namely structural polymorphism, competitive adsorption/desorption, and adaptive recognition of the supramolecular network in response to guest molecules.

Table 1

Variation in the Morphology as well as the Chiral Composition of the System as a Function of Changes in the Stoichiometry (C = 5 × 10-4 M)a

		chiral composition
system	mol % cDBA	% “+” type (CCW)	% “–” type (CW)	morphology
DBA–OC13	0	52 ± 7	48 ± 7	high-density
DBA–OC13/cDBA–OC13(R)	5	82 ± 6	18 ± 6	high-density
DBA–OC13/cDBA–OC13(R)	30	98 ± 1	2 ± 1	high-density
DBA–OC13/cDBA–OC13(R)	50	88 ± 7	12 ± 7	high-density (M) + quasi honeycomb with guests
DBA–OC13/cDBA–OC13(R)	70	0	100	quasi honeycomb with guests
DBA–OC13/cDBA–OC13(R)	90	0	100	low-density with guests
DBA–OC13/cDBA–OC13(R)	95	0	100	low-density with guests
DBA–OC13/cDBA–OC13(R)	99	100	0	low-density without guests
cDBA–OC13	100	100	0	low-density (M) + high-density

The majority phase at a given stoichiometric composition is denoted by (M) when there is more than one structure formed on the surface. The low-density structures are further classified as with and without immobilized guests. In view of relatively high concentrations used, cDBA guests always interact with the nanowells even when they are not immobilized.

The chiral induction experiments were carried out using alkoxy substituted dehydrobenzo[12]annulene (DBA, Scheme , parts A and B) molecules. DBAs are one of the most intensively studied building blocks at the solution-solid interface.[36,37] Various aspects of DBA self-assembly on solid surfaces, namely, concentration dependent structural polymorphism,[21] host–guest chemistry,[38] odd–even effects,[39] chiral induction processes[34,35] and stimulus responsive behavior,[40,41] have been studied in detail in the recent past. DBA derivatives are known to form two different supramolecular networks depending on their concentration in solution.[21] Higher concentrations favor the formation of a densely packed network whereas dilute solutions lead to a low-density honeycomb structure. Both low- as well as high-density networks are sustained by van der Waals interactions between interdigitated alkoxy chains of neighboring molecules.[37] In contrast to the low-density network where all the alkoxy chains are adsorbed on the surface, the high-density structure consists of molecules with only four or five chains on the surface. The relative alignment of the four interdigitated chains per DBA pair (“+” or “–” type interdigitation) governs the handedness of the resultant network.[35] For the low-density network, a virtual “clockwise” (CW) or “counterclockwise” (CCW) nanowell is obtained by combining six “–” type or six “+” type interdigitation patterns, respectively (Scheme C and 1D).

Scheme 1

(A) Molecular Structures of Achiral and Chiral DBA Derivatives Used in This Study, (B) Molecular Models of the Two DBA Derivatives, and (C, D) Molecular Models Showing the Alkyl Chain Interdigitation Motifs That Form the Basis of the Low-Density (C) and the High-Density (D) Network of DBA

For clarity, the carbon atoms of achiral and chiral DBAs are displayed in different colors. In addition, the methyl groups of cDBA–OC13(R) are colored black.

The definitions for + type and – type handedness are based on these interdigitation motifs. Red arrows in part D indicate locations of alkyl chains that are desorbed from the surface in the high-density networks.

The results described in this contribution originate from the sergeants–soldiers experiments aimed at achieving chiral induction in the high-density phase of DBA molecules. While sergeants–soldiers based induction has been demonstrated for the low-density honeycomb phase of DBAs,[35] the applicability of the sergeant design for homochiral induction in the high-density phase has not been tested. Besides, chiral induction experiments that span two different structural polymorphs (using the same sergeants–soldiers pair) have not been reported yet. The present study was initiated to complement previous results obtained on sergeants–soldiers chiral induction in the low-density phase of DBAs.[35]

Methods

Synthesis DBA–OC13 and cDBA–OC13(R)

The DBA derivatives used in this study were synthesized according to a previously reported method.[35,47]

STM Measurements

All STM measurements were performed at room temperature (20–23 °C) using either a PicoLE or a PicoSPM (Molecular imaging/Agilent, now Keysight) machine operating in constant–current mode with the tip immersed in the supernatant liquid. STM tips were prepared by mechanical cutting from Pt/Ir wire (80%/20%, diameter 0.2 mm). Octanoic acid (Sigma-Aldrich, 99% used as received) was used as a solvent to dissolve DBA–OC13 and cDBA–OC13(R). Separate stock solutions (C = 5 × 10–4 M) were prepared for both the DBA derivatives by weighing appropriate amount of solid compound. The solutions for chiral induction experiments were prepared by mixing the stock solutions in appropriate proportions. Prior to imaging, 15 μL solution was applied onto a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, grade ZYB, Advanced Ceramics Inc., Cleveland, OH) held at 80 °C. The sample was maintained at 80 °C for 3 min after deposition. The loss of solvent and thus the changes in the concentration upon annealing were negligible. The sample showed weight loss of 3% 8 h after hot deposition. After this, the sample was allowed to cool down to room temperature. The cooling down to room temperature occurs within 1 min after removal of the sample from the hot plate. The STM measurement was commenced once the sample reached room temperature. For analysis purposes, recording of a monolayer image was followed by imaging the graphite substrate underneath. This was done under the same experimental conditions but by lowering the substrate bias (typically Vbias = −1 mV) and increasing the tunnelling current (typical Iset = 800–900 pA.) The images were corrected for drift via Scanning Probe Image Processor (SPIP) software (Image Metrology ApS), using the graphite lattice, allowing a more accurate unit cell determination. The images are low-pass filtered. The typical imaging parameters for both DBA derivatives are tunneling current (I) = 250–300 pA and sample bias (V) = −200–250 mV.

Data Analysis

The percentage distribution of handedness (“+” or “–” type) on the surface was determined as an average based on more than at least two experimental sessions. Within each session, at least 30 images were obtained by moving the sample plate (after every six images) by a few millimeters. Weighted mean and weighted standard deviation were calculated according to a method mentioned before.[45]

Results and Discussion

A DBA derivative with six peripheral tridecyloxy chains (DBA–OC13) was chosen for this study together with its chiral analogue, cDBA–OC13(R) (Scheme A). Parts A and C of Figures show STM images of the self-assembled network formed by DBA–OC13 and cDBA–OC13(R) upon deposition from an octanoic acid solution (C = 5 × 10–4 M) on highly oriented pyrolytic graphite (HOPG), respectively (Figure S1). While the surface is predominantly occupied by a rather disordered high-density network in the case of DBA–OC13 (Figure A), the chiral analogue formed a mixture of relatively ordered high- and low-density structures (Figure C). Careful inspection of STM images revealed that the DBA–OC13 network contains both “+” as well as “–” type interdigitation patterns whereas cDBA–OC13(R) self- assembly is characterized by exclusive “+” type interdigitation pattern. cDBA–OC13(R) and chiral DBA derivatives in general, exhibit distinctive STM contrast with a dark contour around the aromatic core (Figure E). This characteristic feature has been used to identify cDBAs from achiral DBA derivatives in surface adsorbed monolayers.[35] Further statistical analysis revealed that the percentage of densely packed DBA molecules at this concentration is 84% and 59% for the DBA–OC13 and DBA–OC13(R) system, respectively. This analysis validates our previous observation that the chiral DBAs form a low-density honeycomb network at relatively higher concentrations than the corresponding achiral analogues.[42]

Figure 1

STM images of the self-assembled networks formed by (A) DBA–OC13 and (C) cDBA–OC13(R) upon deposition of the octanoic acid (C = 5 × 10–4 M) solution at room temperature. Representative STM images provided in parts B and D show the change in the surface morphology upon depositing the solutions at 80 °C for DBA–OC13 and cDBA–OC13(R), respectively. The white line in part B highlights the domain border that separates two opposite handed domains of the high-density phase. Graphite symmetry axes are displayed in lower left corners of parts B and C. Scale bars in parts A–D = 5 nm. (E) Digital zoom of the area marked by dashed rectangle from part C. Black arrows show the dark contours around the annulene cores. The black contour appears only at one edge of the triangular core when the molecule is a part of the high-density network. (F) Plot of the percentage of densely packed DBA molecules for the two systems at room temperature and 80 °C. For large scale STM images, see Figure S1 in the Supporting Information.

The “Hot Deposition” Protocol

In order to obtain uniform surface coverage of the high-density network and to increase the size of the molecular domains, we employed a slightly different sample preparation protocol where the DBA solution was applied to HOPG substrate held at an elevated temperature. Systematic experiments where different deposition temperatures were explored (Figure S2), revealed that deposition of the DBA solution on HOPG held at 80 °C followed by continued heating at 80 °C for 3 min furnished the best results. It must be noted that STM data were always obtained on a sample cooled back to room temperature although the solution was deposited at 80 °C. All the experiments described in this paper were carried out using this protocol, unless mentioned otherwise.

Parts B and D of Figure , respectively, show representative STM images of DBA–OC13 and cDBA–OC13(R) monolayers obtained by using the aforementioned “hot deposition” protocol (also see Figure S1). Although both samples showed improved order and increase in domain sizes, only the DBA–OC13 sample provided 100% surface coverage of the high-density phase upon annealing. On the contrary, the surface coverage of densely packed molecules decreased significantly from 59% to 16% for cDBA–OC13(R) with concomitant increase in that of the low-density phase (Figure F). We attribute this discrepancy to the fact that the formation of a high-density pattern involves out-of-plane bending and desorption of one or two alkoxy chains, which may be less favorable for cDBA–OC13(R), due to steric hindrance caused by the stereogenic centers. Molecular model provided in Scheme D clearly shows that the DBA molecules in the high-density pattern are closely packed and there is virtually no space for adsorption of all the alkoxy chains. Furthermore, it has been established that adsorption energy per unit area is a critical parameter that determines the stability of a self-assembled structure at the solution-solid interface. Considering this aspect, the difference in energy per unit area in going from the high- to the low-density phase is expected to be larger for the cDBA derivative compared to achiral DBA. This is due to the lower adsorption energy of cDBAs compared to achiral DBAs. As a consequence, the formation of high-density phase is disfavored for chiral DBA derivatives compared to achiral ones.[42]

The annealing protocol merely changed the morphology of the sample while the chiral composition of the two systems remained the same. This means that the cDBA–OC13(R) network was exclusively made up of “+” type interdigitation pattern whereas DBA–OC13 network remained globally racemic with equal surface coverage of “+” and “–” type interdigitation patterns after deposition on the hot substrate.

Chiral Amplification: High-Density Phase

A series of chiral induction experiments were carried out by mixing the octanoic acid solutions of DBA–OC13 and cDBA–OC13(R) at different mole ratios at a constant overall concentration (C = 5 × 10–4 M). Addition of 5 mol % cDBA–OC13(R) to the DBA–OC13 solution caused significant imbalance in the chiral composition of the system producing 82 ± 6% “+” type high-density phase on the HOPG surface. Further increasing the mole ratio of cDBA–OC13(R) to 30% resulted in a virtual homochiral surface with 98 ± 1% “+” type high-density pattern (Figure A, 2B). These results are comparable to the sergeants–soldiers experiments carried out at lower concentrations where the induction occurs in the low-density honeycomb phase. A peculiar difference however is the surface coverage of cDBA molecules. In the previously reported chiral induction experiments for the low-density phase where the sample was prepared at room temperature, the cDBA molecules remained in the monolayer and their surface coverage could be quantified.[35] In the present case, however, we could not find any evidence of cDBA molecules on the surface, which as mentioned before, can be identified from their characteristic appearance in STM images. This observation indicates desorption of the cDBA–OC13(R) molecules during the annealing step.

Figure 2

(A) STM image of the surface obtained upon deposition of a solution of DBA–OC13 containing 30 mol % cDBA–OC13(R) in octanoic acid. Scale bar =20 nm The surface becomes virtually homochiral with large domains that extend several hundred square nanometers. (B) Corresponding high-resolution image. Scale bar =3 n.m. Graphite symmetry axes are displayed in the lower left corner. (C) Chiral induction plot showing variation in the percentage of the “+”/“–” type interdigitation pattern in response to change in the mole ratio at a constant overall concentration of 5 × 10–4 M. (D) Quasi honeycomb structures observed at 50 mol % cDBA–OC13(R). Dashed blue circles highlight the nanowells occupied by guest molecules. Scale bar = 3 nm.

So at what point in the assembly process does the chiral induction take place? A plausible answer is that the chiral information transfer occurs at the nucleation stage. When the sergeants–soldiers solution is deposited onto a hot HOPG surface, cDBA–OC13(R) molecules dynamically coassemble with DBA–OC13 at the point of nucleation thereby transferring chiral information (“+” type interdigitation) to the supramolecular network. This information is amplified in the subsequent growth step leading to the formation of large homochiral domains. The cDBA–OC13(R) sergeants desorb from the surface in the meantime. Preferential desorption of cDBA molecules in the presence of achiral analogues is known to occur due to their relatively lower adsorption energy.[34] Furthermore, the coadsorption of cDBA molecules within the high-density network of achiral DBAs is expected to be energetically expensive due to unfavorable steric effects of the chiral side groups. A plausible explanation for such virtually homochiral surface devoid of chiral sergeants is the chiral memory effect which has previously been used for describing the preservation of global chirality after removal of the chiral auxiliary.[43,44]

Phase Transition and Chiral Reversal: Low-Density Phase

A plot of the percentage of “+” type versus “–” type high-density pattern in response to variation in the cDBA–OC13(R) mole fraction is displayed in Figure C. The slight decrease in the percentage of “+” type high-density pattern at 50% cDBA–OC13(R) is caused by the presence of porous structures on the surface (Figure D). Such domains consist of isolated hexagonal nanowells close packed together with small patches of honeycomb network. The morphology is fundamentally different from the typical honeycomb network formed by DBAs (see Figure S3 for comparison). We call these structures “quasi honeycomb” and their surface coverage is around 4.5%. Close inspection of STM images revealed that the interdigitation pattern in such low-density domains is exclusively “–” type, which contributed to the small decrease in the induction plot observed at 50 mol %.

Considering that cDBA–OC13(R) always adapts “+” type interdigitation and the porous phase described above does not contain “black contour” molecules in the network, we can exclude the possibility of cDBA–OC13(R) coadsorbed alongside the achiral DBA molecules as a part of such quasi honeycomb network. The nanowells appear to be occupied (blue dashed circles, Figure D) and based on the previous results reported on this system, we assign the guest molecules to be cDBA–OC13(R).[45] We have reported in the past that chiral DBAs adsorb in nanowells by adapting a windmill like conformation through in plane bending of the alkoxy chains.[45] Such conformation allows better van der Waals contact of the cDBA molecule with the HOPG substrate by bending the chiral methyl groups away from the solid surface. Furthermore, cDBAs show a pronounced tendency to adsorb in nanowells with handedness that is opposite to the one they induce on the surface. Thus, cDBA–OC13(R) preferentially adsorbs as a guest in the CW nanowells (“–” type interdigitation) whereas it induces formation of CCW nanowells (“+” type interdigitation) acting as a sergeant. The absolute concentration of DBA–OC13 alone at this mole ratio is 2.5 × 10–4 M. A noteworthy point is that at this concentration, DBA–OC13 is readily expected to form a high-density network upon hot deposition (Figure S4).

The experimental facts presented above strongly suggest that the formation of the porous structures with “–” type interdigitation pattern is a direct consequence of cDBA adsorption in the nanowells. However, the surface coverage of such structures is relatively low despite the presence of 50 mol % of cDBA–OC13(R) in solution. In order to probe the evolution of the surface morphology as a function of annealing time, a control experiment was carried out by reducing the annealing time from 3 min to 5 s after hot deposition. STM images of the surface obtained in this fashion show that both high- as well as low-density structures are present on the surface in the initial stages of self-assembly (Figure S5). The low-density structures are however removed from the surface upon extended annealing.

Increasing the mole fraction of cDBA–OC13(R) to 70% caused a drastic change in the morphology where 90% of the surface was occupied by the honeycomb network (Figure S6). This morphological transition was also accompanied by reversal in the chiral composition of the system as all the nanowells were found to be with CW handedness (“–” type interdigitation). A further increase in the mole fraction of the chiral DBA to 90% lead to complete removal of the high-density structure from the surface, which was replaced by the low-density honeycomb structure with CW nanowells (Figure A). The CW nanowells are always filled by the chiral guests. In some high-resolution STM images, the CCW windmill like conformation of the guest molecule inside the nanowell could be identified (Figure B). The chiral composition of the system remains the same (CW nanowells) up to 95 mol % of cDBA–OC13(R) (Figure S7). A unique aspect is that, despite the large excess of cDBA–OC13(R) in the solution phase, the surface composition is still dominated by adsorption of the achiral DBA–OC13 indicating a high degree of preferential adsorption in this system under the specific experimental condition used. cDBA–OC13(R) merely occupies the nanowells as a guest.

Figure 3

(A) Surface coverage (θ) of the porous structure in response to changes in the mole % of cDBA–OC13(R) in solution. (B) Representative STM image of the surface obtained after deposition of a solution containing 90 mol % cDBA–OC13(R) and 10 mol % DBA–OC13. Inset shows a CW nanowell containing a tightly bound guest molecule. Scale bar = 10 nm. (C) STM image of the porous network obtained from a solution containing 99 mol % cDBA–OC13(R) and 1 mol % DBA–OC13. Scale bar = 10 nm. Inset shows a CCW nanowell containing two cDBA–OC13(R) molecules showing the characteristic “black contour” feature. The nanowells appear fuzzy due to short residence time of molecular (mostly achiral DBA) guests.

An overview of the results presented so far reveals that the presence of the chiral DBA in the solution not only brings about a structural transition in the supramolecular network but it also strongly biases the chiral composition of the system. Molecular guests have been known to cause structural changes in supramolecular networks.[30−33] Typically it is assumed that the coadsorption of guest molecules within the voids of the low-density network compensates for the thermodynamic penalty associated with the formation of less dense structures. Such dynamic reconstitution of the supramolecular networks occurs only if there is substantial in-plane and out-of-plane dynamics possible. In the present case, such dynamics is enhanced due to deposition of the solution at elevated temperatures. The uniqueness of results discussed above is that the coadsorption of the cDBA guests is enantiospecific and thus it also strongly influences the handedness of the DBA–OC13 host network. The cDBA–OC13(R) molecules form a tightly bound complex with the host nanowells. Molecular mechanics calculations performed on host–guest complexes formed by cDBA–OC12(R)/DBA–OC12 model pair revealed that adsorption in the CW nanowells is favored by 6 kcal mol–1.[45] The origin of this energy difference lies in the van der Waals interactions between the alkyl chains of the chiral guest and those of host network. The van der Waals contact between the chains is optimal when the handedness of the host nanowell (CW) does not match with the windmill-like conformation of the guest (CCW) (Scheme B).

Scheme 2

Schematics Showing (A) the Sergeants–Soldiers and (B) the Enantioselective Host–Guest Chiral Induction Pathway and (C) a Summary of the Chiral Induction Experiments

This simple schematic essentially shows changes in the morphology as well as chiral composition of the system upon changing the stoichiometric balance at a constant overall concentration. The system goes through a chiral induction followed by guest induced phase transition. The phase transition is also accompanied by reversal of handedness as evident from the change in the interdigitation pattern. Increase in the cDBA mole fraction further causes another reversal in supramolecular handedness.

Another notable aspect is that, in contrast to the typical guest molecules known to cause structural transitions in supramolecular networks, cDBAs are somewhat unusual guests. In fact, they actually belong to the category of molecules that form the host network. The propensity to form such unique windmill-like conformation arises due to the presence of chiral side chains and specifically from the methyl groups at the stereogenic centers. When cDBA adsorbs as a sergeant (as a part of honeycomb network), every alternating alkyl chain is adsorbed with the stereogenic methyl group facing the HOPG substrate due to the identical absolute configuration of the chiral side chains. However, in the windmill like conformation, twisting of the side chains is favored, because such change in the alkyl chain conformation allows all methyl groups to point away from the HOPG surface, thus effectively reducing the steric repulsion with the substrate (Scheme ). Whether the part of the alkyl chain after the stereogenic center twists to right [cDBA with (S) side chains] or left [cDBA with (R) chains] is determined by the absolute chirality of the stereogenic center.

Scheme 3

Tentative Molecular Models Illustrating the Difference in the “Adsorption Face” of the cDBA–OC13 Molecule When It Adsorbs on the HOPG Surface as a Sergeant versus When It Undergoes Enantioselective Adsorption in the Nanowells as a Chiral Guest

Note that three stereogenic methyl groups point to the HOPG surface when the molecule acts as a sergeant thereby reducing the van der Waals contact between extended alkoxy chains and the HOPG substrate. On the other hand, the windmill-like conformation favors twisting on the alkoxy chains which allows stereogenic methyl groups to point away from the HOPG surface thus allowing favorable van der Waals contact with the substrate.

Chiral Reversal: Low-Density Phase

Finally, a solution containing 99 mol % cDBA–OC13(R) gave rise to a second, total reversal of the surface handedness with exclusive surface coverage of CCW nanowells. The cDBA–OC13(R) molecules could be readily identified in the low-density honeycomb network due to their STM contrast (Figure C). Large excess in solution concentration partially compensates for the preferential adsorption of DBA–OC13, thus bringing cDBA–OC13(R) molecules on the surface. The adsorption of cDBA–OC13(R) as a part of the honeycomb network (as sergeants) is accompanied by chiral induction as defined by the chirality of the sergeant (“+” type, CCW). While the nanowells in this network do not show presence of well-defined immobilized guests, their fuzzy appearance indicates that excess molecules in the solution phase interact with the voids in a transient fashion.

The findings described above were certainly aided by the choice of molecular system as well as experimental parameters. The choice of DBA–OC13 and cDBA–OC13(R) ensured that the high-density structures were accessible. DBAs with longer alkoxy chains tend to form high-density structures relatively easily compared to those with shorter alkoxy chains.[21] While achiral DBA derivatives with alkoxy chain lengths up to – OC30H61 have been synthesized,[46] the choice of DBAs with chiral side chains is limited. Furthermore, octanoic acid was chosen to promote the formation of the high-density network because 1,2,4-trichlorobenzene and 1-phenyloctane are known to favor formation of the porous network.[47] The solubility of DBA–OC13 in 1-octanol on the other hand, was found to be relatively low. The temperature treatment was also essential as without it, ordered network formation was not possible. The evaporative loss of octanoic acid during the “hot deposition” process was minimal (less than 5%) thus causing negligible changes in the solution concentration (Figures S9–S11). Also, we resorted to an ex-situ annealing protocol instead of heating the sample in situ using a liquid cell due to technical challenges associated with the use of octanoic acid.

The gradual transformations discussed above are a convolution of three phenomena that occur during self-assembly at the solution-solid interface namely, preferential adsorption, structural polymorphism, and adaptive recognition of the supramolecular network toward the presence of guest molecules, where the sergeants act as guests. cDBA–OC13(R) molecules influence the handedness of the surface in two distinct ways: either via incorporation into the supramolecular network as a sergeant at different stages during the self-assembly process or by adsorbing into the nanowells of the supramolecular network formed by the achiral analogue (Scheme A,B). Notably, they induce opposite surface handedness in the two roles described above. Whether the cDBA molecule acts as a sergeant in a sergeants–soldiers pathway or as guest in an enantioselective host–guest pathway is determined by the stoichiometric composition of the system.

Recent results obtained on the cDBA–OC12(S)/DBA–OC12 sergeants–soldiers system deserve a special mention here.[34] The evolution of this system was studied by carrying out in situ temperature dependent STM measurements at the 1-phenyloctane/HOPG interface. In this study, changes in the chiral composition of the system were monitored by varying the total concentration at constant sergeants to soldiers (10:90) mole ratio. In contrast to the results discussed in the present study, the concentrations were chosen such that the low-density network was always obtained. The chiral composition of the system was found to depend on the total concentration with amplification of supramolecular handedness (defined by sergeant chirality) observed at low concentrations and reversal at higher concentrations. The chiral reversal observed was explained on the basis of enantioselective host–guest adsorption in nanowells. While there exist some similarities in the reported results on cDBA–OC12(S)/DBA–OC12 system[34] and those presented here, the present results involved fundamentally different type of experiments carried out on relatively more complex system.

Conclusions

Although the fabrication of chiral supramolecular surfaces using the sergeants–soldiers strategy has become a fairly routine practice, the experimental approach remains rather conservative. Most examples reported to date have dealt with low sergeants/soldiers mole ratios in dilute solutions. Such specific experimental conditions were often chosen for practical considerations. In the results described above, we have followed a somewhat unconventional path by accessing higher sergeants/soldiers mole ratios in relatively concentrated solutions. Thanks to the choice of the self-assembling system, the outcome has been equally unconventional. Not only did the system evolve through multiple morphologies, its chiral composition changed drastically in response to changes in the sergeants/soldiers mole ratio in solution. We have illustrated in detail how the unusual chiral reversals observed in the self-assembling system are a direct consequence of the opposite trends for chiral induction and enantioselective adsorption.

The results described here illustrate the level of complexity in self-assembling systems and how one can use it to advantage in creating chiral surfaces. Such insight into chiral induction processes is promising for the development of rational design strategies of chiral templates. Furthermore, while the vast majority of published efforts have focused on the fabrication of chiral surfaces, there is comparatively little known about enantioselective adsorption on such surfaces. The present system offers a beacon of hope in that direction, as we have not only realized enantioselective adsorption but also we could also use it to control the morphology as well as the chiral composition of an achiral system.