Chiral Cyclobutane β-Amino Acid-Based Amphiphiles: Influence of Cis

Article pubs.acs.org/Langmuir

Chiral Cyclobutane β‑Amino Acid-Based Amphiphiles: Influence of Cis/Trans Stereochemistry on Condensed Phase and Monolayer Structure Alessandro Sorrenti,† Ona Illa,† Rosa M. Ortuño,*,† and Ramon Pons*,‡ †

Departament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain Departament de Tecnologia Química i de Tensioactius, Institut de Química Avançada de Catalunya, IQAC−CSIC, c/Jordi Girona 18-26, 08034 Barcelona, Spain

‡

S Supporting Information *

ABSTRACT: New diastereomeric nonionic amphiphiles, cis- and trans-1, based on an optically pure cyclobutane β-amino ester moiety have been investigated to gain insight into the influence exerted by cis/trans stereochemistry and stereochemical constraints on the physicochemical behavior, molecular organization, and morphology of their Langmuir monolayers and dry solid states. All these features are relevant to the rational design of functional materials. trans-1 showed a higher thermal stability than cis-1. For the latter, a higher fluidity of its monolayers was observed when compared with the films formed by trans-1 whose BAM images revealed the formation of condensed phase domains with a dendritic shape, which are chiral, and all of them feature the same chiral sign. Although the formation of LC phase domains was not observed by BAM for cis-1, compact dendritic crystals floating on a fluid subphase were observed beyond the collapse, which are attributable to multilayered 3D structures. These differences can be explained by the formation of hydrogen bonds between the amide groups of consecutive molecules allowing the formation of extended chains for trans-1 giving ordered arrangements. However, for cis-1, this alignment coexists with another one that allows the simultaneous formation of two hydrogen bonds between the amide and the ester groups of adjacent molecules. In addition, the propensity to form intramolecular hydrogen bonds must be considered to justify the formation of different patterns of hydrogen bonding and, consequently, the formation of less ordered phases. Those characteristics are congruent also with the results obtained from SAXS−WAXS experiments which suggest a more bent configuration for cis-1 than for trans-1.

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INTRODUCTION

depict due to their nondirectional nature, although they play a crucial role in controlling the aggregate structures.8−11 As a consequence, it remains a great challenge to predict the final outcome of self-assembly in water. Many investigations focus on elucidating the mechanisms by which the structural information is translated from the molecular to the supramolecular level, with the aim to define new design principles and finding new scaffolds and building blocks. Amphiphiles are the most suited candidates to investigate the complexity of self-

The precise control of the supramolecular organization is a crucial prerequisite when the preparation of functional nanostructures by a “bottom-up” approach is pursued.1−3 In the case of polymolecular self-assemblies in water, such as those formed by amphiphilic monomers, the aggregate structure and organization are determined by a complex weave of molecular recognition events based on a delicate balance of noncovalent interactions (such as hydrogen bonding, electrostatic and hydrophobic interactions) and specific steric effects (stereochemical constraints, diastereomeric interactions).4−7 In particular, the hydrophobic interactions are rather difficult to © XXXX American Chemical Society

Received: April 15, 2016 Revised: June 1, 2016

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overall organization in the bulk solid phase using SAXS−WAXS in order to compare the conformation of those molecules under the conditions of extreme packing with their bidimensional selfassembly.

assembly in water; in fact, they are known to form assemblies with variated structure ranging from simple micelles to highly organized polymolecular aggregates (e.g., fibers, tubes, and helices).4,5,12−14 An additional interest in studying and designing new amphiphiles arises from their growing applications in different fields, such as materials chemistry, nanotechnology, and medicinal chemistry, among others.13,15−20 Recently we reported on the preparation of new anionic chiral amphiphiles based on the cyclobutane β-amino acid scaffold and on the investigation of their self-assembly in diluted water solution as well as in the liquid crystal domain.21 Remarkably, we found that the cis/trans stereochemistry, joined with the rigidity of the four-membered ring, have a dramatic effect on the molecular organization and supramolecular structure of micelles, fibers, and mesophases they form as well as on the enantioselection ability of their micellar aggregates for bilirubin enantiomers. These were ascribed to the influence that the stereochemical constraints have on the head-charge stabilization, molecular packing, and propensity to form hydrogen-bond patterns within the assemblies.21 Unnatural cyclobutane β-amino acids have been also used as versatile scaffolds for the preparation of oligopeptides,22 gelators,23,24 and organic conductors,25 in which the cis/trans stereochemistry determines the possibility of forming specific secondary structures (e.g., extended vs helical conformations) and supramolecular architectures (e.g., superhelices).26 Herein, we investigate new diastereomeric nonionic amphiphiles based on a cyclobutane β-amino ester moiety (amphiphiles 1 in Chart 1) showing how the interplay between

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EXPERIMENTAL SECTION

Preparation of Amphiphiles 1. Diastereomeric esters cis-1 and trans-1 (Chart 1) were prepared in enantiomerically pure form as previously reported.21 The compounds were purified by flash chromatography on neutral silica gel (230−400 mesh) using EtOAc/hexanes 3:7 as eluent. Following crystallization from CH2Cl2/pentane (vapor diffusion) yielded white crystalline solids that were used for Langmuir isotherm measurements. Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Scattering (WAXS). SAXS and WAXS measurements were carried out using a S3-MICRO (Hecus X-ray systems GMBH Graz, Austria) coupled to a GENIX-Fox 3D X-ray source (Xenocs, Grenoble), which provides a detector focused X-ray beam with λ = 0.1542 nm Cu Kα line with more than 97% purity and less than 0.3% Kβ. Transmitted scattering was detected using a PSD 50 Hecus. Temperature was controlled by means of a Peltier TCCS-3 Hecus. The samples were inserted in a glass capillary 1 mm diameter with 10 μm wall thickness. The SAXS and WAXS scattering curves are shown as a function of the scattering vector modulus: q = (4π /λ) sin(θ )

(1)

where 2θ is the scattering angle. The q values with this setup ranged from 0.08 to 6.0 nm−1 in the SAXS regime while that of the WAXS regime spans from 28 to 35 nm−1. The system scattering vector was calibrated by measuring a standard silver behenate sample for SAXS detector and p-bromobenzoic acid for the WAXS detector. Because of the use of a detector focused small beam (300 × 400 μm full width at half-maximum) the scattering curves are mainly smeared by the detector width. This smearing mainly produces a widening of the peaks without noticeable effect on the peak position in the small-angle regime. The instrumentally smeared experimental SAXS curves were fitted to numerically smeared models for beam size and detector width effects. A least-squares routine based on the Levenberg−Marquardt scheme was used. The bilayer electronic density profiles and bilayer thickness were determined using a three-Gaussian profile based on the MCG model.32 Preparation and Characterization of Langmuir Films. Surface pressure versus area isotherms were recorded using a computercontrolled KSV Minitrough (total area 24225.0 mm2) on spread monolayers at room temperature (25 ± 1 °C). The Wilhelmy plate method, using a paper plate, was used for measuring surface pressure. The experiments were conducted on a 2 M NaCl subphase. This subphase was chosen because it allowed obtaining good film stability and reproducibility. Spreading solutions of the cis and trans esters were prepared by dissolving the crystalline compounds directly in chloroform. Concentrations of ∼1 mg/mL were used, resulting in a typical deposition volume of 20−25 μL. The spreading solvent was allowed to evaporate for 15 min, and three compression/expansion cycles in the LE phase of the monolayer were performed before starting the regular run, in order to reduce the effect of hysteresis. Compression was performed at a rate of 10 mm/min, when not differently specified. Brewster angle microscopy images were collected with a MicroBAM during the compression of the Langmuir films. Langmuir−Blodgett films on freshly cleaved mica supports were prepared using a KSV Minitrough dipping accessory. AFM Measurements. Atomic force microscopy (AFM) topographic images were acquired in a Multimode8 AFM attached to a Nanoscope V electronics (Bruker) by using Peak Force mode. Briefly, this imaging mode uses individual force curves in order to sense the presence of the surface by deflection the probe cantilever. This deflection, which, in fact, can be converted into an applied vertical force, is kept constant and used as a feedback signal to track the sample surface. Force curves are acquired at a rate of 2 kHz, and the

Chart 1. Structure of Diastereomeric Nonionic Amphiphiles 1

cis/trans stereochemistry and stereochemical constraints strongly influences the physicochemical behavior, molecular organization, and morphology of their Langmuir monolayers and dry (solid) states. Langmuir monolayers formed at the water/air interface by insoluble (or sparingly soluble) amphiphilic molecules are handy models to study the translation of the molecular information into supramolecular organization in two dimensions.27−30 In addition, they attract a considerable interest for the design of molecular devices and soft materials. The unique feature that makes the Langmuir technique extremely powerful for studying intermolecular interaction is that it allows a precise control of the degree of organization and packing in the monolayer by varying the available area per molecule at the water/air interface.28 Moreover, the use of Brewster angle microscopy (BAM) permits to visualize the morphology of the Langmuir film directly at the interface,31 as we exploited in the present work for studying the monolayer formed by amphiphiles 1. In another aspect, we also investigated the B

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Figure 1. SAXS intensity profiles of pure amphiphiles 1 at different temperatures. The curves have been shifted by successive powers of 2 for clarity. (a) trans-1 from top to bottom at 25, 40, 50, 60, 70, 80, and 90 °C. Note the invariant position of the peaks and the increased background signal for the 80 °C measurement. At 90 °C the melt did not show any distinct features. (b) cis-1 from top to bottom at 25, 40, 50, 55, 60, and 70 °C. Note the asymmetry of the peaks and the changes in relative intensity. The increased background signal for the 60 °C measurement indicates the onset of melting, which is complete at 70 °C, where no distinct features are observed.

Figure 2. WAXS curves for pure amphiphiles 1 at different temperatures. The curves have been shifted by successive powers of 2 for clarity. (a) trans-1 from top to bottom at 25, 40, 50, 60, 70, 80, and 90 °C. (b) cis-1 from top to bottom at 25, 40, 50, 55, 60, and 70 °C.

ature differs strongly. While the trans-1 spectra are compatible with slight deformations of the cell with the peaks moving systematically either to bigger or smaller distances without strong intensity changes (Figure 2a), the alterations in the WAXS pattern of cis-1 are dramatic (Figure 2b). These changes mean that the effect of temperature on the structure of cis-1 implies strong differences in the position of the atoms in the unit cell and/or alterations of the unit cell geometry. However, these changes do not reduce the crystallinity of the sample significantly below 60 °C, which strongly suggests the formation of different allotropes with very close energies, but with extraordinarily high order. Note that these allotropes are also visible in the SAXS regime as shoulders that deform the first reflection slightly and more pronouncedly the second when the peak shapes are observed in closer detail. Fitting of the SAXS curves to a bilayer structure allows the determination of the electronic profile perpendicular to the smectic repetition plane (Figure S1 in the Supporting Information).33,34 Although the information content in the SAXS spectra is relatively poor, the difference in electronic density profile has been found consistently, and forcing the position of the polar head Gaussian to be the same in both cases results in incongruencies concerning the groups electronic densities and molecular volume. The different profiles obtained are congruent with trans-1 having its atoms with higher electronic density located at the extreme of the cell, with possibilities of interaction with the next layer. Differently, for cis-1 results are compatible with a somewhat crooked

trigger vertical force was kept to 300 pN. This low force preserves the integrity both of the sample and the probe apex. Used AFM probes sported triangular silicon nitride levers and a silicon oxide pyramid, with a nominal spring constant of 0.35 nN/nm (SNL-10 probes, Bruker). Images were acquired at 0.5 Hz and 768 × 768 pixels. Particle detection and analysis was performed with Nanoscope Analysis software (Bruker).

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RESULTS AND DISCUSSION Investigation on Dry Samples of trans- and cis-1 by SAXS and WAXS. Both amphiphiles trans-1 and cis-1 show typical smectic order in the SAXS patterns; because of the small repeating distance, only two peaks are present in the observation window as shown in Figure 1a,b, and for this reason, only limited information can be obtained on the bilayer structure. What is apparent is that the smectic repeating distance is very similar for both compounds, 2.46 and 2.58 nm, and that the electron distribution perpendicular to the lamellae could be comparable because of the analogous peaks intensity relationship (Figure 1a,b). There was not any remarkable effect of temperature up to temperatures close to the melting point for trans-1 (81 °C) where an increase in background intensity was apparent. Similar behavior was observed for cis-1 (melting temperature 61 °C) except for the presence of a shoulder in the peaks at 25 °C that is nearly absent at the other temperatures and for the relative difference of intensity of the second peak. The presence of many peaks in the WAXS regime (Figure 2a,b) is indicative of crystalline or quasi-crystalline order for both compounds. However, the behavior as a function of temperC

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On the other hand, in the case of cis-1 a continuous increase of surface pressure with decreasing of the molecular area was observed with the film remaining in the LE phase without any evidence for phase transitions. Nevertheless, between 61 and 33 Å2/molecule, i.e., in the field of the LE phase occurrence, the isotherms of the two amphiphiles are roughly superimposable. Moreover, they reach the collapse at almost the same surface pressure (∼32 mN/m). The more expanded features of the isotherm of cis-1 denote a higher fluidity of its monolayers, when compared with the films formed by trans-1, especially at lower molecular areas for surface pressures above 15 mN/m. This behavior has to be ascribed to differences in packing and orientation of the two diastereomeric species at the interface, which in turn affect the possibility of forming strong patterns of amide−amide hydrogen bonds (see below). With increasing surface pressure, the rotational freedom of molecules strongly decreases. As a consequence, orientation-dependent interactions due to the chiral structure of the molecules, which in turn depend here on the different stereochemistry, became dominant.35 Thus, packing properties seem to be imposed principally by the structure of the chiral headgroups rather than by the van der Waals interactions between the aliphatic chains, analogously to what previously observed for other N-acylamino acids derivatives.35−39 In particular, the difference in the relative configuration of the headgroups of cis-1 and trans-1, jointly with the conformational restrictions imposed by the cyclobutane ring (the latter increasing the orientational demand) may lead to different hydrogen bond patterns in the respective monolayers, thus affecting their thermodynamic behavior, as revealed by the π−A isotherms (see below). In fact, due to the highly cooperative and directional nature of hydrogen bonds, their properties depend not only on the atoms directly involved in bonding but also on the whole pattern in which they exist. In addition, another effect that could also influence is the “E/Zisomerization” of the methyl ester group40 in each of the studied compounds, although, with the present results, this effect cannot be directly inferred. In general, hysteresis in a compression/expansion cycle was observed for both amphiphiles. Moreover, the isotherms are more “compressed”, that is, shifted to lower molecular areas, at the second compression especially in the case of cis-1 (Figures S2 and S3). Nevertheless, analogous differences between the isotherms of the two amphiphiles 1 were also observed by comparing their respective second compressions. This behavior might be attributed to a partially irreversible loss of material at the first collapse due to desorption of the amphiphile or to the formation of small surface aggregates41 or crystallites that do not completely spread back upon expansion. However, the evidence that the isotherm shape does not change substantially in further compression/expansion cycles and that the same collapse pressure is reached indicates that the solubilization of the monolayer materials into the subphase is almost negligible (also taking into account the insolubility of amphiphiles 1 in the chosen subphase). Moreover, the shape and position of the isotherms obtained at the first and second compression are fully reproducible within different experiments. Thus, all of these pieces of evidence suggest the occurrence of slow film reorganization phenomena, and hence the first isotherms could correspond to a not completely equilibrated state. Nonequilibrium effects in the thermodynamic behavior, including hysteresis, relaxation, and the presence of overshoots at the onset of plateau regions (as in the case of trans-1), are often observed in the isotherms of monolayers of amino acid-

conformation, with the electron-rich groups (amino and carboxyl groups) separated from the next layer and the cyclobutane occupying the extreme of the cell. Please note that although the fitting for cis-1 at 25 °C has been performed disregarding the apparent shoulder, the electronic profile show the same features to that of the rest of temperatures as it can be observed for the profile at 40 °C. This conformation also sheds light on the different temperature behavior of the two compounds. The trans-1 diastereoisomer has higher thermal stability because of stronger interactions with the other molecules (intermolecular hydrogen bonding) in-plane and interplane. On the other hand, cis-1, because of the intramolecular hydrogen bonds, interacts with the neighboring molecules mainly via van der Waals interactions, allowing low energy changes in conformation and a lower melting temperature (see below). Langmuir Isotherms and Brewster Angle Microscopy (BAM) Experiments on trans- and cis-1. Langmuir monolayers have been used here as model systems for investigating the effect of the cis/trans stereochemistry and of the conformational constraints imposed by the cyclobutane ring on the supramolecular organization of amphiphiles 1 in twodimensions. Remarkable differences have been observed for the surface pressure vs area per molecule (π−A) isotherms of the two diastereomeric esters trans- and cis-1, suggesting an unlike structural organization of their respective Langmuir films (Figure 3). The compression isotherm of trans-1 monolayers

Figure 3. Surface pressure vs area isotherm of trans-1 (red solid line) and cis-1 (blue dashed line) monolayers recorded on a 2 M NaCl subphase at 25 °C (first compression).

shows distinct regions of different compressibility corresponding to different phases and phase coexistence regions. Namely, a liquid expanded (LE) phase at higher molecular areas (at 61 > Mma > 33 Å2/molecule); a short nonhorizontal plateau region (at 32 > Mma > 27 Å2/molecule) indicating a “first”-order phase transition between the expanded and the condensed (LC) phase (the plateau starting with an overshot); a lower compressibility region below 27 Å2/molecule corresponding to the LC phase and, eventually, to a solid (S) phase at lower molecular areas. Finally, a collapse is observed for surface pressure approaching 32 mN/m, at which molecules are forced out of the monolayer to form multilayered structures. The apparent limiting (close packing) area, obtained by extrapolating the steepest linear part of the isotherm to zero surface pressure, is about 28 Å2/molecule. The transition pressure (considered as the minimum π after the overshot) is about 15 mN/m. D

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Figure 4. Surface pressure vs area isotherm (second compression) and corresponding BAM images of trans-1 Langmuir films on a 2 M NaCl subphase at 25 °C. Image frame: 2126 × 1641 μm.

(panels b and c in Figure 4), and just before the collapse, they fill almost the entire surface forming a rather dense monolayer, which gives homogeneous light reflection (panel d in Figure 4). By further compression to smaller areas per molecule, white regions with high reflectivity appear, indicating the collapse of the monomolecular films to form multilayered structures (panel e in Figure 4). The condensed phase domains are characterized by a main linear strand and three smaller strands (only two of which are visible at the beginning; see panel a in Figure 4) that intersect at one point at almost regular angles. Furthermore, during the domains growth, roughly parallel and equidistant branches appear on the strands (panels b and c in Figure 4). The analysis of domain morphology reveals that they are two-dimensionally chiral, and all of them feature the same handedness, i.e., the same chiral sign (Figure 5), thus

based amphiphiles and have been ascribed to a different time scale between compression and film reorganization.36−38,42 In particular, the occurrence of overshoots along the isotherm was attributed to slow rearrangement of the headgroups in the LC phase following nucleation.38 The latter would also explain the surface pressure relaxation (rapid surface pressure drop) observed upon stopping the barriers after the overshoot, i.e., the in the LE−LC phase coexistence region of the trans-1 at a certain molecular area along the isotherm (no relaxation was observed for the films in the LE phase).37,39 Surface pressure relaxation has been previously reported for a large number of amphiphilic monolayers, featuring regions of supersaturation, and in some cases has been attributed to the nucleation and growth of three-dimensional multilayered structures.42 However, in our case, the BAM images do not evidence the formation of multilayered crystals before the collapse (see below), which is also confirmed by AFM studies of Langmuir− Blodgett films of trans-1 deposited on freshly cleaved mica substrates at surface pressures corresponding to the LC region. In fact, the AFM images (Figure S4) show a flat film with several holes, within which the mica surface is clearly visible, and the cross-section analysis reveals a very homogeneous height of 1.2 ± 0.1 nm for the film, which matches well with a monomolecular layer. On the other hand, the drop in surface pressure observed when barriers are stopped after the collapse can be reasonably justified by the transformation of the monolayer material to overgrown three-dimensional structures as confirmed by the corresponding BAM images (see below). Brewster angle microscopy allowed investigating the morphology of the monolayers formed by amphiphiles 1 directly at the interface and evidenced striking differences between the monolayers of the diastereomers trans- and cis-1, once more ascribable to an underlying different molecular organization. Figure 4 shows the BAM micrographs collected along the isotherm of trans-1 (second compression). In correspondence of the LE−LC phase transition, gray condensed phase domains with a dendritic shape start to appear, floating on the black LE phase (panel a in Figure 4). These domains progressively increase in size along the isotherm

Figure 5. Chiral condensed phase domains in the Langmuir films of trans-1. (left) Same BAM snapshot as in Figure 4b. The yellow arrows indicate the domains in which the chiral morphology is clearly evident. (right) Only the S enantiomer is observed.

evidencing that the chiral information is translated from the molecular level (enantiopure trans-1) to the mesoscopic morphological level. Note that at the air/water interface amphiphilic molecules are vectorially oriented with the chains emerging into the air, which excludes the mirror symmetry element.43 Chiral shapes of the condensed phase domains are often observed in Langmuir monolayer formed by chiral E

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Figure 6. BAM images of the dendritic 3D crystals observed beyond the collapse region along the compression isotherm of cis-1 monolayers on a 2 M NaCl subphase at 25 °C. The three images are consecutive snapshots showing that crystal was floating on a fluid LE surface. Image frame: 4000 × 3600 μm.

Figure 7. Possible hydrogen-bond patterns for (a) trans-1 and (b) cis-1 molecules aligned at the water/air interface. Only three carbons of the hydrophobic chains are reported in figure for clarity. Dimeric structures were optimized for the entire molecules at the PM3 level. The face-to-face disposition for cis-1 (bottom right) is reported at different side views. Note that the “lined-up” disposition for trans-1 and cis-1 are not equivalent because they expose a different edge of the cyclobutane ring to the interface.

amphiphiles and in particular by N-acyl amino acid surfactants.35,43 It has to be remarked that dendritic LC structures were also observed during the first compression of trans-1 monolayers, but in this case the domains are larger and appear as extended dendritic rafts (Figure S5). This contrast can be ascribed to differences in primary nucleation (number of primary nuclei) and successive growth, between first and second compression, confirming once more the nonequilibrium nature of the process.43 On the other hand, the formation of condensed phase domains was not observed by BAM all along the compression isotherm of cis-1, and only black images were collected (not shown), confirming that the monolayer remains in a liquid expanded LE phase even at low areas per molecule. However, beyond the collapse, compact dendritic crystals floating on a dark fluid subphase were observed (Figure 6). The high contrast and reflectivity strongly suggest that they are multilayered 3D structures (i.e., dendritic 3D crystal).42 For two-dimensional systems, such as Langmuir monolayers, specific approaches have been developed to explain the evolution of dendritic structures with anisotropic shapes. It

has been reported that the presence of directional interactions between the particles of the growing dendrites is necessary in order to introduce anisotropy, which leads to preferential growth directions. On the other hand, isotropic van der Waals interactions promote the formation of more compact domain shapes.42−44 In particular, the formation of amide−amide hydrogen bonds between head groups has been recognized as responsible for the formation of dendritic LC domains in Langmuir monolayers of N-acyl amino acid amphiphiles.35,43 Therefore, also the morphological diversity between the monolayers of esters trans- and cis-1 has to be ascribed to different hydrogen-bonding patterns between the chiral heads, as a consequence of their diastereomeric configuration. In particular, in the case of trans-1 the alignment of molecules at the water/air interface, with the cyclobutane rings lined-up in the same direction (Figure 7a), allows the formation of extended chains of hydrogen bonds between the amide groups (N−H---OC) of consecutive molecules. In this configuration, the amides planes run parallel to the interface while the ester groups point outward, and the amphiphile tails can be easily aligned thus favoring the appearance of well-structured LC domains. In the case of cis-1, there is the possibility of a F

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“lined-up” disposition of cyclobutanes that allow the formation of extended amide-bond chains (Figure 7b), but also a stable face-to-face disposition is possible. The latter allows the simultaneous formation of two hydrogen bonds between the amide and the ester groups of adjacent molecules, while hampering the formation of extended hydrogen-bond chains and the correct alignment of the hydrophobic tails. Lastly, also the possibility to form an intramolecular hydrogen bond has to be considered in the case of cis-1. In fact, a strong propensity of the cis family to form intramolecular hydrogen bonds has been previously reported for other systems based on this cyclobutane amino acid moiety.45 To sum up, the possibility for cis-1 to form different patterns of hydrogen bonding, some of which preclude the formation of extended chains, would explain the more expanded (disordered) behavior of its monolayers as well as the lower thermal stability of its solid phase and the presence of different allotropic forms as shown by the SAXS−WAXS experiments on the dry samples. Moreover, the molecular arrangements reported in Figure 7 also account for the different electron density profile perpendicular to the smectic plane as determined from the SAXS curves (Figure S1).

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (R.P.). *E-mail [email protected] (R.M.O.). Present Address

A.S.: Institut de Science et d’Ingénierie Supramoléculaires (ISIS), Université de Strasbourg, 8 allée Gaspard Monge, 67083 Strasbourg (Cedex), France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jaume Caelles, in the SAXS-WAXS service at IQAC, for X-ray measurements, and Dr. Fabrizio Marinelli for the optimization of the dimeric structures in Figure 7. Financial support from MINECO (grants CTQ2013-41514-P and CTQ2013-43754-P) is gratefully acknowledged. Authors also thank the support from Generalitat de Catalunya (2014SGR358 and 2014SGR836).

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REFERENCES

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CONCLUSIONS The investigation on the water-insoluble diastereomeric esters cis- and trans-1 evidenced a dramatic effect of the relative stereochemistry on the physicochemical behavior and morphology of their spread Langmuir monolayers as well as on the structure and melting behavior of their dry states. These differences are directly attributable to the possibility of the two diastereoisomers to form different hydrogen-bonding patterns, which in turn determine a different molecular organization in their condensed phases. The formation of intra- or intermolecular hydrogen bonds modulates the self-organization of these molecules in the dry state; however, in this case the dominant forces are excluded volume. A different situation is encountered in spread monolayers, where the excluded volume interactions are significant only at high surface coverage. Thus, the generation of chiral dendritic domains is present in the case of trans-1 (where the dominant interaction is in lined-up configuration), while the possibility of intramolecular hydrogen bonding and the formation of dimers overcomes the lined-up geometry for cis-1. The results confirm that the cyclobutane βamino ester moiety is a valuable scaffold for the preparation of novel amphiphiles. In fact, the rigidity of the four-membered ring joined with the cis/trans stereochemistry allows obtaining amphiphile assemblies with well-defined molecular organizations and mesoscopic shapes, which is crucial for the development of functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01461. Electron density perpendicular to the smectic plane obtained from SAXS measurements; additional Langmuir isotherms of monolayers of amphiphiles cis-1 and trans1; AFM measurements on Langmuir−Blodgett film of trans-1; BAM images of Langmuir monolayers of amphiphiles 1 (PDF) G

DOI: 10.1021/acs.langmuir.6b01461 Langmuir XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.langmuir.6b01461 Langmuir XXXX, XXX, XXX−XXX