Article pubs.acs.org/Langmuir
Chiral Self-Assembly of Designed Amphiphiles: Influences on Aggregate Morphology Thomas G. Barclay,*,†,∥ Kristina Constantopoulos,† Wei Zhang,‡,⊥ Michiya Fujiki,‡ Nikolai Petrovsky,§ and Janis G. Matisons†,# †
Flinders Centre for Nanoscale Science & Technology, School of Chemical and Physical Sciences, Flinders University, Adelaide, South Australia, 5042, Australia ‡ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan § Vaxine Pty Ltd and Department of Endocrinology, Flinders Medical Centre, Adelaide, South Australia, 5042, Australia S Supporting Information *
ABSTRACT: A series of novel amphiphiles were designed for self-assembly into chiral morphologies, the amphiphiles consisting of a glutamic acid (Glu) headgroup connected through an 11-carbon alkoxy chain to a diphenyldiazenyl (Azo) group and terminated with a variable length alkyl chain (R-Azo-11-Glu, where R denotes the number of carbons in the distal chain). TEM imaging of amphiphile aggregates selfassembled from heated, methanolic, aqueous solution showed that chiral order, expressed as twisted ribbons, helical ribbons, and helically based nanotubes, increased progressively up to a distal chain length containing eight carbons, and then decreased with further increases in distal chain length. TEM and CD showed that the chiral aggregations of single enantiomers were influenced by the molecular chirality of the headgroup. However, the assembly of D,L-10-Azo-11-Glu into nanotubes demonstrated that chiral symmetry breaking effected by the azo group was also relevant to the chiral organization of the amphiphiles. The chiral order of aggregate morphologies was additionally affected by the temperature and solvent composition of assembly in a manner correlated to the mechanism driving assembly; i.e., D,L-10-Azo-11-Glu was sensitive to the temperature of assembly but less so to solvent composition, while L-14-Azo-11-Glu was sensitive to solvent composition and not to temperature. FTIR and UV−vis spectroscopic investigations into the organization of the head and azo groups, in chiral and achiral structures, illustrated that a balance of the influences of the hydrophilic and hydrophobic components on self-assembly was required for the optimization of the chiral organization of the self-assembled structures.
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INTRODUCTION
bilayer helical ribbons sufficiently tightly wound can result in straight, stiff, high-axial-ratio nanotubes.16 These nanostructures are of interest because they can be constructed with a variety of surface chemistries and a range of sizes17,18 that provide an array of useful applications including controlled release of encapsulated substances19 and template directed synthesis or organization of other materials.20−22 Molecular chirality and rigid kinks in molecular structure of the hydrophobic tail organize chiral self-assembled morphologies by promoting a consistent, nonzero angle of packing between neighboring amphiphiles through two different and frequently complementary mechanisms:8 (i) molecular-chirality-directed packing of the molecules, in which the chirality of the molecule determines the direction of packing,6 and (ii) chiral symmetry breaking, in which a rigid, kinked, tilt-inducing
The self-assembly of amphiphilic molecules from aqueous environments is one area in which the “bottom-up” approach to the creation of the nanoscale artifacts observed in nature is readily translated into workable synthetic nanostructures.1−3 For these systems, the molecular structure of the amphiphile largely determines the self-assembled morphology,2−4 and therefore desired morphologies can be constructed through the rational design of amphiphile chemistry. For example, molecular chirality and/or rigid kinks in molecular structure of the hydrophobic tail can result in the chiral organization of amphiphiles within aggregates;5−8 an arrangement that can simply and economically create highly ordered structures from a number of identical molecules.9−11 Chiral self-assembled morphologies driven by a chiral amphiphile arrangement include bilayer structures such as twisted and helical ribbons. Interconnected networks of these ribbons are the basis of many gels,7,12,13 and the helices are also predicted to have utility as springs14 and solenoids15 in micromechanical devices. Further, © 2013 American Chemical Society
Received: November 12, 2012 Revised: July 10, 2013 Published: July 15, 2013 10001
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segment in the hydrophobic chain prevents molecular rotation, reducing the packing possibilities and spontaneously generating chirality upon an increase in order.8,23 Herein, we present a novel family of amphiphiles designed to self-assemble into chirally organized bilayer structures using both mechanisms through use of a chiral glutamic acid headgroup and a rigid, kinked azo group in the hydrophobic tail (Figure 1). The self-
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EXPERIMENTAL SECTION
Synthesis. The synthetic procedures were based on those published elsewhere.24 Detailed synthetic procedures and molecular identification (1H NMR, 13C NMR, FTIR, and elemental analysis) for the 10 amphiphiles and their precursors are in the Supporting Information. Materials. All materials were used as received, and materials used in the organic synthesis are listed in the Supporting Information. Methanol (AR) and hydrochloric acid (AR) were purchased from Ajax Finechem Pty. Ltd. (Australia) and potassium hydroxide volumetric standard in methanol (0.1 N) from Sigma-Aldrich Pty. Ltd. (Australia). All water used for self-assembly and dilution was deionized using a Barnstead E-pure water purification system operating at a resistance of at least 18.0 MΩ. Titration Experiments. A volumetric standard of potassium hydroxide (KOH) in methanol (0.1 N) was diluted with methanol and water (95% methanol w/w; 0.05 M KOH). A solution of hydrochloric acid in aqueous methanol (95% methanol w/w, ∼0.05 M) was prepared and standardized against the potassium hydroxide solution to provide an accurate concentration (0.0486 M). These solutions were then used to titrate L-2-Azo-11-Glu dissolved in aqueous methanol (95% methanol w/w; 0.5 mM amphiphile). Self-Assembly. Heated Self-Assembly Procedure. The amphiphile was dissolved in methanol (2.5 mL, 0.7 mM) by sonication at 63 °C, creating a clear, deeply orange solution. To this was added heated pure water (7.5 mL; 18.0 MΩ; 63 °C) and sonication continued at 50, 65, or 80 °C for 1 h. The temperature for each amphiphile was chosen such that there was a gradual transition to a cloudy, pale yellow dispersion over several minutes (L-2-Azo-11-Glu at 50 °C; L-4-, L-5-, and L-6-Azo-11-Glu at 65 °C; all other amphiphiles at 80 °C). Subsequently, sonication was ceased, but heating continued for a further 5 h. The dispersion was then slowly cooled to room temperature. Samples were visually stable over time, maintaining a homogeneous pale yellow dispersion, with a slight tendency to aggregate and precipitate over several months. Generally, this aggregation could be redispersed using gentle sonication at room temperature. Room Temperature Self-Assembly Procedure (RT). The amphiphile was dissolved (0.35 mM) in methanol (100 mL). Aliquots of this solution (300 μL) were then diluted 10-fold with mixtures of methanol and pure water (2700 μL; water = 18.0 MΩ) such that the final solutions ranged in methanol concentration relative to water from 10
Figure 1. Molecular structures of synthesized amphiphiles; all compounds have a glutamic acid headgroup linked by an amide bond to the hydrophobic tail comprising an 11-carbon proximal alkoxy chain connected to a central azo group, and terminated with an alkyl chain. The distal chain is of variable length, where n = 2, 4, 5, 6, 7, 8, 10, 12, or 14. Amphiphiles with an L-glutamic acid headgroup were constructed for each of the distal chain lengths. In addition, a Dglutamic acid headgroup was used to construct an amphiphile with a 10-carbon distal chain.
assembly was optimized for the desired helical aggregates by systematic variation of the terminal or distal alkyl chain length, tuning the relative molecular volumes of the hydrophobic and hydrophilic components of the amphiphile, an important factor in amphiphile assembly into chiral, bilayer ribbons.4 Furthermore, we investigated the influence of headgroup chirality, temperature, and solvent composition on the self-assembly of specific amphiphiles, providing insights into the mechanisms of assembly.
Table 1. Summary of Chiral Structures Self-Assembled from the R-Azo-11-Glu Family of Amphiphiles Using the Heated Method
a
All amphiphiles self-assembled in H2O:MeOH 75:25 (v/v) and annealed at the temperature given. b(L) = left-handed; (U) = undefined chirality; (R) = right-handed. 10002
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to 100% in increments of 10%. Upon the addition of water in sufficient concentration to precipitate each amphiphile, there was an immediate transition from the orange solution to a pale yellow dispersion with the same stability characteristics as samples assembled using the heated method. Instrumentation. Ultraviolet and Visible Light Spectroscopy. Ultraviolet and visible light (UV−vis) spectroscopy was conducted on methanolic aqueous dispersions of amphiphiles as prepared using the room temperature self-assembly procedure. Spectra were recorded with a Varian Cary 50 Scan UV−visible spectrophotometer using a quartz cuvette with a 10 mm path length. Circular Dichroism. Each enantiomer of L-10-Azo-11-Glu was selfassembled using the heated procedure, and the dispersion was analyzed by circular dichroism (CD) using a JASCO J-725 spectropolarimeter equipped with a Peltier accessory to maintain a temperature of 25 °C. The experiment was conducted using a SQgrade cuvette, with a path length of 10 mm, at a scanning rate of 100 nm/min, a bandwidth of 1 nm, and a response time of 1 s, using a single accumulation. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total reflectance Fourier transform infrared (ATR FTIR) spectroscopy was conducted with a Thermo Electron Corporation Nicolet Nexus 870 spectrophotometer utilizing an Attenuated Total Reflectance (ATR) Smart Collector attachment and a DTGS detector. Self-assembled structures were lyophilized before analysis using 128 scans at a resolution of 2 cm−1, and data generated was manipulated using OMNIC software. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was conducted on methanolic aqueous dispersions, as prepared in the self-assembly procedures; initial imaging occurring within hours of assembly and several samples were reimaged after more than 1 year. The dispersion was diluted 4-fold using the same solvent mixture used in self-assembly, and then, 4 μL of the dilution was deposited onto Formvar covered copper grids and air-dried before imaging using a JEOL JEM-1200EX instrument operated at 80 kV with a spot size of 3.
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TRANSMISSION ELECTRON MICROSCOPY (TEM) TEM of Structures Self-Assembled Using the Heated Method. 2-Azo-11-Glu. The amphiphile L-2-Azo-11-Glu selfassembled into organized structures including tubes often perpendicularly aligned to the edge of a flat ribbon (Figure 2).
Figure 2. TEM image of structures self-assembled from 2-Azo-11-Glu.
These tubes had an average length of 247 ± 98 nm, an average external diameter of 53 ± 8 nm, and an average wall thickness of 19 ± 1.3 nm. The association of the tubes and flat ribbons suggests that the tubes form from the flat ribbon and there was no evidence for or against chiral packing for these structures. Interestingly, these short tubes and ribbons were dissimilar to structures observed in the self-assembly of the rest of the RAzo-11-Glu family of compounds. Instead, they appear more closely related to structures assembled from associated amphiphiles with even numbers of carbons in the proximal chain (Supporting Information, Figure S22). Further investigation of the self-assembly of amphiphiles with even numbers of carbons in the proximal chain is not made here, other than stating that such amphiphiles generally assemble differently from those with odd numbers of carbons in the proximal chain. This behavior can be attributed to an odd−even effect for the length of the proximal chain due to the influence on the relative orientations of the headgroup and the organizing group in the hydrophobic tail, as observed previously for both diacetylenic26 and azo24,27 derived amphiphiles. For 2-Azo-11-Glu, it is possible it does not follow the trend for the other 11-carbon proximal chain amphiphiles, as the short distal chain means that its mesogenic character and the influence of the hydrophobic tail on self-assembly is minimized compared to those with amphiphiles with longer distal chains.28,29 4 to 8-Azo-11-Glu. The amphiphile L-4-Azo-11-Glu selfassembled into distorted twisted ribbons; however, most of the aggregated structures were amorphous. In contrast, L-5-Azo-11Glu self-assembled largely into ribbons without chiral character, but some of the wider and denser ribbons did exhibit a twisted morphology. With the increase in distal chain length came an increase in the chiral order of the aggregates, and L-6-Azo-11Glu assembled into high populations of organized ribbon structures, many having twisted or helical character. A unique variation to these ribbons was that many had fluted edges, as seen in Figure 3. The transition from twisted to helical ribbons as distal chain length increases is significant, as these types of
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RESULTS TEM was used to study the influence of the compositional variables for all of the amphiphiles. On the basis of TEM results, selected structures underwent further analysis using UV−vis, CD, and ATR FTIR spectroscopies, and for these structures, the influence of both the temperature and the methanol concentration of the solvent mixture on their selfassembly was also investigated. The results for the heated selfassembly of all amphiphiles are summarized in Table 1 and in further detail in the Supporting Information (Table S1). The chiral self-assembled structures of all samples reimaged with TEM over time were stable in the self-assembly solvent mixture at room temperature for more than 1 year.
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SOLVENT SELECTION FOR SELF-ASSEMBLY OF R-AZO-11-GLU AMPHIPHILES To establish the ionization state of the headgroup after amphiphile synthesis, a pH titration was conducted on L-2Azo-11-Glu from methanolic aqueous solution (95/5 w/w,25 see the Supporting Information). The titration showed that the amphiphiles were initially uncharged after synthesis, meaning they were essentially insoluble in water. The self-assembly of poorly water-soluble amphiphiles into helically based tubes has previously been improved by the addition of alcohol,26 and a mixed solvent system using methanol and water was successfully adopted for the self-assembly of amphiphiles in this research. 10003
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been used as evidence of the helical basis of self-assembled nanotubes.33 Strengthening the evidence that these helices are the precursors to the tubes is that the diameter of the helix closely approximates the diameters of the tubes, and the straightness of the tubes is suggestive of helical construction, as such assembly has been shown to make stiff, straight tubes that cannot easily bend.16 Finally, these amphiphiles are closely related to others with aspartic acid head groups proven to selfassemble into helically based nanotubes.24 10 to 14-Azo-11-Glu. Further increases in the length of the distal chain beyond eight methylene units for the R-Azo-11-Glu family of compounds for this method of self-assembly progressively diminished the capacity of the amphiphiles to express chirality in the self-assembled structures and increased the amount of amorphous aggregates. In the self-assembly of L10-Azo-11-Glu, high populations of flat, curved, or loosely twisted ribbons were observed, with only a few examples of helical ribbons and no tubes found. The left-handed twisted ribbons had an average ribbon width of 114 ± 30 nm and an average pitch of 1.2 ± 0.3 μm (Figure 5a). In an attempt to better understand why the chiral organization diminished for L10-Azo-11-Glu compared to L-8-Azo-11-Glu, the enantiomer was synthesized and assembled. Pure D-10-Azo-11-Glu selfassembled into structures closely matching those of the Lisomer (D-isomer twisted ribbon width 106 ± 28 nm, pitch 1.1 ± 0.3 μm), except that the chiral ribbons were right-handed (Figure 5b). The consistent left-handed nature of chiral ribbons assembled from the L-amphiphiles and the right-handed twisted ribbons assembled from D-10-Azo-11-Glu demonstrate that molecular-chirality-directed packing of the amphiphiles is responsible for the observed chirality in the aggregated structures of the pure enantiomers. Despite this observation, the azo group was still an important influence in the assembly of the single enantiomers. This is illustrated by the selfassembly from water and water/alcohol mixtures of n-acyl-Lglutamic acid derivatives without azo groups in the hydrophobic tails. These amphiphiles self-assembled into fibrous,34,35 micellar,36 and disc37 shaped morphologies but not into the chiral structures observed in this research. In an interesting development, when the D- and L-isomers of 10-Azo-11-Glu were mixed in a 1:1 ratio prior to self-assembly, nanotubes were the dominant organized self-assembled structure (Figure 6). These nanotubes had an average external
Figure 3. TEM image of fluted, twisted ribbons self-assembled from L6-Azo-11-Glu.
ribbons are closely related chiral morphologies able to transform from one to the other,30 with the helical morphology being found to have greater chiral order.31,32 The amphiphiles L-7- and L-8-Azo-11-Glu both selfassembled into high populations of organized structures, including nanotubes (Figure 4a) as well as twisted and helical ribbons. The nanotubes had a distribution of diameters that included distinct steps. This is highlighted by the histogram in Figure 4b showing the tube diameter distribution for L-7-Azo11-Glu. The tubes fitting under the largest peak of the histogram had an average external diameter of 41 ± 3.4 nm, an average wall thickness of 14 ± 1.5 nm, and a calculated internal diameter of 13 nm. Only two distinct diameters were found for the nanotubes self-assembled from L-8-Azo-11-Glu with the smaller, more populous tubes having an average external diameter of 47 ± 3.4 nm, average tube wall thickness of 15 ± 1.9 nm, and calculated internal diameter of 17 nm, similar in character to those assembled from L-7-Azo-11-Glu. Examples of nanotubes selfassembled from L-8-Azo-11-Glu are shown in Figure 4a, along with an example of a left-handed helical ribbon (average width 52 ± 6.5 nm, average pitch 467 ± 43 nm). The existence of helical and twisted ribbon precursor structures has previously
Figure 4. (a) TEM images of structures self-assembled from L-8-Azo-11-Glu. (b) Histogram showing the distribution of external diameters for nanotubes self-assembled from L-7-Azo-11-Glu. 10004
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Figure 5. TEM images of twisted ribbons self-assembled from (a) L-10-Azo-11-Glu and (b) D-10-Azo-11-Glu.
chirality of the self-assembled structure is best explained by the chiral symmetry breaking mechanism. The self-assembly of L-12-Azo-11-Glu exhibited very similar morphologies to those observed for L-10-Azo-11-Glu except that there were no helical ribbons, fewer twisted ribbons, and more amorphous aggregates. The left-handed twisted ribbons for L-12-Azo-11-Glu had an average width of 96 ± 16 nm and average pitch of 862 ± 152 nm. The trend in decreasing expression of chirality in self-assembled morphology with increasing distal chain length continued for L-14-Azo-11-Glu, which had relatively more flat ribbons and very few chiral structures, as well as a lot of fine amorphous aggregates. The relationship between increasing distal chain length and the observed increasing then decreasing chiral order of structures self-assembled from the R-Azo-11-Glu family of amphiphiles using the heated method can be attributed to the increasing mesogenic character of the amphiphile,28,29 intensifying the influence of the hydrophobic tail on aggregate morphology. For this system, a distal chain length of eight carbons represents an optimized balance between the influence of the hydrophobic tail and hydrophilic headgroup for the desired self-assembly of helically based nanotubes. TEM of Structures Self-Assembled Using the Room Temperature Method. A contributing factor to the poorly organized self-assembly of L-14-Azo-11-Glu using the heated procedure was its relative lack of solubility compared to the other members of the R-Azo-11-Glu family of compounds. The application of more heat to the self-assembly could not be used to better solvate the amphiphile, as the binary mixture of methanol and water (25:75 v/v) has a boiling point of less than 85 °C.46 Consequently, increasing the proportion of methanol in the solvent mixture employed for self-assembly was used to improve the order of assemblies from L-14-Azo-11-Glu, and also to further investigate the differences in the assembly of L10-Azo-11-Glu and D,L-10-Azo-11-Glu. Structures self-assembled using this room temperature (RT) method were analyzed using UV−vis to identify shifts in absorbance due to the aggregation of the azo chromophores, with samples of interest being further analyzed by TEM (summary of results in Table 2, further detail in the Supporting Information, Table S1). For RT self-assembly of L-14-Azo-11-Glu, TEM of the solvent mixture containing 90% water revealed structures that closely matched those found for L-14-Azo-11-Glu using the
Figure 6. TEM image of a nanotube self-assembled from D,L-10-Azo11-Glu.
diameter of 164 ± 43 nm and average tube wall thickness of 49 ± 12 nm, giving a calculated internal diameter of 66 nm. These dimensions are 4 times larger than the most prevalent tubes self-assembled from L-7-Azo-11-Glu and suggest the chiral interactions were less intense for D,L-10-Azo-11-Glu.38 Most previous examples have shown that when each enantiomer of an amphiphile self-assembled separately to form similar chiral aggregates of opposite hand, their racemic mixture led to the self-assembly of achiral aggregates due to the fact that the sum of chiral interactions was zero.39−43 One rare exception to this behavior was the aggregation of 1,2-bis(10,12tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC), for which both the single isomers and isomeric mixtures selfassembled into helically based tubes. This was because chiral symmetry breaking, attributed largely to the geometry of the kinked hydrophobic tail, was shown to be the driving force in the chiral self-assembly of DC8,9PC.8,44 The self-assembly of an achiral, azo-based bolaamphiphile into helically based bilayer ribbon tubes45 shows that the azo group is capable of driving chiral assembly in a similar manner. Consequently, if it is accepted that there is a helical basis to the tubes formed from D,L-10-Azo-11-Glu (certainly the straight, high-aspect-ratio tubes match those shown to be self-assembled from helical ribbons in this and related research24), then in this instance the 10005
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amorphous aggregates resulted from self-assembly using 80% water in the solvent mixture compared to the heated method. Scattered long fibers were found in both experiments, but only the fibers from the single enantiomer had any observed chiral character (Supporting Information, Figures S13 and S18). Using 40% water in the solvent mixture resulted in less amorphous aggregates but otherwise similar assembly to the 80% mixture for both L-10-Azo-11-Glu and the racemic mixture. Self-assembled structures included flat ribbons having a large range of widths and, for the single enantiomer only, lefthanded twisted ribbons (Supporting Information, Figures S12 and S17). Comparison between structures self-assembled from D,L-10-Azo-11-Glu using the heated and RT methods showed that assembly of the racemic mixture was sensitive to the temperature of assembly, contrasting to the results for L-14Azo-11-Glu. The results for D,L-10-Azo-11-Glu can be related to azo-derived bolaamphiphiles that required heat to weaken head group interactions for the optimization of azo organization and the subsequently optimized expression of chirality in the selfassembled structure.47 This suggests that different mechanisms are involved in the self-assembly of D,L-10-Azo-11-Glu and L14-Azo-11-Glu and supports that for L-10-Azo-11-Glu the azo group in the hydrophobic tail directs chiral organization through chiral symmetry breaking.
Table 2. Structures Self-Assembled from the R-Azo-11-Glu Family of Amphiphiles Using the RT Method
a All amphiphiles self-assembled in H2O:MeOH mixture and annealed at room temperature with percentage water given. b(L) = left-handed; (U) = undefined chirality.
heated self-assembly procedure, featuring mostly fine amorphous aggregates along with occasional fibrous aggregates. This demonstrated that annealing using the heated self-assembly procedure was unable to significantly affect the organization for L-14-Azo-11-Glu. However, by decreasing the water content of the solvent mixture to 40%, the structures self-assembled from L-14-Azo-11-Glu revealed a distinct blue shift in its UV−vis spectrum (discussed in detail below), and TEM showed the formation of flat, twisted, and helical ribbons. Figure 7 shows
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SPECTROSCOPIC ANALYSES CD Spectroscopy of Structures Self-Assembled from Single Enantiomers of 10-Azo-11-Glu Using the Heated Method. The circular dichroism (CD) spectral profiles for L10-Azo-11-Glu and D-10-Azo-11-Glu were approximately equal in magnitude and opposite in sign (Figure 8), which is
Figure 7. TEM image of structures self-assembled from L-14-Azo-11Glu using the RT method with a solvent mixture containing 40% water.
each of these types of ribbons with some flat ribbons on the left edge, a collection of twisted ribbons just to the right of the center, and two types of loosely coiled helical ribbons with different ribbon widths. The chiral ribbons were all left-handed, with the twisted ribbons having an average pitch of 188 ± 18 nm and the wide helical ribbons having an average pitch of 338 ± 40 nm and an average width of 126 ± 29 nm. The thinner helical ribbons were by far the most populous chiral aggregates, often found in large networks (Supporting Information, Figure S20), and had an average pitch of 529 ± 94 nm and an average width of 34 ± 3.3 nm. The variation in the self-assembly of L14-Azo-11-Glu using the different solvent concentrations shows that increased methanol better solubilizes the hydrophobic tail of the amphiphile, decreasing the influence of the hydrophobic tail on self-assembly and enabling the expression of molecular chirality in the final morphology. The RT method of self-assembly was used to provide more information on the variation between the assembly of L-10-Azo11-Glu and D ,L-10-Azo-11-Glu, and substantially more
Figure 8. CD spectra for D-10-Azo-11-Glu and L-10-Azo-11-Glu.
consistent with the observed opposite hands of aggregate morphologies using TEM. This means there was an equal but opposite chiral arrangement of the transition moments of aggregated azo chromophores for each enantiomer,47 confirming the influence of molecular chirality on the structures selfassembled from each separate enantiomer using the heated method. Further, the UV−vis peak maximum for L-10-Azo-11Glu self-assembled using the heated procedure was 322 nm (Supporting Information, Figure S11b), at the same wavelength of the crossover of the CD spectrum and indicative of a Cotton effect showing that there was exciton coupling of the chirally packed azo groups.47 10006
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Figure 9. UV−vis spectra for D,L-10-Azo-11-Glu and L-14-Azo-11-Glu monitoring the change in spectral profile for aqueous methanolic aqueous solutions containing varying percentages of water at RT.
Figure 10. ATR FTIR spectra for (a) L-10-Azo-11-Glu and (b) D,L-10-Azo-11-Glu self-assembled using the heated procedure; L-14-Azo-11-Glu selfassembled using the RT procedure with (c) 40% water and (d) 90% water in the solvent mixture. Spectra a and c translated up for clarity.
UV−vis Spectroscopy of Selected Structures SelfAssembled Using the RT Method. The UV−vis spectra for D,L-10-Azo-11-Glu and L-14-Azo-11-Glu self-assembled using the RT method with solvent mixtures containing from 0 to 90% water are shown in Figure 9 (Figure S11a in the Supporting Information shows UV−vis spectra for L-10-Azo-11-Glu). The main π−π* band48 occurring at 350 nm for the dissolved amphiphiles was blue-shifted upon precipitation of the aggregates. This is indicative of H-aggregation of the chromophores,49 the wavelength shift, and reduction in intensity of the band being attributable to exciton effects (confirming the CD results described above).50 Other spectral similarities common to the amphiphiles analyzed were that the intensity of the n−π* transition at 430 nm in the UV−vis spectra was significantly diminished for solvent mixtures containing 60−90% water. The disappearance of this band is indicative of intense and ordered interactions between azo groups,24,51 suggesting a stronger influence of the azo group on self-assembly under these conditions. For 10-Azo-11-Glu, the combined spectral profiles for selfassembly in varying solvent mixtures of the single enantiomer and the racemate are very similar. The transitional nature of the spectra for solvent mixtures with 40 and 50% water, having elements of both solubilized and assembled amphiphiles, illustrates there was a single blue shift upon precipitation (44
nm for D,L-10-Azo-11-Glu and 43 nm for L-10-Azo-11-Glu). This was distinct from the blue shift of 28 nm for L-10-Azo-11Glu self-assembled using the heated method (Supporting Information, Figure S11b). As the extent of blue shift is correlated with aggregate size for the related chromophore organizations possible for ordered bilayer assembly of amphiphiles,52,53 the greater blue shift for the largely amorphous structures assembled at RT confirms that the organization of these structures is more strongly influenced by the azo group. The combined spectral profiles for L-14-Azo-11-Glu vary from those for 10-Azo-11-Glu in that two distinct blue shifts occurred as the amount of water in the solvent mixture was increased. The first blue shift of 32 nm for the solvent mixture with 40% water distinguishes the ordered, chiral assemblies found by TEM, while the amorphous aggregates found for solvent mixtures containing 90% water are characterized by the second, larger blue shift of 44 nm and by the disappearance of the n−π* transition at 430 nm discussed above. This matches the behavior of 10-Azo-11-Glu in that the apparently amorphous aggregates have a greater blue shift and exhibit better azo organization than the visually chirally organized ones. ATR FTIR of Selected Self-Assembled Structures. Figure 10 shows the ATR FTIR spectra for lyophilized structures self-assembled from L-10-Azo-11-Glu and D,L-1010007
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shoulders for the 90% solvent mixture and heated self-assembly (Supporting Information, Figure S21), indicative of weaker hydrogen bonding and hence a reduced headgroup contribution to the organization of aggregates assembled using ≥75% water. This is complementary to the increased azo group organization observed for structures assembled from 90% water using UV−vis.
Azo-11-Glu using the heated method and L-14-Azo-11-Glu using the RT method with 40 and 90% water in the solvent mixture. Highly consistent in all spectra are the vibrations between 1582 and 1584 cm−1 and 1600−1602 cm−1, characteristic for aromatic CC stretching in azo groups within hydrophobic tails of the amphiphiles.54 Also consistent was the presence of carbonyl stretching vibrations above 1700 cm−1, attributed to carboxylic acid groups, confirming the amphiphiles were uncharged and the acid groups not deprotonated.55 More varied were vibrations associated with the amide moieties in the head groups. L-10-Azo-11-Glu selfassembled into multiple structures, as observed by TEM, and this was reflected in the ATR FTIR spectrum (Figure 10a) with multiple amide carbonyl stretching vibrations, including an intense one at 1656, a weaker one at 1673, and a shoulder at 1690 cm−1 (the latter possibly a sideband of the intense peak56), two N−H stretching vibrations at 3283 and 3322 cm−1, and a relatively broad N−H bending and C−N stretching vibration at 1549 cm−1.56,57 These bands are all characteristic of hydrogen bonding between amide groups from adjacent molecules32,57−59 and suggest at least two distinct, ordered hydrogen bonding modes for the different self-assembled structures. In contrast, the ATR FTIR spectrum from the racemic mixture (Figure 10b) is reflective of the dominant nanotube morphology, having a single intense, amide carbonyl stretching vibration at 1694 cm−1, a single N−H stretching vibration at 3342 cm−1, and a relatively sharp N−H bending and C−N stretching vibration at 1552 cm−1. The wavenumber for the amide carbonyl stretching vibration for D,L-10-Azo-11-Glu is uncharacteristically high for amide hydrogen bonding between neighboring amphiphiles in helical bilayers.32,57−59 In this instance, the position of this relatively broad amide band, along with its shoulder that runs into the carboxylic acid carbonyl stretching vibration at 1742 cm−1, is suggestive that conformational stress is imparted onto the C O group due to strong H-bonding interactions from multiple or unaligned sources, such as the carboxylic acid protons.58 This complexity is also reflected in the movement of all the racemic mixture’s amide bands and carboxylic acid carbonyl stretching vibrations to higher wavenumbers than any of the equivalent bands for the single enantiomer. Such movement is usually equated with weaker hydrogen bonding for the N−H stretching vibrations and the amide and carboxylic acid carbonyl stretching vibrations but with stronger hydrogen bonding for the N−H bending and C−N stretching vibrations.57 As such, this phenomenon cannot be explained by a simple transition in headgroup order and shows that the hydrogen bonding interactions between head groups, and accordingly the headgroup organization, are significantly different between structures self-assembled from the single enantiomer and the racemic mixture. This is in accord with the analysis of the different self-assembled structures for the single enantiomer and the racemic mixture made using TEM and UV−vis and provides further support that the alternate chiral symmetry breaking mechanism drives chiral assembly for the racemic mixture. The spectrum for structures self-assembled from L-14-Azo11-Glu using the RT procedure with a solvent mixture containing 40% water (Figure 10c) has an amide carbonyl stretching vibration and an N−H bending and C−N stretching vibration in regions characteristic of hydrogen bonding between amide groups in adjacent amphiphiles, occurring at 1648 and 1549 cm−1, respectively. These amide bands became indistinct
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CONCLUSION The molecular design of the azo amphiphiles with glutamic acid head groups was successful in promoting self-assembly into chiral aggregates. The relationship between increasing distal chain length and the observed increasing then decreasing chiral order of the aggregates, along with the supporting spectroscopic results, demonstrated that the influences of the hydrophilic headgroup and hydrophobic tail on organizing self-assembly must be balanced to achieve the ultimately desired chiral morphologies. For the assembly of the single enantiomers, the chiral order was optimized for L-8-Azo-11Glu, resulting in the formation of helically based nanotubes for which molecular-chirality-directed packing was shown to control the chirality of the aggregate. Concomitantly, the azo group was an important component in the chiral self-assembly of single enantiomers and integral in generating the nanotubes assembled from D,L-10-Azo-11-Glu, for which chiral symmetry breaking was shown to be the mechanism directing chiral assembly. The design of the R-Azo-11-Glu family of amphiphiles, incorporating strongly organizing groups in both the hydrophilic headgroup and the hydrophobic tail, represents an interesting development in the rational design of amphiphiles for self-assembly into chiral ribbons. Specifically, the tension between headgroup and azo group directed order could adjust the chiral order of the self-assembled structures through changes in molecular structure of the amphiphiles and their conditions of self-assembly. This behavior can be used to finetune the self-assembled structure to achieve a specifically desired chiral morphology. Additionally, switchable structures could be designed on the basis of varying solvent and temperature conditions with potential for use in applications such as drug delivery.
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ASSOCIATED CONTENT
S Supporting Information *
Table summarizing self-assembly results, synthetic experimental procedures and product analyses, additional TEM images, and UV−vis and ATR FTIR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +61 8 8302 5695. Fax: +61 8 8302 5639. Present Addresses ∥
Mawson Institute, University of South Australia, Adelaide, South Australia, 5095, Australia. ⊥ College of Chemistry, Chemical Engineering and Materials Science of Soochow (Suzhou) University, Suzhou 215123, China. # Gelest Inc., 11 East Steel Road, Morrisville, Pennsylvania 19067, USA. 10008
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Notes
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The authors declare no competing financial interest.
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