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Tunable Helical Assemblies of L-Alanine Methyl Ester-Containing Polyphenylacetylene Bing Shi Li,*,† Jacky W. Y. Lam,‡ Zhen-Qiang Yu,† and Ben Zhong Tang‡ †

School of Chemistry, Shenzhen University, Shenzhen, 518060, China Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

‡

ABSTRACT: The self-assemblying behaviors of L-alanine methyl ester-containing polyphenylacetylene (PPA-Ala, in Chart 1) were investigated upon the evaporation of its solvent on mica and on air/water interfaces. The introduction of chiral amino acid attachments to the polyphenylacetylene backbone induced a helical conformation of the backbone, which was stabilized by various noncovalent interactions, especially hydrophobic effect and hydrogen bonds. The helicity of the polymer was further amplified in its higher-order selfassemblies as the formation of helical fibers on the surface of mica upon natural evaporation of its THF solution. By LB technique, the polymer chains were guided to form ordered parallel ridges and highly aligned, with their helical conformation still remaining. The reorganization of the chiral polymer chains on air/ water interface was associated with the additional hydrophobic effect of PPA-Ala on an air/water interface. The polymer backbones had to adopt different arrangements to minimize their contact with water, and this adjustment led to the formation of aligned polymer ridges under proper surface pressure.

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INTRODUCTION Biological supramolecular assemblies, which are best exemplified by the double helix structure of DNA and triple helix structure of collagen, teach us important lessons in the construction of helical architectures on the nanoscale.1 Chirality in natural molecular systems is important to induce helical assemblies, which are stabilized by noncovalent interactions of molecules.2 Amino acids are the constitutional components of

in different manners. Following judicious patterns of noncovalent interactions, such as hydrogen bonding and solvophobic repulsion, the synthetic chiral molecules have the capacity to achieve self-organizational architectures,4−7 which play a pivotal role in influencing the properties of the molecules, and thus their ultimate functions. The helical selfassemblies stabilized by the noncovalent interactions are usually sensitive to their environmental conditions and vary with the types of solvents8,9 and different surface/interfaces.10 External conditions play an important role in directing the noncovalent interactions, which further influence the self-assemblies of the chiral molecules and consequently the optical properties. Good characterization of the self-assemblies under different conditions is a critical step toward the successful application of the chiral molecules in the future. In this paper, we study the noncovalent interaction tuned self-assemblies of PPA-Ala formed on the surface of mica and on an air/water interface with the LB technique.11 The LB technique has been successfully applied to prepare monolayers of a broad type of molecules, such as small organic molecules,12−17 polymers,18−21 inorganic molecules,22 inorganic/organic hybrids,23−27 and biological molecules.28−32 By applying an asymmetric force field to the amphiphilic molecules, the hydrophilic parts of the molecules tend to pack on the air/water interface and the hydrophobic portions are inclined to avoid contact with water molecules. This

Chart 1. Chemical Structure of PPA-Ala

proteins with a strong capacity to self-assemble via various noncovalent interactions. Incorporation of the naturally appearing chiral amino acids to the synthetic polyphenylacetylene backbone generates a kind of novel molecule with attractive properties, because such hybridization of chirality and synthetic conjugated polymer is expected to lead to the melding of multiple novel features.3 The introduction of chiral atoms to the pendant groups of the polymer brings bias to the polyacetylene backbone, which can induce asymmetric assemblies of the molecules, giving rise to helical assemblies © 2012 American Chemical Society

Received: January 5, 2012 Revised: March 13, 2012 Published: March 14, 2012 5770

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when the solvent was changed to DCM, the peak was inverted, indicating that the “handedness” of the helical polymers has reversed with the variation of the external solvent. Thus, both the chirality of the chemical structures of the amino acid attachments and the external environments have exerted considerable impact on the chain helicity of the polymer as well.

method is efficient and convenient for the preparation of ordered amphiphilic molecular monolayers. For example, amphiphilic peptides can be successfully tuned to form parallel ridges.28 PPA-Ala bears hydrophilic amino acid attachments and a hydrophobic PPA backbone, so the polymer is also amphiphilic like peptides; therefore, this method is theoretically suitable for the preparation of PPA-Ala film. However, preparation of amphiphilic helical polymer film with this method is a relatively less touched area. Our results indicated that, upon the evaporation of its solvents, PPA-Ala formed helical nanotubes as revealed by AFM and TEM, which are randomly deposited on the surface of mica. With the LB technique, the randomly located helical assemblies of PPA-Ala can be properly tuned to alignment, thus providing the possibility to prepare ordered chiral film.

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EXPERIMENTAL SECTION

Materials. PPA-Ala was synthesized using [Rh(nbd)Cl]2 as a catalyst according to the method in our previous publication.33 CD Spectra. Circular dichroism (CD) measurements were performed on a Jasco J-720 pectropolarimeter in 1 mm quartz cuvettes using a step resolution of 0.2 nm, a scan speed of 50 nm/min, a sensitivity of 0.1°, and a response time of 0.5 s. Each spectrum was the average of 5−10 scans. AFM. AFM samples were prepared by allowing a droplet of 4 μL diluted polymer solution of desired concentration to evaporate on the surface of mica at ambient conditions. AFM investigations were performed on a Nano IIIa atomic force microscope (Digital Instruments, Santa Barbara, CA) operating in tapping mode using hard silicon cantilever tips with a spring constant of ∼40 N/m. TEM. TEM samples were prepared by depositing a droplet of 5 μL polymer solution of desired concentration on the surface of ultrathin carbon film coated copper grids (Ted Pella, Inc.) at ambient conditions. Images were captured with the TEM (Tecnai10, Philips) operating at 100 kV. Langmuir Film. π−A isotherm of PPA-Ala on the subphase of H2O was measured at 23 ± 1 °C, using a Sigma 70 Cam 200 LB 5000 trough (length 364 mm × width 75 mm), equipped with a Wilhelmy platinum plate. The water used as subphase was ultrapure water with the resistance 18.2 Ω. After ensuring that the water surface was clean, ∼210 μL THF solution of PPA-Ala (concentration = 0.033 mg/mL) was spread on a water surface using a microsyringe. π−A isotherms were measured after allowing the film to equilibrate for 15 min by closing the barriers at a constant rate of 10 mm/min. LB films of PPAAla was transferred to a newly cleaved mica at 5 mm/min, at a surface pressure of 10 mN/m.

Figure 1. CD spectra of PPA-Ala in its THF, MeOH, and DCM solution. Polymer concentration (mM): ∼1.5. Temperature: ∼22 °C.

Superhelical Assemblies Formed by PPA-Ala. In nature, the chain helicity is usually amplified in the morphological structures of their high-order assembling architectures. It is expected that the chain helicity of PPA-Ala is further amplified in their superhelical assemblies. We then studied the self-assembling structures of PPA-Ala with AFM. PPA-Ala formed left-handed superhelical fibers which are several micrometers long upon the evaporation of THF, as shown in Figure 2. The fibers have a broad distribution in the widths, which are in the range from ∼4.2 to ∼26.8 nm because thicker helical fibers were formed by the helical braiding of thinner fibers. Helical fibers were also formed upon evaporation of DCM solution of PPA-Ala, which are much thicker than those formed upon the evaporation of THF, with the widths in the range ∼14.7−48.8 nm, as shown in Figure 3. Helical pitches

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RESULTS AND DISCUSSIONS Chain Helicity of PPA-Ala in Solutions. The introduction of chiral amino acid attachments into polymer PPA backbone creates multiple chiral centers, which will exert asymmetric force fields to the PPA backbone, leading to the helical rotation of polymer chains. We then check the chain helicity of the polymer with CD spectroscopy,34 which is a powerful tool to study helical structures. As a control experiment, the CD spectrum of the monomer of PPA-Ala is also measured. Strong Cotton effects associated with the absorptions of the polyacetylene backbone are observed in the spectra of PPAAla in its methanol, THF, and DCM solution at the wavelengths of 310 and 372 nm, whereas the monomer is CD-inactive. This clearly confirms that the polymer chain takes a helical conformation with a large excess in one “handedness”. The peak intensity weakened when the solvent was changed from THF to methanol, suggesting that the relative populations of right- and left-handedness of the polymer have changed;

Figure 2. Phase image of superhelical fibers formed with a left-handed twist by PPA-Ala upon the evaporation of its THF solution (concentration of PPA-Ala: 1−5 μg/mL), imaged on a Nano IIIa atomic force microscope (AFM) in tapping mode. 5771

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apparently bundled in a side-by-side way which facilitated the coalescence of polymer chains to form thick nanofibers, and meanwhile linked up in a head-to-tail manner to lengthen the nanofibers. The superhelical fibers formed by PPA-Ala are randomly oriented, and it will be more desirable for their potential application if the helical assemblies are formed in an ordered way. The amphiphilicity of the chiral polyacetylene provides the possibility to align the polymer chains using the LB technique to form regular assemblies.28,29 Inspired by this idea, we then check the possibility to prepare orderly oriented helical assemblies with LB technique.

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LANGMUIR-BLODGET FILM OF PPA-ALA The PPA backbone is nonpolar and not soluble in water, but its polar amino acid pendants have high affinity to water; thus, the amphiphilic polymer is expected to adopt a different assembly on the water surface different from that in its THF solution. To explore the self-assembly behavior of the polymers on the air/ water interface, we further prepared a single molecular layer of the polymers and check the morphological structures of the assemblies. The THF solution of the polymer was spread on the surface of water droplet by droplet, and the surface pressure of the polymers gradually increased by compressing the water surface until the point that a compact polymer monolayer is formed. The surface pressure−molecular area (π−A) isotherm of PPA-Ala prepared at 20 mN/m is given in Figure 5. The isotherm of PPA-Ala exhibits two regions: one is less than 15 mN/m, where the surface pressure region gradually increased with compression, and the other region is a high surface pressure region, where the surface pressure abruptly increased with compression. The limiting area of PPA-Ala is 0.68 nm2. We then check how the polymer chains assemble on air/ water interfaces. The monolayer of the polymer on the air/ water interface was gently transferred to the surface of newly cleaved mica (with the hydrophilic side of the monolayer facing the substrate and the hydrophobic side facing outward). The morphology of the LB monolayer is then examined with AFM, as shown in Figure 6. For self-assemblies of PPA-Ala on the air/

Figure 3. Height image of superhelical fibers formed by PPA-Ala upon the evaporation of DCM solution (concentration of PPA-Ala: 5 μg/ mL), imaged in tapping mode. Helical fibers labeled with arrows show the most easily discernible helical pitches.

were most easily recognized from the fibers labeled with arrows, and both left-handed and right-handed helical fibers were observed. To further characterize the helical fibers, TEM experiments were also carried out. Upon the evaporation of THF on the carbon-coated copper grid, hierarchical structures of helical fibers are formed as illustrated in Figure 4. The sharp contrast of the edges and the interior of the helical fibers indicates that the helical fibers are helical nanotubes. The thinnest nanotubes observed are ∼4.8 nm wide, which are approximately the width of the double polymer chains. Thin fibers helically rotated together to form rope-like thick fibers. PPA-Ala also formed tubular structures upon the evaporation of DCM solution (images were not provided). Both AFM and TEM images revealed the formation of hierarchical helical nanotubes by PPA-Ala, which have lengths of several micrometers long and widths up to ∼50 nm wide, while comparing with the theoretical length and width of a single polymer chain (provided that the backbone of the polymer has fully extended, the maximum length is ∼410 nm and the width is ∼2 nm). The elementary polymer chains were

Figure 4. TEM image of superhelical fibers formed by PPA-Ala upon the evaporation of its THF solution (concentration of PPA-Ala: 5 μg/mL) on a carbon-coated copper grid. 5772

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the nonpolar PPA backbone and water molecules, and attractions between amino acid attachments and water. The balanced effect of the above two kinds of interactions thus exerted an additional asymmetric force field to the amphiphilic polymer chains and destroyed the already existing balance and tuned the arrangement of the chiral polymer chains. The polymer chains would accordingly reorganize to achieve a new balance between the interpolymer interactions and polymer− water interactions. The nonpolar PPA backbones had to alter their locations to minimize their contacts with the polar water molecules. As a consequence of the adjustment of the polymer chains, superhelical aggregates like that in Figure 2 did not form; instead, parallel ridges of polymers were formed upon a proper surface pressure and pressing angle, To confirm whether the helical conformation of the polymer chains still remains, we also measure the CD spectra of the LB films with 20 layers (data not provided). The LB film displays similar absorption like that in their solution, clearly indicating that the polyphenylacetylene backbones still take a helical conformation while forming LB film. Thus, the LB technique could be employed as a tool to prepare highly aligned chiral films. Self-assemblies of chiral polymers have aroused considerable interest due to their potential application in the field of materials and devices. Our study offers an important approach to revealing the different impacts of solvent and surface/ interface on the helical self-assemblies and helps to better tune the molecular assemblies.

Figure 5. π−A Isotherm of PPA-Ala on the subphase of water; concentration of its THF solution of PPA-Ala: 0.097 mg/mL.

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CONCLUSION

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AUTHOR INFORMATION

In summary, we have reported the tunable assemblies of chiral polymers PPA-Ala. AFM images revealed that PPA-Ala selfassembled into superhelical fibers upon the evaporation of their THF and DCM solutions. TEM imaging further revealed that the helical fibers have tubular interiors. The self-assembling structures of PPA-Ala on the air/water interface prepared with the LB technique showed that PPA-Ala formed aligned ridges. Formation of the diverse morphologies under different conditions is attributed to the different driving forces for selfassembly. In THF, PPA-Ala self-assemble into superhelical fibers, but on the air/water interface, the additional lateral hydrophobic effect was involved and broke the original balance between the polymers and the environment. The polymer chains thus had to reorganize under the guidance of the new balance of noncovalent interactions among polymers, and between polymers and water molecules, and self-associated into different structures.

Figure 6. High-resolution image of the aligned polymer chains on air/ water interface. Regions labeled with 1 and 2 have different orientation and gap widths.

water interface, the majority of the surface is covered with a layer of flat film. By scrutinizing into the flat layer, parallel ridges were found with an even width of ∼9.6 nm and with the same orientations with only a few exceptional ones having wider gaps and slightly deviated orientation. For example, the gap labeled with 1 has the width of ∼18.4 nm, while for gap 2, its orientation is slightly deviated from most of the gaps. There are also defects in the LB layer, such as the small holes among the polymer ridges, thus making it available to measure the height of the ridges, which is ∼0.56−0.73 nm, corresponding to the thickness of a single polymer chain. Thus, we have successfully fabricated an ordered monolayer of chiral polyacetylene. The characteristic morphology of the ridges formed by PPAAla shared great resemblance with that of polypeptide on air/ water surface.28 On the surface of water, PPA-Ala did not associate into randomly oriented superhelical fibers, the typical morphological structures formed in their THF solution. The dramatically different assemblies formed on air/water interfaces were related with the different noncovalent balances on the air/ water interface from that in the THF solution. THF is a good solvent for both the PPA backbone and amino acids, so that the polymer backbone and attachments can extend well in THF solution. While on the air/water interface, because water is a poor solvent for the PPA backbone but a good solvent for amino acid attachments, there exists great repulsions between

Corresponding Author

*Tel: 86-755-26558094, Fax: 86-755-26536141, E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant number: 21104046), Outstanding Youth Investigator fund of Shenzhen (Grant Number: JC201005250038A), and the Key Laboratory of New Lithium-Ion Battery and Mesoporous Materials. 5773

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