Polyacetylene Nanostructures

Encyclopedia of Nanoscience and Nanotechnology

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Polyacetylene Nanostructures Kevin K. L. Cheuk, Bing Shi Li, Ben Zhong Tang Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

CONTENTS 1. Introduction 2. Unsubstituted Polyacetylene 3. Polyacetylenes with Amino Acid Ester Pendants 4. Polyacetylenes with Amino Acid Pendants 5. Polyacetylenes with Ethylene Oxide Pendants 6. Conclusions Glossary References

1. INTRODUCTION Many complicated biological tasks, such as transforming chemicals in localized areas, transporting materials between cells or organs, and defending against foreign enemies, are handled by functionalized biopolymers. As can be seen from enzymes, hemoglobin, liposomes, etc. [1, 2], nature has tailored each of the biomacromolecules with a suitable structure and arranged or assembled the structural components in an organized way. To achieve an organizational structural hierarchy, the living world relies on a noncovalent architecture. Self-assembly is the organizational motif that is ubiquitously utilized by biological systems and underlies the formation of a wide variety of complex biological structures. The process obviously affords high flexibility, high efficiency, etc., but, importantly, still allows precise control over the formation of structural morphologies [3, 4]. Many of the concepts of biological self-assembly are derived from studies of tobacco mosaic virus, which is a helical assembly composed of 2130 identical protein units [5]. Though forming this complex protein requires a delicate management among the molecular interactions (van der Waals, electrostatic, hydrophobic, and steric interactions, hydrogen and coordination bonds, etc.), the assembly mechanism has exemplified that the association process is entropically driven [5, 6] and the final bioassembly ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

occupies the state of a thermodynamic minimum. In general, the success of organizing structures by self-assembly depends on how successfully these interactions can be utilized to bind molecules together. Supramolecular chemistry is inspired by the study of the basic features of these secondary interactions, which contributes to our understanding not only of living systems, but also nonliving systems. Based on these interactions, supramolecular chemistry has assembled a number of well-defined, regular nonbiological architectures [7–16], which are expected to generate new properties and to create new materials [17–19]. Making precise nano- and mesoscopic structures is still one of the greatest challenges now facing scientists and technologists. Molecular self-assembly is one of the feasible ways of approaching this goal. Although self-assembly originated from the study of small molecules, it should also be a strategy that is applicable to big molecules. Through cooperative self-assembly processes, macromolecular chains may also be hierarchically organized into large and complex but precise and ordered structures. Nature has presented wonderful examples in this regard: the
-helix and -sheet of proteins assemble into defined tertiary and quaternary structures. The natural process is truly amazing, and judiciously designed nonnatural systems should be able to mimic the bioprocesses to self-assemble into biomimetic structures. There are many successful reports addressing this idea. In this article, we will review the biomimetic hierarchical structures generated by unsubstituted and substituted polyacetylenes.

2. UNSUBSTITUTED POLYACETYLENE Polyacetylene is the simplest linear conjugated macromolecule and is one of the best-known organic conductors [20–24]. The discovery of the metallic conductivity of doped polyacetylene has opened up a fascinating field of research, conductive macromolecules, which have brought us to a plastic-electronics revolution, with many exciting implications in high technologies, as evidenced by the award of the Nobel Prize in chemistry in 2000. High-molecular-weight Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (703–713)

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polyacetylene can be synthesized from the acetylene polymerization initiated by the Ziegler–Natta catalyst (Chart 1) [25, 26], but under “normal” conditions, only a featureless morphology can be produced [27–29]. The fundamental norm lasted until Akagi, Shirakawa, and co-workers investigated the asymmetric polymerization of acetylene [30]. Through mixing of a chiral nematic mesogen (R)- or (S)-2 (Chart 2) with an organometallic catalyst, multistranded helical polyacetylene fibers, a few hundred nanometers in diameter, were successfully prepared. It was observed that the polymerization conducted in the presence of the (R)-form nematic liquid crystals led to the formation of lefthanded helical fibrillar morphologies (Fig. 1), whereas that in the presence of the (S)-form gave polymer fibrils twisting in the opposite direction. With control over optical purity or the type of chiral dopant, it was possible to generate helical morphologies with different helical pitches.

Figure 2. TEM image of two regular coiled fibers, symmetrically grown on a single copper crystal and prepared by polymerization of acetylene at ∼250  C. Reprinted in part with permission from [31], Y. Qin et al., Org. Lett. 4, 18 (2002). © 2002, American Chemical Society.

catalyst

number, node diameter, node length, helix pitch, and fiber diameter. A small amount of the acetylene monomer may have initially bound to the chiral catalyst in an asymmetrical way, and the chiral chains propagated with other monomer molecules also inserted in an asymmetrical fashion.

HC CH n 1

n HC CH

Chart 1

O O

(CH2)6 O

C5H11

(CH2)6 O

C5H11

(R)-(+)- or (S)-(-)-2

Chart 2

Another helical structure of polyacetylene was prepared at high temperatures (∼250  C) [31]. Similar to the Akagi and Shirakawa reaction, Qin et al. also used an asymmetric condition to build the helical polymers (Fig. 2). However, the catalyst system used by Qin is different from that used by Akagi and Shirakawa: the former used a native asymmetric catalyst, copper tartrate, and the latter used an achiral catalyst blended with a chiral dopant. The fibers obtained were obviously different; the former afforded single-stranded and more regular helical structures. Interestingly, the fibers were formed in such a way that two helical strands simultaneously grew on a single copper nanocrystal. The two strands were opposites in an absolute helical sense but identical in cycle

Figure 1. SEM micrographs of left-handed helical polyacetylene fibers prepared in a chiral nematic reaction field. Reprinted with permission from [30], K. Akagi et al., Science 282, 1683 (1998). © 1998, American Association for the Advancement of Science.

3. POLYACETYLENES WITH AMINO ACID ESTER PENDANTS The two examples discussed above clearly demonstrate that the formation of regular helical structures closely relates to the asymmetrical force field. Chirality is a structural feature of many natural constituent components (amino acids, saccharides, etc.), and biological systems exhibit helicity at all organizational levels (e.g.,
-helix of proteins, double helix of DNA, triple helix of collagen, and spiral bacterium of Spirillum). Obviously, rather than using an external stimulus, nature has put an asymmetrical field in internal building units, permitting better control over the final assembly structures. Many scientists have taken this approach and used asymmetrical building blocks to construct chiral polymers [32–64]. The (unsubstituted) polyacetylene chain is symmetrical and achiral. However, when pendant groups are introduced, the chain symmetry is broken. Theoretical computation suggests that the chain segments of a substituted polyacetylene bearing bulky appendages can take on helical conformations [65]; a majority of the chain segments with long persistence lengths will rotate in one preferred screw sense when the appendages are chiral species. In addition to fusing the chiral species into the polymer structure at the molecular level by covalent bonds, the polyacetylene chains could also spirally rotate in asymmetrical force fields when the  electrons of the conjugated backbone and/or the functional groups of the achiral substituents experience molecular interactions with external chiral species [66]. With the aim of preparing asymmetrical polyacetylenes for the study of hierarchiral structures, Shinohara, Shigeka, and co-workers synthesized an optically active polyphenylacetylene bearing menthoxycarbonylamino groups (Chart 3), which possessed a high molecular mass 1 × 106 Da) and a high stereoregularity (Z content ∼90 mol%) [67]. Under the influence of the

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Polyacetylene Nanostructures

chiral pendants, the achiral main chain of 3 became CD active and absorbed strongly in the long-wavelength region (∼400 nm). A possible reason for this is the formation of a secondary structure with an excess of one-handed helical structure. Further investigation of its higher-order structure by scanning probe microscopy revealed that the polymer afforded a helical quaternary structure. From Figure 3a, it can be seen that the two polymer strands are intertwined to form a right-handed double-helical structure. Each strand is also helically rotating and has a width of 9 Å, which matches the width of the polyphenylacetylene backbone with a Z configuration, as suggested by the molecular mechanics calculation. C

C n

H

N

O

H

3

Chart 3

The
-helix of the polypeptide chain exemplifies nature’s ability to construct complex chiral structures by making use of hydrogen bonding provided by each amino acid residue. Inspired by this phenomenon, in 1997, our group embarked on a research program on the development of

new biomimetic polymers, using amino acids as chiral ingredients. Through the incorporation of the naturally occurring building blocks into the conjugated polymer structure, we synthesized a variety of new amphiphilic polyacetylenes [66], examples of which are given in Charts 4 and 5. The amino acid substituents not only conferred helical conformations on the polyacetylene chains but also endowed them with hydrogen bonding capacity, which is an important secondary force for self-assembly. The polymers were therefore able to self-fold into higher-order structures. An example of the organizational morphologies is shown in Figure 4. By a simple process of solvent evaporation, polymer 4, which bears isoleucine appendages, organized into helical cables with left-handed twists. The atomic force microscope (AFM) image revealed that the helical fibrils were, on average, >500 nm in length and ∼20.5 nm in width. These sizes are much bigger than the dimensions of a single chain, indicating that nanofibrils are assemblies of multiple strands of polymer chains. Similar to the fibrous proteins of
-keratin [68], the helical chains of 4 may have been plaited together via a folding process aided by noncovalent interactions such as interstrand hydrogen bonding [69]. The involvement of noncovalent forces in the assembly process was clearly confirmed by the partially unraveled helixes generated by the repeated horizontal scans of the AFM tip on the surface of mica in the direction perpendicular to the long axes of the nanofibers. The unraveled morphology was obviously due to the breakage of the physical bonds that had glued the chain segments together. C

C

C

n

C

C

n

H

H H N

H

O

N

H

O

O

C

n

H

H

O

C

O

N

O

H

H

O O

N

O

C H 7

C

C

R

H 5

H

O

O

H 4

C

n

H

6

O C

Chart 4 C

C

C

n

H

C

C

n

H N O O

H

H

O

C

n

H

H

N O

H

H

O

N O

O

n

H

H

H

O

N

O

O

H

O

O C

C

H

H

H

H

8

9

10

11

Chart 5

Figure 3. (a) Low-current STM height image of interwound polymer chains of 3 on highly oriented pyrolytic graphite at room temperature. (b) String model of the interwinding polymer chains. Reprinted with permission from [67], K. Shinohara et al., J. Am. Chem. Soc. 123, 3619 (2001). © 2001, American Chemical Society.

Similarly, polymer 5, which bears smaller pendants of alanine groups, also formed helical fibrils. The structural details are different, however; the helical cables of 5 are righthandedly winding, which is different from the left-handed twisting observed in 4, and the cables of 5 (∼100 nm in diameter) are on average thicker than those of 4. Why the assembly behavior of 5 is different from that of 4 is unclear, but it may be related to the packing process benefiting from

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Figure 4. AFM (A) height and (B) phase images of left-handed helixes formed upon natural evaporation of a methanol solution of 4 (36.6 M) on newly cleaved mica. Reprinted with permission from [62], B. S. Li et al., Polym. Prepr. 42, 248 (2001). © 2001, American Chemical Society.

the molecular structure of 5. The smaller pendants in 5 may facilitate close packing and interchain hydrogen bonding between the polymer chains due to a less pronounced steric effect arising from the side groups. Thus, more polymer chains can associate together to form single strands, as verified by the thicker strands, which eventually fold up to form the multistranded cables. With the proline appendages, 6 is incapable of forming hydrogen bonds. It is thus of interest to know whether the macromolecular chains can associate into defined structures without the involvement of this noncovalent interaction. Although 6 in methanol displayed only weak CD activity [70], when its dilute solution (38.9 M) was dropped onto freshly cleaved mica, tortuously twisted cables of doublestranded helical fibrils were formed, an example of which is shown in Figure 6. The morphology is composed of two helical strands (∼150 nm in width), which braid together to form the final supercoils. Each of the strands seems to be the result of winding of many single-stranded fibers, and the whole structure bears a good likeness to the superhelical structure formed by the menthol-containing polymer (3) already discussed (cf. Fig. 3). The similarity suggests that the

Figure 5. AFM (A) height and (B) phase images of self-assembly morphologies of multistranded right-handed helixes formed upon natural evaporation of a dilute methanol solution of 5 (43.2 M) on newly cleaved mica. Reprinted with permission from [62], B. S. Li et al., Polym. Prepr. 42, 248 (2001). © 2001, American Chemical Society.

Polyacetylene Nanostructures

Figure 6. AFM deflection images of tortuously twisted nanofibers formed upon natural evaporation of a methanol solution of 6 (38.9 M) on freshly cleaved mica. Reprinted with permission from [42], K. K. L. Cheuk, Ph.D. Dissertation, Hong Kong University of Science & Technology, 2002.

cyclic molecular structures in the polymers favor the formation of the loose double-helical quaternary structure. Molecular information, such as amino-acid sequence and chain chirality and amphiphilicity, encoded in the primary structures of biomacromolecules plays a primary role in determining their native folding structures; for example, l-glutamic acid segments in proteins often give
-helix structure, whereas l-isoleucine segments most frequently induce -sheet formation [1, 2, 71]. The folding structures can, however, be varied or denatured by the changes in the environmental surroundings of the biopolymers, because of the noncovalent nature of the supramolecular assembly. Loss of body fluid, for example, can transmute organizational structures of proteins by dehydration or deprivation. Similar phenomena have also been observed in the self-assembly processes of the amino acid-containing polyacetylenes. For example, whereas a methanol solution of 7 gave pearlshaped structures upon natural evaporation, its tetrahydrofuran (THF) solution gave helical cables with a clear left-handed twist under similar assembly conditions (Fig. 7). The polymer chains of 7 may take an extended conformation in THF because THF is a good solvent of both the backbone and the pendants of the polymer. During the aggregation process accompanying solvent evaporation, the extended helical chains may twine around each other via interchain

Figure 7. AFM images of (A) clustered pearls, (B) helical cables, and (C) clustered pearls plus helical cables formed upon natural evaporation of (A) methanol, (B) THF, and (C) methanol/THF (1:7 by volume) solutions of 7 on the surfaces of newly cleaved mica. Scale bars (nm): (A) 250, (B) 125, (C) 500. Concentration of polymer solutions: ∼1–5 mM. Reprinted with permission from [72], Li et al., Macromolecules 36, 77 (2003). © 2003, American Chemical Society.

Polyacetylene Nanostructures

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hydrogen bonding to give twisted strands, further association of which in different multiplicities (doublet, triplet, etc.) will give thicker fibrils of different diameters—this assembly mechanism is clearly suggested by the image shown in Figure 7B. It is envisioned that evaporation of a methanol/THF solution of 7 may generate transition morphologies containing both micellar and fibrillar structures. This was indeed the case; as shown in Figure 7C, the organizational morphologies obtained from the mixture solvent system showed combined features of the assembly structures obtained from the individual solvent systems.

4. POLYACETYLENES WITH AMINO ACID PENDANTS Whereas the amino acid pendants of polymers 4–7 are esterified, polymers 8–11 carry “free” amino acid pendants. The change in the molecular structure is expected to affect the assembly structures of the polymers. Different from its “polyester” counterpart 5 (cf. Fig. 5), “polyacid” 8 formed pigtail-like nanofibers upon the natural evaporation of its methanol solution under ambient conditions (Fig. 8). A number of fibrils were bundled together to form plaits with loose tails. Each fibril comprised many polymer chains that were stuck together through interchain hydrogen bonds. “Polyacid” 9 also formed helical fibrils. Close examination by transmission electron microscopy (TEM), however, revealed that the fibrils possessed a hollow structure (Fig. 9); that is, the fibrils were actually nanotubes. The tubes wound around each other to give twisted hollow structures. As marked by the rectangle in Figure 9A, there are many hollow rings or spherical vesicles in addition to the twisting tubes. These structures are remarkable, because nanotubes and vesicles have seldom been formed by homopolymers,

Figure 8. AFM height image of pigtail-like nanofibers formed upon natural evaporation of a methanol solution of 8 (46.0 M) on newly cleaved mica. Reprinted with permission from [62], B. S. Li et al., Polym. Prepr. 42, 248 (2001). © 2001, American Chemical Society.

Figure 9. TEM images of vesicles and nanotubes formed upon natural evaporation of a methanol solution of 9 (19.3 M) on a carbon-coated grid under ambient conditions. Reprinted with permission from [42], K. K. L. Cheuk, Ph.D. Dissertation, Hong Kong University of Science & Technology, 2002.

although they have been observed in a few copolymer systems [13, 73–79]. The coexistence of the vesicular and tubular structures suggests that the vesicles are the building units of the nanotubes; stringing the former together leads to the formation of the latter. In the polar solvent of methanol, the polyacetylene main chains of 9 may associate together with their pendants facing outward, in an effort to minimize the exposure of their hydrophobic backbones to the polar environment [80]. Curling of the bi- or multilayer sheets would result in the formation of a closed vesicular structure. Following the mechanism of coalescence [81], the vesicles may fuse together to form the tubular structure, the shells of which are covered by the isoleucine pendants. During the thickening of the solution accompanying the solvent evaporation, the tubes may grow in length by the linear fusion of more vesicles, and the lengthened nanotubes may twine round each other via hydrogen bonding of the pendants on the outer shells to give the helical cables of nanotubes. The previous mechanism should be solvent dependent; changing the solvent or environment would affect the assembly process because of the perturbation of the interactions between the polymer chains. A more comprehensive study was conducted on the self-assembly processes of 10, which carries leucine pendants [66]. When methanol was used, twisted cables of helical fibrils were formed, an example of which is shown in Figure 10A. Close inspection of the cable structure reveals that the fibrils fold back to form knobs or  bends at two ends of the cable. When a dilute solution was used, the aggregation of the macromolecules was further suppressed and the resultant morphology looked like long threads or helical chains, each pair of which twist over and under each other to form a double helix (inset of Fig. 10A). When the solvent was changed to chloroform, the morphology obtained from the solution of similar concentration (9.6 M) became distinctly different. Instead of forming helical cables, the chloroform solution gave oval eggs or

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Polyacetylene Nanostructures

Figure 10. AFM images of self-assembly morphologies formed upon natural evaporation of dilute solutions of 10 on newly cleaved mica. (A) A multistranded cable from a 12.8 M methanol solution (inset: duplex braids from a 2.6 M solution). (B) Spherical vesicles from a 9.6 M chloroform solution with 0.5% methanol. (C) Extended filaments from 12.8 M methanol containing 25.5 M KOH. Reprinted with permission from [66], B. Z. Tang, Polym. News 26, 262 (2001).

spherical vesicles (Fig. 10B). This difference is, however, not totally unexpected. Unlike methanol, chloroform is a poor solvent of amino acids, and the polymer chains would selffold to minimize their exposure to the solvent. When the solvent evaporates, the elementary “foldamers” may cluster together to form compactly packed larger spheres in an effort to minimize their surface area in contact with the air. The morphology changed not only with solvent but also with pH. The addition of KOH to the methanol solution disassembled the helical cables and gave extended filaments (Fig. 10C). The complexation of the K+ ions with the polymer chains partially dissociates the amino acid groups, thus hindering the hydrogen bond formation. The electrostatic repulsion of the closely located charged chains cannot easily bend but takes an extended conformation. Concentration is also a critical parameter in determining the final morphological structure; overcrowding may cause post-fusion of the polymer aggregates, leading to the formation of even higher-order structures. For example, evaporation of a dilute solution of 10 gave crescent helical fibers (Fig. 11A). The closely located helical cables may be interconnected by interstrand hydrogen bonds of the amino acid groups, but the solvent molecules, on the other hand, try to push the polymer chains apart; the balance of the two antagonistic forces results in the formation of a fence-like structure. If the concentration of the polymer solution is increased, more polymer chains will stick together to construct more closed fences or cages, and the neighboring cages may merge to form a continuous structure. Indeed, when a THF solution of 10 with a relatively high concentration (28.9 M) was used, a mesoporous, honeycomb-like, thin solid film comprising interconnected cages was formed (Fig. 11B). The interconnection of the cages was threedimensional in nature, as clearly revealed by a scanning electron microscope image of the tilted sample (Fig. 11C). Figure 11D shows a partial view of a long fiber of 10

Figure 11. (A) AFM image of crescent helices formed by a diluted solution of 10 (12.8 M). (B) Scanning electron microscope (SEM) image of a honeycomb pattern generated from a concentrated THF solution of 10 (28.9 mM), whose 3D nature is clearly revealed by the enlarged side view (C) with the sample tilted 60 from the electron beam axis. (D) SEM micrograph of a twisted fiber formed upon diffusion of ether into a methanol solution of 10 (3.9 mM). A and D are reprinted with permission from [85], F. Salhi et al., J. Nanosci. Nanotechnol. 1, 137 (2001). © 2001, American Scientific Publishers. B and C are reprinted with permission from [66], B. Z. Tang, Polym. News 26, 262 (2001).

obtained from a diffusion process. When ether was diffused into a methanol solution of 10, left-handed helical ribbons were formed. The image proves that the polymer is capable of building a large helical structure, even in the micrometer domain. The folding structures of biopolymers are affected by external perturbations in addition to internal factors. The supercoiling of DNA is affected, for example, by small changes in the local ionic strength, which can cause the reversal of the helical twist in the DNA coils [82]. Similar phenomena have been observed in our nonnatural polymer system. The pH effect on the assembly behaviors of polymer 11 is shown in Figure 12. Changing the medium pH from “neutral” to “basic” changed the organizational

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Polyacetylene Nanostructures

Figure 12. AFM deflection images of (A and B) helical nanofibers formed by 11 upon natural evaporation of its methanol solution (7 g/ml) on freshly cleaved mica under ambient conditions. The nanofibers can be unraveled by adding a 1 M equivalent of KOH (i.e., KOH/11 = 1 1) to the methanol solution of the polymer (4 g/ml), as shown in C–E. Reprinted with permission from [83], Li et al., NanoLetters 1, 323 (2001). © 2001, American Chemical Society.

morphology of the polymer from continuous helical cables to discrete random threads [83]. The random threads showed almost no macroscopic screw sense. This example illustrates how ionization of the carboxyl groups by the potassium ions breaks the hydrogen bonds, which are believed to maintain the long, thick helical structure of the polymer chains. By partially hydrolyzing “polyester” 7, we succeeded in generating a valine-containing polymer consisting of both ester and acid repeat units, poly[(4-ethynylbenzoyl-l-valine methyl ester)-co-(4-ethynylbenzoyl-l-valine)]. This hybrid polymer afforded self-assembly structures with organizational features of both the “polyester” and the “polyacid.” While “polyester” 7 assembled into pearl-shaped morphological structures upon natural evaporation of its dilute methanol solution (cf. Fig. 7), under similar conditions

“polyacid” 11 associated into helical ropes (cf. Fig. 12). The hybrid polymer gave an intermediate structure with spherical beads strung together on a filamentary string (Fig. 13). In methanol, the coiled chains of 7 may pack together to minimize the exposure of its hydrophobic backbones to the polar solvent, forming micelle-like structures with their outer shells decorated by the hydrophilic amide and ester functional groups. On the other hand, the strong solvation power of the methanol solvent toward 11 may force the helical chains to take on an extended conformation. During the solvent evaporation, the micelles may grow in size and stick together to give the clustered pearls, whereas the individual helical chains may aggregate in a side-by-side fashion to form spirally twisting fibrils. This aggregation is reminiscent of protein folding. Particular types of amino acids tend to form a relatively small number of local structural motifs [84], whereas aggregate in the proteins in such a way that the hydrophobic and hydrophilic regions are respectively out of, and in, contact with water.

5. POLYACETYLENES WITH ETHYLENE OXIDE PENDANTS The previous results clearly show that not only molecular asymmetry but also chain amphiphilicity plays important roles in the self-assembly processes of chiral acetylene polymers. In this section we focus our discussion on the self-organization of polymers solely with the latter property (amphiphilic achiral species). A few examples of such amphiphilic polyacetylenes are given in Chart 6, in which polar hydrophilic substituents are appended to nonpolar polyacetylene backbones. C

C

n

C

C

C

n

O O

(OC2H4)2Cl

O

n

O O

O O

12

C

H

H

H

13

O O

(OCH2CH2)mOCH3 (m ~ 8)

14

Chart 6

Figure 13. AFM images of string beads formed upon natural evaporation of methanol solutions of a partially hydrolyzed 7, poly[(4ethynylbenzoyl-l-valine methyl ester)-co-(4-ethynylbenzoyl-l-valine)], on the surfaces of newly cleaved mica. Scale bars (nm): 100; concentration: ∼2 g/ml; temperature: ∼23  C. Reprinted with permission from [72], Li et al., Macromolecules 36, 77 (2003). © 2003, American Chemical Society.

When a tiny drop of a THF/hexane solution of 12, which bears short ethylene oxide pendants, was deposited on a glass slide, a microporous thin film (∼2–10 m in pore diameter) was instantly formed upon solvent evaporation (Fig. 14A). When the solvent was changed to a mixture of dichloromethane (DCM) and hexane and when the solution concentration was increased, a three-dimensional porous thin solid film with a smaller pore size (∼1 m in diameter) was formed (Fig. 14B). When a dilute solution was used, the evaporation process afforded the elementary building blocks for a three-dimensional higher-order structure: bagel-like vesicles [86, 87], whose sizes varied from several hundred nanometers to a few micrometers (Fig. 14C and D). This primary structure may help us understand the assembly mechanism: when deposited on a glass substrate in the three-dimensional space, the horizontally oriented vesicles may merge into porous layers that are parallel to the substrate accompanying the solvent evaporation, whereas

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Figure 14. Microporous films formed by natural evaporation of (A) THF/hexane (∼1 1 by volume) and (B) DCM/hexane (∼1 1 by volume) solutions of 12 on glass slides. Concentrations of 12: (A) ∼1, (B) ∼2 mg/ml. (C, D) Vesicular structures formed by the evaporation of a dilute solution of 12 (∼03 mg/ml) in the DCM/hexane mixture. The morphologies shown in (A, C) and (B, D) are POM and SEM images, respectively. Reprinted with permission from [85], F. Salhi et al., J. Nanosci. Nanotechnol. 1, 137 (2001). © 2001, American Scientific Publishers.

convergence of the vesicles with other orientations would produce the interlayer pores, leading to the formation of the multilayer porous structure. The self-association of 12 is also solvent-sensitive: when relatively polar solvents were used, discrete microspheres, rather than regular pores, were formed. Upon solvent evaporation, a THF solution of 12 afforded spheres with an

Figure 15. Microspheres formed by natural evaporation of (A, B) THF and (C, D) DCM/acetone (∼1 1 by volume) solutions of 12 on glass slides. Concentration of 12: ∼1 mg/ml. The morphologies shown in (A, C) and (B, D) are POM and SEM images, respectively. Reprinted with permission from [85], F. Salhi et al., J. Nanosci. Nanotechnol. 1, 137 (2001). © 2001, American Scientific Publishers.

Polyacetylene Nanostructures

Figure 16. POM image of the microcrystals of carbazole formed inside microspheres of 12 by natural evaporation of a DCM/acetone solution of 12/carbazole on a glass slide. Concentration of 12/carbazole: ∼6 mg/ml; weight ratio of 12/carbazole: 2:1; volume ratio of DCM/acetone: ∼1 1. Reprinted with permission from [46], K. K. L. Cheuk et al., Polym. Mater. Sci. Eng. 82, 54 (2000).

average diameter of ∼1.5 m (Fig. 15A and B) [85]. Similarly, a DCM/acetone mixture of 12 also gave microspheres (Fig. 15C and D). The hydrophobic polyacetylene main chains of 12 may have aggregated together to form the core of the spheres that are surrounded by its hydrophilic ethylene oxide side chains under polar conditions, whereas the amphiphilic polymer may have formed bi-, tri-, or multilayers and/or even vesicular structures [76, 87] to reduce the surface area in the nonpolar environments. Biomineralization has attracted much interest in recent years [88–95]. Molding of inorganic crystals into appropriate shapes and sizes has created a variety of different material properties. The amphiphilic polyacetylenes were found to regulate the growth of organic crystals. For example, evaporation of a DCM/acetone mixture of 12 and carbazole gave regular spherical microcrystals of carbazole ∼20 m in diameter (Fig. 16). Under similar conditions, when a drop of a DCM/ethanol solution of carbazole/13, which contains crown ether appendages, was deposited on a glass slide, isolated sickle-like crystals, ∼20 m in length, were formed (Fig. 17). Replacing 13 with 14, which bears long oligo(ethylene oxide) appendages, led to the generation of

Figure 17. POM images of the microcrystals of carbazole formed inside microreservoirs of 13 by natural evaporation of DCM/ethanol (∼1 1 by volume) solutions of 13/carbazole on a glass slide. Concentrations of 13/carbazole: ∼(0.25 mg/0.5 mg)/ml. Reprinted with permission from [46], K. K. L. Cheuk et al., Polym. Mater. Sci. Eng. 82, 54 (2000).

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Polyacetylene Nanostructures

are biocompatible, and some of them can even stimulate the growth of living cells [101–103]. Their photoelectronic and biological properties make the polyacetylenes promising candidates for biomaterials; they may be found to be useful in potential applications in such biotechnological systems as artificial nervous networks, photosynthesis devices, and phototherapy processes.

GLOSSARY Figure 18. POM images of microcrystals of carbazole in a polymer matrix of 14 formed by natural evaporation of DCM/ethanol solutions of carbazole/5 on a glass slide. Volume ratio of DCM/ethanol: ∼1:1; concentration of 14: ∼1 mg/ml; weight of carbazole added to 1 ml of the polymer solution: 2 mg. Reprinted with permission from [46], K. K. L. Cheuk et al., Polym. Mater. Sci. Eng. 82, 54 (2000).

crystal bars, ∼8 to 20 m in length (Fig. 18). The formation of these defined crystal structures is probably achieved by a micelle-like self-assembly process. When solvent evaporates, carbazole molecules pack together to form a crystal. However, in the presence of amphiphilic polymers, the packing process may be slowed down because the carbazole molecules may be wrapped in the amphiphilic chains. The polymer backbones may solvate the carbazole molecules in the inner core, with the polar ethylene oxide groups exposed to the solvent environment. The separate micelle-like complexes may thus in turn give the microcrystals that are covered by, or sheathed in, the amphiphilic polymer chains. Different pendants bring about different degrees of repulsion or steric effects. The different properties of the micellelike aggregates may have eventually guided the carbazole molecules to grow in different patterns.

6. CONCLUSIONS It is generally agreed that it is difficult to form supramoleular assemblies of defined architecture from synthetic homopolymers [96]. In this review, we have proved, however, that homopolyacetylenes can self-organize into macromolecular architectures with well-defined three-dimensional structures. Both the unsubstituted and substituted polyacetylenes spontaneously assembled into various mechanically robust organizational morphologies reminiscent of natural hierarchical structures: helixes, vesicles, fibrils, etc. This clearly demonstrates that synthetic homopolymers with rationally designed molecular structures can be used to fabricate biomimetic morphologies, whose size, shape, and pattern can be readily tuned by simple morphosynthetic processes. The organizational morphologies changed with the variations in the pendant hydrophilicity, solvent polarity, and pH value, suggesting a proteomimetic adoptability [97] and demonstrating the tunability of the hierarchical structures by internal perturbations and external stimuli. Unlike “conventional” vinyl polymers, the acetylene polymers consist of alternating double bonds, whose structural cousins are highly photoconductive [98–100]. Cytotoxicity assays have revealed that the amphiphilic polyacetylenes

Achiral An object that is superposable on its mirror image. Amphiphilic A molecule with a hydrophilic end that dissolves in polar solvents and another, hydrophobic end that dissolves in nonpolar solvents. Chiral An object that is nonsuperimposable to its mirror image. Circular dichroism (CD) A change from planar to elliptic polarization when an initially plane-polarized light wave traverses an optically active medium. Helicity The sense of twist of a helix, propeller, or screw. Micelle An aggregate formed by amphiphilic molecules in, e.g., water such that their polar ends are in contact with water and their nonpolar portions are on the interior. Vesicle A small, thin-walled bladderlike cavity. Z (zusammen) One of the stereochemical descriptors for alkenes or for other double-bond systems with at least two nongeminal substituents (other than H) at the two ends of the double bonds. If the pertinent substituents are on the same side, the descriptor is Z. E. (entgegen) denotes that the substituents of highest Cahn-Ingold-Prelog (CIP) priority at each end of the double bond are on opposite sides. In the polyacetylene system, there exist four theoretically possible stereostructures for the repeat units, that is, E–s-E, E–s-Z, Z–s-Z, and Z–s-E isomeric structures. Z content The content (%) of the stereoisomers with a Z configuration.

ACKNOWLEDGMENTS We thank the financial support from the Hong Kong Research Grants Council (projects HKUST 6187/99P, 6121/01P, and 6085/02P), the University Grants Committee of Hong Kong (project AoE/P-10/01-1-A), and Esquel Enterprises Ltd., Hong Kong (project EGC 001.02/03). We are indebted to Prof. Xudong Xiao of the Department of Physics of our university and Prof. Chunli Bai of the Chinese Academy of Sciences, Beijing, for their helpful discussions. We are grateful to Mr. Jacky W. Y. Lam and Drs. Fouad Salhi, Qunhui Sun, Gao Li, Junwu Chen, Jingdong Luo, and John A. K. Cha of our research group for their technical assistance.

REFERENCES 1. G. B. Johnson, “The Living World,” 2nd ed. McGraw-Hill, Boston, 2000. 2. M. K. Campell, “Biochemistry,” 2nd ed. Saunders College Publishing, New York, 1995. 3. D. Philip and J. F. Stoddart, Angew. Chem. Int. Ed. 35, 1155 (1996).

712 4. P. Ball, “The Self-Made Tapestry: Pattern Formation in Nature.” Oxford University Press, Oxford, 1999. 5. A. Klug, Angew. Chem. Int. Ed. Engl. 22, 565 (1983). 6. M. A. Lauffer, “Entropy-Driven Processes.” Springer-Verlag, New York, 1975. 7. G. M. Whitesides and B. Grzybowski, Science 295, 2418 (2002). 8. T. Ishi-I, M. Crego-Calama, P. Timmerman, D. N. Reinhoudt, and S. Shinkai, Angew. Chem. Int. Ed. 41, 1924 (2002). 9. G. M. Whitesides and M. Boncheva, Proc. Natl. Acad. Sci. U.S.A. 99, 4769 (2002). 10. J. Stahl, J. C. Bohling, E. B. Bauer, T. B. Peters, W. Mohr, J. M. Martín-Alvarez, F. Hampel, and J. A. Gladysz, Angew. Chem. Int. Ed. 41, 1872 (2002). 11. E. D. Sone, E. R. Zubarev, and S. I. Stupp, Angew. Chem. Int. Ed. 41, 1706 (2002). 12. L. Brunsveld, B. J. B. Folmer, E. W. Meijer, and R. P. Sijbesma, Chem. Rev. 101, 4071 (2001). 13. D. T. Bong, T. D. Clark, J. R. Granja, and M. R. Ghadiri, Angew. Chem. Int. Ed. 40, 988 (2001). 14. T. Kawasaki, M. Tokuhiro, N. Kimizuka, and T. Kunitake, J. Am. Chem. Soc. 123, 6792 (2001). 15. F. M. Raymo, M. D. Bartberger, and J. F. Stoddart, J. Am. Chem. Soc. 123, 9264 (2001). 16. A. E. Rowan and R. J. M. Nolte, Angew. Chem. Int. Ed. 37, 63 (1998). 17. O. Ikkala and G. T. Brinke, Science 295, 2407 (2002). 18. G. A. Ozin, Chem. Commun. 419 (2000). 19. J. M. Lehn, Science 260, 1762 (1993). 20. M. Aldissi, Ed., “Intrinsically Conducting Polymers: An Emerging Technology.” Kluwer, Dordrecht, the Netherlands, 1993. 21. H. G. Kiess, Ed., “Conjugated Conducting Polymers.” SpringerVerlag, Berlin, 1992. 22. I. V. Krivoshei and V. M. Skorogatov, “Polyacetylene and Polyarylenes: Synthesis and Conducting Properties.” Gordon and Breach Science, New York, 1991. 23. T. A. Skotheim, Ed., “Handbook of Conducting Polymers.” Dekker, New York, 1986. 24. J. C. W. Chien, “Polyacetylene.” Academic Press, New York, 1984. 25. G. Natta, G. Mazzanti, G. Pregaglia, and M. Peraldo, Gazz. Chim. Ital. 89, 465 (1959). 26. G. Natta, G. Mazzanti, and P. Pino, Angew. Chem. 69, 685 (1957). 27. S. Shibahara, M. Yamane, K. Ishikawa, and H. Takezoe, Macromolecules 31, 3756 (1998). 28. J. W. Hall and G. A. Arbuckle, Macromolecules 29, 546 (1996). 29. H. Shirakawa, Y.-X. Zhang, T. Okuda, K. Sakamaki, and K. Akagi, Synth. Met. 65, 93 (1994). 30. K. Akagi, G. Piao, S. Kaneko, K. Sakamaki, H. Shirakawa, and M. Kyotani, Science 282, 1683 (1998). 31. Y. Qin, H. Li, Z. Zhang, and Z. Cui, Org. Lett. 4, 18 (2002). 32. H. Goto, Y. Okamoto, and E. Yashima, Macromolecules 35, 4590 (2002). 33. B. Erdogan, J. N. Wilson, and U. H. F. Bunz, Macromolecules 35, 7863 (2002). 34. K. Morino, K. Maeda, Y. Okamoto, E. Yashima, and T. Sato, Chem. Eur. J. 8, 5112 (2002). 35. T. Sato, K. Terao, A. Teramoto, and M. Fujiki, Macromolecules 35, 2141 (2002). 36. R. Nomura, J. Tabei, and T. Masuda, J. Am. Chem. Soc. 123, 8430 (2001). 37. E. Yashima, K. Maeda, and O. Sato, J. Am. Chem. Soc. 123, 8159 (2001). 38. C.-Y. Huang, J. W. Klemke, Z. Getahun, W. F. DeGrado, and F. Gai, J. Am. Chem. Soc. 123, 9235 (2001). 39. R. B. Prince, J. S. Moore, L. Brunsveld, and E. W. Meijer, Chem. Eur. J. 7, 4150 (2001).

Polyacetylene Nanostructures 40. J. M. Hannink, J. J. L. M. Cornelissen, J. A. Farrera, P. Foubert, F. C. De Schryver, N. A. J. M. Sommerdijk, and R. J. M. Nolte, Angew. Chem. Int. Ed. 40, 4732 (2001). 41. I. M. Khan, Ed., “Synthetic Macromolecules with Higher Structural Order,” ACS Symposium Series, Vol. 812, American Chemical Society, Washington, DC, 2001. 42. K. K. L. Cheuk, Ph.D. Dissertation, Hong Kong University of Science & Technology, 2002. 43. K. K. L. Cheuk, J. W. Y. Lam, Q. Sun, J. A. K. Cha, and B. Z. Tang, Polym. Prepr. 40, 653 (1999). 44. B. S. Li, K. K. L. Cheuk, D. Yang, J. W. Y. Lam, L. Wan, C. Bai, and B. Z. Tang, Macromolecules 36, 5447 (2003). 45. K. K. L. Cheuk, J. W. Y. Lam, Q. Sun, J. A. K. Cha, and B. Z. Tang, Polym. Prepr. 40, 655 (1999). 46. K. K. L. Cheuk, J. W. Y. Lam, J. A. K. Cha, and B. Z. Tang, Polym. Mater. Sci. Eng. 82, 54 (2000). 47. K. K. L. Cheuk, J. W. Y. Lam, and B. Z. Tang, Polym. Mater. Sci. Eng. 82, 56 (2000). 48. K. K. L. Cheuk, J. W. Y. Lam, J. A. K. Cha, and B. Z. Tang, Polym. Prepr. 41, 131 (2000). 49. J. W. Y. Lam, K. K. L. Cheuk, and B. Z. Tang, Polym. Prepr. 41, 912 (2000). 50. J. W. Y. Lam, K. K. L. Cheuk, and B. Z. Tang, Polym. Prepr. 41, 969 (2000). 51. K. K. L. Cheuk, J. W. Y. Lam, Q. Sun, J. A. K. Cha, and B. Z. Tang, Polym. Prepr. 41, 981 (2000). 52. K. K. L. Cheuk, F. Salhi, J. W. Y. Lam, and B. Z. Tang, Polym. Prepr. 41, 1567 (2000). 53. F. Salhi, K. K. L. Cheuk, J. W. Y. Lam, and B. Z. Tang, Polym. Prepr. 41, 1590 (2000). 54. F. Salhi, J. W. Y. Lam, K. K. L. Cheuk, J. A. K. Cha, and B. Z. Tang, Polym. Prepr. 41, 1185 (2000). 55. B. S. Li, K. K. L. Cheuk, J. Zhou, Y. Xie, J. A. K. Cha, X. Xiao, and B. Z. Tang, Polym. Prepr. 42, 543 (2001). 56. K. K. L. Cheuk, J. Luo, J. W. Y. Lam, and B. Z. Tang, Polym. Prepr. 42, 545 (2001). 57. B. S. Li, K. K. L. Cheuk, J. A. K. Cha, X. D. Xiao, and B. Z. Tang, Polym. Mater. Sci. Eng. 84, 396 (2001). 58. K. K. L. Cheuk, B. S. Li, F. Salhi, J. W. Y. Lam, J. A. K. Cha, and B. Z. Tang, Polym. Mater. Sci. Eng. 84, 516 (2001). 59. K. K. L. Cheuk, B. S. Li, F. Salhi, J. W. Y. Lam, J. A. K. Cha, and B. Z. Tang, Polym. Mater. Sci. Eng. 84, 536 (2001). 60. B. S. Li, K. K. L. Cheuk, X. Xiao, C. Bai, and B. Z. Tang, Polym. Mater. Sci. Eng. 85, 39 (2001). 61. K. K. L. Cheuk, B. S. Li, J. Chen, and B. Z. Tang, Polym. Prepr. 42, 234 (2001). 62. B. S. Li, K. K. L. Cheuk, X. Xiao, C. Bai, and B. Z. Tang, Polym. Prepr. 42, 248 (2001). 63. J. W. Chen, J. W. Y. Lam, K. K. L. Cheuk, Z. L. Xie, H. Peng, L. M. Lai, and B. Z. Tang, Polym. Mater. Sci. Eng. 86, 179 (2002). 64. J. W. Chen, K. K. L. Cheuk, Z. L. Xie, J. W. Y. Lam, and B. Z. Tang, Polym. Prepr. 43, 690 (2002). 65. J. W. Y. Lam, L. Y. Ngai, T. W. H. Poon, Z. Y. Lin, and B. Z. Tang, Polym. Prepr. 41, 289 (2000). 66. B. Z. Tang, Polym. News 26, 262 (2001). 67. K. Shinohara, S. Yasuda, G. Kato, M. Fujita, and H. Shigekawa, J. Am. Chem. Soc. 123, 3619 (2001). 68. G. L. Zubay, “Biochemistry,” 4th ed. Wm. C. Brown Publishers, Boston, 1998. 69. B. Brutschy and P. Hobza, Chem. Rev. 100, 3861 (2000). 70. J. Luo, K. K. L Cheuk, J. W. Y. Lam, and B. Z. Tang, Polym. Prepr. 42, 541 (2001). 71. R. H. Pain, Ed., “Mechanisms of Protein Folding,” 2nd ed. Oxford University Press, New York, 2000.

Polyacetylene Nanostructures 72. B. S. Li, K. K. L. Cheuk, L. Ling, J. Chen, X. Xiao, C. Bai, and B. Z. Tang, Macromolecules, 36, 77 (2003). 73. M. S. Spector, R. R. Price, and J. M. Schnur, Adv. Mater. 11, 337 (1999). 74. L. Zhang and A. Eisenberg, J. Am. Chem. Soc. 118, 3168 (1996). 75. Y.-Z. Liang, Z.-C. Li, and F.-M. Li, New J. Chem. 24, 323 (2000). 76. W. Meier, Chem. Soc. Rev. 29, 295 (2000). 77. S. Stewart and G. Liu, Angew. Chem. Int. Ed. 39, 340 (2000). 78. S. Vauthey, S. Santoso, H. Gong, N. Watson, and S. Zhang, Proc. Natl. Acad. Sci. USA 99, 5355 (2002). 79. X. H. Yan, G. J. Liu, F. T. Liu, B. Z. Tang, H. Peng, A. B. Pakhomov, and C. Y. Wong, Angew. Chem. Int. Ed. 40, 3593 (2001). 80. Y. Moroi, “Micelles: Theoretical and Applied Aspects.” Plenum, New York, 1992. 81. K. Yu and A. Eisengerg, Macromolecules 31, 3509 (1998). 82. Z. Reich, L. Zaidmann, S. B. Gutman, T. Arad, and A. Minski, Biochemistry 33, 14177 (1994). 83. B. S. Li, K. K. L. Cheuk, F. Salhi, J. W. Y. Lam, J. A. K. Cha, X. Xiao, C. Bai, and B. Z. Tang, NanoLetters 1, 323 (2001). 84. J. M. Thornton and S. P. Gardner, Trends Biol. Sci. 14, 300 (1989). 85. F. Salhi, K. K. L. Cheuk, Q. Sun, J. W. Y. Lam, J. A. K. Cha, G. Li, B. Li, J. Chen, and B. Z. Tang, J. Nanosci. Nanotechnol. 1, 137 (2001). 86. C. J. Brinker, Y. Lu, A. Sellinger, and H. Fan, Adv. Mater. 11, 579 (1999). 87. H. Ringsdorf, B. Schlarb, and J. Venzmer, Angew. Chem. Int. Ed. Engl. 27, 113 (1988). 88. G. J. de A. A. Soler-Illia, C. Sanchez, B. Lebeau, and J. Patarin, Chem. Rev. 102, 4093 (2002).

713 89. S. Mann, “Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry.” Oxford University Press, New York, 2002. 90. J. Aizenberg, G. Lambert, S. Weiner, and L. Addadi, J. Am. Chem. Soc. 124, 32 (2002). 91. K. Naka and Y. Chujo, Chem. Mater. 13, 3245 (2001). 92. L. A. Estroff and A. D. Hamilton, Chem. Mater. 13, 3227 (2001). 93. S. Mann, Angew. Chem. Int. Ed. 39, 3392 (2000). 94. D. B. DeOliveira and R. A. Laursen, J. Am. Chem. Soc. 119, 10627 (1997). 95. S. Mann, Ed., “Biomimetic Materials.” VCH, New York, 1996. 96. S. M. Yu, C. M. Soto, and D. A. Tirrell, J. Am. Chem. Soc. 122, 6552 (2000). 97. K. B. Storey and J. M. Storey, Eds., “Protein Adaptations and Signal Transduction.” Elsevier, Amsterdam, 2001. 98. B. Z. Tang, J. W. Y. Lam, X. Kong, P. P. S. Lee, X. Wan, H. S. Kwok, Y. M. Huang, W. Ge, H. Chen, R. Xu, and M. Wang, “Liquid Crystals III” (I. Khoo, Ed.), p. 62. SPIE, Bellingham, WA, 1999. 99. B. Z. Tang, H. Chen, R. Xu, J. W. Y. Lam, K. K. L. Cheuk, H. N. C. Wong, and M. Wang, Chem. Mater. 12, 213 (2000). 100. J. Sun, H. Chen, R. Xu, M. Wang, J. W. Y. Lam, and B. Z. Tang, Chem. Commun. 1222 (2002). 101. B. Z. Tang, Polym. Prepr. 43, 48 (2002). 102. K. K. L. Cheuk, B. S. Li, J. W. Chen, Y. Xie, and B. Z. Tang, “Proceedings of the 5th Asian Symposium on Biomedical Materials,” 2001, p. 514. 103. B. S. Li, K. K. L. Cheuk, J. Zhou, Y. Xie, and B. Z. Tang, Polym. Mater. Sci. Eng. 85, 401 (2001).