self-assembled microstructures of functional molecules

cience 12 (2007) 106–120www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface S

Self-assembled microstructures of functional molecules

Katsuhiko Ariga a,⁎, Takashi Nakanishi b,c, Jonathan P. Hill a

a Supermolecules Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japanb MPI-NIMS International Joint Laboratory, Max Planck Institute of Colloids and Interfaces, Germany

c Organic Nanomaterials Center, National Institute for Materials Science (NIMS), Japan

Available online 9 June 2007

Abstract

Recent developments of bottom-up fabrication based on self-assembly processes allow us to construct well-designed nano- and microstrcturessuch as spheres, fibers, tubes, and disks from various functional molecules including biopolymers, conjugated molecules, porphyrins, graphenes,and fullerenes. These assembling techniques do not always require traditional (hydrophilic/hydrophilic) amphiphilic structure. A wide range offunctional molecules can be now applied for the fabrication of desired microstructures.© 2007 Elsevier Ltd. All rights reserved.

Keywords: Self-assembly; Microstructure; Bottom-up fabrication

1. Introduction

Preparation of ultrasmall structures has been investigatedintensively from the viewpoint of development of portabledevices capable of high density information storage. Thestrategies for these fine fabrications can be classified into thetwo main categories of bottom-up and top-down approaches.The top-down-type fabrication from bulk materials has beenvery successful in provision of controlled structure over shortlength scales. Unfortunately, limitations on fabrication size andavailable materials may become serious problems in the top-down approaches. On the other hand, research has also recentlybeen concentrated on bottom-up-type fabrication, which relieson supramolecular self-assembly processes. The latter approachdepends on individual molecules and their structures, and hasbecome a powerful tool for the preparation of supramolecularcomplexes and certain nanostructures. However, in order thatsystems prepared using bottom-up approaches can interfacewith the larger structures fabricated by top-down-type techni-ques, an important intermediary process is required. Self-assembled “micro”-structures could play this role so that colloidand interface sciences will be critical especially in relation totheir structural dimensions.

⁎ Corresponding author. Tel.: +81 29 860 4597; fax: +81 29 852 4832.E-mail address: [email protected] (K. Ariga).

1359-0294/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.cocis.2007.05.008

As summarized in a recent review by Shimizu et al. [1•],amphiphiles with designed structures have been traditionally usedfor the preparation of some kinds of microstructures such asvesicles, fibers, and tubes. Research of this concept is welldeveloped and is at a stage to provide deep insights on finestructural control. For example, Oda et al. recently demonstratedfine-tuning of twisted and helical structures formed from non-chiral dicationic gemini amphiphiles complexed with chiraltartrate anions [2]. The morphologies and dimensions of twistedand helical ribbons, as well as tubules, can be controlled byvarying temperature, by introducing small amounts of additives,or by slight modification of molecular structure. Microstructureformation from non-amphiphilic materials has also been recentlyinstigated. Shimizu et al. extended bola-type molecular design toinclude a photoelectronic unit, oligo(p-phenylenevinylene), inmicron-long helical fibers through complementary hybridizationwith oligonucleotide [3]. Transcription of organic microstructuresinto inorganic materials has been also proposed [4]. For example,Shimizu et al. prepared silica tubular structures through sol–gelreaction on tubular structures composed from tetra-peptidic lipidfollowed by calcination [5]. Changing the template morphologyby in situ addition of ethanol in a silica/lipid xerogel led to theformation of unique tube-in-tube silica structures.

Microstructure formation from typical amphiphiles such assurfactants and lipids is now well matured, and interests ofresearchers are shifting to microstructures from functional com-ponents whose structures are far from conventional amphiphiles.

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http://dx.doi.org/10.1016/j.cocis.2007.05.008

107K. Ariga et al. / Current Opinion in Colloid & Interface Science 12 (2007) 106–120

Indeed, many trials of fabrication of various functionalmolecules contained within microsize-controlled structuresthrough self-assembling processes have been recently reported.In this review, research developments on self-assembledmicrostructures from various kinds of functional moleculesincluding polymers, biocomponents, porphyrins, conjugatestructures, graphenes, and fullerenes within the past few yearsare highlighted.

2. Microstructures of various functional molecules

2.1. Microstructures from designed polymers

Polymeric materials have the advantage of chemicalrobustness as compared with low-molecular weight substances.Therefore, the preparation of microstructures from polymericmaterials has been considered especially with regard to theirpractical uses. Self-assembly processes of well-designed poly-mers are known to provide regular structures in ranges fromnanometer to centimeter. A few unique approaches are intro-duced below.

Yan et al. demonstrated under particular conditions thespontaneous formation of macroscopic tubes of millimeterdiameters and centimeter lengths through simple self-assemblyof an amphiphilic hyperbranched copolymer [6]. The amphi-philic polymers were composed of a hydrophobic hyper-branched poly(3-ethyl-3-oxetanemethanol) core and containedseveral hydrophilic poly(ethylene glycol) arms. Polymers withappropriate length of poly(ethylene glycol) chains and hydro-philic–hydrophobic ratios could form the macroscopic tubes.Macroscopic tube self-assembly was observed only when thedried viscous polymer material was added directly to aparticular volume of acetone under stirring at room temperature.However, careful tuning of the preparation conditions wasnecessary. The formation of macroscopic tubes is affected byfactors such as the nature of the solvent, the copolymerconcentration, additive ions, and the self-assembly procedure.The macroscopic tubes were observed only in acetone and couldnot be found in water, ester, alcohol, and other solvents. Therequired polymer concentration in acetone ranged from 0.01 to1 mg ml− 1. Observation of the macroscopic tubes bytransmission electron microscopy (TEM) revealed the presenceof a lamellar structure with repeated regions of hydrophilic andhydrophobic polymer domains. The tubes formed are flexibleand robust. They can be bent in solvent to form a crossed knotpattern. If the tubes are removed from acetone and dried onglass, they collapsed into ribbon-like objects. Surprisingly, thetubular shape can be recovered upon re-immersion of the driedobject in acetone.

Self-assembly processes of polymeric materials sometimesinduce the formation of microscopic-patterned structures. Yabuet al. reported a fabrication method for honeycomb-patternedmetal films by electroless plating using self-assembledhoneycomb structures of an organic polymer as a template[7]. The latter structure can be formed by the simple casting ofpolymer solutions. Under humid conditions, solvent evapora-tion induces water droplet condensation that are then packed

regularly by lateral capillary forces and by convectional flow.Porous honeycomb patterns can be formed after completeevaporation of the used solvent and water. Metal pincushionstructures were then prepared by simple peeling of thehoneycomb-patterned polymer films and subsequent electrolessplating. The polymer template was removed by sintering orsolvent elution to complete the metallization. The resultingpattern has micrometer-range dimensions and can be employedas a two-dimensional photonic crystal based on its regularityand high refractive index. For instance, these transparentconductive films could be applied as flat panel displays.

Combination of the concepts of artificial fabrication and self-assembling processes sometimes results in more elaboratemicrostructures. Mirkin et al. reported self-assembly processesof metal–polymer amphiphiles that were fabricated bysequential electrodeposition of gold and polypyrrole in aporous aluminum template (Fig. 1A) [8••]. The micron-sizedassemblies have outside diameters of 400 nm for the gold and360 nm for the polypyrrole portion. Hydrophobic attractionbetween polymer moieties in water causes contraction of thepolypyrrole domains resulting in the designed curvature of theself-assembled microstructures of gold–polypyrrole amphiphil-ic rods. They formed mostly tubular structures with diameter inthe range 10 to 100 mm, depending on the ratio between goldand polypyrrole domains. In this case, assembly structures werenot limited to tubular morphologies. For example, planar sheet-like assemblies could be created using rods with polypyrroledomains sandwiched between two gold segments. Their studyintroduced the concept of using polymer segments in nanorodstructures to control their assembly into flat two-dimensionaland curved three-dimensional microstructures. The rod unitscan behave like amphiphilic molecules with good moleculardesigns. Different micron-scale architectures can be generatedupon designing rod structures by controlling the ratio ofdifferent materials.

Layer-by-layer (LbL) assembly is known as a powerfulmethod to prepare self-assembled structures in a controlledmanner, as summarized in a recent review [9]. In particular, theuse of a removable template in the LbL assembly is useful forthe preparation of three-dimensional microstructures. Forexample, Vansco et al. prepared organometallic microcapsulescomposed of anionic and cationic poly(ferrocenylsilane)through LbL self-assembly of the corresponding polymersonto colloidal templates followed by core removal [10].Swelling of the formed capsules was controlled by redoxreaction of the ferrocene functionality, resulting in controlledpermeability of the capsule membrane. The reported systemshould have great potential for application in materials scienceas well as in biorelated technology such as drug deliverysystems. If one selects a template with regular pores, self-assembled microtubes can be fabricated by the LbL assemblingtechnique (Fig. 1B). Sequential assembly of polymers within acontrolled pore followed by template removal can result in self-standing tubular structures. For example, Li et al. reportedfabrication of microtubes based on hydrogen-bonding LbL self-assembly from poly(acrylic acid) and poly(4-vinylpyridine)[11]. They also demonstrated successful removal of the poly

Fig. 1. (A) Synthesis and self-assembly behavior of metal–polymer amphiphiles. (B) Synthesis of microtubes using porous template. (C) SEM image of human serumalbumin microtubes. Reprinted with permission from Ref. [12•], ©2005, American Chemical Society.

108 K. Ariga et al. / Current Opinion in Colloid & Interface Science 12 (2007) 106–120

(acrylic acid) components from the tubes. The resulting porous-walled tubes could also be useful as carriers in drug delivery oras catalyst supports.

2.2. Microstructures from biopolymers and related molecules

Biopolymers such as proteins and DNA have sophisticatedfunctions that cannot be easily mimicked using artificialmaterials. Therefore, incorporation of biopolymers into micro-structures through self-assembly processes is an attractiveresearch target. For this purpose, approaches used for micro-fabrication of conventional polymers can be applied tobiocomponents. Li et al. applied the LbL assembly techniqueto fabrication of microtubes from biocomponents such asproteins and phospholipids [12•]. Human serum albumin has

structural stability under acidic or basic conditions (even at pHdeparting significantly from its isoelectric point) because ofstrong binding sites within its subdomains. Thus, the surfacecharge of human serum albumin can be modified to be eithermore positive or more negative by varying pH. This makes itpossible to form an LbL assembly of human serum albuminitself. Fig. 1C depicts synthetic microtubes of human serumalbumin that have smooth and clean surfaces with a wallthickness of around 30 nm. The length of the resulting tubes isabout 60 μm and they possess flexibility tolerant to free bending.

In peptide segments, amino acid residues containing variousside chains can be connected in a designated sequence.Hydrophilic and hydrophobic units can be linked in a desiredsequence and length, resulting in ideal amphiphilic polymers.Therefore, well-designed peptides can act as amphiphilic


components of polymer vesicles. Bellomo et al. reported theself-assembly of diblock copolypeptides into spherical vesicleswhose micron-range diameter and structure are dictatedprimarily by the ordered conformations of the polymersegments [13]. As shown in Fig. 2A, the polymer used containsoligo(ethyleneoxide)-terminated lysine sequences and leucine–lysine copolymer as hydrophilic and hydrophobic domains,respectively. In the latter parts, the hydrophilic and electrostaticproperties of the lysine residues can be tuned by varying pH.Protonation of the lysine residues in the polypeptide chainconsiderably enhances their hydrophilicity with concurrent

Fig. 2. (A) Spherical vesicle formation from copolypeptide. (B) Memb

destabilization of the α-helical structure of the leucine-richdomain because of electrostatic repulsion. A helix-to-coilconformational transition induced by protonation of the lysinegroups also destabilizes the vesicular assembly, leading toporous membranes. Under basic conditions, leakage of trappedmaterials within the vesicle core was minimized. However,when the pH was lowered by the addition of HCl, a near-instantaneous disruption of the vesicle membranes occurredaccompanied by the release of the trapped molecules. Theabundance of functionality present in amino acids provides apowerful means to design various properties into polymeric

rane channel formation from dipeptide with dendritic side chain.


vesicles, and can be exploited in the preparation of deliverysystems for therapeutic agents. Deming et al. extended thisconcept to an actual cell-uptake experiment using copolypep-tides composed of polyleucine and polylysine portions [14], thelatter of which are expected to interact with cells. Epithelial andendothelial cell lines were examined because of their relevancein oral and intravenous drug delivery, respectively. Treatment ofcultures of both cell types with polypeptide vesicles of 100-nmaverage diameter revealed rapid uptake of the vesicles and theircontents by both cell lines. In addition, since the oligoleucinehydrophobic interactions are stronger than the polyarginine–cell interactions, the vesicles did not disrupt upon cell binding.

Peptide segments can be self-assembled through hydrogenbonding. Formation of secondary structures such as α-helix orβ-sheet often provides specific structures in their self-assem-blies. Matsuura et al. reported a unique approach for preparationof submicron-size spheres through formation of intermolecularantiparallel β-sheets of artificial C3-symmetric peptide con-jugates, trigonal(FKFE)2, where F, K, and E representphenylalanine, lysine, and glutamic acid, respectively [15].Three oligopeptide segments linked through a trimesic acidderivative form an antiparallel-type β-sheet in acidic aqueoussolution, as confirmed by CD and IR spectra. Formation ofspherical structures from the peptide conjugate was proved bymicroscopic observation. AFM and scanning electron micro-scopic (SEM) observations revealed the presence of sphericalstructures of 22–34 nm diameter and concave structures of 50–100 nm size. The observed concave structures might be formedby the collapse of the spherical assemblies on the micasubstrate. This strategy could be applied widely in the designof submicron-scale spherical bioassemblies, which would beused in many fields as gene carriers or as reactors.

Non-spherical structures were also produced by self-assembly processes of peptide derivatives. Percec et al. prepareda membrane channel upon self-assembly of a dipeptide con-taining a dendritic side chain at its tyrosine residue (Fig. 2B)[16]. The resulting channel was reconstituted in phospholipidsliposomes. Proton permeability of liposomes containing anaverage of one to two reconstituted dendritic pores was compa-rable in efficiency to those containing gramicidin channels.Hentschel and Börner proposed an approach of peptide-guidedmicrostructure formation for synthetic polymers based onβ-sheet formation of the peptide moiety [17]. A poly(n-butylacrylate) chainwas linked to a designed sequence of a peptide thatencodes a high tendency to adopt an antiparallel β-sheet motif.The self-assembly process of the peptide segment led to theformation of densely twisted tape-like microstructures, whichcould be observed by AFM and TEM observations. This strategyshould open a new avenue to a variety of polymericmaterials witha defined hierarchical microstructure.

The use of other biopolymers for microstructure formation hasalso been demonstrated. Nucleic acids, variously DNA or RNA,are capable of restoring and transmitting biological informationthrough highly specific complementary base-pairing. This uniqueproperty can be used for programmed formation of nanostructuresand microstructures and has been summarized in the recentreview by Seeman and Lukeman [18•]. Not only structure

formation but also the operation ofmolecular mechanical devices,i.e. nanorobotics, based on programmed DNA assemblies wasproposed byDing and Seeman [19]. In their system, a robotic armcomposed of DNA can rotate on a two-dimensional crystal of aDNA array. Mao et al. proposed the use of virus assembly for thepreparation of one-dimensional inorganic structures [20]. Pep-tides, which exhibit control of composition, size, and phaseduring nanoparticle nucleation, have been expressed on the highlyordered filamentous capsid of the M13 bacteriophage. Mineral-ization on the one-dimensional viral structure and removal of theviral template by means of annealing promoted orientedaggregation-based crystal growth, forming individual crystallinewires. This method was used to realize the synthesis of single-crystal ZnS, CdS, and free-standing chemically ordered CoPt andFePt wires.

2.3. Microstructures from conjugated polymers

Conjugated polymers have unique photoelectronic proper-ties because of their delocalized π and σ electrons. Although theproperties of single-chain conjugated polymers have beeninvestigated, the careful design of organized structures at thenanometer and micrometer scales is also important for theconstruction of functional devices based on functions of theconjugate polymers. One of the hot topics in research onstructural organization of conjugated polymers is to constructmicrostructures through hierarchic self-assembled processes.

A recent review by Goto and Akagi [21] summarized stableoptically active polymers of helical structure, which have beensynthesized by many approaches including introduction of anoptically active substituent to the polymer side chain or additionof active components to the polymers, resulting in variouschiroptical properties. Akagi et al. reported that helicalpolyacetylene with very stable chiroptical properties can besynthesized in the reaction medium of a chiral nematic liquidcrystal [22•]. Recently, Akagi et al. reported the control of themorphology of polyacetylene microbundles using crown ether-type binaphthyl derivatives as chiral dopants [23]. The twistingpowers of the crown ether-type binaphthyl dopant for phenyl-cyclohexane-derived nematic liquid crystals increased withdecreasing ring size of the crown ether. Helical polyacetyleneswere synthesized in the chiral nematic liquid crystalline phase.SEM observation revealed that many spiral domains with ca.100-μm diameter were contained in the film, and the fibrilsturned in a counter-clockwise direction to form a bundle withsubmicron-level width, and the bundle further turned in the samedirection to form a spiral domain. With increasing twistingpower of the crown ether-type binaphthyl dopant, the inter-fibrildistance within the bundles of helical polyacetylene decreasedand the diameter of a fibril bundle decreased but the diameter of afibril did not change. This strategy can be extended to otherconjugated polymers such as polythiophene or polypyrrole. Thepresent chiral dopants can more precisely provide the screwstructures of these conjugated polymers by virtue of the ring sizeeffect of the crown ether and the host–guest interaction.

Ohira et al. reported micron-level morphology control ofpolysilanes [24], which are expected to display unique


photoelectronic properties based on σ-electron conjugationalong their main chains. AFM observation of an isolated helicalpolysilane chain bearing a fluoroalkyl side group revealedchanges from a rod to a circle structure depending on the mainchain length, i.e. the chain topology eventually transforms fromrod to circular structure as the main chain length increased.Semicircular structures intermediate between rod and circlecould be observed in a very narrow region of medium length(400–500 nm). Topological switching between rod and circlestructures is therefore due to a discontinuous transition phe-nomenon. In one of the proposed models of the circularstructure, intramolecular interaction between C–F and Si isexpected to stabilize a perfect closed-circle structure becausethe fluorine atoms on the side chains may contact the Si atomson the main chain close to the end groups with possible for-mation of Si–O–Si bond between the end-termini via hydro-lysis of the Si–H end group.

Valiyaveettil et al. reported the single-step self-organization ofhighly ordered structures of functionalized poly( p-phenylene)swithout using either a controlled environment or expensivefabrication technique [25]. Microporous film structures of poly( p-phenylene) derivatives in a honeycomb pattern were pre-pared by direct spreading under ambient conditions of a dilutepolymer solution on various substrates, such as glass, quartz,silicon wafer, indium tin oxide, gold-coated mica, or water, andexhibit blue-light-emitting properties. These patterns areexpected to reach applications in photonic and optoelectronicdevices. In addition, it was successfully demonstrated that thematerials obtained could be used for micropatterning of carbonnanotubes with good spatial distribution over a large-area film.

2.4. Microstructures from supramolecular polymers

Supramolecular polymers, which are formed by assembly ofsmall functional molecules through non-covalent interactionssuch as metal coordination and hydrogen bonding, have beeninvestigated because of their versatilities of component andstructural design. Several examples of microstructure formationfrom supramolecular polymers have been reported recently.

Oh and Mirkin demonstrated that the simple addition of anappropriate initiation solvent to a precursor solution of metal ionssuch as Zn(II) and Cu(II) ions and a ligand (homochiralcarboxylate-functionalized binapthyl bis-metallotridentate Schiffbase) results in the spontaneous and fully reversible formation of anew class of metal–metalloligand particles (Fig. 3A) [26].Interestingly, initial formation of particles with diameters of afew hundred nanometers was observed followed by coalescenceyielding uniform and smooth microparticles. Detailed observa-tion by SEM suggests a two-step cluster-fusion growthmechanism, where several small particles first aggregate toform large cluster particles, which in a second step undergointraparticle fusion to yield large uniform spherical particles. Thereversible nature of the metal coordination complex permits thesystem to anneal into a smooth particle. Optical properties of theparticles depended on additional coordination ligands such asDMSO, pyridine, DMF, acetone, methanol and water. Differentcolor features can be recognized by the naked eye.

One of the most important features of coordination polymersis structural control by ligand structures. Geometries of unitcoordination structures are defined by ligand structure, possiblyleading to control of assembled structures at the macroscopiclevel. Maeda et al. demonstrated modification of microstructuresof coordination polymer assemblies upon systematic changes ofthe ligand structures [27]. Bisdipyrrin (dipyrromethene) ligandsbridged by rigid spacers were complexed with Zn(II) ions toprovide various coordination polymers whose textures wereobserved by SEM. The use of mpm-type and pmp-type ligandsfor coordination with Zn(II) ions in THF resulted in theformation of submicron-size spherical structures (see images(a) and (b) in Fig. 3B, respectively). Uniform distribution of Zn(II) ions in the former structure was confirmed by Zn elementalmapping using high-resolution TEM energy dispersive X-ray(HRTEM–EDX) analysis (inserted image in Fig. 3B(a)).Interestingly, in the former structure, trains of spheres wereobserved. Uniform distribution of the complex within thestructure was illustrated using fluorescence micrographicimaging of the latter assembly (image inserted in Fig. 3B(b)).Submicron-size polymer particles are normally spherical forminimization of the interfacial free energy between the particleand the solvent. Therefore, appropriate selection of surroundingsolvents should induce morphological changes. Indeed, the useof mixtures of THF and water (2:1, v/v) instead of pure THFafforded bell-shaped objects for the coordination polymer with apmp-type ligand (image (c) in Fig. 3B). The structures self-assembled from mmm-type ligand and Zn(II) ions in 1:1 THF/water solution were similar to micron-scale golf balls (SEMimage in Fig. 3B(d) with TEM image as insert). The sameresearch group also provided examples of morphological controlof self-assembled materials through hydrogen bonding [28],where microstructures of porous, fibrous and sheet-like shapesbased on the hydrogen-bonding interactions of dipyrrolyldike-tones were presented. Morphologies of the self-assembledstructures can be finely tuned by selection of componentmolecules and solvents. These examples demonstrate thatmicron-scale architectures of supramolecular polymers can bedesigned as desired and prepared upon appropriate selection ofcomponent molecules and solvents, with the possibility ofpreparation of functional materials with dimension-controlledstructures.

Kitagawa et al. reported morphological control of coordina-tion polymers at the micrometer and millimeter scales throughcrystallization processes [29]. The coordination polymer inves-tigated was crystallized after the reaction of 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (hat-(CN)6) withcopper(I) ions in the presence of ethanol. The reaction of hat-(CN)6, a metal ion, and a nucleophile can afford a coordinationframework, because the ligand hat-(CN)6 functions not only as aC3 symmetrical chelate ligand, but also as a supplier of a linearbridging ligand (CN). The reactions of hat-(CN)6 with variousalcohols such as ethanol in the presence of the copper(I) com-plex afforded the corresponding coordination frameworkscontaining the respective tricyano trialkoxy derivatives of hatthrough the substitution of cyano groups by alkoxy groups.Crystals first grew along the c-axis into a needle, which

Fig. 3. (A) Microparticle formation from metal–metalloligand coordination polymer. (B) SEM images of coordination polymer assemblies with various ligands:(a) with pmp-ligand from THF (inset: Zn mapping HRTEM–EDX); (b) with mpm-ligand from THF (inset: fluorescence micrograph); (c) with mpm-ligand from THF/water (2/1); (d) with mmm-ligand from THF/water (1/1) (inset: TEM image). Reprinted with permission from Ref. [27], ©2006, American Chemical Society.(C) Unique crystal growth of coordination complex of hat-(CN)6 with copper(I) and ethanol.



transformed into a hexagonal dumbbell. Subsequently, the twobases of the hexagonal dumbbell grew forming hexagonalcogwheels. The six arms then extended and branched at regularintervals, forming the superlattice. Branching occurred at twocenters from which ‘twigs’ sprouted along the hexagonal latticein clockwise or anticlockwise directions. Because no process ofgrowth along the c-axis, thin skeletal plates were formed. Thisexample suggests that application of appropriately designedchemical reaction to a certain crystallization system may induceformation of unprecedented micromorphologies reflecting thesymmetry of the crystal structure.

2.5. Microstructures from porphyrin derivatives

Various photoelectronic functions are expected for porphyrinicmaterials and related compounds such as phthalocyanines, andtheir assembly into controlled microstructures is crucial to thedevelopment of device construction. These pigments have beenwell investigated, especially in order to mimic natural light-harvesting systems. X-ray crystallographic structures of the light-harvesting antenna of the photosynthetic purple bacteria haveinspired research on assemblies of porphyrin derivatives. One ofthe most successful strategies for the formation of supramolecularassemblies of porphyrin derivatives is a route using coordinationto metal ions, as summarized in a recent review by Kobuke [30].Supramolecular assemblies obtained by ligand complimentarycoordination tometal ions at the center or exterior of the porphyrincores resulted in stable and discrete supramolecular structures,providing media appropriate for photophysical studies on energytransfer as systems analogous of the light-harvesting antennae.

Instead of constructing a rather small supramolecular complex,the formation of larger microstructures of porphyrin derivativesrequires a combination of various interactions, as reported byTakeuchi et al. [31]. They designed porphyrin-based organoge-lators, using porphyrin derivatives having hydrogen-bond-donating (carboxylic acid)/hydrogen-bond-accepting (pyridine)substituents or electron-donating (dialkylamino)/electron-with-drawing (pyridine) substituents at their peripheral positions. SEMand TEM observations revealed the superstructures containedwithin the organogels. Porphyrins tend to assemble in one dimen-sion because of their intermolecularπ–π stacking interaction, andthis one-dimensional aggregation mode can be further modulatedby combination of the other interactions between electron-donating/withdrawing groups and hydrogen-bond-donating/accepting groups. Additionally, the important role played by thedipole–dipole interaction in influencing the molecular assemblymode leading to the final morphology was suggested.

Kojima et al. reported the preparation of tubular and ringstructures from a saddle-distorted molybdenum(V)–dodecaphe-nylporphyrin complex by recrystallization from toluene withvapor diffusion of methanol [32]. Straight tubes with ca. 20 nmin diameter were detected. Electron diffraction and EDX spectraconfirmed that the tube observed was composed of molybdenumand oxygen. Dark field imaging and electron diffraction in-dicated the existence of Mo clusters as polycrystals. Formationof ring structures with a diameter of almost 200 nm was alsoobserved. The saddle-distorted porphyrin complexes provide

curved surfaces in assembled structures, resulting in a variety ofself-assembled structures for metalloporphyrins. Shelnutt et al.reported the synthesis of discrete free-standing porphyrinnanosheets using a reprecipitation method [33], in which anethanolic solution of Sn(IV) 5-(4-pyridyl)-10,15,20-triphenyl-porphyrin dichloride in ethanol was simply injected intodeionized water at room temperature under vigorous stirring.A wide range of applications in electronics, photonics, andcatalytic systems can be expected as a result of their uniquemorphology, photocatalytic properties, and large surface areas.

In several examples, porphyrin derivatives were used ascofactors to assist self-assembly of components. Takeuchi et al.recently proposed the use of porphyrin conjugates for alignmentof conjugated polymers within their self-assembled micro-structures through formation of poly-pseudorotaxane structures[34]. Reynhoudt et al. reported the self-assembly behavior ofconjugates of a synthetic polymer and natural protein through aporphyrin cofactor [35]. Biohybrid triblock copolymers weresynthesized by direct coupling of a synthetic diblock copoly-mer, polystyrene-b-polyethylene glycol, prepared using atomtransfer radical polymerization (ATRP), to a hemoprotein(hemoprotein, myoglobin or horse radish peroxidase) by theCu(I) catalyzed azide-alkyne [3+2] cycloaddition reaction (alsocalled “click” reaction) and subsequent cofactor reconstitution.The copolymers self-assembled into vesicles, micellar struc-tures, or lamellae-containing spheres. A fusion process involv-ing two micellar rods resulted in Y-junctions and toroids.

2.6. Microstructures from aromatic oligomers

Interesting photoelectronic properties can be also expectedfor conjugate oligomers of aromatic compounds, and their self-assembly into nanostructures and microstructures has beenrecently investigated. Several examples using compounds listedin Fig. 4 are briefly described below.

Lee et al. reported the formation of ring structures by theaqueous self-assembly of amphiphilic dumbbell-shaped mole-cules based on a rod segment that was grafted with hydrophilicpolyether dendrons at one end and hydrophobic branches at theother (Fig. 4(A)) [36]. TEM images indicated the coexistence ofspherical and curved cylindrical micelles with open ends, whichslowlymetamorphosed into toroidal micelles over a period of oneweek. The cross-sectional diameter was 16 nm, and the ringdiameters ranged from 70 to 300 nm with an average value of120 nm. The enhanced stability of the toroidal micelles relative tothe short cylinders with open ends can be attributed to the stronghydrophobic associationwithin the core consisting of the stiff rod-like units and the longer alkyl chains. Jonkheijm et al. investigatedsystematically the formation mechanism of fibrillar structures ofoligo(p-phenylenevinylene) derivatives with chiral side chainscapped at opposing ends by tridodecyloxybenzene and ureido-triazine groups, which had been tailored for self-complementaryfour-fold hydrogen bonding (Fig. 4(B)) [37]. The experimentaldata support a nucleation-growth pathway that gave rise to aremarkably high degree of cooperativity. Monomers first formeddimers via quadruple hydrogen bonding under fast equilibrium,which continued to shift upon cooling until 10 to 15 stacked

Fig. 4. Aromatic oligomeric molecules for microstructure formation.



dimers. Upon cooling themolecules in the pre-aggregates becamemore restricted in their relative positions. Helix formationtransforms the pre-aggregate into a chiral nucleus, finally leadingto fibrous structures via the elongation-growth pathway.

Ajayaghosh et al. extensively researched microstructureformation of conjugate oligomers and their gel formation. Forexample, they reported the formation of micrometer-sizesupramolecular tapes and helices through self-assembly of oligo(p-phenylenevinylene) derivatives (Fig. 4(C)) [38]. SEM imagesof a dried assembly obtained from decane revealed the formationof superstructures of micrometer size. The gel obtained exhibitedbirefringence when viewed through crossed polarizers indicatingthe molecular anisotropy of the aggregates. Size and morphologyof the assemblies were tunable, resulting in nanoparticles,microspheres, and superstructured blue-light-emitting organo-gels. The same research group also reported transformation ofself-assembled structures of tripodal squaraine dyes (Fig. 4(D)),which formed vesicular structures and then helical architectures[39]. The tripodal squaraines from acetonitrile first self-assembledinto hollow spherical structures that transform to linear helicalstructures upon interaction with Ca2+ or Mg2+.

Control of optical properties based on supramolecularstructure variation is an interesting target in functionalizationof self-assemblies of aromatic oligomers. Fluorescence reso-nance energy transfer (FRET) between the tape-like structure ofa few tailor-made oligo( p-phenylenevinylene) derivatives(donor, Fig. 4(E)) and entrapped rhodamine B (acceptor) wasalso investigated by Ajayaghosh et al. [40]. The efficiency ofFRET was considerably influenced by the ability of the oligo( p-phenylenevinylene) compound to form self-assembled ag-gregates and hence could be controlled by variation of thepolarity of the solvent and temperature. Tong et al. synthesizeda butterfly-shaped pyran derivative (Fig. 4(F)) by attachingtwo cholesteryl groups to a central conjugated aromatic moiety[41]. SEM and TEM observations revealed its self-assembledmorphologies at the submicron scale. Interestingly, this mol-ecule showed aggregation-induced emission, whose color andefficiency can be tuned readily by a morphology change fromcrystalline to amorphous phase. Efficient green emission wasobserved in the crystalline phase, while yellow and red emis-sions were realized in the amorphous phase.

2.7. Microstructures from graphene molecules

Expansion of aromatic conjugate leads to the formation of agraphene sheet, and assembly of the graphene sheets can beregarded as supramolecular nanocarbon analogues. Müllen etal. have been investigating assemblies of an extendedpolycyclic aromatic hydrocarbon, namely hexa-peri-hexaben-zocoronene (HBC). HBC can be regarded as the smallestgraphene fragment consisting of 13 fused benzene rings, and ithas a strong tendency to stack together via π-electronicinteractions. For example, they prepared macroscopic rodstructures by template synthesis [42]. HBC derivatives withlong, branched 2-decyl-tetradecyl alkyl side chains, which haverather low melting points, were filled by melting and coolingprocedures. An aluminum oxide membrane used as the porous

template was then carefully removed by its dissolution in a coldaqueous, dilute, sodium hydroxide solution. SEM images takenafter the complete removal of the alumina template displayedintact, organized structures of graphene assemblies.

Fukushima et al. have developed preparative methods for self-assembled graphene tubes from amphiphilic HBC derivatives.Three examples are shown below. Hill et al. realized preparationof electrically conductive tubular structures through self-assem-bly of an HBC derivative with hydrophobic C12 chains andhydrophilic triethylene glycol chains (Fig. 5A(a)) [43••]. Thismolecule dispersed in appropriate solvent self-assembled intofiber-like structures as confirmed by SEM observation. Detailedinvestigation by TEM revealed tubular structures that werestraight and discrete with no detectable branching, having auniform diameter of 20 nm. Although the intact tube wasessentially insulating, the tubular structure after oxidation withNOBF4 showed a conducting current–voltage profile with anohmic behavior. The resistivity estimated at 285 K is comparablewith that of an inorganic semiconductor nanotube composed ofgallium nitride. Jin et al. investigated chirality of the helical tubesassembled from HBC derivatives with chiral centers in theattached chains (Fig. 5A(b) and (c)) [44]. The tubular structureswith right- and left-handed helical senses were obtained from the(S)- and (R)-enantiomers of the amphiphile, respectively.Chirality of the assemblies prepared from mixtures of the twoamphiphiles at various mixing rations was investigated by CDmeasurements. Even though the enantiomeric excess of the chiralamphiphile was changed over a wide range from 20% to 100%,the CD spectrum of the suspension remained almost unchanged,resulting in a sigmoidal response of the CD intensity to themixingratio between (S)- and (R)-enantiomers. Such a non-linearphenomenon is referred to as chirality amplification, where themajor enantiomer incorporated into each tube determines thehelical sense (majority rule). Yamamoto et al. reported photo-electronic properties of a coaxial tubular structure formed bycontrolled self-assembly of trinitrofluorenone-appended HBCamphiphile (Fig. 5A(d)) [45]. Photo-conducting properties of thetubes formed were evaluated using a two-probe method acrossmicrometer-gap electrodes. Current–voltage profiles of the tubesshowed that the current was markedly enhanced by a factor ofN104 upon photo-irradiation. The tube used has a coaxialconfiguration, where a molecular layer of electron-acceptingtrinitrofluorenone laminates an electron-donating graphitic layerof π-stacked HBC. Photo-excitation of the self-assembled HBCshould result in the generation of a charge-separated stateinvolving radical cations and anions in the inner and outer layersof the tubes, respectively. This spatial separation of charge carriersprevents their rapid recombination, thereby enabling photo-conduction to occur along the tubes.

2.8. Microstructures from fullerene derivatives

Fullerene families have been investigated widely as usefulcandidates for components in supramolecular photoelectronicdevices. Therefore, their assembly into nanostructures andmicrostructures is an attractive research target. Simple strategiesinclude nanowhisker and nanotube formation from C60 and C70

Fig. 5. (A) Amphiphilic HBC molecules for tubular structure formation. (B) Fullerene derivatives for microstructure formation.



fullerene themselves, which have been investigated extensivelyby Miyazawa and coworkers. For example, Tachibana et al.reported the preparation of C60 nanowhiskers by a liquid–liquidinterfacial precipitation method, where the growth of thenanowhiskers is promoted under illumination even with weakambient (fluorescent) lighting [46]. Themaximum length exceeds1 mm while the diameter is about 250 nm.

Modification of fullerene molecules has been used for thepreparation of fullerene microstructures. One of the typicalexamples is the formation of vesicle structures from anionicfullerenes bearing phenyl rings (Fig. 5B(a)), as seen in the reviewby Nakamura and Isobe [47]. One fullerene derivative formedspherical aggregates when dissolved in water. A laser light-scattering study of the association behavior of the potassium saltof the pentaphenyl fullerene in water revealed the formation ofstable spherical vesicles with an average hydrodynamic radiusand a radius of gyration of about 17 nm at a very low criticalaggregation concentration [48•]. One vesicle is composed of ca.12,000 fullerene molecules, and its size distribution is unexpect-edly narrow. Formation of vesicular structures composed offullerene was also reported by Chiang et al. [49]. Theysynthesized water-soluble oligo(ethylene glycolated) derivativesof fullerene (Fig. 5B(b)) that provided nano- to submicron-sizespherical hollow vesicles with a shell width of 15–20 nm, whichcould be observed by TEM.

Non-spherical nanostructures and microstructures composedof fullerene derivatives have also been reported. Hirsch et al.demonstrated morphology changes in assembled structures ofamphiphilic fullerenes (Fig. 5B(c)) upon variation of pH [50].Rod-shaped aggregates with a double-layer ultrastructure werepredominant at neutral pH in water (phosphate buffer pH 7.2).Increasing acidity to around pH 9 (borate buffer pH 9.2)induced formation of globular micelles. Kawaguchi et al.demonstrated hierarchic growth of supramolecular assembliesfrom individual spheres to network structures through stereo-complex formation of a polymer chain attached to fullerene[51]. They synthesized isotactic and syndiotactic C60-end-capped poly(methyl methacrylate)s (PMMA) with a preciselycontrolled structure including molecular weight distribution andtacticity. The stereo-regular PMMA–C60 conjugate self-assem-bled to form a core–shell aggregate with C60 as the core and thePMMA chains as the shell in water/acetonitrile (1/9, v/v)because of the solvophobic interaction of the C60 units. Uponappropriate mixing of isotactic- and syndiotactic-PMMA–C60

conjugates, the core–shell aggregates further self-assembledwith each other into two-dimensional and three-dimensionalnetwork structures through iterative stereo-complex formationof the shell PMMA chains. A unique structure was prepared byGeckeler et al. who reported the synthesis of hollow porouscarbon spheres possessing a large surface area by destructiveoxidation of C60 by KMnO4 under basic conditions [52].

2.9. Diverse control of microstructure formation from a singlemolecule (fullerene derivative)

The abovementioned examples illustrate the possibilities forsynthesis of various nanostructures and microstructures of

functional molecules. However, these approaches tend toprovide only one or two structures from a single molecule. Amost promising technique for science and technology based onself-assembled materials would be a methodology to freelycreate various morphologies from a single compound using asimple procedure. This has been recently accomplished byNakanishi et al. through their research on microstructureformation of a single fullerene molecule [53••], as shownbelow.

Their molecular design is based on a novel concept ofamphiphilicity. As indicated in Fig. 6A, the molecule usedcontains two distinct portions, an sp2-carbon-rich fullerenemoiety and a group containing sp3-carbon-rich aliphatic chains.These parts are both hydrophobic but show different affinitiesfor several solvents, resulting in diverse solvation of these twodiffering groups in solvents. Different solvation of these twoparts should affect curvature of the resulting assemblies, leadingto the formation of versatile-shaped assemblies of the fullerenederivative. Preparation of the fullerene assemblies is rathersimple, i.e., self-assembled supramolecular objects wereprepared by incubation of this molecule in an appropriatesolvent or solvent mixture. When 1,4-dioxane was used, thebrown colored supernatant contained self-assembled singlebilayer disks (Fig. 6A(a)) in its SEM image where the diameterof the disks was observed to be 0.2–1.5 μm. The layer thicknessof ca. 4.4 nm determined from tapping-mode AFM correspondsto the thickness of an alkyl-chain-interdigitated bilayer structureof this fullerene derivative. This fullerene derivative self-assembled into spherical vesicle-like aggregates with anaverage diameter of 250 nm in the 2-propanol/toluene solventsystem. The vesicle structure of these spheres was confirmed bySEM (Fig. 6A(b)), and the presence of empty core and wallstructure was confirmed by HRTEM observation. Furthermore,different solvent conditions, for instance use of 1-propanol,directed the assembly to form one-dimensional structures(Fig. 6A(c)). The resulting fibers and bundles of fibers hadlengths of over 20 μm, which appeared as partially twisted two-dimensional tapes in their TEM images. When this fullerenederivative was dispersed in an equimolar mixture of water/THF,a turbid brown-colored dispersion was obtained. SEM and TEMmicrographs revealed cone-shaped objects of sub-micron size(Fig. 6A(d)). The cone apex has a hole with a diameter ofca. 60 nm. The thickness of the shell of the conical objects isca. 150 nm, which corresponds to a multi-layer film.

In addition to these microstructures, nanostructures and bulkstructures of fullerenes can be tuned using a similar moleculardesign. One-dimensional lamellae, epitaxially oriented alongthe highly oriented pyrolytic graphite (HOPG) lattice (Fig. 6B)were formed by the same fullerene derivative [54]. Tapping-mode AFM observation revealed that the lengths of the lamellaeexceed 100 nm (Fig. 6B(a)). The high-resolution STM imageshowed lamellae composed of C60 arranged in a zigzag-typefashion (Fig. 6B(b)). Slight modification in the position ofalkoxyl chains substituted at the phenyl rings attached tofullerene unexpectedly provided viscous materials (Fig. 6C)[55•]. Detailed studies of the rheological, electrochemical, andcharge carrier transport properties of these materials confirmed

Fig. 6. (A) Diverse control of microstructure from single fullerene derivatives. Reprinted with permission from Ref. [53••], ©2005, Royal Society of Chemistry.(B) Straight lamellar structures on HOPG obtained from the same compound in (A). Reprinted with permission from Ref. [54], ©2006, American Chemical Society.(C) Room temperature liquid fullerene. Reprinted with permission from Ref. [55•], ©2006, American Chemical Society.


that they can be regarded as electrochemically active liquidfullerenes. Examples described in this section can be regardedas some of the ultimate techniques for fabrication of self-

assembled structures. The concepts could be adapted for free-fabrication of nanoscale and microscale objects of any kind offunctional molecule.

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3. Conclusion and future perspectives

The diverse potential of microstructure formation based onself-assembly processes has been illustrated by examples in thisreview. Microstructures of various morphologies such as sphere,fiber, tube, disk, and cone can be prepared by spontaneous self-assembly of many kinds of molecules including biopolymers,conjugated molecules, porphyrins, graphenes, and fullerenes.This suggests that fabrication techniques required fornanoscience and nanotechnology could be attained solelythrough bottom-up-type approaches based on self-assembly.However, this is not true for practical purposes. In order tointegrate functional molecules in miniaturized devices, nanos-tructures and microstructures of functional elements prepared bybottom-up self-assembly need to be immobilized within finelytuned device structures fabricated using top-down-type ap-proaches. Moreover, a combination of top-down and bottom-upstrategies is beneficial for the preparation of microstructuresthemselves. For example, Lee et al. reported the use of amicrochannel containing a Y-shaped micromixer reactor forfullerene nanowhisker preparation [56]. This method providedimproved uniformity in thickness and length of nanowhiskersthan conventional flask reactions. Räder et al. has proposed useof solvent-free matrix-assisted laser desorption/ionization(MALDI) apparatus for assembly of graphene molecules [57].This soft-landing bottom-up technique resulted in the growth ofpolycrystalline layered ultrathin films with high degrees oforientation and purity. This approach should lead to self-assembly of functional molecules in desired locations atsurfaces. Apart from these examples, various possibilities canbe expected by hybridizing top-down and bottom-up fabricationmethods. Techniques developed using top-down and bottom-upapproaches have been predominantly developed independentlyof each other. However, their recent combination has enabledfurther progress in device fabrication technologies.

Acknowledgements

The researches described in this chapter were partiallysupported by Grant-in-Aid for Scientific Research on PriorityAreas “Chemistry of Coordination Space” and a Grant-in-Aidfor Science Research in a Priority Area “Super-HierarchicalStructures” from the Ministry of Education, Science, Sports,and Culture, Japan, and a Grants-in-Aid for Scientific Research(B) from Japan Society for the Promotion of Science.

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self-assembled microstructures of functional molecules

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