Stimulus-Responsive Azobenzene Supramolecules: Fibers, Gels, and

Apr 18, 2013 - Department of Chemical Engineering, Hanyang University, Seoul 133-791, ... Advanced Technology Support Team, RIKEN Advanced Science ...
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Stimulus-Responsive Azobenzene Supramolecules: Fibers, Gels, and Hollow Spheres Sumi Lee,† Seungwhan Oh,† Joosub Lee,† Yashwardhan Malpani,‡,§ Young-Sik Jung,*,‡ Baotao Kang,∥ Jin Yong Lee,*,∥ Kazunari Ozasa,⊥ Takashi Isoshima,⊥ Sang Yun Lee,⊥ Masahiko Hara,⊥ Daisuke Hashizume,# and Jong-Man Kim*,† †

Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea Bio-Organic Science Division, Korea Research Institute of Chemical Technology, Daejeon 305-606, Korea § University of Science and Technology (UST), 217 Gajeong-Ro, Yuesong, Daejeon 305-350, Korea ∥ Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea ⊥ Flucto-Order Functions Research Team, RIKEN-HYU Collaboration Research Center, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan # Advanced Technology Support Team, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: Novel, stimulus-responsive supramolecular structures in the form of fibers, gels, and spheres, derived from an azobenzene-containing benzenetricarboxamide derivative, are described. Self-assembly of tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxamide (Azo-1) in aqueous organic solvent systems results in solvent dependent generation of microfibers (aq DMSO), gels (aq DMF), and hollow spheres (aq THF). The results of a single crystal X-ray diffraction analysis of Azo-1 (crystallized from a mixture of DMSO and H2O) reveal that it possesses supramolecular columnar packing along the b axis. Data obtained from FTIR analysis and density functional theory (DFT) calculation suggest that multiple hydrogen bonding modes exist in the Azo-1 fibers. UV irradiation of the microfibers, formed in aq DMSO, causes complete melting while regeneration of new fibers occurs upon visible light irradiation. In addition to this photoinduced and reversible phase transition, the Azo-1 supramolecules display a reversible, fiber-to-sphere morphological transition upon exposure to pure DMSO or aq THF. The role played by amide hydrogen bonds in the morphological changes occurring in Azo-1 is demonstrated by the behavior of the analogous, ester-containing tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5-tricarboxylate (Azo-2) and by the hydrogen abstraction in the presence of fluoride anions.

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functional groups, including polymerizable moieties,14−16 photoresponsive groups,17−19 and stimulus-sensitive chromogenic dyes,20,21 the resulting supramolecular architectures often have meritorious features that include enhanced stabilities, reversible phase transitions, and sensory functions. In this context, photoresponsive molecular systems containing azo-

INTRODUCTION

The generation of functional supramolecular structures through self-assembly of small organic molecules continues to be a significant and challenging scientific endeavor.1−13 Molecularly ordered states of molecules often possess markedly different optical, chemical, and physical properties compared to those of the individual molecules. When subjected to special selfassembly conditions, individual molecules can aggregate to form various nano/microshapes, such as micelles, vesicles, tubes, wires, and helixes. If the constituent molecules contain © XXXX American Chemical Society

Received: January 15, 2013 Revised: March 18, 2013

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actions in governing the solvent dependent morphology of Azo-1

benzene chromophores are especially interesting, and as a result, they have been extensively studied.22−29 Importantly, azobenzenes are readily prepared, and they participate in efficient, light-driven, reversible trans−cis photoisomerization reactions. Consequently, this group has been incorporated into numerous molecular systems, such as ion gate channels,30 molecular switches,31,32 holographic gratings,33,34 and drug delivery vesicles.35 Owing to the presence of its intriguing C3-symmetry and the presence of three hydrogen-bondable amide groups, members of the 1,3,5-benzenetricarboxamide (BTC) family have been actively explored as scaffolds for supramolecular columnar structures.36−43 During the course of studies aimed at the development of one-dimensional nanostructures using BTC derivatives,44 we observed that the azobenzene-containing benzenetricarboxamide tris(4-((E)-phenyldiazenyl)phenyl)benzene-1,3,5 tricarboxamide (Azo-1) (Scheme 1) not only

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RESULTS AND DISCUSSION Preparation of Azo-1 Supramolecules. Azo-1, an azobenzene-containing BTC, is soluble in organic solvents, such as dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), and tetrahydrofuran (THF). Addition of water to 10 mM solutions of Azo-1 in these solvents results in formation of supramolecular aggregates that have strikingly different morphologies. Formation of aggregates was observed to take place when the amounts of water in the specified solvent were above 17% (for DMSO), 21% (for DMF), and 20% (for THF). As displayed in Figure 1a, Azo-1 supramolecules prepared in

Scheme 1. Structures of Azobenzene-Containing Azo-1 and Azo-2

displays a photoinduced reversible phase transition, but also shows morphological diversity ranging from fibers and gels to spheres depending on the fabrication conditions employed for supramolecular assembly. In addition, and quite surprisingly, self-assembly of the azobenzene derivative Azo-1 in certain environments results in the formation of hollow spherical structures. Although numerous examples of hollow spheres have been described, the majority have been prepared by using inorganic substances45−47 or polymers.48,49 In addition, hollow spheres are typically fabricated by utilizing a template-assisted coating and dissolution strategy.50 In contrast, fabrication of hollow spheres through self-assembly of small organic molecules is a rare occurrence.51−53 In the study, described below, we have explored the fabrication and properties of supramolecular structures derived from the azobenzene-containing BTC derivative Azo-1 and its structural analogue tris(4-((E)phenyldiazenyl)phenyl)benzene-1,3,5 tricarboxylate (Azo-2). Investigations with the latter substance were carried out to gain information about the role played by amide hydrogen bonding interactions in governing supramolecule formation. Also, the results of single crystal X-ray diffraction and FTIR analyses together with other experimentation have provided a clearer understanding the role played by H-bonding inter-

Figure 1. Scanning electron microscope (SEM) images of Azo-1 (10 mM) supramolecules prepared in 50% DMSO−H2O (a), 50% DMFH2O (b), and 50% THF−H2O (c and d). Transmission electron microscope (TEM) images of the spheres (e and f). The inset in part b shows a vial containing the Azo-1 gel.

50% DMSO−H2O are composed of microfibers with 3−5 μm diameters. This is an expected result, considering the fact that the structure of the Azo-1 molecule, containing three hydrogen bondable amide groups and aromatic moieties, would favor formation of one-dimensional columnar aggregates. The diameter of the microfibers was found to be relatively insensitive to the composition of the solvent. Thus, microfibers with similar diameters (3−5 μm) are produced when selfassembly of Azo-1 is carried out in aqueous DMSO solutions containing 20−50% water (data not shown). Interestingly, addition of water to a DMF solution containing Azo-1 results in gelation of the solvent (Figure 1b). Gelation promoted by Azo-1 is highly efficient, occurring when less than 1.0 wt % of Azo-1 is present (Figure S1, Supporting Information). In contrast to fibrous Azo-1 supramolecules formed in aqueous DMSO and DMF solutions, nano/microsized spheres are generated from self-assembly of Azo-1 in mixtures of THF B

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and water (Figure 1c). More intriguing is the fact that the precipitates of Azo-1 formed in aqueous THF solutions are mainly composed of hollow spheres (Figure 1d,e) along with some bulk spheres (Figure 1f). Moreover, as the water content of the solvent increases, smaller sized spheres are generated (Figure S2, Supporting Information). Since Azo-1 is highly soluble in DMSO, DMF, THF, and pyridine, addition of various solvents to these solutions was carried out to investigate the effects on fiber, gel, and sphere formation. Depending on the solvent composition, various morphologies were produced (Figure S3). In order to demonstrate that hollow spheres are indeed generated, an encapsulation experiment using the fluorescent Rhodamine B (RB) dye was carried out (see the encapsulation condition in Supporting Information). The optical and fluorescence images, displayed in Figure 2a,b, clearly show

Figure 3. (a) Optical microscope images of the Azo-1 supramolecules prepared in 50% DMSO−H2O upon UV and visible light irradiation. (b) UV−vis spectra of Azo-1 (10 mM) supramolecules prepared in 50% DMSO−H2O before (black line) and after (red line) UV irradiation (330−385 nm) for 30 s. The absorption spectrum obtained after visible light irradiation (460−490 nm, 2 min) of the UV irradiated sample is presented as a blue line.

Figure 2. Optical (a) and fluorescence (b) microscope images of the Rhodamine B encapsulated spheres. Solvent sensitive emission property of fluorescein (c) and fluorescence images of fluoresceinencapsulated Azo-1 spheres (d).

regenerated fibers causes melting of the supramolecules. These light-induced phase transitions of Azo-1 supramolecules can be repeated numerous times without decomposition of the material. The light-induced reversible morphological transition can be viewed in the movie clip provided as a Supporting Information. The results of UV−vis spectroscopic monitoring of the morphological changes indicate that a reversible trans−cis isomerization process occurs during the phase transition event (Figure 3b). In order to test if local heat is generated by UV irradiation and that this is the cause of the phenomenon, a thermometer was placed under the light source. Heat generation was not observed. In addition, the fibers do not melt at temperatures below 70 °C, indicating that an isomerization reaction is responsible for the phase transition. We also observed that regeneration of the fibers occurs in the dark but at a much lower rate (>30 min). In contrast, growth of the fibers takes place as soon as the melted sample is exposed to visible light. It should be noted that no phase transitions take place when dry Azo-1 fibers are used. An additional, and interesting, observation was made when a photomasked irradiation experiment was performed. Specifically, photomasked UV irradiation (30 s) of the fibrous Azo-1 supramolecules causes selective melting of the fibers in the UV exposed areas. This phenomenon results in the formation of patterned images (Figure 4). Visible light irradiation (3 min) of

that Rhodamine dye molecules are encapsulated within the spheres. To gain further information on the nature of the hollow spheres, we employed the solvent polarity sensitive green fluorescent dye fluorescein (FL) (see the solvent sensitive emission property of fluorescein dye and encapsulation of fluorescein dye in Supporting Information).54 As displayed in Figure 2c, FL in an aqueous environment fluoresces around 530−540 nm while in THF only weak emission is observed. The results of an FL encapsulation experiment show that Azo-1 spheres do not display green emission in their cores (Figure 2d), strongly indicating that their hollow sites are THF-rich. Photoinduced Morphological Transition. The next issue addressed in this investigation was the photoinduced phase transition of the Azo-1 supramolecules. A drop of an aqueous DMSO solution containing fibrous Azo-1 supramolecules was placed between two glass slides. The sample was irradiated with UV light (330−385 nm) while monitoring morphological changes using an optical microscope (Figure 3a). Irradiation for 10 s results in disappearance of the fibrous Azo-1 supramolecules. Regeneration of the fibers takes place when the UV-treated sample is exposed to visible light (460− 490 nm) for 2 min. Finally, UV light irradiation of the C

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the THF solution (Figure 5c). The cycle involving the reversible solvent-induced sphere−fiber transformation can be repeated numerous times. Effect of Amide Groups. In order to probe the effects of amide groups present in Azo-1 on supramolecular architectures, additional experiments were carried out. It is well-known that fluoride anion can abstract certain imide or amide hydrogens in conjugated molecules.55 Amide hydrogen abstraction by fluoride anion often results in significant changes in the electronic absorption and emission properties of the conjugated molecules. This phenomenon has served as the basis for the design of efficient fluoride ion sensitive colorimetric and fluorometric chemosensors.55,56 If fluoride anion abstracts hydrogens from the amide groups of the Azo-1 supramolecules, morphological changes are expected to take place. In order to test this proposal, tetrabutylammonium halides (F, Cl, Br, I, 20 mol equiv) were added to aqueous DMSO solutions containing fibrous Azo-1 aggregates. As displayed in Figure 6a, only the sample exposed to fluoride anion becomes wine-red while those exposed to the other halide anions (Cl, Br, I) retain the original orange-yellow color.

the patterned image results in disappearance of the image. Different image patterns are produced when the write−erase process is repeated.

Figure 4. Patterned images generated by a sequential write (UV light)−erase (visible light) process with Azo-1 supramolecules. A black paper is placed behind the glass slide for optimal viewing of the patterned images.

The phase transition of Azo-1 supramolecules also takes place in aqueous DMF. UV light irradiation of Azo-1 gel, formed by slow addition of water to a DMF solution, results in melting of the gel (Figure S4a, Supporting Information). The reverse process, involving gel formation, occurs upon irradiation with visible light. Unlike the fiber and gel forms of Azo-1, the spheres formed in aqueous THF are stable to UVirradiation. Thus, prolonged (>1 h) UV irradiation of Azo-1 spheres does not cause any detectable morphological change (Figure S4b, Supporting Information), even though trans−cis isomerization of azo chromophore takes place (Figure S5, Supporting Information) Solvent-Induced Morphological Transition. Our attention next focused on the solvent-induced phase transition of the Azo-1 supramolecules. DMSO was added to a suspension of Azo-1 spheres that were prepared in an aqueous THF solution. Interestingly, the spheres immediately disappear and long microfibers slowly form (Figure 5b). After removal of DMSO in vacuo, the fibers were dissolved in THF. Regeneration of the spheres was observed to take place when water was added to

Figure 5. Solvent-induced morphological transition of the Azo-1 supamolecules. Inset shows SEM images of each morphology.

Figure 6. Vials (a), optical microscopic images (b), and UV−vis spectra (c) of Azo-1 fibers upon exposure to anions. D

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information about molecular stacking interactions. The results of a single crystal X-ray diffraction analysis of Azo-1 (crystallized from a mixture of DMSO and H2O) reveal that it possesses supramolecular columnar packing along the b axis (Figure 8). The azobenzene moieties in the crystal exist in a

In addition, the aqueous DMSO solution containing suspended Azo-1 supramolecules also becomes clear upon exposure to fluoride anion. Images observed by using an optical microscope reveal that the fluoride anion-treated sample no longer possesses the fiber morphology (Figure 6b). These results demonstrate that fluoride anion is capable of destroying the aggregates of Azo-1 supramolecules. Inspection of the effect of fluoride anion on the Azo-1 supramolecules by utilizing UV−vis spectroscopy reveals that the presence of this anion results in the appearance of a new absorption maximum at 475 nm, which is responsible for the wine-red color generation (Figure 6c). In contrast to the fibrous Azo-1, treatment of the Azo-1 spheres with fluoride anion does not induce any morphological changes (data not shown), an observation which shows that the amide hydrogens in the spheres are not exposed to the aqueous environment. The effects of amide groups present in Azo-1 on supramolecular architecture were probed using the ester anologue Azo-2, which lacks hydrogen bondable amide groups. Interestingly, fibrous supramolecules are still generated when Azo-2 is subjected to a self-assembly in a 50% DMSO−H2O solution (Figure 7a,b). Also, irradiation (10 s) of Azo-2 fibers

Figure 7. (a) SEM image of Azo-2 supramolecules (10 mM) prepared in 50% DMSO−H2O. (b−d) Optical microscopic images of the Azo-2 supramolecules upon UV and visible light irradiation. The inset in part d is the SEM image of the aggregates. Figure 8. (a) Molecular structure of Azo-1. (b) Stacking structure of Azo-1, where the intermolecular N−H···O hydrogen bonds are drawn with light blue lines. (c) Packing diagram of Azo-1, viewed along the b axis.

with UV light results in disappearance of the fibers (Figure 7c). However, unlike Azo-1 supramolecules, where visible light irradiation regenerates fibers, irradiation (3 min) of the UVtreated sample of Azo-2 supramolecules with visible light leads to generation of amorphous aggregates (Figure 7d). The UV and visible light-induced melting-aggregation cycle can be repeated. It should be noted that, unlike Azo-1 supramolecules, Azo-2 fibers are found to be stable in response to added fluoride anion (Figure S6). Structural and Mechanistic Considerations. In general, when subjected to self-assembly conditions 1,3,5-benzenetricarboxamide (BTC) derivatives favor formation of onedimensional columnar structures. The presence of three hydrogen bondable amide groups enforces the formation of C3-symmetrical supramolecular architectures when the BTC derivatives contain relatively less bulky substituents, as evidenced by single crystal structures of amino acid esterderived BTCs.37 We have obtained crystals of Azo-1 to gain

twisted mode with respect to the plane of the central benzene core (Figure 8a). Interestingly, to reduce steric repulsion, Azo-1 molecules stack in columns with each individual molecule slipped by 3.13 Å with respect to the stacked molecule (Figure 8b). The crystal analysis also demonstrates that only one amide group of each Azo-1 molecule takes part in direct intermolecular hydrogen bonding with the amide group of an adjacent Azo-1; the corresponding geometric parameters are 2.915(7), 2.24 Å, and 134° for N···O and N−H···O distances, and N−H···O angle, respectively (Figure 8b). Data obtained from FTIR analysis also suggest that C3symmetrical hydrogen bonding interactions in Azo-1 supramolecules are not likely to occur in aqueous DMSO (Figure 9). The stretching band of amide carbonyl groups of Azo-1 E

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In D9, two interacting sites have one water molecule as a bridge and the other interacting site has two water molecules (water dimer) as a bridge. Similarly, in D10/D11, all the three interacting sites have water dimer/water−DMSO as a bridge as described in Figure S7. In the solvent mediated hydrogen bondings, the geometries (bond angles) are much closer to common hydrogen bonding networks. The interaction energies (ΔE(dimer) = E(dimer) − 2E(monomer) − ∑E(solvents)) of D1, D2, ..., and D11 were calculated to be −65.4, −144.6, −206.6, −367.4, −189.2, −302.7, −288.6, −510.7, −227.1, −309.9, and −616.6 kcal/mol, respectively. It is worth noting that the direct hydrogen bondings between amides of Azo-1 will not be so favorable as the solvent mediated hydrogen bondings. By comparing ΔE of D3/D4/D6 and D2/D3/D5, it was concluded that DMSO could make the hydrogen bonding stronger than water does. Thus, in DMSO, Azo-1 supramolecule should grow in the form of D8. However, in aqueous DMSO, water molecules could participate in the DMSOmediated hydrogen bonding due to the large distance between amides. The water molecule participation further stabilizes D8 (ΔE = −510.7 kcal/mol) to D11 (ΔE = −616.6 kcal/mol), which would be considered as a favorable form in aqueous DMSO. In addition, CO bond lengths were calculated to be 1.248−1.268 Å for D8, while 1.252−1.268 Å and 1.276−1.292 Å for D11, which support FTIR spectra (Figure 9) showing that there is one CO vibrational frequency at 1680 cm−1 in DMSO, while slightly red-shifted two vibrational frequencies at 1666 and 1652 cm−1 in aqueous DMSO. Aggregation of Azo-1 in aqueous THF solution also results in a shift of the amide carbonyl stretching frequency from 1681 to 1654 cm−1 (Figure 8C,D). Unlike the Azo-1 assembly in aqueous DMSO, the amide carbonyl band in an aqueous THF suspension of this substance is much broader, suggesting that numerous types of hydrogen bonds exist in the aggregated Azo1 molecules under this condition. An extremely intriguing aspect of the aggregation behavior of Azo-1 is that two strikingly different morphologies (fiber and sphere) exist in different solvent systems. It has been suggested that dielectric constant of a solvent significantly affects hydrogen bonding and the rate of self-assembly of molecules.59 The dielectric constants for THF, DMSO, and H2O are 7.5, 46.7, and 80, respectively. Owing to the large difference in the dielectric constant (more than 10 times), addition of water to a THF solution containing Azo-1 molecules brings about an abrupt change in the nature of the solvent system. In order to minimize interfacial free energy, spherical liquid droplets are initially formed, and these intermediates later solidify to yield Azo-1 spheres. In contrast, the small dielectric constant difference between DMSO and water allows relatively slow formation of supramolecular structures and provides sufficient time for the fiber growth. To gain evidence for the proposed effect of the dielectric constant on the morphology of the Azo-1 supramolecules, aggregation was attempted in a mixture of THF and EtOH (dielectric constant: 24.55). Only fibrous morphologies are generated when EtOH is added to a Azo-1 THF solution (Figure S8, Supporting Information). Thus, by replacing the water with a large dielectric constant by EtOH with a relatively small dielelectric constant, a sphere to fiber morphological change occurs. UV irradiation of the Azo-1 fibers formed in aqueous DMSO causes trans to cis isomerization of the azo chromophore. This process likely disrupts favorable intermolecular stacking interactions as a consequence of the formation of conforma-

Figure 9. FTIR spectra of Azo-1 in DMSO (a), aqueous DMSO (b), THF (c), and aqueous THF (d).

molecules in 100% DMSO solution appears at 1680 cm−1 (Figure 9a). Addition of water to the DMSO solution induces aggregation of the Azo-1 molecules, which leads to a shift of carbonyl stretching vibration from 1680 to 1666 and 1652 cm−1 (Figure 9b). The existence of two bands at 1666 and 1652 indicates that Azo-1 molecules form at least two different types of hydrogen bonds. To understand the stacking structures and interactions, quantum simulation was performed by a suite of Gaussian 09 program.57 All the calculations were performed by the M062X58 method with 3-21G* basis set, and the calculated structures were shown in Figure S6. The Azo-1 dimer without any solvent molecules (D1) contains one direct hydrogen bonding (N−H···O) and two π-stacking between azobenzenes of Azo-1. In crystal, the Azo-1 supramolecule should be aggregated in this form. The distances for N···O and N−H···O and N−H···O angle were calculated to be 2.903 Å, 1.902 Å, and 165°, respectively, which are somewhat different from the crystal structure due to the packing effect, etc. The tilted angle along the stacking column was calculated to be about 65°. Beside the direct H-bond, two stacking interactions between azobenzenes were also involved with interlayer distance of 3.2− 3.3 Å. Considering the unfavorable geometry (∠ = 134° in crystal) for hydrogen bonding, solvent molecules such as water and DMSO might be involved in solution. Azo-1 dimers with involvement of solvent molecules (D2−D11) were calculated, and the calculated structures were shown in Supporting Information (Figure S7). It is anticipated that entropy would play an important role in sphere/fiber formation. However, in this work, we concentrated on the final structures (crystallized or in solution phase). Thus, we considered all the possible cases as a final structure of the stacked Azo-1 when water is present in DMSO. For example, three interacting modes are possible for stacking of dimer of Azo-1: no solvent molecule (D1); two solvent molecules [two water (D2), one water, and one DMSO (D3), or two DMSO (D4)]; three solvent molecules [three water (D5), two water and one DMSO (D6 and D7), three DMSO (D8)]. For D1−D8, at most only one solvent molecule is involved in each interacting site of the stacked Azo-1 dimer. F

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tionally different cis-azobenzene moieties. IR monitoring of the process revealed that an increase occurs in hydrogen bonding of the Azo-1 moieties upon UV irradiation (Figure S9). This observation indicates that, along with some completely dissociated free Azo-1 molecules, the fiber structures are partially maintained as a consequence of the formation of various intermolecular and solvent-mediated hydrogen bonds (Scheme 2). The more polar nature of the cis isomer should

supramolecules have been observed. The results of a single crystal X-ray diffraction analysis reveal the supramolecular columnar packing and the data obtained from FTIR studies support the proposal that multiple hydrogen bonds exist in the Azo-1 fibers. The significance of amide hydrogen bonds to the morphological changes occurring in Azo-1 supramolecules has been demonstrated by observing the behavior of the analogous ester-containing Azo-2 and the results of hydrogen abstraction experiments with fluoride anion. The photoinduced, reversible disassembly and reassembly of the one-dimensional Azo-1 supramolecules is interesting in the context of potential applications to supramolecular memory systems. The results of this investigation show that a method employing a combination of supramolecular-favorable unit molecules (BTC derivatives) and photoresponsive moieties (azobenzene) could serve as the foundation for unique strategies to design stimulus-responsive functional supramolecules.

Scheme 2. Schematic Representation of the Photoinduced Reversible Phase Transition of Azo-1 Supramolecules

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of Azo-1 and Azo-2. SEM images of Azo-1 spheres in aqueous THF. UV−vis spectra of Azo-1 spheres before and after UV irradiation. Photographs of vials containing Azo-1 gel. Optical microscopic images of Azo-1 fibers prepared in 50% THF−EtOH. Crystal data for Azo-1. The calculated structures of Azo-1 supramolecules. This material is available free of charge via the Internet at http://pubs.acs.org.

enhance the solubility of the UV-irradiated and disasssembled molecules in an aqueous organic solvent. Visible light irradiation regenerates the relatively nonpolar trans-azo form and leads to recovery of one-dimensional columnar structures. The partially maintained fibers could serve as core templates for the reassembly of Azo-1 molecules after visible light irradiation. The ester containing compound Azo-2 is also capable of forming one-dimensional fibrous structures that result from favorable intermolecular interactions between aromatic rings. The UV-induced fiber-to-melting transition is also observed with Azo-2 supramolecules. However, in contrast to Azo-1 supramolecules, the UV irradiated and melted states of the ester-containing compound do not return to a fibrous form upon visible light irradiation and only amorphous particles are produced. This observation indicates that amide groups are critical for the recovery of the one-dimensional fiber structures. The observation that fluoride anion induces disassembly of the Azo-1 fibers suggests that amide hydrogens in this substance are exposed to the aqueous solution. In addition, the colorimetric change that occurs in the presence of fluoride anion is strongly indicative that hydrogen abstraction takes place from the amide moieties. In contrast to the Azo-1 fibers, no morphological changes are observed when Azo-1 spheres, generated in aqueous THF, are exposed to fluoride anion. Interestingly, the fluoride anion-induced chromic transition observed in Azo-1 fibers does not take place with the corresponding spheres. These results suggest that the amide groups in the Azo-1 spheres are not in contact with the aqueous phase. The fact that no morphological changes are observed upon UV irradiation of the hollow spheres also indicates that the spherical aggregates of Azo-1 supramolecules are relatively stable.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-M.K.); [email protected] (Y.S.J.); [email protected] (J.Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this research was provided by the National Research Foundation of Korea through Basic Science Research Program (Nos. 20120006251 and 2012R1A6A1029029), Center for Next Generation DyeSensitized Solar cells (2012000593). The work at SKKU was supported by the NRF grants (2007-0056343 and 20110015767).

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CONCLUSION The investigation described above has shown that supramolecular structures, in the forms of fiber, gels, and spheres, are readily generated from the azobenzene-containing benzenetricarboxamide derivative Azo-1. In addition, stimulus (photo, solvent, ion)-induced structural transformations of the Azo-1 G

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