Reversible Actuation of Polyelectrolyte Films: Expansion-Induced

Nov 11, 2013 - The capability of the force produced by film expansion for cis–trans azobenzene isomerization can be helpful for designing novel poly...
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Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables cis−trans Isomerization of Azobenzenes Yuanyuan Zhang, Ying Ma, and Junqi Sun* State Key Laboratory of Supramolecular Structure and Material, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzenecontaining polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly{1−4[4-(3-carboxy-4hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)−poly(acrylic acid) (PAA) complexes (noted as PAZO−PAA) with poly(allylamine hydrochloride) (PAH) produces azobenzene-containing PAZO−PAA/PAH films. UV irradiation induces trans−cis isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the cis−trans back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the cis−trans back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for cis−trans azobenzene isomerization can be helpful for designing novel polymeric actuators.

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INTRODUCTION Actuating materials, which can undergo reversible and large deformation in response to external stimuli such as heat, light, electricity, humidity, or chemicals,1−8 have drawn increasing interest because of their potential applications in sensing devices, artificial muscles, and micromechanical robots, among others.4,6,8−10 Among various actuating materials, light-driven actuators exhibit potential because light as an external stimulus can be controlled rapidly and remotely, thereby avoiding the use of wires and connections in building actuators.1,11−16 The reversible trans−cis isomerization of azobenzene compounds upon photoirradiation has enabled the majority of light-driven actuators that are capable of fast deformation and reversion.1,15−20 Light-driven azobenzene actuators based on liquidcrystalline elastomers (LCEs) are unique in fabricating fastresponse actuators because the alignment of the azobenzene mesogens in LCEs allows anisotropic and rapid deformation.1,21,22 Generally, previously reported fast-response azobenzene-containing polymer actuators cannot maintain deformation without UV irradiation because cis−trans back-isomerization occurs rapidly at room temperature either under visible light irradiation or in darkness.15−20,22,23 Studies have not been reported on photodriven actuators that can prolong their deformation without UV irradiation despite the importance of such actuators in microfluidic devices and photoswitching materials such as microbrakes and microstrobes. © 2013 American Chemical Society

The efficiency of azobenzene isomerization is significantly determined by the free volume that surrounds it. The reduction of free volume often hinders the isomerization of azobenzenes in solid state.24,25 Therefore, compact films with azobenzenes can be used for fabricating light-driven actuators that can sustain deformation. However, the key challenge to this process is the restoration of the deformed actuators to their initial state by using other readily available stimulus. Significant progress has recently been achieved by employing mechanical forces to facilitate bond scission, chemical reaction, and conformational changes in force-sensitive mechanophores.26−29 Mechanochemistry, which deals with the chemical and physicochemical changes in substances enabled by mechanical force, provides alternative methods to control chemical reactions.28−33 The attachment of mechanophores on polymer backbone facilitates the utilization of mechanical force through polymer chain motion. This motion is usually induced by stretching, compressing, grinding, and ultrasonication of polymer composites.33 This study aims to determine whether the mechanical force produced by the expansion of azobenzenecontaining polymeric films is sufficiently strong to facilitate cis− trans back-isomerization of azobenzenes and restore deformed Received: August 5, 2013 Revised: October 7, 2013 Published: November 11, 2013 14919

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1 mm ×10 mm strips to produce the actuators. The actuator was placed in a quartz vial with a gas inlet and outlet to control humidity. The UV irradiation-induced bending of the actuators was conducted in an ambient environment with 20% relative humidity (RH). UV light irradiation was performed with a 250 W mercury lamp at a distance of 10 cm through a glass filter that blocks the UV light below 340 nm. Unbending of the actuators was achieved with flowing humid nitrogen through the quartz vial, which increased the relative humidity to ∼80%. Instruments and Methods. Scanning electron microscopy (SEM) images were obtained using an XL30 ESEM FEG scanning electron microscope. The thicknesses of the (PAZO− PAA/PAH)*n films were determined based on their crosssectional SEM images. Digital camera images and videos were captured with a Sony digital video camera recorder (DCRSR62E). UV−vis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Dynamic light scattering (DLS) measurement was performed on a Malvern Zetasizer Nano ZS. The measurements were taken at a scattering angle of θ = 173° at 20 °C by using a He−Ne laser with a wavelength of 633 nm.

actuators to their initial state. Light-driven actuators that bend upon UV irradiation were fabricated in this study through layerby-layer (LbL) assembly of polyelectrolyte films containing azobenzene groups and then releasing them from substrates. These actuators can prolong their deformed state under visible light irradiation and restore their original configuration after exposure to a humid environment. This study is the first to reveal that the force produced by the expansion of thermally cross-linked polyelectrolyte films enables cis−trans backisomerization of azobenzenes, which can effectively induce the unbending motion of polymeric actuators.

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MATERIALS AND METHODS Materials. Poly{1−4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO), poly(acrylic acid) (PAA) (Mw ca. 1800 g mol−1), poly(allylamine hydrochloride) (PAH) (Mw ca. 52 000), and poly(diallyldimethylammonium chloride) (PDDA) (20 wt %, Mw ca. 100 000 to 200 000) were purchased from Sigma− Aldrich. All polyelectrolytes were used without further purification. The solution pH in all experiments was adjusted with either 1 M HCl or 1 M NaOH. Preparation of PAZO−PAA Complexes. The aqueous dispersion of PAZO−PAA complexes was prepared by directly mixing two aqueous solutions of PAA (0.26 mg mL−1) and PAZO (11.7 mg mL−1) with a volume ratio of 9:1. The pH of the mixture was then adjusted to 3.8 with 1 M HCl under continuous stirring. By this process, the feed monomer molar ratio of PAA to PAZO becomes 1:1, corresponding to concentrations of PAA and PAZO at 0.23 and 1.17 mg mL−1, respectively. Preparation of (PAZO−PAA/PAH)*n Films. The freshly cleaned silicon wafer or quartz was immersed in an aqueous cationic solution of 1 mg mL−1 PDDA for 20 min to obtain a positively charged surface. The LbL assembly of (PAZO−PAA/ PAH)*n films on PDDA-modified quartz or silicon wafer was conducted automatically by a programmable dipping machine (Dipping Robot DR-3, Riegler & Kirstein GmbH) at room temperature. The PDDA-modified substrate was first immersed in an aqueous PAZO−PAA solution (pH 3.8) for 15 min to obtain a layer of PAZO−PAA complexes and then rinsed with water four times for 1 min each time to remove the physically adsorbed complexes. The substrate was subsequently immersed in an aqueous PAH solution (1 mg mL−1, pH 7.5) for another 15 min and rinsed in four water baths for 1 min each time. The adsorption and rinsing steps were repeated until the desired number of bilayers was obtained. No drying step was used in the deposition procedure unless it was in the last layer. The (PAA−PAZO/PAH)*n films were thermally cross-linked by heating at 180 °C for 2 h in a vacuum oven. Free-standing (PAZO−PAA/PAH)*30 films were released from the substrates by a previously reported ion-triggered exfoliation method. Briefly stated, thermally cross-linked (PAZO−PAA/ PAH)*30 films deposited on the PDDA-modified silicon wafer were immersed into an aqueous solution with a pH of 2.0 for ∼10 min to release the films from the substrate. To facilitate the release of the films, the edges of the films were cut off with a knife. The LbL-assembled PAA/PAH films were fabricated similarly with PAZO−PAA/PAH films by replacing the aqueous PAZO−PAA solution with the aqueous PAA solution (1 mg mL−1, pH 3.5). Bending/Unbending of PAZO−PAA/PAH Actuators. The (PAZO−PAA/PAH)*30 free-standing films were cut into

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RESULTS AND DISCUSSION Rapid LbL Assembly of Azobenzene-Containing Polyelectrolyte Films. The LbL assembly of polycation and polyanion provides a facile way to the fabrication of polyelectrolyte multilayer films with well-controlled film structures.34−40 The free volume within LbL-assembled polyelectrolyte films can be well suited by controlling polyelectrolyte interpenetration, which conveniently tailors the isomerization of azobenzenes. Commercially available PAZO is selected for the preparation of actuators because carboxylate and hydroxyl groups on azobenzenes can exhibit strong hydrogen bonding and electrostatic interactions with corresponding polyelectrolytes to form stable networks. To rapidly fabricate PAZO-containing micrometer-thick polyelectrolyte films, PAZO is first mixed with PAA in an aqueous solution, thereby producing PAZO−PAA complexes.41−44 As shown in Scheme 1a, hydrogen bonding between hydroxyl groups of PAZO and carboxylic acid groups facilitates the formation of PAZO−PAA complexes in an acidic aqueous solution. PAZO−PAA complexes with a feed monomer molar ratio of 1:1 in an aqueous solution of pH 3.8 have a z-average diameter of 197.2 nm, as determined by dynamic light Scheme 1. (a) Formation of PAZO−PAA Complexes. (b) Schematic of the LbL Assembly of PAZO−PAA Complexes with PAH for the Fabrication of (PAZO−PAA/PAH)*n Films and Their Release from Substrates to Produce FreeStanding Films

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transition of the trans-azobenzenes, increases almost linearly with increasing number of film deposition cycles. This linear increase confirms the incorporation of PAZO in the PAZO− PAA/PAH films. The thicknesses of the (PAZO−PAA/ PAH)*n films with different deposition cycles, which were determined from their corresponding cross-sectional SEM images, also increase linearly with increasing number of film deposition cycles (Figure 2b). The as-prepared (PAZO−PAA/ PAH)*30 film has an average thickness of 1260 ± 34 nm. The rapid construction of PAZO−PAA/PAH films is attributed to the large size of PAZO−PAA complexes. The (PAZO−PAA/ PAH)*30 films were thermally cross-linked at 180 °C for 2 h, producing amide bonds between the amine groups of PAH and the acid groups of PAA as well as enhancing their mechanical stability (Figure S1).45 After immersion of the thermally crosslinked (PAZO−PAA/PAH)*30 film into an aqueous solution with a pH of 2.0 for ∼10 min, a large-area free-standing film was obtained (Figure 2c).46 The thermally cross-linked (PAZO−PAA/PAH)*30 film with a thickness of 1242 ± 68 nm was released by breaking the electrostatic interaction of the first PAA−PAZO layer with the underlying PDDA layer in a highly acidic aqueous solution (inset of Figure 2b). The SEM image (Figure 2d) shows aggregates on the surface of freestanding (PAZO−PAA/PAH)*30 film. The film is translucent with transmittance >60% in the spectral range between 450 and 800 nm. Azobenzene Isomerization in (PAZO−PAA/PAH)*30 Films. Azobenzene photoisomerization in the cross-linked (PAZO−PAA/PAH)*30 films deposited on quartz substrates was investigated by alternately irradiating the films with UV and visible light. As shown in Figure 3a, the absorbance of the (PAZO−PAA/PAH)*30 film at 358 nm is reduced about 0.14 (6% of the original absorbance) after 2 min UV irradiation, indicating the occurrence of the trans−cis isomerization of azobenzenes. The π−π* transition of the trans-azobenzene absorbance at 358 nm decreases with continuous UV irradiation of the film (Figure S2). Only the cis−trans back isomerization of the films after 2 min UV irradiation is investigated because the actuation of the free-standing (PAZO−PAA/PAH)*30 film is accomplished within 2 min UV irradiation. The absorption spectrum of the UV-irradiated (PAZO−PAA/PAH)*30 film shows no change after 2 min visible light irradiation. Visible light irradiation was performed with a 150 W metal halide lamp at a distance of 10 cm through a filter that blocks the light below 450 nm. No back isomerization occurred even after 0.5 h visible light irradiation, suggesting that visible light failed to effectively induce cis−trans back isomerization of azobenzenes. For LbL-assembled PAZO−PAA/PAH films, the interpenetration of PAZO, PAA, and PAH within the films produces compact polyelectrolyte films that lack free volume. The trans−cis isomerization further shortens the azobenzenes and produces resultant polyelectrolyte films that are more compact than those before UV irradiation, leading to irreversibility of cis−trans azobenzene isomerization. Thermal relaxation of cis-azobenzenes at ambient condition (40% to 50% RH, 20 to 25 °C) leads to a very slow, incomplete recovery of the trans configuration, with ∼80% recovery of the trans-azobenzenes after 15 days. However, after exposure of the UV-irradiated (PAZO−PAA/PAH)*30 film to a 80% RH, which was accomplished by flushing N2 gas passing through water to the film, the original trans configuration is completely recovered within 2 min (Figure 3a). Therefore, the azobenzenes in (PAZO−PAA/PAH)*30 films can perform

scattering (DLS) measurement (Figure 1). DLS shows three peaks for the intensity average diameters centered at

Figure 1. Hydrodynamic diameter distribution curve of PAZO−PAA complexes in water with pH of 3.8.

approximately 58, 302, and 4806 nm. A significantly low amount of the largest PAZO−PAA complexes is present in aqueous PAZO−PAA dispersions because DLS scattering intensity is proportional to the sixth power of the particle size. Filtration and centrifugation failed to remove these large complexes because of the equilibrium among complexes of varying sizes in aqueous PAZO−PAA dispersions. PAZO−PAA complexes subsequently underwent LbL assembly with PAH to produce (PAZO−PAA/PAH)*n films (where n represents the number of film deposition cycles) on PDDA-modified substrates (Scheme 1b). PAZO and PAA in PAZO−PAA complexes exhibit electrostatic and hydrogen bonding interactions with PAH, driving the formation of PAZO−PAA/PAH multilayer films. As indicated in Figure 2a, the absorbance at 358 nm, which is ascribed to the π−π*

Figure 2. (a) UV−vis absorption spectra of (PAZO−PAA/PAH)*n films, with n bieng 2, 6, 10, 14, 20, and 30 from the bottom to the top. The inset in (a) shows the absorbance at 358 nm as a function of film deposition cycles. (b) The thickness of the (PAZO−PAA/PAH)*n films as a function of film deposition cycles. The inset in (b) is the cross-sectional SEM image of a thermally cross-linked (PAZO−PAA/ PAH)*30 film.( c) photograph of a free-standing (PAA−PAZO/ PAH)*30 film with an area of 4 cm ×2.8 cm. (d) SEM image of a thermally cross-linked (PAZO−PAA/PAH)*30 film. 14921

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polymeric actuators that can prolong their deformation under visible light irradiation. Reversible Bending/Unbending Motion of FreeStanding Films. The free-standing (PAZO−PAA/PAH)*30 films were cut into 1 mm ×10 mm strips to produce actuators. An actuator with one end clamped and held vertically is capable of large-scale bending motion upon UV irradiation in an ambient condition (20 °C and 20% RH) (Figure 4a and

Figure 4. (a) Time profiles of bending and unbending movements of a (PAZO−PAA/PAH)*30 free-standing film (1 mm ×10 mm) upon UV irradiation (top panel) and humidity (∼80% RH) treatment (bottom panel). (b) Time-dependent displacement of a (PAZO− PAA/PAH)*30 actuator upon UV irradiation (■) and humidity (∼80% RH) treatment (□). The solid cycle represents the N2 drying step after humidity treatment. (c) Multiple reversible bending and unbending processes of a (PAA−PAZO/PAH)*30 actuator.

Figure 3. (a) UV−vis absorption spectra of a thermally cross-linked (PAZO−PAA/PAH)*30 film deposited on a quartz substrate after sequential treatments of UV irradiation for 0 (●) and 2 (■) min, visible light irradiation for 2 min (▽), and exposure to an environment with ∼80% RH (□). For clarity, the curves with 2 min visible light irradiation and with humid treatment are moved down 0.1 and up 0.1, respectively, referring to their original data. The inset in (a) shows the absorbance changes of the film at λmax 358 nm after 2 min UV light irradiation, visible light irradiation and humid treatment. (b) Multiple trans−cis azobenzene isomerization of a free-standing (PAZO−PAA/ PAH)*30 film as monitored by the absorbance at 358 nm. ■ and □ represent the absorbance of the (PAZO−PAA/PAH)*30 film at 358 nm after 2 min UV irradiation and 2 min humid N2 treatment, respectively.

Supporting Information, Movie 1). The displacement, which is the horizontal distance between the original and the final positions of the tip of the actuator, increases with irradiation time and finally reaches ∼6.0 mm after 90 s UV irradiation, with the flexion angle expanding to ∼77°. The slow bending of the actuator during the initial 30 s is partly attributed to the gradual increase in intensity of the mercury lamp with time. The bent actuator cannot revert to its original configuration with 0.5 h visible light irradiation. Thermal relaxation of cisazobenzenes at ambient conditions (40% to 50% RH, 20 to 25 °C) leads to a very slow, incomplete unbending motion of (PAZO−PAA/PAH)*30 actuators, which restores ∼50% of the original bending deformation after 15 days. However, complete unbending occurs upon exposure of the bent actuator to an environment with ∼80% RH at room temperature. The bent actuator becomes almost flattened within 90 s. The actuator returns to its initial configuration after being dried with N2 flow, which removes the absorbed water from the actuator. The bending/unbending process of the actuator as a function of time is depicted in Figure 4b. The multiple bending/unbending processes of the (PAZO−PAA/PAH)*30 actuator were performed by alternate irradiation of the actuator with UV light, exposure to an environment with ∼80% RH and subsequent N2 drying. As shown in Figure 4c, the bending/ unbending processes of the free-standing (PAZO−PAA/

trans−cis isomerization under UV light irradiation and cis−trans back-isomerization in a humid environment. In a control experiment, although the partial trans−cis azobenzene isomerization occurs in a drop-cast PAZO film upon 2 min UV irradiation, the cis−trans isomerization does not occur by either visible light irradiation or exposure of the film to an environment with 80% RH. This result indicates that the cross-linked network in PAZO−PAA/PAH films is critically important for humidity-induced cis−trans back-isomerization of azobenzenes. Moreover, the (PAZO−PAA/PAH)*30 freestanding films undergo the same azobenzene isomerization with those deposited on solid substrates. As indicated in Figure 3b, the trans−cis and the cis−trans azobenzene isomerizations in a free-standing (PAZO−PAA/PAH)*30 film induced by alternate UV irradiation and exposure to a humid environment are highly reversible and reproducible. Therefore, free-standing PAZO−PAA/PAH films can be used for constructing 14922

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Figure 5. Schematic of the bending (a→b) and unbending (b→c) movements of a PAZO−PAA/PAH actuator upon UV irradiation and humidity treatment.

PAH)*30 films are highly reproducible, with an average displacement of 6.3 ± 0.5 mm for the tip in each bending process, which fulfills their functions as actuators. The slow and incomplete unbending motion caused by thermal relaxation confirms that the azobenzene-containing polyelectrolyte actuators can prolong their deformation without constant UV irradiation. Mechanism of Bending/Unbending Motions of the Actuators. UV irradiation isomerizes the azobenzenes on the front surface of the (PAZO−PAA/PAH)*30 actuators from trans to cis, which reduces the molecular length of the azobenzenes from 9 Å to 5.5 Å.47 The azobenzenes on the back surface of the actuator cannot be isomerized because of the rapid reduction in light intensity in the normal direction of the actuator.22 Therefore, the surface layer facing the incident light shrinks, whereas the volume of the remaining films remains constant. The asymmetric shrinkage of the films produces a force that bends the actuator toward the source of UV light (Figure 5a,b). In a control experiment, a thermally cross-linked free-standing (PAA/PAH)*20 films with a thickness of 1.2 μm was fabricated. A 1 mm × 10 mm (PAA/ PAH)*20 strip cannot bend upon UV irradiation, confirming that the bending of the (PAZO−PAA/PAH)*30 actuator is caused by trans−cis azobenzene isomerization but not the thermal effect caused by UV light irradiation (Figure S3). In PAZO−PAA/PAH films, the azobenzenes cannot perform cis−trans back-isomerization under visible light irradiation because of the limited free volume in the compact films. The PAZO−PAA/PAH films comprising hydrophilic PAZO, PAA, and PAH polyelectrolytes can absorb water and expand when exposed to highly humid environment. PAZO−PAA/PAH films can adsorb water to ∼27% of its original mass when the RH increases from 0% to 88% (Supporting Information). The expansion of the films provides free volume and mechanical force to induce the cis−trans back isomerization of azobenzenes. The azobenzenes are fixed in the network of cross-linked PAZO−PAA/PAH films by covalent attachment as well as electrostatic and hydrogen bonding interactions. As depicted in Figure 5b,c, the motion of polymer chains caused by film expansion exerts a mechanical force on both ends of cisazobenzenes, which pulls the azobenzenes in opposite directions. Such force is adequately large and thus can induce the cis−trans back isomerization of azobenzenes in the PAZO− PAA/PAH films, as shown in Figure 3. In the early 1990s, Kim and Reneker showed that repetitive stretching and relaxation of a polyurethane elastomer attached with azobenzenes can change all cis-azobenzenes into trans-azobenzenes.48 Theoretical studies predicted that purely mechanical forces produced by pulling the tips apart or compressing them can attain cis−trans and trans−cis isomerizations of azobenzenes suspended between two gold tips.49 Our recent study demonstrated that thermally cross-linked PAA/PAH films could be used for the

fabrication of powerful bilayer actuators comprising a PAA/ PAH film and a UV-cured Norland Optical Adhesive layer.6 The mechanical force produced by expansion/shrinkage of the thermally cross-linked PAA/PAH film can drive an energetic walking device carrying a load 120 times heavier than the actuator to walk steadily on a ratchet substrate when the surrounding humidity is alternately changed. Therefore, it is reasonable that the mechanical force produced by the motion of polyelectrolyte chains in PAZO−PAA/PAH actuators enables cis−trans back isomerization of azobenzenes. The cis− trans azobenzene isomerization restores the bent actuators to its original configuration (Figure 5c). Our previous study also showed that a film with a higher Young’s modulus produces a larger mechanical force by the polyelectrolyte chain motion during film expansion. Thermal cross-linking enhances the Young’s modulus of the PAZO−PAA/PAH films,6 thereby increasing the strength of the mechanical force generated by the motion of polyelectrolyte chains and facilitating cis−trans azobenzene isomerization. By contrast, the hydrogen bonding between hydroxyl and carboxylic acid groups of azobenzenes is weak. Although the hydrogen bonding leads to the formation of three-dimensional networks in drop-cast PAZO films, the force produced by the expansion of PAZO networks is too weak to enable the cis−trans back isomerization of azobenzene groups (Figure S4). This study is the first to demonstrate that the force produced by film expansion enables cis−trans azobenzene isomerization and can be explored to induce motion of actuators.

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CONCLUSIONS

We have developed a technique for fabricating azobenzenecontaining polyelectrolyte actuators that are capable of reversible bending/unbending movements by UV irradiation and mechanical force produced by film expansion. In contrast to previously reported photodriven actuators, the PAZO− PAA/PAH actuators can prolong their bending deformation without constant UV irradiation because the limited free volume in the actuators inhibits cis−trans azobenzene back isomerization. The expansion of the film upon water adsorption in a humid environment produces a mechanical force that is adequately strong to enable the cis−trans back isomerization of azobenzenes and restore the bent actuators to their original configuration. The integration of mechanochemistry with smart polymer films can facilitate the design of new actuators with unexpected functions. The new azobenzene-containing polyelectrolyte actuators that can prolong their bending deformation can expand the application of actuators into microfluidic devices and photoswitching devices such as microbrakes and microstrobes. 14923

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ASSOCIATED CONTENT

S Supporting Information *

Detail for measurement of water absorption of PAZO−PAA/ PAH films, Figures S1−S4, and movie 1 of bending/unbending motion of a (PAZO−PAA/PAH)*30 actuator. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

State Key Laboratory of Supramolecular Structure and Material, College of Chemistry, Jilin University, Changchun 130012, P. R. China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Grant Nos. 20974037, 20774035) and the National Basic Research Program (2007CB808000).

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REFERENCES

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