Azobenzene Molecular Machine: Light-Induced Wringing Gel

Letter Cite This: ACS Macro Lett. 2018, 7, 576−581

pubs.acs.org/macroletters

Azobenzene Molecular Machine: Light-Induced Wringing Gel Fabricated from Asymmetric Macrogelator Yu-Jin Choi,† Ji-Tae Kim,† Won-Jin Yoon,† Dong-Gue Kang,† Minwook Park,† Dae-Yoon Kim,† Myong-Hoon Lee,† Suk-kyun Ahn,*,‡ and Kwang-Un Jeong*,† †

Department of Polymer-Nano Science and Technology and Department of Flexible and Printable Electronics, Chonbuk National University, Jeonju 54896, Republic of Korea ‡ Department of Polymer Science and Engineering, Pusan National University, Busan 46241, Republic of Korea S Supporting Information *

ABSTRACT: To develop light-triggered wringing gels, an asymmetric macrogelator (1AZ3BP) was newly synthesized by the chemically bridging a photoisomerizable azobenzene (1AZ) molecular machine and a biphenyl-based (3BP) dendron with a 1,4-phenylenediformamide connector. 1AZ3BP was self-assembled into a layered superstructure in the bulk state, but 1AZ3BP formed a three-dimensional (3D) network organogel in solution. Upon irradiating UV light onto the 3D network organogel, the solvent of the organogel was squeezed and the 3D network was converted to the layered morphology. It was realized that the metastable 3D network organogels were fabricated mainly due to the nanophase separation in solution. UV isomerization of 1AZ3BP provided sufficient molecular mobility to form strong hydrogen bonds for the construction of the stable layered superstructure. The lighttriggered wringing gels can be smartly applied in remote-controlled generators, liquid storages, and sensors.

B

Our research group has previously reported a macrogelator with a 3-fold rotational symmetry (C3) based on the benzene1,3,4-tricarboxamide (BTA) supramolecule.17,18 When the functional groups were further tethered to the three amides, the BTA-derivative constructed well-defined superstructures in organic solvents (organogels) and nematic liquid crystalline media (liquid crystal physical gels, LCPGs), respectively. By applying external stimuli such as thermal, UV light and electric field, we demonstrated the remote-controlled sol−gel transition for light shutters. The light scattering state of BTA-based LCPG was reversibly switched to the transmitting state by applying 22 V, 5 times lower than that of the conventional polymer-dispersed liquid crystal (PDLC) optical device.18 We raised a question from a molecular design point of view. “How can asymmetric macrogelators affect physico-chemical properties under stimulation?” In this study, we explore a new class of asymmetric macrogelator to answer this question. Here, asymmetric molecules are limited to two hemistructures with a noncentrosymmetric structure with distinct chemical and morphology on the nanometer scale.19−21 Based on the concept of asymmetric molecule, we newly designed and successfully synthesized an asymmetric macrogelator as shown in Figure 1a. Here, asymmetry means the

ottom-up nanotechnology allows us to tune and maximize material properties by amplifying and adjusting chemical functions and shapes on the different length scales, especially 1 to 100 nm. Self-assembly of molecular machines has been developed to control molecular switches of soft materials by external stimuli.1−3 However, it is still questionable whether the structure−property relationship of molecular switches is fully understood. It is therefore important to ensure that molecular machines programmed at the molecular level are automatically and precisely converted to useful materials with the desired properties. In the case of organogel, the gelator forms a threedimensional (3D) network by noncovalent interactions and captures organic solvents by capillary forces to prevent the flow.4,5 Therefore, the organogel as a physical gel structure can be maintained by physical interactions rather than covalent bonds (chemical gelation). Introducing directional noncovalent interactions such as hydrogen bond allows the on−off switching of gelation induced by external stimuli, called sol−gel transition.6−8 Since the gelator can rapidly reach equilibrium in solutions, the reversible sol−gel transition is quickly detected.9−11 Recently, stimuli-sensitive sol−gel transitions have received much attention in the fields of injectable delivery systems, tissue engineering, and molecular electronics.12,13 The drug or cell can be easily encapsulated in solution by the sol− gel transition and released directly to the target site.14−16 © XXXX American Chemical Society

Received: February 27, 2018 Accepted: April 25, 2018

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DOI: 10.1021/acsmacrolett.8b00167 ACS Macro Lett. 2018, 7, 576−581

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ACS Macro Letters

Figure 1. (a) Chemical structure of 1AZ3BP. (b) POM image taken at 148 °C by cooling at 20 °C/min. (c) DSC curves of cooling and subsequent heating thermal scans for 1AZ3BP at various scanning rates. (d) 1D SAXS and (e) 1D WAXD patterns for a smectic liquid crystal phase at 25 °C. (f) Schematic illustration of molecular packing structure.

large dendritic spherulites are formed at −2.5 °C/min, whereas small multidomains are observed under the cross-polarized light at −20 °C/min. This rate-dependent transition behavior can result from metastable states for kinetic reasons.24,25 Typical POM images of fan-shaped crystals are observed when 1AZ3BP is cooled from isotropic to 130 °C at a rate of −20 °C/min (Figures 1b and S8). Based on the DSC and POM results, it is realized that the endothermic peak at 154.8 °C represents the transition from the isotropic state to a typical smectic LC phase, followed by the crystallization at 143 °C. To investigate the molecular packing structure of 1AZ3BP, 1D/2D WAXD, and 1D SAXS experiments are carried out. SAXS of 1AZ3BP taken at 25 °C is resolved in three peaks with a q-value ratio of 1:2:3 (q = 4π sin θ/λ, where λ is the X-ray wavelength and 2θ is the scattering angle), so the formation of the layer structure is clear (Figure 1d).26,27 From the scattering peak, the corresponding real space domain spacing d1 can be calculated according to d1 = 2π/q1. The d-spacings corresponding to the q-values are 6.35, 3.18, and 2.12 nm, respectively. This result indicates that the d-spacing of 6.35 nm can be assigned to a periodic layer structure slightly smaller than the molecular length (L = 7.9 nm, energy minimized dimension in Figure S9). The shortened layer distance can be attributed to the intercalation of alkyl chains. In the 1D WAXD pattern (Figure 1e), the peaks with d-spacing of 0.43 and 0.41 nm represent the lateral molecular packings between alkyl tails, biphenyl mesogens, and AZ groups. Since the temperaturedependent 1D SAXS pattern (Figure S10) does not change, it is safe to conclude that the smectic layer structure remains below the isotropic phase. The 2D WAXD pattern is obtained by irradiating X-ray perpendicular to the extrusion direction (ED) in Figure S11. The uniaxially oriented sample is prepared by mechanically extruding 1AZ3BP at 150 °C and then annealing at 100 °C. With the ED on the equator, higher order layer reflections appear on the meridian. Two arcs at 0.41 and 0.45 nm on the equator originate from the lateral close packing of mesogens in the layer structure. The azimuthal scanning data of these arcs indicates that the long axis of mesogens is parallel to the layer normal direction. From X-ray experiments and structure analysis, it is realized that 1AZ3BP forms a layer structure by alternating arrangement within the layer, as

difference in function and volume between 1AZ and 3BP. The 3BP dendron is purposely chosen as one face to ensure the solubility of the asymmetric macrogelator in organic solvents and to induce the lateral molecular packing through π−π interaction and van der Waals force. By introducing the 1AZ molecular machine on the other face of asymmetric macrogelator, the ordered phase can be manipulated by photoisomerization. Intermediate compounds (1AZ and 3BP) of 1AZ3BP are obtained according to the procedure described in the Supporting Information (Schemes S1 and S2).17,22 The 1AZ3BP asymmetric macrogelator is synthesized by forming an amide bond through the condensation reaction of the carboxylic acid group of 3BP with the primary amine functional group of 1AZ. Comparing the 1H NMR spectrum of 1AZ3BP with that of the intermediate (Figures S1 and S2), one multiplet and two singlet peaks are detected at δ = 7.82, 7.73, and 8.11 ppm, which confirms successful synthesis of 1AZ3BP (Figure S3). 1H and 13C NMR results (Figure S4) clearly indicate the formation of amide bond between 1AZ and 3BP. The chemical structure and purity of 1AZ3BP are further confirmed by the MALDI-ToF mass spectrum (Figure S5). The observed massto-charge (m/z) values of monoisotropic peaks (2079.6 and 2096.3) are almost identical to the calculated [M + Na]+ and [M + K]+ ones of 1AZ3BP. The phase behaviors of 1AZ3BP in the bulk state are first studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). Since the degradation temperature of 1AZ3BP is 359 °C (Figure S6), DSC experiments are carried out in the temperature range between 30 and 200 °C. Additionally, different heating and subsequent cooling rates are applied to understand the thermal transition behaviors of 1AZ3BP with thermodynamic properties (Figure 1c).23 At 2.5 °C/min heating rate, the onset temperature of the endothermic transition is detected to be 165.2 °C with 124.1 kJ/mol. During the subsequent cooling process of −2.5 °C/min, overlapped exothermic transition peaks are observed at 157.3 and 154.2 °C with 118.5 kJ/mol. At a faster cooling rate of −20 °C/min, merged exothermic transition peaks are noticeably resolved into two peaks: 154.8 °C (40 kJ/mol) and 142.7 °C (79.7 kJ/mol), respectively. As represented in Figure S7a,b, 577

DOI: 10.1021/acsmacrolett.8b00167 ACS Macro Lett. 2018, 7, 576−581

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ACS Macro Letters illustrated in the Figure 1f. The asymmetric molecular shape of 1AZ3BP plays a crucial role in minimizing the free volume and stabilizing the layer structure. 1AZ3BP has an asymmetric structure with two different mesogens, but both 1AZ molecular machine and 3BP dendron have ethylene oxide and alkyl chain groups. Therefore, the 1AZ molecular machine and the 3BP dendron form the AB alternating arrangement in the layer without phase separation. To understand the role of hydrogen bond in the thermal process, FTIR experiments are conducted at various temperatures and compared with DSC results.28 When a hydrogen bond is formed between amide groups, the N−H stretching vibration band as donor and the CO stretching band as acceptor are expected to have their absorption shift toward the lower frequency due to the decrease of the acceptor bonding force.29,30 The temperature-dependent FTIR spectrum of 1AZ3BP is recoded during heating from 30 to 180 °C and is represented in Figure S12a. By increasing the temperature, the N−H band at 3296 cm−1 and the CO band at 1644 cm−1 are slightly shifted to higher frequencies before reaching the isotropic phase, indicating that molecular arrangement through hydrogen bond is considerably stabilized. When the temperature reaches 175 °C, the free N−H band and the free CO stretching band are detected at 3434 and 1675 cm−1, respectively. The magnitude of these shifts as well as the intensity changes are clear evidence of weakening and reducing of inter- and intramolecular hydrogen bonds in the isotropic phase.31,32 The wavenumber shift with respect to temperature is also summarized in Figure S12b. It is concluded that the driving force of the self-assembled 1AZ3BP is hydrogen bonding in the bulk state. The gelation process is performed by heating the sample in a screw-capped vial and subsequently cooling to room temperature.19,33 1AZ3BP asymmetric macrogelator can immobilize the organic solvents (cyclohexane and methanol with 3 wt % 1AZ3BP) and forms a very stable organogel under ambient conditions even for several months (Figure 2a). The gelation capabilities of 1AZ3BP in other organic solvents are summarized in Table S1. Interestingly, when an organogel is exposed to UV light, the color of the organogel changes within 5 min due to the trans-to-cis photoisomerization of AZ molecular machine (inset image of Figure 2a). Upon irradiating UV light longer than 5 min, the organic solvent is completely squeezed out of the gel and phase-separated (Figure 2b).34 This peculiar phenomenon is similar to the “wringing out the clothes” situation. Generally, it is well-known that the trans-tocis photoisomerization of AZ molecular machine can disrupt the physical interactions. Therefore, macrogelators containing the AZ molecular machine show gel−sol transitions by UV light.18,35,36 However, the sol−gel transitions of 1AZ3BP macrogelator are only reversibly occurred by heat, as shown in Figure S13. The organogels of 1AZ3BP show wringing behavior by photostimulation. The wringing behavior can be visualized by scanning electron microscopy (SEM) morphological observation. By freeze-drying process, we can obtain a xerogel state. As shown in Figure 2c, the xerogel shows a threedimensional (3D) network with many micropores, a typical morphology of organogel. Surprisingly, the morphology of xerogel is converted into a layered structure after irradiating UV light (Figure 2d). The UV−vis spectra of the solution and gel state confirm the reversible photoisomerization of AZ molecular machine in alternating UV and Vis light (Figure 2e,f). Note that the absorbance spectra of AZ molecular

Figure 2. Photographic images of organogel formed in the cyclohexane and methanol solution with 3 wt % 1AZ3BP: (a) before and (b) after UV exposure with (c, d) their corresponding SEM images of xerogel, respectively. The inset image of Figure 1a represents the UV-induced color change. Photoisomerization of 1AZ3BP in (e) CHCl3 and (f) organogels under exposing UV and vis lights monitored by the UV−vis spectroscopy with the plot of time-dependent absorbance variation at 450 nm.

machine are generally characterized by the trans-isomer in the 350−360 nm (π−π* transition) and a low energy transition of cis-isomer in the 430−440 nm (n−π* transition).37,38 To understand the UV-induced wringing behavior, we further conducted 1D WAXD experiments as well as FTIR and Raman spectroscopy before and after irradiating UV light to organogels. In the 1D WAXD patterns (Figure 3b), only two broad reflections (d-spacing = 4.5 and 0.41 nm) are detected for the xerogels before UV irradiation. This result indicates that 1AZ3BP molecules are initially nanophase separated to form 3D network in organogels but do not construct the highly ordered crystal structure as shown in Figure 3a. After irradiating UV light, however, the 1D WAXD pattern of xerogel in the Figure S14 shows strong multiple diffractions in the low-angle (d-spacing = 3.3, 2.6, and 1.69 nm) and high-angle regions (dspacing = 0.42 and 0.37 nm). Subsequent irradiation of visible light for 3 weeks, which induces cis-to-trans isomerization, does not retrieve the initial 3D network of organogels. Instead, the layer structure is slightly expanded and further stabilized while recovering trans-isomer of 1AZ3BP molecules. From the FTIR results (Figure 3c), it is found that the significant amount of N−H and CO functions does not participate in the 578

DOI: 10.1021/acsmacrolett.8b00167 ACS Macro Lett. 2018, 7, 576−581

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ACS Macro Letters

more stable layered structure. UV irradiation gives rise to a photoisomerization of AZ molecular machine and provides sufficient molecular mobility to form strong hydrogen bonds between N−H and CO, resulting in the peculiar wringing behavior. Broer’s group recently reported the on-demand LC release based on a photoresponsive LC network. LC networks were macroscopically contracted by squeezing out the nematic LC upon UV irradiation.34 Note that the wringing behavior of 1AZ3BP is originated from the light-induced stabilization of ordered molecular packing structures. In summary, to understand the morphological transition of light-responsive smart gels, a photoresponsible asymmetric macrogelator (1AZ3BP) was newly designed and successfully synthesized by chemically connecting an azobenzene (1AZ) molecular machine and a biphenyl-based (3BP) dendron through a 1,4-phenylenediformamide connector. 1AZ3BP was alternatively self-assembled into the layered structure in the bulk driven by the intermolecular hydrogen bond, while the three-dimensional (3D) networked organogel was constructed in solution. The 3D network organogels in the cyclohexane and methanol solution with 3 wt % 1AZ3BP showed an intriguing wringing behavior upon irradiating UV light. Scattering and morphological investigations of 1AZ3BP xerogels before and after the UV irradiation clearly demonstrated the transformation of the metastable 3D network into the stable layered structure driven by the formation of strong intermolecular hydrogen bonds between N−H and CO, resulting in a peculiar wringing behavior. Understanding the phase transition behavior of photoresponsive wringing gels can be useful for the development of remote-controllable molecular machine.

Figure 3. (a) Schematic illustration of light-induced wringing gel. (b) 1D WAXD patterns, (c) FTIR, and (d) Raman spectra of xerogels before and after UV treatment. Note that the vis exposure time of 1D WAXD pattern is 3 weeks after UV treatment. Additionally, the blue line in Raman spectra indicates the spectrum recorded in the bulk state.

formation of hydrogen bonds during the gelation. This means that the 1AZ3BP organogels are formed in the solution mainly due to the nanophase separation between the ethylene oxide and the alkyl chains. Interestingly, after exposing UV light, the intensity of hydrogen-bonded N−H absorption band at 3284 cm−1 dramatically increases with concomitantly decreasing the free N−H band at 3439 cm−1. As shown in Figure 3d, the Raman spectrum of UV-exposed xerogel exhibits an absorption at 1607 cm−1, a characteristic absorption band of aromatic C C stretching vibration, which is blue-shifted compared with the original xerogel. This result indicates that the π−π interaction between mesogens of 1AZ and 3BP is weakened during the formation of the layered structure.39 This interpretation is confirmed again by the Raman spectrum of the self-assembled bulk sample in the melt state (Figure 3d). From the IR and Raman spectroscopic analyses combined with X-ray scattering and SEM results, it is concluded that nanophase separation is the main driving force for 3D network fiber formation in solution. As schematically illustrated in Figure 4, the formation of 3D network in solution is kinetically favorable and the 3D network of the organogel is metastable. The metastable 3D network of organogel can be activated enough by UV light and overcome the energy barrier to reach a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00167. Experimental method, schemes of synthetic procedures, 1 H and 13C NMR, MALDI-TOF, TGA spectra, energy minimized molecular geometry, POM images, 1D SAXS patterns, 2D WAXD pattern, gelation properties, FTIR spectra, images of sol−gel transition, and 1D WAXD pattern (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Suk-kyun Ahn: 0000-0002-6841-4213 Kwang-Un Jeong: 0000-0001-5455-7224 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by BRL (2015042417), MOTIEKDRC (10051334), Mid-Career Researcher Program (2016R1A2B2011041), Basic Science Research Program

Figure 4. Schematic illustration of the wringing behavior of 1AZ3BP under the light stimulation. 579

DOI: 10.1021/acsmacrolett.8b00167 ACS Macro Lett. 2018, 7, 576−581

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(2016R1D1A3B03931932), and NRF Global Ph.D. Fellowship Program of Korea.

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