Concise Synthesis of Photoresponsive Polyureas Containing Bridged

May 18, 2018 - nm blue light, stable cis-PbAzo converts into metastable trans- ... 405 nm blue light, it initially bends away the light source with th...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Concise Synthesis of Photoresponsive Polyureas Containing Bridged Azobenzenes as Visible-Light-Driven Actuators and Reversible Photopatterning Shenzhen Li,† Guang Han,‡ and Wangqing Zhang*,†,§ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co., Ltd, Beijing 100123, China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Linear photoresponsive polyurea of PbAzo containing bridged-azobenzene moieties in backbone was synthesized via polyaddition reaction between hexamethylene diisocyanate and cis-3,3′-diamino ethylene-bridged azobenzene. The bridged-azobenzene moieties endow the PbAzo polyurea advantages of visible-light-driven isomerization and fast and powerful photoresponse. Under irradiation with 405 nm blue light, stable cis-PbAzo converts into metastable transPbAzo accompanying the amorphous-to-crystalline transition and the yellow-to-red color change via cis-to-trans isomerization. With further illumination with 532 nm green light, trans-to-cis isomerization reversibly takes place. This photoresponsive polyurea is used in photopatterning, in which patterns can be reversibly written or erased alternatively by 405 nm blue light and 532 nm green light or heating. Besides, the polyurea film can act as qualified visible-light-driven actuators. Under irradiation with 405 nm blue light, it initially bends away the light source with the bending angle above 110 deg in several seconds, and then it recovers to its initial state with no attenuation under irradiation with 532 nm green light. Our photoresponsive polyurea is different from photoresponsive polymers including planar azobenzene moieties, and this polyurea is expected to be promising for smart materials. photo-oxidation and other photoinduced side reactions.42−44 Besides, the characteristic absorptions of the azobenzenecontaining polymers in stable and metastable forms are generally close to each other, which results in imperceptible color change when these azobenzene-containing polymers are employed in photopatterning.34,45−47 Just recently, unique azobenzenes with a special bridged structure were synthesized.48−52 These bridged azobenzenes can be reversibly switched by visible light. Contrast to planar azobenzenes, the thermally stable form of bridged azobenzenes is the cis conformation. Exposed under visible light with certain wavelength, these bridged azobenzenes reach their metastable trans conformation of planar state, and the trans-to-cis isomerization reversibly takes place under visible light irradiation. It is now expected that polymers containing such bridged azobenzenes will exhibit photoresponsive properties completely different from those containing planar azobenzenes.

1. INTRODUCTION Photoresponsive polymers undergoing shape and/or color change in response to light are attracting increasing attention.1−5 Generally, photoresponsive polymers contain photochromic moieties of azobenzene,6−21 diarylethene,22−25 and spiropyran.26−30 Upon exposure to light, the chromophores change their molecular structure, and the polymer chains thus give rise to conformation change, and polymer undergoes concomitant changes in physical and chemical properties through photoisomerization. Of all photoresponsive polymers, these polymers containing azobenzene chromophores undergoing photoinduced contractions/expansions are promising photoregulated actuator.31−33 Furthermore, polymers with nonstackable azobenzene chromophores in the side chains are used in rewritable optical storage.34−37 However, the reported azobenzene-containing polymers generally have several drawbacks. For examples, most of polymers are crosslinked in order to improve the elastic modulus,38−41 whereas this cross-linking leads to feeble bending at room temperature and inconvenience in handling. Also, most azobenzenecontaining polymers require ultraviolet light to trigger isomerizations, and this ultraviolet light can cause damage due to © XXXX American Chemical Society

Received: April 1, 2018 Revised: May 18, 2018

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DOI: 10.1021/acs.macromol.8b00687 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of the Bridged Azobenzene bAzo

upon irradiation with 532 nm light changes smoothly and is completed in about 12 and 30 s (Figure S2). This quick isomerization of the monomers makes the polymeric film to be fast photoresponsive actuators and easily rewritable materials, which will be discussed subsequently. Furthermore, the large separation of the n−π* transition of the trans and cis isomers, which is 90 nm, leads to an easily detectable yellow-to-red change for both monomer and polymer via isomerization. 2.2. Synthesis of Bridged-Azobenzene Polyureas. The bridged azobenzene polyureas of PbAzo were synthesized by polyaddition between bAzo (3) and HDI (4) in the presence of DBTDL catalyst. The synthesized PbAzo is soluble in DMSO but insoluble in THF. Therefore, polyaddition in DMSO is a solution polymerization and polyaddition in THF is a precipitate polymerization, respectively. Both solution polymerization in DMSO and precipitate polymerization in THF are checked. The solution polymerization runs slightly slower than the precipitate polymerization (Figure 2A), and the poly-

Herein, linear polyureas containing bridged-azobenzene moieties in the main chains were synthesized by means of polyaddition reaction54,55 between hexamethylene diisocyanate (HDI) and (Z)-11,12-dihydrodibenzo[c,g][1,2]diazocine-3,8diamine (bAzo). This PbAzo polyurea is quite sensitive to light stimulation due to the nonplanar structure of the bridged azobenzenes with little π−π stacking at cis conformation. By illuminating with 405 nm blue light and 532 nm green light, cisto-trans and trans-to-cis isomerizations alternatively take place as indicated by the yellow-to-red color change. The polyurea film is conveniently prepared by casting the polymer solution. The film can bend against the 405 nm blue light source quickly with bending angle above 110 deg, demonstrating a qualified visible-light-driven actuator. Furthermore, the synthesized PbAzo polyureas are also used as photorewritable materials in reversible photopatterning.

2. RESULTS AND DISCUSSION 2.1. Synthesis of Bridged Azobenzenes. The bridged azobenzene monomer of (Z)-11,12-dihydrodibenzo[c,g][1,2]diazocine- 3,8-diamine (3, bAzo) with 15% yield was synthesized by a two-step synthesis following Scheme 1. The synthesized bAzo was characterized by 1H NMR and 13C NMR (Figure 1 and Figure S1) and thin layer chromatography. The

Figure 2. Polymerization kinetics of the polyaddition between bAzo (3) and HDI (4) in THF and DMSO (A), the GPC traces of the PbAzo synthesized in THF and DMSO at 24 h (B), Mw and Đ of PbAzo synthesized in THF at different times (C), and optical images of PbAzo (D).

addition under [3]/[4]/[DBTDL] = 1/1/0.03 runs fastest (Table S1). The PbAzo polyureas synthesized at 24 h polymerization time have a molecular weight Mw of about 5 kg/mol with narrow molecular weight distribution with Đ at about 1.03−1.05 as characterized by GPC (Figure 2B). The molecular weight of PbAzo is also characterized by matrixassisted laser desorption ionization-time-of-flight (MALDI− TOF) mass spectrometry (MS). However, these PbAzo polyureas are almost insoluble in the low boiling point solvents such as THF, ethyl acetate, and MeCN and just soluble in the high boiling point solvents of DMF and DMSO. This made a large noise in the MALDI−TOF analysis of traced PbAzo dissolved in MeCN (Figure S3). Despite this, the molecular weight of PbAzo by MALDI−TOF MS is close to those by

1

Figure 1. H NMR spectra of bAzo and the PbAzo polyureas.

main problem hampering the synthesis of bAzo is the low and unreliable yields in the subsequent reductive ring closure of the dinitro precursors as discussed elsewhere.48−52 The DMF solution of cis-bAzo in the photostationary states (PSS) shows an n-π* transition at approximately 400 nm, and trans-bAzo shows an n−π* transition at 490 nm, respectively (Figure S2). The absorbance spectrum of pure trans-bAzo is calculated following the method reported by Fischer.53 It is found that either the cis-to-trans isomerization of bAzo upon irradiation with 405 nm light or the trans-to-cis isomerization B

DOI: 10.1021/acs.macromol.8b00687 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 3. (A) Absorption spectra of the cis- and trans-PbAzo polyureas in DMF, PSS states of polyurea after 405 nm blue light irradiation and after subsequent 532 nm green light irradiation; (B) time-dependent absorption spectra of cis-PbAzo upon 405 nm light irradiation; (C) time-dependent absorption spectra of trans-PbAzo upon 532 nm light irradiation; (D) absorption at 485 nm with alternating irradiation of 405 and 532 nm light at the reversible photoisomerization cycles.

GPC. The synthesized polyureas were confirmed by 1H NMR (Figure 1), and all the proton signals were nicely assigned, indicating successful synthesis of PbAzo. By comparing the chemical shifts at 6.7 and 6.9 ppm, the molecular weight of the polyureas at 4.0 kg/mol is obtained by assuming the polyurea chain including one azobenzene terminal. This NMR determined molecular weight is slightly lower than the Mn determined by GPC or MALDI−TOF, and the reason is ascribed to three possible terminal sequences in the synthesized polyureas (Scheme S1). The molecular weight of these PbAzo polyureas synthesized by precipitate polymerization via polyaddition in THF at different monomer conversion was also checked. As summarized in Figure 2C, the molecular weight of the PbAzo polyureas increases with monomer conversion and Đ keeps below 1.1. In comparison with the polymers synthesized by polyaddition,54,55 the present polyureas have low molecular weight, and one possible reason is the low basicity of the aromatic diamines in the monomer of bAzo (3), which leads to the decreased activity toward diisocyanate of HDI (4). The PbAzo polyureas are thermally stable at room temperature. The polymer molecular weight dose not decrease after the PbAzo polyureas being left at room temperature for three months. However, as similar as traditional azobenzene polymers,54,55 they start to decompose at temperature above 90 °C, which is slightly higher than the on-set decomposition temperature of the bAzo monomer (3) at 85 °C (Figure S4). No glass transition temperature (Tg) is observed in DSC curves of these PbAzo polyureas below on-set decomposition temperature 90 °C (Figure S5). The endotermic peak in the DSC curve of the PbAzo polyureas after irradiation with 405 nm blue light is possibly ascribed to the crystalline PbAzo melting, which will be discussed subsequently. 2.3. Photoisomerization and Photostability of Bridged-Azobenzene Polyureas. The photoisomerization of the PbAzo polyureas in DMF solution is initially studied. The reversible cis-to-trans transition takes place under irradiation with 405 nm light as indicated by polymer solution

changing from bright yellow to glossy red (Figure S6). The reversible photoisomerization between cis- and trans-PbAzo is illustrated in Figure 3. Cis-PbAzo exhibits a π−π* absorption around 320 nm in the UV range and an n−π* absorption around 405 nm, which are blue-shifted about 30 nm and redshifted about 5 nm compared with the monomer of cis-bAzo, respectively. Isomerization of cis-to-trans transition is achieved using 405 nm blue light, and the resultant trans-PbAzo exhibits a π−π* absorption around 350 nm in the UV range and an n−π* absorption around 485 nm. Note: the absorption spectrum of the resultant trans-PbAzo was obtained following the Fischer method.53 1H NMR indicates a trans/cis ratio at 39/ 61 in the PSS state of PbAzo upon irradiation with 405 nm, in which the trans/cis ratio is calculated by comparing the chemical shifts at 8.45 and 8.63 ppm (Figure 4). This trans/ cis ratio in PbAzo is higher than that in the monomer of bAzo (Figure S2) in the bAzo monomer (34/66), suggesting that cisto-trans transition of bridged-azobenzenes is slightly hindered

Figure 4. 1H NMR spectra of the PbAzo polyureas at PSS before and after 405 nm light irradiation. C

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Macromolecules by the polyurea chains. Further irradiation with 532 nm green light, trans-to-cis transition occurs and the recovered polyurea shows an identical UV−vis spectra with cis-PbAzo. It is noticed that (1) the n−π* band of trans-PbAzo has a long wavelength tail which even extends to 620 nm and (2) the n−π* band of trans-PbAzo (450 to 620 nm) and cis-PbAzo (375 to 450 nm) are clearly separated. Therefore, the photoisomerization of the PbAzo polyureas can be achieved upon irradiation with a broad band light. Indeed, when this glossy red solution of trans-PbAzo is exposed to sunlight at 3 p.m. in autumn for about 50 s, it turns back to bright yellow of cis-PbAzo. Figure 3B indicates that the cis-to-trans isomerization of the PbAzo polyureas takes place quickly under 405 nm light irradiation and is completed within 15 s. In comparison, the trans-to-cis transition under 532 nm light irradiation runs a little slower (Figure 3C), and this is ascribed to the 532 nm blue light far away from the n−π* absorption of trans-PbAzo around 485 nm and therefore having low quantum efficiency. The photoisomerization between cis- and trans-PbAzo is fully reversible, and almost no attenuation in the n−π* absorption at 485 nm is found even after 8 cycles of photoisomerization. In addition, heating could also gave rise to the trans-to-cis transition of PbAzo, and this thermal relaxation is firmly dependent on temperature (Figure 5). At 20 °C room

Figure 6. Polarized optical microscopy (POM) of the PbAzo film under 405 and 532 nm light irradiation.

driven amorphous-to-crystalline transition. Further study and application of this visible-light-driven amorphous-to-crystalline transition are undergoing in our lab. 2.4. Photopatterning and Light-Driven Actuator of Bridged-Azobenzene Polyureas. Conventional photoswitchable compounds including azobenzenes, spiropyrans and diarylethenes have been proposed for optical recording.35,56−61 To meet the requirement of information storage, these photoswitchable compounds should have three characters. First, their stable form should much different from their metastable states, and this difference can be easily detected. Second, the stable to metastable transition should be fast and reversible. Third, both the stable and metastable form should be kept for enough time under certain conditions. As introduced above, the features of the present PbAzo polyureas, e.g., the fast and reversible trans/cis transition upon visible light irradiation or by heating, the different color (yellow or red) of the cis- and trans-PbAzo, the relatively long t1/2 of the metastable transPbAzo at room temperature, make the PbAzo polyureas to be rewritable photopatterning materials. Figure 7A shows the photopatterning procedures through alternative irradiation of PbAzo with 405 nm blue light and 532 nm green light. It indicates that photopatterning with a help of mask can be achieved through irradiation with 405 nm blue light (procedures 1−6) and then the photopatterning can further be erased through irradiation with 532 nm green light

Figure 5. Thermal relaxation dynamics of the PbAzo polyureas in DMF at different temperatures.

temperature, the trans-to-cis transition takes place very slowly with a half-life period t1/2 ≈ 5 h (Figure S7). With temperature increasing, the trans-to-cis transition becomes much faster and the trans-to-cis transition is completed in 8 min with the half-life period t1/2 ≈ 2 min at 80 °C. The photoisomerization of the PbAzo polyureas in bulk film is further investigated. For planar azobenzenes in bulk, π−π stacking can somewhat takes place, which makes cis/trans transition in bulk different from in solution.34 On the other hand, for bridged azobenzenes, they usually have much larger free volume and distorted π-electron system, and therefore, π−π stacking is greatly inhibited.49,51 As shown in Figure S8, the PSS states of the present PbAzo polyureas in solution and in film are almost identical, suggesting minimal π−π stacking in PbAzo. This minimized π−π stacking leads to all azobenzene moieties in nonplanar bridged conformation, and therefore cisPbAzo has an amorphous structure as confirmed by polarized optical microscope (Figure 6A). Under irradiation with 405 nm light, bridged-azobenzene moieties in PbAzo are isomerized via cis-to-trans transition, and crystalline trans-PbAzo is formed (Figure 6B). Our PbAzo polyurea is believed to be a typical photoresponsive polymer, which can undergo visible-light-

Figure 7. (A) Typical photopatterning procedures alternatively employing 532 nm green light and 405 nm blue light. (B) Photopatterning with different masks alternatively by 532/405 nm light and heating. D

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Macromolecules (procedures 7−8). Furthermore, different photopatterning using the PbAzo polyureas can be achieved employing different masks as demonstrated in Figure 7B, in which the pattern can be alternatively written or erased either by 532/405 nm light or heating at 70 °C (Movie 1 in Supporting Information). Polymeric actuators can undergo mechanical deformation in response to various physical stimuli, such as pH, heat, and light, and these actuators are expected to be used for cantilevers, micropumps, and many other micromechanical applications.62−66 Up to now, several light-driven actuators of azobenzene-containing polymers have been designed.50,66,67 Upon irradiation usually with UV light, incident photons are absorbed by the film, and isomerization occurs preferentially in the film surface.68−71 In case of polymers containing planar azobenzenes, light irradiation results in asymmetric uniaxial contraction of polymer film, which causes the film to bend toward the light source.12,39 However, upon irradiation with 405 nm blue light, bridged azobenzenes are isomerized and the cis-to-trans transition leads a nonplanar-to-planar conformation change, which causes the distance between the amino nitrogen atoms to change from 8 to 11 Å.49 As a result, 405 nm blue light irradiation gives rise to photoexpansion of the surface layer, and the PbAzo film bends away from the light source as shown in Figure 8A. This light-driven bending of the polyurea film is recorded by digital camera (Movie 2 in the Supporting Information) and several snapshots depicting the bending at a given irradiation time are extracted and are summarized in Figure 8B, in which the arrows indicate the incident direction of the 405 nm blue light. The first frame in Figure 8B shows the initial state of the PbAzo film, the second and third frames show how the PbAzo film bends against the 405 nm blue light for 1 and 10 s, and the last one shows the final state of the PbAzo film when the light source is eventually removed, respectively. The results summarized in Figure 8C indicate that the PbAzo film (2.0 cm ×2.0 mm ×30 μm) quickly bends away from the 405 nm light source with the bending angle at about 109 degree in 8 s at 20 °C room temperature. The bending speed of the PbAzo film is correlative to the film width, and when the PbAzo film is narrowed to 1.0 mm width, the film bends to about 113 deg in 5 s (Figure S9). The present PbAzo film may be one of the rare examples of soluble polymers used as actuator,72 which is different from the cross-linked polymeric ones.7,38,40,67,73,74 The possible reason is ascribed to the intraand/or intermolecular hydrogen bonding in the present polyureas. Compared with planar azobenzene-containing polymers reported previously,7,38 although the present PbAzo film is relatively thick, it bends faster and has a larger bending angle. Besides, the present PbAzo film can bend at 20 °C room temperature much lower than the Tg of the PbAzo polyureas, which provides great convenience for the practical application of this photoresponsive materials. It is believed that three characters including the bridged structure of the azobenzene moiety, the non-cross-linked polymer backbone and the high content of the linear bridged azobenzenes at 58 wt % are ascribed to the fast and powerful bending. The results in Figure 8D indicated that the bent film can be recovered upon irradiation with 532 nm green light. Different from the PbAzo film bending away the 405 nm light source, herein the PbAzo film bends toward the 532 nm green light source. This 532 nm green light leads to trans-to-cis isomerization and therefore contraction of the film surface, which causes the film to bend toward the 532 nm light source as shown in Figure 8D. It is found that the light-driven

Figure 8. (A) Schematic illustration of the bending mechanism of PbAzo film; (B) a series of pictures illustrating the bending process of the film under 405 nm blue light; (C) bending angle as a function of time under the 405 nm blue light and 532 nm green light; (D) a series of pictures illustrating the unbending process of the PbAzo film under 532 nm green light; (E) bending angle of the PbAzo film in each cycles of irradiation with 405 nm blue light and 532 nm green light. The film size is 2.0 cm (length) × 2.0 mm (width) × 30 μm (thickness).

oscillation of the PbAzo film bending away the 405 nm light and bending toward the 532 nm light can be repeated over 30 times almost with no attenuation in the bending angle or the bending speed (Figure 8E). The recovery of the bent film in dark at room temperature is also checked. It is found the bent film is straightened very slowly until a minimum bending angle of 25 deg is reached in 30 h.

3. CONCLUSIONS Linear photoresponsive polyureas of PbAzo containing bridgedazobenzene moieties in the polymer backbone are synthesized via polyaddition reaction between hexamethylene diisocyanate and ethylene-bridged azobenzene. The molecular weight of PbAzo polyureas slightly increases with monomer conversion. The bridged-azobenzene moieties endow the PbAzo polyureas five characters. First, the thermal stable state is in amorphous cis-PbAzo with little n−π* stacking, and the metastable form is in crystalline trans-PbAzo, which is different from those containing planar azobenzene moieties. Second, trans-to-cis and cis-to-trans isomerization of PbAzo accompanying with yellow-to-red color transition reversibly take place by illuminating with 405 nm blue light and 532 nm green light, E

DOI: 10.1021/acs.macromol.8b00687 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Synthesis of the PbAzo Polyureas

of 3 (0.250 g, 1.05 mmol), HDI (4) (0.177 g, 1.05 mmol), the DBTDL catalyst (20 mg, 0.031 mol) and DMSO (1.0 g) were added and stirred at 25 °C under nitrogen atmosphere. At a given time interval, a quotient solution was taken out, the monomer conversion was analyzed by 1H NMR by comparing the chemical shift at 8.44 ppm (Ar−NH) and 5.06 ppm (Ar−NH). The synthesized PbAzo was precipitated in diethyl ether, collected by centrifugation (10000 rpm, 3 min), and finally dried at room temperature under vacuum. 4.4. Film Preparation and Characterization. The 10 wt % DMF solution of PbAzo was dropped on a clean quartz substrate and then cast by drying at 40 °C under vacuum for 24 h. The thickness of the film (30 μm) was measured by a digital vernier caliper (Mitutoyo MDH-25M). The film was cut as 2.0 cm (length) × 2.0 mm (width) × 30 μm (thickness) or 2.0 cm (length) × 1.0 mm (width) × 30 μm (thickness) for optical observation. Photoisomerization was induced by 405 nm blue laser light and 532 nm green laser light placed 20 cm away from the film, and the light intensity was estimated to be about 30 mW/cm2 at this distance. The bending behavior of the PbAzo film was observed upon irradiation initially with 405 nm blue laser light at room temperature and then with 532 nm green laser light. The photographs and videos of the film bending and unbending were taken by a digital SLR camera (Nikon D7100, full HD, NTSC). The bending/unbending dynamics was quantified with respect to the tip position of the PbAzo film in each frame of the recorded videos. 4.5. Characterization. 1H NMR and 13C NMR were recorded on a Bruker Avance III 400 MHz NMR spectrometer in CDCl3 or DMSO-d6 as solvent. UV−vis spectra were recorded on a Varian 100 UV−vis spectrophotometer at room temperature. The molecular weight and its distribution or the polydispersity index (Đ, Đ = Mw/ Mn) of the synthesized polyureas were determined by gel permeation chromatography (GPC) equipped with a Waters 600E GPC with three SHODEX columns in DMF containing LiBr (0.01 mol/L) operating at a flow rate of 0.8 mL/min at 50.0 °C using an RL 2000 refractive index detector, and the system was calibrated with monodisperse polystyrene standards. Differential scanning calorimetry (DSC) measurements were carried out on a Netzsch DSC 204 differential scanning calorimeter under nitrogen atmosphere with heating rate at 10 °C/min. Thermal decomposition of the polymers were performed using a thermogravimetric analyzer (TGA) Netzsch TG 209 at a heating rate of 10 °C min−1 from 25 to 800 °C under constant nitrogen flow.

which avoids irradiation of UV light. Third, light-driven trans-cis and cis-trans isomerization runs fast. Four, thermal relaxation can also induce trans-to-cis transition of PbAzo, and transPbAzo can keep relatively stable at room temperature. Five, polymer film can be conveniently prepared by casting polymer solution. These characters make the PbAzo polyureas to be rewritable photopatterning materials and qualified visible-lightdriven actuators. In photopatterning, patterns can be reversibly written or erased alternatively by 405 nm blue light and 532 nm green light or heating. Under irradiation with 405 nm blue light and 532 nm green light, the PbAzo film can bend away the 405 nm blue light source and bend toward the 532 nm green light source with the bending angle at above 110 degree in several seconds, and the light-driven oscillation of the PbAzo film can be repeated with no attenuation. Our PbAzo polyureas are believed to be promising smart materials.

4. EXPERIMENTAL SECTION 4.1. Materials. 4,4′-Ethylenedianiline (97%, TCI), 1,6-hexamethylene diisocyanate (HDI; 98%, TCI), and dibutyltin dilaurate (DBTDL; 95%, Adamas) were all used without further purification. Dimethyl sulfoxide (DMSO) was dried over calcium hydride (CaH2) and distilled under reduced pressure before use, and tetrahydrofuran (THF) was dried over 4 Å molecular sieves (MS). All other reagents were purchased from Tianjin Chemical Reagent Factory and used without further purification. 4.2. Synthesis of the Monomer of Bridged Azobenzene. The bridged azobenzene monomer of bAzo (3) was synthesized as shown in Scheme 1 following the protocol of Herges and co-workers with slight modification.49 A solution of 1,2-bis(4-aminophenyl)ethane (1) (2.00 g, 9.42 mmol) in 16 mL of concentrated sulfuric acid was warmed up to 60 °C. Sodium nitrate (1.74 g, 20.42 mmol) dissolved in 18 mL of sulfuric acid was added dropwise to the solution with magnetically stirring. The resulted deep brown solution was kept at 60 °C for 4 h and afterward poured into 100 mL of ice water. Aqueous ammonia solution (32%) was slowly added to neutralize the reaction mixture. The red precipitate was collected by filtration, washed with water and dried in vacuum over CaCl2 to afford 1,2-bis(2-nitro-4aminophenyl)ethane (2) (2.80 g, 9.26 mmol, 98% yield). Sodium hydroxide (17.60 g, 440.00 mmol) dissolved in 72 mL water was added to the suspension of 2 (2.00 g, 6.62 mmol) in 260 mL ethanol, and the mixture was heated to 50 °C. After 10 min, a solution of glucose (17.00 g, 94.36 mmol) in 40 mL water was added, and the reaction mixture was stirred at 50 °C for 5 h. At this time, the raw materials completely disappeared, and the azoxybenzene derivative was obtained as the main product (Figure S10). Then, another portion of glucose (3.50 g, 19.43 mmol) in 40 mL water was added. After 1 h, this mixture was cooled to room temperature, and 800 mL water was added. The resulting mixture was extracted three times with 200 mL ethyl acetate. The combined organic layers were washed with brine and dried with anhydrous Na2SO4. After concentration under vacuum, the residue was purified by flash chromatography on silica gel using pentane/ethyl acetate (1:1 to 1:3) as eluent to afford yellow solid of bAzo (3) (0.250 g, 1.05 mmol, 15.9% yield, seeing 1H NMR and 13C NMR in Figure S1). 4.3. Synthesis of Bridged-Azobenzene Polyureas. Bridgedazobenzene polyureas of PbAzo were synthesized by polyaddition reaction between bAzo (3) and HDI (4) under nitrogen atmosphere in the solvent of DMSO or THF (Scheme 2). Herein, the polyaddition in DMSO was typically introduced. In a 5 mL Schlenk flask with a magnetic, the bridged-azobenzene monomer

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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/acs.macromol.8b00687. Experimental details and supplementary figures (PDF) Movie 1, photopatterning (MOV) Movie 2, light-driven bending of the polyurea film (MOV)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Z.). ORCID

Wangqing Zhang: 0000-0003-2005-6856 Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS Financial support by the National Science Foundation for Distinguished Young Scholars (No. 21525419), the National Science Foundation of China No. 21474054), PCSIRT (IRT125), and the National Key Research and Development Program of China (2016YFA0202503 and 2017YFC1103501) is gratefully acknowledged.

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DOI: 10.1021/acs.macromol.8b00687 Macromolecules XXXX, XXX, XXX−XXX