Hydrogelation landscape engineering and a novel strategy to design

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Surfaces, Interfaces, and Applications

Hydrogelation landscape engineering and a novel strategy to design radically induced healable and stimuli-responsive hydrogels Yanqiu Wang, Xudong Yu, Yajuan Li, Yajun Zhang, Lijun Geng, Fengjuan Shen, and Jujie Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02592 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Hydrogelation landscape engineering and a novel strategy to design radically induced healable and stimuli-responsive hydrogels Yanqiu Wang, Xudong Yu*, Yajuan Li, Yajun Zhang, Lijun Geng, Fengjuan Shen, Jujie Ren College of Science, and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, PR China E-mail for corresponding author: [email protected] (X. Yu) Abstract: Herein, we report the versatile ways to prepare both low-molecular-weight hydrogels and polymeric hydrogels based on various types of supramolecular interactions starting from a simple amphiphilic terpyridine-based molecule TPYA. Notably, we report that stable terpyridine-based radicals could be generated by light or heat irradiation in polymeric hydrogels based on hydrogen bonding interaction between -COOH of PAA and terpyridine motif of TPYA for the first time. The generation of radicals are confirmed by EPR and UV-vis experiments, and the process is accompanied with significant color changes from white to dark purple. The stable radical hydrogels prepared by supramolecular strategy are self-healing, stretchable and self-supporting that are able to be molded to different kinds of geometrical shapes. It is deduced that the generation of terpyridine-based radicals enhances the intermolecular hydrogen bonding and π-π interaction of molecules in hydrogel matrix, which is responsible for the self-healing ability. Finally, we also show that the radical gels could selectively respond to ammonia and stretch with reversible color changes based on the reversible hydrogen bonding interaction. Keywords: hydrogel, radical gel, self-healing, stretchable, stimuli-responsive 1. Introduction Smart hydrogels including both low-molecular-weight hydrogels and polymer hydrogels are of great interest in the past decades for their applications in sensors, tissue engineering, cell culture and drug delivery systems.1-11 In order to prepare these gels with a bulk of functions, multiple interactions such as coordination interaction, hostguest encapsulation, electrostatic interactions as well as hydrogen bonding interaction are introduced to the gel system that responds to external stimuli including ultrasound, heating-cooling, as well as chemical stimuli.12-18 Controlling of gelation pathways in supramolecular assembly also allows the formation of hydrogel materials with distinct 1

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properties and controlled functions, starting from a set of simple organic compounds and assembly conditions. However, it is still a challenge that one simple gelator can form a number of vastly different gel materials with multiple stimuli-responsive properties in engineered interaction pathways.19 Especially, robust, self-healing hydrogels have drawn increasing attention in the past decades owing to its potential application in material science, biological chemistry and industry.20-26 To achieve this goal, different strategies and the corresponding gels are developed such as nanocomposite gels, double network gels, slide-ring gels, as well as the gels with dynamic cross-linkers, reversible covalent bonds and increased network branches.27-29 Despite the great effort in preparing robust and self-healing gels, it is still a big challenge to fabricate adaptive and self-healing gels capable of novel functions especially in visual response to external stimuli. In recent years, radical species have been gradually concerned in material chemistry since the unpaired electron results in superior properties and they find wide application in the field of organic light-emitting diodes (OLED), catalyst, spintronics, magnetic materials, imaging agents, and so forth.30-35 These species are usually unstable, uncontrollable, chaotic, and air disable. Recently, it is found that supramolecular systems are promising candidates for stabilization and generation of transient radicals.36 For example, Zhang and coworkers have demonstrated the NDI radical anions that have been stabilized by host-guest interaction;37 Liu et al have used a gel to generate O radicals which was further utilized for efficient polymerization.38 Notably, viologen (1,1′-disubstituted-4,4′-bipyridinium) radical cation, which possesses electrochromism and photochromism properties in supramolecular aggregation system, has been found extensive applications in the fields of catalysis, sensing, molecular machines, and intelligent materials.39-43 Generally, one or two electron redox species of viologen radical are extensively found in photoactive bipyridinium salts or metal-bipyridinium frameworks for applications in photochromism materials.44-46 In such compounds, the radical should be generated based on ionic or coordination bonding in crystals.47 For example, Wu et al have reported the generation of pyridine-based radicals based on Zn2+-pyridine metal-organic complexes (MOFs) under X ray irradiation.48 However, crystals were difficult to be obtained and the functions of these materials are monotonous; Moreover, the pyridine-based radicals in these materials are unstable for preservation. In the easy design of stable photochromism materials based on pyridinebased radicals with multiple stimuli-responsive properties, it is advantageous to seek novel approaches that the pyridine-based radical production and stabilization in soft matter especially in multifunctional gels via supramolecular strategy. On basis of above considerations, herein, a simple and amphiphilic compound TPYA containing terpyridine and adamantane motifs are synthesized and characterized 2


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(Scheme 1). The molecule is versatile that it has the ability to form different kinds of low-molecular-weight hydrogels when other interactions including coordination and inclusion interactions are introduced; TPYA could be also rationally incorporated into PAA (polyacrylic acid) gel networks through supramolecular hydrogen bonding and hydrophobic interactions (PAA/TPYA hydrogel). Amazingly, we find that terpyridinebased radicals could be generated when the PAA/TPYA hydrogel is triggered by light or heat irradiation. Such process is accompanied with the significant color changes from white to red and then to dark purple. The resulting radicals could be preserved for months in ordered gel assembly, and the radical hydrogels exhibit robust, stretchable and self-healing properties that could be molded to different kinds of self-supporting and self-sustaining geometrical shapes. Finally, as applications in stimuli-responsive materials, we show that the radical gel could selectively respond to ammonia with color fading among test amines and other organic solvents. On the other hand, the radical gel is also developed for constructing stretching force switches through instant and reversible color changes from dark purple to purple. To date, as far as we known, this is the first report about the generation, preservation and function of terpyridine-based radicals based on supramolecular interactions. Scheme 1 (a) The chemical structure of TPYA. (b) The different gelation pathways of TPYA in water and the generation of supramolecular radical gel that shows self-healing and stimuli-responsive properties; ultrasound or heating-cooling treatment of TPYA in the presence of different guest species in water (1-3). 1: In the presence of ZnF2; 2: In the presence of other Zn2+ metal ions and β-cyclodextrin (β-CD); 3: In the presence of Cu2+, Mn2+ or Fe2+ ions and β-CD; 4: polymerization approach.

2. Experimental Section 2.1 Materials Acrylic acid, methacrylic acid, acrylamide and N, N'-methylenebisacrylamide were purchased from Aladdin reagent (Shanghai) Co. Ltd. 2-acetylpyridine, P-hydroxy benzaldehyde, methyl bromoacetate, β-cyclodextrin and 1-adamantanamine 3



hydrochloride were obtained from Sinopharm chemical reagent Co. Ltd. All metal salts and other reagents used for gelation test were purchased from Shanghai Darui fine chemical Co. Ltd without further purification. Distilled water was used for the gelation test for both metallogel and polymeric gels, and there was no obvious impact on the gel properties when running water was used. 2.2 Instrumentation SEM (Scanning Electron Microscope) images of the dry metallogels and polymer gels were obtained by using FE-SEM S-4800 (Hitachi) instruments. After spinning the metallogels or polymer gels on glass slides and coating them with Au, the xerogels were tested. NMR (Nuclear Magnetic Resonance) spectra for the organic molecules and the titration experiments were carried out on a Bruker Advance DRX 400 spectrometer operating at 500 and 125 MHz for 1H NMR and 13C NMR spectroscopy respectively. UV-Vis (Ultraviolet–visible spectroscopy) absorption spectra for all the solutions, metallogels, polymer and radical gels were generated on a UV-Vis 2550 spectroscope (Shimadzu). Fluorescence curves for the solutions, gels and titrations were collected on a Hitachi F-7000 spectrometer. Rheological experiments for both metallogels and polymer gels were carried out on resulted gels using a controlled stress rheometer (Malvern Bohlin GeminiHRnano). Plate and cone geometry of 25 mm and 20 mm diameters respectively were utilized during the rheological test process. The HSXF/UV300 Xe lamp (Beijing NBET Group Corp) was used as UV light irradiation source. Electron paramagnetic resonance (EPR) spectra of the hydrogels were recorded on a Brucker model A 300 ESR spectrometer. 3. Results and discussion 3.1 Low-molecular-weight metallogel formation of TPYA The synthesis of TPYA could be seen in supporting information (SI) via four steps. TPYA was a small and flexible molecule that could both bind ions and β-CD via coordination and host-guest inclusion interactions. Although TPYA showed good solubility in most of the tested organic solvents such as CHCl3, THF, dioxane and alcohols (Table S1), it did not dissolve in water even at high temperature. Previous work demonstrated that introduce of ions to terpyridine-based ligand could change the polarity and solubility of terpyridine ligand, leading to hydrogels.49 Therefore, firstly, we investigated the influence of the metal salts on gelation properties of TPYA in water. The gelation ability of TPYA in water was evaluated with 19 kinds of metal ion salts, which could be seen in table S2. Addition of ZnF2 (one equivalent of TPYA) into the suspension of TPYA leaded to opaque gel with yellow-green emission color (Figure S1a). Upon the addition of other different metal salts, mostly, suspensions were observed. Interestingly, in the presence of β-cyclodextrin, TPYA could form a wide range of hybrid gels with cooperative coordination and inclusion interactions in the 4


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presence of Zn2+ salts, CuCl2, MnCl2, AlCl3, and Bi(NO3)3 (Figure S1b, Table S3). The coordination interactions between TPYA and ions were further certified by 1H NMR experiments (Figure S2). 3.2 Gelation properties of TPYA metallogels The effect of counter anions on the gel properties were further studied. The ZnF2/TPYA hydrogel exhibited green emission color (Figure S1a). When using Cl- instead of F-, the ZnCl2/TPYA hydrogel showed blue emission color (Figure S1b). Fluorescence spectra revealed that the emission wavelengths of Zn2+ hydrogels with different counter anions could be tuned from 514 to 437 nm (Figure S3). This difference was ascribed to the different geometries and chelating abilities of Zn2+ salts, which had significant effects on the ICT process of TPYA.50 The counter anions also affected the binding ability of Zn2+ salts and TPYA in solutions. The binding constants of different Zn2+ salts and TPYA in solutions followed the order: Zn(NO3)2>ZnCl2>ZnF2 (Figure S3, S4 and Table S4), which were calculated according to our previous reports with 1:1 stoichiometric ratio of host and guest.51 Upon the addition of other metal salts including CuCl2, Bi(NO3)3, and MnCl2 in the presence of β-CD, fluorescence quenching phenomena was observed for the resulted metallogels. These gels also displayed tunable wavelengths in the range from 395 nm to 465 nm (Figure S5). The above experiments indicated the different gelation approaches and resulting hydrogels were highly controlled by both ions and counter anions, and the significant emission color difference of ZnF2/TPYA metallogel in comparison with that of other metallogels endowed the selective and visual recognition of ZnF2 via a gelation approach. SEM experiments were carried out in order to gain insight into morphology properties of gels in macroscopic level. Seen from Figure S1c, the TPYA/Zn(NO3)2/βCD hydrogel was dominated by homogenous and entangled fibers with average diameter of about 40 nm, and the fiber aggregations of TPYA/Zn(AcO)2/β-CD and TPYA/ZnF2 hydrogels were more intensive than that of TPYA/Zn(NO3)2 hydrogel (Figure S1d and Figure S6). On the other hand, the TPYA/Al(NO3)3/β-CD and TPYA/AlCl3/β-CD hydrogels gave anisotropic square structures and adhesive micro ribbons were observed for TPYA/CuCl2/β-CD hydrogel (Figure S7). Such results indicated that both the cations and anions highly impacted gelation pathways and the resulting optical and morphology properties. Moreover, the red shift of absorption peak of TPYA/Zn2+ gels centered at around 284 and 322 nm in comparison with that of the corresponding solutions indicated the J aggregation of Zn2+-terpyridine motifs in hydrogel gel matrix (Figure S8, S9). 3.3 PAA/TPYA hydrogel and radical gel preparation The above studies on low-molecular-weight metallogels showed that the hydrogels were prepared with tunable gelation approaches and macro properties controlled by 5



different supramolecular interactions and external stimuli. However, as most of the lowmolecular-weight gels, these hydrogels lacked good mechanical strength that were prone to collapse when subject to shaking or higher mechanical stress. Considering the good mechanical strength of polymeric gels, TPYA assembly was rationally inserted into the gel networks through the hydrogen bonding (pyridine and -COOH motifs) and hydrophobic interactions (adamantane motif and hydrophobic chain of PAA), which were reversible and well-established in supramolecular gels. Such reversible interaction might also result in novel and fascinating functions. The preparation of the PAA/TPYA hydrogels could be seen in supporting information (SI). Firstly, the hydrogen bonding interaction between -COOH of PAA and terpyridine group of TPYA was confirmed by 1H NMR experiments. As seen from Figure S10, the -COOH of acroleic acid at 12.49 ppm was greatly weakened and became broader in the presence of TPYA. Secondly, we observed that the as-prepared hydrogels became opaque with increasing amount of TPYA, which might be due to the introduction of hydrophobic property of adamantane group, resulting in the coassembly of TPYA and PAA through hydrophobic interactions in water. Porous structure was observed for TPYA/PAA hydrogel from SEM experiment, which was the typical morphology for polymer gel networks (Figure S11). The hydrogels displayed a swelling trend in the presence of short chain alcohols and changed to transparent gels (Figure S12), revealing the hydrogen bonding competition between -COOH and alcohols and -COOH and TPYA. The gel in methanol showed the greatest degree of swelling in volume (about 250%). Whereas, other test organic solvents such as benzene and CH2Cl2 did not have significant changes on the gel. The transmittance changes of PAA hydrogels controlled by TPYA and alcohols revealed that the intermolecular hydrogen interaction between TPYA and PAA was reversible in gel networks. Based on the inclusion interaction between β-CD and TPYA, adhensive hydrogel could be prepared as seen in Figure S13, which presented a simple method for preparing integrated gels. When the fresh cut pieces of PAA/TPYA and PAA/β-CD hydrogels were contacted, they would adjoin with each other. The molecule transport via hostguest inclusion interaction on the interface of the two gel blocks could be seen clearly from the interfaces between two gels in dark irradiated by 365 nm.52 Amazingly, we found that this PAA/TPYA hydrogels could undergo obvious color changes from white to red irradiated by sunlight or UV light. As seen from Figure 1a, the color of white PAA/TPYA hydrogels (CTPYA: 0.0075 mol/L) gradually transformed to red when triggered by UV light for 30 min. With increasing concentration of TPYA (CTPYA: 0.015 mol/L), the time for the hydrogel that undergone photochromic transformation from white to purple needed more than 1 h (Figure 1b). However, once the hydrogel became purple after light or heat irradiation for over 30 min, it was clearly 6


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seen that the color continued to turn darker with increasing time even under dark. Finally, the color of the hydrogel turned to dark purple. Notably, if the irradiated time was less than 25 min, the color of the radical gel could be recovered in the dark except for the edge on the gel (Figure S14a-14d). Interestingly, the light responsive properties of PAA/TPYA hydrogel could be remotely controlled. For example, when the PAA/TPYA hydrogel was wrapped by PAA/β-CD hydrogel, it was also able to undergo significant color changes from white to red triggered by UV light (Figure 1c). While, no color change was observed for PAA hydrogel or PAA/β-CD hydrogel irradiated by light. The significant color changes from white to purple could be also obtained efficiently by putting the gel into hot water for minutes (less than 20 min, Figure 1d). All the resulting hydrogels with deep colors were stable for months and highly tolerant to oxygen and light. Similar to previous reports, the color change might be caused by the generation of pyridine radicals due to the electron deficiency of pyridine motifs when they bonded with -COOH groups through hydrogen bonding interactions.48,51-52 As shown in Figure 1e, the in situ UV-irradiated hydyrogel showed a noticeable single-peak radical EPR (electron paramagnetic resonance) signal at g=2.0036 with multiple peaks, which corresponded with the peak of organic radicals.53-55 The multiple peaks might be attributed to coupling effect of adjacent pyridine groups of terpyridine segments. Whereas, the original PAA/TPYA hydrogel exhibited no EPR signal when triggered by light stimuli. Meanwhile, UV-vis spectra displayed a novel absorption band ranging from 450 nm to 715 nm, which informed the existence of terpyridine-based species (Figure 1f). To further confirm the vital role of terpyridine units in the hydrogel networks for the radical production, the terpyridine derivatives 1-4 (Scheme S2) were also incorporated into the PAA hydrogel network instead of TPYA. The results showed that these PAA hydrogels containing the amphiphilic terpyridine derivatives (compound 1 and 2 in Scheme S2) also exhibited similar photochromic properties. On the other hand, using methacrylic acid instead of acrylic acid, the resulting polymethacrylic acid/TPYA hybrid mixture was very hard and dense, which was unfavorable for radical transport. Therefore, only the edge of gel became red (Figure S14e, 14f). Without hydrogen bonding interaction, there was no obvious color change for polyacrylamide (PAM)/TPYA hydrogel triggered by light or heat. To indicate that the photochromism happened in ordered gel assembly, the following experiments were also performed. 1) When the suspension of acrylic acid/TPYA (CAA: 4×10-3 M; CTPYA: 4×10-3 M) in water was triggered by light or heat, no color change was observed; 2) When exposed directly to UV irradiation, the ZnF2/TPYA hydrogel also exhibited photochromism with color changes from white to red (Figure S15). Notably, the gradually losing of water was important for such color change, finally 7



leading to red solid. While, color changes were not observed when the hydrogel was put in a closed test tube followed by irradiation without losing of water; 3) Also, there were no notable changes when the diluted solutions of the Zn2+-TPYA complexes (105 M) were irradiated by light. These revealed that the 3D gel matrix facilitated the charge transfer from environment to terpyridine motifs, leading to production of terpyridine-based radicals. While, diluted solution was unfavorable for the radical generation. time

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Figure 1. (a-c): The color and fluorescent changes of PAA/TPYA hydrogel triggered by UV light in different conditions, for (a) CTPYA=0.0075 mol/L; for b) CTPYA=0.015 mol/L; (c) The PAA/TPYA hydrogel (CTPYA=0.015 mol/L) was wrapped by PAA/βCD hydrogel; (d) The color change process for PAA/TPYA hydrogel (CTPYA=0.0075 mol/L) in boiling water, and then the resulted gel was kept in sealed and water environment for 3 days; (e) EPR spectrum for the radical hydrogel; (f) Absorption spectra of the PAA/TPYA hydrogel and the radical hydrogel. Based on the above results, the possible scenarios for the generation of terpyridinebased centered radical could be illustrated as following: just like the chemical structures of bipyridinium or Zn-pyridine complexes (For example MV and ZnV derivatives),48, 53-55 the electropositivity of pyridine motifs were greatly enhanced when incorporating them orderly into PAA hydrogel networks via supramolecular interactions. The resulting pyridine motifs in the junctions of networks were prone to accept electron from the environment in order to form radicals. Additionally, the gradual color changes from red to dark purple even in dark revealed that once the reaction was inspired at the edge of the gel, 3D gel matrix supplied an efficient channel for the electron transport, which endowed spontaneous radical reaction. Finally, it was presented that the hybrid gel network had the radical stabilized character and gave rise to the stability of the photo-activated state. Two reasons might be responsible for the stable radical species: 8


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1) the ordered stacking of terpyridine motifs was favorable for the charge dispersion; 2) the shielding effect of hydrogel reduced the access of oxygen to unpaired electron. To assess the impact of TPYA and radical on the mechanical properties of PAA hydrogel, the frequency sweep experiments of these hydrogels were carried out and compared. As seen from Figure S16, the G' for the as-prepared PAA gels was around 1626 Pa. When TPYA was inserted into the gel networks, the G' of PAA/TPYA hydrogel reached about 4484 Pa. Furthermore, when radicals generated, the G' of radical hydrogel improved significantly to about 33351 Pa by an order of magnitude. Such results indicated that the PAA/TPYA hydrogel was greatly strengthened after production of radicals, which might be due to the enhanced hydrogen bonding interaction (where the radical terpyridine group had more electron density as hydrogen bonding donor) and attraction interaction among radical molecules. 3.4 Self-healing properties of PAA/TPYA radical gels Intriguingly, we observed that the as-prepared radical hydrogel displayed outstanding self-healing properties. When the two cut blocks of radical hydrogel were contact with each other, they would join together to form a self-supporting bar with approximately 3 h. With time along for 24 h, the color of the bar transformed to dark purple with increasing generation of radicals on the healing interface. It was clearly seen that the joints between the two blocks were strong enough to sustain stretching (about twice times as the initial length), in which the continuous interface radical interaction might offer chances to enhance the interaction between two blocks (Figure 2a-2f). Additionally, when the cut TPYA/PAA hydrogel pieces were contacted with the radical hydrogel block, the radical species could be transferred efficiently to the hydrogel pieces by the chain reaction of radicals. Finally, they adjoined with each other again, revealing another approach for preparing self-healing gels (Figure 2g-2m). Moreover, the deformation recovery was further determined by rheology experiments (Figure S17). When the continuous high strain 150% was applied, the radical hydrogel decreased from about 33351 Pa to about 20583 Pa. The G' value of the gel recovered immediately to its initial state again less than 200 s when the strain decreases to 0.1 %. Such process could be repeated for many times.

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Figure 2. Photos of the healing hydrogels from cut hydrogel blocks. (a) The hydrogel (CTPYA: 0.015 mol/L) was irradiated by UV light for 2.5 h; (b) cut pieces; (c) and (d) The healed hydrogel in water environment; (e) The healed gel after staying for 24 h; (f) The gel of (e) under stretching; (g) Photos of radical PAA/TPYA hydrogel (irradiated by UV light for 2 h and cut pieces of PAA/TPYA hydrogels (CTPYA: 0.015 mol/L); (h) Gel blocks were contacted; for (i-m): The contact hydrogel blocks stayed in water environment along with different times in dark. (i) 24 h; (j) 36 h; (k) 48 h; (l) and (m) 72 h. Moreover, the red radical gels were also self-supporting and strong enough that could be molded into various geometrical shapes, such as alphabets, hearts, circle, pentagram, and even helical fibers (Figure 3a, 3b). Notably, the radical gel was easy to be knotted and showed obvious color changes under stretching (Figure 3c, 3d). Such results indicated the outstanding mechanical properties of the radical hydrogels.

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Figure 3. (a) and (b) The different geometrical shapes of radical PAA/TPYA hydrogels (CTPYA: 0.015 mol/L, irradiated by UV light for over 2.5 h; scale bar for (a) 1 cm, (b): 1 cm; (c) and (d): The knotted radical PAA/TPYA hydrogel under stretching. 3.5 Stimuli-responsive properties of PAA/TPYA radical gels The color changes under stretching inspired us to study the possibility of radical hydrogel for constructing color switches. First, we found that the dark purple radical gels could be stretched up to three times as their initial length, together with color changes from dark purple to purple. When the stretching was released, the shape and color repaired to the initial state again quickly. Such process could be repeated for many circles without fatigue. Color switches controlled by alternating the stretching and losing of stretching was also examined by UV spectra (Figure 4a-4c). The dark purple radical gel exhibited absorption value at 2.28 (λmax=575 nm), which decreased to 0.62 by 3.68-fold when the gel suffered stretching (Figure 4d). The absorption value changes could be repeated by many times with a difference smaller than 10%. Although there are some visualized sensors of xerogels toward physical stimuli such as pressure or grinding,56 as far as we know, this is the first paradigm that hydrogel is applied to visually and reversibly sense stretching stimuli.

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Based on the reversible supramolecular hydrogen interaction of -COOH and terpyridyl units that was necessary for radical generation, herein, the radical hydrogel could be developed to selectively respond to ammonia as a kind of chemical stimuli. The colorimetric change of hydrogels after incubating the radical gels in solvents for 16 h was seen in Figure S18. Ammonia brought about a significant dark purple-tocolorless color changes of radical hydrogels, together with the volume expansion for 45-fold. Such significant hydrogel swelling was rationally due to the repulsive force when radical terpyridyl units lose electrons.57-58 Especially, the gel could retain its initial shape during the volume expansion changes (Figure 4e-4h). After irradiation by light, the swelling gel could become red again. Otherwise, the radical hydrogel displayed weaker swelling trend and color changes in the solvents of short alcohols and THF, acetone and dioxane (Figure S18). These changes did not occur when other organic solvents such as CH2Cl2, ethyl acetate and amines especially aliphatic amines (Et3N, allyamine et al) were used. Aliphatic amines were regard as the base that had stronger alkalinity than that of ammonia, however, the experiments showed that they could not penetrate into the gel networks, which might be due to the bigger volume of these molecules. These clearly indicated that the radical hydrogels displayed high capacity to visually and selectively discriminate ammonia among others. Notably, the hydrogen bonding interaction competition between -COOH and terpyridine and -COOH and NH3 played an important role for such obvious changes in optics.51 From the above results, our findings about the functional assembly of PAA and TPYA could be summarized as following (Figure 5): firstly, TPYA could be coassembled orderly with PAA network through hydrogen bonding and hydrophobic interactions. Upon photo irradiation, the electron deficient terpyridine group which bonded with –COOH of PPA was able to obtain electron from environment, in order to form radicals in gel networks; Subsequently, the radical molecules would attract with each other,57-58 resulting in darkening colors; meanwhile, the hydrogen bonding strength was also enhanced due to the more electron donating ability of radical terpyridine unit, finally leading to robust and malleable gels. It was presented that the π-π stacking of terpyridine radicals had the dynamic and reversible characteristics, which were very sensitive to stretching with fast color changes. As a result, the introduction of ammonia into the hydrogel destroyed the hydrogen bonding interaction of TPYA and PAA, leading to the disappearance of radicals and strong repulsion of molecules with significant volume expansion. Finally, the strong self-healing properties of radical hydrogel could be also deduced: the radical reaction between the surfaces of cut pieces of PAA/TPYA hydrogels with enhanced intermolecular interactions endowed the healing ability of them, which supplied a novel way for preparing robust 12


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and self-healing materials based on continuous radical reaction. Therefore, based on the above results, we proposed that the structure of terpyridine-based radical was similar to that of viologen radicals (ZnV or MV, Figure 5c). Notably, the radicals were neutral because that only hydrogen bonding existed between terpyridine unit and PAA, which was different from that of ZnV and MV compounds where coordination and ionic band existed (Figure 5c). Meanwhile, the radicals were very stable with efficient electron transmission in gel networks. To the best of our knowledge, this is the first report about the preparation of terpyridine-based radicals. 4. Conclusions In summary, by employing different kinds of supramolecular interactions, we demonstrate an engineering pathway in preparing diverse gels within a hydrogelation landscape. We have presented the intended generation of varies of hydrogel materials starting from one initial terpyridine derivative. More importantly, we explore for the first time a new and universal strategy for preparation of supramolecular radicals stemming from the complexation of poly(acrylic acid) with terpyridine derivatives in 3D gel matrix. The radicals are usually stabilized by spin delocalization or steric protection, we have reported here that the reversible radical generation and stabilization could be both performed in polymeric gels via a supramolecular approach, where the radical π units assemble with each other. The resulting radical hydrogels display superior mechanical properties and sensing behavior toward both stretching and ammonia. Therefore, the work not only supplies a novel approach for easily preparation of light responsive supramolecular gels, but also opens a new way for preparing robust and self-healing hydrogels containing enhanced intermolecular interactions originated from the generation of terpyridine-based radicals. C H

n O H

N

a

H C C H

O N

.N

N

H C n O H

O N

N

.

R

e. C H

R

TPYA

heat PAA hydrogel

b

PAA/TPYA hydrogel

Radical hydrogel

Expanded hydrogel

e radical reaction

TPYA

Radical TPYA

-COOH of PAA

c C H

H C

TPYA

C H

n O H

O

Transparent gel

N

H C n O H

O N

C H

UV or heat

.N

H C n O H

N+

O N

N

R Radical gel

R Opaque gel

13


N+

MV

.

.

N

n

O O NH4+

NH3

light or

H C

2+

N Zn2+

Zn N

ZnV


Figure 5. (a) The possible mechanism for the generation of radical hydrogel and responsive ability toward ammonia; (b) Illustration for the self-healing hydrogel via radical reaction approach; (c) The proposed radical structure of TPYA-PAA complex vs reported viologen radical cation precursors (MV and ZnV structures). Acknowledgements Yu and coworkers appreciate for the financial support of NNSFC (No. 21401040, 21771051), High-level talent project of Hebei Province (No. 2016002014), Natural Science Foundation of Hebei Province (No. B2016208115), Excellent Youth Funding of Hebei Province (No. B2018208112) and Young Talent Plan of Hebei Province. Supporting Information Additional data including synthesis details for both organic molecules and gels, SEM images, photos (metallogels, polymer gels and radical gels), other titration experiments for Fluorescent and 1H NMR experiments, as well as rheology tests. These details are available on line via the Internet at http://pubs.acs.org. Notes and references (1) Zhang, X. W.; Wang, J.; Jin, H.; Wang, S. T.; Song, W. L. Bioinspired Supramolecular Lubricating Hydrogel Induced by Shear Force. J. Am. Chem. Soc. 2018, 140, 3186-3189. (2) Yang, Z. M.; Liang, G. L.; Wang, L.; Xu, B. Using a Kinase/Phosphatase Switch to Regulate a Supramolecular Hydrogel and Forming the Supramolecular Hydrogel in Vivo. J. Am. Chem. Soc. 2006, 128, 3038-3043. (3) Li, L.; Yan, B.; Yang, J. Q.; Huang, W. J.; Chen, L. Y.; Zeng, H. B. Injectable Self-Healing Hydrogel with Antimicrobial and Antifouling Properties. ACS Appl. Mater. Interfaces 2017, 9, 9221-9225. (4) Li, Z. Q.; Wang, G. N.; Wang, Y. G.; Li, H. R. Reversible Phase Transition of Robust Luminescent Hybrid Hydrogels. Angew. Chem. Int. Ed. 2018, 57, 2194-2198. (5) Hu, K.; Sun, J. F.; Guo, Z. B.; Wang,P.; Chen, Q.; Ma, M.; Gu, N. A Novel Magnetic Hydrogel with Aligned Magnetic Colloidal Assemblies Showing Controllable Enhancement of Magnetothermal Effect in the Presence of Alternating Magnetic Field. Adv. Mater. 2015, 27, 2507-2509. (6) Alizadehgiashi, M.; Khuu, N.; Khabibullin, A.; Henry, A.; Tebbe, M.; Suzuki, T.; Kumacheva, E. Nanocolloidal Hydrogel for Heavy Metal Scavenging. ACS Nano. 2018, 12, 8160-8168. (7) Song, M. M.; Wang, Y. M.; Wang, B.; Liang, X. Y.; Chang, Z. Y.; Li, B. J.; Zhang, S. Super Tough, Ultrastretchable Hydrogel with Multistimuli Responsiveness. ACS Appl. Mater. Interfaces 2018, 10, 15021-15029.

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Table of Contents graphic supramolecular radical species

C H

.N

H C n O H N

O N

.

.

R

stretchable radical hydrogel

20


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Hydrogelation landscape engineering and a novel strategy to design

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