Article pubs.acs.org/IC
Fluorescent and Electrochemical Supramolecular Coordination Polymer Hydrogels Formed from Ion-Tuned Self-Assembly of Small Bis-Terpyridine Monomer Xudong Yu,* Zengyao Wang, Yajuan Li, Lijun Geng, Jujie Ren, and Guoliang Feng* College of Science, and Hebei Research Centre of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, P. R. China S Supporting Information *
ABSTRACT: Herein, ditopic ligand DTA comprised of terpyridine and acetylene segments with only one aromatic π-conjugated building block was designed and synthesized. Driven by metal−ligand coordination interactions, we presented that the use of metal salts can direct the selfassembly of DTA in the generation of fluorescent and electrochemical polymers that entrapped water to form ambidextrous hydrogels. These were characterized by several approaches including fluorescent titrations, UV−vis, circular dichroism, and X-ray diffraction spectra as well as scanning electron microscopy and transmission electron microscopy experiments. DTA can selectively recognize Zn2+ ions and gelate water in the presence of ZnC6H10O6 (zinc lactate), giving Zn2+specific fluorescent metallogels. Otherwise, DTA/Cu(OAc)2 forms nonfluorescent, electrochemical, and chiral hydrogel that responds to multiple stimuli such as heat, light, shearing, electrolysis, and reducer. The ion-controlled gelation approach, morphology, rheology, as well as fluorescent and chiroptical properties of DTA was studied in detail. Hence, our work demonstrated for the first time the crucial role of metal salts in the supramolecular polymerization and corresponding properties, in which symmetry breaking played an important role for the dynamic assembly difference. highly desirable for construction of photoelectric materials.26 Moreover, although metal−ligand bonds can be facilely adjusted by varying metal ions in solution state, the control of both the formation and corresponding function by changing metal−ligand interaction in hydrogel platform has not yet been reported. The self-assembly of 2,2′:6′,2″-terpyridine (tpy) in medium has recently received notable attention due to their applications as building blocks toward self-assembly into gels, functional interface assemblies, well-defined architectures, and crystals.27−32 Ditopic or tritopic terpyridine-based ligand is especially attractive in the area of fabricating stimuli-responsive gels. For example, Zhukhovitskiy et al. prepared the robust and highly branched polymeric metallogels in dimethyl sulfoxide (DMSO) based on ditopic terpyridine-based ligand; 33 Bellemin-Laponnaz et al. demonstrated the light-powered and self-healable metallo-supramolecular gel in chloroform.34 However, most of the ditopic bis-terpyridine ligand formed metallogels in organic media due to the poor solubility of them in water, and it is still a challenge that the self-assembly of ditopic bis-terpyridines can gel in pure water. Moreover, in aspect of the functions of tpy derivatives, luminescence soft materials of these metallogels were well-recognized in the past
1. INTRODUCTION The preparation of supramolecular hydrogels that are able to respond to external stimuli, such as pH, light, redox, and ions, has attracted significant interest due to their wide applications in biomaterials, drug delivery, optoelectronics, and hybrid materials.1−13 Recently, self-assembly of metallopolymer based on dynamic metal−ligand coordination interaction is a promising way to construct stimuli-responsive gel in aqueous media with perspectives in magnetic, optical, redox, electrochemical, and mechanical properties.14−18 In these gel systems, the different valence states and coordination geometries of metal centers19 and modification of ligand structures20 allow the fabrication of controllable gels in both properties and the corresponding functions. A wide variety of structural ligands have been designed and illustrated in constructing threedimensional networks via metal−ligand coordiantion that can entrap solvents to form gels.21−23 For example, Nitschke reported the polymeric hydrogel formation of pyridine-based Schiff-base derivatives cross-linked by metal−organic cages;24 T. Gunnlaugsson demonstrated the mechanically enforced and fluorescent metallogels of pyridine-2,6-dicarboxylic acid derivative.25 Currently, however, most of the ligands contain hydrogenbonding sites and hydrophobic/hydrophilic groups besides the coordinating segment; very few works focus on the design of functional ligand with pure aromatic π-conjugated unit, which is © XXXX American Chemical Society
Received: April 23, 2017
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DOI: 10.1021/acs.inorgchem.7b01031 Inorg. Chem. XXXX, XXX, XXX−XXX
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Scheme 1. Chemical Structure of DTA, Illustration of the Metallo-Copolymer Gel Triggered by ZnC6H10O6 and Cu(OAc)2, and Photos of Metal-Based Polymeric Gels in Lighta and in Dark
a
Irradiated by 365 nm.
decades.21,22 For example, Maji has successfully acquired whitelight-emitting-diode based on lanthanide polymeric gels.35 However, functional ligands that can form both fluorescent and electrochemical hydrogels are still scarce in present literatures. In previous works, we mainly focused on the development of organogels and hydrogels constructed based on small organic gelators with multiple stimuli-responsive properties.36−42 Herein, we report the design and preparation of Zn2+- and Cu2+based metallo-supramolecular polymer hydrogels with tunable properties and functions based on ditopic terpyridine-acetylene ligand (DTA) (Scheme 1) bearing only one aromatic πconjugated building block. Introduction of diacetylene moiety into the backbone of the ditopic ligand can endow the gel with light-responsive properties because of the polymerization interaction of diacetylene in gel tissue. DTA forms Zn2+specific fluorescent hydrogel among test ions. Otherwise, in the presence of Cu2+, DTA forms chiral, injectable, and electrochemical gel just at room temperature, which shows multistimuli-responsive properties such as UV light, thioxtropy, redox, and heat. We believe that the metal salt controlled gelation approach and that the corresponding properties will pave a new way for constructing novel functional polymeric soft materials.
source. The X-ray diffraction (XRD) data of xerogels of DTA/metal salt aggregates were obtained by a Bruker AXS D8 instrument (Cu target; λ = 0.1542 nm, Germany). UV−vis absorption spectra were obtained on a UV−vis 2550 spectroscope (Shimadzu) just at room temperature. Rheological experiments of these hydrogels were measured using a controlled stress rheometer (Malvern Bohlin GeminiHRnano); a vertebral plate geometry of 24 mm was used for the experiments.
3. RESULTS AND DISCUSSION The synthesis and characterization of DTA could be seen in the Supporting Information. The gelation studies of DTA in a test tube were performed in water. The ligand DTA did not dissolve in water even by heating the mixture to 100 °C, indicating the insoluble property of DTA in water with pure π-conjugated structure. Accidentally, the mixture became transparent sol in the presence of ZnC6H10O6 (zinc lactate, 0.8−1.5 equiv of DTA) at 90 °C, which changed to opaque and fluorescent gel with yellow color after cooling within seconds. Otherwise, the addition of Cu(OAc)2 (>0.7 equiv of DTA) triggered the room-temperature hydrogelation of DTA just at room temperature; Figure S1 showed the gradually gelation process of DTA with Cu(OAc)2. The ion-triggered gelation process was also thermoreversible as observed by the inversed test tube method. The above results indicated the strong, reversible, and dynamic coordination interaction between metal ions and DTA, and the coordination of DTA to Cu2+ or Zn2+ caused great changes in the charge polarization of DTA, leading to cross-linking of ligand DTA, finally resulting in hydrogels rather than precipitate or solution. UV−vis and fluorescent experiments were performed to characterize the specific self-assembly behavior of DTA with ions. We first checked the metal−ligand interaction of DTA with ions in a diluted aqueous medium. The DTA solution displayed three absorption peaks at 249, 285, and 330 nm, which became broader and red-shifted when coordinated with Cu(OAc)2 or ZnC6H10O6 ions (Figure 1), which suggested the metal−ligand coordination interaction. In gel state, the three absorption peaks of DTA/ZnC6H10O6 gel displayed blue shift in comparison to that of the diluted solution of DTA/ ZnC6H10O6 (diluted from the corresponding gel), indicating the H aggregate of DTA/ ZnC6H10O6 assembly (Figure S2), whereas there was no obvious difference between DTA/ Cu(OAc)2 gel and the corresponding diluted solution.
2. EXPERIMENTAL SECTION Materials. The used chemical materials in this work were purchased from commercial companies and used without further purification. 4-Vinylpyridine, 4-bromobenzaldehyde, Cu(OAc)2, Na2S, AgNO3, trimethylsilylacetylene, bis(triphenylphosphine)palladium(II) dichloride, ethylenediaminetetraacetic acid (EDTA), KI, and other metal salts for gelation tests were obtained from Shanghai Darui fine chemical Co. Ltd. Instrumentation. Gel morphologies were measured by using FESEM S-4800 instruments (Hitachi) and a JEOL JEM2011 apparatus operating at 200 kV. Samples for scanning electron microscopy (SEM) tests were prepared by coating a little of the gel samples on glass slides and then coating them with Au. The samples for transmission electron microscopy (TEM) tests were obtained by putting a small amount of diluted gel on a carbon-coated copper grid. All the NMR spectra were performed on a Bruker Advance DRX 400 spectrometer operating at 500 and 125 MHz. The mass spectra of organic molecules were measured on a PlasmaMS 300 instrument. Fluorescence spectra of DTA solution and aggregates were performed on an Edinburgh instrument FLS-920 spectrometer by using a Xe lamp as an excitation B
DOI: 10.1021/acs.inorgchem.7b01031 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Absorption spectra of DTA solution (1 × 10−4 M) and the solution upon the addition of Zn C6H10O6 or Cu(OAc)2 in water (λex = 370 nm). Considering the solubility and polarity of DTA, the experiments were performed in the solvent mixture of THF and water (v/v = 4:1).
Seen from Figure 2a, DTA solution (1 × 10−4 M) displayed emissive peak at 410 nm with a shoulder at 435 nm, which quenched upon the addition of most of the metal salts (50 equiv) except for Zn2+ ions. In the presence of Zn(OAc)2 or ZnC6H10O6, the peak at 410 nm disappeared, and a new broad peak banded at 452 nm arose, which was accompanied by fluorescence enhancement in blue emission color. Figure 2b showed the gradual fluorescence changes of DTA with the increasing concentration of ZnC6H10O6. This result indicated that DTA behaved as selective and “off-on” fluorescence chemosensor toward Zn2+ ion in diluted solution among test ions. In aggregation state with higher concentrations such as 25 mg/mL, the fluorescence emission spectra of DTA/Zn2+ assembly also had relevance to the counterions and phase states. Seen from Figures 3a,b and S3, only ZnC6H10O6 can trigger the gelation of DTA by heating−cooling process in pure water, together with fluorescence enhancement and obvious red shift of the fluorescent peak. The addition of other zinc salts led to suspensions rather than gels, together with fluorescence quenching except for ZnBr2. The above results suggested the specific and selective interaction of DTA toward ZnC6H10O6. On the contrary, when DTA was coordinated with Cu2+, the efficient energy and electron transfer process of terpyridine group was inhibited, resulting in fluorescence quenching phenomena.
Figure 3. (a) The fluorescentce spectra of DTA assembly (25 mg/ mL) in pure water upon the addition of different Zn2+ salts (λex = 370 nm, 1 equiv). For ZnC6H10O6: gel state; for other salts: suspension; (b) the intensity value of DTA and DTA in the presence of Zn2+ salts at λem = 543 nm.
The metal salts also had obvious effect on the macromorphology of the polymeric assembly. TEM and SEM experiments revealed that DTA/ZnC6H10O6 hydrogel was comprised of sheet structure (Figure S4), and DTA with Cu(OAc)2 spontaneously self-assembled to nanospheres with average diameter of ∼70 nm (Figure S5). On the contrary, no regular and homogeneous morphologies were observed in both DTA/Zn(OAc)2 and DTA/zinc gluconate suspensions (Figure S6). The above results presented that the ordered assembly of DTA/ZnC6H10O6 polymer in three-dimensional networks can restrict the molecular rotations, prohibit the aggregation of DTA/ZnC6H10O6, and reduce the energy dissipation, thus facilitating the efficient intramolecular charge transfer (ICT) process from terpyridine unit to Zn2+ ion. Notably, the counteranion was also an important factor for the gelation
Figure 2. (a) Fluorescence spectra of DTA (1 × 10−4 M) upon the addition of metal ions (50 equiv). (photos, inset) left: DTA solution (1 × 10−4 M), right: DTA solution upon the addition of ZnC6H10O6. (b) Fluorescence titrations of DTA (1 × 10−4 M) with ZnC6H10O6. C
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Figure 4. (a) Photos of DTA/Cu(OAc)2 hydrogels with multistimuli-responsive properties; (b) CV curves of DTA/Cu(OAc)2 hydrogel; (inset) photos of injectable and viscous gel; (c) CD spectra of DTA/Cu(OAc)2 hydrogels (12.5 mg/mL) before and after irradiation by light (λ = 355 nm).
process.43−45 Herein, it was deduced that the intermolecular interactions of anions such as van der Waals force and hydrogen-bonding interactions was also favorable for the spontaneous assembly and gelation process of DTA molecules (Table S1). Amazingly, the resulting DTA/Cu(OAc)2 hydrogel showed excellent multistimuli-responsive properties (Figure 4a). By heating or strongly shaking, the gel transformed into a sol. After staying for less than 10 min, the sol became gel again, indicating the thixotropic property and thermal reversibility of the metallogel. Moreover, the reversible gel phase changes could be also controlled by electrolysis process. From Figure 4b, the cyclic voltammetry (CV) curve of DTA/Cu(OAc)2 solution exhibited two oxidation and redox peaks, which increased and changed to one peak in the gel sate, indicating the complete coordination of pyridyl segment with Cu2+. By putting the anode into DTA/Cu(OAc)2 hydrogel and applying current for 100 min, the gel completely collapsed, together with obvious color changes from blackish green to green, presumably because of the reduction of Cu2+-DTA complex to Cu+-DTA complex (Figures S7 and S8). The gel reformed when placing the green precipitate into cathode followed by applying current. Such reversible gel to precipitate process could be repeated at least for three circles. However, the color could not revert to the initial state completely. This reversible gel-to-suspension transition can also be adjusted by chemical oxidants and reducers. For example, when an equal amount (molecular ratio) of sodium ascorbate was added on the gel surface, it was observed that the gel gradually turned into an ocher-yellow suspension after few minutes. Afterward, the gel formed again upon the addition of potassium peroxydisulfate. Further study showed that DTA/Cu(OAc)2 hydrogel could also respond to KI and EDTA with gel-to-precipitate transition, due to the formation of CuI2 (which further changed to CuI and I2) and Cu2+-EDTA complex, respectively (Figure S9). However, the process was irreversible. The above results revealed the multiple stimuli-responsive properties of DTA/Cu(OAc)2 hydrogel with phase and color changes.
Metal ions not only had impact on the gelation approaches and stimuli-responsive properties of DTA but could also affect the chiral properties of the hydrogels. Seen from Figures 4c and S10, selective ion gelation that induced supramolecular chirality from achiral component was observed in the hydrogels. In the diluted solution state, no CD Cotton effect for DTA or DTA/ Cu2+ assembly (diluted from gel) was found (Figure S10a), while the CD spectra of DTA/Cu(OAc)2 hydrogel (12.5 mg/ mL) exhibited three positive peaks at 275, 362, 439 nm and a negative peak at 420 nm, indicating the helical supramolecular assembly, which was ascribed to the chiral symmetry breaking of the coordination polymers.46 The intensity value of the negative peak enhanced with the increasing concentration from 12.5 to 25 mg/mL (Figure S10b), whereas no chiral signal was found in the DTA/ZnC6H10O6 hydrogel. More importantly, the DTA/Cu(OAc)2 hydrogel can respond to light with visual color changes, showing a precise control on the chirality. When the gel was irradiated (λ = 355 nm) for 8 h, the dark green gel and xerogel as prepared turned dark, and the chirality of metallogel was reversed completely (Figures 4c and S11), which might be attributed to the topochemical polymerization reactions of DTA from diacetylene to polydiacetylene in gel networks. As mentioned above, the DTA/Cu(OAc)2 hydrogel showed dual ion- and light-controlled chirality properties, which was scarce in literature. This chirality-reverse phenomenon in gel system represents a typical paradigm that chirality can be controlled by light stimulus through in situ polymerization interaction method. Since supramolecular chirality of artificial systems from nonchiral compound through coordination interactions remains an active area due to their important roles in sensing system, chiral recognition and separation, as well as molecular memory systems,47,48 our finding would be meaningful for constructing ion-controlled chiral soft materials with stimuli-responsive properties. At last, we also examined the ion effect on the gel rheology properties. The static frequency and dynamic strain sweep measurements of the hydrogels could be seen from Figure 5a,b. D
DOI: 10.1021/acs.inorgchem.7b01031 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. (a) Frequency dependency of G′ and G″ for gels of DTA/ion hydrogels (DTA: 25 mg/mL, strain: 0.1%); (b) dynamic strain sweep measurement of DTA/ion hydrogels (DTA: 25 mg/mL, angular frequency at 10 rad/s); (c) recovery test for corresponding DTA/Cu(OAc)2 hydrogels, alternating strain amplitudes of 100% and 0.1%.
Figure 6. Illustration of the hydrogel formation mechanism and ion-tuned gelation properties.
XRD measurements were also performed to better understand the aggregation mode of metal-based supramolecular polymers (Figure S12). The DTA/Cu(OAc)2 and DTA/ ZnC6H10O6 xerogels both showed similar d values, which was tested by three times to confirm the result. The peaks at 1.07 and 1.40 nm were attributed to the monomer length of DTA and DTA/ion pairs, respectively. Moreover, the peaks banded at 1.40, 0.70, and 0.35 were exactly with the ratio of 1:1/2:1/4, indicating the lamellar and order structure of the supramolecular polymers in hydrogel networks. From the above results, our findings could be summarized as follows (Figure 6): DTA could not dissolve in water even by heating, interestingly, in the presence of metal salts such as ZnC6H10O6 and Cu(OAc)2; DTA molecules could be linked by ions through metal−ligand coordination interaction to form more hydrophilic supramolecular polymeric aggregates in water. These aggregates further self-assembled to threedimensional networks that can entrap solvent molecules, finally resulting in metallogels. Moreover, note that π−π stacking interactions among the fluorophores also played important roles in the gelation process.
Both the storage modulus (G′) and loss modulus (G″) of DTA/ZnC6H10O6 hydrogel were much higher than that of DTA/Cu(OAc)2 hydrogel, indicating the stronger coordination interaction between DTA and Zn2+ and “hard” property of DTA/ZnC6H10O6 opaque hydrogel. On the contrary, the strain for the following point (gel-to-suspension transition) of DTA/ Cu(OAc)2 hydrogel was much higher (>100% strain) than that of DTA/ZnC6H10O6 hydrogel (32% strain), indicating the “soft” properties of DTA/Cu(OAc)2 gel that was injectable with continuous phase. Figure 5c showed the recovery experiments of DTA/Cu(OAc)2 hydrogel with reversible shape changes by imposing alternating strain. With the strain at 100%, both G′ and G″ decreased obviously, reflecting the gel deformation under high strain. When the strain was decreased to 0.1%, the gel reconstructed after 200 s, and a quick recovery of ∼80% origin modulus value was observed. Such reversible gel deformation modulated by strain could be repeated for several circles. The above results certified the thixotropic properties of DTA/Cu(OAc)2 hydrogels with dynamic metal− ligand coordination interaction, showing ion-tuned mechanically “hard” and “soft” properties of gels. E
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The binding of DTA with Zn2+ facilitated the ICT process of terpyridine-alkyne segment, resulting in the red shift of DTA both in solution and gel state. In suspensions of DTA assembly with other Zn 2+ salts, the fluorescence quenched by aggregation-caused quenching (ACQ) effect without solvent. In gel state, the efficient energy transfer from DTA to Zn2+ can be attributed to two reasons: (1) the long-range polymerization of DTA with Zn2+ in water inhibited the aggregates of DTA molecules, which accelerated the energy transfer; (2) the molecular rotation was restricted in gel networks, which decreased the energy dissipation. However, when coordinated with Cu(OAc)2, the symmetry breaking happened in the DTA/Cu(OAc)2 aggregates, and the chiral metallopolymer assembly further aggregated to nanosphere structure, leading to injectable and thixotropic hydrogels. The obtained gel was so soft and sticky that it showed multistimuli responsiveness toward both chemical and physical stimuli. The ion salts controlled gelation, and stimuli-responsive properties would open a new way to construct structurally tunable materials with multiple functions.
ACKNOWLEDGMENTS X.Y. and co-workers are very thankful for the financial support from NNSFC (21401040 and 21301047), Natural Science Foundation of Hebei Province (Nos. B2016208115, B2014208160, and B2014208091). Young Talent Plan of Hebei Province and High-Level Talent Project of Hebei Province (2016002014), Key Foundation of Hebei Province Department of Education Fund (ZD2016059).
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01031. Synthesis procedure, photos of gels, SEM and TEM images, and spectra of gels (PDF)
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REFERENCES
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4. CONCLUSIONS In conclusion, supramolecular coordination polymeric hydrogels were obtained from self-assembly of small bis-terpyridine monomer with specific metal salts such as ZnC6H10O6 and Cu(OAc)2. However, other ion salts could not trigger the gelation. DTA can selectively discriminate Zn2+ ions by fluorescence enhancement in blue emission both in solution and gel among test ions. Otherwise, in the presence of Cu(OAc)2, DTA formed injectable, chiral, and electrochemical gels just at room temperature, which showed multistimuli responsive properties including both chemical and physical stimuli such as temperature, thixotropy, light, EDTA, KI, and sodium ascorbate. Therefore, the hydrogelation and corresponding properties such as fluorescence, chirality, and rheology can be modulated just by changing metal salts, which opened a new way for designing hydrogels with multiple needs. Notably, the ditopic ligand was comprised of a πconjugated system without hydrogen-bonding sites or hydrophilic units, which was a typical paradigm for constructing novel π gelators in water. We believe that these dynamic polymeric gels can be used for biologically relevant materials and soft actuators toward external stimuli in the future.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. (X.Y.) *E-mail:
[email protected]. (G.F.) ORCID
Xudong Yu: 0000-0002-9649-5997 Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.inorgchem.7b01031 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b01031 Inorg. Chem. XXXX, XXX, XXX−XXX
