A Linear AIE Supramolecular Polymer Based on a Salicylaldehyde

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A Linear AIE Supramolecular Polymer Based on a Salicylaldehyde Azine-Containing Pillararene and Its Reversible Cross-Linking by CuII and Cyanide Pi Wang,*,† Bicong Liang,† and Danyu Xia*,‡ †

Inorg. Chem. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/30/19. For personal use only.

Ministry of Education Key Laboratory of Interface Science and Engineering in Advanced Materials, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, P. R. China ‡ Scientific Instrument Center, Shanxi University, Taiyuan 030006, P. R. China S Supporting Information *

functionalities.31−36 However, pillararene-based AIE fluorescent supramolecular polymers fabricated by salicylaldehyde azines have never been reported. Herein, a pillar[5]arene dimer (P5D), which is an AA-type monomer, was obtained by introducing a salicylaldehyde azine group into a monofunctionalized pillararene through the CN bond by a facile synthetic route. Therefore, a linear AIE supramolecular polymer was achieved in chloroform by the host−guest interaction between salicylaldehyde azine-containing pillar[5]arene dimer P5D and a homoditopic (BB-type) guest (G), bearing two neutral guests at its two ends, reported by our group.37 Because the salicylaldehyde azine group can coordinate with a CuII ion, the linear supramolecular polymer could be cross-linked with quenching of the fluorescence emission.20,33,38 Meanwhile, the linear AIE supramolecular polymer will recover when cyanide (CN−) was added to the system,39,40 suggesting its reversible cross-linking by CuII and CN− (Scheme 1). First, 1H NMR spectroscopy experiments were performed to study the host−guest interaction between P5D and G. As shown in Figure S6b, compared to P5D and G alone (Figure S6a,c), the peaks related to the protons Ha−Hd of the cyano group part on G shifted upfield, and the peaks related to the protons H1−H7 on P5D shifted downfield, indicating the host−guest interaction between P5D and G. Next, concentration-dependent 1H NMR spectroscopy experiments were carried out to monitor formation of the linear supramolecualr polymer (Figure S7). At low concentrations, the signals for protons Hauc−Hduc of uncomplexed G were clearly observed. As the concentration increased, the peaks of the protons Hauc−Hduc on G became less obvious. With a further increase of the concentration, peaks related to the uncomplexed monomers disappeared, accompanied by a broadening of the main peaks, suggesting the formation of high-molecular-weight aggregates driven by host−guest interactions between P5D and G.28 Two-dimensional diffusion-ordered 1H NMR (2D DOSY NMR) spectroscopy was also used to investigate formation of the linear supramolecular polymer. As shown in Figures S8 and S9, with an increase of the concentrations of P5D and G from 2.50 to 100 mM in chloroform, the measured weight-average diffusion coefficient decreased from 1.05 × 10−9 to 7.08 × 10−11 m2 s−1, indicating the formation of large aggregates.41

ABSTRACT: A linear AIE supramolecular polymer was constructed by a salicylaldehyde azine-containing pillar[5]arene dimer P5D and a homoditopic guest G. The linear supramolecular polymer displayed strong fluorescence at high concentration. It displayed crosslinked structure and fluorescence quenching property after the addition of CuII and recovered after the addition of cyanide. s a new type of fluorescent supramolecular polymer, the aggregation-induced emission (AIE) supramolecular polymer has attracted much attention in the field of advanced fluorescent materials in high concentration or in the solid state, such as fluorescent sensors, bioimaging, and organic lightemitting diodes.1−3 To date, many AIE groups, tetraphenylethylene,4−7 triphenylethylene,8,9 salicylaldehyde azines,10,11 siloes,12−14 azabenzanthrones,15 cyanostilbenes,16,17 and triarylamines,2 have been utilized as building blocks to construct AIEactive functional systems. Among them, salicylaldehyde azines displayed high potential in the application of fluorescent materials because of their convenient syntheses, varied AIE color from green to red depending on the substituents on azines, and high quantum efficiencies at concentrated states.18 However, AIE fluorescent supramolecular polymers based on salicylaldehyde azines have rarely been reported. In addition, the salicylaldehyde azine group also can act as a cross-linker to obtain cross-linked metallosupramolecular polymers through metal coordination with some metal ions, for example, Cu2+ and Zn2+,19,20 which will introduce unique properties, such as catalysis, conductivity, magnetic, bioactivity, and sensitization, benefiting applications in the fields of heterocatalysis, electronics, and biomaterials. 21−26 These two roles of salicylaldehyde azines will facilitate their construction of AIEactive or cross-linked metallosupramolecular polymers. Among various noncovalent interactions that are used to construct supramolecular polymers, host−guest interactions are widely applied because of their unique properties such as high binding ability, stimuli-responsiveness, and special topological structure.22,27,28 Pillar[n]arenes,29,30 a generation of host macrocycles reported in 2008, have shown interesting host− guest binding properties in the construction of supramolecular polymeric materials with various topological structures and

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© XXXX American Chemical Society

Received: October 15, 2018

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DOI: 10.1021/acs.inorgchem.8b02896 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Scheme 1. Chemical Structures of P5D and G and Cartoon Representations of the Linear AIE Supramolecular Polymer and the Reversible Cross-Linking Process of the Linear Polymer by CuII and CN−

Figure 1. AIE property of the linear supramolecular polymer: (a) images of P5D + G at different concentrations under UV light at 365 nm; (b) fluorescence emission spectra of P5D + G at different concentrations. The excitation wavelength was 350 nm.

Importanly, the linear AIE supramolecular polymer can be cross-linked by CuII because of coordination between the salicylaldehyde azine group and CuII ion in a molar ratio of 1:2, accompanied by quenching of the fluorescence.20,38 First, we performed 1H NMR spectroscopy to study the complexation between CuII and P5D. As shown in Figure S12, upon the gradual addition of CuII to the solution of P5D, the peak related to H15 became weak and then disappeared, the peaks related to H12−H14 became weak and broad, and the peaks related to H8 and H9 became broad when the molar ratio of CuII/P5D was 2:1, demonstrating the coordination between CuII and P5D. In addition, the broadening effects of H8−H13 showed the formation of large aggregates, suggesting the linking between P5D molecules.42 In addition, fluorescence titration experiments were carried out to investigate the molar ratio and binding strength between P5D and CuII. As shown in Figures S13−S15, by a mole ratio plot, 1:2 stoichiometry was obtained for the complexation between P5D and CuII. By a nonlinear curvefitting method, the binding strength was estimated to be 9.92 × 104 M−1. Then 1H NMR spectroscopy was used to study the interaction between CuII and the linear supramolecular polymer. As shown in Figure S16a,b, upon the addition of 2.0 mol equiv of CuII to the solution of P5D and G both at 15.0 mM, the peak related to the proton H15 on the salicylaldehyde azine group of P5D disappeared and the peak related to the proton H14 became broad, indicating coordination between CuII and the salicylaldehyde azine group.33 Meanwhile, the signals for the other protons of P5D became broad. These results demonstrated the formation of a cross-linked supramolecular polymer.42 In addition, because cyanide is known to react with Cu ions to form very stable Cu(CN)2 species,39 the linear supramolecular polymer recovered after the addition of tetrabutylammonium cyanide (TBACN; Figure 16c). The signal for the proton H15 on P5D that did not turn back was due to the weak noncovalent interaction between the phenol group of salicylaldehyde azine and the cyanide group.43,44 However, the noncovalent interaction did not affect further coordination between P5D and CuII.40 As shown in Figure S16d, after the further addition of CuII, the peaks related to the protons on P5D and G went back to the cross-linked state, indicating the reversible cross-linking property of the linear supramolecular polymer. Moreover, SEM experiments displayed the cross-linked supramolecular polymer derived from the linear supramolecular polymer in the microscopic structure. The SEM images in Figure 2a,b show an interconnected structure at 15.0 mM of the

In addition, the viscosity of the linear supramolecular polymer was investigated. A double-logarithmic plot of the specific viscosity (Vs) versus concentration was realized, showing a (Figure S10). At low concentrations, a linear relationship between Vs and the concentration was obtained; the slope of the curve is 1.26, indicating noninteracting assemblies of constant size, which was due to the presence of cyclic unimers in dilute solutions. As the concentration increased, a remarkable increase in the specific viscosity was observed as the slope of the curve increased to 2.08, indicating a transition from cyclic oligomers to highly ordered polymers, leading to a restriction of the mobility of the polymeric chains.37 In addition, formation of the supramolecular polymers with high molecular weight and a high degree of linear chain extension by P5D and G in chloroform can be confirmed by scanning electron microscopy (SEM) experiments. As shown in Figure S11, it can be observed that, from a high concentration solution, a rodlike fiber with a diameter of ∼0.5 μm was obtained. This result provided further evidence for formation of the supramolecular polymer. Interestingly, the supramolecular polymer was endowed with the AIE feature by the salicylaldehyde azine group. As shown in Figure 1 a, the fluorescence emission became strong with an increase of the concentrations of P5D and G in chloroform. In addition, after drying, the fluorescence became stronger intensively owing to the closer aggregation of the salicylaldehyde azine groups.20 The absolute fluorescence quantum yield of the dry sample of the AIE supramolecular polymer was 16.3% using the integrating-sphere method.10,20 Fluorescence emission spectroscopy experiments were carried out to investigate the AIE property of the linear supramolecular polymer. As shown in Figure 1 b, with an increase of the concentrations of P5D and G, the maximum fluorescence emission at around 510 nm ascribed to the salicylaldehyde azine group10 enhanced remarkably, suggesting the AIE property of the supramolecular polymer. B

DOI: 10.1021/acs.inorgchem.8b02896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

CuII (Figure 3b), while upon the addition of CN−, the solution turned to light brown with the recovery of fluorescence (Figure 3a) and the maximum fluorescence emission enhanced with a shift phenomenon of the wavelength due to the weak noncovalent interaction between CN− and P5D, which was caused by hydrogen bonding between the phenol group of P5D and the N atom of CN−.43 This weak noncovalent interaction affected the excited-state intramolecular proton transfer of P5D,45 leading to a shift of the maximum fluorescence emission.46 To confirm the noncovalent interaction between P5D and CN−, 1H NMR and fluorescence emission spectroscopy experiments were performed. As shown in Figure S19 , after the addition of CN− to the solution of the linear supramolecular polymer, the signals for the proton H15 of P5D disappeared, indicating the interaction between P5D and CN−. In addition, As shown in Figure S20, upon the addition of 2.0 mol equiv of CN − to the solution of the linear supramolecular polymer, the maximum fluorescence emission at around 510 nm shifted to around 540 nm, confirming the interaction between P5D and CN−. These results confirmed the reversible cross-linking property of the AIE supramolecular polymer. In summary, a linear AIE supramolecular polymer constructed by a salicylaldehyde azine-containing pillar[5]arene dimer P5D and a homoditopic guest G was designed and prepared. The linear supramolecular polymer displayed strong fluorescence at high concentration. After the addition of CuII, the linear supramolecular polymer changed into a cross-linked supramolecular polymer accompanied by quenching of the fluorescence. After the further addition of CN−, the linear AIE supramolecular polymer recovered. This cross-linkable AIE supramolecular polymer fabricated by salicylaldehyde azinecontaining pillararene dimer was first reported and suggested as a promising candidate for advanced material such as metallogels, sensors, and adaptive coatings.

Figure 2. SEM images of the morphology of the gold-coated crosslinked supramolecular polymer from the solutions of P5D + G and CuII: (a) P5D + G (15.0 mM) and CuII (30.0 mM); (b) enlarged image of part a; (c) equimolar mixtures of P5D + G (5.00 mM) and CuII (10.0 mM); (d) enlarged image of part c.

monomers P5D and G, and parts c and d of Figure 2 show a three-dimensional network structure at 5.00 mM, indicating formation of the cross-linked supramolecular network owing to aggregation of the linear supramolecular polymeric chains through CuII coordination.22,42 Moreover, fluorescence emission spectroscopy experiments were carried out to study the reversible cross-linking property of the AIE supramolecular polymer by CuII and CN−. First, the reversible CuII-induced cross-linking phenomenon of the supramolecular polymer is presented in Figure 3 a. After the addition of CuII, the yellow solution of the linear supramolecular polymer became dark brown with quenching of the fluorescence. The fluorescent emission spectra of the linear supramolecular polymer showed a remarkable decrease of the maxmium fluorescence intensity at around 510 nm after the addition of

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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.inorgchem.8b02896. Syntheses, characterizations, and other materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Pi Wang: 0000-0002-8803-7953 Danyu Xia: 0000-0001-6575-6448 Notes

The authors declare no competing financial interest.

Figure 3. Reversible cross-linking property of the AIE fluorescent supramolecular polymer by CuII and CN−: (a) images of the crosslinking property of P5D + G under sunlight and UV light at 365 nm; (b) fluorescence emission spectra of the cross-linking property of P5D + G with the addition of CuII and CN−. The excitation wavelength was 350 nm. The concentrations of P5D and G were both 40.0 mM. The amount of CuII was 2.0 mol equiv of P5D, and that of TBACN was 4.0 mol equiv of P5D.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation for Young Scientists of China (Grant 21704073) and the Natural Science Foundation for Young Scientists of Shanxi Province, China (Grant 201801D221078). C

DOI: 10.1021/acs.inorgchem.8b02896 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02896 Inorg. Chem. XXXX, XXX, XXX−XXX