The fabrication of supramolecular assembly with quadruple switchable fluorescence by ionic self-assembly strategy
Graphical abstract
The switchable assembly was fabricated by ionic self-assembly strategy, the morphological alteration was triggered by β-cyclodextrin (β-CD) inclusion and oxidant. Furthermore, the supramolecular fluorescence switch exhibited quadruple fluorescence tiers.
Introduction
Multi-stimuli responsive functional materials prepared by supramolecular self-assembly approaches (including ionic self-assembly (ISA), π-π conjugation, metal coordination and hydrogen bonding, etc) have become an attractive research issue in recent years [[1], [2], [3]]. Constructing the functional materials by ISA strategy has received much attentions due to its unique advantages of simplicity, reliability and controllability [4,5]. The properties and morphology of obtained supramolecular assemblies can respond to external stimuli, such as temperature [6], redox species [7], electricity [8], pH [9], light [10], CO2 [11] and solvent [12], etc. The multiple stimuli-responsive micromaterials have potential applications in the field of photoelectronic devices, catalyst and biological field [[13], [14], [15], [16]].
Combining fluorescent dye molecules and oppositely charged surfactants via ISA strategy is an effective approach for the fabrication of supramolecular fluorescent materials. Due to the synergistic noncovalent interactions between dye molecules and surfactants, the materials prepared by ISA exhibited diverse microstructures. Lots of related work has been done. By mixing a cationic dipeptide with three different dyes respectively, Ma et al. prepared self-assembled nanostructures with various morphologies including spherical, flower-like, urchin-like and plate-likes structures [17]. Zhao et al. fabricated the star-like micromaterials by a two-step ISA based on the electrostatic interactions generated by double tailed surfactant (AOT) and dye molecule (RB) [18]. The star-like assembly prepared by various molar ratios of AOT and RB showed different morphologies, indicating that molar ratio could influence the structures of assembly. Moreover, the assembly containing fluorophores and responsive blocks showed fluorescence switching behaviors. Yan et al. reported a switchable fluorescent polymer composed of oligoaniline, triphenylamine and fluorene groups, and the fluorescent behavior of the polymer could be modulated by electrochemical method and pH stimuli [19]. Dutta et al. employed ISA strategy to obtain supramolecular fibrillar and vesicular assemblies, and the fluorescence behaviors could be transformed by changing temperature, pH and solvent polarity [20].
Due to their excellent redox characteristics and inclusion complexation with β-CD, ferrocene (Fc) and its derivatives have been widely studied in recent years. A series of redox and photo-responsive ferrocene and azobenzene-based polymer have been obtained and studied by Xia et al. [21]. Ma et al. prepared the host-guest complexes by modifying two β-CD derivatives using Fc groups and the multiple stimulus responsiveness of supramolecular self-assemblies was explored in detail [22]. Zhao et al. synthesized double-headed amphiphilic molecules composed of β-CD and Fc with alkyl chain as linker, and their self-assembly behaviors were investigated [23]. In our previous work, the photoelectric functional hierarchical materials constructed by Fc16AB and AR were obtained by ISA strategy, which showed multi-stimuli response behaviors (including β-CD inclusion, redox species and pH) [24]. In order to further study the influence of molecule structure on the morphology and performance of assembly, another dye molecule with different structure need to be chosen to co-assemble with functional surfactant.
Herein, the supramolecular assemblies with various morphologies (such as rod-like fiber, spindle micelle and fiber bundle) were constructed by mixing 16-alkyl (ferrocenyl-methyl) ammonium bromide (Fc16AB) and Orange G (OG) with different molar ratio and their stimuli-response behaviors upon oxidant and β-CD inclusion were investigated. The supramolecular switch with quadruple fluorescence was obtained successfully and the “on” and “off” states could be transformed simply by adding oxidant and β-CD inclusion. The multi-stimuli responsive fluorescent behavior of the assembly is an intriguing character with great potential application in intelligent photoelectric devices.
Section snippets
Materials
(N, N-Dimethylamino) methylferrocene (purity 98 wt%) and cerium sulfate (Ce(SO4)2) were purchased from Sigma-Aldrich, St. Louis (USA). 1-bromohexadecane and Orange G (>95 %) and β-CD was purchased from Alfa Aesa and Sinopharm Chemicals respectively. All reagents were used without additional purification. The cationic surfactants Fc16AB was prepared by the synthetic method reported in this literature [25].
The preparation of binary complex
In order to prepare binary complex with ferrocene group in reduced form, two aqueous
Results and discussion
The appearance and morphologies of binary complex formed by Fc16AB-OG were influenced by the molar ratio of Fc16AB and OG, as shown in Fig. S1. The mixed solutions were turbid and large number of fibrous aggregates could be observed when the molar ratio of Fc16AB and OG changed from 1:4 to 1:1 (Fig. S1). Then the solution became clear when the ratio was 2:1. Further increase of the ratio would not influence the appearance of the complex significantly. The composition of binary complex was
Conclusion
In conclusion, the switchable fluorescence binary assembly with ferrocene and azo groups was successfully constructed by mixing Fc16AB and OG molecules via ISA strategy. The obtained complexes exhibited different microstructures including rod-like fibers, spindle micelle and fiber bundles. Morphological transformation was triggered by the addition of oxidant and the formation of β-CD inclusion complex. At the same time, the obtained complexes showed quadruple switchable fluorescence behavior,
Credit author statement
Qiuhong Li conceived the ideas. Pengliang Sui and Huajie Yu performed the synthesis of materials and characterization of Pickering emulsion. Pengliang Sui, Qiuhong Li and Dongqin Luo analysed the data and wrote the paper. Pengliang Sui, Qiuhong Li, Zhaoyu Zhang, Weimeng Si and Aixiang Li reviewed and edited the paper. All authors have given approval to the final version of the manuscript.
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgements
We are thankful for the financial support from National Natural Science Foundation Young Investigator Grant Program Project No. 21706148 and Natural Science Foundation of Shandong (Project No. ZR2019MEM010).
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