English

Redox-Regulated Dynamic Self-Assembly of a Lindqvist-Type Polyoxometalate Complex

• Zhang Jing ^{1,2, ,} ,
• Wang Lina ^1,3, ,
• Chen Xiaofei ^4, ,
• Wang Yufeng ^1, ,
• Niu Chengyan ^1, ,
• Wu Lixin ^4, ,
• Tang Zhiyong ^{2, ,}

• 1.

  Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, P. R. China

• 2.

  CAS Key Laboratory of Nanosystem and Hierarchy Fabrication, National Center for Nanoscience and Technology, Beijing 100190, P. R. China

• 3.

  College of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, P. R. China

• 4.

  State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China

Corresponding author: Zhang Jing, jingzhang@sxu.edu.cn Tang Zhiyong, zytang@nanoctr.cn

• Received Date: 02 December 2019
  Accepted Date: 25 December 2019
  Revised Date: 20 December 2019
  Available Online: 15 September 2020
• Fund Project:

  The project was supported by the National Key Basic Research Program of China (2016YFA0200700), the National Natural Science Foundation of China (21972081, 21890381, 21721002, 21502107), the Frontier Science Key Project of Chinese Academy of Sciences (QYZDJ-SSW-SLH038), and the K. C. Wong Education Foundation

Abstract: Dynamic regulation of self-assembly is of vital importance in chemistry, biology and material science thanks to its great potential for development of smart materials and devices. Polyoxometalates (POMs) are a class of functional inorganic nanoclusters, which has become one of the excellent building blocks for supramolecular self-assemblies, especially when covalently or non-covalently modified by organic species. As typical stimuli-responsive functional clusters, the POMs could be photochemically or electrochemically reduced to mixed-valence states, of which the structural integrity remains even after encountering stepwise multi-electron redox process. The intriguing photochromism of the POMs in different states exhibits distinct photophysical properties, which motivates us to exploit the dynamic self-assemblies of POM-based complexes. The divalent Lindqvist-type hexamolybdate cluster [Mo₆O₁₉]^2- is one of the least negative-charged POMs, which is the ideal building blocks to construct novel assembly structures. Based on this motivation, herein, a single chain surfactant-encapsulated polyoxometalate (POM) complex (ODTA)₂[Mo₆O₁₉] was prepared by simple counterion replacement of Lindqvist-type (TBA)₂[Mo₆O₁₉] with octadecyltrimethylammonium (ODTA) in acetonitrile solution. The structure of the POM complex was confirmed by ¹H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and elemental analysis. The solution of complex (ODTA)₂[Mo₆O₁₉] in the mixed solvents of acetonitrile and isopropanol with the volume ration of 4 to 1 exhibited reversible photochromism upon alternate UV light irradiation and air exposure. Upon UV light irradiation, the light yellow transparent solution of (ODTA)₂[Mo₆O₁₉] turned into blue quickly. The new broad absorption band appearing at ca.751 nm assigned to the Mo^V → Mo^VI intervalence charge-transfer (IVCT) transition, indicated the formation of reduced POM, as revealed by UV-Vis absorption spectra. After exposed to air, the blue solution was bleached. The alternate photochromism could be conducted for multiple cycles. Helical self-assembled morphology of (ODTA)₂[Mo₆O₁₉] was formed in acetonitrile/isopropanol, characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. More interestingly, morphology transformation of the complex from helical strips to spherical assemblies occurred accompanied by photochromism occurrence. The morphology evolution during the photochromism process experienced from shortened helical strips through sea urchin-like aggregates to spherical assemblies. Most significantly, the helical assemblies could be recovered again after air oxidation, implying the reversible morphology transformation driven by redox stimulus. The redox-modulated reversible self-assembly is driven by the variation of electrostatic attraction between organic cations and inorganic anions as well as the electrostatic repulsion between inorganic ionic clusters, proved by X-ray photoelectron spectroscopy (XPS) and ¹H NMR spectra. The results will contribute to better understanding the mechanism of dynamic assemblies and inspire the precise fabrication of advanced smart materials.

Key words:
• Polyoxometalate
•  /  Morphology control
•  /  Redox
•  /  Photochromism
•  /  Dynamic self-assembly

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