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

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 MoV → MoVI 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.