Rational design of photo-responsive supramolecular nanostructures based on an azobenzene-derived surfactant-encapsulated polyoxometalate complex

Abstract

Using an ionic self-assembly (ISA) approach, photo-responsive surfactant-encapsulated polyoxometalate complexes (SECs) were fabricated in water from an original Keggin-type polyoxometalate (POM) and a cationic surfactant containing an azobenzene group, viz. phosphotungstic acid (H₃[PW₁₂O₄₀]) and 4-ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB). The driving forces and self-assembly mechanism of the ETAB–POM supramolecular hybrids were investigated by NMR, Fourier transform infrared (FTIR), UV/vis and small angle X-ray scattering (SAXS) characterization methods. Of particular interest is the complex solution which shows an obvious variation upon UV light irradiation. On a macro-scale, its turbidity increases obviously, from a clear solution before UV irradiation to a turbid state. The microcosmic structures of the complex change from coral-like structures to dispersive nanospheres. These phenomena can be ascribed to the transformation of ETAB from trans- to cis-isomers after exposure to UV light. Beyond that, a cyclic voltammetric (CV) method was employed to observe the electrochemical properties of SECs. The results obtained in this work will shed light of the SECs' applications in phase separation, heterogeneous catalysis reactions, the detection of environmental pollutants, etc.