Biologically inspired polyoxometalate–surfactant composite materials. Investigations on the structures of discrete, surfactant-encapsulated clusters, monolayers, and Langmuir–Blodgett films of (DODA)40(NH4)2[(H2O)n⊂Mo132O372(CH3CO2)30(H2O)72]

Abstract

A detailed analysis of the supramolecular architecture of the nanoporous surfactant-encapsulated cluster (SEC) with the empirical formula (DODA)₄₀(NH₄)₂[(H₂O)_n ⊂ Mo₁₃₂O₃₇₂(CH₃CO₂)₃₀(H₂O)₇₂] 1 (n ≈ 50) is presented. The open framework architecture of the Keplerate cluster is investigated by means of small angle neutron scattering (SANS) in CDCl₃ solutions containing discrete SECs. A simplifying core–shell model of 1 is developed, which describes the SEC as a solvent-filled nanocavity, surrounded by two concentric shells (a first polyoxometalate shell of 2.96 nm outer diameter, and a consecutive surfactant shell of 6.18 nm outer diameter, respectively). The model is successfully applied to probe the content of H₂O/D₂O guest molecules in the Keplerate host. Different surface analytical techniques are applied to characterize the hierarchical structures of monolayers and thin films of 1. Monolayers at the air–water interface are investigated by means of optical ellipsometry and Brewster angle microscopy. Electron density profiles of the monolayers of 1 are gained from synchrotron X-ray reflectance (XRR) measurements that provide further evidence for the supramolecular core-shell architecture of the SEC. Within the spatial resolution limits of these analytical methods, the current data support a monolayer model consisting of hexagonal close-packed arrays of discrete SECs, floating at the air–water interface. Langmuir–Blodgett (LB) transfer of compressed monolayers on to a solid substrate leads to homogeneous multilayers. In the XRR spectra of LB multilayers of 1 multiple Bragg reflections appear, thus indicating an intrinsic tendency of the SECs to adapt a 3-dimensional, highly ordered solid state structure. Considering the huge variety of structurally different polyoxometalates and the possibility to tailor the surfactant shell by means of classic organic synthesis, the self-organization of hierarchically structured thin films and solids based on SECs bears promising perspectives towards the engineering of functional materials.