Exploring the self-assembly of dumbbell-shaped polyoxometalate hybrids, from molecular building units to nanostructured soft materials

Abstract

The formation of hierarchical nanostructures using preformed dumbbell-like species made of covalent organic-inorganic polyoxometalate (POM)-based hybrids is herein described. In this system, the presence of charged subunits (POM, metal linkers, and counter ions) in the complex molecular architecture can drive their aggregation, which results from a competition between the solvation energy of the discrete species and intermolecular electrostatic interactions. We show that the nature of the POM and the charge of the metal linker are key parameters for the hierarchical nanoorganization. The experimental findings were corroborated with a computational investigation combining DFT and molecular dynamics simulation methods, which outlines the importance of solvation of the counter ion and POM/counter ion association in the aggregation process. The dumbbell-like species can also form gels, in the presence of a poorer solvent, displaying similar nanoorganization of the aggregates. We show that starting from the designed molecular building units whose internal charges can be controlled by redox trigger we can achieve their implementation into soft nanostructured materials through the control of their supramolecular organization.

Scheme 1. Synthetic route to KSn[tpy] and DSn[tpy] (in the polyhedral representation, the WO₆ octahedra are depicted with oxygen atoms at the vertices and metal cations buried inside). Color code: WO₆ octahedra, blue; PO₄ tetrahedra, green.

Fig. 1. ¹H NMR (300 MHz) in DMSO-d₆ solution containing KSn[tpy] (2 mM, top left) and DSn[tpy] (top right) in the presence of 0.5 equiv. of [Co(H₂O)₆](NO₃)₂ (middle); after oxidation of the Co(ii) with 0.5 equiv. TBABr₃ (down).

Fig. 2. Energy-minimized structures of the dumbbell-like species KSn[tpy].CoII (a), KSn[tpy].CoIII (b), DSn[tpy].CoII (c) and DSn[tpy].CoIII (d).

Fig. 3. SAXS pattern of a 1 mmol solution of the molecular building-units KSn[tpy] (left, black curve) and DSn[tpy] (right, black curve) in DMSO-d₆, and their resulting dumbbell-shaped Co(ii) (red curves) and Co(iii) (blue curves) complexes and the computed SAXS pattern (CRYSOL) of the optimized structures of KSn[tpy].CoIII and DSn[tpy].CoIII (dotted black). The solvent and the capillary were subtracted to obtain these patterns.

Fig. 4. ITC isotherm obtained by injecting a [Co(H₂O)₆](NO₃)₂ solution in DMSO (2.2 mM) into a KSn[tpy] (red), DSn[tpy] (blue, with a vertical shift of −60 kJ.mol⁻¹) or 4′-(4-ethynylphenyl)-2,2′:6′,2′′-terpyridine (black, with a vertical shift of −100 kJ.mol⁻¹) solution in DMSO (0.385, 0.150 and 0.385 mM, respectively) at 25 °C. The lines correspond to fits according to a model considering a direct formation of the 2 : 1 assembly.

Fig. 5. SAXS pattern of a 1 mmol solution of the molecular building-unit KSn[tpy] (left, black curve) and DSn[tpy] (right, black curve) and the resulting aggregates POM.CoII (red curves) and POM.CoIII (blue curves) assemblies in CD₃CN (respectively obtained upon the addition of 0.5 equiv. [Co(H₂O)₆](NO₃)₂ and further oxidation of the Co(ii) with 0.5 equiv. TBABr₃). The solvent and the capillary were subtracted to obtain these patterns.

Fig. 6. Computed electrostatic potential map of KSn[tpy].CoIII.

Fig. 7. Top left: distance distribution functions (g(r)) between a POM (P atom) of one KSn[tpy].CoIII unit and the Co atom of the other unit for the UNPACKED setup in water (blue) and in DMSO (pink). Middle: g(r) between POM (P atom) of one KSn[tpy].CoIII unit and the Co atom of the other unit for initial setups UNPACKED (blue) and PACKED (green) in water. Right: g(r) between two POMs (P atom) of different KSn[tpy].CoIII units for initial setups UNPACKED (blue) and PACKED (green) in water. Bottom left: distance distribution functions g(r) between POM and TBA for the UNPACKED setup in water (orange) and DMSO (purple). Snapshots: spontaneously formed dimer in the UNPACKED setup (blue), and tetramer in the PACKED setup (green). TBA ions colored pinkish; POM oxygens colored red; organic chains colored blue.

Fig. 8. Time-evolution snapshots of dumbbell–dumbbell dimer assisted by TBA counter cations.

Fig. 9. (a) Ethanol organogel of KSn[tpy].CoIII. (b) Thermogravimetric analysis of KSn[tpy] (black) and KSn[tpy].CoIII as an ethanol organogel (red) and hydrogel (blue).

Fig. 10. (Left) SAXS pattern the POM.Con organogels: KSn[tpy].CoIII (plain blue), KSn[tpy].CoII (plain red), DSn[tpy].CoIII (dotted blue) and DSn[tpy].CoIII (dotted red). (Right) Evolution of the SAXS pattern of KSn[tpy].CoIII under successive acquisitions; the bold blue and red lines correspond to the first and last scans respectively. The solvent and the capillary were subtracted to obtain these patterns.