Solution-Phase and Magnetic Approach towards Understanding Iron Gall Ink-like Nanoassemblies

Solution‐Phase Magnetic Mechanistic Study of Ni‐Seamed Pyrogallol[4]arene Nanocapsules Reveal Presence of Novel Cylindrical and Spherical Species

The magnetic properties of nickel‐seamed C‐pyrogallol[4]arene (PgC₃Ni) hexamers and dimers are studied for the first time in solution. The combination of small‐angle neutron scattering and superconducting quantum interference device magnetometer measurements of the solution species reveal their paramagnetic and weakly antiferromagnetic behaviour. Surprisingly, the magnetic results indicated the presence of an unprecedented 13 Å‐radius species, larger than both the dimeric and hexameric nanocapsules with both octahedral and square‐planar metal centers. To confirm the presence of this novel species, we performed a mechanistic study of PgC₃Ni as a function of temperature and solvent and deduced the presence of two additional new species: a) an 11 Å cylinder with Ni atoms seaming the tubular framework and b) an 8 Å‐radius sphere with non‐interacting Ni centers located within the internal cavity. Select parameters that shift the equilibrium towards desired species are also identified.

Solution structure of zinc-seamed C-alkylpyrogallol[4]arene dimeric nanocapsules

The structural stability and solution geometry of zinc-seamed-C-propylpyrogallol[4]arene dimers has been studied in solution using in situ neutron scattering and 2D-DOSY NMR methods. In comparison with the structures of the analogous copper-/nickel-seamed dimeric entities, the spherical geometry of the PgC₃Zn species (R = 9.4 Å; diffusion coefficient = 1.05 × 10^-10 m² s^-1) is larger due to the presence of ligands at the periphery in solution. This enhanced radius in solution due to ligation is also consistent with the findings of model molecular dynamics simulations of the zinc-seamed dimers.

Interconvertible vanadium-seamed hexameric pyrogallol[4]arene nanocapsules

Research into stimuli-responsive controlled self-assembly and reversible transformation of molecular architectures has received much attention recently, because it is important to understand and reproduce this natural self-assembly behavior. Here, we report two coordination nanocapsules with variable cavities: a contracted octahedral V₂₄ capsule and an expanded ball-shaped V₂₄ capsule, both of which are constructed from the same number of subcomponents. The assemblies of these two V₂₄ capsules are solvent-controlled, and capable of reversible conversion between contracted and expanded forms via control of the geometries of the metal centers by association and dissociation with axial water molecules. Following such structural interconversions, the magnetic properties are significantly changed. This work not only provides a strategy for the design and preparation of coordination nanocapsules with adaptable cavities, but also a unique example with which to understand the transformation process and their structure-property relationships.

Adapting the cavity of a coordination capsule generally involves the addition or removal of subcomponents. Here, the authors report two vanadium-organic coordination nanocapsules with the same number of components but variable cavity sizes—an expanded ball and contracted octahedron—whose solvent-controlled interconversion is attributed to the versatile coordination geometry of the vanadium centers.

Bowl-in-bowl complex formation with mixed sized calixarenes: adaptivity towards guest binding

We demonstrated the organization of two differently sized calixarenes C-methylresorcin[4]arene (RsC1) and either calix[6]arene (Calix6) or calix[8]arene (Calix8), where the lower rim of RsC1 partially overlaps with the upper rim of Calix6 or Calix8. An adaptive nature of the heteromacrocyclic assembly towards the binding of a model guest has been observed.

Tunicate-mimetic nanofibrous hydrogel adhesive with improved wet adhesion

The main impediment to medical application of biomaterial-based adhesives is their poor wet adhesion strength due to hydration-induced softening and dissolution. To solve this problem, we mimicked the wound healing process found in tunicates, which use a nanofiber structure and pyrogallol group to heal any damage on its tunic under sea water. We fabricated a tunicate-mimetic hydrogel adhesive based on a chitin nanofiber/gallic acid (a pyrogallol acid) composite. The pyrogallol group-mediated cross-linking and the nanofibrous structures improved the dissolution resistance and cohesion strength of the hydrogel compared to the amorphous polymeric hydrogels in wet condition. The tunicate-mimetic adhesives showed higher adhesion strength between fully hydrated skin tissues than did fibrin glue and mussel-mimetic adhesives. The tunicate mimetic hydrogels were produced at low cost from recyclable and abundant raw materials. This tunicate-mimetic adhesive system is an example of how natural materials can be engineered for biomedical applications.

Solution structures of nanoassemblies based on pyrogallol[4]arenes

Nanoassemblies of hydrogen-bonded and metal-seamed pyrogallol[4]arenes have been shown to possess novel solution-phase geometries. Further, we have demonstrated that both guest encapsulation and structural rearrangements may be studied by solution-phase techniques such as small-angle neutron scattering (SANS) and diffusion NMR. Application of these techniques to pyrogallol[4]arene-based nanoassemblies has allowed (1) differentiation among spherical, ellipsoidal, toroidal, and tubular structures in solution, (2) determination of factors that control the preferred geometrical shape and size of the nanoassemblies, and (3) detection of small variations in metric dimensions distinguishing similarly and differently shaped nanoassemblies in a given solution. Indeed, we have shown that the solution-phase structure of such nanoassemblies is often quite different from what one would predict based on solid-state studies, a result in disagreement with the frequently made assumption that these assemblies have similar structures in the two phases. We instead have predicted solid-state architectures from solution-phase structures by combining the solution-phase analysis with solid-state magnetic and elemental analyses. Specifically, the iron-seamed C-methylpyrogallol[4]arene nanoassembly was found to be tubular in solution and predicted to be tubular in the solid state, but it was found to undergo a rearrangement from a tubular to spherical geometry in solution as a function of base concentration. The absence of metal within a tubular framework affects its stability in both solution and the solid state; however, this instability is not necessarily characteristic of hydrogen-bonded capsular entities. Even metal seaming of the capsules does not guarantee similar solid-state and solution-phase architectures. The rugby ball-shaped gallium-seamed C-butylpyrogallol[4]arene hexamer becomes toroidal on dissolution, as does the spherically shaped gallium/zinc-seamed C-butylpyrogallol[4]arene hexamer. However, the arenes are arranged differently in the two toroids, a variation that accounts for the differences in their sizes and guest encapsulation. Guest encapsulation of biotemplates, such as insulin, has demonstrated the feasibility of synthesizing nanocapsules with a volume three times that of a hexamer. The solution-phase studies have also demonstrated that the self-assembly of dimers versus hexamers can be controlled by the choice of metal, solvent, and temperature. Controlling the size of the host, nature of the metal, and identity of the guest will allow construction of targeted host-guest assemblies having potential uses as drug delivery agents, nanoscale reaction vessels, and radioimaging/radiotherapy agents. Overall, the present series of solid- and solution-phase studies has begun to pave the way toward a more complete understanding of the properties and behavior of complex supramolecular nanoassemblies.

Packing arrangements of copper-seamed C-alkylpyrogallol[4]arene nanocapsules with varying chain lengths

The synthesis and structural elucidation of copper-seamed C-alkylpyrogallol[4]arene hexamers with different tail lengths (n = 3, 7, 9) reveal differences in packing arrangements.