Morphological Evolution of Metal-Organic Frameworks by Crystal Splitting into Sheaf-like Structures

The formation and mechanistic aspects of hierarchically micro-structured metallo-organic architectures are detailed in this study. These fascinating structures have a rod-like core with two fantails to give it a unique sheaf-like appearance. The rod-like cores develop into the sheafs via a crystal splitting growth mechanism. The tips of these growing rods evolve into fantails from basal faces; the addition of material to their shell results in sheaf-like superstructures. We succeeded to observe the formation of hedrites, sheafs, and spherulites. This phenomenon has been observed for zeolites and biominerals but is rare for metal-organic frameworks (MOFs). The here presented MOFs are based on pyridine–nickel d8 coordination chemistry. Electron diffraction analyses indicate that the metallo-organic sheafs are polycrystalline structures consisting of single-crystalline nanorods deviating gradually from each other in their orientation. The crystallographic structure formed from achiral components consists of helical channels with opposite handedness. The chemical accessibility of the nanochannels and the porosity of the superstructures are demonstrated by the diffusion of a chromophore (sodium resorufin) into the channels. The confinement and alignment of the chromophores inside the channels resulted in polarized-light dependent coloration of the crystals; the polycrystallinity generated areas having different optical properties. The morphology and size of the crystals strongly depends on the concentration, molar ratio of starting materials, and the nature of the counter anions of the nickel salts (Cl–, Br–, I-, NO3–, and OAc–).