Morphological Evolution of Metal-Organic Frameworks into Hedrite, Sheaf and Spherulite Superstructures with Localized Different Coloration

The here reported branched metal-organic frameworks (MOFs) are the first known complex superstructures of this kind, and the growth mechanism may explain other crystal shapes. The mechanistic aspects of the formation of fascinating structures having a hedrite, sheaf or spherulite appearance are detailed in this study. The extent of branching can be controlled, resulting in crystals that either exhibit multiple generations of branching or just a single generation. The absence of further branching from the initial branches is the most distinguishing characteristic of a spherulite. These structures might result from an increasing number of defects on initially fast-grown rods. As the basal facets become less reactive over time, material is added to the prism facets, leading to secondary nucleation and branching. These branching forms triangular branches fully connected to the rod surface, eventually growing longer than the central rod. Electron diffraction analyses show that the metal-organic sheafs are polycrystalline structures with their fantails 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.