I discovered an unprecedented assembly case wherein hierarchically porous tubular superstructures with multiple levels of channel sizes can be assembled from a series of hollow tube crystallites during a facile one-pot synthesis at a suitable evolution temperature. In addition, spherulite superstructures were formed within a few hours if the nucleation and growth kinetics were altered. The spherulite superstructures were radially assembled from crystalline MOF-74 nanofibrils with an external spherical envelope. They display a ‘‘Maltese-cross’’ extinction pattern under polarized light, which is the initial example of this phenomena observed in porous materials. To fabricate more sophisticated architectures, such as quaternary superstructures, tertiary seed architectures can be utilized during the assembly process. For example, I observed a secondary growth of MOF-74-II plumose superstructures when MOF-74-III spherulite superstructures were mixed with solutions containing organic linkers with different lengths. This seed-mediated study demonstrates a synthetic approach that produces complex hetero-structured superstructures. The hierarchical MOF structures including atoms, building blocks, unit cells, crystallites, assemblies and superstructures are quite similar to the multi-level arrangement of proteins which contain primary, secondary, tertiary and quaternary structures. These superstructures contain precisely defined modules that are designed to execute specific tasks in sequence.
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