Diffusion-Coupled Molecular Assembly: Structuring of Coordination Polymers Across Multiple Length Scales

Porous coordination polymers (PCPs)
are an intriguing class of
molecular-based materials because of the designability of framework
scaffolds, pore sizes and pore surface functionalities. Besides the
structural designability at the molecular scale, the structuring of
PCPs into mesoscopic/macroscopic morphologies has attracted much attention
due to the significance for the practical applications. The structuring
of PCPs at the mesoscopic/macroscopic scale has been so far demonstrated
by the spatial localization of coordination reactions on the surface
of templates or at the phase boundaries. However, these methodologies
have never been applied to the fabrication of solid-solution or multivariate
metal–organic frameworks (MOFs), in which multiple components
are homogeneously mixed. Herein, we demonstrate the structuring of
a box-type superstructure comprising of a solid-solution PCP by integrating
a bidirectional diffusion of multiple organic ligands into molecular
assembly. The parent crystals of [Zn2(ndc)2(bpy)]n were placed in the DMF solution of additional
organic component of H2bdc, and the temperature was rapidly
elevated up to 80 °C (ndc = 1,4-naphthalenedicarboxylate, bpy
= 4,4′-bipyridyl, bdc = 1,4-benzenedicarboxylate). The dissolution
of the parent crystals induced the outward diffusion of components;
contrariwise, the accumulation of the other organic ligand of H2bdc induced the inward diffusion toward the surface of the
parent crystals. This bidirectional diffusion of multiple components
spatially localized the recrystallization at the surface of cuboid
parent crystals; therefore, the nanocrystals of a solid-solution PCP
([Zn2(bdc)1.5(ndc)0.5(bpy)]n) were organized into a mesoscopic box superstructure.
Furthermore, we demonstrated that the box superstructures enhanced
the mass transfer kinetics for the separation of hydrocarbons.