Probing Molecular Mechanisms of Self-Assembly in Metal-Organic Frameworks

Metal-organic framework materials (MOFs) are a class of nanoporous materials, important to many applications (e.g., gas storage, separation), and their synthesis has received considerable attention. Yet, very little is known about the mechanisms of self-assembly of MOFs. Here, we provide molecular-level insights into the previously unexplored behavior of the self-assembly process, through molecular dynamics simulations, for an archetypal Zn-carboxylate MOF system exhibiting complex vertex topologies (e.g., paddle-wheel clusters). A key finding of this study is the characterization of a stochastic and multistage ordering process intrinsic to self-assembly of the Zn-carboxylate MOF system. A variety of transient intermediate structures consisting of various types of Zn-ion clusters and carboxylate-ligand coordination, and featuring a range of geometric arrangements, are observed during structural evolution. The general features deduced here for the mechanism of the self-assembly of this archetypal MOF system expose the complexities of the various molecular-level events that can occur during the early stages of this process spanning time scales of nano- to microseconds. More generally, we provide fundamental insights that elucidate key aspects of the early stages of the self-assembly mechanism for an important class of nanoporous materials, and of experimental studies exploring nucleation and growth processes in such materials.