The physical and chemical mechanisms of the molecular assembly process are crucial for a deeper understanding of the process. Compared with chemical reactions, molecular assembly processes have characteristics such as complex structures, multiple assembly pathways, and multiple binding sites. Therefore, achieving in situ, dynamic, and high-resolution characterization of the assembly process is extremely challenging. The characterization method for molecular assembly processes requires high energy resolution, in situ, high time resolution, quantitative, and other characteristics. Currently, characterization methods are challenging to meet these requirements. To address this issue, we have developed high-time resolution microfluidic nuclear magnetic resonance (NMR) technology, which improves the time resolution of NMR technology to the second level. We have used this technology to reveal the assembly kinetics of host-guest processes, key intermediates, and physical and chemical mechanisms, including the shuttle process of cucurbituril in azobenzene molecules and the host-guest-based supramolecular self-sorting polymerization process.
In the study of quasi-rotaxane supramolecular systems, using the kinetic information obtained by microfluidic NMR and identification of key intermediate structures, we found that the shuttle movement of cucurbituril is the rate-determining step, requiring the overcoming of a higher energy barrier. There is a sub-process in its assembly pathway that ends in only 10 seconds, which can only be characterized by spatially correlated two-dimensional NMR spectroscopy and cannot be characterized by conventional NMR. With the help of microfluidic NMR, we collected the ROESY spectrum of the system at 1.5 seconds after the start of the assembly process. In the study of supramolecular polymerization systems with self-sorting phenomena, we used microfluidic NMR to identify an intermediate that cannot polymerize in the assembly pathway. The dynamic study of the assembly pathway shows that its self-sorting mechanism is achieved through the "error correction" function in the "non-classical" step-growth polymerization process.