Identifying the Polymorphs of Zr-Based Metal–Organic Frameworks via Time-Resolved Fluorescence Imaging

The presence of polymorphs in metal–organic framework (MOF) crystallites with diverse topologies not only complicates the characterization and computational modeling of the materials, but also dramatically affects their properties. Thus, identifying topological differences to investigate polymorphs in MOFs is of great significance for obtaining phase-pure material. Herein, we demonstrate that fluorescence lifetime imaging (FLIM) is a powerful and nondestructive technique for direct analysis of polymorphs in MOFs by using NU-1000 as a model system. By utilizing intrinsic fluorescence from pyrene-tetracarboxylate based linkers, NU-901 is identified as the structural isomer present within NU-1000. The amount of NU-901 phase is shown to be dependent on the identity of the carboxylic acid modulator used for the synthesis of NU-1000. Moreover, both confocal fluorescence microscopy (CFM) and FLIM identified the presence of NU-901 phase at the center of the crystallites, whereas more NU-1000 phase was observed at both terminus. This work shows the great potential of FLIM in identifying phase impurities in representative samples of NU-1000, that can be easily extrapolated to other MOFs.