Chemical diversity in a metal–organic framework revealed by fluorescence lifetime imaging

It is taken as an article of faith that when a synthetic chemist crystallizes a compound, the crystals correspond to a ‘perfect’ form of matter.

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The paper in Nature Communications is here:

It is taken as an article of faith that when a synthetic chemist crystallizes a compound, the crystals correspond to a ‘perfect’ form of matter. While many suspect that imperfections may exist in crystals, very little has been done to find, assess and determine the spatial arrangements of such imperfections (which we call diversity). These issues arise prominently in the chemistry of metal-organic frameworks (MOFs), where the combination of multiply functionalized organic linkers results in a multivariable system in which the spatial arrangement of both functionalities and defects are unknown. As the complex arrangement of functionalities and defects can lead to properties that are not simply a linear sum of those of the pure components, deciphering chemical diversity promises better ways of designing complexity in materials. This task has not been accomplished by current characterization techniques, which are based on bulk measurements and give only statistical average of the whole sample, leaving diversity among constituent individuals unidentified.

To tackle this challenge, we functionalize the organic linker of UiO-67 with fluorescent dyes, which are then incorporated into the framework along with unfunctionalized linkers through either de novo synthesis or post-synthetic linker exchange. By analyzing Förster resonance energy transfer (FRET) between dye pairs, we identify the arrangement of the incorporated functionalities to be randomly distributed in the framework without any clustering or phase separation, further revealing a correlation between fluorescence lifetime and local defect at the molecular level. With this correlation established, for the first time fluorescence imaging combined with lifetime analysis are employed to map the compositional variation and defect distribution in multivariate UiO-67 with three-dimensional resolution. The developed imaging and analytical method in our study can not only assess the spatial arrangement of functional groups and local defects in a given MOF sample, but also reveals the following principles of MOF chemistry:

1) The larger the amount of bulky functional groups incorporated, the higher the level of defects in the crystal structure; 2) The external surface represents a defect and quenches fluorescence in a similar way as internal defects; 3) The individual crystallites in an aggregate show less diversity than the full sample; 4) Post-synthetic incorporation introduces fewer defects than de novo synthesis; 5) Larger crystals tend to contain fewer defects and are less susceptible to defect formation.

This study presents the power of using fluorescence lifetime imaging and analysis in spatial resolution of chemical diversity in MOFs with fast speed, high sensitivity, and good spatial resolution. This report uses MOF chemistry to demonstrate how the long-standing challenge of assessing diversity in crystals can be addressed. We believe that the power of fluorescence imaging revealed in this study will inspire new insight into other materials as we have done in our MOF examples. 

Stefan Wuttke

Senior Lecturer, University Munich (LMU)