Direct Visualization of Chemically Resolved Multilayered Domains in Mixed-Linker Metal–Organic Frameworks

The modular synthesis approach for assembling inorganic nodes and organic multidentate linkers into reticular solids enables rational engineering in porous materials known as metal–organic frameworks (MOFs). Incorporation of two or more linker types in MOF crystals holds great potential for engineering complex pore functionalities, by virtue of chemically heterogeneous domains. However, deciphering linkers distributions in MOFs crystals is challenging because of the insufficient spatial resolution of conventional, chemically sensitive techniques hinders the verification of rational design. Herein, the high spatial resolution and chemical specificity of infrared nanoscopy is leveraged in combination with high-throughput diffraction-limited hyperspectral photoluminescence imaging to determine the composition of individual multivariate UiO-68 MOF crystals (UiO, Universitetet-i-Oslo), after linker-exchange with optically active tetrazine units. The results reveal that the crystals display a three-layer onion-like structure composed of a core-rich in the parent linker, an intermediate multivariate layer with a gradient in linker composition and a proto-MOF external shell. In this outer layer, a fraction of the linkers’ binding groups is hydrogen bonded rather than coordinated with metal nodes, suggestive of superficial reconstruction during the linker-exchange. This study advances the analytical capabilities for studying and engineering heterogeneous domains in mixed-linker MOF crystals down to the nanoscale.