posted on 2022-12-20, 21:30authored byZhihengyu Chen, Gautam D. Stroscio, Jian Liu, Zhiyong Lu, Joseph T. Hupp, Laura Gagliardi, Karena W. Chapman
Chemically
functionalized series of metal–organic
frameworks
(MOFs), with subtle differences in local structure but divergent properties,
provide a valuable opportunity to explore how local chemistry can
be coupled to long-range structure and functionality. Using in situ synchrotron X-ray total scattering, with powder
diffraction and pair distribution function (PDF) analysis, we investigate
the temperature dependence of the local- and long-range structure
of MOFs based on NU-1000, in which Zr6O8 nodes
are coordinated by different capping ligands (H2O/OH, Cl– ions, formate, acetylacetonate, and hexafluoroacetylacetonate).
We show that the local distortion of the Zr6 nodes depends
on the lability of the ligand and contributes to a negative thermal
expansion (NTE) of the extended framework. Using multivariate data
analyses, involving non-negative matrix factorization (NMF), we demonstrate
a new mechanism for NTE: progressive increase in the population of
a smaller, distorted node state with increasing temperature leads
to global contraction of the framework. The transformation between
discrete node states is noncooperative and not ordered within the
lattice, i.e., a solid solution of regular and distorted nodes. Density
functional theory calculations show that removal of ligands from the
node can lead to distortions consistent with the Zr···Zr
distances observed in the experiment PDF data. Control of the node
distortion imparted by the nonlinker ligand in turn controls the NTE
behavior. These results reveal a mechanism to control the dynamic
structure of MOFs based on local chemistry.