Dynamics of Linkers in Metal–Organic Framework Glasses
Alexander E. Khudozhitkov, Naoki Ogiwara, Masaki Donoshita, Hirokazu Kobayashi, Alexander G. Stepanov, Daniil I. Kolokolov, Hiroshi Kitagawa
Abstract
Metal–organic framework (MOF) glasses have emerged as a new class of organic–inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2 H nuclear magnetic resonance (NMR) spectroscopy. 2 H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle.