Probing the Guest-Mediated Structural Mobility in the UiO-66(Zr) Framework by 2 H NMR Spectroscopy Alexander E. Khudozhitkov, †,‡ Herve ́ Jobic, § Daniil I. Kolokolov,* ,†,‡ Dieter Freude, ∥ Jü rgen Haase, ∥ and Alexander G. Stepanov* ,†,‡ † Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia ‡ Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia § Institut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS, Universite ́ de Lyon, 2. Av. A. Einstein, 69626 Villeurbanne, France ∥ Fakultä t fü r Physik und Geowissenschaften, Universitä t Leipzig, Linne ́ strasse 5, 04103 Leipzig, Germany * S Supporting Information ABSTRACT: The solid-state 2 H NMR technique (analysis of both the spectrum line shape and the spin−lattice relaxation) was used to probe both slow and fast dynamical modes of the phenylene fragments of terephthalate linkers of the UiO-66(Zr) framework affected by the presence of benzene guest in the pores of the material. Such approach allowed us to probe different motions within a broad range of time scale, 10 −3 −10 −11 s. The internal dynamics in the UiO-66(Zr) framework is represented by torsional motions of the phenylene fragment of the linker including 2-site 180° flips (π- flips) of the plane of the phenylene ring and its restricted librations. In the presence of benzene loaded in the MOF pores the rate of π-flips decreases essentially and the activation barrier for this motion increases. The activation barrier has been found to increase almost in a linear fashion on benzene loading. Such observation is surprisingly unique among other MOFs with mobile linkers, like MIL-53(Al) or MOF-5. The fast librational motion occurs on a scale of ∼10 10 Hz and shows no notable dependence on the guest loading. It has been established that anisotropy of T 1 relaxation of the 2 H NMR powder pattern of the phenylene fragments is especially sensitive to the librational motion when this motion is in a range of 10 7 −10 11 Hz. Within this range of libration frequencies, analysis of the anisotropic spin−lattice (T 1 ) relaxation allows quantitative estimation of the rate of librational motion. 1. INTRODUCTION Porous metal−organic frameworks (MOFs) constitute a class of solid materials formed by inorganic nodes (metal cations or metal oxide clusters) bridged together by organic linkers in a regular manner. Such composition makes MOFs unique among other crystalline porous solids because they combine the ordered nature of a crystal and the flexibility of a polymer framework, exhibiting a pronounced local mobility of its building units. This internal structural mobility mediates many fascinating MOFs properties such as optical 1−3 and dielectrical response, 4 the conductivity, 5−7 the adsorption, and the molecular transport of chemical species confined inside their pores. Since the walls of the MOFs pores and the windows interconnecting them are constituted by the linkers, their potential mobility, including rotational and librational motions, could strongly affect both the morphology and the effective size of the frameworks inner space. Indeed, recent studies of the guests mobility in different MOFs have already confirmed the essential role of the framework flexibility in the molecular transport. 8−13 Hence, characterization and control of the structural dynamics in MOFs is key for the rational design of new materials and the search for potential applications of the existing ones. There are several strategies to control the structural dynamics in MOFs, including synthetic modification of the linkers or the frameworks itself, 14,15 temperature regulation, and finally guests inclusion. 16 All three strategies are currently applied to control the structural dynamics. However, if we are interested in particular features of already available systems only the last two options are in the focus. Among the last two, guest introduction offers more options in controlling the internal dynamics by varying either the chemical nature of the guest or the guest loading. There are a number of investigations of guest-mediated internal dynamics in MOFs and related materials. 16−20 It was shown that the chemical nature of the guest and the guest loading could influence the linker rotational dynamics in different ways. For example, it was shown for the ordered porous molecular crystal with phenylenes as the mobile Received: April 6, 2017 Revised: May 4, 2017 Published: May 5, 2017 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b03259 J. Phys. Chem. C XXXX, XXX, XXX−XXX