Local Static and Dynamic Field Fluctuations Near the Cu-Ion in HKUST-1 from 63cu-NMR

Abstract

We report on the local field fluctuations, both of static and dynamic character, at the position of the Cu ions in the Cu(II)-based porous metal-organic framework (MOF) HKUST-1 Cu2(BTC)3 powder. We will compare measurements done in the HKUST-1 pristine form of the material and HKUST-1 infiltrated with tetracyano-quinodimethane (TCNQ). NMR Spectroscopy is a sensitive local probe which can sample both magnetic and charge degrees of freedom of spin bearing nuclei, whose behavior is strongly affected by the magnetic and/or electric activity of neighboring atoms and electrons. Therefore, the loading of TCNQ into the open porous framework of HKUST-1 MOF should have a significant influence in the 63Cu-NMR linewidth. We present data from 63Cu-NMR linewidth, frequency-shift and spin-lattice relaxation experiments in pristine HKUST-1, which display unusual temperature dependence: the linewidth indicating on the one hand a potential interaction between the 63Cu nuclei and the unpaired electrons in the paramagnetic system of Cu(II), and on the other hand, the frequency-shift together with the spin-lattice relaxation time indicating the existence of a substantial density of electron states at the 63Cu sites, with concomitant changes due to temperature variations. We will discuss these data and those obtained from similar experiments in HKUST-1 when infiltrated with TCNQ. Work supported by the National Science Foundation under a Partnership for Research and Education in Materials PREM, grant DMR-1523588.