title: Molecular relaxation processes in a MOF-5 structure revealed by broadband dielectric spectroscopy: signature of phenylene ring fluctuations.
abstract: The molecular mobility of a MOF-5 metal-organic framework was investigated by broadband dielectric spectroscopy. Three relaxation processes were revealed. The temperature dependence of their relaxation rates follows an Arrhenius law. The process observed at lower temperatures is attributed to bending fluctuations of the edges of the cages involving the Zn-O clusters. The processes ("region II") at higher temperatures were assigned to fluctuations of phenyl rings in agreement with the NMR data found by Gould et al. (J. Am. Chem. Soc. 2008, 130, 3246). The carboxylate groups might also be involved. The rotational fluctuations of the phenyl rings leading to the low frequency part of relaxation region II might be hindered either by some solvent molecules entrapped in the cages or by an interpenetrated structure and have a broad distribution of activation energies. The high frequency part of region II corresponds nearly to a Debye-like process: This is explained by a well-defined structure of empty pores.
authors.affiliation: National Institute of Materials Physics, R-077125 Magurele, Romania.
authors.forename: Stefan
authors.initials: S
authors.lastname: Frunza
authors.forename: Andreas
authors.initials: A
authors.lastname: Schönhals
authors.forename: Ligia
authors.initials: L
authors.lastname: Frunza
authors.forename: Paul
authors.initials: P
authors.lastname: Ganea
authors.forename: Hendrik
authors.initials: H
authors.lastname: Kosslick
authors.forename: Jörg
authors.initials: J
authors.lastname: Harloff
authors.forename: Axel
authors.initials: A
authors.lastname: Schulz