Understanding guest and pressure-induced porosity through structural transition in flexible interpenetrated MOF by Raman spectroscopy

Abstract

Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non-porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4-Bipy)(0.5)DMF(H2O)(0.5)} was selected for study. We have investigated structural transformation in MOF-508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF-508 framework is driven by ligand conformation change and inter-linker interactions. The presence of Raman signatures of adsorbed CO2 and its librational mode at 149cm(-1) suggests cooperative adsorption of CO2 in the MOF-508 framework, which is also confirmed from DFT calculations that give a binding energy of 34kJ/mol. Copyright (c) 2015 John Wiley & Sons, Ltd.