Metal-Organic Frameworks (MOFs) are crystalline porous materials constructed of metal ions connected by organic linkers. These materials possess many interesting features, like well-defined pore size, pore shape and ultra-high porosity. A characteristic example of MOFs with one dimensional pores is Al-MIL-53 ([Al(OH)(BDC), BDC = terephthalate or 1,4-benzenedicarboxylate]. The 3D framework of as-synthesized Al-MIL-53 (Al-MIL-53as) is built up of infinite chains of corner-sharing AlO4(OH)2 octahedra. The chains are connected by the organic BDC linkers creating one-dimensional rhombically shaped porous channels. In the Al-MIL-53as the channels are filled with uncoordinated terephthalic acid molecules. These, together with residual solvent or water molecules, can be removed by calcination or solvent extraction, which is referred to as activation of the MOF. The activated Al-MIL-53 structure exhibits breathing, the structure can reversibly change from a large open pore (LP) to a narrow pore form (NP) by changing the temperature and/or pressure conditions.
The breathing effect triggered by temperature was investigated in Al-MIL-53 doped with V. Since the V4+ dopant ions have one unpaired electron (3d1 configuration) they exhibit an Electron Paramagnetic Resonance (EPR) spectrum. EPR spectroscopy is a nondestructive analytical technique very sensitive to local environment of the paramagnetic ion. Spectrum analysis often allows chemical identification of the central paramagnetic ion and of its nearest environment.
The breathing effect in Al-MIL-53 was monitored with in situ EPR, recording spectra as a function of temperature in air and in vacuum. In Figure 1 EPR spectra of V4+ in the NP form (A) and the LP form (B) are presented together with the NP and LP structures (1 and 2 respectively). We show that VIV centers can be used as local probe to detect these phase transitions in the Al-MIL-53 framework.