EPR characterization of vanadium dopant sites in DUT-5(Al)

The structural environment of the V-IV ions doped in the metal-organic framework (MOF) DUT-5(Al) (((AlOH)-O-III) BPDC) was studied with electron paramagnetic resonance (EPR). At low V concentrations, two different vanadyl components are observed in the EPR spectrum: a broad-line component, best visible at ambient air conditions, and a narrow-line component, becoming more prominent at lower pressures. Recording the EPR spectra at both X- and Q-band, spin-Hamiltonian parameters were derived for both components. Similarities between the spectrum of the narrow-line component and the spectrum of V-IV centers in the large pore state of a related MOF, MIL-53(Al) (((AlOH)-O-III)BDC), are discussed, including their sensitivity to oxygen. The broad-line component cannot conclusively be identified by comparison with V-IV centers in MIL-53(Al). Increasing the concentration of V-IV in the DUT-5(Al) framework, a third vanadyl component appears in the spectrum, together with a broad feature related to the spectrum of COMOC-2(V) ((V-IV =O)BPDC). Surprisingly, this third component appears to be linked with a narrow pore state of the mixed-metal MOF.