Revealing the N2 gas-induced gate-opening in Mn-ZIF-8 and Cu-ZIF-8 from the perspective of in situ EPR spectroscopy

Abstract

The structural transition of ZIF-8 under N₂ gas adsorption/desorption processes has been already investigated using various characterization techniques. This study demonstrates that electron paramagnetic resonance (EPR) spectroscopy is an alternative and powerful method that provides valuable local structure insights into the gate opening from ambient pressure (AP) to high pressure (HP) structural phases of the ZIF-8 framework during N₂ gas adsorption/desorption. Our in situ EPR experiments reveal distinct distortions in the environment of the tetrahedral metal ion framework sites for Mn²⁺ and Cu²⁺ dopants, demonstrating their different responsiveness to the gate opening transition. We were able to detect the AP to HP structural transition by monitoring the changes in the zero-field splitting of the paramagnetic Mn²⁺ (S = 5/2) probe ions. The results are complemented by employing Cu²⁺ (S = 1/2) centres as alternative spin probes where the Cu²⁺ g-tensor and hyperfine parameters change likewise at the AP to HP structural transition. Our findings validate the use of paramagnetic centres as spin probes for locally monitoring the N₂ gas-induced structural changes within the ZIF-8 framework. Notably, in situ EPR spectroscopy was successfully utilised to observe such transitions during N₂ gas adsorption/desorption processes in ZIF-8. The acquired spectroscopic results are consistent with previous reports on the gate-opening of ZIF-8, confirming the reliability and potential of this local spectroscopic method.