The large cell volume changes of switchable metal–organic frameworks (MOFs) render them as promising functional materials. Low-frequency phonon modes are known to influence the dynamic response of these materials. The pillared layer DUT-8(M) materials are prototypical examples of switchable MOFs, enabling switching between the closed and open pore phases, largely depending on the metal ions constituting the paddle wheel unit. However, the role of specific phonon modes in the softness of these materials is still rather unexplored. This study combines complementary spectroscopic techniques such as Raman spectroscopy, inelastic neutron scattering, and phonon acoustic spectroscopy (PAS) with density functional theory calculations (DFT) to unravel the vibrational properties of DUT-8(M) with different metal nodes (M = Ni, Co, Zn, Cu) to address these open questions. After analysis of the various experimental and theoretical spectroscopic data, the closed pore phase of DUT-8(Ni) appeared to be stiffer than that of the materials with Co and Zn. Experiments also show that the open pore phase of the Ni based compound is softer than those containing Zn and Co, although these findings could not be supported by theory. Nevertheless, DFT calculations could explain that changing the metal atom has mainly an impact on the phonon modes inducing changes in the paddle wheel unit. These results yield valuable insights into the role of the metal node on the observed flexibility in DUT-8(M) materials and can help to understand the mechanisms behind the phase transition in switchable MOFs.
Abstract