Low-frequency lattice vibrational modes have been discussed to play a crucial role in the phase transformation process of flexible metal–organic frameworks (MOFs). Therefore, Raman spectroscopy was applied to study the lattice dynamics of a pillared layer DUT-8(Ni) framework (DUT – Dresden University of Technology), existing in rigid and flexible forms. Both the open and the close pore phases could be unambiguously identified by breathing mode bands at 23 cm⁻¹ and 60 cm⁻¹ in the corresponding Raman spectra, showing the efficiency of the technique for monitoring the flexibility of MOF materials as well as the differences in the lattice vibrations of the two phases. Born–Oppenheimer Molecular Dynamics simulations showed that observed low-frequency bands indeed correspond to the oscillation of the breathing mode along the diagonals of the pore channels. Moreover, the directional character of low-frequency vibrations in the flexible version of DUT-8(Ni) could be visualized by the orientation dependent Raman spectroscopy experiment.

DUT-8(Co) shows a selective structural transition for CH ₂ Cl ₂ vs. CHCl ₃ and CCl ₄ at 298 K.

The behaviours of the open pore (op) and closed pore (cp) phases of the flexible Ni₂(ndc)₂(dabco) (ndc – 2,6-naphthalene dicarboxylate, dabco – 1,4-diazabicyclo[2.2.2]octane, DUT-8(Ni)) metal–organic framework under high hydrostatic pressures up to 10 GPa in isopropanol and silicone oil were studied by Raman spectroscopy. Ab initio simulations of vibrational spectra were performed for the open and closed pore phases, which allowed us to disclose the characteristic vibrational modes affected by the structural transitions under pressure. Analysis of theoretical and experimental Raman data suggests that the op–cp transition involves gateway vibrations at 25 and 67 cm⁻¹, corresponding to trampoline/rotational motions of aromatic linkers. The experiments reveal the formation of new distorted cp phases at pressures higher than 2 GPa, which are formed without amorphisation. The transition between the cp phase and the distorted cp phase is reversible. The experiments also reveal the pivotal role of the pressure transmitting medium on the phase transition behaviour.