Highly porous materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and hydrogen-bonded organic frameworks (HOFs) have recently been suggested to have promising electronic and dielectric properties. However, high levels of porosity often couple with low structural and mechanical stability. Therefore, the low-frequency lattice dynamics of porous framework materials can reveal a diversity of valuable information relating to the structural flexibility and stability and can explain the mechanistic origins of anomalous mechanical phenomena. Spectroscopic techniques such as Raman and infrared (IR) spectroscopy and inelastic neutron scattering (INS) in conjunction with density functional theory (DFT), can be used to study the terahertz (THz) vibrations of various porous framework materials. Intriguing structural motions and transitions are revealed, including gate-opening and breathing dynamics, ‘trampoline-like’ mechanisms and molecular rotors reminiscent of negative thermal expansion (NTE), and buckling of 2D layers. The work has also recently advanced to investigate the effect of external stimuli (pressure and temperature) and has revealed the nature and underlying mechanisms behind stimuli-induced phase changes and amorphization.
Dr. Matthias Krack