Benzothiadiazole-based rotation and possible antipolar order in carboxylate-based metal-organic frameworks.

By modifying organic ligands of metal-organic framework with dipolar units, they turn suitable for various applications, e.g., in the field of sensor systems or switching of gas permeation. Dipolar linkers in the organic ligand are capable to rotate in certain temperature and frequency ranges. The copper-bearing paddlewheel shaped metal-organic frameworks ZJNU-40 and JLU-Liu30 possess such a polarizable dipole moment due to their benzothiadiazole moiety in the organic ligands. Here, we investigate the molecular rotor behavior of benzothiadiazole units of the two carboxylate-based MOFs by dielectric spectroscopy and computational simulation. Our dielectric results provide clear evidence for significant reorientational relaxation dynamics of these rotors, revealing various characteristics of glasslike freezing upon cooling. The calculated rotational energy barriers are consistent with experimentally determined barriers for single-dipole dynamics. Moreover, for JLU-Liu30 we find hints at antipolar ordering below about 300 K.