Temperature Driven Transformation of the Flexible Metal-Organic Framework DUT-8(Ni)

DUT-8(Ni) belongs to the flexible pillared layer MOFs, which solvent free variant can exist in the open pore (op, rigid) form or in the closed pore (cp, flexible) form depending on the crystal size regime. In present work, we report on response of desolvated DUT-8(Ni) against elevated temperature. For both variants, heating leads to structural transition, involving interpenetration of the framework and resulting in a new crystalline contracted closed pore phase (DUT-8(Ni)_ccp). The new compound was characterized by powder X-ray diffraction and spectroscopic techniques, such as IR, Raman spectroscopy, EXAFS and positron annihilation lifetime spectroscopy (PALS). State of the framework before transition (op vs. cp) influences the transition temperature: the small particles of the op phase transform at significantly lower temperature in comparison to the macroparticles of the cp phase, transforming just before decomposition. Thermal effects of structural cp to ccp transitions were studied using differential scanning calorimetry (DSC), showing an overall exothermic effect of the process, necessarily involving bond breaking and reformation. The theoretical calculations reveal the energetics driving the observed temperature induced phase transition.