CO2 and Temperature Induced Switching of a Flexible Metal-Organic Framework with Surface-Mounted Nanoparticles

The stimuli responsiveness of a material defines its potential application. Within the class of metal-organic frameworks (MOFs) the subclass of flexible MOFs (flexMOFs) has attracted great attention, showing large structural flexibility as a response to external stimuli such as guest adsorption, temperature, and pressure. Derived hybrid composites like nanoparticle (NP) loaded flexible MOFs, which stand to potentially combine advantageous properties of both are yet largely unexplored. Here we report the synthesis of flexible MOFs with surface mounted nanoparticles, e. g. NP@Zn2(BME-bdc)2dabco composites (NP = Pt and SiO2 nanoparticles, BME-bdc2- = 2,5-bismethoxyethoxy-1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane), studying the impact of nanoparticles on the stimulus-responsiveness of a flexible MOF. We show that CO2 physisorption triggered flexibility of the MOF is fully retained and reversible for all NP@flexMOF composites. Additionally, we observe that NPs stabilize the large pore state of the MOF, slightly increasing and shifting the switching pressure window. This effect is also observed during temperature-induced switching but Pt@flexMOF composites partially lose long-range order during the reversion to their narrow pore state, while attached SiO2 NPs allow for a fully reversible transition. These findings suggest that the total exerted material strain triggering the switching is heavily dependent on NP size and the applied stimulus and that guest-induced switchability can be fully realized in NP@flexMOF hybrid materials.