Tuning Gas Adsorption by Metal Node Blocking in Photoresponsive Metal-Organic Frameworks.

By combining first-principles calculations and classical molecular simulations, an atomistic-level of understanding was provided towards the notable change in CO2 adsorption upon light treatment in two recently reported photoactive metal-organic frameworks, PCN-123 and Cu2 (AzoBPDC)2 (AzoBiPyB). It was demonstrated that the reversible decrease in gas adsorption upon isomerization can be primarily attributed to the blocking of the strong adsorbing sites at the metal nodes by azobenzene molecules in a cis configuration. The same mechanism was found to apply also to other molecules, for example, alkanes and toxic gases. Such understandings are instrumental to the future design of photoresponsive metal-organic frameworks. For example, the metal node-blocking mechanism can be leveraged to achieve optimal adsorption properties as a function of metal substitution and/or ligand functionalization. As a proof of concept, it was shown that the working capacity could be increased by a factor of two in PCN-123 by replacing the Zn4 O node with the more strongly adsorbing Mg4 O.