Photoactive metal-organic frameworks for gas separation

Metal-organic frameworks (MOFs) are attracting much attention in recent years for their potential use in CO2 capture technologies. Recently, it has been shown that an efficient capture-and-release process can be obtained upon light treatment in pho- toactive MOFs [1,2]. We demonstrated that in these MOFs the notable change in gas uptake, upon light irradiation, is due to the blocking of the strongly adsorbing metal sites upon isomerization of the azo groups from trans to cis [3]. Interestingly, our study suggests a large fraction of cis at the photostationary state. A large S1/S2 absorption band separation between trans and cis within the MOF could support this hypothesis. In order to address this, we have computed the optical absorption spectra of these MOFs and their ligands using embedded GW/BSE calculations as implemented in FIESTA. Embedding is considered first at the DFT level (COSMO solvation model and ESP charges) and then, in the calculation of the modified screened Coulomb potential. CASSCF/CASPT2 and periodic GW/BSE are performed to benchmark, respectively, the DFT starting point and the choice of the fragment. Work supported by ANR-15-CE06-0003-01. [1] Park et al, JACS 134, 99 (2012); [2] Wang et al, Nature Commun. 7, 13872 (2016);