Host-Guest Interaction Optimization through Cavity Functionalization for Ultra-Fast and Efficient Water Purification by a Metal-Organic Framework.

To achieve optimized host-guest interactions, a "cavity functionalization" approach has been applied using two isostructure MOFs, [Zn(OBA)(BPDB)] ·2DMF (TMU-4) and [Zn(OBA)(HDPT)] ·DMF (TMU-34), where HOBA = 4,4'-oxybis(benzoic acid), BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, and HDPT = 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine. TMU-4 and TMU-34 are functionalized with azine and dihydro-tetrazine groups, respectively. Both of these functions can act as Lewis basic sites, but only the dihydro-tetrazine function inside the cavities of TMU-34 can act as a hydrogen bond donor site. These frameworks were applied in removal of pollutant dyes. The results indicate that TMU-34 can remove rose-bengal B (RB-B) from aqueous solution much more than TMU-4, through size selective and optimized host-guest interaction mechanisms. We believe that the preferred hydrogen bond interaction between RB-B free phenolate/carboxylate and dihydro-tetrazine hydrogens of TMU-34 is responsible for the drastic enhancement in the adsorption capacity and removal kinetics of TMU-34 rather than TMU-4. Our strategy in this work clearly shows that (i) high surface area and porosity as well as rational decoration of MOF cavities with organic functions are the key methods for highly efficient and fast dye removal and (ii) both adsorption and desorption times can be optimized very fast. Also, optimized interaction between TMU-34 and RB-B is strong and effective, yet not so much that it leads to long desorption time. To the best of our knowledge, this is the first paper on removal of highly toxic rose-bengal B dye from aqueous solutions using MOFs.