To achieve optimized host-guest interactions, “cavity functionalization” approach has been applied by two isostructure MOFs, [Zn(OBA)(BPDB)0.5]n·2DMF (TMU-4) and [Zn(OBA)(H2DPT)0.5]n·DMF (TMU-34) where H2OBA = 4,4'-oxybis(benzoic acid), BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene and H2DPT = 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 dihydro-tetrazine function inside the cavities of TMU-34 can act as 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 mechanism. We believe that preferred hydrogen bond interaction between RB-B free phenolate/carboxylate and dihydro-tetrazine hydrogens of TMU-34 is responsible of drastic enhancement in the adsorption capacity and removal kinetic of TMU-34 rather than TMU-4. Our strategy in this work clearly shows that: (i) not only the high surface area and porosity, but rational decoration of MOF cavities with organic functions is the key method for highly efficient and fast dye removal; (ii) both adsorption and desorption time can be optimized very fast. Also, optimized interaction between TMU-34 and RB-B is strong and effective, but not so much that leads to long desorption time. To the best of our knowledge, this is first paper for removal of highly toxic rose-bengal B dye from aqueous solution by MOFs.
