Nanochannel-Based Heterometallic {Co^IITb^III}-Organic Framework for Fluorescence Recognition of Tryptophan and Catalytic Cycloaddition of Epoxides with CO₂

To pave the way for the salient application of metal–organic frameworks (MOFs) as realistic sensors, it is critical to screen or design the customized functional structure with specified prominent synergistic effects provided by its constituents, voids, specific surfaces, functional sites, etc. This prompts us to study the sensing performance of a honeycomb nanochannel of heterometallic MOFs previously invented. This structure has excellent physical and chemical properties of high specific surface area, good chemical stability, and highly open coexistence of Lewis acid–base sites. In this work, the highly robust sky-blue [CoTb(CO2)6(OH2)]-based heterometallic framework of {[(CH3)2NH2][CoTb(TDP)(H2O)]·3H2O·4DMF}n (NUC-31; H6TDP = 2,4,6-tri(2′,4′-dicarboxyphenyl)pyridine) was synthesized. The results of the fluorescence recognition experiment show that, compared with other amino acids, NUC-31 has an ultrastrong fluorescence quenching for tryptophan with a detection limit as low as 0.11 mM, which means that NUC-31 can be used as a potential fluorescence probe for the targeted detection of tryptophan of ecosystems. In addition, the catalytic experiment results indicated that NUC-31 has high activity for catalyzing the cycloaddition reaction of epoxides with CO2 under 75 °C and 1 atm. It is precisely due to NUC-31 having extremely unsaturated tetracoordinated Co(II) and hepta-coordinated Tb(III) metal ions as well as a high pore volume (65.1%), which makes the catalytic reaction conditions relatively mild. Therefore, this work certificated that nanoporous MOFs assembled from a multifunctional ligand with the highly open coexistent Lewis acid–base sites had a potential application not only in monitoring tryptophan in clinical scenarios but also as an effective heterogeneous catalyst.