Heterometallic 3-D Zn/Ca Metal-Organic Frameworks Based on V-Shaped Angular Tetracarboxylic Ligands as Selective Fluorescence Sensors for Nitroaromatic Explosive Vapors.

A family of heterometallic Zn/Ca MOFs based on angular tetracarboxylic ligands with formulas [ZnCa(L)(S)(S')]n (S, S' = H2O or S = H2O, S' = DMF) UCY-18(L) (H4L = 4,4'-hexafluoroisopropilidene diphthalic acid (H4HFPD), 3,3',4,4'-benzophenone tetracarboxylic acid (H4BPTC), 4,4'-oxydiphthalic acid (H4ODPA), 4,4'-azanediyl diphthalic acid (H4ADPA)) is reported. The crystal structures of UCY-18(L) comprise 3-D networks based on helical one-dimensional chain SBUs [ZnCa(L4-)] containing tetragonal channels along the crystallographic a-axis. Gas sorption studies on activated UCY-18(L) indicated appreciable BET surface areas ranging from 1338 to 2134 m2 g-1. Selected vapor sorption studies indicated the affinity of these materials toward representative C6-aromatic and nonaromatic organic molecules. Thin films of UCY-18(HFPD), UCY-18(ODPA), and UCY-18(ADPA) embedded in poly(vinylidene fluoride) were fabricated and evaluated for their sensing capability for vapors of selected nitroaromatic compounds. The thin films exhibit high selectivity in detecting 2,4,6-trinitrophenol and 2,4,6-trinitrotoluene vapors among other nitroaromatic compounds and interferents at equal concentration. The crystal structures of UCY-18(HFPD) loaded with nitrobenzene and 2-nitrotoluene provided useful information about the interactions of the MOFs with these nitroaromatic molecules. Overall, this work highlights the potential of heterometallic Zn/Ca MOF chemistry to afford microporous MOFs containing highly accessible metal centers and displaying the capability to recognize vapors of selected analytes.