Auxiliary Ligand-Assisted Structural Variation of Cd(II) Metal–Organic Frameworks Showing 2D → 3D Polycatenation and Interpenetration: Synthesis, Structure, Luminescence Properties, and Selective Sensing of Trinitrophenol

Four new metal–organic frameworks (MOFs) of Cd(II) ion, [Cd(fdc)(bipy)1.5](1), [{Cd(fdc)(bpee)1.5}.3(H2O)](2), [Cd(fdc)(3bpdb)(H2O)](3), and [{Cd2(fdc)2(4bpdb)3}.1.5(4bpdb).2(H2O)](4) (where, fdc = 2,5-furandicarboxylate dianion, bipy = 4,4′-bipyridine, bpee = 1,2-bis(4-pyridyl)ethylene, 3bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene, and 4bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) have been successfully synthesized using mixed ligand systems and characterized by single crystal X-ray analysis and other physicochemical studies. Structural determination revealed that compounds 1 and 4 possess a rare 2D → 3D polycatenated pillared-bilayer structure with {48.62} SP 2-periodic net topology. Compound 2 has a novel threefold interpenetrating 3D hexagonal framework structure with {46.64} sqc-net topology, whereas compound 3 features a 2D Zig-Zag network with {44.62} sql-net topology. Interestingly, compounds 1 and 4 are the first examples of Cd(II) MOFs based on fdc ligand and bipy/4bpdb spacers exhibiting an unusual 2D → 3D polycatenation of bilayers. Photoluminescence investigation revealed emissions from compounds 1–4 owing to ligand based charge transfer (n → π* and π → π*) transitions. The luminescent emission of 2 can be quenched by addition of trace amount of 2,4,6-trinitrophenol (TNP), demonstrating its potential application for highly selective and sensitive detection of TNP. The influence of N,N′-donor spacers on the structure, topology, and the functionality of the resulting MOFs has been demonstrated.