Subtle Ligand-Substituent Steric Modifications Control Framework Interpenetration with Distinct Luminescent Responses to Coinage Metal Ions

Framework interpenetration is often considered detrimental to porosity in metal-organic frameworks (MOFs), yet it can rigidify lattices and create confined environments that enhance guest-responsive photophysics. Here, a subtle 9-position substituent change on fluorene-based dipyridyl ligands regulates interpenetration and ion-sensing behavior in a unified Zn(II) platform. Combining 9,9-diethyl-9H-fluorene-2,7-dicarboxylic acid (H2L1) with 4,4′-(9,9-diethyl-9H-fluorene-2,7-diyl)dipyridine (L2) or 2,7-di(pyridin-4-yl)-9H-fluorene (L3) affords two topologically identical frameworks with distinct interpenetration degrees: the 3-fold interpenetrated [Zn4(L1)4(L2)2]n (1) and the 4-fold interpenetrated [Zn4(L1)4(L3)2]n (2). Single-crystal X-ray analysis shows that 1 contains fully coordinated dinuclear paddlewheel clusters, whereas 2 features both fully and partially coordinated clusters, leaving exposed carboxylate oxygen atoms as accessible interaction sites. In aqueous media, compound 1 selectively detects Au3+ via fluorescence quenching with a 0.99 μM detection limit, while compound 2 shows multi-ion quenching toward Au3+, Ag+, and Cu2+ with detection limits of 2.42, 2.51, and 2.15 μM, respectively. These results establish a clear structure–property relationship linking subtle linker sterics to interpenetration degree, the saturation and presence of open sites on the clusters, and coinage-metal-dependent fluorescence quenching.