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-9<i>H</i>-fluorene-2,7-dicarboxylic acid (H₂L1) with 4,4'-(9,9-diethyl-9<i>H</i>-fluorene-2,7-diyl)dipyridine (L2) or 2,7-di(pyridin-4-yl)-9<i>H</i>-fluorene (L3) affords two topologically identical frameworks with distinct interpenetration degrees: the 3-fold interpenetrated [Zn₄(L1)₄(L2₎₂]<i>_n</i> (<b>1</b>) and the 4-fold interpenetrated [Zn₄(L1)₄(L3)₂]<i>_n</i> (<b>2</b>). Single-crystal X-ray analysis shows that <b>1</b> contains fully coordinated dinuclear paddlewheel clusters, whereas <b>2</b> features both fully and partially coordinated clusters, leaving exposed carboxylate oxygen atoms as accessible interaction sites. In aqueous media, compound <b>1</b> selectively detects Au³⁺ via fluorescence quenching with a 0.99 μM detection limit, while compound <b>2</b> 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.