Ligand-Driven Turn-On Fluorescence in Zn/Cd(II) MOFs: Enhancing Emission via Aggregation-Induced Mechanism

To explore the influence of flexible auxiliary ligands on the construction of Zn/Cd(II) coordination polymers with the rigid π-conjugated ligand H2IDPA (5-(1-oxoisoindolin-2-yl)isophthalic acid), five new compounds were synthesized: [Cd2(IDPA)2(bpa)(H2O)2]n·2n(H2O) (1), [Cd(IDPA)(bpe)1/2(H2O)]n·n(H2O) (2), [Zn(IDPA)(bpa)]n·5n(H2O) (3), [Zn(IDPA)(bpe)1/2]n (4), and [Zn(IDPA)(bpp)]n·n(H2O) (5). Structural analyses reveal diverse architectures ranging from 2D layers (1-3) with sql topology ({44·62}) to 3D frameworks: a {4·6·8}{4·62·83} net (4) and a dia-type network (5). These variations highlight the critical role of auxiliary ligand flexibility in directing topology. Photoluminescence studies show distinct emissions (violet to green) under UV light. Notably, compound 3 exhibits an enhanced quantum yield (14.21%) due to aggregation-induced emission (AIE). All compounds display excellent structural stability upon solvent removal. This study demonstrates that combining flexible auxiliary ligands with rigid π-conjugated linkers offers an effective strategy to tune both framework structures and luminescent properties, facilitating the design of functional luminescent materials.