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.