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]_<i>n</i>·2<i>n</i>(H₂O) (<b>1</b>), [Cd(IDPA)(bpe)_1/2(H₂O)]_<i>n</i>·<i>n</i>(H₂O) (<b>2</b>), [Zn(IDPA)(bpa)]_<i>n</i>·5<i>n</i>(H₂O) (<b>3</b>), [Zn(IDPA)(bpe)_1/2]_<i>n</i> (<b>4</b>), and [Zn(IDPA)(bpp)]_<i>n</i>·<i>n</i>(H₂O) (<b>5</b>). Structural analyses reveal diverse architectures ranging from 2D layers (<b>1</b>-<b>3</b>) with sql topology ({4⁴·6²}) to 3D frameworks: a {4·6·8}{4·6²·8³} net (<b>4</b>) and a <b>dia</b>-type network (<b>5</b>). 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 <b>3</b> 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.