Highly Selective Ratiometric Sensors for Pb2+ Based on Luminescent Zn(II)-Coordination Polymers with Thiophenedicarboxylate. Crystal Structures and Spectroscopic Studies.

The development of luminescent coordination polymers for the selective sensing of Pb²⁺ in water constitutes an active area of research that impacts analytical, environmental, and inorganic chemistry. Herein, two novel water-stable 2D Zn-coordination polymers {[Zn₂(H₂O)₂(tdc)₂(bpy)]·(H₂O)}_n 1 and [Zn(tdc)(tmb)]_n 2 (tdc = thiophenedicarboxylate; bpy = 4,4'-bipyridine and tmb = 4,4'-trimethylenebipyridine) were synthesized, structurally determined by single crystal X-ray diffraction, and studied in-depth as luminescent sensors for a series of cations (Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ Cd²⁺, Hg²⁺ and Pb²⁺) in 20% aqueous ethanol. These Zn-polymers possess photostability in 20% aqueous ethanol with a strong emission at 410 upon excitation at 330 nm and quantum yields of around Φ = 0.09. Under these conditions, Pb⁺² can be efficiently sensed with polymer 2 through a fluorescent ratiometric response with selectivity over common interfering metal ions such as Cu²⁺, Cd²⁺ and Hg²⁺ in the micromolar concentration range (detection limit = 1.78 ± 10 μM). Such selectivity/affinity of Pb²⁺ over Hg²⁺ for luminescent chemosensors is still rare. On the basis of spectroscopic tools (¹H NMR, far ATR-IR, PXRD), the X-ray crystal structure of 2, and Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopic analysis, the ratiometric fluorescent response is proposed via an efficient metal-ion exchange driven through interactions between thiophenedicarboxylate rings and Pb²⁺ ions. The use of flexible luminescent Zn-coordination polymers as sensors for selective and direct detection of Pb²⁺ in aqueous media has been unexplored until now.