Pb(II) Coordination Polymers: Crystal Engineering, Photophysical Behavior, and Mechanistic Insights Into Acetone Detection

Two new Pb(II)-based coordination polymers (CPs) with chemical formulas, [Pb(L₁)₂(H₂O)]_n (CP1) and [Pb(L₂)(DMF)]_n (CP2), were synthesized under solvothermal conditions using adamantane-1-carboxylic acid (HL₁) and 5-nitroisophthalic acid (H₂L₂) as organic linkers. Single-crystal X-ray diffraction shows that CP1 adopts a 1D chain structure built from mixed chelating/bridging μ₄-adamantane carboxylates and aqua ligands, while CP2 forms a robust 2D sheet network supported by μ₃/μ₄ carboxylate modes of H₂L₂ {L₂^2-} and coordinated DMF. Photophysical studies reveal ligand-centered enhanced fluorescence upon coordination to Pb(II), attributed to framework-induced rigidity of the HL₁ and π-conjugated H₂L₂ linkers. Both CPs show negligible response toward metal ions, anions, and amino acids. However, they display highly selective fluorescence quenching toward acetone. Both CP1 and CP2 exhibit low detection limits (LOD) of 24.22 μM and 3.32 μM, respectively. Stern–Volmer analysis, complemented by density functional theory calculations, indicates that acetone-induced quenching is attributed to a photoinduced electron transfer (PET) pathway, facilitated by the energy-level alignment and weak host–guest interactions including possible hydrogen bonding between acetone and the framework of CPs 1 and 2. Notably, both CP1 and CP2 are recyclable over five cycles with minimal loss in the sensing activity.