Near-White Light Emission from Lead(II) Metal-Organic Frameworks

Reaction of bpy (bpy = 4,4'-bipyridine) with Pb(OAc)₂·3H₂O in DMF (DMF = dimethylformamide) afforded a metal-organic framework (MOF), [Pb₂(μ-bpy)(μ-O₂CCH₃)₂(μ-O₂CCH₃)₂]·H₂O (1). Reaction of bpy with Pb(O₂CCF₃)₂ in a methanol and chloroform mixture furnished another MOF, [Pb(μ-bpy)(μ-O₂CCF₃)₂]·¹/₂CHCl₃ (2). However, the reaction of bpy with Pb(OAc)₂·3H₂O in the presence of trifluoroacetic acid in a similar reaction condition yielded a hydrogen-bonded zwitter-ionic complex of Pb(II), [Pb(bpy-H)₂(O₂CCF₃)₄] (3). All compounds have been characterized by single crystal X-ray crystallography, FT-IR, and ¹H NMR spectroscopies. Compound 1 forms four heptacoordinated Pb(II) joined by (OCCH₃)-O- linkages, resulting in a 3D noninterpenetrated MOF net with a four-connected uninodal sra (SrAl₂) topology. However, in 2, tetra-connected Pb₄(O₂CCF₃)₈ cluster units are linked further through eight bpy ligands to furnish a doubly interpenetrated MOF with a new topology but having the very similar connectivity of 1, whereas 3 forms a zigzag hydrogen-bonded chain structure. The variation of carboxylate anions, pH of the reaction medium, and the ratio of the reactants profoundly affected the final topological structure of the compounds synthesized. The solid-state photoluminescence of 1-3 was investigated at room temperature. Interestingly 1, 2, and 3 achieved close to white light emission when excited at 329, 376, and 330 nm, respectively. The systematic understanding of the photophysical properties of analogous Pb-based compounds may open new perspectives for developing single-phase white-light-emitting materials using Pb(II) based MOFs.