Luminescent lanthanide-2-phenylpyrimidine-carboxylate frameworks: structure and luminescence tuning

Six novel luminescent lanthanide-2-phenylpyrimidine-carboxylate frameworks, namely, [Ln(ppmc)₃(phen)] (Ln = Eu (1), Tb (2)), [Ln(ppmc)₂(C₂O₄)_0.5(H₂O)] (Ln = Eu (3), Tb (4)), {[Eu(ppmdc)(phen)(C₂O₄)_0.5]·0.5H₂O} (5) and [Tb(ppmdc)(benzoate)(phen)] (6) (Hppmc = 2-phenylpyrimidine-4-carboxylic acid, H₂ppmdc = 2-phenylpyrimidine-4,6-dicarboxylic acid) have been synthesized. The X-ray structure analyses reveal that all of the compounds contain carboxylate-bridged dimer units; however, coordination environments of Ln³⁺ ions, bridging modes of carboxylate, and the linkage between them are different. In the isostructural compounds 1 and 2, extensive π–π stacking exists between the anti–anti carboxylate bridged dimer. However, syn–anti carboxylate bridges are found in compounds 3 and 4 in which oxalate connects the dimers to form 1D chains. Compound 5 has a 2D structure connected by both μ₄-ppmdc²⁻ and oxalate in which the carboxylates adopt both anti–anti and chelate-bridging modes. Compound 6 featuring 1D chains also contains anti–anti carboxylate-bridged dimers that are linked by the μ₃-ppmdc²⁻ ligand. All six compounds show characteristic red or green emission attributed to Eu³⁺ and Tb³⁺. The luminescence lifetime and quantum yield are presented and discussed in detail. The use of different ligands and reaction conditions has led to distinct coordination environments of lanthanide ions and the aggregation of dimeric units, which are considered as the dominating factors for the resulting photoluminescence behaviour of solid samples.