Europium and Terbium Coordination Polymers Assembled from Hexacarboxylate Ligands: Structures and Luminescent Properties

Six lanthanide coordination polymers of the formula [Ln­(L¹)_0.5(H₂O)₂]·2H₂O [where Ln³⁺: Eu³⁺ (1), Tb³⁺ (2), and Gd³⁺(3)] and [Me₂NH₂]­[Ln­(H₂L²)­(H₂O)₄]·0.5DMF·xH₂O [where Ln³⁺: Eu³⁺ (4), Tb³⁺ (5), and Gd³⁺(6)], based on p-terphenyl-2,2″,2‴,5,5″,5‴-hexacarboxylate acid (H₆L¹), and p-terphenyl-3,2″,3″,5,5″,5‴,-hexacarboxylate acid (H₆L²), have been solvothermally synthesized and structurally characterized. Complexes 1–3 are 3D frameworks exhibiting 6-connected pcu alpha-Po primitive cubic network with topology (4¹².6³), while complexes 4–6 show two-dimensional (2D) architectures showing simplified 3,4-connected binodal net and (4.6²)­(4².6².8²) topology. Detailed photophysical behaviors have been explored on Eu³⁺, Tb³⁺, and Gd³⁺ complexes. The calculated triplet state energies of H₆L¹ and H₆L² lie above the emissive levels of Eu³⁺ or Tb³⁺ in an ideal range for sensitizing. Furthermore, it is demonstrated that the optimum energy gap between the triplet state of ligand H₆L¹ and the emissive level of Tb³⁺ ion makes the overall quantum yield of Tb³⁺ complex (2) larger than its corresponding Eu³⁺ complex (1). In addition, the coordinated water in the inner sphere has a significant negative influence on the overall quantum yield, especially for the Eu³⁺ complex (4) compared to the Tb³⁺ complex (5), due to the deactivation process caused by vibrational OH oscillators.