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.