Lanthanide-Based Coordination Polymers Assembled from Derivatives of 3,5-Dihydroxy Benzoates: Syntheses, Crystal Structures, and Photophysical Properties

Two new aromatic carboxylic acids, namely, 3,5-bis(benzyloxy)benzoic acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), have been prepared by replacing the hydroxyl hydrogens of 3,5-dihydroxy benzoic acid with benzyl and pyridyl moieties, respectively. The anions derived from HL1 and HL2 have been used for the support of a series of lanthanide coordination compounds [Eu³⁺ = 1–2; Tb³⁺ = 3–4; Gd³⁺ = 5–6]. The new lanthanide complexes have been characterized on the basis of a variety of spectroscopic techniques in conjunction with an assessment of their photophysical properties. Lanthanide complexes 2, 4, and 6, which were synthesized from 3,5-bis(pyridine-2-ylmethoxy)benzoic acid, were structurally authenticated by single-crystal X-ray diffraction. All three complexes were found to exist as infinite one-dimensional (1-D) coordination polymers with the general formula {[Ln(L2)₃(H₂O)₂]·xH₂O}_n. Scrutiny of the packing diagrams for 2, 4, and 6 revealed the existence of interesting two-dimensional molecular arrays held together by intermolecular hydrogen-bonding interactions. Furthermore, the coordinated benzoate ligands serve as efficient light harvesting chromophores. In the cases of 1–4, the lowest energy maxima fall in the range 280–340 nm [molar absorption coefficient (ε) = (0.39–1.01) × 10⁴ M^–1 cm^–1]. Moreover, the Tb³⁺ complexes 3 and 4 exhibit bright green luminescence efficiencies in the solid state (Φ_overall = 60% for 3; 27% for 4) and possess longer excited state lifetimes than the other complexes (τ = 1.16 ms for 3; 1.38 ms for 4). In contrast to the foregoing, the Eu³⁺ complexes 1 and 2 feature poor luminescence efficiencies.