Ion Site Substitution in a Sulfonylcalix[4]arene-Supported Ln8 (Ln = Tb and Eu) Coordination Wheel with Tunable Luminescence

Abstract

Lanthanide coordination clusters (LCCs), namely, [Ln₈(TC4A-SO₂)₄(TA)₂(DMF)₄(H₂O)₁₀] (solvents) (Ln₈ = Tb₈, Eu₈, Tb₄Eu₄; H₄TC4A-SO₂ = p-tert-butylsulfonylcalix[4]arene; H₄TA = dl-tartaric acid), were synthesized via the solvothermal method and were characterized by single-crystal X-ray diffraction. Ln₈ is a wheel-shaped framework built from four cone-shaped mononuclear units (Ln^III-TC4A-SO₂) and four bridging-Ln^III ions with two tartrates located in the center serving as “spokes”. Tb₈ exhibits an extraordinary luminescence quantum yield (QY_s) of 53.3% in the solid state under the excitation of 370 nm, while Eu₈ cannot emit under similar conditions. Furthermore, Tb₈ shows concentration-dependent emission in a methanol solution with the best luminescence QY_l of 61.8%, representing one of the best luminescent Tb-based coordination clusters. The Eu ions can be rationally doped in the Tb ring (Tb_xEu_8–x, 4 ≤ x < 8) by a stoichiometric ratio on specific sites to tune the emission color output, which reveals the efficient energy transfer from ⁵D₄-⁷F₅ and ⁵D₄-⁷F₆ levels of Tb^III to the ⁵D₀ level of Eu^III. This work presents a unique method of controlling the fabrication and tunable luminescence of LCCs.