Heteronuclear lanthanide titanium-oxygen cluster luminescence thermometer with adjustable operating range and sensitivity

Given customizable crystal structure and intriguing optical properties, lanthanide titanium-oxygen clusters (LTOCs) with atomic-level accuracy have gained a lot of interest. In this study, we prepared [Ln₉Ti₂(μ₄–O)(μ₃–OH)₁₄(acac)₁₇(CH₃O)₂(CH₃OH)₃] (Ln = Tb_xEu_9−x (x = 0, 4, 6, 7, 8, 9), Hacac = acetylacetone), Tb³⁺ and Eu³⁺ co-doped LTOCs, to modify the optical properties for the luminescence thermometer. In detail, the serial LTOCs display dual characteristic emission peaks of ⁵D₄ → ⁷F₅ for Tb³⁺ and ⁵D₀ → ⁷F₂ for Eu³⁺ at 548 and 616 nm, respectively, under 330 nm excitation. Effective energy transfer (ET) between Tb³⁺ ions and Eu³⁺ ions was revealed in terms of both emission spectra and luminescence lifetime. The ⁵D₀ → ⁷F₂ emission intensity of Eu³⁺ ions at 616 nm is maximally enhanced (by a factor of 11.2) with a change in the relative molar ratio of Tb³⁺ to Eu³⁺, along with a change in the ET efficiency of Tb³⁺  → Eu³⁺. In addition, the luminescent color changes from red, orange, yellow, to green. This precise control of the ET process between rare-earth ions allows {Tb₆Eu₃Ti₂} to reach a maximum relative sensitivity of 1.241 K⁻¹ at 355 K, which is an enhancement of up to 4.6-fold with respect to the previously reported homonuclear emission, holding great potential in the optical thermometers.