Multimetal rare earth MOFs for lighting and thermometry: tailoring color and optimal temperature range through enhanced disulfobenzoic triplet phosphorescence

Abstract

New isostructural compounds [Ln₇(3,5-DSB)₄(OH)₉(H₂O)₁₅]·4H₂O (Ln = Eu, Gd and Tb) (DSB = 3,5-disulfobenzoate), with excellent optical properties, have been obtained and studied. By combining the lanthanide cations with the DSB ligand the formation of a singular heptanuclear [Ln₇(OH)₉]⁺¹² metallic core secondary building unit (SBU) has been promoted. This core is defined as a bi-capped dicubane SBU, and is a 4-connected node in a bidimensional net with (4⁴·6²) topology. The highly efficient emission from the triplet state of the disulfobenzoic ligand in the Gd-MOF and the exponential temperature dependence of I_Triplet/I_Ln = I_o exp(−E_A/k_BT) allow the design of thermometers with different color ranges through the adequate multimetal combination that cover almost the whole visible spectrum and are useful in a wide temperature range (10–300 K) with unprecedented sensitivities up to 32% K⁻¹. The Eu-DSB and Tb-DSB MOFs present an optimum antenna effect, a temperature independent f–f emission temporal decay and, in spite of the close packing of the Ln ions inside the MOF, no concentration quenching. The external efficiency is drastically increased in MOFs with a combination of a few percent of Eu and/or Tb with Gd by means of radiative and non-radiative energy transfers from the Gd ions and Gd connected ligands.