Observation of divergent La³⁺ ion dilute effect in two series of 3-D fluorescent lanthanide-MOFs-based molecular alloys RE_xLa_1-x-EBTC (RE³⁺=Eu³⁺ or Tb³⁺; EBTC^4-=1,1′-ethynebenzene-3,3′,5,5′-etracarboxylate)

Abstract The lanthanide metal-organic frameworks (MOFs) [Ln₂(EBTC)_1.5(CH₃OH)₄]·6H₂O are isostructural to each other, where EBTC^4-=1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate; Ln³⁺=La³⁺, Eu³⁺ and Tb³⁺; and the corresponding MOF is abbreviated as Ln-EBTC. MOFs Eu-EBTC and Tb-EBTC emit intense red and green luminescence (visible by bare eyes), respectively. The molecular alloys of Eu_xLa_1-x-EBTC and Tb_xLa_1-x-EBTC have been successfully prepared by mixing Eu³⁺/Tb³⁺ and La³⁺ salts with the desired molar ratio in the starting material. Two series of Eu_xLa_1-x-EBTC and Tb_xLa_1-x-EBTC molecular alloys are isomorphic to the parent Ln-EBTC MOFs, while exhibit divergent La³⁺ ion diluting effect, namely, with increasing the relative amount of La³⁺, the intensity of characteristic emission arising from Tb³⁺ ion monotonely increases in Tb_xLa_1-x-EBTC molecular alloys, whereas that of Eu³⁺ ion shows non-monotone decrease in Eu_xLa_1-x-EBTC molecular alloys. The possible origin is discussed for such a divergent behavior between Eu_xLa_1-x-EBTC and Tb_xLa_1-x-EBTC molecular alloys.