Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs

A new series of luminescent heterometallic europium(III)–lutetium(III) terephthalate metal–organic frameworks, namely (Eu(x)Lu(1−x))(2)bdc(3)·nH(2)O, was synthesized using a direct reaction in a water solution. At the Eu(3+) concentration of 1–40 at %, the MOFs were formed as a binary mixture of the (Eu(x)Lu(1−x))(2)bdc(3) and (Eu(x)Lu(1−x))(2)bdc(3)·4H(2)O crystalline phases, where the Ln(2)bdc(3)·4H(2)O crystalline phase was enriched by europium(III) ions. At an Eu(3+) concentration of more than 40 at %, only one crystalline phase was formed: (Eu(x)Lu(1−x))(2)bdc(3)·4H(2)O. All MOFs containing Eu(3+) exhibited sensitization of bright Eu(3+)-centered luminescence upon the 280 nm excitation into a (1)ππ* excited state of the terephthalate ion. The fine structure of the emission spectra of Eu(3+ 5)D(0)-(7)F(J) (J = 0–4) significantly depended on the Eu(3+) concentration. The luminescence quantum yield of Eu(3+) was significantly larger for Eu-Lu terephthalates containing a low concentration of Eu(3+) due to the absence of Eu-Eu energy migration and the presence of the Ln(2)bdc(3) crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln(2)bdc(3)·4H(2)O.