The Structure and Optical Properties of Luminescent Europium Terephthalate Antenna Metal–Organic Frameworks Doped by Yttrium, Gadolinium, and Lanthanum Ions

New heterometallic antenna terephthalate MOFs, namely, (EuxM1−x)2bdc3·4H2O (M = Y, La, Gd) (x = 0.001–1), were synthesized by a one-step method from aqueous solutions. The resulting compounds are isomorphic to each other; the crystalline phase corresponds to Ln2bdc3∙4H2O. Upon 300 nm excitation to the singlet excited state of terephthalate ions, all compounds exhibit a bright red emission corresponding to the of 5D0–7FJ (J = 0–4) f-f transitions of Eu3+ ions. The Eu(III) concentration dependence of the photophysical properties was carefully studied. We revealed that Gd-doping results in photoluminescence enhancement due to the heavy atom effect. To quantitatively compare the antenna effect among different compounds, we proposed the new approach, where the quantum yield of the 5D0 formation is used to characterize the efficiency of energy transfer from the ligand antenna to the Eu3+ emitter.