Highly Luminescent and Thermally Stable Lanthanide Coordination Polymers Designed from 4‑(Dipyridin-2-yl)aminobenzoate: Efficient Energy Transfer from Tb3+ to Eu3+ in a Mixed Lanthanide Coordination Compound

Herein, a new aromatic carboxylate ligand, namely, 4-(dipyridin-2-yl)­aminobenzoic acid (HL), has been designed and employed for the construction of a series of lanthanide complexes (Eu3+ = 1, Tb3+ = 2, and Gd3+ = 3). Complexes of 1 and 2 were structurally authenticated by single-crystal X-ray diffraction and were found to exist as infinite 1D coordination polymers with the general formulas {[Eu­(L)3(H2O)2]}n (1) and {[Tb­(L)3(H2O)].(H2O)}n (2). Both compounds crystallize in monoclinic space group C2/c. The photophysical properties demonstrated that the developed 4-(dipyridin-2-yl)­aminobenzoate ligand is well suited for the sensitization of Tb3+ emission (Φoverall = 64%) thanks to the favorable position of the triplet state (3ππ*) of the ligand [the energy difference between the triplet state of the ligand and the excited state of Tb3+ (ΔE) = 3ππ* – 5D4 = 3197 cm–1], as investigated in the Gd3+ complex. On the other hand, the corresponding Eu3+ complex shows weak luminescence efficiency (Φoverall = 7%) due to poor matching of the triplet state of the ligand with that of the emissive excited states of the metal ion (ΔE = 3ππ* – 5D0 = 6447 cm–1). Furthermore, in the present work, a mixed lanthanide system featuring Eu3+ and Tb3+ ions with the general formula {[Eu0.5Tb0.5(L)3(H2O)2]}n (4) was also synthesized, and the luminescent properties were evaluated and compared with those of the analogous single-lanthanide-ion systems (1 and 2). The lifetime measurements for 4 strongly support the premise that efficient energy transfer occurs between Tb3+ and Eu3+ in a mixed lanthanide system (η = 86%).