Highly luminescent and thermally stable lanthanide coordination polymers designed from 4-(Dipyridin-2-yl)aminobenzoate: Efficient energy transfer from Tb ~(3+) to Eu ~(3+) in a mixed lanthanide coordination compound

摘要

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 (Eu ~(3+) = 1, Tb ~(3+) = 2, and Gd ~(3+) = 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(H _2O) _2]} n (1) and {[Tb(L) _3(H _2O)].(H _2O)} _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 Tb ~(3+) emission (φ _(overall) = 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 Tb ~(3+) (ΔE) = ~3ππ* - ~5D _4 = 3197 cm ~(-1)], as investigated in the Gd ~(3+) complex. On the other hand, the corresponding Eu ~(3+) complex shows weak luminescence efficiency (φ _(overall) = 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* - ~5D _0 = 6447 cm ~(-1)). Furthermore, in the present work, a mixed lanthanide system featuring Eu ~(3+) and Tb ~(3+) ions with the general formula {[Eu _(0.5)Tb _(0.5)(L) _3(H _2O) _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 Tb ~(3+) and Eu ~(3+) in a mixed lanthanide system (≠= 86%).