Crystal structure and photoluminescence of europium, terbium and samarium compounds with halogen-benzoate and 2,4,6-tri(2-pyridyl)-s-triazine

Three crystal compounds of Eu3+, Tb3+ and Sm3+ with o-chlorobenzoate (o-ClBA), o-bromobenzoate (o-BrBA) and 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) have been synthesized. These compounds were characterized by elemental analysis, UV-vis absorption spectroscopy and IR spectroscopy. The crystal structures of compound 1 [Eu2(TPTZ)2(o-ClBA)6(H2O)2]·2C2H5OH, 2 Tb2(TPTZ)2(o-BrBA)6(H2O)2 and 3 Sm2(TPTZ)2(o-BrBA)6(H2O)2 were determined by single-crystal X-ray diffraction. All the three monocrystals belong to the triclinic system, and space group P-1 with the following unit cell parameters. 1: a=10.276(5) Å, b=14.283(7) Å, c=14.439(7) Å; α=91.110(9)°, β=98.228(6)°, γ=105.605(11)°, V=2016.4(17) Å3, Z=1; 2: a=12.299(3) Å, b=13.202(4) Å, c=13.286(4) Å; α=112.904(3)°, β=99.038(4)°, γ=95.720(4)°, V=1931.6(10) Å3, Z=1; 3: a=12.314(4) Å, b=13.235(5) Å, c=13.352(5) Å; α=112.987(4)°, β=99.189(5)°, γ=95.701(4)°, V=1946.0(12) Å3, Z=1. All of them are dimeric complexes with each metal ion coordinately bonding to three nitrogen atoms of one TPTZ, six oxygen atoms of four halogen-benzoate ions and one coordinated water molecule. Furthermore, there exist three types of carboxylate coordination in all of the three crystals. One is unidentate coordination, the second is bidentate chelation, and the third is bridging coordination. The triplet energy levels of o-ClHBA and o-BrHBA were measured to be about 2.29×104 cm−1 (436 nm) and 2.34×104 cm−1 (427 nm), respectively. The energy gaps between the triplet energy levels of the two ligands (o-ClHBA and TPTZ) and the lowest excitation state energy levels of the Eu3+ ions are suitable for an effective intramolecular ligand-to-metal energy transfer. Therefore, the europium compound shows superior photoluminescent properties with long luminescence lifetime (1.72 ms) and high luminescence quantum efficiency (67%).