Crystal structure and photoluminescence of two europium compounds with phenoxyacetic acid and 2,4,6-tri(2-pyridyl)-s-triazine

Abstract

Two novel crystal compounds of Eu³⁺ and Gd³⁺/Eu³⁺ with phenoxyacetic acid (HPOA) and 2,4,6-tri(2-pyridyl)-s-triazine (TPTZ) have been synthesized. The two compounds are characterized by elemental analysis, rare earth coordination titrations, molar conductivity measurements, UV-vis absorption spectroscopy and IR spectroscopy. The crystal structures of compounds 1 Eu₂(TPTZ)₂(POA)₆·2CH₃OH and 2 EuGd(TPTZ)₂(POA)₆·2CH₃OH were determined by single-crystal X-ray diffraction. The two monocrystals belong to the triclinic system and space group P with the following unit cell parameters: a = 12.2448(10), 12.2476(6) Å; b = 13.2214(11), 13.2260(7) Å; c = 13.5248(12), 13.5210(7) Å; α = 74.8544(15), 74.8810(10)°; β = 83.0605(16), 83.0465(8)°; γ = 87.1996(14), 87.2126(8)°; V = 2097.7(7), 2098.5(19) ų and Z = 1, respectively. They are both dinuclear: one is homonuclear and the other is heteronuclear. Each metal ion is coordinately bonded to three nitrogen atoms of one TPTZ and seven oxygen atoms of three phenoxyacetate ions. Furthermore, there exist two coordinate forms between C₆H₅OCH₂COO⁻ and metal ions. One is chelating bidentate and the other is chelating and bridge coordinating. The triplet energy level of phenoxyacetic acid was measured, which is approximately 22 500 cm⁻¹, indicating that the lowest excitation state energy level of Eu(III) and the triplet state energy level of phenoxyacetic acid match well with each other. The luminescent emission intensity of both compounds was very strong. Besides, the results indicate that the luminescent emission intensity, luminescence lifetimes and the emission quantum efficiencies of Gd³⁺/Eu³⁺ compound 2 are remarkably superior to those of compound 1, respectively. This phenomenon may mainly result from the decrease of the concentration quenching effect of Eu³⁺ ions, and the intramolecular energy transfer from the ligands coordinated with Gd³⁺ ions to Eu³⁺ ions.