Eu(III) and Tb(III) Luminescence Sensitized by Thiophenyl-Derivatized Nitrobenzoato Antennas

Thiophenyl-derivatized nitrobenzoic acid ligands have been evaluated as possible sensitizers of Eu(III) and Tb(III) luminescence. The resulting solution and solid-state species were isolated and characterized by luminescence spectroscopy and X-ray crystallography. The Eu(III) complex with 2-nitro-3-thiophen-3-yl-benzoic acid, 1, crystallizes in the monoclinic space group C2/c with a = 28.569(3) Å, b = 17.7726(18) Å, c = 17.7073(18) Å, β = 126.849(2)°, and V = 7194.6(13) Å3. The Tb(III) complex with this ligand, 2, is isostructural, and its cell parameters are a = 29.755(6) Å, b = 18.123(4) Å, c = 19.519(4) Å, β = 130.35(3)°, and V = 8021(3) Å3. Eu(III) crystallizes with 3-nitro-2-thiophen-3-yl-benzoic acid as a triclinic complex, 3, in the space group P1̄ with a = 11.045(2) Å, b = 12.547(3) Å, c = 15.500(3) Å, α = 109.06(3)°, β = 94.79(3)°, γ = 107.72(3)°. and V = 1893.5(7) Å3. With the ligand 5-nitro-2-thiophen-3-yl-benzoic acid, Eu(III) yields another molecular compound, 4, triclinic P1̄, with a = 10.649(2) Å, b = 14.009(3) Å, c = 15.205(3) Å, α = 112.15(3)°, β = 100.25(3)°, γ = 106.96(3)°, and V = 1900.5(7) Å3. All compounds dissolve in water and methanol, and the methanolic solutions are luminescent. The solution species have a metal ion-to-ligand ratio of 1:1. The quantum yields have been determined to be in the range of 0.9−3.1% for Eu(III) and 4.7−9.8% for Tb(III). The highest values of these correspond to the most intense luminescence reported for Ln(III) solutions with this type of sensitizer. The lifetimes of luminescence are in the range of 248.3−338.9 μs for Eu(III) and 208.6−724.9 μs for Tb(III). The stability constants are in the range of log β11 = 2.73−4.30 for Eu(III) and 3.34−4.18 for Tb(III) and, along with the energy migration pathways, are responsible for the reported efficiency of sensitization.