Azido- and amino-substituted dipicolinates for the sensitization of the luminescent lanthanides EuIII and TbIII

Photocytotoxicity of Thiophene- and Bithiophene-Dipicolinato Luminescent Lanthanide Complexes

New thiophene-dipicolinato-based compounds, K₂nTdpa (n = 1, 2), were isolated. Their anions are sensitizers of lanthanide ion (Ln^III) luminescence and singlet oxygen generation (¹O₂). Emission in the visible and near-infrared regions was observed for the Ln^III complexes with efficiencies (ϕ^Ln) ϕ^Eu = 33% and ϕ^Yb = 0.31% for 1Tdpa^2- and ϕ^Yb = 0.07% for 2Tdpa^2-. The latter does not sensitize Eu^III emission. Fluorescence imaging of HeLa live cells incubated with K₃[Eu(1Tdpa)₃] indicates that the complex permeates the cell membrane and localizes in the mitochondria. All complexes generate ¹O₂ in solution with efficiencies (ϕO12) as high as 13 and 23% for the Gd^III complexes of 1Tdpa^2- and 2Tdpa^2-, respectively. [Ln(nTdpa)₃]^3- (n = 1, 2) are phototoxic to HeLa cells when irradiated with UV light with IC₅₀ values as low as 4.2 μM for [Gd(2Tdpa)₃]^3- and 91.8 μM for [Eu(1Tdpa)₃]^3-. Flow cytometric analyses indicate both apoptotic and necrotic cell death pathways.

Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) Å3. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) Å3. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm-1 and efficient energy transfer is observed for EuIII and TbIII. Solutions of [Ln(PyboxPh)3]3+ in dichloromethane display an emission efficiency of 37.2% for Ln[double bond, length as m-dash]Eu and 24.0% for Ln[double bond, length as m-dash]Tb. The excited state lifetimes for EuIII and TbIII are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm-1 and is therefore too low to efficiently sensitize TbIII emission. However, the sensitization of EuIII is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with EuIII (3) and TbIII (4) were also isolated and crystallize in the triclinic space group P1[combining macron] with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) Å3 and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) Å3, respectively.