Photocytotoxicity of Thiophene- and Bithiophene-Dipicolinato Luminescent Lanthanide Complexes

Abstract

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 (${\varphi }_{{{}^{1}O}_2}$) 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.