Sensitised near-IR lanthanide luminescence exploiting anthraquinone-derived chromophores: syntheses and spectroscopic properties

Abstract

The syntheses of two anthraquinone-derived tetraaza macrocyclic ligands (L1via 1-amino-9,10-anthraquinone and L2via 1-amino-4-hydroxy-9,10-anthraquinone) together with their corresponding Ln^III complexes, Ln-L1/2 (Ln = Nd^III, Gd^III, Er^III, Yb^III), are described. Both Ln-L1 (λ_max≈ 380 nm) and Ln-L2 (λ_max≈ 450 nm) complexes absorb in the visible region with good extinction coefficients (ε_vis > 2 × 10³ M⁻¹ cm⁻¹). Phosphorescence measurements on Gd-L1/2 at 77 K allowed the ligand-centred triplet states to be estimated at ca. 22000 cm⁻¹ and 19800 cm⁻¹ for Gd-L1 and Gd-L2 respectively. Steady state and time-resolved measurements showed that both chromophores sensitised Nd^III, Er^III and Yb^III ions, resulting in observable near-IR emission. Preliminary studies on the pH-dependent behaviour of the L2 derivatives demonstrated that deprotonation of the 4-hydroxyl group at pH 12 resulted in a significant bathochromic shift in the absorption profile, thus allowing sensitised near-IR emission utilising λ_ex = 575 nm.