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

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 LnIII complexes, Ln-L1/2 (Ln = NdIII, GdIII, ErIII, YbIII), are described. Both Ln-L1 (λmax≈ 380 nm) and Ln-L2 (λmax≈ 450 nm) complexes absorb in the visible region with good extinction coefficients (εvis > 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 NdIII, ErIII and YbIII 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 λex = 575 nm.