Ln(iii)-cored complexes based on boron dipyrromethene (Bodipy) ligands for NIR emission

Abstract

Two Bodipy-based chromophores have been synthesized, which bear two methoxyphenyl substituents in the 1,7 positions and either hydrogen (L1) or bromine (L2) atoms in the 2,6 positions, with the aim of testing their ability to sensitize the luminescence of near-infrared emitting LnIII ions. Despite the presence of the two halogen substituents in L2, the two chromophores display very similar absorption and emission properties, the only difference being a drop in the quantum yield from 31% for L1 to 21% for L2. In addition, no triplet state emission could be evidenced for both chromophores as well as for their tris complexes with GdIII ions. This is traced back to unfavorable conformation of the molecule, preventing efficient intersystem crossing. Complexes with stoichiometry [Ln(Li)3(tpy)] have been isolated for Ln = Gd, Er, Yb and fully characterized. Ligand emission is partially quenched (20–40%) in the ErIII and YbIII complexes and typical metal-centred luminescence is detected at 1530 (ErIII) and 978 (YbIII) nm upon excitation in the orange portion of the visible spectrum (583 nm). In these complexes, the main energy transfer pathway involves the singlet state(s) of the ligands: the lowest 1ππ state emission overlaps the Er(4F9/2 ← 4I15/2) absorption band and the 1ππ–Yb(2F5/2) gap amounts to only 5700 cm−1. This study demonstrates that the Bodipy framework is adequate for sensitizing the luminescence of NIR-emitting lanthanide ions, allowing excitation wavelengths extending into the orange portion of the visible spectrum and yielding complexes with large molar absorption coefficients (log e ≈ 5.0–5.2). Additionally it also points to the importance of the conformation of the chromophores on the yield of intersystem crossing.