Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1‑Hydroxypyridin-2-one Complexes in Aqueous Solution

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu­(III) and Sm­(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φ<sub>tot</sub>) were found to vary with the linker length, and the same trend was observed for the Eu­(III) and Sm­(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φ<sub>tot</sub>xε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φ<sub>tot</sub> trends of the Eu­(III) and Sm­(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (<i>n</i>LI) is increased in length.