Article pubs.acs.org/IC
Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1‑Hydroxypyridin-2-one Complexes in Aqueous Solution Lena J. Daumann, David S. Tatum, Christopher M. Andolina, Joseph I. Pacold, Anthony D’Aléo, Ga-lai Law, Jide Xu, and Kenneth N. Raymond* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemistry, University of California, Berkeley, California 94720-1460, United States S Supporting Information *
ABSTRACT: A series of 10 tetradentate 1-hydroxy-pyridin-2one (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 (Φtot) 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 (Φtotxε). 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 Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (nLI) is increased in length.
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INTRODUCTION Photoluminescent lanthanide(III) complexes have attracted attention because of their use in a wide variety of applications, such as biofluoroimmunoassays,1,2 as sensors,3,4 in light-emitting diodes,5,6 and other materials.7−18 Interest in lanthanide photoluminescence is spurred by typically long-lived excited state lifetimes, large pseudo-Stokes shifts, and insensitivity to quenching by molecular oxygen compared to organic fluorophores.19 The f−f transitions of the lanthanides are Laporteforbidden, resulting in small molar absorption coefficients (90% in organic solvents have been reported.28−30 Biological assays require large Φtot in aqueous solution, and recently complexes have been reported in this medium with remarkable Φtot as high as 37%.15,16,31 For Sm(III) compounds, enhancing the photoluminescent output is more difficult to achieve, because of the © XXXX American Chemical Society
closely spaced excited states, which favor nonradiative decay processes.24,32−35 The energy gap between the lowest lying excited state and highest lying ground state of Sm(III) is relatively small (∼7400 cm−1), which reduces the lifetime of the Sm(III) excited state, which is a result of more efficient nonradiative decay via vibronic coupling of the nearby X−H oscillators. These processes lead to lower photoluminescence Φtot, which are typically
