Enhanced Near-Infrared Emission from Eight-Coordinate vs Nine-Coordinate YbIII Complexes Using 2-(5-Methylpyridin-2-yl)-8-hydroxyquinoline

Bowie S K Chong; Divya Rajah; Matthew F Allen; Laura Abad Galán; Massimiliano Massi; Mark Ogden; Evan G Moore

doi:10.1021/acs.inorgchem.0c01802

Enhanced Near-Infrared Emission from Eight-Coordinate vs Nine-Coordinate Yb^III Complexes Using 2-(5-Methylpyridin-2-yl)-8-hydroxyquinoline

Inorg Chem. 2020 Nov 16;59(22):16194-16204. doi: 10.1021/acs.inorgchem.0c01802. Epub 2020 Oct 29.

Authors

Bowie S K Chong¹, Divya Rajah¹, Matthew F Allen¹, Laura Abad Galán², Massimiliano Massi², Mark Ogden², Evan G Moore¹

Affiliations

¹ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia.
² School of Molecular and Life Sciences, and Curtin Institute of Functional Molecules and Interfaces, Curtin University, Kent Street, Bentley, WA 6102, Australia.

PMID: 33121245
DOI: 10.1021/acs.inorgchem.0c01802

Abstract

Enhanced near-infrared (NIR) luminescence from two structurally related heterobinuclear Na^IYb^III eight-cooridnate and heterobinuclear Yb^IIINa^I eight-coordinate (CN = 8) complexes is reported and compared to a nine-coordinate (CN = 9) homoleptic complex. For the heteroleptic complex, [Yb(MPQ₂)(acac)], the Yb^III cation is coordinated to two tridentate 2-(5-methylpyridin-2-yl)-8-quinolinate (MPQ) anions, with a bidentate acetylacetonate (acac) anion completing the coordination sphere. Instead, the heterobinuclear [NaYb(MPQ)₄] complex comprises a total of four anionic MPQ ligands, two of which exhibit κ³-coordination to the Yb^III cation. The remaining two MPQ anions are unidentate toward the lanthanide and form μ₂-bridges via the deprotonated quinolinate oxygens to a bound Na^I cation which is also coordinated to the remaining nitrogen donor atoms. The structural properties of these complexes were evaluated by single-crystal X-ray diffraction (SXRD), continuous shape measure (CShM) analysis, and ¹H NMR spectroscopy using a diamagnetic Lu^III analogue. The corresponding photophysical properties were examined in CH₂Cl₂ solution by using absorption and emission spectroscopy. For both the complexes, characteristic Yb^III emission is observed at ca. 980 nm, with recorded photoluminescence quantum yields (Φ_obs) and NIR luminescence lifetimes (τ_obs) of 2.0% and 14.0 μs vs 1.5% and 11.6 μs for the [NaYb(MPQ)₄] and [Yb(MPQ)₂(acac)] complexes, respectively. Interestingly, the eight-coordinate Yb^III complexes both have higher photoluminescence quantum yields when compared to the homoleptic [Yb(MPQ)₃] complex, which has a reported quantum yield of 1.0% and a NIR lifetime determined herein of 13.3 μs under identical conditions. These results have been rationalized by considering the overall efficiency of the ligand-centered sensitization process (η_sens = Φ_isc × Φ_eet), together with subsequent radiative (k_r) and nonradiative (k_nr) deactivation of the Yb^III cation. Moreover, the efficiency of the intersystem crossing (Φ_isc) and electronic energy transfer (Φ_eet) processes involved in the antennae effect have been quantified for the new complexes using a combination of nanosecond and femtosecond transient absorption techniques and have been compared to our previous results using [Ln(MPQ)₃] complexes with Ln = Yb and Lu.