Visible, Near-Infrared, and Dual-Range Luminescence Spanning the 4f Series Sensitized by a Gallium(III)/Lanthanide(III) Metallacrown Structure

Lanthanide­(III)
ions (Ln3+) in coordination compounds
exhibit unique luminescence properties with narrow and characteristic
f-f transitions throughout the visible and near-infrared (NIR) ranges.
In addition, some Ln3+ such as Pr3+, Sm3+, Dy3+, Ho3+, Er3+, and
Tm3+ possess an exceptional ability, although less explored,
to exhibit dual-range emissions. Such remarkable features allow highly
specific use in materials science and biology, for example, for the
creation of sophisticated barcode modules or for the next generation
of optical imaging applications. Herein, a series of Ga3+/Ln3+ metallacrowns (MCs) with the general composition
[LnGa8(shi)8(OH)4]­Na·xCH3OH·yH2O
(Ln-1, Ln = Pr3+, Nd3+, Sm3+–Yb3+ and analogue Y3+; H3shi = salicylhydroxamic acid) is presented. Ln-1 were obtained by reacting Ga3+ and Ln3+ nitrate
salts with the H3shi ligand. X-ray single crystal unit
cell analysis confirmed that all MCs are isostructural. The crystal
structure was solved for the Nd3+ analogue and revealed
that Nd3+ is centered between two [12-MCGaIIIN(shi)-4] MC rings and bound to eight hydroximate
oxygen ions (four from each ring) in a pseudosquare antiprismatic
fashion adopting a pseudo-D4h symmetry.
Pulsed gradient spin echo diffusion ordered 1H NMR spectroscopy
and electrospray ionization mass spectrometry confirmed that the structure
of Ln-1 remains intact in methanol solutions while mass
spectrometry suggests that four OH– bridges are
exchanged with CH3O–/CD3O–. An exceptional ability of this series of MCs to sensitize
the characteristic emission of Ln3+ was confirmed with
the observation of bright red and green emission signals of Eu-1 and Tb-1, NIR emissions of Yb-1 and Nd-1, and dual-range emissions of Pr-1, Sm-1, Dy-1, Ho-1, Er-1, and Tm-1 in the solid state upon excitation
into ligand-centered bands at 340 nm. The luminescence properties
of Ln-1 (Ln = Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, and Yb3+)
were also investigated in CH3OH and CD3OD solutions.
For Eu-1 and Yb-1 MCs, more extensive analyses
of the photophysical properties were performed, which included the
determination of radiative lifetimes, intrinsic quantum yields, and
sensitization efficiencies. The absolute quantum yields (QLnL) of Ln-1 in the visible and NIR ranges have been determined. In
the case of Sm-1, the values of QLnL in CH3OH and CD3OD solutions are exceptionally high, that is,
10.1(5) and 83(1) %. Values obtained for Yb-1, that is,
0.78(4) % in CH3OH and 8.4(1)% in CD3OD, are
among the highest ones reported today for Yb3+ complexes
formed with nondeuterated and nonhalogenated ligands.