Highly Emitting Near-Infrared Lanthanide “Encapsulated Sandwich” Metallacrown Complexes with Excitation Shifted Toward Lower Energy

Abstract

Near-infrared (NIR) luminescent lanthanide complexes hold great promise for practical applications, as their optical properties have several complementary advantages over organic fluorophores and semiconductor nanoparticles. The fundamental challenge for lanthanide luminescence is their sensitization through suitable chromophores. The use of the metallacrown (MC) motif is an innovative strategy to arrange several organic sensitizers at a well-controlled distance from a lanthanide cation. Herein we report a series of lanthanide “encapsulated sandwich” MC complexes of the form Ln3+[12-MCZn(II),quinHA-4]2[24-MCZn(II),quinHA-8] (Ln3+[Zn(II)MCquinHA]) in which the MC framework is formed by the self-assembly of Zn2+ ions and tetradentate chromophoric ligands based on quinaldichydroxamic acid (quinHA). A first-generation of luminescent MCs was presented previously but was limited due to excitation wavelengths in the UV. We report here that through the design of the chromophore of the MC assembly, we have significantly shifted the absorption wavelength toward lower energy (450 nm). In addition to this near-visible inter- and/or intraligand charge transfer absorption, Ln3+[Zn(II)MCquinHA] exhibits remarkably high quantum yields, long luminescence lifetimes (CD3OD; Yb3+, QLnL = 2.88(2)%, τobs = 150.7(2) μs; Nd3+, QLnL = 1.35(1)%, τobs = 4.11(3) μs; Er3+, QLnL = 3.60(6)·10–2%, τobs = 11.40(3) μs), and excellent photostability. Quantum yields of Nd3+ and Er3+ MCs in the solid state and in deuterated solvents, upon excitation at low energy, are the highest values among NIR-emitting lanthanide complexes containing C–H bonds. The versatility of the MC strategy allows modifications in the excitation wavelength and absorptivity through the appropriate design of the ligand sensitizer, providing a highly efficient platform with tunable properties.

Synthesis, Structural, and Magnetic Characterization of a Mixed 3d/4f 12-Metallacrown-4 Family of Complexes

“…Usually, in the central MC cavity sits a transition metal ion, even though there are also reports where alkali and alkaline earth metals occupy the cavity [15,16]. Lately, the focus has been turned into the incorporation of lanthanide ions in the center, since these compounds have been proposed as excellent candidates for molecular recognition [17,18], molecular magnetism [19] and luminescent [1,[19][20][21][22] technologies. Single-crystal diffraction studies revealed that compounds 1 and 2 are isostructural and crystallize in the P4/n tetragonal space group, while complex 3 crystallizes in the P4cc tetragonal space group (Table S1).…”

Seven‐Coordinate Luminophores: Brilliant Luminescence of Lanthanide Complexes with C3v Geometrical Structures

“…[7][8][9] Various types of luminescent lanthanide complexes have been reported to date. [10][11][12][13][14][15][16][17][18][19][20][21] Lanthanide complexes with asymmetric coordination structures exhibit high emission quantum yields (Φff) and large radiative rate constants (kr). [22,23] Lanthanide complexes generally provide eightcoordinate square anti-prism structures that are categorized as D4d in point group theory (8-SAP, Figure 1a).…”

Crystal structure of N-hydroxyquinoline-2-carboxamide monohydrate

“…N-Hydroxyquinoline-2-carboxamide, also known as quinoline-2-hydroxamic acid (QuinHA), has been used for the preparation of various metallacrown complexes (Stemmler et al, 1999;Trivedi et al, 2014;Jankolovits et al, 2013). Presently, the Cambridge Structural Database (Groom et al, 2016) contains ten entries on coordination compounds based on N-hydroxyquinoline-2-carboxamide, nine of which have been reported within the past four years.…”

“…chloride in the presence of N-methylmorpholine according to the reported procedure (Trivedi et al, 2014). Light-yellow crystals suitable for X-ray diffraction were obtained from aqueous solution by slow evaporation at room temperature (yield 76%).…”