title: 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-MC(Zn(II),quinHA)-4]2[24-MC(Zn(II),quinHA)-8] (Ln3+ [Zn(II)MC(quinHA)]) 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)MC(quinHA)] exhibits remarkably high quantum yields, long luminescence lifetimes (CD3OD; Yb3+, QLn(L) = 2.88(2)%, τobs = 150.7(2) μs; Nd3+, QLn(L) = 1.35(1)%, τobs = 4.11(3) μs; Er3+, QLn(L) = 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.
authors.affiliation: Department of Chemistry, Willard H. Dow Laboratories, University of Michigan , Ann Arbor, Michigan 48109, United States.
authors.forename: Evan R
authors.initials: ER
authors.lastname: Trivedi
authors.forename: Svetlana V
authors.initials: SV
authors.lastname: Eliseeva
authors.forename: Joseph
authors.initials: J
authors.lastname: Jankolovits
authors.forename: Marilyn M
authors.initials: MM
authors.lastname: Olmstead
authors.forename: Stéphane
authors.initials: S
authors.lastname: Petoud
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Pecoraro
mesh_terms: D018360:Crystallography, X-Ray; D007259:Infrared Rays; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D015032:Zinc / Q000737:chemistry
chemical_list: D028581:Lanthanoid Series Elements; D009942:Organometallic Compounds; D015032:Zinc