Highly emitting near-infrared lanthanide “encapsulated sandwich” metallacrown complexes with excitation shifted toward lower energy
Résumé
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 Ln$^{3+}$[12-MC$_{Zn(II),quinHA}$-4}]$_2$[24-MC$_{Zn(II),quinHA}$$^{-8}$] (Ln$^{3+}$[$Zn(II)MC_{quinHA}$]) in which the MC framework is formed by the self-assembly of Zn$^{2+}$ 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, Ln$^{3+}$[Zn(II)MC$_{quinHA}$] exhibits remarkably high quantum yields, long luminescence lifetimes (CD$_3$OD; Yb$^{3+}$, QLn$^L$ = 2.88(2)%, $\tau _{obs}$ = 150.7(2) $\mu$s; Nd$^{3+}$, QLn$^L$ = 1.35(1)%, $\tau _{obs}$ = 4.11(3) $\mu$s; Er$^{3+}$, QLn$^L$ = 3.60(6)·10$^{-2}$%, $\tau _{obs}$ = 11.40(3) $\mu$s), and excellent photostability. Quantum yields of Nd$^{3+}$ and Er$^{3+}$ 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.