Optimizing Millisecond Time Scale Near-Infra red Emission in Polynuclear Chrome(Ⅲ)-Lanthanide(Ⅲ) Complexes

摘要

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using stivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF_3SO_3)_2 (M = Cr, Zn) and Ln(CF_3SO_3)_3 (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D3-symmetrical trinuclear [MLnM(L2)_3](CF_3SO_3)_n complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(Ⅲ) chromophores in [CrLnCr(L2)_3]~(9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinudear complexes [CrLn(L1)_3]~(6+), the connection of a second strong-field [CrN_6] sensitizer in [CrLnCr(L2)_3]~(9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)_3]~(9+) under reasonable pumping powers.