Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)–Lanthanide(III) Complexes

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF3SO3)(2) (M = Cr, Zn) and Ln(CF3SO3)(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D-3-symmetrical trinuclear MLnM(L2)(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) 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 dinuclear complexes CrLn(L1)(3), the connection of a second strong-field [CrN6] sensitizer in CrLnCr(L2)(3) 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 CrLnCr(L2)(3) under reasonable pumping powers.