Structure and efficient luminescence upconversion of Ln(iii) aromatic N-oxide coordination polymers

Abstract

A series of lanthanide-based coordination polymers {[Yb_1−xEr_x(4,4′-bpdo)₃(H₂O)₂](CF₃SO₃)₃}_∞ were synthesised by solvent diffusion techniques, where 4,4′-bpdo = 4,4′-bipyridine-N,N′-dioxide, and using differing mole fractions of Yb(III) and Er(III) which were systematically varied (x = 0, 0.05, 0.20, 0.50 and 1). All of the materials obtained were characterised using elemental analyses, single-crystal X-ray diffraction (SXRD) and solid-state photoluminescence studies. Structurally, the coordination polymers crystallise as an isomorphous series of infinite 2D sheets, which contain two inner sphere water molecules, and are isostructural with a previously characterised homometallic Yb(III) compound. In addition to the normal Near Infra-Red (NIR) luminescence, these compounds also demonstrate upconversion emission upon 980 nm excitation. Upconversion luminescence measurements reveal visible emission in the red, green, and blue regions corresponding to the ²H_11/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2 and ²H_9/2 → ⁴I_15/2 transitions of the Er(III) cation upon two and three-photon excitation. We also observed weak emission from the Er(III) cation in the UV region for the first time in a Ln-MOF based material.