Near-infrared emitting single- and mixed-ligand MOFs that can be excited in the visible region: synthesis, crystal structures and sensitization of Nd3+, Er3+ and Yb3+

Abstract

Near-infrared (NIR) luminescent lanthanide(III) (Ln³⁺) metal–organic frameworks (MOFs) constitute a class of materials possessing unique advantages for a wide range of applications due to their photophysical and structural properties. In this work, we present the synthesis and characterization of two series of Ln³⁺-based MOFs emitting in the NIR region (Ln-MOFs) constructed using 2,6-naphthalene dicarboxylic acid (H₂NDC) and its two amino derivatives, 4-aminonaphthalene-2,6-dicarboxylic acid (H₂ANDC) and 4,8-diaminonaphthalene-2,6-dicarboxylic acid (H₂DANDC). The first series comprises mixed-ligand and mixed-metal MOFs with the general formula [La_1−xLn_x(NDC)_1−y(ANDC)_yCl(DMF)] (Ln = Nd³⁺, Yb³⁺, x = 0.005–1, y = 0–0.2), while the second series features MOFs corresponding to the formulae [La_1−xLn_x((NDC)_1−y(ANDC)_y)_1.5(DMF)₂]·DMF (Ln = Nd³⁺, Er³⁺, Yb³⁺, Lu³⁺, x = 0.1 or 1, y = 0.45 or 1) and [Ln(DANDC)_1.5(DMF)₂]·DMF (Ln = Nd³⁺, Er³⁺, Yb³⁺). Detailed studies of the photophysical properties of Ln-MOFs have demonstrated that MOF scaffolds provide an efficient sensitization of NIR-emitting Ln³⁺ ions (η_sens up to 38% for Nd³⁺ analogues). In addition, Ln-MOFs possess long Ln³⁺ luminescence lifetimes (up to 12 μs for Yb³⁺ analogues) and exhibit Ln³⁺ luminescence quantum yields comparable with the highest values reported today for NIR-emitting Ln-MOFs. We discuss and compare the photophysical processes that lead to Ln³⁺ sensitization in two series of Ln-MOFs. Most importantly, the studied Ln-MOFs are among the few examples of Ln³⁺-based NIR-emitting MOFs where Ln³⁺ emission can be sensitized upon excitation in the visible range (up to 450 nm).