Nanosheets of Two-Dimensional Neutral Coordination Polymers Based on Near-Infrared-Emitting Lanthanides and a Chlorocyananilate Ligand

The synthesis, structural characterization, photophysical studies, and exfoliation of two-dimensional (2D) layered coordination polymers, formulated as {[Ln2(ClCNAn)3(DMF)6]·(DCM) x } n (Ln­(III) = Yb­(x = 0), Nd, and Er (x = 2)) based on the heterosubstituted chlorocyananilate ligand, are reported. These compounds consist of neutral polymeric 2D networks of the chlorocyananilate ligand alternating with Ln­(III) ions. They form six-membered rings with rectangular cavities, where neighbor layers are eclipsed along the a axis (Yb), and a regular honeycomb-like structure, with hexagonal cavities filled by dichloromethane solvent molecules (Nd and Er), where neighbor layers alternate along the c axis. Several interlayer interactions between lanthanide centers and dimethylformamide molecules, facing the cavities, are present in all compounds. Free-standing nanosheets, obtained by a top-down strategy involving sonication-assisted solution synthesis and characterized by atomic force microscopy and high-resolution transmission electron microscopy, show lateral dimensions on the micrometer scale, thicknesses down to the monolayer, and the presence of lattice fringes. Time-resolved photoluminescence studies performed on both the bulk and nanosheets clearly demonstrate that the chlorocyananilate ligand acts as an efficient antenna toward Ln­(III) ions and that emission sensitization occurs as a multistep relaxation process involving, in sequence, intersystem crossing and energy transfer from ligand triplet states to the Ln­(III) ions. Effects induced by the exfoliation process on the photophysical properties of the nanosheets are also discussed.