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)xn(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.