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.