Synthesis, crystal structure, NMR and near infra-red luminescence studies of nine-coordinate Nd and Yb complexes based on fluorinated β‑diketone and a tridentate antenna chromophore, 2, 4, 6-Tris-(2-pyridyl)-s-triazine

Abstract

Three nine-coordinate ternary complexes of La(III), Nd(III) and Yb(III) of general composition, [Ln(hfaa)3(tptz)] {hfaa = hexafluoroacetylacetone and tptz = 2, 4, 6-tris-(2-pyridyl)-s-triazine} are synthesised and comprehensively characterised. The X-ray single crystal structure and NMR studies show that the coordination structure of the complexes is nine-coordinate monocapped square antiprismitic in both solid and solution states with LnO6N3 (six oxygen atoms from three hfaa and three nitrogen atoms from one tptz) polyhedra. The complexes are stable in non-coordinating (chloroform) and coordinating (methanol) solvents as indicated by NMR spectrum where no dissociation of any ligand/s is observed. The longitudinal relaxation rate (ρ = 1/T1) is very fast in the cases of paramagnetic Nd(III) and Yb(III) complexes as compared to their diamagnetic La(III) analogue, suggesting their possible use as a relaxation probe in the magnetic resonance imaging. The complexes, on UV light excitation, show characteristic near infra-red emission transitions; 4F3/2→4Ij=9/2, 11/2, 15/2 of Nd(III) and 2F5/2→2F7/2 of Yb(III) ions. The tptz ligand has proved very efficient antenna as it increased the photoluminescence quantum yield of Nd(III) complex. The complexes showed considerable improved photophysical properties in NIR region, which could lead them as significant candidates in developing new materials for more efficient laser and optical amplifier systems, and windows transparent to the biological tissues