Sensitization of Visible and NIR Emitting Lanthanide(III) Ions in Noncentrosymmetric Complexes of Hexafluoroacetylacetone and Unsubstituted Monodentate Pyrazole

A series of highly volatile eight-coordinate air and moisture stable lanthanide complexes of the type [Ln­(hfaa)₃(L)₂] (Ln = Pr <b>(1)</b>, Nd (<b>2</b>), Eu (<b>3</b>), Gd (<b>4</b>), Tb (<b>5</b>), Dy (<b>6</b>), Ho (<b>7</b>), Er (<b>8</b>), Tm (<b>9</b>), and Yb (<b>10</b>); hfaa = anion of hexafluoroacetylacetone and L = pyrazole) have been synthesized and characterized by elemental analysis, IR, ESI-MS⁺, and NMR studies. Single-crystal X-ray structures have been determined for the Eu­(III) and Dy­(III) complexes. These complexes crystallize in the monoclinic space group <i>P</i>2₁/<i>c</i>. The lanthanide ion in each of these complexes is eight-coordinate with six oxygen atoms from three hfaa and two N-atoms from two pyrazole units, forming a coordination polyhedron best describable as a distorted square antiprism. The NMR spectra reveal that both the pyrazole units remain attached to the metal in solution and the β-diketonate and pyrazole protons are shifted in opposite directions in the case of paramagnetic complexes. The lanthanide-induced chemical shifts are dipolar in nature. The hypersensitive transitions of Nd­(III), Ho­(III), and Er­(III) are sensitive to the environment (solvent), which is reflected by the oscillator strength and band shape of these transitions. The band shape due to the hypersensitive transition of Nd­(III) in noncoordinating chloroform and dichloromethane is similar to those of the typical eight-coordinate Nd­(III) β-diketonate complexes. The quantum yield and lifetime of Pr­(III), Eu­(III), Tb­(III), Dy­(III), and Tm­(III) in visible and Pr­(III), Nd­(III), Dy­(III), Ho­(III), Er­(III) Tm­(III), and Yb­(III) in the NIR region are sizable. The environment around these metal ions is asymmetric, which leads to increased radiative rates and luminescence efficiencies. The quantum yield of the complexes reveal that ligand-to-metal energy transfer follows the order Eu­(III) > Tb­(III) ≫ Pr­(III) > Dy­(III) > Tm­(III). Both ligands (hfaa and pyrazole) are good sensitizers for all the visible and NIR emitters effectively, except for Tb­(III), Dy­(III), and Tm­(III), where pyrazole gave a negative effect (e.g., energy back-transfer) that is due to poor intramolecular energy transfer match. The good luminescent properties make these NIR-luminescent complexes to have potential application in optical communication, telecommunications, and fluoroimmunoassays