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 (1), Nd (2), Eu (3), Gd (4), Tb (5), Dy (6),
Ho (7), Er (8), Tm (9), and
Yb (10); 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 P21/c. 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.