Photoluminescent and Slow Magnetic Relaxation Studies on Lanthanide(III)-2,5-pyrazinedicarboxylate Frameworks

In the series described in this work,
the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate
anion to the 2,5-pyrazinedicarboxylate dianion (2,5-pzdc) allowing
the preparation of three-dimensional (3D) lanthanide­(III) organic
frameworks of formula {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction
on 1–5 reveals that they crystallize
in the triclinic system, P1̅ space group with
the series 1–4 being isostructural.
The crystal structure of the five compounds are 3D with the lanthanide­(III)
ions linked through 2,5-pzdc2– dianions acting as
two- and fourfold connectors, building a binodal 4,4-connected (4·648)­(426282)-mog network. The photophysical properties of the Nd­(III) (3) and Eu­(III) (4) complexes exhibit sensitized photoluminescence
in the near-infrared and visible regions, respectively. The photoluminescence
intensity and lifetime of 4 were very sensitive due to
the luminescence quenching of the 5D0 level
by O–H oscillators of four water molecules in the first coordination
sphere leading to a quantum efficiency of 11%. Variable-temperature
magnetic susceptibility measurements for 1–5 reveal behaviors as expected for the ground terms of the
magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce­(III), Pr­(III),
Nd­(III), Eu­(III), and Er­(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature
corroborate these facts. Frequency-dependent alternating-current magnetic
susceptibility signals under external direct-current fields in the
range of 100–2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic
relaxation (single-ion magnet) behavior. In these compounds, τ–1 decreases with decreasing temperature at any magnetic
field, but no Arrhenius law can simulate such a dependence in all
the temperature range. This dependence can be reproduced by the contributions
of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers
system.