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

Abstract

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 {[Ln(2,5-pzdc)(HO)]·6HO} [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er(2,5-pzdc)(HO)]·5HO} (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-pzdc dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·68)(468)-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 D 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 [F, H, I, F, and I for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with M = 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 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.

Magnetic Properties of LnIII–CuII 15‐Metallacrown‐5 Dimers with Terephthalate (LnIII = Pr, Nd, Sm, Eu)

“…During the fitting procedure for complexes 1 – 3 some of the parameter values were fixed using typical values previously reported in the literature [for 1 : g Pr = 0.89 (theoretical g J for Pr III is 0.80); for 2 : g Cu = 2.0; for 3 : g Sm = 0.29 (theoretical g J for Sm III is 0.29), λ Sm = 220 cm –1 ; Table ] , , , . The values obtained for Δ and g Nd for 1 and 2 (Table ) are in good correspondence with those reported previously using the same interpretative approach , , , , , . However, in many prior cases, the fitting procedures were simplified for Pr III ‐containing complexes by using only equation (5) (the χ ⊥ term was neglected) , .…”

Dinuclear LnIII Complexes with 9‐Anthracenecarboxylate Showing Field‐Induced SMM and Visible/NIR Luminescence

“…The fitting afforded values of C = 1.03 s –1 K – n , n ≈ 6, and A = 823.60 s –1 K –1 . These values are comparable to those found for other Nd III compounds . Although usually n = 9 for Kramers ions, n values between 1 and 6 can be considered acceptable, as a result of the structure of the levels …”

From Paramagnetic to Single‐Molecule Magnet Behaviour in Heterobimetallic Compounds Containing the Tetrakis(thiocyanato‐κN)cobaltate(II) Anion

“…The best‐fit parameters obtained by using Equation (2) are listed in Table . This mechanism has been recently proposed for an octahedral cobalt(II) complex as well for some compounds with lanthanide(III) ions…”