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 {[Ln(2)(2,5-pzdc)(3)(H2O)(4)]center dot 6H(2)O}(n) [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er-2(2,5-pzdc)(3)(H2O)(4)]center dot 5H(2)O}(n) (5). Single-crystal X-ray diffraction on 1-5 reveals that they crystallize in the triclinic system, P (1) over bar 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(2-) dianions acting as two- and fourfold connectors, building a binodal 4,4 -connected (4.6(4)8)-(4(2)6(2)8(2))-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-5(0) 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-2(5/2), H-2(4), I-4(9/2), F-7(0), and I-4(15/2) for Ce(III); Pr(LII), Nd(III), Eu(III), and Er(III), respectively] with M-J = 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, tau(-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.