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 (H 2 O) 4 ]·6H 2 O} n [Ln = Ce ( 1 ), Pr ( 2 ), Nd ( 3 ), and Eu ( 4 )] and {[Er 2 (2,5-pzdc) 3 (H 2 O) 4 ]·5H 2 O} n ( 5 ). Single-crystal X-ray diffraction on 1 – 5 reveals that they crystallize in the triclinic system, P 1̅ 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 5 D 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 [ 2 F 5/2, 2 H 4, 4 I 9/2, 7 F 0, and 4 I 15/2 for Ce(III), Pr(III), 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, 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.