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

Maria Vanda Marinho; Daniella O Reis; Willian X C Oliveira; Lippy F Marques; Humberto O Stumpf; Mariadel Déniz; Jorge Pasán; Catalina Ruiz-Pérez; Joan Cano; Francesc Lloret; Miguel Julve

doi:10.1021/acs.inorgchem.6b02774

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

Inorg Chem. 2017 Feb 20;56(4):2108-2123. doi: 10.1021/acs.inorgchem.6b02774. Epub 2017 Feb 3.

Authors

Affiliations

¹ Instituto de Química, Universidade Federal de Alfenas , Campus Sede, Alfenas, MG 37130-000, Brazil.
² Departamento de Química, Universidade Federal de Minas Gerais , Av Antônio Carlos, 6627, Belo Horizonte, MG 31270-901, Brazil.
³ Grupo de Materiais Inorgânicos Multifuncionais, Instituto de Química, Universidade do Estado do Rio de Janeiro , Rio de Janeiro, RJ 20550-900, Brazil.
⁴ Laboratorio de Rayos X y Materiales Moleculares, Departamento de Física, Facultad de Ciencias (Sección Física), Universidad de La Laguna , Edifício de Física y Matemáticas, Apdo. 456, E-38200 La Laguna, Tenerife, Spain.
⁵ Instituto de Ciencia Molecular-Departament de Química Inorgànica, Universitat de València , C/Catedrático José Beltrán 2, 46980 Paterna, València, Spain.
⁶ Fundació General de la Universitat de València, Universitat de València , 46010 València, Spain.

PMID: 28157308
DOI: 10.1021/acs.inorgchem.6b02774

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 {[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-pzdc^2- 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 [²F_5/2, ²H₄, ⁴I_9/2, ⁷F₀, and ⁴I_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.