Highly Luminescent Europium-Based Heteroleptic Coordination Polymers with Phenantroline and Glutarate Ligands

Isostructural lanthanide-based coordination polymers with general chemical formula [Ln(phen)(glu)(NO₃)]_∞ with Ln = La-Tm (except Ce and Pm) have been synthesized by hydrothermal methods (H₂glu stands for glutaric acid and phen stands for 1,10-phenantroline). They crystallize in the monoclinic system with the P2₁/c (no. 14) space group. The crystal structure has been solved on the basis of the La derivative. It can be described as the superimposition of molecular chains of dimeric La(phen)(NO₃)-La(phen)(NO₃) units bridged by glutarate ligands. Luminescent properties have been explored and show that the Eu derivative exhibits the highest luminance observed for Eu-based coordination polymers (85 to 105 cd·m^-2). Effects of the dilution of the Eu³⁺ and Tb³⁺ luminescent ions by Gd³⁺ optically inactive ions are unexpected and to the best of our knowledge unprecedented. This could be related to the different intermetallic energy-transfer mechanisms in competition and to the nonisotropic distribution of the lanthanide ions in these molecular alloys. The investigation of molecular alloys with general chemical formula [Eu_1-Tb(phen)(glu)(NO₃)]_∞ with 0 ≤ x ≤ 1 highlights a very sizable and constant Eu³⁺ luminescence whatever the x value, which further confirms the existence of very strong intermetallic energy transfers in this family of compounds. It is also noticeable that some coordination polymers based on weakly emissive lanthanide ions exhibit very well defined emission spectra.