Synthesis, crystal structure and luminescent properties of isoreticular lanthanide–organic frameworks based on a tetramethyl-substituted terphenyldicarboxylic acid

Abstract

A series of 3D metal–organic frameworks derived from 2′,3′,5′,6′-tetramethyl-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic acid (H₂L) as organic linker and various lanthanides have been prepared. The resulting coordination polymers [Ln₃L₄(NO₃)(DMF)_x(H₂O)]_n (where x = 2 or 3) were characterized by elemental analysis, thermogravimetric investigations, IR and TG-FTIR spectroscopy, as well as powder X-ray diffraction measurements. The crystal structure for six of these isostructural networks based on La(III), Ce(III), Nd(III), Sm(III), Tb(III) and Dy(III) was established using single crystal X-ray crystallography. As an example, the asymmetric part of [La₃L₄(NO₃)(DMF)₃(H₂O)]_n consists of two deprotonated ligand molecules L^2- and two La(III) ions, one of them occupying a general position with full site occupancy, while the other one has a fractional occupancy of 0.5. Investigation of the luminescence of these coordination polymers showed that compounds containing La³⁺, Nd³⁺ and Gd³⁺ possess only the emission associated with the anion of the organic ligand (L^2-), polymers containing Eu³⁺ and Tb³⁺ show pure metal-centered radiation, while networks containing Dy³⁺ and Sm³⁺ display dual metal- and ligand-centered luminescence. The organic linker acts as a good antenna for Tb³⁺, Eu³⁺, Dy³⁺ and Sm³⁺ ions.