A family of luminescent heterometallic coordination polymers based on lanthanide(iii) ions and 6-methyl-2-oxonicotinate: near-infrared/visible emitters and colour fine-tuning†

The work presented herein reports on the synthesis, structural and physicochemical characterization and luminescence properties of a family of isostructural coordination polymers (CPs) with a general formula {[Ln(6m2onic)₄Na(H₂O)₃]·8H₂O}_n (where Ln(III) = Nd (1_Nd), Sm (2_Sm), Dy (3_Dy), Er (4_Er), Tm (5_Tm), Yb (6_Yb) and Dy_0.77Eu_0.12Y_0.11 for the multi-metal compound (7_DyEuY) and 6m2onic = 6-methyl-2-oxonicotinate). The crystal structures consist of one-dimensional heterometallic arrays where octacoordinated lanthanide and hexacoordinated sodium centres are sequentially linked and which are held together into a 3D architecture by an extensive hydrogen bonding network formed by the crystallisation water molecules. Photoluminescence measurements in the solid state at variable temperature reveal good properties based on the capacity of the 6m2onic ligand to provide ligand-centred excitation, as suggested by time-dependent density functional theory (TDDFT), and promote efficient energy transfers to the lanthanide(III) ions, to eventually present intense emissions in both the visible and near-infrared (NIR) regions. On the one hand, compound 4_Er displays characteristic lanthanide-centred bands in the NIR region even at room temperature, meaning that the framework is able to isolate the excitons from the vibrational quenching component. On the other hand, regarding the compounds emitting in the visible region, the almost white light emitted by compound 3_Dy with a quantum yield (QY) of 6.2% should be noted. Both a purer white emission and an improved QY (up to 15.8%) may be achieved by means of a doping strategy of europium and yttrium ions into the Dy counterpart. Finally, taking advantage of white light emitted by compound 3_Dy, the chemical and optical stabilities in water have been confirmed by the photophysical study performed in solution.