Heterolanthanide Terephthalate Coordination Polymers: From the Fight against Counterfeiting to Plastic Waste Recycling

The world’s plastics production continues to grow and could triple by 2060. Unfortunately, only a small proportion of these plastics are currently recycled (around 30%). The remainder is incinerated (around 40%), causing high greenhouse gas emissions, or buried (around 30%), resulting in high levels of microplastic pollution. Ambitious national and international policies have been put in place to increase the proportion of recycled plastics, and major research efforts are underway to improve plastic recycling processes. Unfortunately, all recycling processes (chemical, physical, and biological) require batches of plastics to be recycled that are as homogeneous as possible. Rigorous waste sorting is therefore essential, and marking plastics with luminescent markers could provide a solution. It could also enable circular and short-loop recycling in which an object is recycled into an identical object.

Heterolanthanide coordination polymers are particularly promising candidates for this application. They have demonstrated their effectiveness in the field of anticounterfeiting marking. However, their use in materials traceability requires other assets, such as markers with luminescence properties that are sufficiently different and sufficiently intense for them to be easily identified on a rapid sorting line, as laboratory analysis is no longer relevant for this application.

To prepare such a range of compounds, it is necessary to master the mechanisms that govern luminescence properties. The choice of ligand and metal centers, crystal structure, and particle shaping all have a major influence on luminescence properties. Our group has been working on understanding these phenomena for some 20 years.

Using as an example the family of heterolanthanide coordination polymers with general chemical formula [Ln₂(bdc)₃(H₂O)₄]_∞, where bdc^2– represents benzene-1,4-dicarboxylate, we wish to present here the various levers that can be used to modulate the emission colors and increase the luminescence intensity of heterolanthanide coordination polymers and describe a family of markers that can be used in the field of materials traceability. Beyond the choice of the metallic centers and of the ligands, markers can be designed in the form of core–shell particles, with an intermediate optically nonactive insulating shell that separates the core from the shell that are both made of optically active molecular alloys. Intermetallic energy transfers are therefore minimized, resulting in increased luminescence intensity and emission color modulation. In conclusion, we would like to draw up a rough sketch of what could be the markers used in the field of plastics traceability.