1 School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China.
2 Department of Chemistry, College of Sciences, Shanghai University, Shanghai, 200444, China.
3 Department of Chemistry and Biochemistry and Centre for NanoScience Research, Concordia University, Montreal, QC H4B 1R6, Canada.
4 State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China.

Lanthanide upconversion luminescence in nanoparticles has prompted continuous breakthroughs in information storage, temperature sensing, and biomedical applications, among others. Achieving upconversion luminescence at the molecular scale is still a critical challenge in modern chemistry. In this work, we explored the upconversion luminescence of solution dispersions of co-crystals composed of discrete mononuclear Yb(DBM)₃ Bpy and Eu(DBM)₃ Bpy complexes (DBM: dibenzoylmethane, Bpy: 2,2'-bipyridine). The 613 nm emission of Eu³⁺ was observed under excitation of Yb³⁺ at 980 nm. From the series of molecular assemblies studied, the most intense luminescence was obtained for a 1 : 1 molar ratio of Yb³⁺ : Eu³⁺ , resulting in a high quantum yield of 0.67 % at 2.1 W cm^-2 . The structure and energy transfer mechanism of the assemblies were fully characterized. This is the first example of an Eu³⁺ -based upconverting system composed of two discrete mononuclear lanthanide complexes present as co-crystals in non-deuterated solution.