Water Soluble Rare-Earth(III) based Coordination Polymers with Luminescence Versatility in Various Media: Reaching the Sky of Quantum Efficiency

The preparation of highly efficient photoluminescent rare-earth based coordination compounds, characterized by photoluminescence quantum yield (PLQY) over 90% and thermal/chemical stability that allows their processing in various media (aqueous solutions, polymeric films, etc.) holds enormous significance in their applicability. Herein, a family of isostructural coordination polymers (CPs) with 6-methyl-2-oxonicotinate (6m2onic) ligand, chemically and structurally characterized as {[M(6m2onic)4Na(H2O)3]<middle dot>8H2O}n [where M(III) = Eu (1Eu), Tb (2Tb), Gd (3Gd), Y (4Y) and Eu0.5Tb0.5 (5Eu-Tb)], are reported. Their peculiar crystal structure, based on a hydrogen-bonded framework of 1D arrays in which octacoordinated metal centers (established by four chelating 6m2onic ligands) and Na centers are sequentially linked, gives rise to an excellent metal-organic system benefitting from not only bright PL emissions in solid state but also enough chemical and thermal stability as to yield PL water-soluble complexes and photostable thin-films. In particular, the terbium(III)-based counterpart highlights for its first-in-class PLQY and versatility, which imbues the compound with efficiencies of 97% in bulk state, 35% in aqueous solution, 85% in polymer-based thin-films and 15% after its calcination at 250 degrees C. The experimental photophysical characterization in those media, involving also pH-responsive behavior, is well supported by a solid theoretical analysis of their intramolecular transfers and electronic transitions.