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

Abstract

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]·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 °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.