Intensity enhancement of circularly polarized luminescence in TbPO4·0.64H2O nanocrystals induced by the antenna effect.

We report the synthesis and the structural, morphological, and spectroscopic characterization of hydrated enantiomorphic trigonal Tb3+ orthophosphate nanocrystals (TbPO4·0.64H2O, 45 nm in size) functionalized in aqueous environment with chromophoric organic antenna ligands, namely dipicolinate (DPA) and phthalate (PHTA). During the functionalization protocol, a partial dissolution of the orthophosphate and a digestive ripening process were observed. Remarkably, these inorganic-organic hybrid materials retain Circularly Polarized Luminescence (CPL) activity. Although the CPL intensity of the functionalized samples shows a slight deterioration compared to the unfunctionalized orthophosphate, the ligand molecules, present only in very small amounts (2-3 wt%, as determined by thermal analysis), are sufficient to significantly enhance the Tb3+ emission in the green region around 542 nm, reaching nearly a 40-fold increase under ligand-mediated excitation in the case of DPA functionalization. Among the investigated orthophosphates, the best-performing green chiroptical phosphor is the DPA-functionalized sample prepared using a nominal Tb/ligand molar ratio of 2. The reasons underlying the more efficient Tb3+ luminescence sensitization by DPA are discussed, together with the deterioration of this sensitization observed for larger nanocrystals.