Amplification of Chirality in the Synthesis of Chiral Lanthanide Phosphate Nanocrystals

We have developed an enantio-selective synthesis of intrinsically chiral lanthanide phosphate hydrate (LnPO₄∙nH₂O) nanocrystals. These materials crystalize in the same space-group as quartz, and thus can be either left- or right-handed. Eu³⁺-doped TbPO₄ nanocrystals were synthesized via simple colloidal chemistry, and their handedness was controlled by using several chiral di-carboxylic acid containing molecules, where the most effective was tartaric acid (TA). We measured high circularly polarized luminescence (CPL) activity at the lanthanide emission lines. When using pure TA, the nanocrystals displayed a dissymmetry factor of up to 0.4 (20% circularly polarized light). Interestingly, when synthesized with increasing ratios of D- and L-TA (increasing enantiomeric excess), the increase in CPL activity was non-linear, saturating at ~40% enantiomeric excess of TA. We believe that this saturation in optical activity implies enantiopurity of the resulting nanocrystals. This chiral amplification occurs after an initial nucleation of small chiral crystalline seeds, and is probably related to the enantioselective adsorption of the chiral ligands on the seed particles surfaces.