Circularly polarized and total luminescence as probes of nucleation and growth in chiral nanocrystals

Abstract

Nucleation of crystals as well as their growth is difficult to study experimentally. We have recently demonstrated that chiral Eu³⁺ -doped terbium phosphate nanocrystals are an interesting system for studying nanocrystal formation mechanisms and chiral symmetry breaking, occurring during their formation, directed by chiral ligands, such as tartaric acid. In this paper, we show how simultaneous, in situ monitoring of both total emission intensity and circularly polarized luminescence magnitude and sign versus time during nanocrystal formation provides considerable information on the mechanisms of nanocrystal nucleation and growth. Specifically, we show that the presence of tartaric acid leads to the formation of chiral prenucleation clusters, which deterministically transform into nanocrystals of a specific handedness. Additionally, we demonstrate that both unseeded and seeded nanocrystal syntheses behave differently mechanistically and that the addition of seed nanocrystals catalyses both enantio-specific (also called secondary nucleation) as well as nonspecific nucleation.

Keywords: chiral nanocrystals; circularly polarized luminescence; crystal nucleation and growth; nanocrystals.

FIGURE 1

(A) Total luminescence intensity (I), (B) circularly polarized luminescence (CPL) (ΔI) spectrum, and (C) dissymmetry factor (g _lum ) evolution with time. In a colloidal synthesis of Eu³⁺‐doped TbPO₄·D₂O NCs with L‐TA at 50°C in D₂O, measured at 704 nm (excitation wavelength = 365 nm). The 704‐nm emission line is attributed to the ⁵D₀ → ⁷F₄ transitions of the Eu³⁺ ion. The curves were normalized to their peak magnitude during the measurement period. The negative CPL is obtained due to the use of L‐TA and would be positive if D‐TA is used.

FIGURE 2

(A) Luminescence intensity and (B) circularly polarized luminescence (CPL) versus time measured at 50°C in D₂O, following the addition of L‐NC seed particles (20 μl of seed solution) in the presence of D‐ or L‐TA. L‐seeds with L‐TA synthesis yields 100% ee of the NCs, whereas L‐seeds with D‐TA yields a lower ee (~20%). CPL curves were normalized to the value of the enantiomerically pure NC sample (Figure 1B). The inset in (A) is a magnification of the first minutes of luminescence increase, showing that the two curves overlap at the first ~5 min.

FIGURE 3

(A) Luminescence intensity versus time of both seeded (20‐μl L‐NCs added after 26 min) and unseeded experiments, where the synthesis solution contained D‐TA. (B) Circularly polarized luminescence (CPL) versus time of the seeded experiment measured at 50°C, yielding a very high ee (92%). The CPL curve was normalized to the value obtained for enantiomerically pure sample as obtained in Figure 1B.