Chiral templating of self-assembling nanostructures by circularly polarized light

Abstract

The high optical and chemical activity of nanoparticles (NPs) signifies the possibility of converting the spin angular momenta of photons into structural changes in matter. Here, we demonstrate that illumination of dispersions of racemic CdTe NPs with right- (left-)handed circularly polarized light (CPL) induces the formation of right- (left-)handed twisted nanoribbons with an enantiomeric excess exceeding 30%, which is ∼10 times higher than that of typical CPL-induced reactions. Linearly polarized light or dark conditions led instead to straight nanoribbons. CPL 'templating' of NP assemblies is based on the enantio-selective photoactivation of chiral NPs and clusters, followed by their photooxidation and self-assembly into nanoribbons with specific helicity as a result of chirality-sensitive interactions between the NPs. The ability of NPs to retain the polarization information of incident photons should open pathways for the synthesis of chiral photonic materials and allow a better understanding of the origins of biomolecular homochirality.

Figure 1. Self-assembly of CdTe NPs into twisted nanoribbons induced by illumination with CPL

A and B, SEM images of the ribbons assembled with LCP (A) and RCP (B) as a function of time exposure for 1 h, 12 h, 28 h and 50 h. All scale bars are 1 μm. C and D, tapping mode atomic force microscopy (AFM) topography (left) and phase (right) images of LH nanoribbon (C) and RH nanoribbon (D). E, Distributions of LH, RH, and non-twisted nanoribbons obtained after 50 h illumination with RCP, LCP, UnP, LinP light, and in the dark. F, ensemble CD spectra (solid line) and g factor (dotted line) of dispersions of left-handed (LH) nanoribbons and right-handed (RH) nanoribbons obtained after 50 h of CPL illumination. Linear dichroism effects that could be associated with adsorption on the walls of the cuvette and other spontaneous alignment of linear nanostructures have negligible contribution to the chiroptical properties as indicated by the identity of the CD spectra obtained with and without stirring of the dispersion.

Figure 2. Chirality of single nanoribbons

A and B, Surface rendering of 3D TEM tomographic reconstruction of LH nanoribbon (A) and RH nanoribbon (B). Scale bars are 100 nm. C and D, SEM images of single LH (C) and RH nanoribbons (D). Scale bars are 500 nm. E and F, CD spectra and calculated g-factor spectra for single LH (E) and RH (F) nanoribbons in C and D, respectively. G and H, Computational models of the LH (G) and RH (H) nanoribbons used in the FEM calculations of chiroptical properties based on numerical solutions of Maxwell equations. All scales are nm.

Figure 3. Mechanism of enantioselective assembly of NPs

A, FTIR spectra of original CdTe NPs, purified nanoribbons, and supernatant obtained after 50 h of illumination time. B, Schematic illustration of CdTe phase transition to CdS. C, and E, Models of chiral NPs (C) and chiral NP clusters (E) used in calculations of chiroptical properties. D, and F, Simulated spectra and g-factors for (D) L/R-NPs and (F) L/R-clusters of NPs. Nomenclature for NPs and their clusters is based on the positive (L) and negative (R) optical activity. G, and H, HAADF STEM images of TGA-stabilized truncated tetrahedral CdTe NPs. Scale bars are 15 nm (G) and 5 nm (H). I, High resolution TEM image of TGA-stabilized truncated tetrahedral CdTe NPs.

Figure 4. Molecular dynamic and experimental studies of self-assembly of chiral NPs

A, Atomistic models of NPs with LH and RH truncations used in MD simulations. B, Detailed view of a single NP in aqueous environment with counter ions used in MD simulations. C, A fragment of the simulated self-assembled ribbon from (top view) displaying packing of NPs. Scale bars in A, B, and C are 1 nm. D, and E, The side views of simulated NP ribbon with LH (D) and RH (E) truncated NPs. Dihedral angle θ determines the pitch of the nanoribbons. F, and G, SEM images of experimental assemblies spontaneously formed in dark from chiral CdTe NPs stabilized by L-cysteine (F) and D-cysteine (G). Scale bars are 1 μm.