Electromagnetic Enantiomer: Chiral Nanophotonic Cavities for Inducing Chemical Asymmetry

Abstract

Chiral molecules, a cornerstone of chemical sciences with applications ranging from pharmaceuticals to molecular electronics, come in mirror-image pairs called enantiomers. However, their synthesis often requires complex control of their molecular geometry. We propose a strategy called "electromagnetic enantiomers" for inducing chirality in molecules located within engineered nanocavities using light, eliminating the need for intricate molecular design. This approach works by exploiting the strong coupling between a nonchiral molecule and a chiral mode within a nanocavity. We provide evidence for this strong coupling through angular emission patterns verified by numerical simulations and with complementary evidence provided by luminescence lifetime measurements. In simpler terms, our hypothesis suggests that chiral properties can be conveyed on to a molecule with a suitable chromophore by placing it within a specially designed chiral nanocavity that is significantly larger (hundreds of nanometers) than the molecule itself. To demonstrate this concept, we showcase an application in display technology, achieving efficient emission of circularly polarized light from a nonchiral molecule. The electromagnetic enantiomer concept offers a simpler approach to chiral control, potentially opening doors for asymmetric synthesis.

Keywords: asymmetric chemistry; chiral induction; nanophotonics; polaritonic chemistry; strong coupling; superchirality.

Figure 1

(a) Pair of enantiomers with mirror-image geometries. (b) Interaction of chiral near-fields with achiral molecules results in EM enantiomers through the symmetry-dependent creation of polariton states. (c) Schematic representation of strong coupling between the dark mode of an asymmetric photonic nanocavity and excited state of an achiral molecule placed within its vicinity, leading to the production of hybrid light–matter chiral polariton states.

Figure 2

(a) Chemical structure of MHeB14. (b) Schematic showing orthographic (left top), side view (left bottom), and SEM images (right) of the RH nanocavity. The white scale bar at the bottom represents 500 nm on the micrograph. (c) Self-normalized absorbance (solid black) and luminescence (solid red) of MHeB14 in toluene, imaginary part of refractive index (dashed black), and luminescence (dashed red) of 50% (w/w) film.

Figure 3

Three luminescence bands transitions, labeled as bands I, II, and III, are derived from Gaussian fitting of the photoluminescence (PL) spectrum of MHeB14 on a flat gold substrate. Two dark eigenmodes of nanocavities are show as violet and yellow solid line filled bands. The black arrows indicate the field profiles (top) illustrating the nondipolar y-component of the electric field associated with these modes.

Figure 4

(a) In-plane (E_y) and out-of-plane (E_z) electric-field distributions for 1000 (top) and 1500 nm (bottom) metafilms at the center of the cavity at eigenmodes A and B. The incident E-fields are left and right circularly polarized, and the field values are normalized with respect to the corresponding values of flat gold. The numerical annotation at the bottom indicates the field scaling factor; for instance, “/2.5” implies that the maximum and minimum values are 2.5 times lower than the values indicated on the colorbar. (b) Averaged CPL emission from a single-point electrical dipole oriented at various angles and positioned at the center of the LH cavity is depicted for a metafilm at 1000 nm (top) and 1500 nm (bottom).

Figure 5

Left circularly polarized emission (top) and right circularly polarized emission (bottom) for (a) 1000 and (b) 1500 nm metafilms. The olive and blue colored solid dash filled spectra correspond to luminescence band I of molecules on flat gold and dark eigenmode A of nanocavity. The dashed black vertical lines are guidance for eyes, highlighting the correspondence between PL peak, maxima of dark eigenmode and luminescence band I. ΔE shows the rabi splitting of energy level for the matched combination. (c) Enlarged version of LH-LCP and LH-RCP spectra, marked as red and blue boxes in the original graph, showing PL peak splitting and enhancement for the matched and mismatched combinations, respectively.

Figure 6

Experimental and simulated angle-resolved LCP and RCP emission along with dichroism (LCP–RCP) of the MHeB14-doped LH 1000 nm metafilm. In the experiment, the colors represent the relative intensity count with respect to the flat background. In the simulation, the colors represent the volume-integrated E-field magnitude in the 200 nm film.

Figure 7

Experimental and simulated angle-resolved LCP and RCP emission along with dichroism (LCP–RCP) of the MHeB14-doped LH 1500 nm metafilm. In the experiment, the colors represent the relative intensity count w.r.t. the flat background. In the simulation, the colors represent the volume-integrated E-field magnitude in the 200 nm film.

Figure 8

Fluorescence lifetime spatial map for MHeB14 molecules (top) and the distribution of lifetimes obtained from the highlighted area—black dashed box for molecules on flat gold and red dashed for structures—(bottom) for (a) 1000 and (b) 1500 nm metafilms. τ_max corresponds to the lifetime of the peak of normalized pixel distribution. The white scale bar at the bottom on lifetime images corresponds to 50 μm.