Electronic Circular Dichroism Imaging (ECDi) Casts a New Light on the Origin of Solid‐State Chiroptical Properties

Abstract

Solid‐state ECD (ss‐ECD) spectra of a model microcrystalline solid, finasteride, dispersed into a KCl pellet were recorded by using the synchrotron radiation source at the Diamond B23 beamline. Scanning a surface of 36 mm2 with a step of 0.5 mm, we measured a set of ECD imaging (ECDi) spectra very different from each other and from the ss‐ECD recorded with a bench‐top instrument (1 cm2 area). This is due to the anisotropic part of the ECD (ACD), which averages to zero in solution or on a large number of randomly oriented crystallites, but can otherwise be extremely large. Two‐way singular value decomposition (SVD) analysis, through experimental and simulated TDDFT spectra, disclosed that the measured and theoretical principal components are in line with each other. This finding demonstrates that the observed isotropic ss‐ECD spectrum is governed by the anisotropy of locally oriented crystals. It also introduces a new quality for ss‐ECD measurements and opens a new future for probing and mapping chiral materials in the solid state such as active pharmaceutical ingredients (APIs).

A 2D chiral microcavity based on apparent circular dichroism

“…ACD is also intrinsically 2D chiral, due entirely to the symmetries of the sample and present at normal incidence of light, in contrast with extrinsic chirality that is due to the oblique light incidence imposing a net chirality on the system 17 , 38 , 39 . We also note that ACD should not be confused with the similarly named anisotropic CD 40 , 41 and also that ACD is a form of conversion circular dichroism 17 wherein the net absorption of light occurs due to conversion from LCP to RCP or vice versa.…”

A Chiral Microcavity based on Apparent Circular Dichroism

“…This property is due to "apparent" circular dichroism (ACD), which does not intrinsically originate at the molecular or microscopic level as in the case of natural optical activity, but rather originates from macroscopic ordering due to nonparallel axes of linear dichroism and linear birefringence, which occurs in oriented, low symmetry systems [39][40][41] . We also note that ACD should not be confused with the similarly-named anisotropic CD 42,43 . ACD possesses Lorentz reciprocity but categorically lacks that of directional reciprocity, Figure 1a, which offers the opportunity to induce directionally dependent polarization rotation, thus overcoming the critical limitation in microcavities that prevents amplification since this symmetry breaking counteracts the inversion of circular polarization of normal mirrors.…”

A Perspective on the Simulation of Electronic Circular Dichroism and Circularly Polarized Luminescence Spectra in Chiral Solid Materials

“…Because of the nature of the technological applications of these materials, their characterization often relies on spectroscopic techniques based on their interaction with chiral light, i.e., circularly polarized light. The dominant techniques are electronic circular dichroism (ECD), ,− which measures the difference in absorption between left and right circularly polarized light, and its emission analogue, circularly polarized luminescence (CPL). ,,,,− ECD is sensitive to both the intrinsic chiral nature of the chromophoric units and the supramolecular chirality of the assembly. CPL can provide important information about the structural reorganization of the material after absorption and charge transport dynamics.…”