AI Article Synopsis
- The observed relationship between circular dichroism (CD) and enantiomeric excess (ee) in chiral systems is usually linear, but there are rare cases of negative nonlinear (NN) behavior that are not well understood.
- Researchers found a strong NN CD-ee dependence in polycrystalline thin films of BINOL using advanced techniques like SHG-CD and CD spectroscopy.
- This NN behavior is linked to changes in supramolecular chirality and could enhance the characterization of enantiomeric excess, benefiting sectors like pharmaceuticals and asymmetric catalysis, suggesting many racemate crystals may exhibit similar behavior.
Article Abstract
Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD-ee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD-ee dependence are rare and not well understood. Here, we present a strong NN CD-ee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD-ee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD-ee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.
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