Circular extinction imaging: Determination of the absolute orientation of embedded chromophores in enantiomorphously twinned LiKSO₄ crystals

Crystals of lithium potassium sulfate dyed during growth with Chicago sky blue or pyranine were investigated with a recently developed circular extinction imaging microscope originally intended for capturing circular dichroism. The resulting micrographs provide evidence of anomalous circular extinction, an effect that can arise when strong oscillators embedded in an otherwise homogeneous, anisotropic medium are oriented and rotated in the same sense with respect to the eigenmodes of the host. Our images reflect the previously determined twin laws in LiKSO4 and establish the assignment of the absolute orientation of the dye molecules in the crystal, a supramolecular stereochemical subtlety that we have long struggled with.