Dark-Field Absorbance Circular Dichroism of Oriented Chiral Thin Films

Dark-field and confocal approaches to circular dichroism (CD) spectroscopy of uniaxial thin films examine the relationship between symmetry and incoherence in the nonreciprocal CD response, or the component that is antisymmetric about the light propagation direction. Modifying a conventional CD spectrometer for low-angle scattering detection isolates incoherent contributions to nonreciprocal CD of drop-cast thin films, boasting 5-to-10-fold enhancements in CD dissymmetry parameters. Conversely, confocal detection suppresses the nonreciprocal CD response. These collective measurements provide the first compelling evidence of early predictions by Hecht and Barron, which indicate large chiral- and interface-specific CD observables from scattered signals in uniaxially oriented assemblies. According to this theory, nonreciprocal CD is possible within the electric dipole approximation, leading to chiral-specific observables exceeding reciprocal, isotropic contributions. Dark-field absorbance CD (DCD) spectroscopy thus offers new insights into molecular and macromolecular arrangements with interface selectivity and chiral specificity.