Incoherent Nonreciprocal Absorbance Circular Dichroism of Uniaxial Assemblies

Analytical theory is proposed predicting remarkably large and fully electric-dipole-allowed circular dichroism (CD) in electronic ultraviolet-visible (UV-vis) absorbance spectroscopy of uniaxial surface assemblies. Partial depolarization of the transmitted beam provides a pathway for surface-specific and chiral-specific dissymmetry parameters that are orders of magnitude greater than those from analogous measurements of isotropic systems. Predictions of the model generated using ab initio quantum chemical calculations with no adjustable parameters agreed with UV-vis absorbance CD measurements of naproxen microcrystals prepared on hydrophilic substrates. Notably, these calculations correctly predicted (i) the key spectroscopic features, (ii) the relative magnitudes of chiral-specific peaks in the CD spectrum, (iii) the absolute CD sign, and (iv) the reciprocal CD sign inversion arising from sample reorientation in the instrument. These results connect the molecular structure and orientation to large CD observable in oriented thin-film assemblies, with the potential for further extension to broad classes of chiral specific spectral analyses.

Automated summary

An analytical theory is proposed to predict remarkably large electric-dipole-allowed circular dichroism (CD) in electronic ultraviolet-visible absorbance spectroscopy. The theory suggests that partial depolarization of the transmitted beam provides a pathway for surface-specific and chiral-specific dissymmetry parameters. Ab initio quantum chemical calculations are used to generate predictions that align with experimental measurements. These results connect molecular structure and orientation to the observed CD in oriented thin-film assemblies.