Theoretical study of the large linear dichroism of herapathite

The remarkable linear dichroism of herapathite (HPT), the active component of polaroid, resisted explanation for more than 150 years because the crystal structure was not solved until very recently. The crystal structure with a formula unit of ${({C}₂₀{H}₂₄{N}₂{O}₂{H}₂)}₄{·}{C}₂{H}₄{O}₂{·}3{SO}₄{·}2{I}₃{·}6{H}₂O$ in an orthorhombic cell has a slight disorder related to the positions of the six water molecules and the acetic-acid molecule. The electronic and optical properties of this complex crystal are here calculated on the basis of the newly described x-ray structure using a density-functional theory based method with local-orbital basis. The theoretical optical spectrum of HPT shows giant optical anisotropy as observed experimentally with an anisotropy factor on the order of 385 that can be ascribed to transitions between molecular levels of the 2I₃^- chains that are oriented along the crystalline b axis. It is shown that the key to achieve large anisotropy is to align the iodine ions in a quasi-one-dimensional chain via confinement in a clatharate channel formed by the quinine molecules. The solvent molecules in the crystal have a minimal effect. The implications of this work on biologically relevant systems are discussed.