Chiroptical Spectroscopy, Oriented Molecules and Anisotropic Systems

Chirality and circular dichroism of oriented molecules and anisotropic phases

“…Thus, an absorption measurement is not a suitable experiment to obtain a pseudoscalar property, i.e., a value of a chirality measurement. In principle, these absorption phenomena of oriented and or-dered molecules can be explained with the concept of eigenstates of light [2][3][4] : Every sample can transmit light only in special eigenstates which are determined by the symmetry and the material constants of the sample. Instead of the two circularly polarized eigenstates (Fig.…”

“…Kelvin's definition of chirality 5 does neither provide a way to define what "the anisotropy of chirality" means nor answers the question what a "chirality measurement" really means in an anisotropic system. On the basis of the definition of a chirality measurement for an isotropic dissymmetric phase by Ruch and Schönhofer, 6 an equivalent definition can be given for an anisotropic system 4,7,8 which includes the case of chirality information for selected viewing directions with respect to a molecule or a macroscopic phase: "a chirality measurement of a property of an oriented ensemble of orientationally and positionally ordered molecules, i.e., of an oriented chiral anisotropic phase, is a measurement of a property of an ensemble of 'anisotropic phases' rotationally symmetrically distributed about the viewing direction, which leads for an enantiomer of the 'chiral anisotropic phase' to a result of the same absolute value but with an opposite sign." This latter definition can be applied for properties of a single molecule as well as for properties of macroscopic anisotropic phases.…”

“…S* and D* are the Saupe order parameters which can be described by the coordinates of the order tensor given in its principal axes by S* = (3g* 3333 − 1)/2, [4] and D* = √3(g* 2233 − g* 1133 )/2.…”

“…From the theoretical point of view, the decomposition of the elliptical birefringence and dichroism into a pseudoscalar chiral effect and a scalar achiral effect needs new definitions. 4 It is important to realize that there is also a large experimental problem if the "linear effects" are large in comparison to the "circular effects," i.e., the elliptical eigenstates of the sample are far away from the circular eigenstates, because in this case CD instruments some- times produce artifacts in the process of decomposing elliptical eigenstates in "linear and circular contributions. "…”