The psi-type circular dichroism of large molecular aggregates. III: Calculations

Computations have been carried out to determine how the magnitude and shape of the polymer and salt induced (psi)-type CD spectra depend on the structural properties of a collection of randomly oriented large chiral aggregates. Uniaxial polarizable groups located at the cubic lattice points have been used to model the aggregates. The structure of the model is similar to that of a cholesteric liquid crystal. All computations have been carried out for the case of polarizable groups possessing only one electronic transition between 200 and 320 nm. It is found that the radiation and intermediate couplings between the chromophores in the aggregate which are neglected in previous theories play an important role in determining the shape and magnitude of the psi-type CD spectrum. It is shown that when these couplings are included, only three-dimensional large chiral aggregates show huge and nonconservative psi-type CD spectra. It is shown that the magnitude of the psi-type CD spectrum is controlled by the volume, the chromophore density, and the pitch of the aggregate, while the shape of the psi-type CD spectrum is determined mostly by the pitch and the handedness of an aggregate. When the pitch is close to the center of the absorption band of the chromophore in the aggregate the most distorted (least conservative) psi-type CD spectrum is obtained. The CD spectra of aggregates with opposite handedness are mirror images of each other. It is shown that a rotationally disordered collection of chiral aggregates cannot give rise to a selective reflection of one circular polarization over the other as shown by liquid crystals. The results obtained confirm the theoretical predictions of the two previous papers in this series.