Theory of Absorption and Circular Dichroism of Large Inhomogeneous Molecular Aggregates

Abstract

A general method to describe the spectroscopy of large, internally inhomogeneous particles is presented. The theory utilizes an approach similar to the one used by De Voe in the treatment of the optical properties of polymers. It is found that if the particle is dense the intermediate and radiation coupling mechanisms must be included in addition to the dipole-dipole coupling. Through these coupling mechanisms it is found that the excitation generated at each group in the chromophore can delocalize over regions comparable to the size of the wavelength of light. The spatially averaged equations of the absorbance for a collection of large inhomogeneous arbitrarily shaped aggregates will be presented. This theory is then applied to the polymer and salt induced Psi-type circular dichroism observed in DNA aggregates. Using the formalism developed, it is shown that the anomalously large signals observed in the circular dichroism of certain molecular aggregates result from: (a) the presence of a long-range chiral structure in the aggregate; (b) delocalization throughout the entire particle of the light-induced excitations in the chromophores. This is the first successful attempt to explain the physical origin of the psi-type CD effect. Useful information regarding the chiral structure of the aggregates can be inferred from the theory.