Molecular Exciton Approach to Anisotropic Absorption and Circular Dichroism I. General Formulation

Summary.

The history of the molecular exciton approach and its application to optical rotatory power is outlined, followed by a summary of the general theory of electronic anisotropic chiroptical spectroscopy. The summary includes intensity expressions that are valid regardless of molecular dimensions as well as expressions valid only in the socalled small molecule limit, and presents the multipolar representation of the intensity expressions in the later case, emphasizing the importance of including electric dipole–magnetic dipole and electric dipole–electric quadrupole optical coupling terms on an equal footing in the description of anisotropic optical rotatory power. Expressions for isotropic chiroptical intensities are also included, and the difference in structural and point group symmetry requirements for anisotropic and isotropic optical rotatory power is discussed. On this basis, we present the molecular exciton approach for anisotropic chiroptical spectra, including interchromophoric electric dipole–electric dipole couplings and inter- and intra-chromophoric higher multipole couplings, allowing for the explicit treatment of coupling between achiral as well as chiral chromophores. Expressions are developed both for the partial intensities of the individual excitonic excitations and for the effective band shapes resulting from summation over all excitations within the exciton band. We discuss the importance of the choice of the reference points required for the description of the relative locations of the chromophoric units and for the definition of the local chromophoric multipolar transition moments, and we discuss the limitations of the present formulation. An appendix outlines the basic rules for the dyadic tensor notations utilized in the theoretical intensity formulations.