Electronic Circular Dichroism in Exciton-Coupled Dimers: Vibronic Spectra from a General All-Coordinates Quantum-Dynamical Approach

We present a computational approach of general applicability to simulate the vibronic line shapes of absorption and electronic circular dichroism (ECD) spectra in rigid exciton-coupled dimers based on a time dependent expression of the spectra and quantum dynamical calculations. We adopt a diabatic model of interacting states localized on the monomers whose electronic potential energy surfaces are described within harmonic approximation, including the effect of displacements, frequency changes, and normal-mode mixings. Spectra that fully account for the effect of all nuclear degrees of freedom of the system are obtained through a hierarchical representation of the Hamiltonian in blocks, defined so that few blocks accurately describe the short time dynamics. of the system. With this approach, on the ground of time dependent density functional theory calculations, we simulate the absorption and ECD spectra of a covalent compound representing a ``dimer'' of anthracene, in the spectral region of the L-1(a) monomer transition, obtaining results in good agreement with the experiment.