Phthalocyanine (Pc)-based molecular thin films have emerged in recent years as a promising class of organic semiconductor materials for optoelectronic applications owing to their long exciton coherence length and fast exciton diffusion. However, the dependence of their exciton properties on the dimensionality and thermodynamic conditions, presence of metal ions, and effects of chemical modifications to the PC systems is not yet fully understood. As a first step towards a more comprehensive theoretical understanding of the excitonic properties of Pc thin films, we model their low-temperature exciton absorption spectra by employing the Frenkel Hamiltonian. The latter is derived from quantum-chemical estimates of site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy phthalocyanine (H2OBPc) is found to be strongly dependent on the dimensionality of the model as well as the distance cutoff for including the monomer-monomer exciton coupling. We also caution that the widely used dipole-dipole approximation could substantially overestimate the excitonic coupling between different monomers compared to a more accurate evaluation using the respective transition densities.
Sengupta et al. (Thu,) studied this question.