Towards a Theoretical Understanding of Excitonic Properties of Pthalocyanine Thin Films. I. Low-Temperature Exciton Absorption Spectra

Phthalocyanine (Pc)-based molecular thin films have emerged in recent years as a promising class of organic semiconductor materials for achieving a long exciton coherence length and a high exciton diffusion coefficient. However, the dependence of their exciton properties on the dimensionality and temperature of the Pc systems, existence of metal ions, and chemical modifications to the Pc molecule 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 the low-temperature exciton absorption spectra through employing the Frenkel Hamiltonian and incorporating quantum chem- istry based site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy phthalocyanine (H2-OBPc) was found to be strongly dependent on the dimensionality of the model as well as the distance cutoff for including the monomer-monomer exciton coupling. The best match to experimental low-temperature absorption spectra requires a 2D or 3D model and an inclusion of at least the nearest-neighbor and second-nearest-neighbor exciton couplings. We also caution that the widely used dipole-dipole coupling approximation can substantially overestimate the coupling of excitonic transition densities on different monomers.