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 fast exciton diffusion. However, the dependence of their exciton properties on the dimensionality and temper- ature 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 understand- ing of the excitonic properties of Pc thin films, we model the low-temperature exciton absorption spectra through employing the Frenkel Hamiltonian and incorporating quantum-chemistry-based site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy ph- thalocyanine (H2OBPc) 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 could substantially overestimate the coupling of excitonic transition densities on different monomers.