Relating Crystal Structure and the Charge-Transfer Nature of Excitons in Pentacene from First Principles

The nature of low energy optical excitations within pentacene has been the subject of many experimental and theoretical studies, with much disagreement as to the degree of their charge-transfer character. Here, we use many-body perturbation theory to study singlet excitons within different solid phases of pentacene and demonstrate that inter-molecular interactions lead to delocalized, charge-transfer-like excitations in the bulk crystalline phase. Using the Bethe-Salpeter two-particle correlation function, we demonstrate that the interplay between intermolecular hybridization, local exchange interactions, and attractive electron/hole interactions controls the nature of the exciton. Additionally, we explore simple models to understand and predict the nature of the excitonic wavefunction, in particular whether it has charge-transfer character.