Charge-Transfer States in Pentacene, from Dimer to Crystal
John Herbert, Ohio State University
Pentacene thin films are common constituents of organic photovoltaic materials and a prototypical example of a material that undergoes singlet exciton fission, but significant questions remain regarding the mechanism. In particular, theoretical studies have reached differing conclusions regarding the role (and even the presence) of low-energy charge-transfer (CT) states in this material. Periodic electronic structure calculations predict low-energy CT states in crystalline pentacene but correlated wave function calculations on cluster models (typically dimers) have generally failed to find evidence of CT states at energies relevant to singlet fission. We will address this question using a novel form of ab initio exciton model as well as time-dependent density functional theory calculations. Both approaches support the emergence of CT states, at energies relevant to singlet fission, in small cluster models, although notably these states are sensitive to cluster geometry and much higher in energy in dimer models.