Charge Separation and Charge Transfer in the Low-Lying Excited States of Pentacene
Bushra Alam, Adrian F. Morrison, John M. Herbert
Abstract
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. Here, we use an ab initio exciton model to examine size-dependent trends in low-energy CT states, in models ranging from pentacene dimer to hexamer. We complement these results with additional calculations using time-dependent density functional theory. Our calculations support the idea that dielectric stabilization leads to the appearance of low-energy CT states in the crystalline material that are absent in dimer models, but which (in larger models) become accessible at photon energies relevant to singlet fission. Optimally-tuned and screened range-separated hybrid functionals, which set the frontier orbital energies in a nonempirical way, predict a greater degree of charge separation as compared to other common range-separated hybrid functionals. We examine electron–hole correlations in these calculations, which reveal underlying charge separation in the excited states that may go undetected by other qualitative analysis tools. These results help to connect dimer quantum chemistry to periodic calculations, and they suggest that the former are inadequate models for singlet fission.