Charge Separation and Charge Transfer in the Low-Lying Excited States of Pentacene

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.