A novel nonadiabatic molecular dynamics
scheme is applied to study
the singlet fission (SF) process in pentacene dimers as a function
of longitudinal and lateral displacements of the molecular backbones.
Detailed two-dimensional mappings of both instantaneous and long-term
triplet yields are obtained, characterizing the advantageous and unfavorable
stacking arrangements, which can be achieved by chemical substitutions
to the bare pentacene molecule. We show that the SF rate can be increased
by more than an order of magnitude through tuning the intermolecular
packing, most notably when going from cofacial to the slipped stacked
arrangements encountered in some pentacene derivatives. The simulations
indicate that the SF process is driven by thermal electron–phonon
fluctuations at ambient and high temperatures, expected in solar cell
applications. Although charge-transfer states are key to construct
continuous channels for SF, a large charge-transfer character of the
photoexcited state is found to be not essential for efficient SF.
The reported time domain study mimics directly numerous laser experiments
and provides novel guidelines for designing efficient photovoltaic
systems exploiting the SF process with optimum intermolecular packing
