  In the preceding paper, we assembled the theoretical components necessary for
a unified framework of singlet fission, a type of multiexciton generation
producing two triplet excitons from one singlet exciton. In this paper, we
apply our methodology to molecular dimers of pentacene, a widely studied
material that exhibits singlet fission. We address a longstanding theoretical
issue, namely whether singlet fission proceeds via two sequential electron
transfer steps mediated by a charge-transfer state or via a direct two-electron
transfer process. We find evidence for a superexchange mediated mechanism,
whereby the fission process proceeds through virtual charge-transfer states
which may be very high in energy. In particular, this mechanism predicts
efficient singlet fission on the sub-picosecond timescale, in reasonable
agreement with experiment. We investigate the role played by molecular
vibrations in mediating relaxation and decoherence, finding that different
physically reasonable forms for the bath relaxation function give similar
results. We also examine the competing direct coupling mechanism and find it to
yield fission rates slower in comparison with the superexchange mechanism for
the dimer. We discuss implications for crystalline pentacene, including the
limitations of the dimer model.
