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In the preceding paper, we assembled the theoretical components necessary fora unified framework of singlet fission, a type of multiexciton generationproducing two triplet excitons from one singlet exciton. In this paper, weapply our methodology to molecular dimers of pentacene, a widely studiedmaterial that exhibits singlet fission. We address a longstanding theoreticalissue, namely whether singlet fission proceeds via two sequential electrontransfer steps mediated by a charge-transfer state or via a direct two-electrontransfer process. We find evidence for a superexchange mediated mechanism,whereby the fission process proceeds through virtual charge-transfer stateswhich may be very high in energy. In particular, this mechanism predictsefficient singlet fission on the sub-picosecond timescale, in reasonableagreement with experiment. We investigate the role played by molecularvibrations in mediating relaxation and decoherence, finding that differentphysically reasonable forms for the bath relaxation function give similarresults. We also examine the competing direct coupling mechanism and find it toyield fission rates slower in comparison with the superexchange mechanism forthe dimer. We discuss implications for crystalline pentacene, including thelimitations of the dimer model.

Microscopic theory of singlet exciton fission. II. Application to pentacene dimers and the role of superexchange