  We investigate theoretically the influence of strong light-matter coupling on
the initial steps of the photo-triggered singlet-fission process. In particular
we focus on intra-molecular singlet fission in a TIPS-pentacene dimer
derivative described by a vibronic Hamiltonian including the optically active
singlet excited states, doubly excited and charge transfer states, as well as
the final triplet-triplet pair state. Quantum dynamics simulations of up to
four dimers in the cavity indicate that the modified resonance condition
imposed by the cavity strongly quenches the passage through the intermediate
charge transfer and double-excitation states, thus largely reducing the
triplet-triplet yield in the bare system. Subsequently, we modify the system
parameters and construct a model Hamiltonian where the optically-active singlet
excitation lies below the final triplet-triplet state such that the yield of
the bare system becomes insignificant. In this case we find that using the
upper polariton as the doorway state for photo-excitation can lead to a much
enhanced yield. This pathway is operative provided that the system is
sufficiently rigid to prevent vibronic losses from the upper polariton to the
dark-states manifold.
