  The mechanisms underlying coherent and thermally activated singlet exciton
fission in pi-stacked acene crystals are clarified based on quantum dynamics
simulations parameterized against a highly correlated description of the
electronic excitations and their couplings to intramolecular and intermolecular
vibrations. In TIPS-pentacene crystals, the relative longitudinal shift of the
molecular backbones yields large electronic couplings of the triplet exciton
pair with both the singlet exciton and charge transfer (CT) states. CT-mediated
superexchange and direct pathways are found to contribute synergetically to the
ultrafast (100fs) singlet fission process driven by vibronic coherences. By
contrast, the electronic couplings for singlet fission strictly vanish at the
equilibrium pi-stacking of rubrene that exhibits C2h symmetry. In this case,
the process is incoherent and driven by excitations of symmetry-breaking
intermolecular vibrations, rationalizing the experimentally observed
temperature dependence.
