  We report on large-scale simulations of intrachain exciton dynamics in
poly(para-phenylene vinylene). Our coarse-grained model describes Frenkel
exciton coupling to both fast, quantized C-C bond vibrations and slow,
classical torsional modes. We also incorporate system-bath interactions. The
dynamics are simulated using the Time Evolution Block Decimation method, which
avoids the failures of the Ehrenfest approximation to describe decoherence
processes and nonadiabatic interstate conversion. System-bath interactions are
modeled using quantum trajectories and Lindblad quantum jump operators. We find
that following photoexcitation, the quantum mechanical entanglement of the
exciton and C-C bond phonons causes exciton-site decoherence. Next, system-bath
interactions cause the stochastic collapse of high-energy delocalized excitons
onto chromophores. Finally, torsional relaxation causes additional
exciton-density localization. We relate these dynamical processes to the
predicted fluorescence depolarization and extract the timescales corresponding
to them.
