  The size dependent exciton dynamics of one-dimensional aggregates of
substituted perylene bisimides are studied by ultrafast transient absorption
spectroscopy and kinetic Monte-Carlo simulations in dependence on the
temperature and the excitation density. For low temperatures the aggregates can
be treated as infinite chains and the dynamics is dominated by diffusion driven
exciton-exciton annihilation. With increasing temperature the aggregates
decompose into small fragments consisting of very few monomers. This scenario
is also supported by the time dependent anisotropy deduced from polarization
dependent experiments.
