  This is a review of exciton dynamics in conjugated polymers. Exciton dynamics
encompass multiple time and length scales. Ultrafast femtosecond processes are
intrachain and involve a quantum mechanical correlation of the exciton and
nuclear degrees of freedom. In contrast, post-picosecond processes involve the
incoherent Forster transfer of excitons between polymer chains. Exciton
dynamics is also strongly determined by the spatial and temporal disorder that
is ubiquitous in conjugated polymers. Since excitons are delocalized over
hundreds of atoms, a theoretical understanding of these processes is only
realistically possible by employing suitably parametrized coarse-grained
exciton-phonon models. Moreover, to correctly account for ultrafast processes,
the exciton and phonon modes must be treated on the same quantum mechanical
basis and the Ehrenfest approximation must be abandoned. This further implies
that sophisticated numerical techniques must be employed to solve these models.
We begin by describing the energetic and spatial distribution of excitons in
disordered polymer systems. Next, we discuss ultrafast intrachain exciton
decoherence caused by exciton-phonon entanglement, which leads to fluorescence
depolarization on the timescale of 10-fs. Interactions of the polymer with its
environment causes the stochastic relaxation and localization of high-energy
delocalized excitons onto chromophores. The coupling of excitons with torsional
modes also leads to various dynamical processes. On sub-ps timescales it causes
exciton density localization and local polymer planarization, while on post-ps
timescales stochastic torsional fluctuations cause exciton diffusion along the
polymer chain. Finally, we describe a first-principles, Forster-type model of
intrachain exciton transfer and diffusion, whose starting point is a realistic
description of the donor and acceptor chromophores.
