  An improved real-time quantum Monte Carlo procedure is presented and applied
to describe the electronic transfer dynamics along molecular chains. The model
consists of discrete electronic sites coupled to a thermal environment which is
integrated out exactly within the path integral formulation. The approach is
numerically exact and its results reduce to known analytical findings (Marcus
theory, golden rule) in proper limits. Special attention is paid to the role of
superexchange and sequential hopping at lower temperatures in symmetric
donor-bridge-acceptor systems. In contrast to previous approximate studies,
superexchange turns out to play a significant role only for extremely high
lying bridges where the transfer is basically frozen or for extremely low
temperatures where for weaker dissipation a description in terms of rate
constants is no longer feasible. For bridges with increasing length an
algebraic decrease of the yield is found for short as well as for longer
bridges. The approach can be extended to electronic systems with more
complicated topologies including impurities and in presence of external time
dependent forces.
