  Phenomena involving electron transfer are ubiquitous in nature,
photosynthesis and enzymes or protein activity being prominent examples. Their
deep understanding thus represents a mandatory scientific goal. Moreover,
controlling the separation of photogenerated charges is a crucial prerequisite
in many applicative contexts, including quantum electronics,
photo-electrochemical water splitting, photocatalytic dye degradation, and
energy conversion. In particular, photoinduced charge separation is the pivotal
step driving the storage of sun light into electrical or chemical energy. If
properly mastered, these processes may also allow us to achieve a better
command of information storage at the nanoscale, as required for the
development of molecular electronics, optical switching, or quantum
technologies, amongst others. In this Topical review we survey recent progress
in the understanding of ultrafast charge separation from photoexcited states.
We report the state-of-the-art of the observation and theoretical description
of charge separation phenomena in the ultrafast regime mainly focusing on
molecular- and nano-sized solar energy conversion systems. In particular, we
examine different proposed mechanisms driving ultrafast charge dynamics, with
particular regard to the role of quantum coherence and electron-nuclear
coupling, and link experimental observations to theoretical approaches based
either on model Hamiltonians or on first principles simulations.
