  The concept of an exciton as a quasiparticle that represents collective
excited states was originally adapted from solid-state physics and has been
successfully applied to molecular aggregates by relying on the well-established
limits of the Wannier exciton and the Frenkel exciton. However, the study of
excitons in more complex chemical systems and solid materials over the past two
decades has made it clear that simple concepts based on Wannier or Frenkel
excitons are not sufficient to describe detailed excitonic behavior, especially
in nano-structured solid materials, multichromophoric macromolecules, and
complex molecular aggregates. In addition, important effects such as vibronic
coupling, the influence of charge-transfer (CT) components, spin-state
interconversion, and electronic correlation, which had long been studied but
not fully understood, have turned out to play a central role in many systems.
This has motivated new experimental approaches and theoretical studies of
increasing sophistication. This article provides an overview of works
addressing these issues that were published for A Special Topic of the Journal
of Chemical Physics on "Excitons: Energetics and spatio-temporal dynamics" and
discusses their implications.
