The Exciton Concept

The exciton picture is exploited for the description of molecular systems interacting with light. All the spectroscopically observed phenomena—light absorption, emission, electrostatic interaction of molecules, transitions between ground and exited electronic states, transfer of the excitation energy through the molecular aggregate, etc.—directly involve the formation and time evolution of the excitonic states. Generally, exciton is a collective molecular excitation that, depending on the strength of the intermolecular interaction, can be delocalized over several molecules. In this chapter, we discuss the basic aspects of the excitonic states in molecular dimer, aggregate, and crystal that define the spectroscopic properties of these molecular systems. We also outline various theoretical models that are widely used to describe exciton dynamics—Redfield, modified Redfield, Förster, Lindblad, and HEOM theories—as well as the conditions for their applicability.