Introduction: Theoretical Modeling of Excited State Processes

W do plasma, solar panels, and fireflies have in common? The fundamental physics of these phenomena is governed by excited state processes initiated by light. When a photon is absorbed by a molecule, it promotes an electron to a higher energy level leading to a new electron distribution, which often features an open-shell pattern. This event initiates a variety of processes: radiative and radiationless relaxation, photochemical transformations, electron ejection, or attachment. The competition between these processes determines the fate of an electronically excited system some emit light back, some effectively convert excess electronic energy into heat, some produce charge carriers, and some change their chemical identity. From the quantum-mechanical point of view, these processes entail coupled electronic and nuclear dynamics. Understanding how these quantum processes unfold in systems with many degrees of freedom, usually coupled to the environment, is of great fundamental importance. Ultimately, we want to know how the chemical structure of the molecule and the environment affect branching ratios and time scales of various excited state processes as shown in Fig 1. From a practical point of view, the ability to control these processes is the key to the successful design of new photovoltaic materials, bioimaging probes, photodynamic therapies, and materials for high-energy applications (e.g., fusion reactors). Moreover, precise understanding of lightmatter interactions and the ability to describe them quantitatively allows us to utilize radiation as a tool for interrogating properties of molecules and materials. Spectroscopy is indeed the most common and the most powerful tool for deciphering molecular structure. The techniques vary from classic UV−vis and photoelectron spectroscopies to novel nonlinear approaches and high-energy X-ray attosecond pulses. All these phenomena are governed by the same law: the Schrödinger equation: tantalizingly simple, yet notoriously difficult to solve. Although theory has been very successful in developing practical approaches for electronic structure and coupled nuclear-electron dynamics, challenges still abound. This topical issue of Chemical Reviews highlights recent progress as well as outstanding challenges. The review articles written by a worldwide group of experts span the following subjects: