Excited‐state dynamics

Abstract

Excited-state dynamics is the field of theoretical and physical chemistry devoted to simulating molecular processes induced upon UV-visible light absorption. This involves nuclear dynamics methods to determine the time evolution of the molecular geometry used in concert with electronic structure methods capable of computing electronic excited-state potential energy surfaces. Applications concern photochemistry (see Chapter CMS-030: Computational photochemistry) and electronic spectroscopy. Most of the work in this field looks at unsaturated organic molecules as these provide widely used chromophores with a straightforward photochemistry that can be described by a small number (usually two) of electronic states. The electronic ground state of closed-shell organic molecules is a singlet (electronic spin zero) termed S 0 . Molecules are promoted to their electronic excited states through absorption of UV-visible light (200-700 nm), usually to the first or second singlet, S 1 or S 2 . Typical examples are well represented as a one-electron transition from the π or n highest occupied molecular orbital to a π * or σ * low-lying unoccupied molecular orbital. The photo-excited system will deactivate and return to the electronic ground state over a timescale that can be as short as about 100 fs for ultrafast mechanisms. For example, the initial event of vision is a photo-isomerization of the retinal chromophore in the rhodopsine protein that occurs in ca. 200 fs. 1,2 The goal of a computational approach to the simulation of photo-induced processes is the complete description of what happens at the molecular level from the promotion to the excited electronic state to the formation of products or regeneration of reactants back in the electronic ground state.A fter light absorption, several photophysical or photochemical deactivation mechanisms can compete depending on their relative efficiencies. These are summarized in Figure 1, a typical Jablonski diagram. The excess energy can be released through light emission (fluorescence from a singlet or phosphorescence from a triplet) or through radiationless decay processes (internal conversion between singlets or intersystem crossing between a singlet and a triplet) that convert it into vibrational energy in S 0 . The spectral position, shape, and width of the absorption or emission spectra will depend on all the competing events that dominate the short-timescale dynamics of the molecule (changes in the molecular geometry through nuclear motion coupled to changes in

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