Ultrafast excited-state dynamics at a conical intersection: the role of environmental effects

We review some of the concepts which we have recently developed in order to describe the influence of an environment on the ultrafast dynamical events at a conical intersection (CI). In particular, we propose a description in terms of effective environmental modes, which accurately account for the short-time dynamics at the conical intersection. We address the construction of these modes (i) for intramolecular situations, where the modes in question result from an orthogonal coordinate transformation for the overall N-mode system, and (ii) for solute-solvent interactions in polar solvents, where a description in terms of a Marcus-like solvent coordinate is obtained. In both cases, the dynamical process is determined by the combined evolution of the internal molecular modes and the environmental coordinates, on an ultrafast timescale. We discuss examples related to an intramolecular multi-mode situation in pyrazine, and the polar solvation dynamics for protonated Schiff bases.