The simplest picture of a chemical reaction through a reaction coordinate supposes a system begins as a set of reactants whose energy must fluctuate to overcome some activation barrier en route to the reacted products. These fluctuations are atypical in that they have large energy changes relative to the available thermal energy and thus reactions are exceedingly rare. Often, though, these reactions can occur after some initiation event, which removes this barrier and allows the reaction to freely proceed. Such is the case for photoinduced chemical reactions in which light-matter coupling instantaneously pumps energy into the system and serves as this initiator. These reactions are prevalent in all walks of the chemical sciences.The generic understanding of these reactions is, however, complicated by the many strongly-interacting degrees-of-freedom typically involved. Worse yet, these reactions, except in the most controlled of cases, mostly occur in condensed phase environments such as solvents, solid frameworks, or protein complexes, which dramatically increase the complexity of these dynamics while having important consequences on the reactions.

In this thesis we have applied the tools of nonequilibrium statistical mechanics to understand these photoinduced chemical reactions. These approaches rely on the accurate, but efficient treatment of multi-dimensional quantum systems through rigorous application of approximate theories consistent with fundamental thermodynamic relations. When satisfied, the resulting picture of the dynamics is numerically efficient and quantitatively accurate for many observables. Furthermore, path sampling approaches can be applied through the use of a quantum trajectory framework of the dynamics, which unravels the complex dynamics into a set of reduced simplified reaction coordinates. These techniques, as is demonstrated on model systems of photoinduced phenomena, can describe the complex dynamics and simulate the experimental observations of real condensed phase systems.