Nonequilibrium thermodynamics of light-induced reactions

Abstract

Current formulations of nonequilibrium thermodynamics of open chemical reaction networks only consider chemostats as free-energy sources sustaining nonequilibrium behaviours. Here, we extend the theory to include incoherent light as a source of free energy. We do so by relying on a local equilibrium assumption to derive the chemical potential of photons relative to the system they interact with. This allows us to identify the thermodynamic potential and the thermodynamic forces driving light-reacting chemical systems out of equilibrium. We use this framework to treat two paradigmatic photochemical mechanisms describing light-induced unimolecular reactions -namely the adiabatic and diabatic mechanisms -and highlight the different thermodynamics they lead to. Furthermore, using a thermodynamic coarse-graining procedure, we express our findings in terms of commonly measured experimental quantities such as quantum yields.

Kinetic Barrier Diagrams to Visualize and Engineer Molecular Nonequilibrium Systems

“…The relevance of thermal exergonic steps is a direct consequence of the different principles underlying light-driven systems and chemically driven ones. 54,55 Indeed systems based on photoswitches allow the simultaneous occurrence of information and energy ratchet effects, which is prevented in chemically driven systems. 19 When analyzing the rotation rate, the less stable TS are two effective photochemical states characterized by a practically degenerate span.…”