Coarse graining photo-isomerization reactions: Thermodynamic consistency and implications for molecular ratchets

We formulate thermodynamically consistent coarse-graining procedures for molecular systems undergoing thermally and photo-induced transitions: starting from elementary vibronic transitions, we derive effective photo-isomerization reactions interconverting ground-state species. Crucially, the local detailed balance condition, which constrains reaction kinetics to thermodynamics, remains satisfied throughout the coarse-graining procedures. It applies to the effective photo-isomerization reactions just as it does to the elementary vibronic transitions. We then demonstrate that autonomous photo-driven molecular ratchets operate via the same fundamental mechanism as chemically driven ones. Because the local detailed balance remains satisfied, autonomous photo-driven molecular ratchets, similar to chemically driven ones, operate exclusively through an information ratchet mechanism. This reveals that their design and optimization should prioritize molecular properties governing the information ratchet mechanism, rather than those influencing energetic bias.