  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.
